**Applications**

## **THz QCLs Design Toward Real Applications THz QCLs Design Toward Real Applications**

## Tsung‐Tse Lin Tsung‐Tse Lin

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65351

## **Abstract**

For highly desired THz applications, we discuss the design and fabrication of THz quantum cascade lasers (QCLs) toward the high temperature and large average output power operations for the real applications with the relatively compact portable size cryogenic cooling systems. We describe the temperature performance parameters of THz QCLs and introduce the recent results of an indirect injection design scheme in the THz region and modulation height active structure design with different barriers and wells for the further design direction. The recent fabricated THz QCLs are combined with the liquid nitrogen (LN) cooling Dewar condenser to demonstrate the relatively compact THz source unit by QCLs. The different injection schemes in THz and barriers‐ wells height design in the active region introduce one of the directions for the further high temperature and large output power operation of THz QCLs. The relatively compact size THz source unit with a cryogenic system demonstrates the THz QCLs for real applications with the milliwatt order average output operation near liquid nitrogen temperature.

**Keywords:** Terahertz, quantum cascade laser, semiconductor THz source

## **1. Introduction**

The terahertz (THz) region in the electromagnetic spectrum has drawn much attention due to its wide range of applications in various fields such as spectroscopy, imaging, remote sensing, and communications. Compact THz semiconductor sources are also extremely promising for use in future high‐speed and large‐capacity local telecommunications applications, especially for those applications operating in the range from sub‐THz to a few THz (0.2–2 THz) [1]. The output power of conventional mature radio frequency (RF) electronic devices reduces by 4 orders of magnitude with frequency, which is close to 1 THz in the order of a few microwatts (µW).

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

High‐output‐power continuous‐wave (CW) operation of optical semiconductor devices is very attractive for overcoming this problem. Quantum cascade lasers (QCLs) [2] are compact semiconductor light sources that utilize carrier recycling and intersubband transitions in repeating quantum well (QW) structures, and they have been demonstrated to operate successfully in the mid‐infrared (mid‐IR) [2] and THz [3] regions. They are also arguably the only THz solid‐state sources with average optical output power levels much greater than one milliwatt (mW). This property of high optical output power with a narrow emission line width is quite attractive for a wide range of THz applications.

The current status of THz QCLs that operate without an external magnetic field has been reported in the spectral range from 1.2 to 5.2 THz [4, 5] with a maximum output power of around 1.01 W in pulsed mode [6]. In contrast to the room temperature operation of mid‐IR QCLs, the maximum operating temperature (*T*max) of THz QCLs is 199.5 K [7] at 3.2 THz. This performance was realized by an optimized state‐of‐the‐art Al0.15Ga0.85As/GaAs structure utilizing longitudinal optical (LO) phonon depopulation for the extraction scheme with resonant tunneling (RT) injection and diagonal emission. However, even this temperature performance still limits their practical use in some applications. Higher operating temperatures for THz QCLs with wider frequency that can at least function at temperatures attainable using Peltier coolers (230–250 K) have been one of the most important topics in recent research. The commonly reported high‐temperature performance of THz QCLs roughly follows an empirical limitation depending on their operating frequency, such that *T*max ∼ *h*ω/*k*B. Compared with the near 200 K operation of THz QCLs at 3–4 THz, at low frequencies (<2 THz), THz QCLs are expected to exhibit poorer temperature performance due to this limitation when the frequency decreases. THz QCLs that operate at low frequencies suffer from design difficulties due to the narrow radiative energy separation between the two lasing‐subband levels, which is related to the narrow dynamic range of the current density of the laser. Large thermal perturbation effects within the narrow energy level spaces have prevented the expansion of THz QCLs to low‐frequency high‐temperature operation. This reduction in the dynamic range of the current density with operating frequency can be qualitatively explained by the dependence of RT injection scheme THz QCLs on *kBT*max ∼ *h*ω [8]. It is known to be difficult to achieve both low‐ frequency lasing and high‐temperature operation.

On the basis of some successful solutions in mid‐infrared QCLs, the different barrier height shows some solution to improve the device performance for different directions. For example, utilizing the height barriers to reduce the high temperature parasitic leakage currents [9, 10] and step wells for improving internal quantum efficiencies [11]. Furthermore, with the high technique of crystal growth, the optimization of the individual barrier and well's height at the suitable place is possible to give the solution and one more design freedom of recent stagnated structure design of THz QCLs. Considering these research and our previous studies on high Al composition AlxGa1\_xAs/GaAs design [9], it indicates one of the possible directions for combining the high Al composition structure with variable well‐barrier height design [12] in order to achieve the thermoelectric cooling and higher temperature operation of THz QCLs. For this kind of modulation well‐barrier height modulation height active structure. It is also expected to utilize the further indirect injection design.

