Photo-generated THz antennas | Scientific Reports

ABSTRACT Electromagnetic resonances in conducting structures give rise to the enhancement of local fields and extinction efficiencies. Conducting structures are conventionally fabricated

with a fixed geometry that determines their resonant response. Here, we challenge this conventional approach by demonstrating the photo-generation of THz linear antennas on a flat

semiconductor layer by the structured optical illumination through a spatial light modulator. Free charge carriers are photo-excited only on selected areas, which enables the realization of

different conducting antennas on the same sample by simply changing the illumination pattern, thus without the need of physically structuring the sample. These results open a wide range of

possibilities for the all-optical spatial control of resonances on surfaces and the concomitant control of THz extinction and local fields. SIMILAR CONTENT BEING VIEWED BY OTHERS

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METAMATERIAL SURFACE WAVES Article Open access 25 August 2023 INTRODUCTION The possibility of confining electromagnetic fields in subwavelength volumes has been a main motivation driving the

current interest on optical plasmonics1,2,3. Optical antennas with localized resonances resulting from the coherent oscillation of conduction electrons are characterized by large local

field enhancements in subwavelength volumes. One of the greatest challenges for optical antennas and plasmonics in general is the efficient and fast active control of resonant frequencies

and local fields. Coherent control of local fields by temporally shaping optical pulses4,5 or by phase shaping of beams6,7,8 has been demonstrated. Ultrafast active control of surface waves

has been also achieved by the transient modulation of the dielectric function. This modulation is attained by a pump laser that induces changes in the electron distribution function and the

optical properties of the metal9,10,11,12,13,14, or by modifying the permittivity of the surrounding dielectric15. An alternative to metals for the active control of localized resonances at

THz frequencies are high mobility doped semiconductors with metallic behavior in this frequency range16. Doped semiconductors have charge carrier densities orders of magnitude lower than in

metals17,18,19, which enables the excitation of resonances at THz frequencies with a similar behavior to those of metals at near- and mid-infrared frequencies20,21,22,23. Moreover, this

characteristic offers the possibility of actively tuning THz resonances from a plasmonic behavior to that of surface currents in perfect electrical conductors by controlling the carrier

density16. This concept has been used to modify the propagation of surface waves24 and the resonant response of THz antennas27 and of metamaterials structured on top of semiconducting

substrates25,26 by the photo-excitation of electrons across the semiconductor bandgap. Common to all these works is that electromagnetic fields are controlled in surfaces that have been

physically structured with nano and micro-structures. In this manuscript we demonstrate experimentally a full all-optical generation of linear antennas at THz frequencies. The

photo-generation is realized by illuminating a thin GaAs layer with a laser beam shaped by a Spatial Light Modulator (SLM) to contain several micro antennas. This approach does not require

any physical structuring of the sample and offers the unique possibility of controlling spatially and spectrally resonances and local fields by modifying the illumination pattern. Okada and

coworkers have recently demonstrated the photo-generation of THz diffraction gratings on Si by illumination through a SLM28,29. Chatzakis _et al._ extended this work to GaAs gratings

generated by illumination through an optical mask30. Also, THz beam steering using photoactive semiconductors has been recently demonstrated by Busch _et al._31 and photo-generated

metamaterials have been theoretically proposed by Rizza _et al._32. These works have laid a solid background for the realization of all-optical active THz devices; However, they have not

demonstrated experimentally the possibility of inducing resonant phenomena by the structured illumination of flat surfaces. Therefore, this manuscript constitutes a bridge between the

emerging fields of active photonics and THz metamaterials. RESULTS SETUP AND SAMPLE DESCRIPTION The measurements have been performed using time-resolved THz time-domain spectroscopy (see

Methods). With this technique an optical pulse is used to pump a semiconductor while a time-delayed THz pulse probes the photoinduced changes in the conductivity of the semiconductor. A key

element in our setup is a computer controlled SLM, which is used to structure the pump by spatially shaping the beam. The operation principle of the structured illumination is illustrated in

