Advances in diamond nanofabrication for ultrasensitive devices

ABSTRACT This paper reviews some of the major recent advances in single-crystal diamond nanofabrication and its impact in nano- and micro-mechanical, nanophotonics and optomechanical

components. These constituents of integrated devices incorporating specific dopants in the material provide the capacity to enhance the sensitivity in detecting mass and forces as well as

magnetic field down to quantum mechanical limits and will lead pioneering innovations in ultrasensitive sensing and precision measurements in the realm of the medical sciences, quantum

sciences and related technologies. SIMILAR CONTENT BEING VIEWED BY OTHERS NANODIAMONDS ENABLE FEMTOSECOND-PROCESSED ULTRATHIN GLASS AS A HYBRID QUANTUM SENSOR Article Open access 18 April

2023 RECENT PROGRESS IN HYBRID DIAMOND PHOTONICS FOR QUANTUM INFORMATION PROCESSING AND SENSING Article Open access 08 May 2025 NANOSCALE DIAMOND QUANTUM SENSORS FOR MANY-BODY PHYSICS

Article 11 November 2024 INTRODUCTION Diamond is an ideal platform for nano- and microfabrication leading to a range of robust sensors. This is due to the outstanding mechanical and wide

spectral optical transparency of diamond, combined with biocompatibility and lack of toxicity in both bulk and nanostructures. Moreover, diamond can be fabricated with high purity and

control using chemical vapor deposition. However, due to its hardness and cost, some of its applications in nanomechanics, optomechanics and nanophotonics reside mainly in its

polycrystalline or nanocrystalline forms, which do not always retain the nuance of the outstanding properties of monocrystalline diamond. A recent review1 describes diamond optical and

mechanical properties compared to other materials currently used for optomechanical components (Si, Si3N4, SiC, and α-Al2O3), showing that, in principle, there are more favorable properties

of diamond for nanomechanical and optomechanical realizations. Therefore, a considerable effort has been taking place over the last five years to exploit these properties in a wide spectrum

of diamond nanosystems. To date, single crystal diamond utilization for nanomechanical components2, (for example, nano electro-mechanical systems (NEMS) and micro electro-mechanical systems

(MEMS)), as well for nanophotonics3,4 compared to above mentioned materials, has been limited due to its growth requirements. In fact, a single crystal diamond cannot be grown on any other

substrate than itself. This prevents wafer-scale processing and it represents a challenge in the application of conventional diamond nanofabrication methods. On the other hand,

polycrystalline diamond films, which can be grown in high quality from 4 to 6 inches’ wafers5, permit to fabricate opto-mechanical components with quality factors and oscillation frequencies

rivaling single crystal diamond6. Thus, diamond nanomechanical systems, such as nanomechanical resonators and nanocantilevers, can provide the means for ultrasensitivity in detecting

nanomechanical motions in force microscopy7, and to establish optomechanical systems6, where optical modes and mechanical modes can be coupled. Furthermore, nanofabrication has led to

photonic components in diamond, such as waveguides, photonic crystals (PHC) and optical nanoresonators, to advance light manipulation, propagation and confinement8–10, while their

combination with mechanical modes can advance cavity optomechanics11,12. Another important feature in diamond is the harboring of isolated atomic color centers that can be used as

fluorescent and spin probes for single molecule detection and imaging as well as for fundamental investigation into quantum science. For example, the nitrogen vacancy (NV) center13,14 in

diamond possesses the exceptional optical signatures of an associated electron spin, observed and subsequently exploited for applications including quantum information processing15 and

magnetometry16, with a prominent application being nano–magnetic resonance imaging (nano-MRI) and microscopy17. It has now been established that to reach exceptional sensitivity in diamond

devices with integrated fluorescence and spin probes, the main thrust resides in its single crystal nanofabrication and material purity. Many challenges regarding this accomplishment have

recently been addressed, and a considerable number of advances in this field have now brought forward representative nano- and micro-systems that promise a paradigm shift in sensing, and to

a certain extent surpasses other conventional micro- and nano-devices developed years ago. Proof of the principal demonstrations of integration of these diamond probes within nanoresonators,

based on other outstanding materials more amenable to nanofabrication such as Si3N4, Si nanophotonics or SiC cantilevers, have shown the possibility to couple nanomechanical and

nanophotonic systems with atomic scales probes, such NV center in nano-diamond, bulk diamond or thin diamond membranes (for a review on this specific hybrid systems, see Ref. 18). However,

novel nanosystems sculpted directly in diamond with integrated spin and fluorescent atomic probes are an advanced platform for the realization of ultrahigh sensitivity or strong coupling of

mechanical, optical and spin modes to coherently transfer information from the atomic level to the macroscopic level in a photonic quantum network made of these components19 or for force

microscopy and nano-MRI. In this paper, we focus on a review of the main concepts that lie behind nano-mechanical, nanophotonics and opto-mechanical all-diamond components and recent

advances in their fabrication in single crystal diamond, required in the field of ultrahigh-sensitivity sensors. Furthermore, we highlight the nanofabrication of single crystal diamond with

integrated spin and optical centers acting as multifunctional atomic size probes for ultrahigh-sensitivity devices and leading to exciting applications in field of quantum science and

technology. NANOMECHANICAL COMPONENTS Nanomechanical resonator sensing devices have a very small mass and thus high responsivity to changes in the local environment. Nanomechanical systems

are therefore relevant in force measurements via scanning probe magnetic and atomic force microscopy (AFM), yielding present sensitivities at the attonewton (aN) level (~10–18 N Hz−1/2) and

mass detection of single molecules and proteins20–23. The minimum force detectable by a nanomechanical cantilever, equivalent to a force noise associated with mechanical dissipation, can be

calculated if the cantilever behaves as a simple harmonic oscillator24. It is limited by thermomechanical noise, which is governed by dissipation of the mechanical energy and sample

temperature, and is given by: F min = 2 k B T B k c π f c · Q m Here _k_ c is the spring constant, _f_ c is the resonator frequency, _B_ is the resonator bandwidth and _Q_ m is the

mechanical quality factor. _Q_ m characterizes a resonator's bandwidth relative to its center frequency _f_ c. Higher _Q_ m indicates a lowest rate at which the resonator loses

mechanical energy. For a simple rectangular cantilever, this minimum detectable force can also be expressed in terms of the cantilever width (_w_), thickness (_t_), length (_l_), mass

density (_ρ_) and Young Modulus _E_, in the case of non-internal stress, as F min = w t 2 k B T B l Q m E ρ Therefore, to increase sensitivity, a mass reduction is a suitable solution, in

addition to making the cantilever narrow, thin and long. However, _Q_ m needs to be preserved in the scaling of the cantilever, and to this end, many (yet not fully known) factors at the

micro- and nanoscale tend to introduce other mechanisms responsible for energy dissipation. Recent progress has consisted in fabricating thinner resonators with lower spring constants _k_ c

in conventional materials, such as Si25, or by changing the resonators geometry using doubly clamped beams26. However, reducing the nano-resonators, thickness introduces a decreased _Q_ m

