Terahertz research is mainly concentrated in the 0.1-10 THz band. This is a spectral area that covers a wide range and is very special. Initially, this band was called the â€œTHz Gapâ€ because the band was sandwiched between two relatively mature frequencies, the electronic spectrum and the optical spectrum. The low frequency band overlaps with the millimeter wave band in the field of electronics, and the high frequency band overlaps with the far infrared band (wavelength 0.03-1.0 mm) in the optical field. Due to the particularity of this field, a blank area for early research was formed. However, with the development of research, the importance of terahertz spectrum and technology in the fields of physics, chemistry, biology, electronics, radio astronomy, etc. has gradually emerged, and its application has begun to penetrate into many aspects of social economy and national security, such as bio-imaging, THz spectrum fast detection, high-speed communication, through-wall radar, etc. Terahertz has good application prospects, mainly due to its spectral resolution, safety, perspective, transient and broadband characteristics.
For example: The vibration and rotation frequencies of many biological macromolecules in nature are in the terahertz frequency band, which provides an effective means for detecting biological information; the photon energy in the terahertz band is low, and it will not damage the detector. Non-destructive testing is realized; terahertz waves have good penetrating ability to dielectric materials, which can be used as a means to detect hidden objects; typical pulse width of terahertz pulses is on the order of picoseconds, and terahertz time domain with high SNR can be obtained. Spectrum, easy to spectral analysis of various materials; In addition, the bandwidth of the terahertz band is very wide, from 0.1-10 THz can provide rich spectrum resources for ultra-high speed communication.
Compared with millimeter wave technology, the research of terahertz technology is still in the exploration stage. Terahertz technology mainly includes terahertz wave source, terahertz transmission and terahertz detection, and its key components can be divided into passive components and active devices. Passive components include terahertz transmission lines, filters, couplers, antennas, etc., while active devices include terahertz mixers, frequency multipliers, detectors, amplifiers, oscillators, and the like.
1, terahertz source
With the development of terahertz wave generation technology, there have been many valuable new developments in the research of terahertz sources. The development of low-cost, high-power, room-temperature-stabilized terahertz sources is the basis for the development of terahertz technology. The classification of terahertz sources is diverse, and according to the generation mechanism, it can be divided into optical effects and electronic-based terahertz sources. According to the source type, it can be divided into three categories: incoherent heat radiation sources, broadband terahertz radiation sources, and narrow-band terahertz continuous wave sources.
1.1 Non-coherent heat radiation source
An incoherent heat radiation source converts thermal energy into light energy in the case of thermal equilibrium, producing a continuous spectrum. The main examples are the sun in everyday life, and incandescent lamps. Due to the low power of terahertz waves generated, the application prospects are limited.
1.2 Broadband terahertz radiation source
Broadband terahertz radiation sources are currently mainly used in spectroscopy systems, mainly generated by pulses with a period of tens to hundreds of femtoseconds, and contain ultra-wide spectral components of up to several tens of terahertz in the spectrum. The production methods include:
a) Photoconductive antenna: The main mechanism of photo-guide antenna for terahertz radiation is that the photo-conducting antenna generates carriers under the illumination of light pulses and accelerates under the action of an electric field, generating transient currents on the surface, and then radiating terahertz electromagnetic waves. It is characterized by a high output energy. In recent years, many studies on the generation of broadband terahertz waves by photoconductive antennas have been carried out at home and abroad.
b) Optical rectification method: The optical rectification method utilizes a nonlinear optical rectification effect to generate a difference frequency or a frequency oscillation when two beams or a high-intensity monochromatic beam propagate in a medium, which is characterized in that terahertz can be realized. Ultra-wideband output, but the output energy is relatively low. Based on this principle, the terahertz radiation source has been greatly developed.
c) Air Plasma Method: The principle of the air plasma method is to use the laser to focus the breakdown air to generate terahertz radiation.
d) Semiconductor surface: The basic working principle of terahertz radiation source based on semiconductor surface can be summarized as surface electric field effect and photo-generated Dan Pei effect. For some wide-bandgap semiconductor materials, the surface state exists on the surface, and the surface electric field is formed due to the inconsistent Fermi level of the surface and the inside. Under this electric field, the carriers excited by the laser form a transient current, thereby forming terahertz radiation. For some narrow-bandgap semiconductor materials, due to their large absorption coefficient, a large number of carriers are formed on the surface of the semiconductor, and the electrons and holes in the semiconductor diffuse into the semiconductor, causing the positive and negative charges to separate in space. Dan Pei electric field, radiating terahertz waves. This method is characterized by simple operation and low radiation power.
1.3 narrow-band terahertz continuous wave source
The goal of a narrowband terahertz source is to produce a continuous terahertz wave with a narrow linewidth. Common methods include:
a) Design the oscillator using electronic devices, especially based on the sub-millimeter oscillator, to increase the operating frequency of the oscillator to design an oscillator suitable for the terahertz band. Due to this feature, the currently reported operating frequencies of terahertz sources are mainly concentrated in the lower terahertz band. However, on this basis, terahertz waves of about 1 THz or higher have been obtained by using the frequency doubling chain.
b) Terahertz quantum cascade laser (THz-QCL), as a kind of coherent light source, is based on conduction band subband electronic energy state transition and phonon resonance assisted tunneling to achieve particle number inversion. With the rapid development of quantum cascade lasers, it can be used to study small-scale material motions, such as electron microscopic transport, nanophotonics, and so on. At the same time, due to its compact structure, it has high application value in many fields, such as astrophysics and atmospheric science, space communication, precision spectrometry, security inspection and terahertz imaging.
c) A free electron laser converts the kinetic energy of a relativistic electron beam moving in a magnetic field into photon energy, thereby producing a laser characterized by high energy and high coherence. Due to its continuity, the wavelength of the radiation can be tuned to any wavelength, which is very suitable for use as a terahertz source. However, the disadvantages of free electron lasers are high power consumption, large size and high cost. Therefore, free electron lasers are basically used in laboratory environments. .
d) Optical pump terahertz laser: The terahertz band meets the rotational energy levels of many polar molecules, and the optical pump terahertz laser reverses the number of particles between the rotational levels of these polar molecules, producing terahertz radiation. In the related work at home and abroad, commonly used gases are CH3F, NH3, D2O, CH3OH and so on.
e) Difference frequency terahertz radiation source: The difference frequency terahertz radiation source mainly uses the difference frequency effect of the nonlinear crystal to generate the coherent narrowband terahertz radiation. In this method, two different wavelengths of laser light, that is, different frequencies, are used to pump nonlinear crystals at a certain angle, such as GaSe, ZnGeP2, GaAs, GaP, LiNbO3, and organic crystal DAST. The frequency of the terahertz wave depends on the wavelength of the pump light and can be easily tuned.
f) Optical parametric method: The optical parametric method uses a beam of pump light to enter the crystal to excite the Stokes light and the electromagnetic coupler. Under the combined action of pump light and Stokes light, the electromagnetic coupler undergoes stimulated Raman scattering to achieve terahertz radiation.
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