ADVANCEMENTS IN PHOTOMIXING AND PHOTCONDUCTIVE SWITCHING FOR THZ

ADVANCEMENTS IN PHOTOMIXING AND PHOTCONDUCTIVE SWITCHING FOR THZ SPECTROSCOPY E. R. Brown, Ph. D. Physical Domains, LLC, Glendale, CA physicaldomains@earthlink. net Joe Demers, Ph. D. EMCORE Corp. , Alhambra, CA erbrown@ece. ucsb. edu Acknowledgement: this work was supported by the U. S. Army Research Office (Dr. Dwight Woolard).

Excess Carrier Concentration Ultrafast Photoconductivity teh 1 ns Normal Ultrafast teh < 1 ps tp Log Time Normal Photoconductor hn hn + tp Ultrafast Photoconductor + i VB tp i Matrix of Recombination Centers VB

THz Photoconductive Sources Electrical Bias Photoconduti ve Gap Photoconductive Mode-Locked Pulsed Train time Switch Antenna Metallization hn 1 Photomixer Coherent Pump Beams Electrical Bias THz Output Semiconductor. Beams Substrate n 3 = |n 2 – n 1| hn 2 Interdigita l Electrode

Low-Temperature-Grown (LTG) Ga. As (the first THz photomixer material) + gap 2 mm Decreasing Field Intensity Buffer Layer Arsenic precipitates in Ga. As Matrix SI Ga. As Substrate To achieve sub-picosecond lifetime, growth temperature must be ~200 o. C or less, which is difficult to control and reproduce

Er. As: Ga. As Nanocomposite (normal MBE growth temperature ~580 o. C) hn + hn 1 2 Gold interdigitated electrodes - + - Embedded Er. As Island Layers In Ga. As Al. Ga. As Buffer Layer Semi-insulating Ga. As substrate THz Output Beam

Plan View and X-sec TEM of Single Er. As Layer Sample TG = 580 o. C • Estimated particle density of 4. 02 x 1011 cm-2 • Er. As coverage of 25% • For a 1 ML deposition, particles range in size from 30 nm x 4 nm to as small as 2 nm x 2 nm • Overgrowth is epitaxial 1 ML Images from Elisabeth Muller, Paul Scherrer Institut Wuerenlingen und Villigen, Switzerland

Self-complementary Square Spiral Antenna

Typical Photomixing Setup Fixed Freq DFB Laser l > 780 nm Optical Isolator Variable Freq Optical DFB Laser Isolator l ~ 780 nm Photomixer chip Silicon hyper-hemisphere Calibrated Golay cell or LHe bolometer Optical Diplexer Microscope Objective Wavemeter Mounting Yoke THz output beam

Typical Power Spectrum: Room-Temperature Detector RC time only= 0. 11 ps Experiment Golay noise floor RC + lifetime t eh = 0. 38 ps

Typical Power Spectrum: Cryogenic Bolometer (and demonstrating “zoom-in” capability) -20 Power [d. Bm] -30 -40 30 GHz -50 -60 Zoom-In

Comparison between Ga. As Photomixer Results Er. As-Ga. As (J. Bjarnason et al. , vol. 85, p. 3983 [2004]. ) Power (m W) 10. 0 1. 0 0. 1 Lincoln Lab Group (Duffy et al. , IEEE Trans MTT, 2001) 0. 01 0. 1 1 Difference Frequency (THz) 10

Photomixing Coherent (Homodyne) Transceiver Wavemeter Transmit Photomixer Fixed DFB Isolator Laser l 780 nm Beam Combiner Tunable Laser Isolator l > 780 nm Microscope Objective Chopper THz Aspheres Receive Photomixer Sample Under Test Hyperhemispherical Lens Transimpedance Lock-In Amp

Room-Temperature Commercial System http: //www. emcore. com/product/terahz. php

Power [Arb Units] Signal-to-Noise Ratio 80 d. B 60 d. B 40 d. B Frequency [GHz]

Coherence Experiment. Linewidth Instantaneous 20 MHz 5 d. B FWHM ~ 5 MHz

Solid-State High-Resolution Spectroscopy Lactose Monohydrate (Milk Sugar) Photomixng Photomixing Transceiver Gaussian fit Water Line Lorentzian fit Attenuation Coefficient [1/cm] FWHM = 23 GHz Frequency [THz] 4 FWHM = 23 GHz Water Line Time-Domain Spectroscopy 2 FWHM = 70 GHz 0 Frequency [THz]

Solid-State Explosive

H 2 O 2 -Air Mixture

High-Resolution Vapor-Phase Spectroscopy Methanol and MEK (@STP) Transmitted Power Background (two empty PE containers) Methanol Vapor MEK Vapor

Summary • Er. As: Ga. As now provides the best THz photomixers (and (photoconductive switches too !) for 780 nm drive lasers • Room-temperature operation realizable with coherent (homodyne) operation…. 80 d. B SNR at 200 GHz, 60 d. B SNR at 1. 0 THz, 40 d. B SNR at 1. 8 THz… all @ 1. 0 s integration. • Narrow linewidth (<100 MHz), valuable in high-resolution and “zoom-in” spectroscopy