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.
- quantum cascade laser
- semiconductor THz source
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) . 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). High‐output‐power continuous‐wave (CW) operation of optical semiconductor devices is very attractive for overcoming this problem. Quantum cascade lasers (QCLs)  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)  and THz  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 . In contrast to the room temperature operation of mid‐IR QCLs, the maximum operating temperature (
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 . 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 , it indicates one of the possible directions for combining the high Al composition structure with variable well‐barrier height design  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 characteristics 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 . 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.
2. Indirect injection THz QCLs for low frequency high T operation
An indirect injection scheme was first reported for mid‐IR QCLs among the different kinds of injection schemes . Illustrative band diagrams of typical resonant tunneling injection schemes are shown in Figure 1: (a) for a simplified three‐level system and (b) for a simplified four‐level indirect scattering‐assisted injection scheme with diagonal THz emission, which is used in this work. Improvements in performance have been successfully achieved using fast and smooth LO phonon injection without carrier accumulation. This injection scheme is also expected to circumvent the limitations of the RT injection scheme and to enable the realization of low‐frequency THz QCLs operating at high temperatures. However, the much larger radiative energy in the mid‐IR region causes different subband transport characteristics that need to be considered in designs for THz frequencies. It is difficult to implement an indirect injection design with the correct carrier injection and narrow radiative energy at THz frequencies. Even this design scheme still suffers difficulties in the THz region compared with mid‐IR QCLs. After several theoretical proposals [15–17], a few recent experimental reports [8, 18–20] of indirect injection designs show promising results and demonstrate that there is high potential for attaining low‐frequency, high‐temperature THz QCLs.
Here, we demonstrate an Al0.175Ga0.825As/GaAs QCL design that uses a combination of indirect injection and a less vertical diagonal for emission in the THz region. Structures were grown on semi‐insulating GaAs (100) substrates by solid‐source molecular beam epitaxy (MBE). The growth sequence started with a 250‐nm‐thick Al0.6Ga0.4As etch‐stop layer. The active/injection layers, which are sandwiched between two 100‐nm‐thick
In practice, this kind of simplified four‐level indirect injection scheme depends mainly on the external electrical field. Figure 2 shows the dependence of the energy separation of the subbands on the external electric field for our structure. In the low electric field region around 12–14 kV/cm, the energy separation and band‐diagram conditions are similar to the conventional RT injection scheme, as shown in Figure 3(a): it shows a simplified three‐level‐type design including two lasing levels and one injection level with a subband alignment injection process; the band diagram related to this is shown in Figure 1(a). The injection process occurs from
We focus only on the realization of a radiative process at high electric fields for 1.9 THz lasing. This occurs in the range of 17–19 kV/cm, as shown in Figure 2, which is related to the designed band diagram presented in Figure 3(b). Here, we utilize the LO phonon scattering process to selectively inject carriers from previous injection/extraction levels (
This combination of injection with the emission process also overcomes the critical injection barrier thickness in the RT injection scheme, and achieves a wider dynamic range for the operating current density. In the RT injection design, the dynamic range of the operating current density is related to the emission energy of the QCLs because the resonant condition for lasing corresponds to the subband alignment from
When we replace the injection process by a scattering process, as shown in Figure 1(b), the injection from
Here, we demonstrate 1.9 THz Al0.175Ga0.825As/GaAs QCLs with
3. Modulation barrier‐well active structure design THz QCLs
In order to design the further indirect injection THz QCLs with more design freedom in narrow energy THz frequency, which traditionally usual suffered from opposite thickness optimization. Here, we introduce the active structure design with different heights of barriers and wells. First, we present primary experimental results by arranging the recently recorded three‐well resonant tunneling structure design; modulate the Al the height of barriers with the different emission barrier compositions and introduce an external thin barrier in the widest extraction/injection well. This modulation barrier THz QCL sample succeeds lasing at 3.7 THz with the maximum operation temperature of up to 145K.
Currently, the best performing designs are mainly based on the resonant phonon depopulation scheme for extraction utilizing the AlGaAs/GaAs material systems with one kind of Al barrier concentration 15%. The modulation Al composition barrier THz QCL samples are grown by solid‐state MBE with multiple Al cells. The thickness and Al compositions are precisely controlled within 1% difference. Then the devices are fabricated in the Cu‐Cu metal‐metal waveguide by photolithograph and dry etching. The active region structure design and band diagram are shown in Figure 4.
First, we follow the three‐well resonant tunneling injection design , achieve diagonal emission and increase the spatial separation between upper and lower lasing levels in order to reduce the parasitic injection leakage comes from nonradiative thermally activated longitudinal phonon scattering at high temperature operation. At the widest extraction/injection well, we expected to add an external thin and high layer in order to improve extraction by interface roughness (IR) scattering, which is one solution used in mid‐infrared QCL design . The simulated Al composition of this external barrier with an amount of IR scattering, oscillator strength between the extraction/injection level (
For the further design, we also would like to increase all the barrier compositions, so here we first keep this thin barrier composition the same with others barriers at 15%. The lower emission barrier between two emission wells is modulated in order to reduce the nonradiative IR scattering at the emission layers. The Al composition of the emission barrier with the amount of IR scattering and oscillator strength between two lasing levels (
We utilize the lower emission barrier to reduce the IR scattering at the emission region, add an external thin barrier to improve the injection efficiency and reduce plastic leakage current from
4. Relatively compact‐size potable condenser type THz source by QCLs
After introduce the recent fabricated THz QCLs toward high temperature operation at low frequency (<2 THz) and the 3–4 THz region. For THz applications, the output power is more critical than the operation temperature. Even the recent devices cannot operate at the room temperature. Utilizing some kinds of cryogenic systems, it is still possible to achieve the relatively compact size potable THz application by THz QCLs. Here, we introduce the liquid nitrogen Dewar condenser to combine the fabricated THz QCLs as mentioned above. The size of Dewar is 14 cm2 with 28 cm high without the outside external power supply (Figure 7(a)), the QCL sample array (Figure 7(c)) is fixed under vacuum conditions with cooling holder direct cooled by heat conduction from LN2. The QCL samples are combined with hyperhemispherical Si lens in front of operated mesa and an adjustable inside parabolic mirror in order to focus the output of THz QCLs (Figure 7(d)) and give the near collimated THz wave outside the measurement Tsurupica windows from the widely diverged MMW QCLs.
