Quantum cascade lasers (QCLs) are an emerging laser technology unlocking advanced spectroscopy, healthcare, and communications applications. These ultra-compact semiconductor lasers are based on unique quantum effects and can be engineered to emit specialized wavelengths from the mid-infrared to the terahertz range. This article offers insights into QCLs, their working principle, applications, and recent advancements in the field.
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What are Quantum Cascade Lasers?
Quantum cascade lasers (QCLs) are a special type of semiconductor laser that emits single charge carrier (electrons) photons in the mid-to-far infrared region of the electromagnetic spectrum. Unlike traditional semiconductor lasers, QCLs operate on intersubband transitions within quantum wells, enabling them to achieve efficient and precise unipolar light emission.
A key advantage of QCLs is their tunability across various wavelengths in the mid-infrared spectrum, from 5.5 to 11.0 µm. This tunability is achieved by controlling the structure of the layers rather than relying solely on the lasing material, providing greater flexibility in tailoring their emission wavelengths.
In addition, QCLs offer rapid response times and exceptional spectral brightness, outperforming other light sources such as synchrotrons.
The origins of QCLs trace back to the early 1970s when Esaki and Tsu designed the first one-dimensional periodic potential multilayer using epitaxial growth, resulting in a superlattice structure. This marked a crucial step in the development of QCLs.
Subsequently, Kazarinov and Suris proposed the idea of quantum cascade lasers, and it wasn't until 1994 that Faist and colleagues at Bell Laboratories successfully demonstrated the first working QCL device.
Working Principle of Quantum Cascade Lasers
While conventional semiconductor lasers rely on electron-hole recombination across bandgaps for stimulated emission, QCLs employ quantum energy wells to confine electrons into specific energy states.
This is achieved by creating a periodic structure of thin alternating semiconductor layers forming quantum wells. The quantum confinement in these nanoscale wells causes discrete quantized energy subbands to form in the conduction band.
Applying voltage propels electrons across the device, transitioning them from one quantum well to the next within the "active region," where they descend to lower energy levels and release photons. As electrons proceed through subsequent active regions, they repeatedly transition and emit photons.
A single QCL may consist of as many as 75 active regions, enabling each electron to generate multiple photons as it traverses the structure. This cascading effect amplifies photon emission and allows QCLs to emit high-power light at specific wavelengths.
What Are Quantum Cascade Lasers Used For?
One major application lies in medical diagnostics, a field transformed by the spectroscopic abilities of QCLs.
When exposed to mid-infrared light emitted by a QCL, biological tissues exhibit unique absorption patterns, creating a distinctive "fingerprint" spectrum. Analyzing this fingerprint allows the identification of proteins, sugars, and biomarkers related to diseases such as cancer.
QCLs can detect these molecules at concentrations up to 1000 times lower than conventional infrared sources while offering high selectivity due to their narrowband emission.
Real-Time Environmental and Industrial Gas Sensing
Similar to medical applications, QCL-based sensors are ideal for detecting greenhouse gases and airborne pollutants selectively.
Portable QCL devices are being deployed for real-time air quality monitoring, revolutionizing environmental regulation. These robust, compact sensors can also continually monitor industrial facilities, exhaust stacks, and pipelines for process control, safety, and regulatory compliance without gas sampling.
Terahertz QCLs, operating at longer wavelengths, are now being commercialized for intricate gas analysis, providing heightened measurement accuracy in the 100 to 150 µm range.
QCLs' narrow laser light bandwidth in automobiles enables direct real-time measurement of critical nitrogen-containing exhaust gas components (NO, N2O, NO2, and NH3) with high precision and sensitivity.
Ultra-Fast Optical Data Transmissions
Quantum cascade lasers offer groundbreaking capabilities in optical communications. Compact QCL transceivers could enable wireless links to surpass 100 Gbps over short distances, while in long-haul fiber networks, QCLs efficiently amplify signals at specific wavelengths.
As a result, QCL components will become essential for meeting the increasing bandwidth demands of 5G networks, video streaming, cloud computing, and other high-speed applications.
Recent Research and Development
Opto-Acoustic Modulation of Quantum Cascade Lasers Could Enable 100 Gbps Data Transmission
In a study published in Nature Communications, researchers successfully controlled terahertz quantum cascade lasers, potentially enabling data transmission rates of 100 gigabits per second.
They combined acoustic and light waves to modulate the quantum cascade laser output. This approach demonstrated the potential to control light output by a few percent, marking a significant step forward in developing fully controlled photon emissions from these lasers.
"We believe that with further refinement, we will be able to develop a new mechanism for complete control of the photon emissions from the laser, and perhaps even integrate structures generating sound with the terahertz laser so that no external sound source is needed." Professor Cunningham, co-author of the study.