Fig. 1(a). A horizontally polarized optical pulse (λ = 800 nm), indicated by the red beam in the figure, is transmitted through a polarizing beam splitter (PBS) and reflected back by the

SLM. The SLM is a pixelated liquid crystal device (1920 × 1200 pixels), which rotates the polarization of the incident light. Light reflected from the so-called bright pixels undergoes a

rotation of the polarization by 90°, being reflected by the PBS towards the sample; while light reflected from dark pixels maintains its polarization and it is transmitted back through the

PBS, thus not reaching the sample. The intensity of the light reflected by each pixel of the SLM and by the PBS can be changed continuously from a maximum value (bright pixel) to a minimum

(dark pixel). A lens (not shown in Fig. 1(a)) with a focal length of 150 mm is used to project (1:1) the structured beam onto the surface of the sample, which is a thin GaAs layer, where

electrons are photo-excited from the valence to the conduction band only on the illuminated regions. A THz pulse, indicated by the yellow beam in Fig. 1(a), propagates collinearly with the

optical pulse and transmitted through the sample. The pixel size on the surface of the sample is 8 × 8 μm2, being much smaller than the wavelength of THz radiation. Therefore, this technique

is ideally suited for the optical generation of subwavelength THz structures. The sample used for the experiments consists of a layer of single crystalline undoped GaAs with a thickness of

1 μm bonded to a SiO2 substrate (see Methods). A photograph of the sample is shown in Fig. 1(b). Intrinsic GaAs has a dielectric behavior at THz frequencies. However, its real component of

the permittivity becomes negative, hence GaAs becomes conducting, at 1 THz for carrier densities above 1.5 × 1016 cm−3. These carrier densities are easy to reach by pumping the sample with

an optical pulse of moderate fluence. The high electron mobility of intrinsic GaAs at room temperature (~ 7000 cm2V−1s−1), favors the excitation of localized SPPs. Moreover, the small

thickness of the semiconductor slab, which is comparable to the optical absorption length of GaAs (_L__a_ = 0.7 μm at λ = 800 nm), allows the (nearly) homogeneous excitation of carriers as a

function of the depth in the layer. For the experiments we used pump fluences up to 80 μJ/cm2, which excites ~ 1018 cm−3 free carriers on the bright pixels of the illuminated pattern. The

carrier density in GaAs at the regions illuminated by dark pixels is lower than 1016 cm−3. Therefore, optical pumping of GaAs with a shaped beam results into the local change of the

permittivity from an insulating to a conducting state. PHOTO-GENERATED THZ ANTENNAS To demonstrate the photo-generation of THz antennas on the GaAs layer we have measured the THz extinction

spectra of random arrays of rods with the same orientation generated at a fixed optical fluence while varying their lengths. We have also measured arrays of rods with the same length but

generated with different fluences. Figure 2(a) shows an image of an array of rods. This image is taken by placing a CCD camera at the sample position. Their random distribution suppresses

any effect due to periodicity in the THz extinction, while their horizontal alignment enables the excitation of antenna resonances for THz radiation incident with an horizontal polarization.

A close view of a single linear antenna rod is shown in Fig. 2(b). The effective dimensions of the photo-excided antennas were defined by the FWHM of the pumped fluence. The illuminated

area (filling fraction of the rods) corresponds to 18% of the surface. The spacing between two consecutive rods was chosen to be larger than 100 μm along the long axis of the rod and 40 μm

along the short axis. These distances are large enough to minimize the near field coupling between consecutive rods33, while maximizing their filling fraction to increase the THz extinction.

For the THz extinction measurements the sample was first illuminated with a pump pulse with a center wavelength λ = 800 nm and a duration of 100 fs. Subsequently, a THz pulse was

transmitted through the sample. The time delay between the optical pump and the arrival of the THz pulse was Δτ_p_−p = 10 ps, which is sufficiently long to enable the relaxation of hot

electrons to the lowest energy state of the conduction band34. Furthermore, this time delay was much shorter than the carrier recombination time, which was experimentally determined to be

