Over the years many important applications have been identified for the millimeter and submillimeter (mm/submm) spectral region. These include studies of:
Many of these applications, especially those of interest to the Microwave Laboratory, exploit the very strong interaction between mm/submm radiation and the rotational degrees of freedom of small, fundamental molecular species. The figure shows a typical case, CH3Cl. Here the absorption coefficient rises as the cube of the frequency (due to increasing photon size, the Boltzmann factor, and state degeneracy) before reaching a peak and falling exponentially due to the Boltzmann population). (Blake, et al.) (Warner, et al.) (Bogey, et al.)
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A substantial share of all of the scientific studies in the mm/submm spectral region have been done using the nonlinear harmonic generation and cooled detector methodology first put forth in our 1970 paper and more recently extended. (283, 268) Briefly, our basic spectroscopic system uses klystrons or YIG oscillator driven TWT's in the region around 50 GHz to drive nonlinear harmonic generators. The resulting mm/submm energy is radiated quasi-optically through absorption/emission cells and detected by an InSb detector operating at 1.5 K or a 0.3 K germanium bolometer. Associated electronics are used for frequency measurement and signal recovery, providing in effect a synthesized system for the 100 - 1000 GHz region. Systems of this type have proven to be reliable, easy to operate, and reasonably inexpensive. Because of these attributes, a number of similar systems have now been built at laboratories around the world and have become the standard for a wide range of molecular studies in this spectral region. The figure shows the YIG oscillator - TWT broadband version of this basic system.In this system a computer controlled YIG-tuned oscillator/Traveling Wave Tube combination provides the fundamental microwave power. The spectral purity and tuning linearity of the YIG oscillator make it straightforward for the computer to automatically lock the oscillator to a frequency derived from the synthesizer. Since it is possible for this system to record a very large number of lines in a short period, we have developed a number of routines that enable the host computer to automatically locate and measure lines, subtract baseline, calculate pressure broadening parameters, etc. For additional implementations: (Anderson and Ziurys) (Pickett, et al.) (Demuyuck)
Related information: Claude Woods, or Lucy Zuirys.
The pulse train from the femtosecond laser drives an integrated semiconductor switch/antenna. The current which flows in the antenna produces mm/submm radiation whose frequency spectrum is given by the Fourier transform of the temporal optical pulse train. More specifically, this spectrum consists of a large number of high spectral purity (~3/108) components, each of which is an exact multiple of the mode lock frequency. Components of the mm/submm radiation are selected by passive mm/submm quasioptical techniques for spectroscopic application. One of the most important features of this system is that the frequency of the components is easily calibrated by simple electronic means (the counting of the mode lock frequency near 100 MHz). Additionally, because the mm/submm frequencies are directly related to the length of the laser cavity rather than the difference between two optical cavities, the spectral purity of the source is very high, even without active or passive stabilization schemes.See also Elliot Brown.
A series of Backward Wave Oscillator Tubes (BWOs) have been developed by the ISTOK corporation of Fryazino, Russia which span the microwave spectrum from below 100 GHz to above 1000 GHz. These devices are broadly electrically tunable and have good noise characteristics. As such they are excellent spectroscopic sources. They have been used as the basis of a number of spectroscopic systems. Among the earliest were those developed in Nizhny Novogorod, Russia at the Institute of Applied Physics and reviewed by Krupnov and Burenin (Krupnov and Burenin).
For high resolution spectroscopy, frequency control has ordinarily been achieved by the use of frequency multiplier chains and phase lock loops (Krupnov et al., Belou et al.). For frequencies through about 200 GHz, commercial versions of these sources have becom available from KVARTZ and AMC. An alternative approach has been developed in which the BWO is locked to a Fabry-Perot cavity, which is mechanically scanned (Rad III).
We have developed another alternative (FAST Scan Submillimeter Spectroscopy Technique, FASSST) which takes advantage of the high spectral purity of the BWO and uses a fast sweep (~ 105 linewidths/sec) to "freeze" instabilites associated with power supply ripples, thermal drift, etc. Because ~ 106 points are recorded (in 1 - 10 seconds), it is not possible to display a spectrum in its entirety here. However, the sequence of successive blow-ups illustrates the results.
Because of the fast sweep, ~ 106 Hz of detection bandwidth was used to record the spectrum. The use of a bandwidth typical of high resolution microwave spectra (1 Hz) would increase the signal to noise ratio by ~ 1000.
This section is still under construction. In the mean time you may be interested in our recent papers (462, 465). You may also find these sites informative:
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