This could enable data transmission rates up to 100 gigabits per second for applications like ultra-fast wireless links between facilities.
Bound States in the Continuum Enable Compact, Stable THz Quantum Cascade Lasers
Nanyang Technological University (NTU) researchers have achieved a significant breakthrough in developing electrically pumped, topological, bulk quantum cascade lasers operating in the terahertz (THz) frequency range. The results are published in Light: Science & Applications.
The designed THz QCL uses topological bound states in the continuum (BIC states) to confine light, enabling the laser to operate in a stable single mode while producing a compact, high-quality beam.
Previously, most topological lasers have relied on edge states confined to surfaces or interfaces. However, the NTU researchers focused on topological bulk band edges inside the QCL's active region.
The team demonstrated that exploiting these topological bulk BICs enables vertical and lateral confinement of the lasing mode in a very compact cavity. As a result, the miniaturized design (~10 microns wide) produced a narrow, single-mode THz beam with circular polarization.
This new topological bulk BIC design could enable more robust, single-mode QCL devices across infrared and THz frequencies and provide strong confinement for high beam quality without requiring large external cavities.
Silicon-Integrated Quantum Cascade Lasers for Mid-Infrared Optoelectronics
In recent research published in Applied Physics Letters, US scientists achieved a major milestone by growing 8.1 μm wavelength quantum cascade lasers on silicon using metal-organic chemical vapor deposition (MOCVD).
According to the researchers, this achievement stands out as: "There are no prior reports of QCLs grown by metal-organic chemical vapor deposition (MOCVD) on silicon substrates."
The designed QCL device demonstrated structural integrity without cracking due to the modest sample size and the thick silicon substrate that mitigated curvature accumulation.
This innovation could revolutionize silicon photonic integrated circuits (PICs), offering cost-effective and efficient mid-infrared optoelectronics.
Challenges and Future Outlook
Quantum cascade lasers offer unique capabilities for mid-infrared sensing and spectroscopy applications due to their high-power output, wavelength agility, and narrow line widths. However, complex temperature control electronics and high production costs challenge their broader adoption.
Despite these challenges, the global quantum cascade laser market is projected to exhibit substantial growth, with a projected value of USD 533 million by 2028 and a CAGR of 4.4%. This growth trajectory underscores the growing recognition of QCLs as a transformative technology with promising prospects for various industries.
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References and Further Reading
Dr. Rüdiger Paschotta. (2021). Quantum Cascade Lasers. [Online]. Available at: https://www.rp-photonics.com/quantum_cascade_lasers.html
Dunn, A., Poyser, C., Dean, P., Demić, A., Valavanis, A., Indjin, D., ... & Kent, A. (2020). High-speed modulation of a terahertz quantum cascade laser by coherent acoustic phonon pulses. Nature communications, 11(1), 835. https://doi.org/10.1038/s41467-020-14662-w
Edmund optics. (2023). Quantum Cascade Lasers. [Online]. Available at: https://www.edmundoptics.com/knowledge-center/application-notes/lasers/quantum-cascade-lasers/
Frąckiewicz, M. (2023). The Future of Quantum Cascade Lasers: Applications and Advancements. [Online]. Available at: https://ts2.space/en/the-future-of-quantum-cascade-lasers-applications-and-advancements/
Han, S., Cui, J., Chua, Y., Zeng, Y., Hu, L., Dai, M., ... & Wang, Q. J. (2023). Electrically-pumped compact topological bulk lasers driven by band-inverted bound states in the continuum. Light: Science & Applications, 12(1), 145. https://doi.org/10.1038/s41377-023-01200-8
Paschotta, R. (2008). Quantum Cascade Lasers-Field guide to laser pulse generation (Vol. 14). Bellingham: SPIE press. Available at: https://spie.org/publications/fg12_p45_quantum_cascade_lasers?SSO=1
Pecharroman-Gallego, R. (2017). An Overview on Quantum Cascade Lasers: Origins and Development. InTech. https://doi.org/10.5772/65003
Raspa, A. & Moglia, F. (2021). Looking to the future of quantum cascade lasers. Available at: https://www.laserfocusworld.com/lasers-sources/article/14211918/looking-to-the-future-of-quantum-cascade-lasers
Xu, S., Zhang, S., Kirch, J. D., Gao, H., Wang, Y., Lee, M. L., ... & Mawst, L. J. (2023). 8.1 μm-emitting InP-based quantum cascade laser grown on Si by metal-organic chemical vapor deposition. Applied Physics Letters, 123(3). https://doi.org/10.1063/5.0155202