High Efficiency Interdigitated Terahertz Photoconductive Antenna Integrated With Cylindrical Lens Array

Cylindrical lens array (CLA) and plasmonic nano-grating structures were integrated with a large area interdigitated photoconductive antenna (PCA) for high-efficiency terahertz (THz) emitter applications. The peak-to-peak amplitude of THz radiation pulse from the CLA-integrated plasmonic PCA was enhanced 5.5 times compared with that from the conventional PCA without the plasmonic nano-grating and CLA. The highly localized electric field near the nanoscale grating shows enhanced optical absorption near the plasmonic nano-grating. The CLA focuses incident pulse laser light onto the interlaced active area to avoid destructive interference of the emitted THz waves. The enhanced optical absorption near the plasmonic nano-grating and the higher degree of laser light utilization increased the photo-generated carrier density at the device surface, leading to enhanced THz radiation power.

properties, unique spectral signatures of molecules, and ability to penetrate nonpolar materials.With the development of femtosecond lasers, THz time-domain spectroscopy (THz-TDS) was realized in 1989 by Grischkowsky et al. [1] THz-TDS has become an important technology for nondestructive materials testing, medical diagnostics [2].Photoconductive sources and detectors are most commonly utilized [3], [4] in THz-TDS and provide sufficient dynamic range over 1 THz.These photoconductive sources and detectors are based on photoconductive antennas (PCAs) or switches and were demonstrated, for the first time, by the photoconductive Hertzian dipoles in 1984 [5].PCAs can also generate THz continuous-wave (CW) by using the beating signal of laser beams having two different wavelengths [6], [7].
PCAs consist of a couple of metallic antennas and a photoabsorbing semiconductor.A femtosecond laser optical pulse illuminates the area between two antenna electrodes, and the photo-generated carriers are separated by the electric field established between the antenna gap via external bias voltage.An ultrafast photocurrent is fed to the antenna, and electromagnetic THz pulses radiate into free space.In order to achieve high-efficiency PCA, the transit time of the photo-generated carrier must be reduced, which can be achieved by reducing the distance between electrodes.However, a narrow gap between the electrodes results in the reduced active area interacting with the optical pump beam, which may result in the reduced light intensity.Antenna structure optimization is required to achieve large bandwidth as well as high THz radiation power [8].
Interdigitated antenna structure was employed to overcome the tradeoff between the bandwidth and radiation power [9].To enable the unidirectional carrier acceleration, even-numbered electrode gaps in the interdigitated structure are masked and only the odd-numbered electrode gaps are utilized to absorb the incident optical pump beam.Consequently, the destructive interference in the far field can be suppressed.However, due to the 75% metallic surface and high reflectivity of the gallium arsenide (GaAs) surface, only under 25% of the optical pump beam was used for the THz generation [10].
Recently, a microlens was integrated with an interdigitated PCA to replace the shadow mask.The hexagonal microlens array focused 73.4% of the incident light on the odd-numbered electrode gaps in the interdigitated structure and exhibited the better performance than the shadow mask [10].A cylindrical lens was applied to the single dipole antenna PCA for the line excitation [11].The 300 µm-pitch cylindrical lens array (CLA) was placed on the interdigitated PCA [12].The CLA focused more than 95% of the incident light onto the active area of the PCA without a shadow mask.The device with CLA exhibited an order of magnitude higher THz radiation than the PCA with the hexagonal microlens array at the same optical pump power.Considering that the antenna gap of commonly used interdigitated PCA is around 5 µm, the smaller pitch CLA is required and the technique enabling the precise alignment and distance control between the CLA and interdigitated PCA is also required.
In the past decade, to overcome the low efficiency of PCAs, the research works on the plasmonic structure have been conducted [13], [14], [15].Plasmonic contact electrodes were processed between the bow-tie antenna electrodes to increase the collection efficiency of the photocarriers, thereby increasing the THz radiation power by 50 times [14].Plasmonic contact electrodes were also integrated into the logarithmic spiral antenna to radiate CWs up to 0.8 mW at 1 THz [15].Combined with nanoscale etching technology, a three-dimensional plasmonic contact electrode was fabricated, and an optical-THz conversion efficiency of 7.5% was achieved at an input pump power of 1.4 mW [16], [17].At the photoconductive region, nanorod [18] and nanodisk [19] structures were fabricated to enhance optical absorption and increase the THz radiation of the PCA.In addition, a dipole PCA employing hexagonal, rectangular, and slit plasmonic structures processed by focus ion beam etching exhibited THz radiation 60% higher than that of a commercial device [20], [21], [22].
These plasmonic structures were also employed in the large area interdigitated PCAs.The plasmonic contact electrode was also integrated into the large area interdigitated antenna.The photo-generated carriers were more efficiently collected by the plasmonic electrode, which resulted in the enhanced THz power emission of up to 6.7 mW at the optical pump power of 700 mW [23], [24].Various plasmonic structures, such as a nanoelectrode, nanogap, and shifted nanogap, were introduced in the interdigitated PCA [25].The higher radiation efficiency is ascribed to the enhanced bias field rather than the improved plasmonic absorption [25].
Large-area interdigitated PCAs offer numerous advantages in THz radiation and detection [9].Recently, a multipixel interdigitated PCAs was reported.It demonstrated the controllable generation of azimuthal and radial THz beam [26].However, the metal electrode and shadow mask [10] block the optical pump beam so that a significant portion of the optical pump cannot reach the photo-absorption layer.It is necessary to develop a structure that replaces the shadow mask and condenses incident light into an active area of the large-area PCA.
We fabricated and characterized a large area interdigitated plasmonic PCA integrated with a CLA.We fabricated and characterized a large area interdigitated plasmonic PCA integrated with a CLA.The CLA structure is employed to prevent destructive interference in a large area interdigitated PCA and substitute for the shadow mask.The thickness of CLA has been optimized for the interdigitated antenna structure and successfully replaced the conventional shadow mask.In addition, a plasmonic nano-grating structure was fabricated in the active area, and absorption of the surface was dramatically improved to maximize efficiency.The fabricated device was compared with an identical interdigitated PCA integrated with a shadow mask.At an optical pump power of 80 mW and a bias voltage of 10 V, THz radiation enhancement was measured to be 558% by the effect of the CLA and plasmonic nano-grating

