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Miniaturization has been an everlasting theme in the development of semiconductor lasers. One important breakthrough in this process in recent years is the use of metal-dielectric composite structures that made truly subwavelength lasers possible. Many different designs of metallic cavity semiconductor nanolasers have been proposed and demonstrated. In this article, we will review some of the most exciting progresses in this newly emerging field. In particular, we will focus on metallic-cavity nanolasers with volume smaller than wavelength cubed under electrical injection with emphasis on high-temperature operation. Such devices will serve as an important component in the future integrated nanophotonic systems due to its ultra-small size. Lasers: Rise of the nanolaser Semiconductor nanolasers based on subwavelength-scale metal cavities could become important light sources for integrated optical circuitry on silicon. In this paper, Kang Ding and Cun-Zheng Ning from Arizona State University in the USA review progress in this exciting and rapidly evolving field. They cover the achievement of milestones such as the reduction of cavity sizes to below the scale of one wavelength, operation at room temperature, continuous-wave emission and the use of electrical injection. They describe the design and operating principles of nanolasers, as well as the challenges faced in terms of device fabrication, overcoming cavity loss, high-temperature operation and waveguide integration. Future improvements in fabrication technology to address issues such as surface passivation and material deposition will bring further advances in device performance.

Rare-earth optical materials with large optical gain are of great importance for a wide variety of applications in photonics and quantum information due to their long carrier lifetimes and quantum coherence times, especially in the realization of efficient lasers and amplifiers. Until now, such materials have achieved a gain of less than a few dB cm –1 , rendering them unsuitable for applications in nanophotonic integrated circuits. Here, we report the results of the signal enhancement and transmission experiments on a single-crystal erbium chloride silicate nanowire. Our experiments demonstrate that a net material gain over 100 dB cm –1 at wavelengths around 1,530 nm is possible due to the nanowire's single-crystalline material quality and its high erbium concentration. Our results establish that such rare-earth-compound nanowires are a potentially important class of nanomaterials for a variety of applications including, for example, subwavelength-scale optical amplifiers and lasers for integrated nanophotonics, and quantum information. Erbium chloride silicate nanowire promises optical gain for nanophotonic circuits.

Monolayer transition-metal dichalcogenides (TMDs) have the potential to become efficient optical-gain materials for low-energy-consumption nanolasers with the smallest gain media because of strong excitonic emission. However, until now TMD-based lasing has been realized only at low temperatures. Here we demonstrate for the first time a room-temperature laser operation in the infrared region from a monolayer of molybdenum ditelluride on a silicon photonic-crystal cavity. The observation is enabled by the unique combination of a TMD monolayer with an emission wavelength transparent to silicon, and a high- Q cavity of the silicon nanobeam. The laser is pumped by a continuous-wave excitation, with a threshold density of 6.6 W cm –2 . Its linewidth is as narrow as 0.202 nm with a corresponding Q of 5,603, the largest value reported for a TMD laser. This demonstration establishes TMDs as practical materials for integrated TMD–silicon nanolasers suitable for silicon-based nanophotonic applications in silicon-transparent wavelengths. A silicon nanobeam cavity laser integrated with monolayer molybdenum ditelluride enables room-temperature lasing in the near infrared.

Monolithic semiconductor lasers capable of emitting over the full visible-colour spectrum have a wide range of important applications, such as solid-state lighting, full-colour displays, visible colour communications and multi-colour fluorescence sensing. The ultimate form of such a light source would be a monolithic white laser. However, realizing such a device has been challenging because of intrinsic difficulties in achieving epitaxial growth of the mismatched materials required for different colour emission. Here, we demonstrate a monolithic multi-segment semiconductor nanosheet based on a quaternary alloy of ZnCdSSe that simultaneously lases in the red, green and blue. This is made possible by a novel nanomaterial growth strategy that enables separate control of the composition, morphology and therefore bandgaps of the segments. Our nanolaser can be dynamically tuned to emit over the full visible-colour range, covering 70% more perceptible colours than the most commonly used illuminants. A monolithic heterostructure nanosheet composed of a ZnCdSSe multi-segment quaternary alloy can simultaneously emit laser light in the red, green and blue.

