Lasers can emit laser light, and the global laser market is mainly distributed in the United States, Europe, and China. According to Qianzhan's forecast, the global laser sales revenue is expected to reach 20.63 billion US dollars by 2024. In recent years, China's laser industry has shown rapid growth. Among them, laser components, including optical components for lasers, nonlinear crystals, and laser power supplies, had a market size of 28.8 billion yuan in 2018, a year-on-year increase of 22%. Among them, quantum cascade lasers have superior characteristics, and this article will specifically talk about things related to quantum cascade lasers. 1. The First Proposal of Quantum Cascade Lasers Benefiting from the development of semiconductor materials and the emergence of quantum well lasers, Soviet scientists Kazarinov R. and Suris R. put forward the concept of Quantum Cascade Laser (QCL) in 1971. As shown in Figure 1 is a schematic diagram of the embryonic idea of quantum cascade lasers. First, it is necessary to construct a periodic quantum well, which is a superlattice structure. In the figure, U is the potential barrier, e is the charge of a single electron, α is the length of the superlattice periodic structure, and F is the external static electric field strength. Then eαF represents the single electron energy, and its value must first satisfy the tunneling condition between the quantum wells. According to the uncertainty relation, ℏ/2 ≤ ΔEΔt, so ℏ/t ≤ eαF, where t is the time required for free electron transition. However, eαF cannot be too large. If it exceeds the width I0 of the lower-level quantum well, the electrons cannot be bound to the energy level of the upper-level quantum well. Therefore, ℏ/t ≤ eαF ≤ I0. Then, under a certain bias voltage, electrons transition from the ground state between the subbands of the quantum well to the excited state of the next quantum well, releasing photons, and then transition to the ground state of the same quantum well through non-radiative relaxation, repeating such transitions to achieve cascaded amplification of light. Non-radiative relaxation transitions rely on electron scattering, converting energy into lattice vibrations or transversely moving phonons, and the released phonons can assist electrons in tunneling from the ground state to the excited state of the next quantum well. Although the idea of quantum cascade lasers was put forward earlier, due to the design defects of its own structure and the limitations of material growth technology, this idea did not receive much attention at that time. 2. The First Realization of Quantum Cascade Lasers It was not until breakthroughs were made in material growth technology (molecular beam epitaxy technology) by Zhu Yihe and others, and the development of structural design theory by Capasso F. and others, that the first quantum cascade laser was born in Bell Labs in 1994. It was developed by Faist J. and Capasso F. using the InAlAs/InGaAs/InP material system, so the design of its active region is a three-well coupled diagonal transition structure, as shown in Figure 2. Different from the method in quantum well lasers where electrons and holes recombine to generate photons, quantum cascade lasers are unipolar devices with only electrons involved. Therefore, it was difficult to truly realize quantum cascade lasers in the initially envisaged single quantum well. Similar to lasers, population inversion is required to generate stimulated radiation. The three-well coupled diagonal transition structure successfully designed a three-level structure through different materials and quantum well widths. Among them, energy level 3 is the excited state. Since energy levels 3 and 2 are located in different quantum wells, the overlap of wave functions is small, which increases the non-radiative lifetime of electrons at energy level 3. The strong coupling between energy levels 2 and 1 reduces the lifetime of electrons at energy level 2, making it easy to achieve population inversion. The longer injection/relaxation region can well help electrons tunnel to the next cycle, and also restrict the escape of electrons in high excited states. This 25-level quantum cascade laser achieved an output of 8.5mW at a temperature of 10K, with an emission wavelength around 4.2mm, a maximum operating temperature of 90K, and a threshold current density of 14kA/cm². Although the quantum cascade laser at this time still had shortcomings, it laid a foundation, and in the subsequent more than 20 years, quantum cascade lasers have achieved rapid development. 3. NASA Develops New Quantum Cascade Laser: Capable of Finding Water on the Moon This year, an engineering team from NASA's Goddard Space Flight Center developed a small but powerful laser that will help astronauts find water on the moon in the future. The laser is smaller than a penny and uses quantum mechanical effects to generate beams in the terahertz (THz) range, which is used to find hidden water. Over the past more than a decade, through missions such as Chandrayaan-1, we have confirmed the existence of water on the moon. The orbiter uses a spectrometer to image the lunar surface, which measures the reflection and absorption of light of different wavelengths, thereby revealing the composition of existing substances, including water molecules. Although these instruments are very practical, their sensitivity cannot distinguish between water and similar forms (such as free hydrogen ions and hydroxyl groups). A more precise instrument called heterodyne spectrometers