II. DESIGN AND FABRICATION OF THE DEVICES
The conventional interdigitated PCA uses an opaque shadow mask to achieve unidirectional carrier acceleration on a large area for high excitation powers [12].Fig. 1 shows schematics of the conventional interdigitated PCA used in this study.As shown in Fig. 1(a), the interdigitated finger electrodes are fabricated by optical lithography on a semi-insulating GaAs substrate.The metal shadow mask is electrically isolated from the finger electrodes by a silicon dioxide (SiO 2 ) layer.Fig. 1(b) is the cross-sectional view of the yellow line (A-B) in Fig. 1(a).The optical pump beam is incident only between the odd-numbered electrode gaps (active area), and the photo-generated current flows only in the positive x direction in Fig. 1(b).In this way, THz radiation no longer causes destructive interference in the far field.However, the metal covering 80% of the device surface reflects the incoming optical pump beam and imposes a limit on the optical-to-THz efficiency of PCA.
We fabricated a CLA and integrated it into the interdigitated PCA to reduce the light loss.In addition, a plasmonic nano-grating that enhances the absorption of the GaAs surface was fabricated for the active area [27].The CLA focuses the incident light on the active area where the plasmonic nano-grating is located.Fig. 2(d) is an optical microscope image of the fabricated PCA chip, and Fig. 2(e) and (f) are SEM images of the regions where the plasmonic nanogratings are located.The plasmonic nano-grating is fabricated on the active area using electron beam lithography, electron beam evaporation of metallization followed by a lift-off process.The plasmonic nano-grating is 7.6 µm long apart from the anode and cathode by 144 nm.
The fabrication steps of the CLA film are described in Fig. 3. On the poly-silicon (p-Si) deposited 1.1-mm-thick quartz substrate, the photoresist (PR) was spin-coated.The PR was patterned using photo-lithography and developed to make mask patterns.To transfer the PR mask onto the p-Si layer, reactiveion-etching (RIE) was processed in carbon tetrafluoride (CF 4 ) gas.After the RIE process, the remaining PR was removed using acetone in an ultrasonic agitator.Then, the quartz substrate with the p-Si etching mask was wet-etched in a 49% hydrofluoric (HF) acid solution at room temperature for 20 min to form the CLA mold structure.To facilitate the demolding of the cured polydimethylsiloxane (PDMS, Silgard 184a), the surface of the mold was coated with a self-assembled monolayer of trichloro silane in a vacuum chamber.Finally, PDMS was spin-coated and cured at 150 °C for 30 min.
Fig. 4(a) shows SEM images of the fabricated quartz mold.The lens spacing was altered from 37 to 62 µm.The quartz molds having pitch as short as 37 µm were successfully fabricated.The focusing efficiency depending on the CLA film thickness, was calculated using COMSOL software.Fig. 5(a) shows the distribution of the norm of the time-averaged Poynting vector at the x-z cross-section of the CLA structure.The white dotted arch is the boundary between PDMS (n = 1.41) [28] and air (n = 1).Fig. 5   each curve was shifted for the better comparison.The shaded area in Fig. 5(b) is the active area corresponding to the antenna gap of 8 µm.Fig. 5(c) shows the ratio of the power incident within the gap (−4 µm < x < 4 µm) which is calculated by using the intensity profile given in Fig. 5(b).Based on this calculation result, the thickness of the CLA film should be in the range of 62.4-89.4µm was required to achieve collection efficiency higher than 80%.The spin coating speed and time were controlled to achieve the CLA thickness results in this range after the PDMS curing process.
Four device structures were fabricated for a comparative study of the effects of CLA and plasmonic nano-grating.Sample A (reference sample) was designed as a conventional PCA with a shadow mask.Sample B had a plasmonic nano-grating added to the design of sample A. With sample C, instead of the shadow mask, the CLA structure was integrated into the design of sample A. Sample D was a device that integrated both the CLA structure and plasmonic nano-grating.

