Plasmonic Antireflection Coating for Photoconductive Terahertz

Letter pubs.acs.org/journal/apchd5

Plasmonic Antireflection Coating for Photoconductive Terahertz Generation Faezeh Fesharaki, Afshin Jooshesh, Vahid Bahrami-Yekta, Mahsa Mahtab, Tom Tiedje, Thomas E. Darcie, and Reuven Gordon* Department of Electrical and Computer Engineering, University of Victoria, Victoria, British Columbia V8P 5C2, Canada ABSTRACT: Plasmon-enhanced photoconductive antennas allow for improved performance, particularly in below-band-gap absorption devices using low-temperature-grown GaAs. Here we design the plasmonic nanostructures to act as antireflection coatings as well, achieving below 10% reflection at 1570 nm wavelength in an optimized device. Quantitative agreement is seen between experiment and theory. Terahertz emission field amplitudes demonstrate 18 times enhancement compared to that of a conventional terahertz photoconductive antenna on the same substrate. KEYWORDS: antireflection, nanoantenna, photoconductive, plasmonic, terahertz

N

optical reflection was measured experimentally to verify the simulation. Results demonstrated the improved antenna efficiency. In antenna theory,34 radiation efficiency is defined as the ratio of the radiated power to the input power accepted by the antenna. In a terahertz photoconductive antenna, the optical to electrical power conversion efficiency can be estimated from35

anoantennas and plasmonic structures have a remarkable ability to localize and intensify light into deepsubwavelength dimensions.1−8 These elements act as reflectors, absorbers, resonators, and filters, thereby enabling a wide range of applications in optical beam manipulation, energy harvesting, nonlinear optics, biomedical technologies, and sensing.9−17 Plasmonic nanostructures also provide exceptional prospects in THz wave emission. Whereas various techniques have been used for generating THz radiation,18−25 ultrafast switching of photoconductive antennas has proven to be a promising approach for the generation and detection of terahertz waves.26,27 One of the major problems is that the efficiency of these essential devices in converting the laser source power to THz power is very low.28 In this work, plasmon-enhanced photoconductive antennas with plasmonic nanostructure arrays as antireflection coatings and field-enhancing elements are demonstrated. Low-temperature-grown GaAs (LT-GaAs) is used as the substrate. LTGaAs is widely used for photoconductive emitter and detector devices because of its unique properties, such as the ultrashort carrier lifetime, large resistivity, and relatively good carrier mobility.29−31 In order to benefit from mature fiber technology at 1550 nm, researchers have optimized the carrier lifetime and conductivity of InGaAs, but the properties are inferior to LTGaAs. It has been shown that LT-GaAs can be used to absorb 1.55 μm light by means of midgap arsenic states.32 The substantial benefit of applying plasmon enhancement for the two-stage midgap absorption process associated with LT-GaAs with 1.57 μm laser sources has been demonstrated by our group.33 In this work, a systematic approach was used to design a nanoplasmonic structure optimized primarily for the purpose of significantly reducing the surface reflection. Three-dimensional simulation was conducted to examine the reflection characteristic. For experimental evaluation, conventional and plasmonic photoconductive antennas were fabricated. The © 2017 American Chemical Society

ηOE =

eVb 2μe τ 2ηLfR hfl l 2

(1)

where e is the electron charge (=1.6602 × 10−19 coulombs), Vb is the applied bias voltage, μe is the free carrier mobility of the photoconductor, τ is the photocurrent decay time, ηL is the illumination efficiency, f R is the laser repetition frequency, h is Planck’s constant (=6.626 × 10−34 J·s), f L is the laser frequency, and l is the gap length. According to this equation, the efficiency is proportional to bias voltage squared and illumination efficiency, whereas it is inversely proportional to the gap length squared. Therefore, in order to improve the antenna efficiency, the design must be done in such a way that the gap length is decreased and at the same time illumination efficiency increased. Additionally, the capability to tolerate more bias voltage significantly enhances the radiation efficiency. The portion of optical energy reflected from the bare LTGaAs surface is given by the reflection coefficient R =

(n1 − n2)2 (n1 + n2)2

,

where n1 is the refractive index of air (≈1). For LT-GaAs, the refractive index n2 = 3.5 such that R ≈ 0.3. A conventional approach applies an antireflection-matched dielectric layer based on the principle of constructive and destructive interference; however, this approach presents some challenges, Received: April 21, 2017 Published: May 31, 2017 1350