τ_r_ ~ 450 ps. The THz transmission amplitude is measured within a time window of 12 ps. Over this time window the carrier diffusion length is much shorter than all the characteristic

lengths in the experiment, i.e., the dimensions of the structures and the THz wavelength. Therefore, the photo-excited antennas can be described in a first approximation as having stationary

dimensions and carrier density. We measure the zeroth order differential transmission transients, Δ_E_(_t_), i.e., the THz amplitude transient transmitted through the optically pumped

sample in the forward direction minus the transmitted transient of the unpumped sample Δ_E_(_t_) = _E_P(_t_) − _E_NP(_t_). These measurements are done by chopping the pump beam at half the

repetition rate of the laser, i.e., 500 Hz44. To gain spectral information about this transmission, the transients are Fourier transformed and squared to obtain the transmittance. The

extinction , defined as the sum of scattering and absorption, is given according to the optical theorem as one minus the zeroth-order transmittance. The latter is equal to the ratio of the

pumped to the unpumped transmittance . Therefore, . Figure 3 is the main result of this manuscript. Figure 3 (a) shows the extinction spectra of photo-generated THz antennas with a fixed

dimension of 230 × 40 μm2 and varying optical pumping fluences. The polarization of the THz beam was set parallel to the long axis of the rods. A resonance appears in the extinction spectrum

for optical fluences higher than 12 μJ/cm2. At this fluence the number of photo-excited carriers in GaAs is large enough to give a metallic behavior to the semiconductor in the THz

frequency range. The maximum extinction reaches values higher than 65% while the illuminated fraction of the GaAs is only 18%. This enhanced extinction can be explained in terms of the large

THz scattering cross section of the rods resulting from their antenna like behavior. The resonance can be associated to the fundamental antenna mode, which occurs when the length of the rod

is approximately equal to half the effective wavelength of the mode, i.e., _L_ = λ/(2_n_eff) − 2δ, where λ is the vacuum wavelength, _n_eff the effective refractive index of the THz antenna

mode defining its phase velocity35 and δ is a parameter that leads to an apparent increase of the antenna length to _L_ + 2δ due to the reactance at the ends35,36. This parameter depends in

principle on the antenna width and on the materials defining the antenna and its surroundings. It is thus expected to change with the pump fluence as a consequence of the graded intensity

along the edges of the rods. There is also a clear blue-shift of the resonance as the pump fluence increases. This blue-shift is a consequence of the increase in conductivity of the pumped

GaAs and the evolution of the resonance from a plasmonic behavior to a behavior that approaches that of a perfect conductor16. Indeed, a larger conductivity results in a weaker penetration

of the THz field in the pumped GaAs and a reduction of _n_eff35. In the limit of perfect electric conductor the field does not penetrate into the metal and _n_eff is defined by the

dielectrics surrounding antenna. The extinction spectra of photo-generated THz linear antennas with various lengths and a fixed width of 40 μm pumped at 70 μJ/cm2 is shown in Fig. 3 (b). A

large red-shift of the resonance and increase of the extinction is observed as the length of the photo-generated rods is increased. In Fig. 4 we plot the rod length as a function of the

resonant wavelength of maximum extinction. From the slope of the linear fit, illustrated by the solid line, we obtain _n_eff = 1.84 ± 0.07, while the intersection of this fit with the

ordinate axis equals −2δ with δ = 20.8 ± 3.9 μm. A closer look to Fig. 3(b) reveals a shoulder at shorter wavelengths (around 300 μm) in the extinction spectrum of the longer rods. We

attribute this shoulder, which is absent in the spectra of the shorter rods, to the excitation of the next higher order antenna mode in a rod by normal incident THz radiation, i.e., the

3λ/2_n_eff mode37. The observation of multipolar photo-generated modes is a consequence of the high quality of the GaAs layer used for the experiments. DISCUSSION To further investigate the

localized modes associated to the photo-generated rods, we have performed Finite Difference in Time Domain (FDTD) simulations using a commercial software package (Lumerical Solutions).

