Perfect coupling of light to surface plasmons by coherent absorption

We show theoretically that coherent light can be completely absorbed and transferred to surface plasmons in a two- or three-dimensional metallic nanostructure by exciting it with the time-reversed mode of the corresponding surface plasmon laser ("spaser"). The narrow-band perfect absorption is a generalization and application of the concept of critical coupling to a nanocavity with surface plasmon resonances. Perfect coupling of light to nanostructures has potential applications to nanoscale probing as well as background-free spectroscopy and ultrasensitive detection or sensing.     

Total light absorption in a wide range of incidence by nanostructured metals without plasmons

Metals structured by nanocavities have recently been demonstrated to efficiently absorb light in a wide range of angles of incidence. It has been assumed that nanovoid plasmons are at the origin of the strong absorption. It is shown that it is possible to totally absorb incident light without plasmons. To avoid their excitation, a diffraction grating consisting of cylindrical cavities in a metallic substrate is illuminated in transverse electric polarization. It is found that cylindrical cavities can sustain cavity resonances with a high enhancement of the light intensity, provoking a total absorption of light in a wide range of incidence.     

The effect of gain and absorption on surface plasmons in metal nanoparticles

The compensation of loss in metal by gain in interfacing dielectric has been demonstrated in a mixture of aggregated silver nanoparticles and rhodamine 6G dye. An increase of the quality factor of surface plasmon (SP) resonance was evidenced by the sixfold enhancement of Rayleigh scattering. The compensation of plasmonic losses with gain enables a host of new applications for metallic nanostructures, including low- or no-loss negative-index metamaterials. We have also predicted and experimentally observed a suppression of SP resonance in metallic nanoparticles embedded in dielectric host with absorption. PACS 61.46.Df; 73.20.Mf; 78.67.Bf     

Resonant surface plasmons of a metal nanosphere treated as propagating surface plasmons

On the assumption that the resonant surface plasmons on a spherical nanoparticle are formed by standing waves of two counter-propagating surface plasmon waves along the surface, by using Mie theory simulation, we find that the dispersions of surface plasmon resonant modes supported by silver nanospheres match with those of the surface plasmons on a semiinfinite medium-silver interface very well. This suggests that the resonant surface plasmons of a metal nanosphere can be treated as a propagating surface plasmon wave.     

Photoprocesses on a surface of nanoporous quartz under resonant laser radiation

The photoexcitation of iodine molecules on surfaces of solid (nonporous) and nanoporous quartz by resonant laser radiation in the visible region has been studied. We have detected and studied the high-energy photodesorption of iodine molecules with a translational energy of 1.4 to 1.8 eV from nanoporous quartz surfaces at an exciting photon energy ranging between 1.9 and 2.3 eV, as well as the nonequilibrium surface dissociation of molecules. Unlike the photoprocesses, which are observed only on the surface of nanoporous quartz, the thermal desorption of I₂ molecules with a considerably lower kinetic energy has also been detected on the surface of solid quartz. We have suggested a physical mechanism of photodesorption, under which electronic excitation of an iodine molecule in the confined volume of a nanopore is accompanied by a Franck-Condon electronic transition of a molecule-surface complex to a state with a higher potential energy and subsequent release of this energy in the form of kinetic energy. It has been concluded that photoprocesses on a nanostructured surface are radically different from ordinary surface photoprocesses. Zh. éksp. Teor. Fiz. 114, 114–124 (July 1998)     

Optical absorption by surface plasmons in deep sub-wavelength channels

The interaction of visible light with surface plasmons in a periodic array of silver channels is studied experimentally and theoretically. Channels of width 130 nm and depth 280 nm on a pitch of 468 nm were fabricated using electron beam lithography. Measurements of the reflectance of the channels show strong absorption for a specific angle of incidence at a given wavelength of the light. It is shown using a simple geometric model that this is consistent with the excitation and propagation of surface plasmons in the deep sub-wavelength channels. A numerical simulation of the electromagnetic fields excited in the channels confirms this result.     

