We demonstrate an efficient terahertz (THz) detector based on an optical hybrid cavity, which consists of an optically thin photoconductive layer between a distributed Bragg reflector (DBR) and an array of electrically isolated nanoantennas. Using a combination of numerical simulations and optical experiments, we find a hybrid cavity design which absorbs <75% of incident light with a 50 nm photoconductive layer. By integrating this optical hybrid cavity design into a THz detector, we see enhanced detection sensitivity at the operation wavelength (∼815 nm) over designs which do not include the nanoantenna array

Brener, I

Glass, S

Luk, TS

Mitrofanov, O

Reno, JL

Siday, T

Thompson, RJ

UCL Discovery

PROCEEDINGS OF SPIESPIEDigitalLibrary.org/conference-proceedings-of-spieAn efficient terahertz detector basedon an optical hybrid cavityThomas  Siday, Robert J. Thompson, Samuel  Glass,Ting-Shan  Luk, John L. Reno, et al.Thomas  Siday, Robert J. Thompson, Samuel  Glass,  Ting-Shan  Luk, JohnL. Reno, Igal  Brener, Oleg   Mitrofanov, "An efficient terahertz detector basedon an optical hybrid cavity," Proc. SPIE 10531, Terahertz, RF, Millimeter, andSubmillimeter-Wave Technology and Applications XI, 1053109 (23 February2018); doi: 10.1117/12.2290971Event: SPIE OPTO, 2018, San Francisco, California, United StatesDownloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 10/30/2018  Terms of Use: https://www.spiedigitallibrary.org/terms-of-use  An efficient terahertz detector based on an optical hybrid cavity  Thomas Siday*a, Robert J Thompsona,b Samuel Glassa, Ting-Shan Lukc,d, John L Renoc,d, Igal Brenerc,d and Oleg Mitrofanova,c aElectronic and Electrical Engineering, University College London, London, UK; bLondon Center for Nanotechnology, University College London, London, WC1H 0AH; cCenter for Integrated Technologies, Sandia National Laboratory, Albuquerque, NM 87185, USA; dSandia National Laboratory, Albuquerque, NM 87185, USA ABSTRACT   We demonstrate an efficient terahertz (THz) detector based on an optical hybrid cavity, which consists of an optically thin photoconductive layer between a distributed Bragg reflector (DBR) and an array of electrically isolated nanoantennas. Using a combination of numerical simulations and optical experiments, we find a hybrid cavity design which absorbs >75% of incident light with a 50 nm photoconductive layer. By integrating this optical hybrid cavity design into a THz detector, we see enhanced detection sensitivity at the operation wavelength (~815 nm) over designs which do not include the nanoantenna array.   Keywords: Terahertz detectors, photonic nanostructures, optical cavity 1. INTRODUCTION A significant technological challenge in the terahertz (THz) frequency range is the development of efficient nanoscale THz detectors. The efficiency of photoconductive (PC) THz detectors based on low temperature grown (LT) GaAs is limited by the bulk material properties: the mean free path of photo-excited carriers, and the optical absorption length. A popular method to mitigate the material limitations is to modify the photoconductive antenna to incorporate plasmonic nano-electrodes1–6. These electrodes support an optical plasmon resonance, which strongly localizes the generation of electron-hole pairs to the vicinity of the nano-electrodes. This modification means that when a THz field is present, significantly more carriers can reach an electrode before recombining within the semiconductor. However, the close spacing of the electrically connected nano-electrodes may lead to increased dark noise.  