In our case, without the hyperhemispherical Si lens, the far‐field pattern angle of MMW QCLs is about 40°. If coupled with the Si lens, it is possible to adjust the maximum output near 0°. This is very helpful for the adjusting the THz wave from the small size measurement window and use for real applications. The far‐field pattern is also shown in Figure 7(b), which measured using the NEC THz camera IR/V‐T0831. The adjustment of the position of Si lens is also quite important for the increasing the output power, with and without the Si lens, the measured out power can be increased about 2.5–3 times. The active region design is based on the four well‐resonant tunneling injection LO depopulation scheme with the variable barrier height modulation active structure THz QCLs as discussed above, which originally toward the high temperature operation. The waveguides also use the high temperature operation MMW. The output peak power is 0.1 mW with a duty cycle of 0.02% (1 kHz repeated frequency with 200 ns pulse width). When the duty cycle increases to 5% (10 kHz repeated frequency with 5 µs pulse width), the device gives the 37 µW average power with >1 mW peak power. The changes in duty cycle by changing the repeated input pulse frequency and pulse width are related to the peak output power of THz QCLs when the pulse width is below 1 µs. The too short pulse did not supply the enough input energy for devices. The peak power is become stable when the input pulse is larger than 1 µs. But the average power can be increased continuously with the increasing duty cycle.
Recently, our stable pulse generator (AVTECH AVO‐6HZ‐B) with impedance match can only operate up to 5%. The device can possibly operate at higher duty cycle with larger average output power. But when the duty cycle increases, the heat generated from the device is quite large during the operation, comparing with the measurement condition 0.02%, even with the use of continuous flow liquid helium cryostat. The device heat sink temperature is increased up to 30 K during the operation. For the Dewar case, the duty cycle is larger than 1%. The recent contact electrode by indium ball and 0.07 mm‐thin cupper wire for electric connection is easy to be melted. It causes the difficulty for stable CW operation. The measurement spectrum and the stable operation of current density‐voltage, current density‐light output characteristics direct from the Dewar condenser, are shown in the inset of Figure 8. We can find that the Dewar gives the clear single color 3.8 THz lasing spectrum, stable milliwatt order peak power and microwatts order average power. The maximum peak power is 3.1 mW and maximum average power is 6.2 µW under recent Dewar setting conditions.
For the CW operation with milliwatt average output power, the improvement for the recent Dewar system mainly obtained by two directions: first is the THz QCL device, recently the QCLs used in this condenser are toward the high temperature operation MMW QCLs with narrow ridge width and small mesa size. The output performance of THz QCLs generally decreases when the operation temperature is above 100 K. Upon cooling with the LN2, the performance do not dramatically change to a large extent under low temperature conditions. The few tens K temperature operation is not important for this Dewar setting. The device fabrication move to the larger mesa size SI‐SP waveguide can improve the output performance with the better far‐field pattern. It is suitable for the high output 77 K Dewar condenser with the easier optical aliment in applications. Second direction is the improvement and consideration of cooling efficiency of the Dewar condenser. The recent operation duty cycle is limited by the wire connection and the electrode of the mounted sample holder not by the device itself. If we can reduce the temporary heat increase with large duty cycle by wide size heat conduction and good thermal conductivity, it is possible to improve the operation duty cycle of the Dewar condenser.
On the basis of the first direction, we introduce the recent fabricated modulation active structure THz QCL cooling and setting with the active structure design by more vertical emission and large current injection. This way we achieve the large output THz QCLs with a peak power of 250 mW and an average power of 2.2 mW under the condition of duty cycle 1%. L‐I‐V characteristics, lasing spectrum, and far‐field pattern measured using THz camera are shown in Figure 9. The LN2 Dewar condense demonstrated as a robust compact size THz source unit with stable milliwatt order peak power and microwatts order average power operation by THz QCLs. And the large output power devices without high temperature operation (>100 K) also show the potential for the more powerful THz source unit toward the target of CW operation with milliwatt average output power. For further improvement of Dewar condenser, we replace the recent fabrication by SI‐SP waveguide with large mesa size and high output power active region design, increase the current duty cycle limitation comes from the power supply, cooling efficiency of connect wire and electrode.
Here, we discuss the design and fabrication of THz QCLs toward the high temperature and large average output power operations for the real THz applications with the relatively compact portable size cryogenic cooling systems. We also describe the temperature performance parameters of THz QCLs, introduce the results of an indirect injection design scheme in the THz region and modulation height active structure design with different barriers and wells for further design direction. The recent fabricated THz QCLs are combined with the liquid nitrogen cooling Dewar condenser to demonstrate the relatively compact potable THz source unit by QCLs. The different injection schemes in THz and barrier‐well height design in the active region introduce one of the directions for the further high temperature and large output power operation of THz QCLs for real applications.