Controlling electric fields spatially by graded metamaterials: Implication on enhanced nonlinear optical responses

The local electric field of a graded metal-dielectric cylinder, with complex permittivity given by a spatially dependent Drude model, has been derived analytically in terms of hypergeometric functions. Our results show that the electric field inside the cylinder can be confined in desired positions. Thus one can achieve the control of electric fields by fabricating graded metamaterials with specific material parameters. The enhanced nonlinear optical responses are also examined. The results suggest that the gradation-controlled field distribution may be a consequence of a combination of surface plasmon resonance and microgeometry in graded metamaterials.     

A hybrid model for the acoustic response of a two-dimensional rough surface to an impulse incident from a refracting medium

A numerical model to calculate the impulse response of a two-dimensional, impenetrable, rough surface directly in the time domain has been recently introduced [J. Acoust. Soc. Am. (2000) 107, 27]. This model is based on wedge diffraction theory and assumes that the half-space containing the source and receiver is homogeneous. In this work, the model is extended to handle media where the index of refraction varies with the distance to the surface by merging the scattering model with a ray-based propagation model. The resulting hybrid model is tested against a Finite-Difference Time-Domain (FDTD) method for the problem of backscattering from a corrugated surface in the presence of a refractive layer. This new model can be applied, for example, to calculate acoustic reverberation from the sea surface in cases where the water mass is inhomogeneous and in the presence of a subsurface bubble layer at low frequencies where dispersion is negligible. It can also be used for atmospheric propagation problems where there is a sound speed gradient overlying rough terrain.     

等离激元增强的石墨烯光吸收

石墨烯中等离激元具有特殊的光电性质,其和入射光的强烈耦合可以引起光吸收的增强.本文基于时域有限差分法和多体自洽场理论研究了等离激元对处于光学谐振腔中的石墨烯光吸收的影响.由于石墨烯中等离激元与入射光动量和能量不匹配而不能直接相互作用,因此石墨烯上施加了金属光栅结构.研究发现光栅结构能够对入射光进行动量补偿并且能够引起其下石墨烯中的电场强度产生很大程度增强,从而导致在该石墨烯结构中太赫兹等离激元和入射光发生强烈耦合而产生太赫兹等离极化激元,同时引起石墨烯光吸收的增强.希望本文能够加深对石墨烯光电特性的理解以及可以为基于石墨烯的太赫兹光电装置提供一定的理论依据.     

SPM of nonlinear surface plasmon waveguides

Pulse propagation equation of nonlinear dispersion surface plasmon waveguide is educed strictly from wave equation. The nonlinear coefficient is defined and then used to assess and compare the nonlinear characteristic of three popular 1-D surface plasmon waveguides: the single metal–dielectric interface, the metal slab bounded by dielectric and the dielectric slab bounded by metal. SPM (self-phase modulation) of the typical surface plasmon waveguide is predicted and discussed.     

Polarization controlled couplings of surface plasmon with asymmetrical nanoslit pairs

Light-driven surface plasmons offer an opportunity to ultrafast information processing combining the compactness of electric circuits with the bandwidth of photonic networks. For practical applications, the efficient and controllable conversion from signal light to surface plasmons is essential. This leads to the recent developments in the polarization controlled couplings of surface plasmons. Currently, most works only tailor the orientation and arrangement of nanoslits to control the launching of surface plasmons. In this paper, we consider both the orientation and size of each slit in a one-dimensional array of nanoslit dimers. We first realize the unidirectional propagation of surface plasmons with designed wavefronts. Next, the unidirectional coupling and bi-directional coupling of surface plasmons are realized for a pair of orthogonal polarizations, respectively. This is quite different from the conventional opposite propagating surface plasmons excited by two orthogonal polarizations. The manipulation of both orientation and size of nanoslits allows additional freedom in the photon-plasmon conversions.     

Linear and nonlinear optical response of g-C_3N_4-based quantum dots

Graphite carbon nitride(g-C_3N_4) attracts wide-ranging research interest due to its extraordinary physicochemical properties and promising applications ranging from heterogeneous catalysis to fuel cells. In this work, we design different g-C_3N_4-based quantum dots(g CNQDs), carry out a systematic study of optical properties, and elucidate the shape selectivity, composite nanostructure, and outfield effect. In particular, composites of g CNQDs and metal nanochains present excellent optical response, making it applicable to bioimaging, nano-plasma devices, and metalloenzyme in infrared light related fields. Besides, QDs which original bridging nitrogen atoms are replaced by amino(–NH_2), hydroxyl(–OH),and methyl(–CH_3) functional groups respectively, have excellent spectral selectivity in the deep ultraviolet region. More interestingly, in the study of the laser interaction with materials, the g CNQDs exhibit extremely high stability and light corrosion resistance. Phase transition from insulation to metal is observed under the critical condition of about 5 e V intensity or 337 nm wavelength. All provided theoretical support for designs and applications in g-C_3N_4 quantum devices.     

