Xiangeng Meng

Random lasers with coherent feedback from highly transparent polymer films embedded with silver nanoparticles

We report on observations of random lasers with coherent feedback in highly transparent polymer films embedded with silver nanoparticles. The hybrid materials were fabricated via in situ synthesis method, through which silver nanoparticles were precipitated by thermal treatment. Sharp peaks with linewidth ∼0.5nm were observed to emerge on the broad emission background when the pump energy reached a threshold, together with unidirectional laser irradiation. Random lasers with coherent feedback induced by silver nanoparticles have been rarely reported, hence, we expect that this work will add an aspect to random lasers by using metal nanostructures to obtain coherent feedback.

Wavelength-Tunable Spasing in the Visible

A SPASER, short for surface plasmon amplification by stimulated emission of radiation, is key to accessing coherent optical fields at the nanoscale. Nevertheless, the realization of a SPASER in the visible range still remains a great challenge because of strong dissipative losses. Here, we demonstrate that room-temperature SPASER emission can be achieved by amplifying longitudinal surface plasmon modes supported in gold nanorods as plasmon nanocavities and utilizing laser dyes to supply optical gain for compensation of plasmon losses. By choosing a particular organic dye and adjusting the doping level, the resonant wavelength of the SPASER emission can be tuned from 562 to 627 nm with a spectral line width narrowed down to 5-11 nm. This work provides a versatile route toward SPASERs at extended wavelength regimes.

Plasmonically controlled lasing resonance with metallic-dielectric core-shell nanoparticles.

We experimentally demonstrate the capability of tailoring lasing resonance properties by manipulating the coupling between surface plasmons and photons in random lasing media composed of metallic-dielectric core-shell nanoparticles and organic dyes. It is revealed that core-shell nanoparticle-based systems exhibit optical feedback features distinctive from those containing pure metallic nanoparticles, provided that the scattering strength is weak enough. The pump threshold increases with an increment in the shell thickness, which can provide a direct proof that the local field enhancement plays a central role in the emergence of coherent feedback. The anomalous behavior in both threshold and optical feedback is discussed in terms of the modification of fluorescent properties of fluorophores close to metallic surface.

Unidirectional Spaser in Symmetry-Broken Plasmonic Core-Shell Nanocavity

The spaser, a quantum amplifier of surface plasmons by stimulated emission of radiation, is recognized as a coherent light source capable of confining optical fields at subwavelength scale. The control over the directionality of spasing has not been addressed so far, especially for a single-particle spasing nanocavity where optical feedback is solely provided by a plasmon resonance. In this work we numerically examine an asymmetric spaser – a resonant system comprising a dielectric core capped by a metal semishell. The proposed spaser emits unidirectionally along the axis of the semishell; this directionality depends neither on the incident polarization nor on the incident angle of the pump. The spasing efficiency of the semishell-capped resonator is one order of magnitude higher than that in the closed core-shell counterpart. Our calculations indicate that symmetry breaking can serve as a route to create unidirectional, highly intense, single-particle, coherent light sources at subwavelength scale.

Controlling Random Lasing with Three-Dimensional Plasmonic Nanorod Metamaterials

Plasmonics has brought revolutionary advances to laser science by enabling deeply subwavelength nanolasers through surface plasmon amplification. However, the impact of plasmonics on other promising laser systems has so far remained elusive. Here, we present a class of random lasers enabled by three-dimensional plasmonic nanorod metamaterials. While dense metallic nanostructures are usually detrimental to laser performance due to absorption losses, here the lasing threshold keeps decreasing as the volume fraction of metal is increased up to ∼0.07. This is ∼460 times higher than the optimal volume fraction reported thus far. The laser supports spatially confined lasing modes and allows for efficient modulation of spectral profiles by simply tuning the polarization of the pump light. Full-field speckle-free imaging at micron-scales has been achieved by using plasmonic random lasers as the illumination sources. Our findings show that plasmonic metamaterials hold potential to enable intriguing coherent optical sources.

Random lasing in ballistic and diffusive regimes for macroporous silica-based systems with tunable scattering strength

We have systematically investigated random lasing properties in weakly scattering systems composed of a macroporous silica disk immersed in a dye solution where the solvent is a mixture of two alcohols. Controlling the refractive index of the mixed solvent allows us to vary the scattering strength over a wide range. We have found two different scattering regimes where sharp spectral spikes with linewidth less than 1.0 nm, i.e., random laser with coherent feedback, appear in emission spectra. When the refractive index contrast between the solvent and the silica is very small, random lasing with coherent feedback is observed although the system appears nearly transparent. The coherent feedback vanishes when the refractive index contrast is increased up to a critical value, while further increase in the refractive index contrast results in the revival of the coherent feedback. We suggest that the existence of underlying microcavities plays an important role in the very weakly scattering regime (ballistic) while other mechanisms such as amplified extended modes may lead to the coherent feedback in lasing oscillation when the scattering strength increases.

Intense greenish emission from d0 transition metal ion Ti4+ in oxide glass

Ti4+-doped oxide glass is revealed to yield intense greenish emission by excitations with ultraviolet (UV) light and near-infrared femtosecond pulsed laser (NIFPL). The emission profile obtained by UV excitation can be well traced by NIFPL. Compared with Ta5+-doped oxide glasses reported previously, Ti4+-doped glasses exhibit distinctive characteristics in optical absorption, excitation, and emission spectra, indicating that intense tunable emissions can be readily achieved by selecting nd0 ions as dopants in glasses. The glass materials containing nd0 ions are expected to find applications in high density optical storage and three-dimensional color displays.

Intense blue emission from tantalum-doped silicate glass

The authors report on an intense blue emission centered at 420nm under ultraviolet excitation from pentavalent tantalum ion-doped silicate glasses. The blue emission is distinctly different from emission due to defects in silica glass, which is also reflected in both absorption spectrum and remarkably enhanced emission. The authors suggest that localized Ta 5d0 energy level is responsible for the blue emission.

Two-photon-excited fluorescence from silicate glass containing tantalum ions pumped by a near-infrared femtosecond pulsed laser

We report a novel phenomenon in sodium-calcium-silicate glass doped with Ta(5+). Under irradiation with a 780 nm femtosecond pulsed laser, strong blue emission centered at about 420 nm could be observed. The spectral characteristics are similar to those pumped by ultraviolet photons. The log-log correlation between integrated emission intensity and pump power reveals that a two-photon absorption process is involved in the phenomenon. It is suggested that the presence of localized Ta(5+)5d(0) energy levels is responsible for the appearance of the blue emission. The results indicate that transition metal ions without d electrons play an important role in fields of optics when embedded into glass hosts.

Multi-color light emissions from mesoporous silica particles embedded with Ga 2 O 3 nanocrystals

We report on novel light–emitting properties from monodispersed mesoporous silica particles embedded with β–Ga2O3 nanocrystals that were fabricated through a chemical approach followed by thermal annealing in specific atmosphere. The emission spectrum of such nanocomposites consists of several sharp peaks where the dominant one regularly shifts with variation of the excitation wavelength, leading to observation of multiple–color light emissions ranging from blue, green, to white light wavelength regions. We suggest that the donor levels created by oxygen vacancy while multiple acceptor levels induced by gallium vacancy or gallium oxide vacancy account for the emission features of multiple bands.