Researchers from the HSE International Laboratory of Quantum Optoelectronics in St Petersburg have explored the impact of resonator size on the operating temperature of a microdisk laser with quantum dots in a two-level generation mode. Their findings reveal that microlasers can produce radiation across multiple frequencies, even under elevated temperatures. In the future, this breakthrough will enable the integration of microlasers into photonic circuits, potentially doubling information transmission capabilities. The study findings have been published in Nanomaterials.
The study was carried out as part of 'Research on Frequency Generation Switching in Microdisk Lasers with Quantum Dots for High-Speed Data Transmission' in the framework of the HSE Strategic Project 'Digital Transformation: Technologies, Effects, and Performance' under the Priority 2030 Programme.
In integrated circuits, such as those found in home computer processors, information is conveyed through electrical signals traveling along metal traces. For long-distance data transmission, such as delivering information from the internet to a computer, optical fibre is used, where information is conveyed through a light signal that propagates 100 times faster than electricity. This has prompted scientists to seek ways to harness light for data transfer within chips, thereby enhancing their performance.
The light signal is generated by lasers that transform electrical energy into light with a precisely defined wavelength and directionality. In conventional semiconductor lasers, energy conversion into radiation involves the use of a Fabry-Pérot resonator, typically measuring approximately 1 mm in size and consisting of two facing mirrors. This kind of resonator is too large for modern processor integrated circuits. However, attempts to scale it down to the desired size of hundreds of micrometres (μm) result in a significant loss of resonator performance.
In 1992, a group of scientists from AT&T Bell Laboratories introduced a novel paradigm using a disk or ring resonator. Within the disk or ring, the light beam disperses in a circular fashion, nearly entirely reflecting off the resonator's edge, a phenomenon known as the 'whispering gallery' effect. Lasers of this type continue to function effectively even when scaled down to micron-sized dimensions, making them well-suited for contemporary electronics applications, such as data transmission between microcircuit components.
In the circular galleries of certain buildings, a whisper can travel effectively along the walls but remains inaudible in the rest of the room. The ‘whispering gallery’ phenomenon in circular rooms is linked to the propagation of an acoustic wave along the wall, with multiple occurrences of total internal reflection. Lord Rayleigh was the first to investigate this effect in the whispering gallery of St Paul's Cathedral in London.
Typically, lasers emit radiation at a single wavelength, producing monochromatic light. In 1999–2000, two groups of scientists put forth the idea of incorporating layers containing quantum dots into the active region of microlasers, similar to what had already been done in classical semiconductor lasers. Under specific conditions, quantum dots enable a microlaser to emit two easily distinguishable wavelengths. This phenomenon is known as two-level generation.
In the two-level generation mode, the laser has the capability to simultaneously produce radiation at two distinct wavelengths, such as red and orange. We can control the laser's emitted colour: exclusively red, exclusively orange, or both at once. This presents vast opportunities for encoding transmitted information and, consequently, boosting the throughput of such systems.
The researchers do acknowledge, however, that dual-wavelength generation in microdisk lasers with quantum dots is not always possible. At critical temperatures specific to each resonator diameter, one of the two wavelengths disappears. Researchers at HSE in St Petersburg conducted a series of experiments to determine how the resonator’s size influences the critical temperature, focusing on the temperature range of 20°C to 110°C.
It was discovered that as the temperature increased, the laser required progressively less energy to simultaneously generate two distinguishable radiations. The researchers also observed that the critical temperature decreased from 107°C to 37°C as the diameter of the microdisk reduced from 28 to 20 μm.
Drawing upon the findings of the experiment, equations were formulated making it possible to determine the critical temperature and threshold current density for two-level generation across various resonator sizes. According to the researchers, with the knowledge of the operating conditions, the equation will enable the selection of the optimal laser resonator size for electronics.
The authors also suggest another application for two-level lasers. They can be used in neuromorphic neural networks to simulate the behaviour of neurons in the brain, with one wavelength corresponding to an excitation pulse and the other one representing an inhibition pulse.