Details on the simulations can be found in the Methods. Figure 5 (a) shows the extinction spectra of the 230 μm and 80 μm long rods. To approximate the simulated geometry as much as possible

to the experiment, we have considered a graded variation of the carrier concentration in the GaAs film at the boundaries of the rod from _N_ = 2 × 1018 cm−3 at the core to _N_ = 5 × 1015 

cm−3 in the unpumped surrounding region, which resembles the illumination profiles shown in Fig. 2(b). As can be appreciated in Fig. 5(a), this geometry reproduces reasonably well the

resonance wavelength and magnitude of the extinction. Figures 5 (b) and (c) show the near field enhancement at the GaAs-air interface and at the wavelengths of the maximum extinction for the

230 μm and 80 μm long rods respectively. The enhancement, defined as the near field intensity normalized to the incident intensity at the GaAs-air interface, has the dipolar character

expected for the λ/(2_n_eff) mode in the rod-shaped antennas. The maximum field enhancement, which is larger for the shorter rods, is achieved at the edges of the rods where the charge

density is maximum. It is worthwhile to stress that the near-field enhancement in photo-generated antennas can be fully controlled in magnitude and spatial position by simply changing the

illumination pattern defined with the SLM. By controlling the time delay between the optical pump and THz probe pulse, it is also possible to tune the magnitude of the enhancement. This

approach could be exploited to enhance the sensitivity of locally functionalized surfaces or to realize spectroscopy of subwavelength structures by resonant enhancement of the local fields.

Moreover, larger field enhancements could be achieved by coupling rods to form dimers22 or by defining bowtie antennas with sharp tips and small gaps27. In conclusion, we have demonstrated

the photo-generation of THz antennas by the structured illumination of a thin layer of undoped GaAs. This illumination is accomplished with a spatial light modulator that allows a full

optical control of resonant frequencies and local field enhancements. This approach can be extended to the photo-generation of metamaterials exhibiting magnetic resonances, i.e., split ring

resonators38, metasurfaces for active beam steering39 and THz wave guiding structures40. METHODS GAAS LAYER FABRICATION The sample was prepared utilizing epitaxial growth in an Aixtron 200

low pressure metal organic chemical vapour deposition reactor and subsequent layer transfer. The epitaxial structure composed of a 10 nm thick sacrificial AlAs layer followed by a 1 μm thick

undoped GaAs layer was grown at a temperature of 650°C and a pressure of 20 mbar on a 2-inch diameter (001) GaAs wafer, 2 degrees off towards <110>. Source materials were

trimethyl-gallium and trimethyl-aluminium as group-III precursors and arsine as group V precursor. After growth a flexible plastic support carrier was mounted on top of the GaAs epi-layer

and the sample was subjected to a 20% HF solution in water for selective etching of the AlAs layer41. During the process the plastic carrier is used as a handle to bent away the GaAs

epi-layer from the wafer ensuring optimal access of the HF solution to the 10 nm high etch front of the AlAs release layer42. After separation the GaAs thin-film is bonded to a 1 mm thick

SiO2 substrate using a mercapto-ester based polymer. The thickness of this bonding layer is approximately 40 μm. Finally the plastic support carrier is removed leaving the 1 μm single

crystal GaAs layer on a SiO2 substrate. PERMITTIVITY VALUES The values of the permittivity of the SiO2 substrate and the bonding layer were experimentally determined in the frequency range

of the measurements by measuring the time-domain THz transmission. This phase sensitive technique allows to obtain the complex permittivity from a single measurement provided that the

thickness of the layers are precisely defined. These values of the permittivity are and for the bonding layer and the substrate, respectively. The permittivity of the GaAs layer could not be

accurately determined in the same way due to its small thickness compared to the wavelength of THz radiation. Therefore, we approximate the permittivity of GaAs by that of a free electron

gas described by the Drude model. This approximation has been proven to be valid for semiconductors at THz frequencies34. The frequency dependent complex permittivity resulting from the

