Name: Design, Fabrication, and Experimental Characterization of Plasmonic Photoconductive Terahertz Emitters (Video) | JoVE Journal
Uploaded: 2013-07-08T13:00:00.000+00:00
Duration: 10 min 54 s
Description: Learn to fabricate plasmonic photoconductive terahertz emitters using electron beam lithography. This method achieves two orders of magnitude higher radiation power than conventional designs for advanced imaging applications.

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5 minutes

The authors would like to thank Picometrix for providing the LT-GaAs substrate and gratefully acknowledge the financial support from Michigan Space Grant Consortium, DARPA Young Faculty Award managed by Dr. John Albrecht (contract # N66001-10-1-4027), NSF CAREER Award managed by Dr. Samir El-Ghazaly (contract # N00014-11-1-0096), ONR Young Investigator Award managed by Dr. Paul Maki (contract # N00014-12-1-0947), and ARO Young Investigator Award managed by Dr. Dev Palmer (contract # W911NF-12-1-0253).

1. Plasmonic Photoconductive Emitter Fabrication
<ol>
 <li>Fabricate plasmonic gratings.
 <ol>
 <li>Clean the semiconductor wafer by immersing in acetone (2 min) followed by isopropanol (2 min), and rinsing with deionized water (10 sec).</li>
 <li>Dry the sample with nitrogen and heat it on a hotplate at 115 &deg;C for 90 sec to remove any remaining water.</li>
 <li>Spin MicroChem 950K PMMA A4 on the sample at 4,000 rpm for 45 sec. Pre-bake the resist on a hotplate at 180 &deg;C for 3 min.</li>
 <li>Load the sample into an electron beam lithography tool (JEOL JBX-6300-FS). Expose the plasmonic grating pattern at a base dose around 650 &mu;C/cm2, using a 100 kV acceleration voltage.</li>
 <li>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.</li>
 <li>Rinse the sample with deionized water for 10 sec and then dry the sample with nitrogen.</li>
 <li>Load the sample into a plasma stripper (YES-CV200RFS). Descum the sample using 30 W RF power at 30 &deg;C with a 100 sccm O2 flow rate for 10 sec.</li>
 <li>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.</li>
 <li>Transfer the sample to a beaker of deionized water to minimize exposure to atmospheric oxygen before metal deposition.</li>
 <li>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).</li>
 <li>Pump the chamber to a pressure below 2x10-6 Torr. Deposit Ti/Au (50/450 &Aring;).</li>
 <li>Vent the chamber and remove the sample.</li>
 <li>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).</li>
 </ol>
 </li>
 <li>Deposit SiO2 passivation.

 <ol>
 <li>Clean the sample as in Steps 1.1.1 - 1.1.2.</li>
 <li>Load the sample in a plasma-enhanced chemical vapor deposition tool (GSI PECVD). Deposit 1500 &Aring; of SiO2 at 200 &deg;C.</li>
 </ol>
 </li>
 <li>Open contact vias through SiO2.

 <ol>
 <li>Clean the sample as in Steps 1.1.1 - 1.1.2.</li>
 <li>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 &deg;C for 90 sec.</li>
 <li>Load the sample and mask plate into projection lithography stepper (GCA AutoStep 200). Align the sample and expose.</li>
 <li>Post-bake the exposed photoresist on a hotplate at 115 &deg;C for 90 sec.</li>
 <li>Develop resist in AZ 300 MIF developer for 60 sec.</li>
 <li>Immediately move the sample to a cascade rinse of deionized water for 4 min. Dry the sample with nitrogen.</li>
 <li>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.</li>
 <li>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.</li>
 <li>Remove the residual photoresist by loading the sample in a plasma stripper (YES-CV200RFS). Remove the photoresist using 800 W RF power at 30 &deg;C with a 100 sccm O2 flow rate for 5 min.</li>
 </ol>
 </li>
 <li>Fabricate antennas and bias lines.
 <ol>
 <li>Repeat Steps 1.3.1 - 1.3.6 to pattern antennas and bias lines.</li>
 <li>Repeat Steps 1.1.8 - 1.1.9 to remove surface oxide.</li>
 <li>Take the beaker containing the sample and deionized water to a metal evaporator (Denton SJ-20).</li>
 <li>Vent the chamber and then quickly remove, dry, and load the sample into the chamber.</li>
 <li>Pump the chamber to a pressure below 2x10-6 Torr. Deposit Ti/Au (10/4,000 &Aring;).</li>
 <li>Vent the chamber and remove the sample.</li>
 <li>Repeat Step 1.1.13 to lift-off the deposited metal.</li>
 </ol>
 </li>
 <li>Package the sample.
 <ol>
 <li>Glue the edges of a 12 mm diameter hyper-hemispherical silicon lens to a 2 inch aluminum washer with 8 mm hole.</li>
 <li>Glue a PCB board with metal traces, to which one can easily solder, to the aluminum washer.</li>
 <li>Mount the fabricated plasmonic photoconductive terahertz emitter prototypes on the silicon lens using thin epoxy.</li>
 <li>Wire bond the device contact pads to a PCB board glued on the same aluminum washer.</li>
 <li>Solder wires to the metal traces on the PCB board.</li>
 <li>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.</li>
 </ol>
 </li>
</ol>

