This Review provides an introduction to the compensation of loss and amplification of surface plasmons in waveguides and resonators. Future challenges, including how to overcome the large losses present in plasmonic systems that offer strong electromagnetic confinement, are also discussed.

Surface plasmon–polariton amplifiers and lasers

A nanolaser is a key component for on-chip optical communications and computing systems. Here, we report on the low-threshold, continuous-wave operation of a subdiffraction nanolaser based on surface plasmon amplification by stimulated emission of radiation. The plasmonic nanocavity is formed between an atomically smooth epitaxial silver film and a single optically pumped nanorod consisting of an epitaxial gallium nitride shell and an indium gallium nitride core acting as gain medium. The atomic smoothness of the metallic film is crucial for reducing the modal volume and plasmonic losses. Bimodal lasing with similar pumping thresholds was experimentally observed, and polarization properties of the two modes were used to unambiguously identify them with theoretically predicted modes. The all-epitaxial approach opens a scalable platform for low-loss, active nanoplasmonics.

https://ir.nctu.edu.tw:443/bitstream/11536/16618/1/000306802300038.pdf

Plasmonic Nanolaser Using Epitaxially Grown Silver Film

Optical communications and computing require on-chip nonreciprocal light propagation to isolate and stabilize different chip-scale optical components. We have designed and fabricated a metallic-silicon waveguide system in which the optical potential is modulated along the length of the waveguide such that nonreciprocal light propagation is obtained on a silicon photonic chip. Nonreciprocal light transport and one-way photonic mode conversion are demonstrated at the wavelength of 1.55 micrometers in both simulations and experiments. Our system is compatible with conventional complementary metal-oxide-semiconductor processing, providing a way to chip-scale optical isolators for optical communications and computing.

http://science.sciencemag.org/content/sci/333/6043/729.full.pdf

Nonreciprocal Light Propagation in a Silicon Photonic Circuit

The effects of cavity quantum electrodynamics (QED), caused by the interaction of matter and the electromagnetic field in subwavelength resonant structures, have been the subject of intense research in recent years. The generation of coherent radiation by subwavelength resonant structures has attracted considerable interest, not only as a means of exploring the QED effects that emerge at small volume, but also for its potential in applications ranging from on-chip optical communication to ultrahigh-resolution and high-throughput imaging, sensing and spectroscopy. One such strand of research is aimed at developing the 'ultimate' nanolaser: a scalable, low-threshold, efficient source of radiation that operates at room temperature and occupies a small volume on a chip. Different resonators have been proposed for the realization of such a nanolaser--microdisk and photonic bandgap resonators, and, more recently, metallic, metallo-dielectric and plasmonic resonators. But progress towards realizing the ultimate nanolaser has been hindered by the lack of a systematic approach to scaling down the size of the laser cavity without significantly increasing the threshold power required for lasing. Here we describe a family of coaxial nanostructured cavities that potentially solve the resonator scalability challenge by means of their geometry and metal composition. Using these coaxial nanocavities, we demonstrate the smallest room-temperature, continuous-wave telecommunications-frequency laser to date. In addition, by further modifying the design of these coaxial nanocavities, we achieve thresholdless lasing with a broadband gain medium. In addition to enabling laser applications, these nanoscale resonators should provide a powerful platform for the development of other QED devices and metamaterials in which atom-field interactions generate new functionalities.

Thresholdless nanoscale coaxial lasers

Plasmon lasers can operate at dimensions well below the diffraction limit. Their small size promises uses in nanophotonic circuits and for other size-critical applications. The demonstration of a sub-diffraction-limited plasmon laser with low losses, which enables its room-temperature operation, takes a significant step towards realizing the potential of these lasers.