due to increased surface friction by surface defects24,25. _Q_ m can be restored by reducing the temperature27 (the surface friction being less at cryogenic temperatures), which reduces

practicability. Another option is to use materials that naturally lend themselves to high _Q_ m and high _f_ c (noting that Q m , f c ∝ k c ; k c ∝E), due to higher _k_ c, because of high

stiffness (i.e., a high Young Modulus _E_, _E_=1220 GPa and _ρ_=3530 kg m−3 for example single crystal diamond)24. The same materials are, however, more challenging in establishing micro-

and nanofabrication procedures, which can introduce unexpected mechanical losses. The motivation behind the efforts for fabricating nanomechanical components in diamond is twofold. One is

based on its excellent mechanical and thermal properties and the other is related to the sub-bandgap optical and spin-carrying color centers due to a large bandgap (5.47 eV). These color

centers facilitate the optical readout of the nano-resonator sensors on one side (the electrical readout in nano-mechanical resonators is more cumbersome than in larger micro mechanical

resonators), and they introduce a direct magnetic sensing degree of freedom and can permit optomechanics in the same device. The first motivation relies on the fact that diamond has the

largest Young modulus (inferior only to graphene), which is responsible for a high mechanical resonance frequency and low mechanical losses. Furthermore, the high thermal conductivity (2200 

W mK−1) and low thermal expansion coefficient permit high thermal dissipations. Diamond therefore offers a strategy for exploiting the intrinsically high _f_ c and achievable high _Q_

m=6·106 at the millikelvin level27. These properties remain good at room temperature (_Q_ m=1.2×106), making possible ultrahigh sensitivities without the need of cooling. Combined with

diamond biocompatibility, this can extend the above-mentioned applications at the current sensitivity to living cells and tissues by mere substitution of the material for nano-resonators.

The detection of nuclear magnetic resonance signals using spin magnetic resonance force microscopy28 requires a sensitivity in the zeptonewton (zN) range (~10−21 N Hz−1/2) about force

detection, which is currently achieved using trapped ions29 and nano-tubes mechanical resonators30. The integration of spin sensitive color centers in diamond within high-sensitivity single

crystal nanomechanical resonators could allow a more practical approach to achieve single nuclear spin sensing at room temperature; this has already been demonstrated in a dilute ensemble of

nuclear spins without resorting to the use of ultrasensitive nanomechanical resonators (for a recent review, see Ref. 17 and citations therein). A competitive material to diamond that is

already replacing Si NEMS/MEMS for harsh environments is SiC. NEMS are based on epilayers SiC and have been fabricated for some years but with a lower _Q_ m=500 than current diamond can

realize, due to surface roughness, with a record frequency of _f_ c=1 GHz (Ref.31). It is important to note that a figure of merit which should be considered to enhance sensitivity in

nanomechanical resonators is based on the _f_ c _· Q_ m product. In this respect, in single crystal diamond nanomechanical resonators, the present record achieved is of _f_ c _· Q_ m=2.2 THz

at 5 K in doubly clamped nano-beam waveguides32, while for diamond nanocantilevers _f_ c _· Q_ m=48.3 GHz at 3 K (Ref. 27). A more remarkable record is achieved in optomechanical single

crystal diamond micro disk resonators operating in ambient condition12,33, with _f_ c _· Q_ m=19 THz, while similarly _f_ c _· Q_ m=9.5 THz at room temperature in polycrystalline 3C-SiC

optomechanical micro-resonators has been achieved (the theoretical maximum achievable in SiC being _f_ c _· Q_ m=300 THz at room temperature)34. It is expected that a similar race could

start regarding MEMS/NEMS35 in reconsidering conventional materials, hosting color centers as diamond, such as SiC, for applications envisaged for current diamond nanomechanical resonators.

Some exemplary common nanomechanical resonators realized in diamond for ultrahigh force sensitivity are shown in Figure 1. They are based on nanocantilevers, which are clamped on one end

only (Figure 1a) or nanoscale beams that are clamped on both ends (Figures 1b–e). Centrosymmetric mechanical resonators, such as nano-ring and disk resonators (Figures 1f and g), which are

free standing, can be designed to host mechanical and optical resonance at the same time. They have potentially higher optical performance compared to nano-cantilevers once transferred in

free-standing integrated optical components. Finally, another nano-mechanical system in single crystal diamond named the ‘dome’ resonator (Figure 1h) appears as a shell-type resonator which

is comprised sub-100 nm thickness diamond, and could be used in the development of future nano-electromechanical devices in diamond using grapheme-based electrical components36. In the realm

of magnetic force microscopy, the fabrication of single diamond nano-tips or nano-wires37 has recently been sought after and has been considered a challenge in the field. Diamond nanotips

made of single crystals (i, ii in Figure 1) permit the ability to reach <100 nm thickness in nano-mechanical resonators to enhance scanning probe microscopes in terms of their imaging

resolution. Ultimately, the integration of NV-centers in diamond nanotips will provide the ability to achieve higher sensitivity in MRI of an ensemble of nuclear spins even at room

temperature. DIAMOND NANOCANTILEVERS The key paradigm shift on the material fabrication relied on novel approaches for cantilever fabrication together with the very recent commercial

availability of templates based on optical-grade (doping concentration of Boron and Nitrogen substitutional B, Ns<5 ppm) and electronic-grade (doping concentration and B, Ns<5 ppb)

single crystal diamond plates of size 5·5·(0.02–0.04) mm3, with an initial surface roughness of <5 nm. These commercially available templates have considerable thickness variation of 10 

μm over the plate. This requires protocols for template-assisted precision re-polishing to achieve thickness uniformity better than 1 μm, together with methods for handling such thin

plates27,38. After plate re-polishing, two methods can be used for handling and subsequent etching of the nanocantilevers. One method is based on developing a diamond-on-insulator structure

by water bonding of the plates on thermally oxidized Si substrates, thus permitting a further thinning of the diamond plate to <500 nm by Ar/Cl plasma reactive-ion-etching (RIE). The

cantilevers are then patterned by standard optical lithography and released by substrate etching. A similar wafer bonding–based technique has also been used to form a diamond/silica/silicon

multilayer structure39. The other method consisted of clamping the diamond plates to two SiO2 substrate central apertures. This provides a diamond silica sandwich for optical lithography on

one side and thinning on the other side, resulting in a residual 5 μm diamond ledge with reduced clamping losses at the base of the cantilevers (Figure 1a). Another technique to fabricate

free standing mechanical and photonics nanostructures is the so called angle etching technique26. It is based on electron-beam-lithography (EBL) and anisotropic plasma etching in two steps,

the second step being at an oblique angle to the sample surface. It consists of first patterning a 200 nm titanium etch mask by EBL; an oxygen-based plasma etching is first used to transfer

the mask to the bulk diamond, which is etched down to 600 nm. A second anisotropic etching is then performed at an oblique angle to release the structure. This method is also used to

fabricate free-standing nanoscale components in bulk single crystal diamond, including optical waveguides, PHC and microdisk cavities26. Single and double clamped cantilevers were fabricated

