Nanolasers, like conventional lasers, are based on Einstein’s stimulated emission theory; the key distinction between nanolasers and ordinary lasers in terms of mechanism is light confinement. To achieve light confinement, the resonator or cavity plays a crucial role in selecting the light with a specific frequency and direction as the most important amplification and suppressing the other light. A Fabry–Pérot cavity with two parallel reflection mirrors is used in conventional lasers. A microdisk laser is a miniature laser that consists of a disc with quantum well structures embedded in it. Its dimensions might be either micro or nano-scale. A whispering-gallery mode resonant cavity is used in microdisk lasers.
Periodic dielectric structures with various refractive indices are used in photonic crystal lasers, and light can be contained using a photonic crystal microcavity. There is an organized spatial distribution in dielectric materials. The Fano resonance phenomenon occurs when a flaw in the periodic structure causes the two-dimensional or three-dimensional photonic crystal structure to confine the light in the space of the diffractive limit, resulting in a high-quality factor and strong light confinement for lasers. The plasmonic nanolaser is a nanolaser based on surface plasmon that has a size that much exceeds the diffraction limit of light. When a plasmonic nanolaser is nanoscopic in three dimensions, it is also known as a spacer, because it has the smallest cavity and mode size. At this time, designing a plasmonic nanolaser is one of the most effective technology methods for laser shrinking. Nanowire lasers made of semiconductors have a semi-one-layered structure with breadths going from a couple of nanometers to two or three hundred nanometers and lengths going from many nanometers to a couple of microns.
In recent years, new types of nanolasers have been produced that approach the diffraction limit. In a coupled cavity system, parity-time symmetry is related to a balance of optical gain and loss. The phase transition of lasing modes happens at an extraordinary moment when the gain–loss contrast and coupling constant between two identical, closely positioned cavities are controlled. A Fabry–Pérot cavity with two parallel reflection mirrors is used in conventional lasers. The light could be restricted to a maximum of half its wavelength in this situation, and this limit is known as the diffraction limit of light. Improving the reflectivity of the gain medium, such as employing photonic bandgap and nanowires, is one strategy to approach or decrease the diffraction limit of light. Converting light into surface plasmons in nano-structuralized metals for cavity amplification is another viable technique to exceed the diffraction limit.
The enhancement effect in non-linear optics or surface-enhanced-raman-scattering is also possible with such a laser’s powerful optical fields (SERS). The Internet is growing at a breakneck pace, consuming e normous amounts of energy in the process. Plasmonic nanolaser sensors that can detect certain molecules in the air and be utilized for optical biosensors have recently been developed. Nanolasers have considerable potential for practical applications in the fields of materials characterization, integrated optical interconnects, and sensing due to their unique features, which include low lasing thresholds, high energy efficiency, and rapid modulation speeds. Although nanolasers have shown significant promise, there are still certain obstacles to their widespread application, such as electrically injected nanolasers, cavity layout engineering, and improved metal quality. Realizing electrically injected or pumped operation at room temperature for nanolasers is a critical step toward their practical implementation. However, the majority of nanolasers are optically pumped, and electrically injected nanolasers are still a major technical problem.n