Long range plasmon assisted all-optical switching at telecommunication wavelengths

We exploit the properties of ultranarrow, Fano-like resonances generated by the coupling of long range surface plasmons with ultrathin (～10?nm), metallic, subwavelength gratings embedded in a nonlinear, cubic material to obtain all-optical switching at telecommunication wavelengths for extremely low input power. We provide an example of a silver metallic grating embedded in a chalcogenide glass (As₂S₃), and we show the concrete possibility to achieve all-optical switching at local field intensities compatible with the photo-darkening threshold of the material.     

All-optical switching at the Fano resonances in subwavelength gratings with very narrow slits

We theoretically discuss all-optical switching at the Fano resonances of subwavelength gratings made of a chalcogenide glass (As(2)S(3)). Particular attention is devoted to the case in which the grating possesses extremely narrow slits (channels ranging from a～10 nm to a～40 nm). The remarkable local field enhancement available in these situations conspires to yield low-threshold switching intensities (~50 MW/cm(2)) at telecommunication wavelengths for extremely thin (d～200 nm) gratings when a realistic value of the As(2)S(3) cubic nonlinearity is used.     

Asymmetric Fano resonance and bistability in a two-ring resonator optical switch with high extinction ratio and low switching threshold

We propose a two-ring resonator configuration that can provide optical switching with both high extinction ratio (ER) and low switching threshold. The achievable input threshold is n ₂I_IN ~ 10^?5 or smaller, which is one order lower than that of the conventional one-ring configuration, while maintaining an ER of at least 10dB over a 10-GHz (0.1 nm) optical bandwidth. This performance is enabled by using an asymmetric Fano resonance as opposed to the usual symmetric resonance. The low switching threshold and the high ER are related to the sharpness and the asymmetricity of the Fano resonance, respectively.     

Apodization‐Assisted Subdiffraction Near‐Field Localization in 2D Phase Diffraction Grating

Conventional phase diffraction gratings can be used to localize the incoming optical radiation in the near‐field region. A new design of the binary phase diffraction grating is proposed with embedded pupil opaque mask inside each stripe. By means of numerical simulations, it is shown that with this masked phase grating the spatial resolution of the near‐field localization can be substantially improved and brought even beyond the solid immersion limit (λ/2n). Moreover, due to anomalous apodization effect, the subdiffraction field localization is accompanied by intensity enhancement as compared to the non‐masked design. The pupil mask rearranges the optical fluxes within the stripes and promotes the Fano resonances excitation in the periodic step lattice. This can be important for advancing the phase grating‐based super‐resolution technologies, including subdiffraction imaging, interferometry, and surface fabrication.     

Analysis of the phase lapse problem in closed interferometers

We investigate the connection between the asymmetry of the Fano resonances in a mesoscopic interferometer with an embedded quantum dot and the π lapses in the phase of the “bare” dot transmittance. Consecutive Fano resonances with the same (opposite) sign of the Fano parameter imply the presence (absence) of a phase lapse with π between the corresponding resonances of the dot. Our results suggest that the famous “phase lapse” problem, first reported by Schuster et al. [R. Schuster, E. Buks, M. Heiblum, D. Mahalu, V. Umansky, H. Shtrikman, Nature 385 (1997) 417], can therefore be experimentally addressed in closed interferometers. It is also proposed that the Fano effect can be used to extract the phase distributions of the eigenfunctions for a mesoscopic 2D shape, via the parity of the resonances. In the presence of electron–electron interaction, one can calculate the phases of the T-matrix elements. The numerical results lead to the same conclusions as for the non-interacting case.     

Photoinduced phenomena in amorphous As<Subscript>4</Subscript>S<Subscript>3</Subscript>Se<Subscript>3</Subscript>–Sn films

We investigate the kinetics of photodarkening and recording of holographic diffraction gratings in amorphous As₄S₃Se₃ thin-film structures doped with tin (Sn) in concentrations of 0–10 at %. It is established that an increase in the Sn concentration leads to a decrease in the photodarkening rate and degree. The photodarkening kinetics is approximated by a stretched exponential function. It is found that an increase in the Sn concentration leads to a decrease in the transmission (photodarkening) variation in the investigated As₄S₃Se₃–Sn films. It is determined that, in the recording of holographic diffraction gratings at a Sn concentration of 3–8 at %, the As₄S₃Se₃–Sn films exhibit the maximum sensitivity and diffraction efficiency of the recorded gratings. It is shown that the dependence of diffraction efficiency on the As₄S₃Se₃ film thickness has the maximum at a film thickness of 4 µm.     