focuses on a more compact frequency range by combining incident light with a laser in the device and then measuring the difference between the two light sources. Goddard engineers designed a device that can be tuned to the terahertz frequency where water is located. Existing oscillators and lasers that generate terahertz waves are bulky, heavy, and energy-consuming, but they managed to shrink the design to the size of a coin. To this end, the team used some strange quantum tricks. The device designed by the team is called a quantum cascade laser, which consists of a series of ultra-thin semiconductor material layers. The emitted photons enter this barrier - because these layers are too thin, photons are more likely to ignore the barrier and appear on the other side, a phenomenon called quantum tunneling. When a photon reaches the other side, it excites other photons, so when they pass through 80 to 100 stacked layers in the device, the final result is a cascade of photons with terahertz energy. Waveguides and thin optical antennas keep the beam focused for a longer time. The team said that even with hardware such as a power supply, processor, and spectrometer, the entire system can fit into a teapot-sized device. This means that future astronauts may use a handheld version to find water on the moon, Mars, or other celestial bodies. 4. Subsequent Development and Advantages of Quantum Cascade Lasers Lasers made of semiconductor materials have the most obvious advantages of small size and high integration, as shown in Figure 3 for their physical diagram. And because quantum cascade lasers are unipolar devices, the direction of the electric field of the emitted electromagnetic wave is consistent with the direction of the applied electric field, that is, the emitted light has good linear polarization characteristics. Secondly, a large range of wavelength coverage can be achieved. Unlike conventional semiconductor lasers, whose emission wavelength is limited by the band gap width of the material itself and is difficult to reach the far-infrared and even terahertz bands, the emission wavelength of QCL is determined by the energy level spacing between subbands in the conduction band. The energy level spacing between subbands can be changed by adjusting the materials used and the width of the quantum well, thereby changing the emission wavelength. At present, the shortest mid-infrared band that can be covered is 2.6mm, and the longest is 24mm. In the THz band, it can cover (60-300mm), but it can only work at temperatures lower than 200K. Another essential advantage is operation at room temperature. Unlike conventional semiconductor lasers where the distribution of electrons and holes is very sensitive to temperature, the curvature of subband wave functions in the active region of quantum cascade lasers is nearly the same, making it less prone to the Auger effect. In 1997, Faist J. and others realized pulsed operation at room temperature through distributed feedback quantum cascade lasers (DFB-QCL), which was a major breakthrough in quantum cascade lasers. To realize continuous operation of quantum cascade lasers, not only the power supply needs to be designed for continuous power supply, but also temperature is another major issue. Faist J. and others were also the first to realize continuous operation, achieving continuous output energies of 15mW and 2mW at temperatures of 50K and 85K respectively in 1996, with an output wavelength of 4.6mm. However, the need for bulky cooling equipment is extremely unfavorable for the application of miniaturized quantum cascade lasers, and the output power is not high enough. A major breakthrough in realizing continuous operation at room temperature was made by Beck M. and others in 2002, who achieved room-temperature continuous operation with a wavelength of 9.1mm. The device had an output power of 17mW at 292K, and the maximum continuous operating temperature was 321K. Since temperature itself can affect the thickness of the active region of the quantum cascade laser, thereby affecting its output characteristics and shifting the emission of its central wavelength, the output wavelength can be tuned through temperature within an appropriate temperature range, and also through controlling the operating current. Due to the characteristics of quantum cascade lasers, one electron can excite the same number of electrons as the number of stages. Therefore, compared with conventional semiconductor lasers, their internal quantum efficiency is higher and the output power is greater. At present, Northwest University in China is in a leading position in the world in terms of high-power operation of quantum cascade lasers. In 2009, Bai Y. and others produced an FP cavity quantum cascade laser with a ridge width of 400mm and a wavelength of 4.45mm, which had a peak power of 120W in pulsed operation and a threshold current of 20A. In 2011, Lu Quanyong and others prepared a quantum cascade laser operating in a single longitudinal mode around 4.8mm by means of surface gratings, with a maximum output average power of 2.4W. Due to the superior characteristics of quantum cascade lasers, quantum cascade lasers in the infrared band can be widely used in important fields such as atmospheric detection and infrared countermeasures, while quantum cascade lasers in the THz band can be applied in important fields such as biological detection. Up to now, they are still in rapid development. Article sources: Nonlinear Optics Course Group, Enterprise Magnifying Glass Official, cnBeta