Hybrid vertical directional coupling between a long range surface plasmon polariton waveguide and a dielectric waveguide

To investigate light coupling between a long range surface plasmon polariton (LRSPP) waveguide and a conventional integrated optical component, a hybrid vertical directional coupler consisting of a LRSPP waveguides and a dielectric waveguide is investigated and fabricated. In the proposed coupler the dielectric waveguide and LRSPP waveguide are vertically configured for dense integration and strong coupling. The characteristics of the even and odd super-modes of the coupler are also analyzed to design the device. The fabricated device exhibits damped sinusoidal behavior along the coupling length due to propagation loss of the LRSPP waveguide. The maximum power transfer of 86% from the LRSPP waveguide to the dielectric waveguide is achieved at the coupling length of 600 μm. The measured characteristics of the device are in relatively good agreement with a theoretical analysis.     

Dual mode near-field scanning optical microscopy for near-field imaging of surface plasmon polariton

In this paper, we theoretically and experimentally demonstrate that metal coated apertured probes are efficient near-field probes on surfaces with high reflectivity for the scattering as well as for the collection mode near-field scanning optical microscopy (NSOM). We show that a blunt apertured metal coated tip is very effective in suppressing image dipoles which affect strongly the signals scattered from frequently used sharp metal tips or gold nanoparticle attached probes. By using a simultaneous collection and scattering mode (dual mode) NSOM we measure the near-field images of surface plasmon polariton (SPP) launched from a slit. The collection mode measures propagating SPP along lateral distance in a long scan range with high signal-to-noise ratio, and the scattering mode measures the polarization resolved near-field of SPP. Comparisons of the measured data obtained in the dual mode enable to easily characterize SPP and to separate the measured near-field into the propagating SPP and the directly transmitted light.     

A time convolution less density matrix approach to the nonlinear optical response of a coupled system-bath complex

Time convolution less density matrix theory (TCL) is a powerful and well established tool to investigate strong system-bath coupling for linear optical spectra. We show that TCL equations can be generalised to the nonlinear optical response up to a chosen order in the optical field. This goal is achieved via an time convolution less perturbation scheme for the reduced density matrices of the electronic system. In our approach, the most important results are the inclusion of a electron-phonon coupling non-diagonal in the electronic states and memory effects of the bath: First, the considered model system is introduced. Second, the time evolution of the statistical operator is expanded with respect to the external optical field. This expansion is the starting point to explain how a TCL theory can treat the response up to in a certain order in the external field. Third, new TCL equations, including bath memory effects, are derived and the problem of information loss in the reduced density matrix is analysed. For this purpose, new dimensions are added to the reduced statistical operator to compensate lack of information in comparison with the full statistical operator. The theory is benchmarked with a two level system and applied to a three level system including non-diagonal phonon coupling. In our analysis of pump-probe experiments, the bath memory is influenced by the system state occupied between pump and probe pulse. In particular, the memory of the bath influences the dephasing process of electronic coherences developing during the time interval between pump and probe pulses.     

Bistable switching using an optical Tamm cavity with a Kerr medium

All-optical bistable switching is proposed to realize by using an optical Tamm cavity in which a Kerr medium is sandwiched between a metal layer and a Bragg mirror. Results show that the excitation of clockwise and counterclockwise bistable switching under the control of external optical injection is due to the presence of optical Tamm states. In addition, the bistable characteristics of optical Tamm cavities are found to be sensitive to the polarization of external optical injection.     

The optical transmission characteristics through gold nanoslit arrays embedded in nonlinear medium

We present the numerical investigation of the optical transmission through a periodic gold nanoslit array embedded in the Kerr type nonlinear medium by using a developed two-dimensional Finite Different Time Domain (FDTD) method. The enhanced transmission in the nonlinear structure is attributed to the collaboration of the surface plasmon resonance and the localized waveguide resonance. We show that, in a certain intensity range, with the increase of the incident intensity, the transmission resonance peaks redshift obviously, and peak values decrease firstly and then increase; with the gold film thickness and the embedded depth becoming larger, transmission resonance peaks of both types redshift significantly, and the peak number, peak value and the half peak width change obviously. The electric fields distributions for different embedded depths of the gold slits at various resonance wavelengths are simulated to illuminate the underlying physical mechanisms. It is expected that these results obtained here will help to design nonlinear subwavelength metallic grating devices.     

光子角动量在环形金属纳米孔异常透射过程中的作用

在环形凹槽包围环形金属纳米孔的异常透射器件的研究中,环形凹槽可以将携带光子角动量的入射光转化为涡旋表面等离极化激元,这些涡旋表面等离极化激元传向几何中心并与直接照射在环形纳米孔上的光子发生干涉,当相互干涉的光子满足相位匹配条件时,环形纳米孔的透射率得到显著增强.本文利用理论分析和数值计算的方法研究了光子角动量和凹槽半径对环形纳米孔透射过程的影响.我们发现调节环形凹槽的半径和入射光携带的光子角动量可以调节光子在金膜上表面传输时的径向传播相位,进而影响了环形纳米孔附近的干涉电场强度,最终决定了环形纳米孔的透射率,进而可以通过调节凹槽的半径来调节携带不同光子角动量的光束在环形纳米孔的透射率.本文的研究结果对基于涡旋表面等离极化激元的异常透射器件的设计具有重要的指导意义.     

