Overview

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]. 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.

In the literature there are two basic technologies that have been widely used, frequency multiplication of electronic sources from millimeter wavelengths [3].


Solid state power available in the mm/submm.

Figure: Available Solid State Power shows an overview of the power available from representative sources as a function of frequency. While this graph is for specific classes of sources, the aforementioned limitations are more general, leading to the so-called ‘gap’ in the electromagnetic spectrum. Since it is not the purpose of this web site to review the technologies for the generation of radiation, we will simply observe that with increasing frequency and decreasing wavelength, the sizes of single mode electronic devices (especially the volumes in which the radiation is generated) decrease rapidly with increasing frequency. For interested readers a review has recently appeared [13]. Additionally, fundamental losses associated with conductivity increase, as does the impact of fabrication imperfections. From the high frequency side, the decrease in power with decreasing wavelength of Quantum Cascade Lasers (QCLs) and other optical quantum devices is a consequence of the difficulty of obtaining population inversions between energetically similar states.

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 following equation shows that relatively small amounts of power, concentrated in the narrow bandwidths of heterodyne receivers or the small Doppler linewidths of the SMM/THz corresponded to very high temperatures. \begin{equation}P_N = kT\Delta v\end{equation} Stated another way, these small amounts of power are orders of magnitude higher than the thermal and thermal noise powers (See section 2) and the noise in real detectors (See Section 3.2). Indeed, we will see below that source brightness, expressed in either W/Hz or K, is often a better figure of merit than source power. This is true not only for narrow linewidth gas phase spectroscopy, but also for technical applications such as imaging for which a narrow bandwidth can be defined by a receiver. As a result, spectral purity, frequency agility, and frequency control have become at least as important as the generation of power for successful systems. This has made electronic sources of radiation the dominant source for applications that serve the broader community: astronomy, atmospheric remote sensing, non-proximate active imaging, and laboratory studies [14].

The initial measurements in this spectral region were made by the use of point contact detectors [3]. Heterodyne detectors can be even more sensitive and in many cases can approach the quantum limit. However, for laboratory spectroscopy they in some sense beg the question of available power because of their requirements for local oscillator power. However, for remote sensing applications they have been developed to a very high degree and we will discuss them in this context below.

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 [3].

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.

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