Fengfeng Yao

Polarization and polarization control of random lasers from dye-doped nematic liquid crystals

A polarimetric study of random laser (RL) emitted from dye-doped nematic liquid crystals (NLCs) is presented. We observed linearly polarized light, the orientation of which is in proximity to the bisection between the polarization direction at the maximal scattering in NLCs and the nematic director. Any arbitrary linear polarization of RLs can be obtained by rotating the NLC sample. The efficiency and output uniformity over the complete direction angle of 2π can be optimized by choosing a proper pump polarization.

Light controlled diffraction gratings in C60-doped nematic liquid crystals

The persistent hidden photorefractive gratings were produced in a homeotropic C60-doped liquid crystal cell by two 488nm writing beams. The persistent gratings can be hidden and revealed by turning off and on an incident laser beam at 488, 532, or 632nm at any incident angle in the presence of a dc electric field or by turning off and on the applied dc electric field in the presence of an illuminating beam. These hidden gratings can be rejuvenated after keeping as long as several hours in the presence of the dc electric field. The possible underlying mechanism involving the surface-mediated effect and the bulk-mediated Carr-Helfrich effect is proposed. Consistent with this mechanism, the diffraction efficiency of the hidden gratings shows a dependence on the power of the illuminating beam, and the hidden grating can be revealed by applying a higher voltage without any illumination.

Enhanced magnetic response in a gold nanowire pair array through coupling with Bloch surface waves

We numerically study the coupling of magnetic plasmon polaritons (MPPs) with Bloch surface waves (BSWs) in a system composed of a one-dimensional gold nanowire pair array lying on a periodic dielectric multilayer. At an appropriate period of the dielectric multilayer, maximum coupling takes place between the MPP and the BSW. It results in two branches of hybridized MPPs with a Rabi-type splitting as large as 125 meV. The maximal magnetic field intensity achieved in the center of nanowire pairs is enhanced greatly and an enhancement factor >1.5 is observed compared with that achieved by a nanowire pair array lying directly on a substrate. This has potential applications in nonlinear optics and near-field enhanced spectroscopy.

Controllable Coupling of Localized and Propagating Surface Plasmons to Tamm Plasmons

We theoretically investigate the coupling between Tamm plasmons and localized surface plasmons (LSPs) as well as propagating surface plasmons (PSPs) in a multilayer structure consisting of a metallic nanowire array and a spatially separated metal–dielectric Bragg reflector (DBR). A clear anticrossing behavior of the resonances is observed in the dispersion diagram resulting from the coupling, which is well explained by the coupled oscillator model. The coupling also creates new hybrid LSP or PSP modes with narrow bandwidths and unique spectral features. Upon the excitation of these hybrid modes, the local fields underneath the nanowires for the hybrid LSPs or near the lower metal layer surface for the hybrid PSPs are both enhanced greatly as compared with those achieved in the structure without DBR, which has potential applications in nonlinear optics and surface-enhanced spectroscopies.

Optical magnetic field enhancement through coupling magnetic plasmons to Tamm plasmons

We report on a theoretical investigation of the coupling between magnetic plasmons (MPs) and Tamm plasmons (TPs) in a metal-dielectric Bragg reflector (DBR) containing a gold nanowire pair array embedded in the low refractive index layer closest to the metal film. Strong coupling between MPs and TPs is observed, manifested by large anticrossings in the dispersion diagram. It creates a narrow-band hybridized MP mode with a Rabi-type splitting as large as 290 meV. Upon the excitation of this hybridized MP mode, a 2.5-fold enhancement of the magnetic field in the center of nanowire pairs is achieved as compared with the pure MP of the nanowire pairs embedded in a bare DBR structure (without the metal film). This result holds a promising potential application in magnetic nonlinearity and sensors.

Optical amplification in multilayer photorefractive liquid crystal films

Photorefractive two-beam coupling was performed in fullerene C60-doped nematic liquid crystal cell. Photorefractive gain coefficient as high as 1386cm−1 was obtained at an input pump power as weak as 16mW. However, the thin film nature led to a small gain of 16. By the use of several such cells stacked together to increase the overall interaction length, the optical gain was improved greatly, and the pump power to obtain the highest optical gain was reduced. At 1.2V, the highest gain of 141 was obtained for the 40μW signal at an input pump power of 12mW.

Optically controlled random lasing based on photothermal effect in dye-doped nematic liquid crystals

We presented a way to control the output of the nematic liquid crystal (NLC) random laser with a response time of hundreds of milliseconds by a continuous-wave laser beam with the power of several milliwatts. The lasing intensity can be tuned continuously by variation of the control power in the range from 0 to 5 mW or the lasing output can be switched on and off by turning off and on the control beam when the control power reaches 6 mW. The control power-dependent lasing polarisation and the self-phase modulation of the control beam indicate that the photothermal effect, which leads to the change of the order of the NLC and subsequently the change of the long-range fluctuation of the dielectric tensor, is mainly responsible for the controllability of the random lasing.

Refractive index of silver nanoparticles dispersed in polyvinyl pyrrolidone nanocomposite

A silver nanoparticles/polymer composite was fabricated by reduction of silver nitrate in the presence of the polymer polyvinyl pyrrolidone. The plasmonic resonance of silver nanoparticles leads to a remarkable change of the dielectric dispersion of the polymer in the visible range. The refractive indexes of the nanocomposite with different fractions of the precursor silver nitrate at the wavelengths of 488, 532, and 633 nm were investigated by using reflectometry. At a fraction larger than 0.122, the refractive index change was larger than 0.1, and the maximal refractive index change of 0.84 was achieved at a wavelength of 488 nm with a fraction of 0.422. The experimental results were consistent with the prediction of Maxwell Garnett theory. The solid-state nanocomposite with a strong refractive index dispersion and tunability has promising applications in optics and communication devices.

High diffraction efficiency and a quasi-permanent grating in photorefractive nematic liquid crystal at low temperature

At a temperature lower than the melting point of pure pentyl-cyanobiphenyl liquid crystal (5CB) we studied the photorefractive effect of 5CB heavily doped with fullerene (C60) and obtained a diffraction efficiency of the photorefractive grating of more than 40%, which is more than twice as high as the best data reported to our knowledge. A quasi-permanent grating was observed with a diffraction efficiency of 11.2% after the grating was kept in the dark for a month.

Random lasing in unbounded dye-doped nematic liquid crystals

ABSTRACT Random lasing in dye-doped nematic liquid crystals (NLCs) contained in a quartz cuvette was studied. With the pumping energy rising, random lasing with nonresonant feedback and resonant feedback emerge successively with the lasing threshold of 20 and 40 μJ, respectively. The lasing properties including fluctuation of the emission spectrum, divergence angle of emission, independence of the emission on the polarisation of the pump beam and the depolarisation of emission are distinct from those of dye-doped nematics confined in the sandwich structure cell or the capillary tube. The distinctions are associated with the thermal fluctuation of dielectric tensor since there is negligible surface anchoring for the nematics in the pump volume. This indicates that the anchoring state of NLCs is an important factor determining the random lasing properties. GRAPHICAL ABSTRACT