DOI: 10.1021/acsphotonics.7b00410 ACS Photonics 2017, 4, 1350−1354

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Table 1. Plasmonic Antireflection Coating Samples and Simulated and Measured Reflection at 1570 nm

Figure 1. Scanning electron microscope image of plasmonic photoconductive antenna: (a) straight slit with 590 nm periodicity, (b) square patch with 580 and 420 nm periodicity in different directions, (c) hexagonal with 690 nm periodicity, (d) capture image of entire dipole.

as a material with an ideal refractive index and/or accurate thickness is not always obtainable. A 25% reflection has been reported in the literature with application of a single dielectric layer.35 In this paper, plasmonic nanoantennas with different geometries are designed in order to improve their illumination efficiency. Initial dimensions for nanoantennas with specific geometry were calculated theoretically, and HFSS (highfrequency structural simulator) three-dimensional electromagnetic full-wave analysis was used to optimize the periodicity and metal thickness for minimum reflection. A parametric sweep was performed to create an initial good design, and a quasi Newton optimizer was used for fine-tuning. Our dimension is limited to structures with element dimensions on the order of half a wavelength or less. In order to achieve enhanced THz radiation, sharp edges are preferable to provide stronger localization of the field near the metal surface.36 Different geometries of diagonal, hexagonal, and octagonal patches were studied and optimized, and field intensity was monitored in each simulation using the finite element method. In this case, the maximum plasmonic field enhancement falls over those reduced volumes and midgap absorption is enhanced. In addition, it is preferable to have polarizationindependent geometries. Therefore, a variety of structures including straight slits, square patch, and hexagon patch have been designed and fabricated. A numerical model was constructed in HFSS to optimize the structure and minimize

the reflection coefficient (S11). A unit cell with master−slave boundary conditions set in the x- and y-directions was designed, and a broadband plane wave polarized in the x-axis was injected toward the structure at normal incidence. For modeling gold, Johnson and Christy permittivity values were used and Palik permittivity values were used for GaAs. Lengthbased mesh operation was applied in order to achieve a high degree of accuracy. Table 1 shows the simulated plasmonic nanostructures. In this kind of simulation, the reflection coefficient (S11) looking into a single driven element will be the same as S11 looking into that element when its neighbors are also energized. In this case, S11 fairly represents how well matched the array is, since mutual coupling between neighboring elements improves the match of a given element when its neighbors are also driven, and minimal backscatter is indicative of a well-matched nanoantenna. For the proof of concept and to verify the reflection reduction from the LT-GaAs substrate with the aid of the proposed procedure, different nanoplasmonic photoconductive antennas are designed to operate at the standard telecommunication wavelength of 1570 nm. Optical wave reflection was measured by illuminating the nanoantenna and a UDT S380 power meter at the corresponding wavelength. As shown in Table 1, the first unit cell is a straight slit with 590 nm periodicity and 100 nm gap, showing a 36% measured reflection coefficient. The second design is a 380 nm square patch with gap sizes of 40 and 200 nm showing 25% reflection. The final 1351


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Figure 2. (a) Experimental setup for measuring THz generation in photoconductive switches. THz detection using commercial LT-GaAs photoconductive switch for all samples. (b) Time domain THz-driven current detected at receiver; curves are offset for clarity. (c) Power spectrum of THz response obtained from results in (b).

design is a hexagonal patch with 690 nm periodicity and 120 nm gap, and its reflection decreases to 9%. The proposed dimensions provide a more than 12 × 12 element array in what is considered the active region and therefore has a simulated reflection that is in excellent agreement with measurement. Slight discrepancies are predictable due to fabrication tolerances. Therefore, coupling efficiency is significantly improved. Figure 1 shows a scanning electron microscope image of the fabricated nanoantennas. Figure 1a is the SEM image of the plasmonic square patch nanostructure, and Figure 1b shows the hexagon patch pattern. Figure 1c is one of the tested photoconductive devices including the plasmonic structure on the active region. The devices consist of 1 μm of LT-GaAs grown on GaAs. The growth temperature is 226 °C followed by annealing at 600 °C for 1 min. Standard photolithography and negative resist layers were used to outline terahertz dipole antennas. After developing the resist, subsequent deposition of 10 nm Ti/100 nm Au, and lift-off processes, the active regions were patterned using a high-resolution Hitachi FB-2100 focused ion beam system (FIB/SEM) to establish the plasmonic nanostructures. Finally, a hydrochloric acid etch was used to remove excess gallium deposited by the ion beam. Another factor that significantly affects terahertz radiation is bias voltage. The breakdown voltage is significantly increased in the nanoplasmonic structures, whereas at the same time the localized electric field is significantly enhanced in the structure with sharp edges. These nanostructures concentrate electric field and at the same time reduce the antenna operating temperature via enhanced infrared thermal emission. To examine the terahertz emission of the designed nanoplasmonic antennas, conventional open-gap photoconductive emitters were also fabricated besides plasmonic closed gaps. The conventional structure consists of an ultrafast photoconductor with a 5 μm gap between anode and cathode contacts, whereas