III. MEASUREMENT
The fabricated PCA was measured using a THz TDS system as a THz emitter.An 800-nm-wavelength laser with a pulse width of 200 fs and a repetition rate of 80 MHz was utilized.The polarization of the laser used in the measurement was parallel to the interdigitated antenna (y-pol in Fig. 1) and the spot-size of beam was 500 µm.A commercial PCA (PCA-40-05-10-800-x, BATOP GmbH) was used for the detector.The performance of devices was measured as a THz emitter in the THz TDS system.Fig. 6  peak-to-peak amplitude.As with previous research [27], this was analyzed as the effect of the dramatic absorption improvement on the GaAs surface.The effect of CLA was examined by comparing sample A and sample C. The peak-to-peak amplitude of the pulse was measured to be 272% of the reference sample.The radiated THz power enhancement was calculated from the power spectrum density in Fig. 6(e).The total radiated THz power of sample C was 480% of that of sample A. Sample A that has the shadow mask used only 20% of the incident light.On the other hand, sample C, which has a CLA structure, can use 80%-100% of the incident light.At the same optical pump power, the light power density can be enhanced more than four times, and this is well matched with the experimental results.Finally, sample D, which has both the CLA structure and the plasmonic nano-grating, contained all the improvements in each element.The peak-to-peak amplitude of sample D was measured as 558% of sample A. As shown in Fig. 6(f) and (g), due to the plasmonic grating, the frequency spectra of PCAs with the plasmonic grating (sample B, D) are different from those of the PCAs without the plasmonic grating (sample A, C).The photo-absorption occurs relatively near the surface for samples with the plasmonic grating [27].Because the carrier lifetime at the surface is shorter than that at the bulk [29] substrate, the devices with plasmonic grating exhibit the enhanced power spectral density at 0.2-0.4THz.
The peak-to-peak amplitudes as a function of optical pump powers and applied voltages are shown in Fig. 7  increase of the peak-to-peak amplitude was slightly decreased at 80 mW.This can be analyzed as a result of screening due to excessive photocarriers generated.If there was no screening, the peak-to-peak amplitude would appear to increase proportionally as the optical pump increased [30].However, unlike other samples, in sample D, more photocarriers were generated at the same input optical pump power, which would have inhibited the acceleration effect by the bias field [31].