with rectangular or triangular cross section. For triangular cross section cantilevers fabricated with the ‘angle etching’ method, the nano-beam thickness, _t_, is linked to the width, _w_,

via the relationship40 t= w 2 tan θ where ≈50°. For rectangular singly nanobeams, the measured frequencies modeled by f c =0.162 t l 2 E ρ (Figure 2) experimentally follow a _f_ c∝_l_ −2

scaling law. For double clamped cantilevers, the frequency dependence on the length is not monotonic due to uniaxial compressive stress along the length of the beam _σ_. Thus, for a

rectangular doubly clamped cantilever, the vacuum frequency will be reduced by the term under square root42 f c =1.03 w l 2 E ρ × 1 + σ l 2 3.4 E w 2 For doubly clumped triangular cross

sectional cantilevers, a similar behavior to the in plane and out-of-plane frequencies occurs depending on _σ_ (Ref. 40). In Table 1 we summarize the most relevant results on the single

clamped cantilevers dimensions, for a given frequency and _Q_ m compared at room temperature and vacuum conditions for electronic-grade single crystal diamond27,38,41 and optical-grade

single crystal diamond40. Figure 2 shows these data for clarity. The geometry of the cantilever, mostly thickness and length, introduces the major variability on _Q_ m and _f_ c. In Ref. 39

cantilevers were realized with lengths _l_=60 and 80 μm, _w_=15 and 20 μm and with varying thickness _t_=0.76/2.1 μm. Room temperature _Q_ m=338 000 at _f_ c=730 kHz (size 80×20×1.5 μm) was

obtained while the maximum room temperature27 _Q_ m=1 200 000 for _l_=800 nm in electronic-grade single crystal diamond, with _Q_ m≈5.8×106 at _f_ c=32 kHz in the millikelvin regime. In Ref.

40, free-standing single crystal diamond singly and doubly clamped diamond nanobeams with triangular cross section were fabricated, both by the angled-etching methodology26. Compared to the

singly clumped cantilevers of the same length and width where the axial stress is released by the free-ended structure, the doubly clumped cantilevers provide a lower frequency and a lower

_Q_ m. For example, a 675 nm wide, 55 μm long doubly clamped diamond nano-beam shows a maximum _Q_ m~19 000 for 6.1 MHz at room-temperature in a high vacuum, showing a discrepancy from

Euler-Bernoulli theory, which strongly indicates a compressive stress due to the fabrication process. In Ref. 27, nano-cantilevers with thickness _t_=80–800 nm, _l_=20–240 μm and _w_=8–16 μm

were realized and in Ref. 41 the nanobeams realized were fixed at a height of _h_=530 nm, _w_=820 nm, 1.3 μm and _l_=4.8–48 μm, the dimensions _w_ and _l_ being comparable to the other

methods. _Q_ m increases for longer devices due to reduced clamping losses, while _f_ c reduces for longer cantilevers. The temperature effects on the cantilevers and their thickness are the

major reasons for the large variability of _Q_ m and _f_ c due to mechanical dissipation. It has been observed that _Q_ m∝_t_, thus indicating the presence of surface friction. Surface

friction is reduced at cryogenic temperatures, providing the higher _Q_ m. Surface friction mechanisms were tested27 by modifying the cantilever’s surface species by oxygen and fluorine

termination on both optical-grade and electronic-grade single crystal diamond cantilevers. In the first case, an improvement factor of 10 for _Q_ m was observed for the same temperature and

thickness, indicating that specific atomic species layers on the surface can reduce/increase surface friction. Figure 3a shows a summary of _Q_ m measured values versus temperature and

cantilevers thickness, for electronic-grade and optical-grade single crystal diamond. Polycrystalline diamond (PCD) and single crystal Si are also shown for comparison. This comparison

determines the material role, temperature and thickness optimization to achieve the highest force sensitivity. The sensitivity in rectangular cantilevers is F min = w t 2 k B T B l Q m E ρ =

w k B T B l α , where α= t E ρ Q m is related to the mechanical dissipation as discussed in Ref. 27. It has been inferred by determining the noise temperature of the cantilevers from their

measured displacements T= k c x 2 k B and using measured values of _f_ c, _Q_ m, _k_ c. In Figure 3b, we show a summary of the force sensitivity inferred and projected. At present, diamond

nanocantilevers’ best sensitivity has been measured to be in the order of 0.54 aN Hz−1/2 at a temperature of 100 mK for a 660-nm thick cantilever27. This is already better than a

single-crystal Si of 290 nm thickness at 220 mK and roughly the same material purity (0.82 aN Hz−1/2)43. The projected limit of the technology based on these recent works (240·1·0.05 μm)

would bring the thermal noise floor to the 10–100 zN Hz−1/2 range27. The thinnest built 60·15·0.75 μm cantilever in Ref. 39 shows a best sensitivity of 0.4 fN Hz−1/2 at room temperature and

0.1 fN Hz−1/2 at 10 K. In Ref. 37 a sensitivity of 2.4 aN Hz−1/2 at 4 K is achieved using a diamond nanowire integrated with an ultrahigh sensitivity Si cantilever and a projected

sensitivity of 0.65 aN Hz−1/2 at 300 mK. The nanocantilever dynamics for applications, such as mass sensing, magnetometry and scanning probe microscopy, needs to be measured in nanoscale

fluids, as biochemical analysis of samples is usually performed in ambient air or liquids. Ref. 41 measured the pressure-dependent mechanical dissipation 1/ Q m g , l of the triangular cross

section singly clumped nanobeams, operating in light (He) and heavy gases (N2 and Ar) ( 1 / Q m g ) and water ( 1 / Q m l ). The _Q_ m and frequency were measured in vacuum conditions and

compared to the gas and fluid conditions to determine the fluidic dissipation. For a cantilever of 29·0.820·0.530 μm3, as reported in Table 1, by going from vacuum to gas, the resonance

frequency does not shift significantly (_f_ g=1.208 MHz) while the dissipation increases, Q m g =100; in water both frequency and dissipations change significantly, _f_ w=0.43 MHz and Q m w

=1.1. The interaction of the gas with the cantilever was studied, but changing the pressure _p_, 1/ Q m g ∝p for low pressure (_p_<_p_ c in the condition of molecular flow, that is, the

gas molecules interact only with the surface of the cantilever), while 1/ Q m g ∝ p (_p_>_p_ c, indicating the transition to viscous flow). It has been observed that _p_ c depends

linearly on the frequency for high-frequency cantilevers (_f_ c>10 MHz). The wider cantilevers with the same length, and hence the same frequency, provide a lower _p_ c at which the

transition occurs. In the low-frequency regime, the transition from molecular flow to viscous flow occurs when _w_~_λ_, where _λ_ is the mean free-path of the gas molecules. When using the

same cantilever in different gases (He and Ar), _p_ c is lower for Ar due to its larger molecular diameter. DIAMOND NANOWIRES In scanning probe microscopy, the force sensitivities

deteriorate as the tip becomes close to a surface due to noncontact friction effects, depending on dielectric fluctuations44, radiative heat transfer and the presence of static or

fluctuating surface charge37. Apart from the use of Si nano-wires39, nano-mechanical detection of ultrasmall forces was achieved with freely vibrating nanobeams or particles that were ≳100 

nm from a surface. Diamond due to a low dielectric constant has the potential for low noncontact friction. In Ref. 37 diamond nanowires of a few micrometers long and diameters of around 100 

nm were fabricated using two different masking schemes and applying masked etching of single crystal substrates by inductively coupled plasma reactive ion etching. One scheme was based on

micromachining of received diamond surface defects acting as intrinsic masking, the other by deterministic fabrication of diamond nanowires using an electron beam–defined alumina etch mask.

