Letter
Plasmonic Anti-reflection Coating for Photoconductive Terahertz Generation Faezeh Fesharaki, Afshin Jooshesh, Vahid Bahrami-Yekta, Mahsa Mahtab, Tom Tiedje, Thomas E. Darcie, and Reuven Gordon ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 1, 2017
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Plasmonic Anti-reflection 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 Plasmon-enhanced photoconductive antennas allow for improved performance, particularly in below-bandgap absorption devices using low-temperature grown GaAs. Here we design the plasmonic nanostructures to act as anti-reflection 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 eighteen times enhancement compared to that of a conventional terahertz photoconductive antenna on the same substrate.
Keywords: Anti-reflection, Nanoantenna, Photo-conductive, Plasmonic, Terahertz
Nanoantennas and plasmonic structures have a remarkable ability to localize and intensify light into deepsubwavelength dimensions1-8. 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 sensing9-17. 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 waves26-27. One of the major problems is that the efficiency of these essential devices in converting the laser source power to THz power is very low28. In this work, plasmon-enhanced photoconductive antennas with plasmonic nanostructure arrays as antireflection coatings and field-enhancing elements are demonstrated. Lowtemperature-grown GaAs (LT-GaAs) is used as the substrate. LT-GaAs is widely used for the 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 LT-GaAs. It has been shown that LT-GaAs can be used to absorb 1.55 µm light by means of mid-gap arsenic states32. The substantial benefit of applying plasmon enhancement for the two-stage mid-gap absorption process associated with LT-GaAs with 1.57 µm laser sources has been demonstrated by our group33. 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 plasmonicphotoconductive antennas were fabricated. The optical reflection was measured experimentally to verify the simulation. Results demonstrated the improved antenna efficiency. In antenna theory34, 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:
ߟைா =
ୣ್ మ ఓ ఛమ ఎಽ ೃ మ
(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, fR is the laser repetition frequency, h is Planck’s constant (= 6.626×10-34 J⋅s), fLis 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 ܴ = (୬భ ି୬మ )మ (୬భ ା୬మ )మ
, 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 as a material with an ideal refractive index and/or accurate thickness is not always obtainable. 25% reflection has been reported in the literature with application of a single dielectric layer35. 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 (high frequency 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 Quasi Newton optimizer was used for fine tuning. Our dimension is limited to structures with element dimension 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 surface36. 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
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maximum plasmonic field enhancement falls over those reduced volumes and midgap absorption is enhanced. In addition, it is preferable to have polarization independent 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 condition set in x-and y-directions was designed, and a broadband plane wave polarized in x-axis was injected towards the structure at normal incidence. For modeling gold, Johnson and Christy per-
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mittivity values were used and Palik permittivity values were used for GaAs. Length-based mesh operation was applied in order to achieve a high degree of accuracy. Table I 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.
Table 1. Plasmonic anti-reflection coating samples, and simulated and measured reflection at 1570nm.
(a)
Simulated Structure
Periodicity (nm)
Reflection from simulation
Reflection from measurement
Slits
590
30%
32%
Square
580 / 420
21%
25%
Hexagonal
690
9%
9%
Sample
(b)
(c)
(d)
Figure 1. Scanning electron microscope image of plasmonic photoconductive antenna: (a) Straight slit with 590 nm periodicity (b) Square patch with 580 nm and 420nm periodicity in different directions (c) Hexagonal with 690 nm periodicity (d) capture image of entire dipole.
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 I, 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
the gap sizes of 40 nm and 200 nm showing 25% reflection. The final design is a hexagonal patch with 690 nm periodicity and 120 nm gap and its reflection decreases to 9%. The proposed dimension provides a more than 12 x 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.
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(a)
(b)
(c)
Figure. 2. (a) Experimental setup for measuring THz generation in photoconductive switches. THz detection using commercial LTGaAs 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).
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 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 in 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 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 femtosecond 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 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 (>100V) or device failure. The higher bias is allowed in plasmon-enhanced LT-GaAs devices due to the larger resistivity and thermal breakdown33. 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.
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Depending on the application we may need to work with structures under different polarization or a structure which is polarization independent. Therefore, polarization analyses were also performed on the designed and fabricated plasmonic devices. Table II 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 only transmit TM polarization.
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Table 2. Plasmonic nanostructures’ dimensions and polar6
Sample
Width (nm)
Gap(nm)
Polarization
Slits
490
100
TM 7
Square
380
40/200
TM
Hexagonal
560
120
TE/TM
8
9
10
ization dependence 11
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 anti-reflection elements. Below 10 % reflection was demonstrated and the performance has been doubled with respect to our recent publication33. The demonstrated structures are not uniquely suited to THz applications, and we expect that they may also benefit other application such as solar cells, plasmon-enhanced nonlinear optics such as applications of graphene-based or other nonlinear material nanocomposite structures38-Error! Reference source not found..
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Corresponding Author *
[email protected] 18
Author Contributions FF did simulations, fabricated structures and performed measurements. AJ fabricated structures and performed measurements. VBY and MM grow substrates under the supervision of TT. TD and RG supervised the project, providing ideas and facilities.
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Funding Sources This work is supported by an NSERC Ideas to Innovation grant.
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ACKNOWLEDGMENT The authors acknowledge support of an NSERC I2I grant.
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TOC Figure 88x34mm (96 x 96 DPI)
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