Recent advances in terahertz imaging

We review recent progress in the field of terahertz “T-ray” imaging. This relatively new imaging technique, based on terahertz time-domain spectroscopy, has the potential to be the first portable far-infrared imaging spectrometer. We give several examples which illustrate the possible applications of this technology, using both the amplitude and phase information contained in the THz waveforms. We describe the latest results in tomographic imaging, in which waveforms reflected from an object can be used to form a three-dimensional representation. Advanced signal processing tools are exploited for the purposes of extracting tomographic results, including spectroscopic information about each reflecting layer of a sample. We also describe the application of optical near-field techniques to the THz imaging system. Substantial improvements in the spatial resolution are demonstrated. PACS: 07.57.Pt; 42.65.Re 1 Terahertz time-domain spectroscopy The far-infrared, or terahertz, region is one of the least explored ranges of the electromagnetic spectrum. Until relatively recently, it was difficult to efficiently generate and detect terahertz (THz) radiation. Most THz sources were either low-brightness emitters such as thermal sources, or cumbersome, single-frequency molecular vapor lasers. Detection usually relied on bolometric methods, which require cryogenic operation and generally provided low sensitivity. Recently, however, there has been a revolution in THz technology, as a number of newly discovered or re-discovered generation and detection schemes have revitalized the field. These techniques, based on frequency conversion using nonlinear optics [1–6], are often simpler, more reliable, and ∗ Present address: Electrical Engineering Dept., Stanford University, Stanford, CA potentially much less expensive than the more traditional approaches. One of the first and most interesting of these non-linear optical techniques is terahertz time-domain spectroscopy, or THz-TDS [7]. The key components of a THzTDS system are a femtosecond laser and a pair of specially designed transducers. By gating these transducers with ultrafast optical pulses, one can generate sub-ps bursts of THz radiation, and subsequently detect them with high signalto-noise. These THz transients consist of only one or two cycles of the electromagnetic field, and they consequently span a very broad bandwidth. Bandwidths extending from ≈ 100 GHz to 2 or 3 THz are routine, and more than 5 THz has been demonstrated [8]. Furthermore, although the average intensity of the radiation is quite low, the high spatial coherence produces a brightness that exceeds that of conventional thermal sources. Finally, the gated detection is orders of magnitude more sensitive than typical bolometric detection, and requires no cooling or shielding of any kind. The design, construction, and characterization of a THz-TDS system, including in particular the semiconductor transducers [7, 9, 10], has been described elsewhere [11, 12]. Due to the high signal-to-noise and broad bandwidth, the THz-TDS system is ideal for spectroscopic studies of many different physical systems. Many condensed-phase systems exhibit interesting phenomena in the THz spectral range, and, in many cases, these are not yet understood. Indeed, the THzTDS system provides opportunities for spectroscopic studies that would not otherwise be possible. For example, the coherent detection permits the investigation of hot samples such as flames. These would be impossible to study using any detection system that is sensitive to the radiation from the sample. The coherent gated detection of the THz-TDS system is insensitive to incoherent radiation, and is therefore the only farinfrared system capable of flame spectroscopy [13]. A second example is the class of experiments which exploit the extremely short temporal duration of the THz pulses. A typical example is the ‘visible-pump, THz-probe’ experiment, in which a sample is optically prepared with a fs optical pulse, and the evolution of the far-infrared spectrum is measured