the closed-gap designs include a nanoscale plasmonic pattern between them. All designs connect to a long dipole antenna of 30 μm length, fabricated on the same LT-GaAs substrate. The incident optical pump from a femtosecond laser with a central wavelength of 1570 nm, 40 MHz repetition rate, and 80 fs pulse width was firmly focused onto the gap area of each fabricated device. The performance of the fabricated devices was examined as THz sources. Figure 2 shows figures of the experiment schematic and the measured terahertz radiation from the plasmonic nanostructure and conventional terahertz emitters. All samples were biased at 20% lower than the onset of breakdown (>100 V) or device failure. The higher bias is allowed in plasmon-enhanced LT-GaAs devices due to the larger resistivity and thermal breakdown.33 Terahertz radiation enhancement of ∼12 times from the straight slits, more than 14 times from the square patch, and more than 18 times was observed from the hexagonal plasmonic photoconductive emitter. At maximum, the plasmonic photoconductive emitter produced 18.22 nA terahertz field current compared to the 1.02 nA of the conventional photoconductive emitter. This significant radiation power enhancement is due to the higher illumination efficiency, enhanced localized electric field due to sharp metal edges, increased level of generated photocurrent, and higher thermal breakdown. Depending on the application, we may need to work with structures under different polarization or a structure that is polarization independent. Therefore, polarization analyses were also performed on the designed and fabricated plasmonic devices. Table 2 demonstrates the dimensions and polarization dependency of all structures. Straight slit and square patch structures show strong polarization dependence with extinction reduced to close to zero. This was also predicated from the numerical simulation, as these nanostructures can transmit only TM polarization. 1352


