Introductory Chapter: Origin of Terahertz Technology

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 (δτ) less than 100 femtoseconds (fs) and high coherence within several tens of micrometers (i.e., δx = Cδτ, where δ x and C are, respectively, a light speed and a coherent length in free space). The presented THz radiation from time-varying Hertzian dipoles also has high coherence and can be characterized by amplitudes and time- and spatial-dependent wave phases. This fs-laser-excited THz radiation thus satisfies the fundamentals of wave optics [1], called THz waves in literatures.

In the pioneering era during or before the 1980s, THz waves were the picosecond (ps) electric pulse signals measured from the integrated circuits on silicon-on-sapphire (SOS) substrates [2], but not from air-free space. For general electronic devices, ps electric pulses are extremely fast response or switching performance, which results from the rapid capture of optically injected carriers by the large density of material structural defects. Such the time-varying electric pulse currents, or electric oscillation, to dynamic switch one circuit, are presented on the basis of laser illuminance, called a photoconductive performance. The integrated circuits attach the surfaces of photoconductive layers to generate photocurrents for the functions of Hertzian dipoles [2], transmission lines [3, 4], and planar-integrated optoelectronic antennas [5]. For SOS substrates, silicon is an example of the photoconductive layer to be patterned with metal circuits or electrodes. The active spot of the photoconductor surface was one miniature metal gap of an integrated circuit at which fs optical pulses are focused [2].

For the presentation of picosecond photoconductive Hertzian dipoles, two identical Hertzian dipoles were used as the transmitting and receiving dipoles for ps electric pulses within a 1.1-mm-thick insulating material (alumina) [2]. The configuration is shown in Figure 1(a) had two Hertzian dipoles that were oppositely and symmetrically attached to one 1.1-mm-thick alumina slab. The alumina slab was used as the propagation space of ps electric pulses. The electric pulse duration within the full width at half maximum was 2.3 ps, corresponding to a coherent length of 0.69 mm and less than the 1.1 mm propagation distance of a dielectric medium (alumina). Compared to the photoconductive electric pulses on the coplanar transmission lines [3, 4, 7] or coaxial cables [8], this is the first demonstration for far-field detecting a free-propagating THz wave without any waveguide support.

Figure 1.

Configurations to coherently and synchronously generate, detect ps second electric pulses: (a) Hertzian dipoles, (Reprinted from reference [2]. © 1984 AIP publishing) and (b) coplanar transmission lines. (Reprinted from reference [6]. © 1988 AIP publishing).

Oscilloscopes cannot resolve these ps electric pulses in a time domain. The time-varying photocurrent or electric field should be performed from the correlation of electric signals between the pump laser wave and generated THz wave at the receiving Hertzian dipole. Figure 1(a) shows the THz waves were synchronously excited by one fs-pulse laser beam at the transmitting and receiving Hertzian dipoles, respectively, calling excitation and monitor laser beams. A pulse shot of the monitor laser beam [I(t + τ)] at the receiving photoconductor was delayed relative to that of the excitation laser beam at the transmitting photoconductor [I(t)], which is controlled by a precise translation stage. The transmitting dipole on the left side of Figure 1(a) had a DC bias, and the receiving dipole, which was directly connected to a low-frequency voltage amplifier, detect the bias of the transmitting dipole.

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-ps level on the coplanar transmission lines, as shown in Figure 1(b) [3]. Such the sub-ps electric pulse was critically observed when the excitation and monitor laser beam spots are separated as close as possible on the metal circuit due to the lowest substrate dispersion or distortion to the electric pulses.

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 ps-scaled pulse widths. It proves that THz wave radiating from one laser spot corresponds one-point source of a sphere wave, whose dimensions are approximate to those of metal gaps but much smaller than those of THz wavelengths, i.e., a subwavelength scale. To efficiently collimate THz waves from the laser excitation on the photoconductive dipoles, high-refractive-index sphere lenses were, therefore, requested to attach to the substrate backsides [6]. The free-space propagation of THz waves was then realized with a high directional feature, like a laser beam, which is shown in Figure 2(b) [9]. Based on the photoconductive scheme, THz wave spectrum over 1 THz bandwidth was performed for a 10 cm propagation distance [9]. High-brightness THz waves with a power signal-to-noise ratio (SNR) above 10⁵ were then presented when transmission lines and Hertzian dipoles were assembled as the integrated circuits of photoconductive antennas (PCAs), which are shown in Figure 3(a) [10]. Furthermore, a suitable optic set up to collect and focus THz waves along the system optic axis was also expressed [10]. Figure 3(b) shows the usage of off-axis parabolic mirrors to achieve the mode matching between a THz beam and a fs laser beam at a PCA detector [10]. From the development progress of point source THz optics (Figure 2(a)), THz beams (Figure 2(b)), and the high-brightness schemes (Figure 3), the free-propagating THz waves were completely demonstrated and became useful for advanced applications.

Figure 2.

Configurations of (a) point source THz optics, (Reprinted from reference [6]. © 1988 AIP publishing) and (b) THz beams. (Reprinted from reference [9]. © 1989 AIP publishing).

Figure 3.

Configurations of (a) transmission-line-integrated dipole antenna, and (b) off-axis parabolic mirrors between two PCAs. (Reprinted from reference [10]. © 1989 AIP publishing).

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.