Terahertz (THz) radiation is like a treasure chest, which has not been fully opened by humans. Terahertz radiation is an electromagnetic spectrum located between the infrared and microwave regions. It has a series of ideal characteristics, and its application is worth looking forward to. Terahertz radiation opens a "window" for obtaining unique spectral information of molecules and solids. It can penetrate non-conductive materials such as textiles and biological tissues without ionization, so it will not cause damage to the object being studied. This advantage opens up attractive prospects for applications such as non-invasive imaging and non-destructive quality control. However, despite the various ideas about the potential uses of terahertz radiation, these ideas have become difficult to implement due to the lack of practical technologies for generating and detecting terahertz radiation.
Schematic diagram of a single laser ridge bar in a thermoelectrically cooled terahertz quantum cascade laser
It is exciting that Lorenzo Bosco, Martin Franckie and colleagues of the Jerome Faist group of the ETH Institute of Quantum Electronics in Zurich have realized a terahertz quantum cascade laser that can operate at 210K (-63 ° C), reaching The highest operating temperature of this device to date. More importantly, this operation is the first demonstration in a temperature range that does not require cryogenic coolant. Bosco et al. Adopted thermoelectric cooling, which is more compact, cheaper, and easier to maintain than refrigeration equipment. This progress removes major obstacles on the way to various practical applications.
Quantum cascade lasers for practical applications
For a long time, quantum cascade lasers (QCL) have been considered as a natural concept for terahertz devices. Like many lasers widely used in the visible to infrared range, quantum cascade lasers are also based on semiconductor materials. However, compared to typical semiconductor lasers used in bar code readers or laser pointers, quantum cascade lasers work completely differently from the principle of light emission. In short, quantum cascade lasers are built from repeated stacks of precisely designed semiconductor structures (see Figure c), which are designed to achieve appropriate electronic transitions (see Figure d).
Figure a: The thermoelectric cooling laser box of the laser installed on the top of the Peltier element (white square) allows operation between 195K and 210.5K. The laser is emitted vertically through the window on the top cover.
Figure b: The laser chip mounted in the laser box is in contact with the fine gold wire connected to the top of multiple laser ridge strips.
Figure c: Schematic diagram of a single laser ridge strip. Horizontal lines show the quantum well structure formed by layered semiconductors. Ridged strips (150 microns wide) are sandwiched between thin copper layers to form a "sandwich" structure.
Figure d: Pressurized at the edge of the conduction band (white line), the electron density is analyzed by the energy displayed in different colors. The bias-driven electron passes a non-radiative transition indicated by the dotted arrow. This will pump the energy states in the thin wells more densely than the wider well energy states indicated by the green arrows, allowing for net stimulated emission of terahertz photons.
The concept of a quantum cascade laser was proposed in 1971, but it was not proven for the first time by Faist and colleagues in 1994, and was subsequently successfully developed by Bell Labs in the United States. This method provides value for many basic experiments and application experiments, especially in the infrared band. Since 2001, quantum cascade lasers for terahertz emission have also made substantial progress. But the need for cryogenic coolants (usually liquid helium) greatly increases complexity and cost, and makes the equipment larger and more difficult to move, preventing it from being widely used. Seven years ago, the operating temperature reached about 200K (-73 ° C), and progress in the pursuit of higher temperature operation of terahertz quantum cascade lasers has stagnated there.
Breaking down barriers that rely on cryogenic refrigeration technology
Reaching 200K is already an impressive feat. This temperature is just below the limit where cryogenic technology can be replaced by thermoelectric cooling. Since 2012, the temperature record has not changed, which also means that some kind of "psychological obstacle" has begun to be established, and many scenes have begun to accept the reality that terahertz quantum cascade lasers must work with cryocoolers.
Today, the ETH team has broken this barrier. In the journal Applied Physics Letters, they proposed a thermoelectrically cooled terahertz quantum cascade laser with an operating temperature increased to 210K. In addition, the emitted laser light is strong enough to be measured with a room temperature detector. This means that the entire device can work normally without low-temperature cooling, further enhancing the potential of the method in practical applications.
Bosco, Franckie, and colleagues eliminated the "cooling barrier" on two fronts. First, they used the simplest single structure in the design of a quantum cascade laser stack, with two quantum wells per cycle (Figure d). This approach is considered a way to achieve higher operating temperatures, but at the same time this dual-well design is also very sensitive to minimal changes in semiconductor geometry. The optimization of one parameter will lead to the deterioration of another parameter. Since the experimental optimization of the system is not a viable option, they have to rely on numerical simulation.
The team's second substantial progress has been confirmed in recent research, and they can accurately model complex experimental quantum cascade lasers using a method called an unbalanced Green's function model. The calculation must be performed on a powerful computer cluster, which is efficient enough to systematically search for the optimal design. The team has the ability to accurately predict device performance and manufacture devices to exact specifications, enabling a series of lasers that operate continuously over the temperature range achievable with thermoelectric cooling (see figures a and b). Moreover, the existing method is by no means an end, and the Faist team has the idea to further increase the operating temperature, and the preliminary results do look promising.
Filling the terahertz gap
The first demonstration of a terahertz quantum cascade laser capable of operating without cryogenic cooling, filling the "terahertz gap". Terahertz gaps have long existed between proven microwave and infrared radiation technologies. Without any moving parts or circulating liquids, thermoelectrically cooled terahertz quantum cascade lasers introduced by ETH physicists are more likely to break through the limitations of professional laboratories for application and maintenance, thereby further opening up the "Terahertz treasure chest" of " cover".