J. R. Middendorf

THz generation using extrinsic photoconductivity at 1550 nm

1550-nm pulses from a fiber-mode-locked laser are used to drive an ErAs:GaAs photoconductive switch, resulting in easily measured THz radiation with average broadband (~0.1 to 1.0 THz) power of ≈0.1 mW. The new THz switching mechanism is attributed to fast extrinsic photoconductivity that generates photocarriers (probably electrons) from the ErAs nanoparticles embedded in the material with a lifetime of ~0.45 ps (354 GHz bandwidth). This is the first known demonstration of useful THz power generation by extrinsic photoconductivity.

Extraordinary optical transmission and extinction in a Terahertz wire-grid polarizer

A THz wire grid polarizer is simulated and demonstrated consisting of 40-μm periodic aluminum strips mounted on a polycarbonate substrate with a variable metal-to-gap ratio. Full-wave numerical simulations were performed from 100 GHz to 550 GHz predicting that the transmission in perpendicular (parallel) polarization is much higher (lower) than that predicted by geometric optics, leading to a very high extinction ratio of ∼60 dB between 100 and 550 GHz when the gaps become very small (<5 μm). This behavior is confirmed qualitatively in experiments between 100 and 530 GHz where extinction ratios exceeding 40 dB are achieved. These results are explained physically as an electromagnetic concentration effect in the gaps consistent with plasmonic-like behavior. The effect depends critically on gap width and weakly on frequency.

High Fill-Factor Substrate-Based Wire-Grid Polarizers With High Extinction Ratios

Low-cost, substrate-based, millimeter-wave-to-THz wire-grid polarizers have been fabricated on crystalline-quartz substrates using planar processing techniques. The polarizers achieve high extinction ratios (at least 60 dB) with a single layer, while maintaining low insertion loss (a few decibels) by taking advantage of previously under-utilized “spoof” surface-plasmon effects. Full-wave finite-element simulations done with High Frequency Structure Simulator and experiments both show that metal fill-factors upwards of 90% greatly improve polarizer extinction ratio. The extinction ratio of high fill-factor polarizers exceeds that of a commercial free-standing wire grid by up to 20 dB at normal incidence.

Finite-element simulation and design of a high-extinction-ratio THz wire-grid polarizer

A THz wire grid polarizer was simulated, designed, and demonstrated. The polarizer consists of 40-micron periodic aluminum strips mounted on a polycarbonate substrate. Finite element numerical simulations were performed from 100 GHz to 550 GHz. The results of these simulations predicted that the transmission in perpendicular polarization would be much higher than that predicted by geometric optics, leading to a very high extinction ratio of ~ 60 dB at high fill factors (~ 90%). This behavior was qualitatively demonstrated in experiments between 100 and 530 GHz where extinction ratios exceeding 40 dB were achieved. These results are explained physically as an electromagnetic field concentration effect in the gaps characteristic of plasmonic-like behavior. The effect is strongly dependent on gap width and weakly dependent on frequency.

1550-nm time-domain study of ErAs:GaAs photoconductive switches as a function of the erbium concentration

We present the performance of ultrafast ErAs:GaAs photoconductive-switch antennas measured with a 1550-nm time-domain spectrometer. A 1%- and 2%-Er ErAs:GaAs PC switch were tested and showed detected transmitted frequencies up to at least 2.5 and 4.0 THz, respectively, with a dynamic range of ~50 and ~55 dB below ~200 GHz.

Fast-scanning continuously variable THz spectrum analyzers based on the Fabry-Perot

A THz Fabry-Perot spectrum analyzer is presented that uses substrate-based, wire-grid polarizers as the reflectors. Previous THz scanning Fabry-Perot concepts have used wiremeshes or thin metallic layers as the reflectors, both of which are effective to some degree, but lack tunability. The use of wire-grid polarizers allows for fully tunable resolution and throughput, thus enhancing the Fabry-Perot by increasing its operational flexibility. Here the Fabry-Perot concept for a fast-scanning THz spectrum analyzer is presented, potential applications are shown, and the wire-grid reflector concept is demonstrated.

ErAs:GaAs extrinsic photoconductivity: a new alternative for 1550-nm-driven THz sources

This paper summarizes our recent progress on the discovery and THz performance of ErAs:GaAs photoconductive devices driven around 1550 nm. We will present the impulse response of such device in a time-domain spectrometer where the detection is realized with a GaAs electro-optic crystal. The full width at half-maximum of the temporal pulse is 500 fs and the corresponding bandwidth is greater than 2.5 THz. We also present different 1550-nm properties of this material including carrier lifetime by pump-probe phototransmission. All evidence to date suggests that the 1550-nm ultrafast behavior in ErAs:GaAs occurs by extrinsic photoconductivity, not two-photon effect.