Drude model of free charge carriers is given by where both electron-electron and electron-phonon interactions are taken into account. The relative high and low (DC) frequency dielectric

constants are and , respectively. Moreover, the transverse optical phonon absorption is at νTO = 8.03 THz with a damping coefficient Γp = 0.072 THz. The plasma frequency of GaAs is given by

where _N_ is the carrier concentration per unit volume and _m_eff = 0.063 _m_e is the effective electron mass. The e-e collision rate is given by Γe = _e_/(_m_effμ), where μ is the carrier

concentration dependent mobility, which can be approximated by the empirical relation43 TIME-RESOLVED THZ TIME-DOMAIN SPECTROSCOPY The measurements have been performed with a modified

time-resolved THz time-domain spectrometer. With this pump-probe technique, a pulsed laser beam from an amplified oscillator (λ = 800 nm, repetition rate = 1 KHz, pulse duration = 100 fs) is

split in three beams. One of the beams is used to generate THz radiation by optical rectification in a 0.5 mm thick ZnTe crystal. The THz pulse is collected by parabolic gold mirrors and

weakly focused onto the sample. The beam size onto the sample has a FWHM of 2.5 mm. The transmitted THz radiation is focused onto a 1 mm thick ZnTe crystal. The THz field amplitude in this

crystal is probed by the second optical beam, which detects changes in the refractive index of the ZnTe crystal induced by the THz pulse (electro-optical sampling). By controlling the time

delay between the optical beam generating the THz pulse and the optical beam probing the THz field amplitude, it is possible to measure the THz amplitude transients. These transients can be

Fourier transformed to obtain the amplitude or the power spectra. The third optical beam in the setup is used as an optical pump for the sample. Controlling the time delay between the

optical pump and the THz pulse probing the sample allows an accurate investigation of carrier dymanics in photo-excited samples44. FDTD SIMULATIONS The extinction and near fields were

simulated using a commercial 3D - Finite Difference in Time Domain (FDTD) software. The simulated structures were chosen to be as close as possible to the experimental conditions, i.e., a

multi-layered structure consisting of air - silicon oxide - bonding polymer - GaAs - air. The experimentally determined values of the permittivity of the bonding layer and the substrate were

used for the simulations. An homogeneous carrier concentration was considered through the thickness of 1 μm of the GaAs layer. The illuminated rods were simulated by considering a

rectangular region with rounded corners and a permittivity given by the Drude model. A graded illumination across the edges of the rods (see side graphs in Fig. 2(b)) was taken into account

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ACKNOWLEDGEMENTS We are thankful to M.C. Schaafsma and J. Versluis for valuable discussions. This work has been supported by the ERC through grant no 259727 THZ-PLASMON and by the

Netherlands Foundation for Fundamental Research on Matter (FOM) and the Netherlands Organisation for Scientific Research (NWO). AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Center for

Nanophotonics, FOM Institute AMOLF, Science Park 102, 1098 XG, Amsterdam, The Netherlands G. Georgiou, H. K. Tyagi & J. Gómez Rivas * Institute for Molecules and Materials, Radboud

University Nijmegen, Heyendaalseweg 135, 6525 AJ, Nijmegen, The Netherlands P. Mulder, G. J. Bauhuis & J. J. Schermer * COBRA Research Institute, Eindhoven University of Technology, P.O.

Box 513, 5600 MB, Eindhoven, The Netherlands J. Gómez Rivas Authors * G. Georgiou View author publications You can also search for this author inPubMed Google Scholar * H. K. Tyagi View

author publications You can also search for this author inPubMed Google Scholar * P. Mulder View author publications You can also search for this author inPubMed Google Scholar * G. J.

Bauhuis View author publications You can also search for this author inPubMed Google Scholar * J. J. Schermer View author publications You can also search for this author inPubMed Google

Scholar * J. Gómez Rivas View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS G.G. did the measurements and simulations and partially wrote the

manuscript, H.K.T. helped with the measurements, P.M., G.J.B. and J.J.S. fabricated the sample, J.G.R. conceived the experiment and partially wrote the manuscript. All the authors

participated in the discussions and revised the manuscript. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing financial interests. RIGHTS AND PERMISSIONS This work is

licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ Reprints

and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Georgiou, G., Tyagi, H., Mulder, P. _et al._ Photo-generated THz antennas. _Sci Rep_ 4, 3584 (2014). https://doi.org/10.1038/srep03584

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