The terahertz (THz) spectral range has considerable potential for application in numerous fields, including communications, non-destructive imaging, spectroscopy, and biological engineering. One major obstacle preventing widespread commercial deployment of this technology is the lack of a compact and mass-producible high-performance semiconductor source operating in the THz range. As shown in figure 1, on the low-frequency side (from the electron device), Resonant tunnel diode (RTD) oscillators have operated at room temperature. On the other hand, on the high frequency side (from the optical device), THz quantum cascade lasers (THz-QCL) have been reported; however, they require a cryogenic-cooling.
Figure 1 Room-temperature semiconductor THz sources (Output power vs Frequency)
Quantum cascade lasers (QCLs) are semiconductor light emitting device (Figure 2(a)). This light sources are one of the best known devices created by engineering of electron wavefunctions in semiconductor heterostructures grown by molecular beam epitaxy or metal organic vapor phase epitaxy crystal growth techniques. Unlike devices based on band-to-band transitions, the electrical and optical properties of QCLs, such as optical transition energy and dipole moment, upper and lower laser states lifetimes, and electron transport are determined by their heterostructure design. The working principle is shown in Figure 2(b). Over the past two decades, QCLs have become the most attractive semiconductor sources in the mid-infrared and terahertz regions. THz QCLs nevertheless still require cryogenic cooling for operation.
Figure 2 (a)Quantum cascade laser device
(b)Working principle of QCL
An alternative approach to THz generation from QCLs is based on intracavity difference-frequency generation (DFG) in a dual-wavelength mid-IR QCL. These devices, known as THz DFG-QCLs, use a mid-IR QCL active region engineered to exhibit a giant intersubband nonlinear susceptibility χ(2) for an efficient THz DFG process (Figure 3). Upon application of bias current, THz DFG-QCLs produce two mid-IR pump frequencies as well as the THz frequency, that corresponds to the difference of the mid-IR pump frequencies, via the intra-cavity nonlinear mixing process in the device active region. The optical nonlinearity of the DFG-QCL active region does not require population inversion across the THz transition. As a result, THz DFG-QCLs can operate at room temperature, unlike THz QCLs. THz DFG-QCLs are currently the only electrically-pumped, monolithic, mass-producible semiconductor sources operable at room temperature.
Figure 3 Schematics of working principle of THz QCL source based on nonlinear optical effect
In our group, by using our original concept “Anticross DAU active region (Figure 4(a)),” we achieved mW-level output power at room temperature . THz output power can be detected with room-temperature THz thermo-electric detector. Also, ultra-broadband THz emission spectrum spanning over one octave (Figure 4(b)) have been obtained based on the DAU active region, and as a result, we have successfully demonstrated THz imaging with these THz sources.
Figure ４ (a) AnticrossDAU structure
(b) Typical characteristics of room-temperature broadband THz nonlinear QCL device
Figure 5 Spectroscopic imaging using broadband THz-QCL source (Polyethylene, D-histidine, DL-histidine, reprint from Reference 1)
The THz frequency range is very important for many applications, such as imaging, chem/bio sensing, heterodyne detection, and spectroscopy. In the field of ICT, ultra-broadband wireless communications are expected for short distance applications, while long-distance transmission is difficult due to strong water absorption.
In the field of nondestructive testing and analysis, THz frequency-range radiation has been used to demonstrate imaging of objects that are opaque at optical frequencies. There are many THz applications, including screening of chemical substances, nondestructive imaging for industrial material and historical arts object. By building a compact imaging system using THz nonlinear QCLs, much research on THz imaging technology will be performed in the future.
In the field of astronomy, THz frequency signal is very important since interstellar gas and dust can be detected in this frequency range. With the fine structure line of interstellar gases, the star formation process can be investigated. For these purposes, an astronomical receiver requires a compact local oscillator. Since THz nonlinear QCL sources can generate single frequency THz signal, it will be used as a LO to analyze a THz wave signal coming from space.
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