The pillar geometry is made of a pyramidal base connected to a nanowire-like tip via a thin waist of ~10 nm (Figure 1 i–ii). The diamond nanotips were integrated with a Si cantilever with a

sensitivity of a few aN·Hz−1/2, _Q_ m≈130 000 and _f_ c≈7.6 kHz at 4 K. The cantilever tip _f_ c and _Q_ m reduced only when the diamond tip was ≤10 nm from a gold surface (_Q_ m≈40 000 at 1

 nm from the surface). The values of noncontact friction were estimated to be comparable with high-frequency (1 MHz) Si nanowire resonators45. In force detected MRI methods, a sample should

be positioned as close as possible to a nanoscale ferromagnetic tip, generating a strong magnetic force on a spin, which determines the spatial imaging resolution. At ≲10 nm a magnetic force

gradient of 150 Gauss/nm can be achieved, creating a force on a single proton spin of ≈0.21 aN. This force value corresponds to a diamond nanowire tip cantilever sensitivity at 10 nm

distance and at 4 K of 2.4 aN Hz−1/2, while at 300 mK, the sensitivity of the diamond nanowire tip cantilever can be 0.65 aN Hz−1/2. DIAMOND DOME RESONATORS The first nanomechanical

resonators micro fabricated in single-crystal diamond were dome-type resonators (Figure 1h) built with a 70-nm thick shell36 (the current thinner single crystal diamond cantilevers were 76 

nm thick), demonstrating a quality factor _Q_ m≈4000 at room temperature (_f_≈27 MHz), increasing to _Q_ m≈20 000 at 10 K and covering a wide range of resonant frequencies from 10 to 600 

MHz. Here the main loss mechanisms were introduced by the fabrication process, based on C+ ion implantation to create a loss sacrificial layer. In addition, a high Ns concentration substrate

was used. The resonance frequency of this resonator is modeled as a clamped circular plate46 whose mechanical frequency depends linearly on the height and inversely on the square of the

radius of the plate. Energy dissipation in single crystal diamond annular plate resonators (175 nm thickness) has been measured for temperatures ranging from 4 to 300 K at resonant

frequencies around 50–55 MHz (Ref. 46). An order of magnitude reduction in dissipation is observed as the temperature is lowered from room temperature (1/_Q_ m=5×10−4) to 30 K (1/_Q_

m=5×10−5). NANOPHOTONICS COMPONENTS Nanophotonics is an interface between optical elements, such as optical fibers and lenses used to transfer optical signals, and solid-state physical

objects, such as currents and spins in a material, color centers and nanomechanical objects47. Diamond also provides the opportunity to harness quantum nanophotonics when photons are used to

interconnect quantum systems such as spin states associated with luminescent individual color centers in diamond, such as NV-centers. A recent review with a focus on quantum nanophotonics

and the integration of color centers in diamond can be found in Ref. 48. These color centers can constitute quantum bits of information and their readout occurs by the observation of a

single-photon emission associated with the manipulation of their electron spin state and of the nearby coupled nuclear spin sublevels. To make these and other applications viable, the

efficiency of information exchanged between the NV-center’s electron spin and a photon is of central importance. While the realization of nanophotonic components in diamond has been an

active area of research for almost a decade1, in this review, only the latest nano-photonic components achieved in single crystal diamond are considered. We focused on broadband photon

collection efficiency enhancement components such as diamond nanopillars, narrow bandwidth devices such as disk resonators, ring resonators and one-dimensional (1D) and two-dimensional (2D)

PHC cavities. The latter components are characterized by their optical _Q_-factor ~_λ_/Δ_λ_ and mode volume _V_~(_λ_/_n_)3. Nano-cavities can be used to modify the spontaneous emission (SE)

of color centers in the diamond49. To increase the possibility to use NV-centers in quantum networks or in magnetic sensing more efficiently, it is necessary to force the SE to be mostly in

the Zero Phonon Line (ZPL; naturally only 3% of its photoluminescence is in the ZPL). To achieve this, it is necessary to couple the SE with one optical cavity mode to generate a Purcell

enhancement F p , max = 3 4 π 2 ( λ cav n ) 3 Q V while the SE enhancement is given by 50,51,52, F SE =ξ F p , max 1 1 + 4 Q 2 ( λ ZPL λ cav − 1 ) 2 where _ξ_ accounts for the spatial and

angular overlap between the emitter dipole moment and the cavity mode intensity. Typically, F SE ≪ F p , max , even if the theoretical value can differ from the actual measured _F_ SE. While

the total fraction of emission into the ZPL once in the cavity is given by _ξ_ ZPL _F_ p, max, where _ξ_ ZPL is the fraction of emission of the defect in the ZPL in the bulk material, about

3% for NV center. However, since not all the optical resonators are coupled with a specific color center, we compared them with respect to the measured _Q_ and theoretical _F_ p, max based

on the cavity designs. PHC cavities are characterized by high _F_ p, max due to a smaller mode volume. The design of the cavities requires optimization of _F_ p, max while the integration of

the center in the cavity requires the optimization of _F_ SE to achieve maximum SE enhancement. In Figure 4, we show scanning electron microscopic images and details of nanophotonics

components fabricated in single crystal diamond10,50,53–55. Among the optical resonators described in the following sections, we show those currently achieving the highest _Q_ s at specific

wavelengths in the near infrared and in the visible spectrum at around 637 nm. This is in the ZPL of the NV center, or in the spectral region 600–750 nm, where known color centers56 in

diamond with single photon emission can be integrated. In Figure 5, data from Ref. 1,10,50,51,54,55,57,58,59,60,61,62,63,64,65,66,67,68, we summarize the measured _Q_ s and the predicted _F_

p,max enhancement at specific wavelengths corresponding to ZPLs color centers in diamond (visible) and for typical infrared resonance modes. It is observed that higher _Q_ s can be obtained

in the near-infrared due to larger cavity dimensions less limited by the fabrication techniques and in larger resonators, such as ring resonators. Among the PHC cavities, the 1D-PHC

cavities have achieved the highest observed Q-factors and _F_ p, max due to smaller mode volume, compared to the same parameters achieved in 2D-PHC cavities. Thus, the fabrication procedures

are still a major hurdle in the achievement of the expected Purcell enhancement as well as the placement of the color center at the exact locations within the maximum intensity of the

cavity modes volume. The fabrication of photonic components in single crystal diamond for nanomechanical resonators suffers from the fact that a thin film or a freestanding diamond membrane