One alternative is to use a photonic structure to trap light within an optically thin semiconductor layer, a method utilized previously for photovoltaics7–10. A structure for THz detection, known as an optical hybrid cavity is shown in Fig. 1(a)11. Here, an optically thin photoconductive layer of LT-GaAs is sandwiched between a DBR and a Gold nanoantenna array, which is electrically isolated from the photoconductive layer with a 15 nm thick layer of Al2O3. This prevents the nanoantennas interacting with the electrical properties of the device. Details of the fabrication process can be found in12. The operation of the device is as follows: infra-red (IR) light (λ=800 nm) excites a dipolar resonance in the nanoantennas, which scatter the light into the photoconductive layer. Photons which escape the photoconductive layer are reflected back by the DBR, forming a cavity mode similar to an optical microcavity within the photoconductive layer11. The DBR was designed using 5 pairs of alternating 60 nm Al0:55Ga0:45As (n=3.25 at 800nm) and 55 nm GaAs (n=2.68 at 800 nm) layers, resulting in a stop band centered near 810 nm.         *tom.siday.15@ucl.ac.uk Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications XI,edited by Laurence P. Sadwick, Tianxin Yang, Proc. of SPIE Vol. 10531, 1053109© 2018 SPIE · CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2290971Proc. of SPIE Vol. 10531  1053109-1Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 10/30/2018Terms of Use: https://www.spiedigitallibrary.org/terms-of-use(a)Contact-C(1)EpoxyNanoantenna/**%.1im 14,71 446.1=1 141.M11 %%I=(b)PC layer Air Dielectric DBRLGaAsLDBRILhtHybrid Cavity Array Reflectivity Hybrid Cavity Array Transmission- - - - DBR Reflectivity - - - - DBR Transmission(a)0.70.60.543; 0.4, 0.3E) 0.20.10.0,-, i'\ ,_%,stop band-,- ...\\,, ,.,,vi_, --/,-700 750 80 850 900Wavelength(nm)1.00.80.60.40.20.0(b) 0.70.60.50.40.3d`c) 0.20.10.0- stop band700 750 800 850 900Wavelength(nm)1.00.8 co0.6 Cl)0.40.20.0  In this work, we vary several critical parameters of the hybrid cavity design: The thickness of the photoconductive layer LGaAs, the periodicity of the nanoantenna array a and b, and the size the individual nanoantennas La and Lb. These parameters are shown in Fig. 1(b).   Figure 1. (a) Visualization of the optical hybrid cavity. (b) Cross-section of the cavity, showing the critical parameters of the device. The red line represents the optical field within the cavity12 Figure 2 shows optical reflectivity and transmission spectra for the hybrid cavity, where (a) are numerical spectra calculated using the finite-difference time-domain (FDTD) method13, and (b) are experimental reflectivity spectra. Transmission (T) and reflectivity (R) spectra are evaluated throughout this chapter to ease comparison between experimental and numerical measurements. Where device absorption is required, this is approximated as 1 − ሺܴ + ܶሻ. As can be seen Fig. 2, without the nanoantennas, the structure behaves identically to a DBR, with a stop-band centered at 800 nm (black dashed line). When the nanoantenna array is introduced, the reflectivity (solid black line) reduces to <10% at ~810 nm, whilst transmission through the device remains low. This reflectivity suppression clearly indicates the presence of the hybrid cavity mode discussed above.  Figure 2. Spectra showing in black the reflectivity, and in red the transmission for the hybrid cavity (solid lines). The dashed lines show the same results when no nanoantenna array is present. The results in (a) are simulated using the FDTD method, and (b) are experimental measurements. The hybrid cavity parameters are a =250 nm, b =350 nm and LGaAs = 270 nm12. Proc. of SPIE Vol. 10531  1053109-2Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 10/30/2018Terms of Use: https://www.spiedigitallibrary.org/terms-of-useReflectivity for varying GaAs layer thicknessesE 300CO 250a)CÑ 100CT 50700 750 800 850 900Wavelength (nm)Ío.60.11 -(R +T) for varying GaAs layer thicknesses300- 0.82502001501000.6 ó50700 750 800 850 900Wavelength (nm)0.22. THICKNESS OF THE PHOTOCONDUCTIVE LAYERWe first consider the photoconductive layer thickness, LGaAs. Figure 3 shows reflectivity (a) and absorption spectra (b) for varying LGaAs thickness. Three zones of high absorption are visible, with maxima between 800 and 810 nm for LGaAs = 270, 160 and 50 nm. This effect can be understood by considering the field distribution of the