Design, Fabrication, and Experimental Characterization of Plasmonic Photoconductive Terahertz Emitters

Summary

We describe methods for the design, fabrication, and experimental characterization of plasmonic photoconductive emitters, which offer two orders of magnitude higher terahertz power levels compared to conventional photoconductive emitters.

Abstract

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on photoconduction, which has been one of the most commonly used techniques for terahertz generation ^1-8. Terahertz generation in a photoconductive emitter is achieved by pumping an ultrafast photoconductor with a pulsed or heterodyned laser illumination. The induced photocurrent, which follows the envelope of the pump laser, is routed to a terahertz radiating antenna connected to the photoconductor contact electrodes to generate terahertz radiation. Although the quantum efficiency of a photoconductive emitter can theoretically reach 100%, the relatively long transport path lengths of photo-generated carriers to the contact electrodes of conventional photoconductors have severely limited their quantum efficiency. Additionally, the carrier screening effect and thermal breakdown strictly limit the maximum output power of conventional photoconductive terahertz sources. To address the quantum efficiency limitations of conventional photoconductive terahertz emitters, we have developed a new photoconductive emitter concept which incorporates a plasmonic contact electrode configuration to offer high quantum-efficiency and ultrafast operation simultaneously. By using nano-scale plasmonic contact electrodes, we significantly reduce the average photo-generated carrier transport path to photoconductor contact electrodes compared to conventional photoconductors ⁹. Our method also allows increasing photoconductor active area without a considerable increase in the capacitive loading to the antenna, boosting the maximum terahertz radiation power by preventing the carrier screening effect and thermal breakdown at high optical pump powers. By incorporating plasmonic contact electrodes, we demonstrate enhancing the optical-to-terahertz power conversion efficiency of a conventional photoconductive terahertz emitter by a factor of 50 ¹⁰.

Introduction

We present a novel photoconductive terahertz emitter that uses a plasmonic contact electrode configuration to enhance the optical-to-terahertz conversion efficiency by two orders of magnitude. Our technique addresses the most important limitations of conventional photoconductive terahertz emitters, namely low output power and poor power efficiency, which originate from the inherent tradeoff between high quantum efficiency and ultrafast operation of conventional photoconductors.

One of the key novelties in our design that led to this leapfrog performance improvement is to design a contact electrode configuration that accumulates a large number of photo-generated carriers in close proximity to the contact electrodes, such that they can be collected within a sub-picosecond timescale. In other words, the tradeoff between photoconductor ultrafast operation and high quantum efficiency is mitigated by spatial manipulation of the photo-generated carriers. Plasmonic contact electrodes offer this unique capability by (1) allowing light confinement into nanoscale device active areas between the plasmonic electrodes (beyond diffraction limit), (2) extraordinary light enhancement at the metal contact and photo-absorbing semiconductor interface ^{10, 11}. Another important attribute of our solution is that it accommodates large photoconductor active areas without a considerable increase in the parasitic loading to the terahertz radiating antenna. Utilizing large photoconductor active areas enable mitigating the carrier screening effect and thermal breakdown, which are the ultimate limitations for the maximum radiation power from conventional photoconductive emitters. This video article is concentrated on the unique attributes of our presented solution by describing the governing physics, numerical modeling, and experimental verification. We experimentally demonstrate 50 times higher terahertz powers from a plasmonic photoconductive emitter in comparison with a similar photoconductive emitter with non-plasmonic contact electrodes.