Infrared absorption by surface phonons and surface plasmons in small crystals

A continuum model is developed which describes the infrared absorption of interacting spheres very much smaller than the wavelength of light. A mode with uniform polarization inside the sphere is assumed responsible for the absorption. The frequency of this mode in a small isolated sphere can be greatly shifted from the bulk value due to the surface polarization charge. If the particles are not isolated, this frequency is also influenced by the dipolar interaction of neighboring particles. We find that it is possible to define an average dielectric function for a layer of small interacting, absorbing spheres which has the Lorentz form for either phonon or plasmon excitations. If both plasmon and phonon excitations are possible in the same particle, the absorption coefficient of the coupled system displays two resonances and an antiresonance.     

Physisorption on a metal surface probed by surface state resonant second harmonic generation

Second harmonic generation from a Ag(1 1 0) surface, resonantly enhanced by the surface state transition at 1.74 eV, is found to be greatly affected by submonolayer adsorption. The physisorption of water or methanol causes a monotonic, exponential-like decay of the SH intensity that can be described by a model treating the adsorbate as a delocalized, weak perturbation in the resonantly enhanced SHG. On the other hand weak chemisorption of aniline generates a complex response in the SH intensity that eludes the predictability of the model. Analysis of the SH intensity has determined that water or methanol adsorption causes an upward shift in the minimum energies of the pair of surface states on Ag(1 1 0) and an increase in the transition linewidth. The sensitive response of the surface states to the presence of adsorbates provides a basis for SHG resonantly enhanced by surface state transition as a highly sensitive probe of submonolayer coverage.     

Particle-in-cell investigation on the resonant absorption of a plasma surface wave

The resonant absorption of a plasma surface wave is supposed to be an important and efficient mechanism of power deposition for a surface wave plasma source. In this paper, by using the particle-in-cell method and Monte Carlo simulation, the resonance absorption mechanism is investigated. Simulation results demonstrate the existence of surface wave resonance and show the high efficiency of heating electrons. The positions of resonant points, the resonance width and the spatio-temporal evolution of the resonant electric field are presented, which accord well with the theoretical results. The paper also discusses the effect of pressure on the resonance electric field and the plasma density.     

Making metals transparent for white light by spoof surface plasmons

From first-principles computations we reveal that metallic gratings consisting of narrow slits may become transparent for extremely broad bandwidths under oblique incidence. This phenomenon can be explained by a concrete picture in which the incident wave drives free electrons on the conducting surfaces and part of the slit walls to form spoof surface plasmons (SSPs). The SSPs then propagate on the slit walls but are abruptly discontinued by the bottom edges to form oscillating charges that emit the transmitted wave. This picture explicitly demonstrates the conversion between light and SSPs and indicates clear guidelines for enhancing SSP excitation and propagation. Making structured metals transparent may lead to a variety of applications.     

Far-wing light absorption induced by resonant or near-resonant collisions

Summary We have studied the absorption of light induced by a resonant or near-resonant collision between two atoms. The calculations have been performed by taking into account also the magnetic sublevels, which makes our theoretical predictions more applicable to realistic cases. Analytical expressions for the far-wing absorption cross-section have been obtained.     

Life-time of surface plasmons in small metal particles

Existing theories concerning the life-time of surface plasmons in small metal particles fail to explain recent experimental data for silver. Therefore, two of these theories were critically re-examined, and some numerical errors were found. The semi-classical approach by Lozovik et al. yields, after correction, physically reasonable results, which are nearly identical with theoretical results by Kreibig. The corrected RPA-result by Lushnikov et al. is in quantitative agreement with recent experimental data for silver.     

Modified method of surface plasmons in metal superlattices

We present a modified method to solve the surface plasmons(SPs) of semi-infinite metal/dielectric superlattices and predicted new SP modes in physics. We find that four dispersion-equation sets and all possible SP modes are determined by them. Our analysis and numerical calculations indicate that besides the SP mode obtained in the original theory, the other two SP modes are predicted, which have either a positive group velocity or a negative group velocity. We also point out the possible defect in the previous theoretical method in accordance to the linear algebra principle.     