cavity mode, depicted in Fig. 1(b). When LGaAs is varied, the wavelength of the mode supported by the hybrid cavity also varies. The resonance within the hybrid cavity can be approximated with the k-vector condition:  ݇ = 2ߨߣ =݉௥ߨܮீ௔஺௦݊ீ௔஺௦ + ݉஽஻ோܮ஽஻ோ݊஽஻ோHere, mr is a half integer multiple representing the mode order of resonance in the cavity, nGaAs is the refractive index of the photoconductive layer, mDBR is the number of DBR pairs of thickness LDBR, and nDBR is the averaged refractive index of the DBR. This k-vector condition is plotted in Fig. 3(b) for mr = 5.5, 6.5, and 7.5. Within the DBR stopband (750-875 nm), this k-vector condition coincides with the absorption peak, confirming the hybrid cavity mode is excited. From this k-vector condition, it is possible to express the optimum thickness of the hybrid cavity: ܮீ௔஺௦ = ቀଵସ +ଵଶܰቁ ߣ/݊ீ௔஺௦, where N = 0, 1, 2.... Figure 3. Normalized reflectivity (a) and absorption (b) when the photoconductive layer thicknesses is varied12. 3. PERIODICITY OF THE NANOANTENNA ARRAYThe second parameter investigated is the periodicity of the nanoantenna array a and b (Fig. 1(b)), as it was suggested11 that the periodicity could affect the coupling of light into the hybrid cavity. A number of hybrid cavity structures were fabricated to investigate this. Figure 4(a) shows scanning electron microscope (SEM) images of nanoantenna arrays with periodicities b = 350 nm and 450 nm (a = 250 nm). Figure 4(b) and (c) show numerically calculated absorption spectra for varying periodicity b and a, respectively. What is interesting here is there is practically no change in the position or maximum absorption of the hybrid cavity mode for a wide range of periodicities. Only at large periodicities, larger than the effective wavelength (λ = nGaAs) does the spectrum significantly change. This behavior is similar to that seen in the context of photovoltaics by Munday et al.14 Figure 4(d) and (e) show the wavelength and peak absorption for the cavity mode respectively for different nanoantenna array periodicities. For smaller k-vector (large periodicities) the center wavelength of the mode shifts significantly, which indicates the presence of multiple modes. For larger k, where ݇௫	ଶ + ݇௬	ଶ > ݇଴	ଶ݊ଶ, the mode is significantly more stable, and shifts by only ~15 nm across the range of periodicities simulated. Therefore, the range of periodicities where the hybrid cavity mode is supported can be approximately expressed as ቀଶగ௔ ቁଶ + ቀଶగ௕ ቁଶ = ݇଴	ଶ݊ଶ.Figure 4(f) shows experimental reflectivity measurements confirming the stability of the hybrid cavity mode across a range of periodicities. Proc. of SPIE Vol. 10531  1053109-3Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 10/30/2018Terms of Use: https://www.spiedigitallibrary.org/terms-of-use(1 +21)-6CO N 'Cr M NÓ Ci Ó Ó Ci Ó ÓOOo Opm=U") ó 1,0n CO CO NC) (WU) D(1 +a) -6_4oÑco f0 LO V CO N.-Ó Ó Ó Ó C Ó Ó ÓoEco --rcom.-o co Nro -6oN-ooonECÓ es MoO)LO C rnm `tI It.I IO O OoV 7D (wu) q.0.114...::.. ............4........44u44...v4..4a4.o)oNOOOC3.0I-1I209 É 3.0Eç ó1.5 2.0 2.5kx (107m-1)800 805 810 815 820Wavelength (nm)i ou Z 0. 5209=0.4dç i:Cÿ 0.3314 o 0.2Z251 a4191.5 2.0 2.5 30kx (107m-1)0.45 0.65 0.85Absorption0.10.0750 800 850 900Wavelength (nm)   Figure 4. The effect of nanoantenna periodicity on optical absorption within the hybrid cavity. (a) SEM images of hybrid cavities with different nanoantenna periodicity. Here LGaAs = 270 nm, and the electron beam current density is 500 μC/cm2. (b) And (c) show optical absorption spectra as the periodicity in b and a is varied, respectively. (d) Wavelength of maximum absorption and (e) the normalized peak absorption when the periodicity is varied for both a and b. (f) experimental measurements for different nanoantenna periodicities.12 4. SIZE OF THE NANOANTENNAS The final parameter of interest in optimizing the hybrid cavity structure is the size of the individual nanoantennas. Figure 5(a) shows numerically calculated reflectivity spectra for nanoantenna