Novel non-linear optical techniques for diagnostics: laser-induced gratings and two-colour four-wave mixing

Electrostriction, population transfer caused by absorption of laser energy and subsequent thermalization are the most important mechanisms responsible for the formation of laser-induced gratings. Applying time resolved light diffraction from electrostrictive gratings allows for the measurements of thermodynamic properties like temperature, gas composition, and flow velocity. Thermal gratings can be employed as a very sensitive spectroscopic tool.The Two-Colour Four-Wave Mixing (TCFWM) signal formed by a non-resonant pump laser and a resonant probe laser in an acetylene/air diffusion flame comprises of contributions due to soot and C₂, the latter is generated by vaporising the soot. Profile measurements across the flame have shown that the signal composition changes within the flame.Intermediate level labelling by Two-Colour Resonant Four-Wave Mixing Spectroscopy (TC-RFWM) is applied to reduce the spectral complexity of molecular spectra. In addition, we observe extra resonances forbidden by a conventional three-level scheme in OH- and NH-containing flames. A detailed investigation shows that the newly observed TC-RFWM resonances are induced by collisional energy transfer within the population, alignment and orientation gratings formed in both electronic states that are coupled to the laser fields.     

Observation of Fano-Type Interference in a Coupled Cavity-Atom System

We present the experimental observation of the Fano-type interference in a coupled cavity-atom system by confining the laser-cooled ~(85)Rb atoms in an optical cavity.The asymmetric Fano profile is obtained through quantum interference in a three-level atomic system coherently coupled to a single mode cavity field.The observed Fano profile can be explained by the interference between the intra-cavity dark state and the polariton state of the coupled cavity-atom system.The possible applications of our observations include all-optical switching,optical sensing and narrow band optical filters.     

基于激发态吸收的C60分子全光开关特性研究

用５３２．０ｎｍ的脉冲Ｎｄ：ＹＡＧ激光作为泵浦光，７４７．０ｎｍ的连续半导体激光作为探测光，在Ｃ₆₀甲苯溶液中，利用Ｃ₆₀分子的三重态－三重态非线性吸收效应，获得Ｃ₆₀分子的全光开关和全光调制特性，并用速率方程进行了动力学数值模拟，得到与实验相一致的结果。 关键词：     

Long period gratings and rocking filters written with a CO2 laser in highly-birefringent boron-doped photonic crystal fibers for sensing applications

In this work, we demonstrate the possibility of fabricating short-length long-period gratings and rocking filters in highly birefringent Photonic Crystal Fiber using a CO₂ laser. In our experiments both kinds of gratings were made in the same Boron doped highly birefringent PCF using similar exposure parameters. We also present the sensing capabilities of both fabricated gratings to temperature, strain and hydrostatic pressure by interrogation of the wavelength shifts at different resonances.     

基于硅基光子器件的Fano共振研究进展

硅基光子技术的发展为新型微纳光学功能器件和片上系统提供了高可靠、高精度的实现手段.采用硅基光子技术构建的具有连续(准连续)模式微腔与离散模式的微腔耦合产生的Fano共振现象得到了广泛关注.Fano共振光谱在共振波长附近具有不对称且尖锐的谐振峰,传输光的强度在共振波长附近从0突变为1,该机制可显著提高硅基光开关、探测器、传感器,以及光非互易性全光信号处理的性能.本综述分析了Fano共振的一般数学表述,总结了当前硅基光子微腔耦合产生Fano共振的理论模型研究现状,讨论了不同类型硅光器件实现Fano共振的方法,比较各种方案优劣及适用场合,梳理了Fano共振在全光信号处理方面的应用研究情况.最后探讨存在的一些问题及未来可能的相关研究方向.     

Optical Fiber Excitation of Fano Resonances in a Silicon Microsphere

In this article, Fano lineshape whispering gallery modes were observed in the light scattering spectrum of a silicon microsphere in near-infrared telecommunication wavelengths. A simple model is presented to explain the transition from Lorentzian lineshape to the Fano lineshape resonances with the coupled-mode theory of multiple whispering gallery modes. Polar mode spacing of 0.23 nm is observed in the spectra, which correlates well with the calculated value. The quality factor of the Lorentzian and Fano resonances are on the order of 10⁵. By using an appropriate interface design for the microsphere coupling geometries, Fano lineshape optical resonances herald novel device applications for silicon volumetric lightwave circuits.     

