1."Extension of Microwave Absorption Spectroscopy to 0.37-Millimeter Wavelength," Phys. Rev. Lett. 25, 1397-1399 (1970).
- The standard spectrometer for much of SMM/THz spectroscopy: Frequency multiplication from a lower frequency source and a sensitive cryogenic detector
2. "Molecular Beam Maser for the ShortMillimeter Wave Region: Spectral Constants of HCN and DCN," Phys. Rev. 187, 58-65 (1969).
- A high resolution molecular beam maser for the SMM/THz
3 "Microwave Generation from Picosecond Demodulation Sources," Appl. Phys. Lett. 47, 894-896 (1985)."Femtosecond Demodulation Source for High Resolution Submillimeter Spectroscopy," Appl. Phys. Lett. 67, 3810 (1995). "Modulating and scanning the mode-lock frequency of an 800-MHz femtosecond Ti:sapphire laser," Optics Letters 24, 250 - 252 (1999).
- Femtosecond demo sdulation for comb generation in the SMM/THz
4. "A Fast Scan Submillimeter Spectroscopic Technique," Rev. Sci. Instrum. 68, 1675 (1997).
- The Fast Scan Submillimeter Spectroscopy Technique (FASSST) for the rapid (a few seconds) acquisition of 'complete' spectra in the SMM/THz
5. "Noise, detectors, and submillimeter-terahertz system performance in nonambient environments," J. Opt. Soc. 21A, 1273 (2004).
- Noise in the SMM/THz: Contrary to conventional wisdom, the SMM/THz is a remarkably quiet spectral region for spectroscopy and sensing
6. "Continuously tunable coherent spectroscopy for the 0.1 to 1.0 THz region," Appl. Phys. Lett. 42, 309-310 (1983).
As is well known, the THz spectral region is by far the least explored portion of the electromagnetic spectrum, largely because of the difficulty of generating and detecting radiation at these frequencies. In fact, Townes has pointed out that his original motivation for the development of the maser/laser was his desire to make a 'molecular generator' to overcome this problem.1 Although thermal, or nearly thermal, 2 photons have always existed in the THz, the problem is one of generating 'appropriate' radiation, not only in terms of power, but also spectral purity. In this section , we will focus on those properties important for the development of spectroscopic applications and the development of useful spectroscopic systems. Because there is an intimate interplay between the development of appropriate THz sources and the spectroscopic studies themselves, we will also use this section to discuss laboratory spectroscopic results. These will include studies of the basic spectroscopy of small, fundamental species, the production and study of free radicals and ions, and the study of collisional processes. Figure III-1 shows an overview of the power available from representative sources as a function of frequency. 3 While at first it might seem that these power levels are so low as to preclude scientific work over much of the region, this is far from the case. Most spectroscopic measurements are linear, and as a result it is possible to trade detector sensitivity for source power. The initial measurements in this spectral region were made by the use of point contact detectors, 4 which in their more modern microelectronic fabricated form 5 have been rechristened as Schottky barrier diodes and are still an important tool. The next important step in the development of spectroscopic systems in this region was the introduction of cryogenic detectors, (6)
especially the InSb hot electron bolometer because of its speed ( t < 10-6 sec) and sensitivity (~10-12 W/Hz-1/2) . Additionally, the much higher reliability of the cryogenic detectors made work in this spectral region much less of a 'black art' and made THz science more accessible to non-specialists. In most
configurations the sensitivity of the InSb detectors begins to decrease above ~300 GHz and the slower (~10-3 sec) Si and Ge bolometers become more advantageous in
many systems.7 Additionally, bolometers rapidly gain sensitivity with decreasing temperature and cooling them to 3He temperatures (~300 mK) can increase their NEPs
A very large majority of THz studies have taken advantage of the high Q 's (~106 ) associated with spectral lines of low pressure gases. Consequently, correspondingly high spectral purity has been a requirement for most THz laboratory spectroscopic systems, as well as for their corresponding field applications. Approaches to the high Q source problem have included a series of advances in nonlinear frequency multiplication and cooled detector development, 4,6, 8, 13 (4) (6) (8) the extension of fundamental electron beam oscillators 14,15 and fundamental solid state oscillators to higher frequency, 16,17 (16) (17) optical heterodyne down conversion, 18-22(18)(19) (21) (22) the production of microwave sidebands on FIR laser sources, 23-27 (23) (24) (25) (26) (27) and the demodulation of femtosecond laser pulse trains. ( 28) (29)
In the following sections we will discuss those techniques which have been the most widely used to illustrate results typical of high resolution spectroscopy and its applications in the THz. Because virtually all of the systems we will discuss make use of the cryogenic detectors discussed above, we will organize this section according to source technology.
| OSU Physics Department | |The College of Mathand Physical Sciences | The Ohio State University |
References
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