Protocol

1. Plasmonic Photoconductive Emitter Fabrication

1. Fabricate plasmonic gratings.
   1. Clean the semiconductor wafer by immersing in acetone (2 min) followed by isopropanol (2 min), and rinsing with deionized water (10 sec).
   2. Dry the sample with nitrogen and heat it on a hotplate at 115 °C for 90 sec to remove any remaining water.
   3. Spin MicroChem 950K PMMA A4 on the sample at 4,000 rpm for 45 sec. Pre-bake the resist on a hotplate at 180 °C for 3 min.
   4. Load the sample into an electron beam lithography tool (JEOL JBX-6300-FS). Expose the plasmonic grating pattern at a base dose around 650 μC/cm², using a 100 kV acceleration voltage.
   5. Develop PMMA by immersing the sample in a MIBK:IPA 1:3 mixture for 90 sec. Immediately transfer the sample to a solution of pure isopropanol for 60 sec.
   6. Rinse the sample with deionized water for 10 sec and then dry the sample with nitrogen.
   7. Load the sample into a plasma stripper (YES-CV200RFS). Descum the sample using 30 W RF power at 30 °C with a 100 sccm O₂ flow rate for 10 sec.
   8. Remove surface oxide by immersing in a HCl:H₂0 3:10 mixture for 30 sec. Immediately transfer the sample to a cascade rinse of deionized water for 4 min.
   9. Transfer the sample to a beaker of deionized water to minimize exposure to atmospheric oxygen before metal deposition.
   10. Take beaker containing the sample in deionized water to a metal evaporator (Denton SJ-20). Vent the chamber and then remove, dry, and load the sample into the chamber (these steps should be followed without interruption to prevent surface oxide formation on the sample).
   11. Pump the chamber to a pressure below 2x10^-6 Torr. Deposit Ti/Au (50/450 Å).
   12. Vent the chamber and remove the sample.
   13. In order to lift-off the deposited metal, place the sample on a Teflon holder in a beaker of acetone, cover, and leave overnight. Uncover the beaker, place it in an ultrasonic agitator, and wait until all unwanted metal is removed (typically 30 sec).
2. Deposit SiO₂ passivation.
   1. Clean the sample as in Steps 1.1.1 - 1.1.2.
   2. Load the sample in a plasma-enhanced chemical vapor deposition tool (GSI PECVD). Deposit 1500 Å of SiO₂ at 200 °C.
3. Open contact vias through SiO₂.
   1. Clean the sample as in Steps 1.1.1 - 1.1.2.
   2. Spin on HMDS at 4,000 rpm for 30 sec. Spin on Megaposit SPR 220-3.0 photoresist at 4,000 rpm for 30 sec. Pre-bake the resist on a hotplate at 115 °C for 90 sec.
   3. Load the sample and mask plate into projection lithography stepper (GCA AutoStep 200). Align the sample and expose.
   4. Post-bake the exposed photoresist on a hotplate at 115 °C for 90 sec.
   5. Develop resist in AZ 300 MIF developer for 60 sec.
   6. Immediately move the sample to a cascade rinse of deionized water for 4 min. Dry the sample with nitrogen.
   7. Load the sample into a reactive ion etcher (LAM 9400). Etch SiO₂ using a TCP RF power of 500 W, a Bias RF power of 100 W, 15 sccm of SF6-, 50 sccm of C₄F₈, 50 sccm of He, 50 sccm of Ar for 80 sec.
   8. Remove the bulk of the photoresist by placing the sample in acetone (5 min) followed by isopropanol (2 min). Rinse in deionized water (10 sec). Dry with nitrogen.
   9. Remove the residual photoresist by loading the sample in a plasma stripper (YES-CV200RFS). Remove the photoresist using 800 W RF power at 30 °C with a 100 sccm O₂ flow rate for 5 min.
4. Fabricate antennas and bias lines.
   1. Repeat Steps 1.3.1 - 1.3.6 to pattern antennas and bias lines.
   2. Repeat Steps 1.1.8 - 1.1.9 to remove surface oxide.
   3. Take the beaker containing the sample and deionized water to a metal evaporator (Denton SJ-20).
   4. Vent the chamber and then quickly remove, dry, and load the sample into the chamber.
   5. Pump the chamber to a pressure below 2x10^-6 Torr. Deposit Ti/Au (10/4,000 Å).
   6. Vent the chamber and remove the sample.
   7. Repeat Step 1.1.13 to lift-off the deposited metal.
5. Package the sample.
   1. Glue the edges of a 12 mm diameter hyper-hemispherical silicon lens to a 2 inch aluminum washer with 8 mm hole.
   2. Glue a PCB board with metal traces, to which one can easily solder, to the aluminum washer.
   3. Mount the fabricated plasmonic photoconductive terahertz emitter prototypes on the silicon lens using thin epoxy.
   4. Wire bond the device contact pads to a PCB board glued on the same aluminum washer.
   5. Solder wires to the metal traces on the PCB board.
   6. Connect device contact pads to a parametric analyzer (Hewlett Packard 4155A) using wires soldered to the corresponding pads of the PCB board for testing purposes.

