Microwave and terahertz spectroscopy

8 1.4.1 Introduction Spectroscopy, or the study of the interaction of light with matter. has become one of the major tools of the natural and physical ciences during this century. As the wavelength of the radiation is varied across the electromagnetic spectrum, characteristic properties of atoms, molecule , liquid and solids are probed. In the optical and ultraviolet regions (A. ""' I J-Lm up to 100 nm) it is the electronic structure of the material that is investigated, whi le at infrared wavelengths (""' 1-30 J-Lm) the vibrational degrees of freedom dominate. Microwave spectroscopy began in 1934 with the observation of the "-'20 GHz absorption spectrum of ammonia by Cleeton and Williams. Here we will consider the mjcrowave region of the electromagnetic spectrum to cover the l to I 00 x I 0 9 Hz, or I to I 00 GHz (A. ""' 30 em down to 3 mm). range. While the ammonia mjcrowave spectrum probes the inversion motion of this unique pyramidal molecule, more typically microwave spectroscopy is associated with the pure rotational motion of gas phase species. The section of the electromagnetic spectrum extending roughly from 0.1 to I 0 x 10 12 Hz (0.1-1 0 THz, 3-300 cm-1) is commonly known as the far-infrared (FIR) , submjllimetre or terahertz (THz) region, and therefore lies between the mjcrowave and infrared windows. Accordingly, THz spectroscopy shares both cientific and technological characteri tic with its longer-and shorter-wavelength neighbours. While rich in cientific information, the FIR or THz region of the spectrum has, until recently, been notoriously lacking in good radiation ources-earning the dubious nickname 'the gap in the electromagnetic spectrum'. At its high-frequency boundary, most coherent photonic device (e.g. diode lasers) cease to radiate due to the long lifetimes associated with spontaneous emission at these wavelengths, while at its low-frequency boundary parasitic losses reduce the oscillatory output from most electronic devices to in ignificant levels. A a result, exi ting coherent sources suffer from a number of limjtations. This situation i unfortunate since many scientific disciplines-including chemical physics, astrophy ic , cosmochemistry and planetary/atmospheric cience to name but a few-rely on high-resolution THz spectroscopy (both in a pectral and temporal sense). In addition, technological applications such as ultrafast signal processing and massive data transmjssion would derive tremendous enhancements in rate and volume throughput from frequency-agile THz synthe izers. In general, THz frequencies are uitable for probing low-energy light-matter interactions, such …