/pdf/room-temperature-sub-diffraction-limited-plasmon-laser-by-4dy3wuhx8l.pdf

Room-temperature sub-diffraction-limited plasmon laser by total internal reflection

We demonstrate room-temperature pulsed laser emission from optically pumped metallo-dielectric cavities that are smaller than their emission wavelength in all three dimensions. The cavity consists of an aluminium/silica bi-layer shield surrounding an InGaAsP disk in which the thickness of the silica layer is optimized to minimize the gain threshold of the laser. The lasers are pumped using a 1,064-nm pulsed fibre laser with a pulse width of 12 ns and repetition rate of 300 kHz. Lasing emission at 1.43 µm is observed from a laser with slightly elliptical gain core with major and minor diameters of 490 and 420 nm, respectively. Owing to the isolation provided by the aluminium shield, this laser design approach can be used to create arrays of uncoupled lasers with emitter densities that are close to the Rayleigh resolution limit. Room-temperature lasing from metallo-dielectric cavities that are smaller than their emission wavelength in all three dimensions is reported. The cavity consists of an aluminium/silica bi-layer shield that surrounds an InGaAsP disk. The gain threshold of the laser is minimized by optimizing the thickness of the silica layer.

Room-temperature subwavelength metallo-dielectric lasers

Electrically driven subwavelength scale metallo-dielectric pedestal pillar lasers are designed and experimentally demonstrated. The metallo-dielectric cavity significantly enhances the quality factor (Q > 1500) of the wavelength and subwavelength scale lasers and the pedestal structure significantly reduces the threshold gain (< 400 cm(-1)) which can potentially enable laser operation at room temperature. We observed continuous wave lasing in 750 nm gain core radius laser at temperatures between 77 K and 140 K with a threshold current of 50 μA (at 77 K). We also observed lasing from a 355 nm gain core radius laser at temperatures between 77 K and 100 K.

Electrically pumped sub-wavelength metallo-dielectric pedestal pillar lasers

An etch-free fabrication technique for creating low loss silicon waveguides in the silicon-on-insulator material system is proposed and demonstrated. The approach consists of local oxidation of a silicon-on-insulator chip covered with a e-beam patterned hydrogen silsesquioxane mask. A single oxidation step converts hydrogen silsesquioxane to a glass-like compound and simultaneously defines the waveguides, bypassing the need for any wet or dry etching steps. The spectral response of ring resonators fabricated using this technique was used to characterize the waveguide losses. Intrinsic Q-factors as high as 1.57 × 10(6), corresponding to a waveguide loss of 0.35 dB/cm, were measured.

Etch-free low loss silicon waveguides using hydrogen silsesquioxane oxidation masks

Metal-clad subwavelength lasers have recently become excellent candidates for light sources in densely packed chip-scale photonic circuits. In this review, we summarize recent research efforts in the theory, design, fabrication, and characterization of such lasers. We detail advancements of both the metallo-dielectric and the coaxial type lasers: for the metallo-dielectric type, we discuss operation with both optical pumping and electrical pumping. For the coaxial type, we discuss operation with all spontaneous emission coupled into the lasing mode, as well as the smallest metal-clad lasers to date operating at room temperature. A formal treatment of the Purcell effect, the modification of the spontaneous emission rate by a subwavelength cavity, is then presented to assist in better understanding the quantum effects in these nanoscale semiconductor lasers. This formalism is developed for the transparent medium condition, using the emitter-field-reservoir model in the quantum theory of damping. We show its utility through the analysis and design of subwavelength lasers. Finally, we discuss future research directions toward high-efficiency nanolasers and potential applications, such as creating planar arrays of uncoupled lasers with emitter densities near the resolution limit.

Subwavelength semiconductor lasers for dense chip-scale integration

As the field of semiconductor nanolasers becomes mature in terms of both the miniaturization to the true sub-wavelength scale, and the realization of room temperature devices, the integrated treatment of multi- ple design aspects beyond pure electromagnetic consid- eration becomes necessary to further advance the field. In this review, we focus on one such design aspect: tempera- ture effects in nanolasers. We summarize recent efforts in understanding the interplay of various temperature- dependent parameters, and study their effects on opti- cal mode and emission characteristics. Building on this knowledge, nanolasers with improved thermal perfor- mance can be designed, and their performance evaluated. Although this review focuses on metal-clad semiconduc- tor lasers because of their suitability for dense chip-scale integration, these thermal considerations also apply to the broader field of nanolasers.

Olesya Bondarenko

Papers

Room-temperature subwavelength metallo-dielectric lasers

Electrically pumped sub-wavelength metallo-dielectric pedestal pillar lasers

Etch-free low loss silicon waveguides using hydrogen silsesquioxane oxidation masks

Subwavelength semiconductor lasers for dense chip-scale integration

Temperature effects in metal-clad semiconductor nanolasers