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

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.