of 200 nm is necessary for photonic devices. The adopted methods to achieve a diamond membrane of 200 nm or less are common to the fabrication of nanocantilevers and they can be grouped

under the following methods (Figure 6, images from Refs. 32,57,69): * i The ‘thin-down’ method, where a diamond substrate with a thickness of tens of microns from commercial suppliers is

thinned down by dry etching52,57 (Figure 6a), or RIE, to a thickness of some hundreds of nanometers59. * ii The ‘lift-off’ technique65,70, which consists of creating a sacrificial layer by

irradiating the diamond with ions. This layer is removed after graphitizing it by annealing. A regrowth of a pristine diamond layer is performed to remove any left diamond damaged and the

thin layer is then transferred to a different substrate to create nano-structures71. * iii The ‘angle-etching’ technique, described earlier10. * iv The ‘undercut’ etching method, where a

resist is patterned by EBL and transferred on the bulk diamond; a final undercut of the structures being required to achieve freestanding devices on top of the bulk diamond. The undercut

typically introduces damage to the devices. A process flow for creating diamond nano-beams using quasi-isotropic reactive ion undercut etching is shown in Ref. 72 * v Quasi-isotropic oxygen

undercut, based on conventional vertical RIE and oxygen plasma etching at elevated sample temperature12,32,55 (Figure 6b). * vi Focused ion beam (FIB) milling typically uses Ga or O2 ions to

directly cut the diamond without the need for generating a mask. Ref. 61 uses for example Ga FIB to fabricate nanostructures. Its main drawbacks are related to damage due to implanted ions

and lack of scalability of the process due to its long milling time73. A new approach to diamond nanofabrication has been recently proposed in Ref. 74. They employed reactive ion beam angled

etching (RIBE) to manufacture optical resonators. They report _Q_ of ≈30 000 in bulk polycrystalline and ≈286 000 in single crystal diamond. The technique may have advantages over

comparable techniques to produce freestanding nanostructures with better scalability. For nanopatterning of the cavities, the following methods are used: (a) FIB milling defines and

transfers the photonics pattern directly in the diamond but its accuracy is limited to 10 nm in addition to introducing damage to the diamond61; (b) EBL defines a pattern in an electron beam

resist layer and dry etching is used to transfer the photonic structures into the diamond64; (c) Si mask lithography has also been used due to the relatively mature fabrication process of

silicon-on-insulator (SOI), where a Si mask is patterned on an Si insulator that is released and transferred to a diamond membrane, the diamond membrane then being patterned using oxygen

RIE62,69. DIAMOND NANO-PILLARS Diamond nano-pillars and nano-wires as nano-photonics component are used to increase the collection efficiency of photons emitted from an NV or can be used as

scanning probes for magnetometry and diamond cantilevers for force-sensing. The first single crystal diamond nano-wire75 was 2 μm tall and 200 nm wide, acting as a waveguide in a dielectric

antenna, with a single NV center photons collection enhancement factor of 10. Similar enhancements can be achieved by using solid immersion lenses etched into the diamond surface76. The

fabrication is scalable as nanowires can be fabricated in parallel on the same chip77. One of the limitations is due to the fabrication of nano-pillars on a (100) facet single crystal

diamond as ideally, an NV dipole should be perpendicular to the pillar’s axis for maximum collection efficiencies78, while it forms an angle with the pillars, axis due to the NV-center

orientation aligning along one of the four equivalent <111> crystal-directions. As high-quality, high-purity (111)-oriented chemical vapor deposition diamond has become available, Ref.

53 demonstrated the fabrication of single crystal diamond nano-pillars on this (111)-oriented substrate. This crystal orientation provides a factor of 2–3 further enhancement of the

collection efficiency compared to previous demonstrations. RING RESONATORS Ring resonators are photonic elements etched into single diamond-on-insulator (a SiO2 substrate) or free standing

(suspended over air) diamond thin films. Ring resonators are generally coupled to waveguides and are relevant for complex architectures enabling on-chip photonic functionalities as opposed

to stand-alone components. Ref. 58 integrated a ring resonator coupled to an optical waveguide with grating in- and out-couplers to direct a NV-center inside the ring resonator as a source

of photons to the waveguide output. Ring resonators have high quality factors of _Q_=12 600, while _Q_=3200 when NV-center single photons in the resonator are coupled out of the waveguide.

Similarly, Ref. 57 fabricated a microring resonator coupled to a ridge waveguide with similar uncoupled and coupled _Q_-factors, providing a measured _F_ SE=12 at 637 nm. Ref. 59 developed a

high-quality factor single crystal diamond race-track resonator, operating at near-infrared wavelengths (1550 nm) using a very similar fabrication method with _Q_~250 000. In contrast to

the above methods, in Ref. 10 suspended racetrack resonators and a nanobeam photonics crystal cavity was realized from bulk diamond using the angle etching technique with _Q_=151 000 at 1622

 nm. Single crystal diamond ultrahigh-quality-factor (106) diamond ring resonators operating at telecom wavelengths were fabricated54, used as a diamond non-linear photonics platform where

optical parametric oscillations were observed via a four-wave mixing. MICRODISKS Microdisk resonators sustain ‘whispering gallery’ electromagnetic modes and they are typically stand-alone

devices where the emission is out-coupled using evanescent fiber couplings. Hence, they have limited applications in integrated nanophotonics. Disk resonators are mostly considered due to

the simplicity of fabrication. They were the first optical cavity type studied in diamond. Ref. 60 fabricated single crystal diamond microdisks by thinning down a diamond membrane with

further epitaxial overgrowth of a thin single crystal diamond film. The microdisk was coupled with the ZPL of an ensemble of Si vacancy defects (SiV) with ZPL at 738 nm, a loaded _Q_=2200, a

very moderate FSE of 1.3 and a mode volume _V_~9.6·(_λ_/_n_)3. Ref. 55 developed microdisks supporting TE- and TM-like optical modes with _Q_>1.1·105 and V<11·(_λ_/_n_)3 at a

wavelength of 1.5 μm. PHC CAVITIES The realization of interconnected high-_Q_ cavities for quantum information technologies should be in the form of a 1D nano-beam or 2D-slab PHC. In fact,

these architectures could permit to produce large scale systems with multiple emitters per cavity strongly coupled as well as arrays of multiple emitter-cavity nodes to implement

entanglement generation, quantum memories and photonic or spin qubits quantum gates. An example of entangling two SiV centers strongly coupled in diamond nano-cavities is shown in Ref. 68,

as a first tangible step in this direction. Similarly, efficient coupling of germanium vacancy in diamond waveguides has been achieved, demonstrating non-linear properties at the single

photon9. The first 1D nano-beam was fabricated in diamond using FIB61. The holes’ distance defines a lattice constant _a_=205 nm for visible modes confinement with very low quality factors

of _Q_=220. A triangular nano-beam PHC cavity by varying the crystal lattice constant a0-a=220÷240 nm with predicted _Q_~2×106, was fabricated by Ref. 62, yielding a measured _Q_~3000.