Optical generation of terahertz plasmons on comb-shaped surface of metal

The possibilities of a recently proposed (Opt. Express 17, 9323 (2009)) method for generating terahertz surface plasmons on a microstructured (comb-shaped) metal surface using a nonlinear polarization pulse that moves with a superluminal velocity and is induced by an ultrashort laser pulse in a strip of electrooptic material deposited on the surface are theoretically studied. For an arbitrary direction of motion of the nonlinear source along the comb-shaped surface, fields of excited terahertz plasmons and the angular spectral distribution of the radiated energy are calculated. It is shown that the spectral and energy characteristics of plasmons can be efficiently controlled by varying the direction of motion of the source. Conditions (parameters of the comb-shaped structure, direction and velocity of motion of the source) that ensure the maximal efficiency of the optical-to-terahertz conversion are found. The developed method of terahertz generation is promising for surface terahertz spectroscopy.     

周期排列的电介质小球所诱发的金属-电介质表面上的表面等离子激元的光学性质

研究了金属板的上下表面附近各放置一层按周期排列的电介质小球的体系的光学性质.用多重散射法计算的结果显示金属上侧的周期性排列的电介质小球可诱发金属-电介质表面上的表面等离子激元.这些表面等离子激元的存在可通过非常尖锐的吸收峰反映出来.对于无限厚的金属板，这些吸收峰的峰值位置主要与电介质小球的周期有关，且与解析理论符合得相当好.在有限厚度的金属板中，金属板的两侧表面会产生对称和反对称的两种表面等离子激元，从而使原来在无限厚的金属表面上所出现的单一频率的表面等离子激元劈裂为双频率.由于对称和反对称的表面等离子激元模式在金属板的两侧表面均有相当强的电磁场，因而它们可导致强的电磁波穿透.通过在金属板的下侧加入玻璃球层可将表面等离子激元的电磁场引导出金属，并产生透射波.用多重散射法计算的结果证实，在此体系中由表面等离子激元所引起的透射可达到相当的强度. 对该体系中的物理机理进行了详细分析，从而能够通过调节该体系中的一些参数来控制表面等离子激元出现的频率，使强吸收峰或强透射峰出现在所希望的频率上. 关键词： 表面等离子激元 吸收谱 透射     

Ultrafast control of grating-assisted light coupling to surface plasmons

We demonstrate subpicosecond control over the coupling of free-space radiation to surface-plasmon polaritons using 830 and 500 nm period gold gratings. Thermal changes to the electron distribution following irradiation by 100 fs, 810 nm pulses produce a shift of the 570 nm plasmon resonance by ~0.75 nm with reflectivity change up to 6% and decay time of ~1 ps.     

Red shift of surface plasmons in small metal particles

It is shown that the experimentally found red shift of the surface plasmon resonance in a small metal particle is a direct measure of the first moment of the induced surface charge.     

Terahertz-pulse emission through laser excitation of surface plasmons in a metal grating

The second-order processes of optical-rectification and photoconduction are well known and widely used to produce ultrafast electromagnetic pulses in the terahertz frequency domain. We present a new form of rectification that relies on the excitation of surface plasmons in metal films deposited on a shallow grating. Multiphoton ionization and ponderomotive acceleration of electrons in the enhanced evanescent field of the surface plasmons results in a femtosecond current surge and emission of terahertz electromagnetic radiation. Using gold, this rectification process is third or higher-order in the incident field.     

Resonant tunneling mediated by resonant emission of intersubband plasmons

We study tunneling through resonant tunneling diodes (RTD) with very long emitter drift regions (up to 2 microm). In such diodes, charge accumulation occurs near the double barrier on the emitter side, in a self-induced potential pocket. This leads to a substantial enhancement of the wave function overlap between states of the pocket and the RTD, and, consequently, to increased off-resonant current mediated by various scattering processes. For RTD with the longest drift region (2 microm), an additional strong current peak is observed between the first and the second resonant peaks. We attribute this pronounced feature to the intersubband transitions mediated by resonant emission of intersubband plasmons.