Notes1-3

Notes1-3 - THz Science, Technology, and Systems The best...

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1 THz Science, Technology, and Systems The best motivation for students and researchers entering a new field is often the scientific history and systems applications. This first set of notes provides some highlights from three scientific and application areas: (1) spectroscopy, (2) radio astronomy, and (3) concealed-object imaging, and (4) biomedical imaging. We will also contrast the investment perspectives from the Government and from Private Industry. Both sectors are actively involved in the THz field, more so than any other time in history. Will also summarize some of the “grand challenges” that THz researchers and engineers are presently facing – challenges that certainly qualify as good Ph.D. Thesis topics !
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2 THz Spectroscopy • Historically was the first application in THz region and based on the rotational transitions of many vapor-phase molecules that occur in this region. 1950s : THz spectroscopy of gas-phase molecules started using frequency multiplication of microwave and mm-wave vacuum-tubes (W. Gordy et al). Continual growth of frequency multiplier technology since that time. 1960s : THz spectroscopy of solids and liquids started in the 1960s with the advent of a simple instrument – the Fourier transform spectrometer. Much pioneering work done on the dielectric properties of plastics, semiconductors, and ceramics (H. Gebbie, P. Richards, K. Button, et al.) 1970s : High-resolution (i.e., “line”) astronomy with advent of ultrasensitive cryogenic heterodyne receivers above 100 GHz the discovery of CO rotational ladder and other small molecules in nebular regions (T.G. Phillips et al.) 1970s: Low-resolution astronomy with the advent of ultrasensitive cryogenic (composite) bolometers and the discovery of the cosmic background blackbody peak (P. Richards et al). 1990s: Spectrometric imaging becomes possible with the advent of ultrafast photoconductive techniques: time-domain and photomixer spectrometry
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3 (from Website of Prof. F. DeLucia; http://www.physics.ohio-state.edu/~uwave/energyspec.html) An Early High-Resolution THz Spectrometer
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4 Spectroscopic Figures-of-Merit • Resolution, δν (instantaneous linewidth ) - Low resolution δν > 0.1 cm -1 (3 GHz or higher); Fourier Transform and Time-Domain - Moderate resolution 0.0001 cm -1 (3 MHz) < δν < 0.1 cm -1 (3 GHz) photomixer spectroscopy, FASSST (vacuum-tube) - High resolution δν < 3 MHz frequency multiplier-based spectroscopy • Frequency tuning ∆ν = ν max - ν min - Broadband incoherent (frequency multiplexed), ∆ν >> 1 THz Fourier transform and time-domain - Broadly tunable coherent ∆ν > 1 THz (photomixing) - Moderatlly tunable coherent - ∆ν > 100 GHz (BWOs) - Slightly tunable coherent ∆ν < 100 GHz (Frequency multiplier chains) • Average power P ave (at ~ 1 THz) - “High”: P ave > 1 mW (BWOs) - “Moderate” 0.1 < P ave < 1 mW (Frequency multiplier chains) -“Low”: P ave < 0.1 mW (time domain switches, photomixers)
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5 Interference fringes Spectrum InSb detector 1 InSb detector 2 Ring cavity: L~15 m Mylar beam splitter 1 Mylar beam splitter 2 High voltage power supply Slow wave structure sweeper
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This note was uploaded on 12/02/2009 for the course ECE 000 taught by Professor O during the Spring '09 term at UCSB.

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Notes1-3 - THz Science, Technology, and Systems The best...

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