IV. CONCLUSION
We fabricated and characterized an interdigitated PCA integrated with CLA.The CLA successfully substituted the shadow mask and increased the THz pulse amplitude up to 272% by concentrating the light on the active area of the device.By integrating both the plasmonic nano-grating and the CLA into the device, the peak-to-peak amplitude of the THz wave was improved by 558%.Plasmonic-nano grating enhanced photoabsorption near the device surface and the CLA improved the usage of the optical pump beam, so that the output THz wave intensity was dramatically enhanced.His research interest focuses on plasmonic photoconductive antennas.
Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

Fig. 1 .
Fig. 1.Schematic diagram of the large area interdigitated PCA coupled to the shadow mask.(a) Top view and (b) cross-section view of the white line shown in (a).

Fig. 2 .
Fig. 2. Schematic diagram of (a) the manufactured PCA module.(b) Top view.(c) Cross-section view of the PCA chip.(d) Microscopic image of the manufacture PCA chip.(e) and (f) SEM images of the active area.

Fig. 2
shows a schematic diagram, optical microscopy image, and scanning electron microscope (SEM) image of the plasmonic PCA coupled to the CLA.The manufactured PCA module composed of a PCA chip, PCB substrate, Si lens, and the lens mount is shown in Fig. 2(a).The diameter and thickness of the undoped high resistivity float zone (HRFZ) silicon hyper-hemispherical lens are 12 mm and 7.1 mm respectively (P/N: LHS-HRFZ-Si-D12-T7.1).The schematic diagram showing the antenna part of the PCA chip is shown in Fig. 2(b).Fig. 2(c) shows the cross-section of the white dotted line in Fig. 2(b).The arch-shaped structure located on the top of the electrode is the cross-section of the fabricated CLA.

Fig. 4 (
b) and (c) are a micrograph and a photograph of CLA film realized by using PDMS, respectively.

Fig. 4 .
Fig. 4. (a) SEM images of the quartz mold.The number in figure is the distance between the adjacent cylindrical lens.(b) Optical microscope image of the CLA film.(c) Photograph of the PDMS CLA film.
(a)-(d) shows the time-domain signal of the devices.When the applied voltage was 10 V and the optical pump power was 80 mW, the power spectral densities of the devices are shown in Fig. 6(e).THz radiation enhancement in the peak-to-peak amplitude of samples B-D was compared with sample A (reference sample) under a 10 V bias voltage and 80 mW optical pump.Compared to the reference sample, sample B with plasmonic nano-grating was enhanced to 208% in the Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

Fig. 6 .
Fig. 6.Time-domain signal from the (a) sample A, (b) sample B, (c) sample C, and (d) sample D. (e) Power spectral density obtained from the sample A-D at bias voltage of 10 V and optical pump power 80 mW.Normalized power spectral density of (f) sample A and B; (g) sample C and D.
(a)  and (b).The peak-to-peak amplitudes were proportional to the optical pump powers and applied voltages.Especially in sample D, the

Fig. 7 .
Fig. 7. Peak-to-peak values as a function of (a) optical pump powers at applied voltage 10 V, and (b) applied voltages at optical pump power 80 mW.

Gyejung
Lee received the B.S. degree in electronics and communication engineering from Hanynag University, Ansan-si, South Korea, in 2015, and the M.S. degree in 2017 from the School of Electrical Engineering and Computer Science, Gwangju Institute of Science and Technology (GIST), Gwangju, South Korea, where he is currently working toward the Ph.D. degree.