Investigation of the propagation properties of terahertz waves through a semiconductor subwavelength slit

The propagation properties of terahertz (THz) waves through semiconductor conductor-dielectric-conductor (CDC) structure have been investigated. The influence of geometrical parameters, radiation frequencies and temperatures on the propagation properties have been shown and discussed. The contour results demonstrate that as the length of the dielectric filling materials increase, the effective indices of the propagation modes increase. Compared with the results of the metal structure, the effective indices of surface modes through semiconductor InSb slits increase, the propagation length decrease. The effective indices of the surface modes decrease, the propagation lengths increase with the increasing of the carrier concentration. It is expected that the numerical results may be very helpful to have better understand the propagation mechanism of THz waves through semiconductor subwavelength slit.     

Difference scattering field properties between periodic defect particles and three-dimensional slightly rough optical surface

Based on the practical situation of nondestructive examination, the calculation model of the composite scattering is established by using a three-dimensional half-space finite difference time domain, and the Monte Carlo method is used to solve the problem of the optical surface with roughness in the proposed scheme. Moreover, the defect particles are observed as periodic particles for a more complex situation. In order to obtain the scattering contribution of defects inside the optical surface, a difference radar cross section is added into the model to analyze the selected calculations on the effects of numbers, separation distances, different depths and different materials of defects. The effects of different incident angles are also discussed. The numerical results are analyzed in detail to demonstrate the best position to find the defects in the optical surface by detecting in steps of a fixed degree for the incident angle.     

A novel Raman optical activity instrument operating in the deep ultraviolet spectral region

Raman optical activity (ROA) has been exclusively observed in the visible (VIS) and near‐infrared (NIR) spectral regions to date. During the last few years, we have designed, constructed and tested the first ROA instrument, operating in the deep‐ultraviolet (DUV) spectral region employing 244‐nm excitation. This novel DUV ROA instrument is based on a backscattering geometry and incident circular polarization modulation (ICP); it makes use of a fast DUV imaging lens‐based spectrograph and specially designed DUV grade polarization optics. The performance of this instrument has been evaluated by analysing measured non‐resonant DUV ROA spectra of non‐absorbing enantiomeric liquid samples and by comparing these with corresponding ROA spectra recorded in the visible spectral region. Copyright © 2015 John Wiley & Sons, Ltd.     

Silver and gold films and fibers several nanometers thick: Very slow optical surface plasmons

On films and fibers of well-reflecting metals (silver, gold) several nanometers thick, surface plasmons could be slowed down by 10 to 40 times. The plasmons are additionally slowed down when the nanofilms (nanofibers) are placed into a medium with a dielectric constant that is approximately equal to but still smaller than the modulus of the negative dielectric constant of the metal. As the result, the optical frequency waves prove to have wavelengths of ∼4 nm, i.e., as in soft x-ray. The propagation losses of these waves are moderately high. We propose to develop the optics (the optical transformations — deviation, focusing, photonic crystals, etc.) of these waves on thin metal layers integrated into nanodevices. In particular, we calculated the probability of spontaneous emission of a photon by an atom (molecule) into the surface plasmon of a nanoparticle. This probability proved to be increased by many orders of magnitude. This work interprets experiments that show a higher (14 orders of magnitude and more) probability of spontaneous Raman scattering of a molecule on the surface of a silver nanoparticle. The molecule is in the field of a surface plasmon, owing to which the local field and density of states of the field prove to be increased to such an extent as to give a rise of 12 or 13 orders of magnitude. An additional increase by one or two orders of magnitude is due to the antenna effect of a pair of nanoparticles, one of which is extremely small and the other is sufficiently large to serve as an efficient transceiver antenna. The possibility of developing sources of light pulses of exceptionally short duration arises.     

Surface solitons at the interface between semi-infinite linear and thermal nonlinear optical media

We investigate numerically surface-wave solitons occurring at the interface between semi-infinite linear and thermal nonlinear optical media, with the refractive index of the linear medium being greater than that of the nonlinear medium (in the absence of light). We find that the threshold energy flows of the existence of the surface solitons depend on the linear refractive index difference of the two media. Their fitting empirical formula has been obtained. Furthermore, we elucidate that the optical beams propagating in thermal nonlinear optical media, either as a single surface soliton or as a dipole surface soliton, can be attracted to the surface, even when launched from far away.     

Fundamentals of nonlinear optical materials

In this article, we briefly review the fundamental aspects of nonlinear optical materials and their role in modern communication.