The recent two methods are introduced for improving the performance of THz QCLs. We demonstrate a relative compact‐size semiconductor THz source unit by the recent fabrication of THz QCL devices for real applications. The operation temperature largely limited the real compact size potable THz applications by semiconductor‐based QCLs. A compromise solution is to capitalize on the liquid nitrogen (LN2) cooling Dewar condenser, keep the useful charac‐ teristics of THz QCLs and reduce the cooling system size, and realize the robust portable compact size THz source unit by QCLs. The output power is one of the most important characteristics required for the different real THz applications. The recently large peak output power THz QCLs are realized by a large size mesa with a semi‐insulated surface plasmon (SI‐ SP) waveguide and achieved the 1 W peak power. But this still limited on the pulse operation, the average output of THz QCLs still recorded by the previous report [13]. Here, we introduce a Dewar condenser cooling system with our recent fabricated metal‐metal waveguide (MMW) modulation height active structure THz QCLs and the premier measurement results of few microwatts average power with milliwatt order peak power, larger output power THz QCLs with the peak output power of 250 mW and the average output power of 2.2 mW, indicating the further improvement in the direction of the continuous‐wave (CW) mW order average power operation. In the following paragraphs, we introduce the details of indirect injection low frequency THz QCLs, modulation height active structure design, and a Dewar condenser type THz source unit by THz QCLs.

High‐output‐power continuous‐wave (CW) operation of optical semiconductor devices is very attractive for overcoming this problem. Quantum cascade lasers (QCLs) [2] are compact semiconductor light sources that utilize carrier recycling and intersubband transitions in repeating quantum well (QW) structures, and they have been demonstrated to operate successfully in the mid‐infrared (mid‐IR) [2] and THz [3] regions. They are also arguably the only THz solid‐state sources with average optical output power levels much greater than one milliwatt (mW). This property of high optical output power with a narrow emission line width

The current status of THz QCLs that operate without an external magnetic field has been reported in the spectral range from 1.2 to 5.2 THz [4, 5] with a maximum output power of around 1.01 W in pulsed mode [6]. In contrast to the room temperature operation of mid‐IR QCLs, the maximum operating temperature (*T*max) of THz QCLs is 199.5 K [7] at 3.2 THz. This performance was realized by an optimized state‐of‐the‐art Al0.15Ga0.85As/GaAs structure utilizing longitudinal optical (LO) phonon depopulation for the extraction scheme with resonant tunneling (RT) injection and diagonal emission. However, even this temperature performance still limits their practical use in some applications. Higher operating temperatures for THz QCLs with wider frequency that can at least function at temperatures attainable using Peltier coolers (230–250 K) have been one of the most important topics in recent research. The commonly reported high‐temperature performance of THz QCLs roughly follows an empirical limitation depending on their operating frequency, such that *T*max ∼ *h*ω/*k*B. Compared with the near 200 K operation of THz QCLs at 3–4 THz, at low frequencies (<2 THz), THz QCLs are expected to exhibit poorer temperature performance due to this limitation when the frequency decreases. THz QCLs that operate at low frequencies suffer from design difficulties due to the narrow radiative energy separation between the two lasing‐subband levels, which is related to the narrow dynamic range of the current density of the laser. Large thermal perturbation effects within the narrow energy level spaces have prevented the expansion of THz QCLs to low‐frequency high‐temperature operation. This reduction in the dynamic range of the current density with operating frequency can be qualitatively explained by the dependence of RT injection scheme THz QCLs on *kBT*max ∼ *h*ω [8]. It is known to be difficult to achieve both low‐

On the basis of some successful solutions in mid‐infrared QCLs, the different barrier height shows some solution to improve the device performance for different directions. For example, utilizing the height barriers to reduce the high temperature parasitic leakage currents [9, 10] and step wells for improving internal quantum efficiencies [11]. Furthermore, with the high technique of crystal growth, the optimization of the individual barrier and well's height at the suitable place is possible to give the solution and one more design freedom of recent stagnated structure design of THz QCLs. Considering these research and our previous studies on high Al composition AlxGa1\_xAs/GaAs design [9], it indicates one of the possible directions for combining the high Al composition structure with variable well‐barrier height design [12] in order to achieve the thermoelectric cooling and higher temperature operation of THz QCLs. For this kind of modulation well‐barrier height modulation height active structure. It is also

is quite attractive for a wide range of THz applications.

72 Quantum Cascade Lasers

frequency lasing and high‐temperature operation.

expected to utilize the further indirect injection design.