1D-PHC cavities were fabricated with a lattice constant _a_=200 nm and a hole size deterministically varying within the nano-beam, achieving a small _V_~0.7ˑ(_λ_/_n_)3 designed to be in

resonance with the NV-center ZPL. The maximum observed quality factor was 700 and no coupling to color centers was observed for the 1D-PHC cavity63. A 1D-PHC cavity64 consisting of a series

of holes etched through a diamond ridge waveguide had _Q_=6000 and was demonstrated to couple the cavity modes of a single Nitrogen NV with _F_ SE=7. Nanobeam PHC cavities were fabricated

both in the near IR and at 637 nm by adapting the angle-etching techniques10. These devices consist of a waveguide with elliptically shaped holes designed to have the highest _Q_ and the

smallest mode volumes. The fundamental resonance at _λ_~1, 680 nm has a _Q_~3×106 and a _V_~2.26·(_λ_/_n_)3. For _λ_~637 nm the cavity parameters were scaled down with _Q_=33, 000/59, 000,

while the best _Q_~8200 for the coupled waveguide cavity mode at 710 nm. In addition to improving the cavities parameters, attention has been paid to deterministically incorporate NV in the

diamond close to the cavity mode. PHC nano-beam cavities65 were coupled to the NV’s ensemble by incorporating a delta-doping technique, based on growing a nanometer-thick diamond

nitrogen-doped layer. The cavities were fabricated with a linearly tapered lattice constant with a _Q_~24 000 and _V_~0.47·(λ/_n_)3. Regardless of such a small volume, the _F_ SE=20 for the

ZPL of the NVs ensemble, results much lower than expected. In Ref. 51 it is reported that the highest _F_ SE=62 is achieved for a single NV in a 1D-PHC cavity with a measured _Q_~9900 and

_V_~1.05·(λ/_n_)3 (Figure 7). A linear increase of the lattice constant from 0.9·_a_ to _a_ in increments of 0.02·_a_ per period away from the center, defines the cavity defect state. When

strongly coupled to a single NV, the cavity _Q_~3300. Recently a breakthrough in integrated diamond nano-photonics has been achieved by integrating SiV in a 1D-PHC cavity with _V_~2.5

(_λ_/_n_)3 and _Q_~7200, by implanting Si ions at the center of the cavity to create the color centers. The strong coupling of single SiV with the cavity permitted to observe optical

nonlinearity at the single photon level68. 2D-PHC cavities were fabricated in PCD, initially, with very low _Q_-factors1,4,48. An experimental _Q_~1000 and a current _F_ SE=70 related to the

coupling of the cavity with a single NV-center, was achieved in Ref. 50. The cavity is a linear three-hole defect PHC cavity fabricated in a triangular lattice with a period of _a_≈218 nm.

Theoretical quality factors as high as _Q_~6000 can be achieved with this design with very small _V_~0.88·(_λ_/_n_)3. Ref. 63 develops 2D-PHC cavities with triangular lattice of air holes

with lattice constant _a_=275 nm and _V_=1.5·(_λ_/_n_)3, coupled, for the first time, to an ensemble of SiV centers. In Ref. 65 a 2D-PHC cavity with a triangular lattice of air holes and a

one-hole type of defect was fabricated with a measured _Q_ ~1000. The deterministic coupling to a single SiV center was recently observed in a triangular lattice of air holes with a crystal

lattice _a_ ≈283 nm (Ref. 66). The nanocavity is formed by a linear one or seven holes’ defect in the photonic lattice. The fabrication of the cavity procedure was like the one reported in

Ref. 63; however, the cavity was fabricated around a preselected single SiV color center in the membrane. This achieved a _Q_~320 once the cavity was coupled for a defect _F_ SE=19, while

the theoretical _V_~1.7·(_λ_/_n_)3. A cavity was coupled with a single NV deterministically implanted at the center of the cavities64 achieving a maximum _Q_ ~1200 and _V_~1·(_λ_/_n_)3. The

cavity was fabricated with the same procedure as described in Ref. 66; however, only a modest _F_ SE was observed. For application in magnetometry79 NV-centers should be placed as close as

possible to the diamond surface, while the above-described high-_Q_ cavities have their field maxima located centrally within the cavity. Hybrid metal-diamond PHC regardless of a smaller _Q_

could provide better field localization. A hybrid metal-diamond PHC80 was fabricated using silver as the substrate and single-crystal diamond nano-pillars arranged in a hexagonal lattice

with a cavity defect. The diamond nano-pillars are on top of a silver substrate, with a 5-nm layer of Al2O3 (_n_=1.8) sandwiched in between as the gap dielectric (Figure 8). This hybrid

cavity, whose fabrication is very challenging, had a measured _Q_=170 with a simulated V~0.1·(_λ_/_n_)3. INTEGRATED NANO-MECHANICAL AND OPTOMECHANICS SYSTEMS There is currently a significant

interest into the integration of nanomechanical components with the NV-center. An NV-center integrated in nanomechanical resonators could enhance current nano-MRI in scanning probe

microscopy81. In addition, the coupling of the mechanical modes of the resonators with NV phonons permits modification and/or control of the spin coherence of the defect. The spin-phonons,

interaction can lead to a new tool for imaging strain at the nanoscale and for coherent information transfer and manipulation between spin and mechanical modes as phonon-induced spin-spin

interactions82. In addition, the spin coherence time of the single NV-center, normally up to 2 ms (Ref. 13), in bulk diamond at room temperature but collapsing to a few microseconds in

shallow NV-centers due to surface effects83, could be significantly enhanced to improve magnetic sensitivity. Finally, the combination of nano-mechanical systems with optical nanocavities

naturally leads to opto-mechanical systems that, enriched with color centers with quantum mechanical properties, provide a route to undertaking a fundamental study in the domain of quantum

optomechanics. INTEGRATED NANO-MECHANICAL SYSTEMS NV diamond probes for magnetic imaging scanning probe microscopy are an example of integrated nano-mechanical systems. Scanning probe

magnetic imaging permits the mapping of magnetic fields at the nanoscale, and was demonstrated at room temperature compatible with living cells by using a single diamond NV-center spin as a

magnetometer84. A functionalized nano-diamond with NV-center magnetic spin is placed on a regular atomic force microscopy (AFM) tip, which provides the scanning function. Alternatively, a

single NV-center was positioned at 10 nm from the end of a high-purity diamond nanopillar mounted on an AFM85. This method enhanced the spin coherence time to 75 μs, and the NV collection

efficiency due to wave guiding with 25 nm imaging resolution of the magnetic domains and magnetic field sensitivity of 56 nT Hz−1/2. In Ref. 81, the de-coherence of a NV-center a few tens of

nanometers from the tip of a 200 nm diameter scanning nanopillar was employed to image the randomly fluctuating magnetic fields from paramagnetic impurities on an underlying diamond

surface. A monolithic diamond pillar is a scanning probe based on a NV-center used to image the magnetic domains on a hard disk surface and the vortices in an iron pnictide superconductor

with 30 K critical temperature86. At 6 K temperature, the probe achieved a sub-100 nm spatial resolution for 3 μT Hz−1/2 direct current (DC) field sensitivity. The magnetic stray field of

single Ni nanorods was imaged with a resolution of 70 nm for scanning probes consisting of a scanning nanopillar (200 nm diameter, 1–2 μm length) on a diamond thin (<1 μm) cantilever

structure (see Figure 9(d))87. The devices (diamond cantilever plus nano-pillar with integrated NV-center) with the best magnetic field sensitivity (50±20 nT·Hz−1/2 in an AC magnetic field

and 200 nT·Hz−1/2 in a DC magnetic field) were transferred to an AFM head. The transfer of the scanning probe to the AFM tip is done using micromanipulators based on quartz micropipettes.