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(9) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Biosensing with Plasmonic Nanosensors. Nat. Mater. 2008, 7, 442−453. (10) Jadidi, M. M.; Sushkov, A. B.; Myers-Ward, R. L.; Boyd, A. K.; Daniels, K. M.; Gaskill, D. K.; Fuhrer, M. S.; Drew, H. D.; Murphy, T. E. Tunable Terahertz Hybrid Metal−Graphene Plasmons. Nano Lett. 2015, 15, 7099−7104. (11) Jadidi, M. M.; König-Otto, J. C.; Winnerl, S.; Sushkov, A. B.; Drew, H. D.; Murphy, T. E.; Mittendorff, M. Nonlinear terahertz absorption of graphene plasmons. Nano Lett. 2016, 16, 2734−2738. (12) Wang, F.; Shen, Y. R. General Properties of Local Plasmons in Metal Nanostructures. Phys. Rev. Lett. 2006, 97, 206806. (13) Stockman, M. I.; Shalaev, V. M.; Moskovits, M.; Botet, R.; George, T. F. Enhanced Raman Scattering by Fractal Clusters: ScaleInvariant Theory. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46, 2821. (14) Li, K.; Stockman, M. I.; Bergman, D. J. Self-Similar Chain of Metal Nanospheres as an Efficient Nanolens. Phys. Rev. Lett. 2003, 91, 227402. (15) Ditlbacher, H.; Hohenau, A.; Wagner, D.; Kreibig, U.; Rogers, M.; Hofer, F.; Krenn, J. R. Silver Nanowires as Surface Plasmon Resonators. Phys. Rev. Lett. 2005, 95, 257403. (16) Søndergaard, T.; Bozhevolnyi, S. Slow-Plasmon Resonant Nanostructures: Scattering and Field Enhancements. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 073402. (17) Crozier, K. B.; Sundaramurthy, A.; Kino, G. S.; Quate, C. F. Optical Antennas: Resonators for Local Field Enhancement. J. Appl. Phys. 2003, 94, 4632−4642. (18) Auston, D. H.; Cheung, K. P.; Smith, P. R. Picosecond Photoconducting Hertzian Dipoles. Appl. Phys. Lett. 1984, 45, 284− 286. (19) Grischkowsky, D.; Keiding, S.; Van Exter, M.; Fattinger, C. FarInfrared Time-Domain Spectroscopy with Terahertz Beams of Dielectrics and Semiconductors. J. Opt. Soc. Am. B 1990, 7, 200610.1364/JOSAB.7.002006. (20) Zhang, X. C.; Auston, D. H. Optoelectronic Measurement of Semiconductor Surfaces and Interfaces with Femtosecond Optics. J. Appl. Phys. 1992, 71, 326−338. (21) Hu, B. B.; Zhang, X. C.; Auston, D. H.; Smith, P. R. Free-Space Radiation from Electro-Optic Crystals. Appl. Phys. Lett. 1990, 56, 506− 508. (22) Roskos, H. G.; Nuss, M. C.; Shah, J.; Leo, K.; Miller, D. A.; Fox, A. M.; Köhler, K. Coherent. Submillimeter-Wave Emission from Charge Oscillations in a Double-Well Potential. Phys. Rev. Lett. 1992, 68, 2216. (23) Mittendorff, M.; Xu, M.; Dietz, R. J.; Künzel, H.; Sartorius, B.; Schneider, H.; Helm, M.; Winnerl, S. Large Area Photoconductive Terahertz Emitter for 1.55 μm Excitation Based on an Ingaas Heterostructure. Nanotechnology 2013, 24, 214007. (24) Jadidi, M. M.; König-Otto, J. C.; Winnerl, S.; Sushkov, A. B.; Drew, H. D.; Murphy, T. E.; Mittendorff, M. Nonlinear Terahertz Absorption of Graphene Plasmons. Nano Lett. 2016, 16, 2734−2738. (25) Tani, M.; Fukasawa, R.; Abe, H.; Matsuura, S.; Sakai, K.; Nakashima, S. Terahertz Radiation from Coherent Phonons Excited in Semiconductors. J. Appl. Phys. 1998, 83, 2473−2477. (26) Jepsen, P. U.; Jacobsen, R. H.; Keiding, S. R. Generation and Detection of Terahertz Pulses from Biased Semiconductor Antennas. J. Opt. Soc. Am. B 1996, 13, 2424−2436. (27) Verghese, S.; McIntosh, K. A.; Calawa, S.; Dinatale, W. F.; Duerr, E. K.; Molvar, K. A. Generation and Detection of Coherent Terahertz Waves using Two Photomixers. Appl. Phys. Lett. 1998, 73, 3824−3826. (28) Lepeshov, S. I.; Gorodetsky, A.; Krasnok, A. E.; Rafailov, E. U.; Belov, P. A. Enhancement of Terahertz Photoconductive Antennas and Photomixers Operation by Optical Nanoantennas. arXiv preprint. 2016, 1607.07233. (29) Tani, M.; Matsuura, S.; Sakai, K.; Nakashima, S. I. Emission Characteristics of Photoconductive Antennas based on Low-Temper-

Table 2. Plasmonic Nanostructures’ Dimensions and Polarization Dependence sample

width (nm)

gap (nm)

polarization

slits square hexagonal

490 380 560

100 40/200 120

TM TM TE/TM

On the other hand, the hexagonal plasmonic structure is not polarization dependent and couples light with both TE and TM polarizations. In summary, we have demonstrated dual functionality from nanoplasmonic structures: both as field enhancing and antireflection elements. Below 10% reflection was demonstrated, and the performance has been doubled with respect to our recent publication.33 The demonstrated structures are not uniquely suited to THz applications, and we expect that they may also benefit other application such as solar cells and plasmon-enhanced nonlinear optics such as applications of graphene-based or other nonlinear material nanocomposite structures.37−39

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Reuven Gordon: 0000-0002-1485-6067 Author Contributions

F.F. did simulations, fabricated structures, and performed measurements. A.J. fabricated structures and performed measurements. V.B.Y. and M.M. grew substrates under the supervision of T.T. T.D. and R.G. supervised the project, providing ideas and facilities. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support of an NSERC I2I grant. REFERENCES

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Plasmonic Antireflection Coating for Photoconductive Terahertz

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