1. Photoconductive schemes
Terahertz (THz) technology in history comes from the time-varying Hertzian dipoles based on optical excitation of high-peak-power lasers. The high-peak-power lasers have pulse durations (
In the pioneering era during or before the 1980s, THz waves were the picosecond (
For the presentation of picosecond photoconductive Hertzian dipoles, two identical Hertzian dipoles were used as the transmitting and receiving dipoles for
Oscilloscopes cannot resolve these
The low-frequency amplifier, i.e., a lock-in voltage amplifier, at the receiving dipole measured the average currents when the optical delay was scanned. The lock-in amplifier integrated one mechanical chopper with a frequency around several KHz. The chopper modulated the pump laser at the transmitting dipole, and the chopping frequency is synchronous to the lock-in amplifier. Based on the signal extraction method, the electric pulse was found down to sub-
The point source, THz optics, was first presented from the coplanar transmission lines (Figure 1(b)) [6] when one sphere mirror was attached on the sapphire side of an SOS substrate, as shown in Figure 2(a) [6]. Different from the electric pulses measured from the alumina-based [2] and transmission-line-based THz devices [3, 4, 7], the electric-pulse waveforms had positive and negative amplitudes with
2. Bridging THz gaps in optics, photonics, and molecular spectroscopy
In the 1980s, pioneering THz Technology, therefore, opens from the aforementioned THz radiation with the properties of high coherence, high brightness, and high directionality. In 1989, THz spectroscopy based on the photoconductive scheme, called THz time-domain spectroscopy (THz-TDS), was successfully demonstrated to present the molecular absorption lines of ambient water vapor in 0.5–1.5 THz [11]. THz-TDS of water vapor is the milestone of molecular sensing with a fingerprint spectral feature in the THz region. These absorption lines provide the researchers or engineers to calibrate the precision of laser time delay, i.e., exactly extracting THz waveforms in their THz-TDS systems. Using the natural calibrator of THz-TDS—ambient water vapor, THz technology is then extended till the present.
In the 1990s, PCAs were engineered with various integrated circuits [12, 13, 14] and various photoconductive layers [15, 16, 17, 18, 19] to approach tunable bandwidths and ultrashort pulse widths of THz waves. Besides a photoconductive scheme, nonlinear optics and specified crystals were presented for their fundamentals in THz wave generation [20, 21] and detection [22, 23, 24, 25, 26, 27, 28, 29]. The detection limit of PCAs was also discussed in the 1990s [30, 31], and several schemes in the year 2000 were specified to expand the available spectral range of THz-TDS [32, 33, 34]. THz imaging concepts were also presented in the 1990s to express more applications of THz waves [35, 36]. In the meantime, the spectral range of THz-TDS, 0.1–3 THz, was considered as one part of the far infrared-ray spectrum to reveal spectral dielectric constants of materials, such as the superconductors [37, 38], toxic chemicals [39], nonpolar/polar liquids [40, 41], vapors/gases [42], and semiconductors (bulks [43], thin film [44] and quantum-well structures [45]). These specified issues lead to the modern THz technology as expressed in this book. For THz-TDS technology, this book introduces the latest progress of THz waves to sense biological water and the possible propagation along photonic crystal fibers. For imaging technology, this book introduces the application of food inspection, the novelty of THz special light modulator, and near-field imaging with a nanometer scale. For PCA technology, the concept of an interdigitated photoconductive antenna is highlighted. For semiconductor technology, the conductivity models are reviewed in this book for the THz field and the relating carrier transition. For nonlinear optics technology, the novelty of THz wave power detection through a harmonic wave conversion and a nonlinear crystal, MgO:LiNbO3, is introduced in this book. These technologies not only bridge THz gaps in optics, photonics, and molecular spectroscopy but also potentially become available in life.
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