The scanning probes were initially glued to the quartz tip using UV curable glue (Figure 9(a)), after a scanning probe was released from the substrate. Finally, the quartz tip carrying the

scanning probe was glued to a tuning fork attached to an AFM head (Figures 9(c and d)). The use of the AFM tip for magnetic scanning with single NV-centers usually brings about issues with

the unavailability of simultaneous topographic scanning and the lengthening of the acquisition time. These were recently tackled by employing a nanospin ensemble of 100 NV-centers hosted in

a nano-diamond (Figure 10), providing up to an order of magnitude gain in the signal-to-noise ratio (and thus acquisition time) with respect to a single NV-center, while preserving sub-100 

nm spatial resolution88. When integrated with nanomechanical systems, NV-centers show strong spin-phonon coupling in nanoscale dimension resonators89–91 mediated by the lattice strain effect

as shown by incorporating photo-stable NV-centers in diamond cantilevers at different location from the cantilever base90. Experiments on NV-centers embedded at the base of 60 μm long, 15 

μm wide and ~1 μm thick single-crystal diamond cantilevers39, where NV interacts with the fundamental mechanical mode of the cantilever via strain, permitted spin-based strain imaging with a

sensitivity of 3·10−6 strain·Hz−1/2 (Ref. 90). The cantilevers have ~MHz mechanical frequency and _Q_ m⩾3˙105. The amplitude of the cantilever motion has been shown to impact the NV spin

evolution. The strain susceptibility parameters parallel and perpendicular to the NV symmetry axis were determined from spin evolution measurements. Similar measurements were performed in

Ref. 91. To enhance the spin-phonon coupling strength, a smaller mechanical mode volume can be achieved by reducing the cantilever thickness and width to a few hundreds of nanometers while

doubling its length. As demonstrated in Ref. 89 a single NV-phonon coupling rate of approximately 2 Hz was achieved. The cantilevers were fabricated using angle-etching technique26 with

_w_=580 nm, _t=_170 nm and _l_=19 μm, _Q_ m~10 000 and _f_ m=937 kHz. It has also been shown that it is possible to control NV-center spins coherently by applying a stress in resonance with

spin state splitting. This also permits access to spin transitions that are not accessible with AC magnetic field control92,93. Recently, coherent control of the NV-center triplet state was

shown in high-overtone bulk acoustic resonators (HBARs) by high-frequency mechanical stress resonators with a split frequency92,93. This parity nonconserving process enables access to

forbidden transitions by common optical and microwave spin sequence excitations. Internal time-varying strain fields in diamond cantilevers can also induce coherent oscillations in an

embedded NV-center, reaching the strong-driving regime at room temperature and enhancing the spin coherence time94. In fact, diamond nanocantilevers coupled with NV-centers gain access to

the strong spin-phonon coupling regime, where the mechanical dynamics can be detected by electron spin resonance and spin echo measurements in the time domain. INTEGRATED OPTOMECHANICS

Optomechanics is the study of the mechanical interaction between light and matter, mediated by radiation pressure. When the interaction is enhanced by introducing an optical resonator, it is

called cavity optomechanics11. Due to the ability to detect tiny mechanical forces, opto-mechanical systems have applications for the detection of gravitational waves, and are useful

experimental systems to test quantum mechanics and decoherence models6 (see Figure 11). The detection of the actively modulated nano-mechanical response of up to 115 MHz at room temperature

condition has been shown in integrated electro opto-mechanical circuits, featuring sensitive interferometric motion readouts and mechanical resonators with high _Q_ s up to 9600 using PCD95.

The H-beam resonator is electrically driven on one side by metal electrodes, while the motion is detected on the opposite side by an optical Mach-Zehnder interferometer; the two systems are

optically separated by a PHC engraved into the beam by EBL (see Figure 12). This enables opto-mechanical devices for a broad range of applications, such as optical actuators, vibration

sensors and reconfigurable optical elements. A single-crystal diamond opto-mechanical system was shown to excite mechanical oscillations in nano-beams with lengths from 50 to 80 μm, with

amplitudes of more than 200 nm and a _Q_ m of 7·105, allowing stress fields strong enough to potentially couple to the NV-center spin transitions over a 150-nm bandwidth with a coupling

coefficient up to 45 GHz/nm and a displacement sensitivity of 9.5 fm·Hz−1/2 (Ref. 32). The tunable coupling of single NV-center spins to external electromagnetic fields for quantum

information applications was recently demonstrated in a hybrid spin-electro-mechanical device, where, by integrating a NV-embedded diamond beam with a superconducting coplanar waveguide

cavity, a single NV-center spin is coupled to the single microwave cavity photons, enabling the mediation of coherent information transfer by mechanically dark polaritons96. In analogy with

optical PHC, diamond crystals (OMCs) have been proposed, where a quasi-periodic diamond nanostructure leads to coupling of an optical cavity field to a mechanical mode via the radiation

pressure of light. In contrast to other material systems, diamond OMCs operating in the resolved sideband regime possess large intra cavity photon capacity (>105) and a sufficient

opto-mechanical coupling rate to exceed a cooperativity of ~1 at room temperature12,32,33. In the above examples, the NV spin ground state is coupled with mechanical modes and read out

optically, whereas other methods are employed when the NV excited state is coupled to the cavity mechanical modes. NV-excited state presents a much stronger electron-phonon interaction

compared to its ground state. This property makes it ideal for optomechanical control of the center quantum states via phonon-assisted optical transitions or sideband transitions, at the

expenses of cryogenic operation. Further work in this direction at 8 K is contained in Ref. 97, where the NV excited-state electron-phonon interaction permit to couple its direct dipole

optical transitions to surface acoustic waves, achieved on a patterned layer of piezoelectric ZnO, sputtered on a bulk diamond surface. This method permits the control of the internal

electron states of the NV corresponding to the ground and excited state spin states and it can be extended to control the motional states of a coupled NV nano-mechanical system through these

optomechanical processes. CONCLUSIONS AND OUTLOOK As discussed in this review, recent advances in diamond synthesis and nanofabrication have enabled high-quality nano-mechanical systems for

force microscopy and nano-fluidics as well as nano-photonic devices for increased photon collection and tailored light-matter interaction. In the classical application of nano-cantilevers

for AFM, diamond has proved to be better than Si, with a force sensitivity of 0.54 aN Hz−1/2 at 100 mK (Ref. 27). However, using thinner cantilevers to increase sensitivity, the

mechanical-_Q_ tends to be reduced due to surface friction. A hybrid solution of a diamond nanowire with a Si cantilever provides the current record sensitivity of 2.4 aN·Hz−1/2 at 4 K (Ref.