The manipulation of photons in structures smaller than the wavelength of light is central to the development of nanoscale integrated photonic systems for computing, communications, and sensing. We assemble small groups of freestanding, chemically synthesized nanoribbons and nanowires into model structures that illustrate how light is exchanged between subwavelength cavities made of three different semiconductors. The coupling strength of the optical linkages formed when nanowires are brought into contact depends both on their volume of interaction and angle of intersection. With simple coupling schemes, lasing nanowires can launch coherent pulses of light through ribbon waveguides that are up to a millimeter in length. Also, interwire coupling losses are low enough to allow light to propagate across several right-angle bends in a grid of crossed ribbons. The fraction of the guided wave traveling outside the wire/ribbon cavities is used to link nanowires through space and to separate colors within multiribbon networks. In addition, we find that nanoribbons function efficiently as waveguides in liquid media and provide a unique means for probing molecules in solution or in proximity to the waveguide surface. Our results lay the spadework for photonic devices based on assemblies of active and passive nanowire elements and presage the use of nanowire waveguides in microfluidics and biology.

Monolayer transition metal dichalcogenides (TMDs) provide the most efficient optical gain materials and have potential for making nanolasers with the smallest gain media with lowest energy consumption. But lasing demonstrations based on TMDs have so far been limited to low temperatures. Here, we demonstrate the first room-temperature laser operation in the infrared wavelengths from a monolayer of molybdenum ditelluride on a silicon photonic-crystal nanobeam cavity. Our demonstration is made possible by a unique choice of TMD material with emission wavelength below silicon absorption, combined with the high Q-cavity design by silicon nanobeam. Lasing at 1132 nm is demonstrated at room-temperature pumped by a continuous-wave laser, with a threshold density at 6.6 W/cm2. The room-temperature linewidth of 0.202 nm is the narrowest with the corresponding Q of 5603, the largest observed for a TMD laser. This demonstration establishes TMDs as practical nanolaser materials. The silicon structures provide additional benefits for silicon-compatible nanophotonic applications in the important infrared wavelengths.

Metallic-Cavity lasers or plasmonic nanolasers of sub-wavelength sizes have attracted great attentions in recent years, with the ultimate goal of achieving continuous wave (CW), room temperature (RT) operation under electrical injection. Despite great efforts, a conclusive and convincing demonstration of this goal has proven challenging. By overcoming several fabrication challenges imposed by the stringent requirement of such small scale devices, we were finally able to achieve this ultimate goal. Our metallic nanolaser with a cavity volume of 0.67{\\lambda}3 ({\\lambda}=1591 nm) shows a linewidth of 0.5 nm at RT, which corresponds to a Q-value of 3182 compared to 235 of the cavity Q, the highest Q under lasing condition for RT CW operation of any sub-wavelength laser. Such record performance provides convincing evidences of the feasibility of RT CW metallic nanolasers, thus opening a wide range of practical possibilities of novel nanophotonic devices based on metal-semiconductor structures.

We investigated surface plasmon polariton (SPP) propagation in a metal-semiconductor-metal structure where semiconductor is highly excited to have optical gain. We show that near the SPP resonance, the imaginary part of the propagation wavevector changes from positive to hugely negative, corresponding to an amplified SPP propagation. The SPP experiences a giant gain that is 1000 times of material gain in the excited semiconductor. We show that such a giant gain is related to the slowing down of average energy propagation in the structure

Metallic nanocavity lasers provide important technological advancement towards even smaller integrable light sources. They give access to widely unexplored lasing physics in which the distinction between different operational regimes, like those of thermal or a coherent light emission, becomes increasingly challenging upon approaching a device with a near-perfect spontaneous-emission coupling factor \\(\\beta\\). In fact, quantum-optical studies have to be employed to reveal a transition to coherent emission in the intensity fluctuation behavior of nanolasers when the input-output characteristic appears thresholdless for \\(\\beta = 1\\) nanolasers. Here, we identify a new indicator for lasing operation in high-\\(\\beta\\) lasers by showing that stimulated emission can give rise to a lineshape anomaly manifesting as a transition from a Lorentzian to a Gaussian component in the emission linewidth that dominates the spectrum above the lasing threshold.