2. Plasmonic Photoconductive Emitter Characterization

<ol>
 <li>Device alignment.
 <ol>
 <li>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.</li>
 <li>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).</li>
 <li>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.</li>
 </ol>
 </li>
 <li>Output power measurement.
 <ol>
 <li>Use an optical chopper (Thorlabs MC2000) to modulate the optical pump from the mode-locked pump laser incident on each device.</li>
 <li>Measure the output power of the plasmonic photoconductive terahertz emitter prototypes using a pyroelectric detector (Spectrum Detector, Inc. SPI-A-65 THz).</li>
 <li>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.</li>
 </ol>
 </li>
 <li>Radiation spectral characterization.
 <ol>
 <li>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.</li>
 <li>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.</li>
 <li>Collimate the generated terahertz beam using a first polyethylene spherical lens. Focus the collimated terahertz beam using a second polyethylene spherical lens.</li>
 <li>Before the focus of the terahertz beam, combine the collimated terahertz beam with the probe optical beam using an ITO coated glass filter.</li>
 <li>Place a 1 mm thick, &lt;110&gt; ZnTe crystal mounted on a rotation stage at the combined focus of the optical and terahertz beam.</li>
 <li>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.</li>
 <li>Using a half-waveplate in the probe path, rotate the polarization of the optical probe to be at a 45&deg; angle relative to the terahertz polarization direction.</li>
 <li>Use a quarter-waveplate after the ZnTe crystal, convert the optical beam polarization into circular polarization.</li>
 <li>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.</li>
 <li>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.</li>
 <li>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.</li>
 </ol>
 </li>
</ol>

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</ol>

No conflicts of interest declared.

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...

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.

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design, fabrication, and experimental characterization of plasmonic photoconductive terahertz emitters

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

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.

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 μm and 3...

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.

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on ...