37). However, this is not yet in the region of the zN range for nuclear magnetic resonance signals using spin magnetic resonance microscopy although projected values indicate the

possibility. The integration of NV center in diamond nano-cantilevers and nanopillars, and their combination, has advanced the field of diamond probes for scanning probe microscopy of

magnetic fields, reaching resolutions of 25 nm for an AC magnetic field sensitivity of 50 nT·Hz−1/2 at room temperature. This is due to the improved coherence time of the NV spin in the

nanostructures, based on high-purity single crystal diamond. The coupling of NV spin with the mechanical mode of a nano-cantilever permits the control of the NV spin coherence by induced

strain and thus strain imaging via spin detection with a sensitivity of 3×10−6 strain·Hz−1/2. By reducing the mode volume of the cantilever’s mechanical mode, it is possible to increase the

coupling strength between NV spin and diamond phonons to reach strong spin-phonon coupling and quantum coherence control of spins via mechanical dynamics. This opens new architectures for

spin-spin interaction via phonons. In optical and photonics applications, high Qs up to 106 have been obtained in ring resonators and up to 105 in disk resonators at 1550 nm. However, due to

large mode volumes, the maximum spontaneous emission enhancement tends to be low. In the visible region, _Q_-values tend to be much lower even for ring resonators, and, in this spectral

region 1D-PHC cavities maximum _Q_ is 24 000. The maximum coupling of single NV with a 1D-PHC is achieved with a _Q_=9900 with a loaded _Q_=3000 when strongly coupled to NV51, while it

appears that the best performance in the direction to operate non-linear quantum optics and integrated optical quantum networks is achieved by entangling SiV color centers in diamond 1D

nano-cavities68. A combination of these advances has led to optomechanical devices in diamond, and we can foresee novel applications in spin optomechanics where optical and mechanical modes

are coupled in the same devices with spin defects; additionally spin control can be achieved in diamond crystals by photonics approach at the expenses of operating at cryogenic temperature.

While initial approach to cavity optomechanics was achieved in PCD, SCD provides lower mechanical dissipation and higher NV coherence, demonstrated in SCD microdisk12 with opto-mechanical

cooperativity C~3 at room temperature. This is currently the direction of most expansion for diamond micro- and nano-mechanical devices applications for sensing, particularly with the

promise of higher temperature operation. Another path to optomechanics could be the use of a bulk diamond at low temperature, where the Brillouin interactions are altered and a bulk crystal

can be regarded as a coherent macroscopic opto-mechanical system, providing access to ultrahigh _Q_-factor (~107/109) and phonon modes at very high frequencies (12 GHz)98. However, in this

case coupling with atomic color centers may result challenging and the low-temperature operation will limit sensing applications, while being an avenue for fundamental physics studies. In

summary, ideally, an integrated nanoscale mechanics or photonics platform in diamond should mimic the planar technologies which have been developed in SOI since the mid-1990s99,100, all

operating at fiber optic telecommunication wavelength, _λ_~1550 nm. Despite the progress in diamond nanofabrication, there are still fundamental challenges to be overcome regarding their

employment in some of the foreseen applications, particularly in the quantum technology domain where the nanofabricated structures need to have integrated color centers. The first limit is

the lack of color centers in diamond for the infrared region. Quantum photonic network implementations are based on diamond photonic nano-structures operating in the visible range, and the

integrated color centers require long spin coherence times and lifetime limited emission line widths. In this case, more developments must be undertaken to produce nanostructures with both

high intrinsic quality (high _Q_ values and low mode volumes) and high defect quality to be able to operate at higher temperature. Similarly, application of integrated nanomechanical systems

and nanosensors based on NV require a long coherence time. Some of the nanofabrication procedures, such as dry etching, can introduce degradation and increase the surface area near defects,

leading to the degradation of defect properties. For this reason, proper surface termination methods must be developed. Furthermore, the realization of large-scale quantum photonic systems

will depend on scalable fabrication techniques and on the tunability to match the resonance wavelength with the defect transition frequency without degrading the cavity _Q_. Finally,

following the recent success with diamond, other materials hosting color centers with similar quantum properties are emerging, with suitable infrared operation, low-cost single crystal

fabrication and CMOS compatibility such as SiC101, or 2D materials, which could provide a higher light confinement and control if integrated in nanostructures102, albeit these materials have

not reached the same level of maturity in the here targeted applications. AUTHOR CONTRIBUTIONS SC and AB designed the manuscript, processed the references, prepared the original figures and

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INFORMATION AUTHORS AND AFFILIATIONS * School of Engineering, RMIT University, Bundoora, 3083, Victoria, Australia Stefania Castelletto * Swinburne University of Technology, Centre for

Micro-Photonics (H74), Hawthorn, 3122, Victoria, Australia Stefania Castelletto & Lorenzo Rosa * Department of Information Engineering, University of Parma, Parma, 43121, Italy Lorenzo

Rosa * Military Technological College, Muscat, 111, Sultanate of Oman Jonathan Blackledge & Albert Boretti * Dublin Institute of Technology, Rathmines Road, Dublin, 6, Ireland Jonathan

Blackledge * Department of Petroleum and Chemical Engineering, Sultan Qaboos University, PO Box 33, Al-Khoud, Muscat, 123, Sultanate of Oman Mohammed Zaher Al Abri * Water Research Center,

Sultan Qaboos University, PO Box 17, Al-Khoud, Muscat, 123, Sultanate of Oman Mohammed Zaher Al Abri * Department of Mechanical and Aerospace Engineering, Benjamin M. Statler College of

Engineering and Mineral Resources, West Virginia University, P.O. Box 6106, 325 Engineering Sciences Building, Morgantown, 26506, WV, USA Albert Boretti Authors * Stefania Castelletto View

author publications You can also search for this author inPubMed Google Scholar * Lorenzo Rosa View author publications You can also search for this author inPubMed Google Scholar * Jonathan

Blackledge View author publications You can also search for this author inPubMed Google Scholar * Mohammed Zaher Al Abri View author publications You can also search for this author

inPubMed Google Scholar * Albert Boretti View author publications You can also search for this author inPubMed Google Scholar CORRESPONDING AUTHOR Correspondence to Albert Boretti. ETHICS

DECLARATIONS COMPETING INTERESTS The authors declare no conflict of interest. RIGHTS AND PERMISSIONS This work is licensed under a Creative Commons Attribution 4.0 International License. The

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http://creativecommons.org/licenses/by/4.0/ Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Castelletto, S., Rosa, L., Blackledge, J. _et al._ Advances in diamond

nanofabrication for ultrasensitive devices. _Microsyst Nanoeng_ 3, 17061 (2017). https://doi.org/10.1038/micronano.2017.61 Download citation * Received: 03 October 2016 * Revised: 16 June

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