THz Generation & THz Physics
A photoconductive emitter consists of an engineered semiconductor substrate with an antenna structure patterned onto it. When the output of a femtosecond laser is focused onto the emitter, the light is absorbed within the material and charge carriers (electron-hole pairs) are generated. The properties of the semiconductor substrate are switched from an insulating to a conducting state for the duration of the charge carrier lifetime. A bias voltage applied to the antenna leads to a photocurrent across the structure, and the accelerated charge carriers emit THz electrical field proportional to the time-derivative of the photocurrent. In order to detect the THz radiation the inverse process is exploited. There, the generated charge carriers are accelerated by the THz field towards the electrodes, leading to a weak photo current which is measured after amplification. The photocurrent detected is proportional to the amplitude of the THz electric field.
Fiber-coupled antennas for ultrafast coherent terahertz spectroscopy in low temperatures and high magnetic fields
REVIEW OF SCIENTIFIC INSTRUMENTS 73 (9): 3258-3264 SEP 2002
For the purposes of measuring the high-frequency complex conductivity of correlated-electron materials at low temperatures and high magnetic fields, a method is introduced for performing coherent time-domain terahertz spectroscopy directly in the cryogenic bore of existing dc and pulsed magnets. Miniature fiber-coupled THz emitters and receivers are constructed and are demonstrated to work down to 1.5 K and up to 17 T, for eventual use in higher-field magnets. Maintaining the submicron alignment between fiber and antenna during thermal cycling, obtaining ultrafast (<200 fs) optical gating pulses at the end of long optical fibers, and designing highly efficient devices that work well with low-power optical gating pulses constitute the major technical challenges of this project. Data on a YBCO superconducting thin film and a high mobility two-dimensional electron gas are shown. (C) 2002 American Institute of Physics.
B. Ferguson, X. C. Zhang
NATURE MATERIALS 1 (1): 26-33 September 2002
Terahertz spectroscopy systems use far-infrared radiation to extract molecular spectral information in an otherwise inaccessible portion of the electromagnetic spectrum. Materials research is an essential component of modem terahertz systems: novel, higher-power terahertz sources rely heavily on new materials such as quantum cascade structures. At the same time, terahertz spectroscopy and imaging provide a powerful tool for the characterization of a broad range of materials, including semiconductors and biomolecules.
D. Dragoman, M. Dragoman
PROGRESS IN QUANTUM ELECTRONICS 28 (1): 1-66 2004
Terahertz signals were until recently an almost unexplored area of research due to the difficulties in generation and detection of electromagnetic fields at these wavelengths. Neither optical nor microwave techniques are directly applicable in the terahertz range since optical wavelengths are too short and microwave wavelengths are too long compared to terahertz field wavelengths. The development of ultrafast optical techniques, the manufacturing of semi-insulating semiconductors with very short lifetimes and of band-engineered heterostructures, as well as the micromachining techniques and nanotechnology have boosted the terahertz fields as a new area of research in quantum electronics with many important applications. The paper reviews the most recent results in THz fields and is focused on the physical principles of terahertz generators and receivers, underlining the link between terahertz devices and modern technologies such as micromachining and nanotechnology. (C) 2003 Elsevier Ltd. All rights reserved.
Pulsed THz emission from low temperature grown Be-doped InGaAs/InAlAs photoconductive switches at 1030 nm excitation
R. J. B. Dietz, R.Wilk, B. Globisch, H. Roehle, D. Stanze, S. Ullrich, S. Schumann, N. Born, N. Voss, M. Stecher, M. Koch, B. Sartorius and M. Schell
Conference Proceedings, IRMMW 20 Conference Proceedings, IRMMW 20 (): Sep 2012
We present first results of pulsed THz emission from low temperature (LT) MBE grown Be-doped InGaAs/InAlAs multi-nanolayer structures at an excitation wavelength of 1030 nm. The spectra obtained reach 3 THz. We further investigate the material’s relaxation time constants by differential transmission experiments.
H. Roehle, R. J. B. Dietz, H. J. Hensel, J. Böttcher, H. Künzel, D. Stanze, M. Schell, and B. Sartorius
Optics Express 18 (3): 2296-2301 Feb 2010
Mesa-structuring of InGaAs/InAlAs photoconductive layers is performed employing a chemical assisted ion beam etching (CAIBE) process. Terahertz photoconductive antennas for 1.5 μm operation are fabricated and evaluated in a time domain spectrometer. Order-of-magnitude improvements versus planar antennas are demonstrated in terms of emitter power, dark current and receiver sensitivity.