Miniature, but powerful mid-infrared (IR) quantum cascade lasers (QCLs) have been developed and commercialized recently. Their unique properties make it possible to advance the analytical problem-solving capabilities of IR spectroscopy in many ways. This includes highly sensitive measurements as well as novel applications. Bernhard Lendl of the Institute of Chemical Technologies and Analytics at Vienna University of Technology, Austria, has been researching and developing QCLs for many applications. Spectroscopy recently spoke to Lendl, and to Markus Brandstetter, a member of Lendl's research group, about how QCLs work, the current state of the art, and practical applications of the technology.
Could you summarize the principles of how quantum cascade lasers (QCLs) work, and the state to which they have evolved since their invention in 1994?
Like all lasers, QCLs have to meet three conditions to obtain a stimulated emission and thus light amplification — or in other words, to obtain a gain from a laser:
Population inversion: a state of the gain medium of the laser, where higher energetic levels are more populated than the lower ones.
Feedback: the light emission in the gain medium under the first condition is stimulated by photons present in it. Therefore, there must be some portion of emitted photons reinjected into the gain medium.
Energy supply: a means to maintain the population inversion. In the case of QCLs, like the majority of semiconductor lasers, this is an electrical current.
The main difference between standard semiconductor lasers and QCLs lies in the way population inversion is achieved. Standard semiconductor lasers are designed to have population inversion between the conduction and the valence band. Hence, electrons can undergo transitions between these energetically different levels, causing photon emission (interband transition). Here, the photon energy, and thus the wavelength of the emitted light, is determined by the size of the band gap. Because the band gap itself is more or less determined by the two materials used as gain medium, only a certain emission range is feasible. Unfortunately, the available materials exclude the major part of the mid-infrared (IR) spectral region. In contrast to this conventional approach using bulk semiconductor materials, QCLs are designed to obtain population inversion within the conduction band where transitions can occur (inter-subband transition). This is achieved by using a periodic series of very thin (nanometer-level) layers of different semiconductor materials. By applying an electrical voltage across this structure, a cascade of quantum wells of different size and depth develops. Radiative electron transitions between these wells or subbands can occur, inducing laser emission. The nice thing about this concept is that the energy difference between the subbands can be adjusted by changing the thickness of the thin layers. Hence, layer thicknesses are designed to achieve band-gap sizes that correspond to mid-IR radiation. This is also known as band-gap engineering. Using different designs, narrow- or broad-band media can be designed and produced, mainly using a technique called molecular beam epitaxy.
Figure 1: A cascade of quantum wells within the conduction band is the basis for mid-IR laser emission of quantum cascade lasers.
A scheme of a QCL is shown in Figure 1. The gain medium of the laser is realized as a cascade of active cell and injector pairs. The electrons are injected (by means of the electrical current) into the energetic level E3. The system is inverted: The higher level (E3) is more populated than the lower level (E2). In a favorable case, the electrons undergo a radiative transition from E3 to E2. The wavelength of the photon is determined by the energetic spacing between these two levels. The subsequent transition (from E2 to E1) is tailored to be in resonance and thus is very fast and efficient. It serves for extraction of the electrons from level E2 and thus creates conditions for population inversion between levels E3 and E2. After that, the electrons are evacuated by the injector of the following period. By passing a series of such injection–extraction patterns a single electron can cause multiphoton emission.
QCLs are available in Fabry–Pérot (FP), distributed feedback (DFB), and external-cavity (EC) configurations. FP–type QCLs basically consist of the bare QCL chip and show multimode emission at all wavenumbers that meet the Fabry–Pérot condition within the spectral range supported by the QCL's gain curve. DFB QCLs typically have a Bragg grating on top of the QCL chip supporting only a specific emission wavelength. In that way, single-mode emission is achieved and spectral tuning is limited to a few wavenumbers only, which can be realized by changing the operating temperature of the laser chip. Broadband tuning can be achieved by creating an external cavity, which requires placement of a diffraction grating on one end of the laser output (Figure 2). By changing the grating angle, the resonance conditions change and different modes are amplified, leading to spectral tuning of the laser.
Figure 2: Scheme of a possible EC configuration (Littrow) used for broadband tuning of QCLs.
In theory, every emission wavenumber can be fabricated by proper design of the quantum well structure. Today's situation is that commercially available EC QCLs cover the 4–12 µm range. In practice, however, some wavenumbers are easier to fabricate than others. It can therefore occur that QCLs for specific applications, such as for certain gas absorption lines, occasionally are not commercially available. In addition to room-temperature-operated QCLs in the mid-IR range, liquid-nitrogen-cooled QCLs are available for the terahertz spectral region.