The overall goal of this procedure is to demonstrate a highly efficient method for generating terahertz radiation. This is accomplished by incorporating plasmonic contact electrodes in photo conductive ertz emitters to enable high quantum efficiency and ultra fast device operation simultaneously. The first step is to fabricate plasmonic photo conductive emitter prototypes using electron beam lithography.Next, the radiation power of the implemented emitter prototypes is measured and compared with identical conventional photo conductive ertz emitters without plasmonic electrodes. The final step is to characterize the spectral properties of the radiation from plasmonic photo conductive ertz emitter prototypes. Ultimately, results are obtained, which show that incorporating plasmonic electrodes in a photo conductive emitter could enhance the radiation power and efficiency by two orders of magnitude.This method can help address the main limitation of conventional for the conductive ertz TERs, which is the tradeoff between high quantum efficiency and ultrafast operation of conventional. For the conductors. Conventional photo conductive ertz emitters consist of a ertz antenna integrated with an ultra fast photo conductor.Terahertz.Electrical currents are produced for this antenna when an optical pump beam with terahertz frequency components is focused on the ultra fast photo conductor. This beam generates photo carriers at a rate proportional to the envelope of the optical pump. With the application of a bias electric field, the photo carriers drift towards the contact electrodes, feeding the antenna, and generating ertz radiation.The main advantage of plasmonic over conventional photo conductive ertz emitters is that they allow the accumulation of a much larger number of photo generated carriers in close proximity to the plasmonic contact electrodes. These carriers can then drift to the ertz antenna within a sub picosecond timescale. Plasmonic contact electrodes achieve this greater carrier accumulation because they allow the confinement of light in nanoscale device active regions, and offer extraordinary light enhancement at the metal contact and photo absorbing semiconductor interface.To implement a plasmonic photo conductive terror herz emitter first pattern, the nanoscale plasmonic contact electrodes using electron beam lithography followed by metal deposition and liftoff. Next, deposit a silicon dioxide ation layer using plasma enhanced chemical vapor deposition etch contact via through the silicon dioxide layer using optical photolithography and dry plasma etching. Finally, pattern the antennas and bias lines using optical lithography, followed by metal deposition and liftoff.Christopher Berry, a graduate student from my lab, will demonstrate the rahter characterization procedure. After fabricating the plasmonic grading, it must be packaged and characterized. The devices are mounted on a silicon lens that is attached to an aluminum washer and then placed on a rotation mount.Tightly focus the optical pump beam from a titanium sapphire mode locked laser onto the active area of the device. Adjust the rotation mount so that the electric field of the optical pump is perpendicular to the plasmonic gradings for efficient excitation of surface plasma waves. Now use the parametric analyzer to simultaneously apply a bias voltage to the device and measure its induced electrical current.Ensure the optimum pump alignment and polarization by maximizing the photo current of the device. To measure the output power, use an optical chopper to modulate the beam from the mode locked pump laser incident on the device. Connect the output of a pyroelectric detector to a lock-in amplifier.Using the optical chopper's reference frequency, use the pyroelectric detector to measure the output power of the plasmonic terahertz emitter prototype. Begin spectral characterization by taking the beam from a titanium sapphire mode locked laser and employing a beam splitter to split it into a pump beam and a probe beam modulate the beam in the pump path with an electro optic modulator. Focus the pump beam onto the active area of the emitter under test.To generate terahertz radiation, focus the generated ertz beam using a polyethylene spherical lens at a point before the focal plane of the ertz beam, combine it with the optical probe beam using an indium tin oxide coated glass filter mount a one millimeter thick zinc telluride crystal on a rotation stage. Place this at the combined focus of the optical and terahertz beam to vary the time delay between the optical and terahertz pulses interacting in the zinc Telluride crystal. Insert a controllable optical delay line in the optical probe path by using a motorized linear stage.Use a half wave plate in the probe beam path to rotate the polarization of the optical probe to be at 45 degrees relative the ertz polarization direction. Next, use a quarter wave plate after the zinc telluride crystal to convert the optical beam polarization into circular polarization. Then split the circularly polarized optical beam into two branches with a wallin prism.Measure the optical beam power in each branch using two balanced detectors connected to a lock-in amplifier. For experiment control and data collection. Connect the motorized delay line and lock in amplifier to computer programmed to iteratively move the motorized delay line.Pause and read data from the lock-in. Amplifier shown here are examples of the conventional left and plasmonic right emitters that were fabricated and tested. The difference is the plasmonic emitter incorporates two nanoscale plasmonic contact gradings in the input port of the bow tie antenna as shown on the inset at right the incident.Optical pump was tightly focused on each fabricated device and positioned near the anode contact electrode to maximize radiated power. The orientation of the optical electric field for each device was also chosen to maximize the radiated power. Shown in blue is the measured te hertz radiation power from the plasmonic terahertz emitter as a function of optical pump power.The red curve is for the conventional emitter note, the vertical axis is logarithmic. The inset uses the same color scheme to show the photo current as a function of the optical pump power. The significant radiation power enhancement is due to the higher photo current levels generated when employing plasmonic contact electrodes.Here, the measured terahertz radiation power is plotted versus the collected photo current for various bias voltages and optical pump powers. Again, blue corresponds to the plasmonic emitter red to the conventional emitter. The data points are all curve fitted to the same line with a slope of two.This confirms the quadratic dependence of the radiation power on the induced photo current and the fact that all operational conditions and antenna properties are the same for the two emitters. The ertz power enhancement defined as the ratio of the ertz power emitted by the plasmonic ertz emitter to the conventional ertz emitter is shown here as a function of the bias voltage for different optical pump power levels at low optical pump power levels and a bias voltage of 30 volts output power enhancement factors up to 50 are observed. The enhancement factor decreases slightly at higher optical pump power levels and higher voltages.This can be explained by the carrier screening effect, which should affect the plasmonic photo conductor more since it is generating more photo current and separating a larger number of electron hole pairs. This plot shows the radiated power in the time domain from the plasmonic photo conductive emitter. It was measured in response to a 200 femtosecond optical pulse from a mode locked titanium sapphire laser with 800 nanometer central wavelength and 76 megahertz repetition rate.The radiated power in the frequency domain shows peaks around 0.35 ertz and 0.55 ertz that are associated with the resonance peaks of the bow tie antenna. The peak around 0.1 terahertz is associated with the resonance peak of the dipole antenna formed by the bow tie antenna bias lines. In this video, we present a novel photo conductive rah Hz generation technique that uses a plasmonic contact electrode configuration to enhance the optical to rah Hz conversion efficiency by two orders of magnitude.The significant increase in the rah Hz radiation power from the presented plasmonic photo conductive emitter is very valuable for future high sensitivity terahertz imaging spectroscopy and spectrometry systems for advanced chemical identification, medical imaging, biological sensing astronomy, security screening, and material characterization.