lengths La = 70-110 nm. The location of minimum reflectivity is seen to shift to longer wavelengths for increasing nanoantenna length,  as would be expected for a dipole antenna15. Experimental verification was provided by altering the electron beam dosage during fabrication, which has the effect of altering the nanoantenna size, and can be seen in the SEM in Fig. 5(b) for electron beam doses 450 μC/cm2 and 650 μC=cm2. Figure 5(c) shows experimental reflectivity measurements for the electron beam doses shown in the SEM images, plus 550 μC/cm2. This effect of nanoantenna size can be explained intuitively by considering the phase of light reflected from the hybrid cavity. We can consider light reflected from the hybrid cavity consist of two components: reflections form the nanoantenna array, and reflection from the DBR. While the phase of light reflected by the DBR remains approximately constant across the stopband, the phase of light reflected by the nanoantennas shifts significantly across the nanoantenna resonance. Therefore, by altering the length (La) of the nanoantennas, their resonance can be tuned such that light Proc. of SPIE Vol. 10531  1053109-4Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 10/30/2018Terms of Use: https://www.spiedigitallibrary.org/terms-of-use(a) (b)- 70x30 nm 80x40 nm90x50 nm 100x60 nm 110x70 nm0.80.6cc0.20.0700 750 800Reflectivity (nm)450 µC/cm2IP waaIl aa aIl A-500 nm II(e) 0.4'5'0.3Úa 0.2NW0.1450 µC/cm2550 pC/cm2600 µC/cm2850 900 750 775 800 825 850Wavelength (nm)  reflected from the nanoantennas destructively interferes with light reflected from the DBR. For the hybrid cavity design where LGaAs = 250 nm, a = 340 nm and b = 250 nm, this occurs most efficiently when La = 90 nm.  Figure 5. (a) Simulated reflectivity spectra for different nanoantenna sizes La and Lb. (b) SEM images showing nanoantenna arrays with different electron beam exposure. (c) Reflectivity spectra of the cavities displayed in (b)12. 5. TERAHERTZ CHARACTERISATION To evaluate the performance of the hybrid cavity design for THz detection, the hybrid cavity design was integrated into a photoconductive THz detector. For comparison, an identical photoconductive antenna was fabricated without the nanoantenna array. Figure 6(a) shows THz waveforms, generated with a ZnTe crystal in a THz time-domain spectroscopy (TDS) system (the experimental configuration is inset), and measured with (red) and without (black) the nanoantenna array. The IR pulse from the Ti:Sapphire laser is tuned to 815 nm. In both cases the shape and spectrum (Fig. 6(a) and 6(c)) of the pulse is the same, with the peak photocurrent higher when the nanoantenna array is present.  Figure 6(b) shows the performance of the hybrid cavity THz detector when the wavelength of the IR gate pulse is varied. In this experiment, a THz detector consisting of a 1 μm thick layer of InAs on a 30 μm layer of GaAs replaces the ZnTe crystal to prevent the THz beam becoming misaligned as the wavelength of the IR gate pulse is varied. Relative to the detector without the nanoantenna array, the sensitivity of the hybrid cavity detector is enhanced by 17% at 815 nm. Proc. of SPIE Vol. 10531  1053109-5Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 10/30/2018Terms of Use: https://www.spiedigitallibrary.org/terms-of-use600400200Q0-200(a) ZnTeTHzSi LensD1.201.15r Q 1.10. z-4000i iNo ArrayNA-Array1.051.00e e e(b)-GaAs THzInAs DD= Detectori . i .2 4 6 8 800 810 820Trw,e \ 1AIn ilnr. +h /rim\830om -20736 -400cl- -60-80I Ill IG lNJ/ V V V I I lyll I k 1 1 1 1 /1NA -ArrayNo Array1 I 11 2 3Frequency (THz)4 5   Figure 6. THz characterization of the hybrid cavity design. (a) The THz pulse detected with the nanoantenna array (red) and without (black).  (b) Peak THz field detected with the hybrid cavity design, relative to the design without the nanoantenna array when the laser wavelength is varied. (c) Power spectrum of the waveforms shown in (a).12 6. CONCLUSIONS We have analyzed the performance of the hybrid cavity design when three device parameters are varied: The thickness of the photoconductive layer, the periodicity of the nanoantenna array, and the individual nanoantenna length. We found the thickness of the photoconductive layer was of critical importance, as the hybrid cavity mode is only supported when LGaAs = 50, 160 and 270 nm. While the periodicity does not significantly affect the device performance, the length of the individual nanoantennas does. This must be chosen such that light reflected by the nanoantennas destructively interferes with light reflected by the DBR. THz characterization shows the nanoantennas do improve device performance at the operation wavelength (~815 nm). Proc. of SPIE Vol. 10531  1053109-6Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 10/30/2018Terms of Use: https://www.spiedigitallibrary.org/terms-of-use  REFERENCES [1]  Berry, C. W., Wang, N., Hashemi, M. R., Unlu, M. and Jarrahi, M., “Significant performance enhancement in photoconductive terahertz optoelectronics by incorporating plasmonic contact electrodes.,” Nat. Commun. 4, 1622 (2013). [2]  Berry, C. W., Hashemi, M. R. and Jarrahi, M., “Generation of high power pulsed terahertz radiation using a plasmonic photoconductive emitter array with logarithmic spiral antennas,” Appl. Phys. Lett. 104(8), 81122 (2014). [3]  Heshmat, B., Pahlevaninezhad, H., Pang, Y., Masnadi-Shirazi, M., Burton Lewis, R., Tiedje, T., Gordon, R. and Darcie, T. E., “Nanoplasmonic terahertz photoconductive switch on GaAs,” Nano Lett. 12(12), 6255–6259 (2012). [4]  Matsuura, S., Tani, M. and Sakai, K., “Generation of coherent terahertz radiation by photomixing in dipole photoconductive antennas,” Appl. Phys. Lett. 70(5), 559 (1997). [5]  Moon, K., Lee, I.-M., Shin, J.-H., Lee, E. S., Kim, N., Lee, W.-H., Ko, H., Han, S.-P. and Park, K. H., “Bias field tailored plasmonic nano-electrode for high-power terahertz photonic devices.,” Sci. Rep. 5, 13817 (2015). [6]  Wu, Q. Y. S., Tanoto, H., Ding, L., Choy Chum, C., Wang, B., Bian Chew, A., Banas, A., Banas, K., Jin Chua, S. and Teng, J., “Branchlike nano-electrodes for enhanced terahertz emission in photomixers.,” Nanotechnology 26(25), 255201 (2015). [7]  Briscoe, J. L. and Cho, S.-Y., “Hybridised extraordinary optical transmission in plasmonic cavity,” Electron. Lett. 50(24), 1860–1862 (2014). [8]  Atwater, H. a and Polman, A., “Plasmonics for improved photovoltaic devices.,” Nat. Mater. 9(3), 205–213 (2010). [9]  Kim, S. J., Thomann, I., Park, J., Kang, J. H., Vasudev, A. P. and Brongersma, M. L., “Light trapping for solar fuel generation with Mie resonances,” Nano Lett. 14(3), 1446–1452 (2014). [10]  Simovski, C., Morits, D., Voroshilov, P., Guzhva, M., Belov, P. and Kivshar, Y., “Enhanced efficiency of light-trapping nanoantenna arrays for thin-film solar cells.,” Opt. Express 21 Suppl 4(104), A714–25 (2013). [11]  Mitrofanov, O., Brener, I., Luk, T. and Reno, J. L., “Photoconductive Terahertz Near-Field Detector with a Hybrid Nanoantenna Array Cavity,” ACS Photonics 2(12), 1763–1768 (2015). [12]  Thompson, R. J., Siday, T., Glass, S., Luk, T. S., Reno, J. L., Brener, I. and Mitrofanov, O., “Optically thin hybrid cavity for terahertz photo-conductive detectors,” Appl. Phys. Lett. 110(4), 41105 (2017). [13]  Lumerical Solutions Inc., http://www.lumerical.com/tcad-products/fdtd/ [14]  Munday, J. N. and Atwater, H. A., “Large Integrated Absorption Enhancement in Plasmonic Solar Cells by Combining Metallic Gratings and Antireflection Coatings,” Nano Lett. 11(6), 2195–2201 (2011). [15]  Balanis, C. A., Antenna Theory: Analysis and Design, John Wiley & Sons (2016).  Proc. of SPIE Vol. 10531  1053109-7Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 10/30/2018Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

An efficient terahertz detector based on an optical hybrid cavity

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An efficient terahertz detector based on an optical hybrid cavity

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