Benjamin Graber

Failure mechanism of THz GaAs photoconductive antenna

We investigated the failure mechanism of THz GaAs photoconductive antenna using high resolution x-ray diffraction topography. From these studies, it was found that grain boundaries are formed during the high frequency device operation. This results in the segregation of gold at the boundaries causing electromigration of the metal between the gold micro-strips. This disrupts the photocurrents from being produced by femtosecond laser thus preventing terahertz beam generation from the photoconductive antennae leading to device failure.

Coherent and incoherent terahertz beams measured from a terahertz photoconductive antenna

We have systematically measured and analyzed the terahertz beams of a photoconductive antenna fabricated on a GaAs substrate. Our data indicate that the antenna produces both coherent and incoherent terahertz beams. The former is produced largely by the plasmon, and the latter is believed to be due to the black body radiation resulting from the thermal excitations and Joule heating by both the femto-second laser and the bias voltage, applied across the electrodes of the antenna. The terahertz-beam property is greatly affected by the operating condition of the photoconductive antenna.

High SNR single measurements of trace gas phase spectra at THz frequencies

Fast detection and identification of trace gases in ambient conditions demand a high signal-to-ratio (SNR) and superior resolution from a single measurement. We performed time-domain terahertz (THz) spectroscopy on gas (or vapor) phase samples of carbon monoxide, methanol, water, and acetonitrile at concentrations less than 10 ppm and demonstrated 50–60 dB SNR on a single measurement. Using data measured at different sample concentrations and terahertz probe beam powers, we investigated the interplay between the SNR, the probe beam power, the sample concentration, and the electric dipole moment of molecules, for which we extended an absorbance theory to THz frequencies. When comparing our data with references and a theoretical model, we found some discrepancy in certain spectral line intensities, suggesting that certain rotational resonance quantum state may have higher populations or transition probabilities than our model predicts at atmospheric conditions. We could achieve the above results largely due...

Failure analysis of THz GaAs photoconductive antenna by means of high resolution X-ray topography

Time domain terahertz spectroscopy is currently being widely used for various applications. A photoconductive antenna, which is also known as Auston-switch, is popular choice to produce a wideband terahertz beam for the time domain terahertz spectrometer. The device consists of two gold micro-strips deposited on GaAs (001) substrate and is designed to produce a terahertz beam over broadband spectrum with a relatively high conversion efficiency. However the photoconductive antenna often experiences noticeable degradation and failure, especially when a high voltage bias is applied to it or when it is exposed to a strong laser beam whether the laser beam is a femto-second laser pulse to generate terahertz beam or from other lasers.