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Research

JoVE Journal

Engineering

Polycrystalline silicon thin-film solar cells on glass are fabricated by deposition of boron and phosphorous doped silicon layers followed by crystallisation, defect passivation and metallisation. Plasmonic light-trapping is introduced by forming Ag nanoparticles on the silicon cell surface capped with a diffused reflector resulting in ~45% photocurrent enhancement.

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

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Fabricating Metamaterials Using the Fiber Drawing Method

The procedure for implementing a refractive index sensor for terahertz frequencies based on a grooved parallel-plate waveguide geometry is described here.  The method yields a measurement of the refractive index of a small volume of liquid through monitoring of the shift in the resonant frequency of the waveguide...

Terahertz Microfluidic Sensing Using a Parallel-plate Waveguide Sensor

This protocol outlines the simulation, fabrication and characterization of THz metamaterial absorbers. Such absorbers, when coupled with an appropriate sensor, have applications in THz imaging and spectroscopy.

Simulation, Fabrication and Characterization of THz Metamaterial Absorbers

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Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

The fabrication of high contrast gratings as the parallel spectrum splitting dispersive element in a concentrated photovoltaic system is demonstrated. Fabrication processes including nanoimprint lithography, TiO2 sputtering and reactive ion etching are described. Reflectance measurement results are used to characterize the optical performance.

Fabrication of High Contrast Gratings for the Spectrum Splitting Dispersive Element in a Concentrated Photovoltaic System

A protocol for the colloidal synthesis of silver nanocubes and fabrication of plasmonic nanoscale patch antennas with sub-10 nm gaps is presented.

Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics

A protocol for the design and fabrication of a novel nanopillar-based split ring resonator (SRR) is presented.

Fabrication of Nanopillar-Based Split Ring Resonators for Displacement Current Mediated Resonances in Terahertz Metamaterials

We demonstrate the fabrication of periodic gold nanocup arrays using colloidal lithographic techniques and discuss the importance of nanoplasmonic films.

Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography

This protocol describes the instrumentation for determining the excitation and coupling rates between light emitters and Bloch-like surface plasmon polaritons arising from periodic arrays.

Determination of the Excitation and Coupling Rates Between Light Emitters and Surface Plasmon Polaritons

Presented here is a protocol for the synthesis of silver-palladium (Ag-Pd) alloy nanoparticles (NPs) supported on ZrO2 (Ag-Pd/ZrO2). This system allows for harvesting energy from visible light irradiation to accelerate and control molecular transformations. This is illustrated by nitrobenzene reduction under light irradiation catalyzed by Ag-Pd/ZrO2 NPs.

Preparation of Silver-Palladium Alloyed Nanoparticles for Plasmonic Catalysis under Visible-Light Illumination

Learn to fabricate plasmonic photoconductive terahertz emitters using electron beam lithography. This method achieves two orders of magnitude higher radiation power than conventional designs for advanced imaging applications.

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

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