2. Plasmonic Photoconductive Emitter Characterization

1. Device alignment.
   1. Place the aluminum washer carrying the plasmonic photoconductive terahertz emitter prototypes on a rotation mount and tightly focus the optical pump from a Ti:Sapphire mode-locked laser (MIRA 900D V10 XW OPT 110V) onto the active area of each device.
   2. Adjust the rotation mount such that the electric field of the optical pump is oriented for efficient excitation of surface plasmon waves (normal to the plasmonic gratings).
   3. Use the parametric analyzer to simultaneously apply bias voltages to each device and measure the induced electrical current in each device. Confirm the optimum optical pump alignment and polarization adjustment by maximizing the photocurrent of each device under test.
2. Output power measurement.
   1. Use an optical chopper (Thorlabs MC2000) to modulate the optical pump from the mode-locked pump laser incident on each device.
   2. Measure the output power of the plasmonic photoconductive terahertz emitter prototypes using a pyroelectric detector (Spectrum Detector, Inc. SPI-A-65 THz).
   3. Connect the output of the pyroelectric detector to a lock-in amplifier (Stanford Research Systems SR830) with the optical chopper's reference frequency to recover terahertz power data at low noise levels.
3. Radiation spectral characterization.
   1. Start with a Ti:Sapphire mode-locked laser and use a beam splitter to split the output of the mode-locked laser into a pump beam and a probe beam.
   2. Use an electrooptic modulator (Thorlabs EO-AM-NR-C2) to modulate the optical beam in the pump path. Focus the pump beam onto the active area of the photoconductive emitter under test to generate terahertz radiation.
   3. Collimate the generated terahertz beam using a first polyethylene spherical lens. Focus the collimated terahertz beam using a second polyethylene spherical lens.
   4. Before the focus of the terahertz beam, combine the collimated terahertz beam with the probe optical beam using an ITO coated glass filter.
   5. Place a 1 mm thick, <110> ZnTe crystal mounted on a rotation stage at the combined focus of the optical and terahertz beam.
   6. Insert a controllable optical delay line in the optical probe path by using a motorized linear stage (Thorlabs NRT100) to vary the time delay between the optical and terahertz pulses interacting inside the ZnTe crystal.
   7. Using a half-waveplate in the probe path, rotate the polarization of the optical probe to be at a 45° angle relative to the terahertz polarization direction.
   8. Use a quarter-waveplate after the ZnTe crystal, convert the optical beam polarization into circular polarization.
   9. Split the circularly polarized optical beam into two branches by a Wollaston prism. Measure the optical beam power in each branch using two balanced detectors connected to a lock-in amplifier.
   10. Connect the motorized delay line and lock-in amplifier to a computer. Write a Matlab script to iteratively move the position of the motorized delay line, pause, and read the signal magnitude from the lock-in amplifier.
   11. Convert the stage position to the time domain, through dividing the total optical delay length by the speed of light, followed by a discreet Fourier transform (using Matlab) to obtain the frequency domain data.

Results

To demonstrate the potential of plasmonic electrodes for terahertz power enhancement, we fabricated two terahertz emitters: a conventional (Figure 1a) and plasmonic (Figure 1b) photoconductive emitter incorporating plasmonic contact electrodes to reduce carrier transport times to contact electrodes. Both designs consist of an ultrafast photoconductor with 20 μm gap between anode and cathode contacts, connected to a 60 μm long bowtie antenna with maximum and minimum widths of 100 &m...

Discussion

In this video article, we present a novel photoconductive terahertz generation technique that uses a plasmonic contact electrode configuration to enhance the optical-to-terahertz conversion efficiency by two orders of magnitude. The significant increase in the terahertz radiation power from the presented plasmonic photoconductive emitters is very valuable for future high-sensitivity terahertz imaging, spectroscopy and spectrometry systems used for advanced chemical identification, medical imaging, biological sensing, ast...

Materials

Name	Company	Catalog Number	Comments
Reagent
Polymethyl Methacrylate (PMMA)	MicroChem	950K PMMA A4
Hexamethyldisilazane (HMDS)	Shin-Etsu MicroSI	MicroPrime HP Primer
Optical Photoresist	Dow Chemical	Megaposit SPR 220-3.0
Photoresist Developer	AZ Electronic Materials	AZ 300 MIF Developer
Methyl Iso-Butyl Keytone (MIBK)	Avantor Performance Materials	9322-03
Equipment
Ti:Sapphire Mode-Locked Laser	Coherent	MIRA 900D V10 XW OPT 110V
Pyr–lectric Detector	Spectrum Detector	SPI-A-65 THz
Electron-Beam Lithography Tool	JEOL	JBX-6300-FS
Plasma Stripper	Yield Engineering Systems	YES-CV200RFS
Metal Evaporator	Denton Vacuum	SJ-20
Plasma Enhanced Chemical Vapor Deposition Tool	GSI	GSI PECVD System
Projection Lithography Stepper	GCA	AutoStep 200
Reactive Ion Etcher	LAM Research	9400
Parameter Analyzer	Hewlett Packard	4155A
Optical Chopper	Thorlabs	MC2000
Lock-in Amplifier	Stanford Research Systems	SR830
Electrooptic Modulator	Thorlabs	EO-AM-NR-C2
Motorized Linear Stage	Thorlabs	NRT100