Shaping and switching of polychromatic light in arrays of periodically curved nonlinear waveguides

We overview our recent theoretical results on spatio-spectral control, diffraction management, and broadband all-optical switching of polychromatic light in periodically curved one and two dimensional arrays of coupled optical waveguides. In particular, we show that polychromatic light beams and patterns produced by white-light and supercontinuum sources can experience wavelength-independent normal, anomalous, or zero diffraction in specially designed structures. We also demonstrate that in the nonlinear regime, it is possible to achieve broadband all-optical switching of polychromatic light in a directional waveguide coupler with special bending of the waveguide axes. Our results suggest novel opportunities for creation of all-optical logical gates and switches which can operate in a very broad frequency region, e.g., covering the entire visible spectrum. Presented at 9-th International Workshop on Nonlinear Optics Applications, NOA 2007, May 17–20, 2007, Świnoujście, Poland     

Collapse of resonance in quasi-one-dimensional quantum channels

We study the resonance structure of the conductance (transmissivity) of a quasi-one-dimensional channel that contains an attractive impurity of finite dimensions and derive an exact expression for the scattering matrix. We show that an impurity of finite dimensions may cause a set of Fano resonances to appear in the transmissivity. We also find that due to the coherent interaction the Fano resonances can collapse and discrete levels may appear in the continuum. Finally, we establish the wave function of the discrete levels and study the channel transmissivity in the critical regime. Zh. éksp. Teor. Fiz. 116, 263–275 (July 1999)     

Transmission gaps,trapped modes and Fano resonances in Aharonov–Bohm connected mesoscopic loops

A simple mesoscopic structure consisting of a double symmetric loops coupled by a segment of length $d_0$ in the presence of an Aharonov–Bohm flux is designed to obtain transmission band gaps and Fano resonances. A general analytical expression for the transmission coefficient and the density of states (DOS) are obtained for various systems of this kind within the framework of the Green's function method in the presence of the magnetic flux. In this work, the amplitude of the transmission and DOS are discussed as a function of the wave vector. We show that the transmission spectrum of the whole structure may exhibit a band gap and a resonance of Fano type without introducing any impurity in one arm of the loop. In particular, we show that for specific values of the magnetic flux and the lengths of the arms constituting the loops, the Fano resonance collapses giving rise to the so-called trapped states or bound in continuum (BIC) states. These states appear when the width of the Fano resonance vanishes in the transmission coefficient as well as in the density of states. Also, we show that the shape of the Fano resonances and the width of the band gaps are very sensitive to the value of the magnetic flux and the geometry of the structure. These results may have important applications for electronic transport in mesoscopic systems.     

Optimization of near-field optical field of multi-layer dielectric gratings for pulse compressor

Multi-layer dielectric (MLD) gratings for pulse compressors in high-energy laser systems should provide high diffraction efficiency as well as high laser induced damage thresholds (LIDT). Nonuniform optical near-field distribution is one of the important factors to limit their damage resistant capabilities. Electric field distributions in the gratings and multi-layer film region are analyzed by using Fourier modal method. Optimization of peak electric field in the gratings ridge is performed with a merit function, including both diffraction efficiency and electric field enhancement when the top layer material is HfO₂ and SiO₂, respectively. A set of optimized gratings parameters is obtained for each structure, which reduce the peak electric field within the gratings ridge to being respective 1.39 and 1.84 times the value of incident light respectively. Finally, we also discuss the effects of gratings refractive index, gratings sidewall angle and incident angle on peak electric field in the gratings ridge.     

Doubly excited 2s2p 1,3P1 resonances in photoionization of helium

The multi-configuration Dirac-Fock（MCDF） method is implemented to study doubly excited 2s2p ^1,3P₁ resonances of the helium atom and the interference between photoionization and photoexcitation autoionization processes.In order to reproduce the total photoionization sprectra,the excited energies from the ground 1s^{2 1}S₀ state to the doubly excited 2s2p ^1,3P₁ states and the relevant Auger decay rates and widths are calculated in detail.Furthermore,the interference profile determined by the so-called Fano parameters q and ρ² is also reproduced.Good agreement is found between the present results and other available theoretical and experimental results.This indeed shows a promising way to investigate the Fano resonances in photoionization of atoms within the MCDF scheme,although there are some discrepancies in the present calculations of the 2s2p ³P₁ state.     

Laser gain in femtosecond microstructured Nd:MgO:LiNbO3 crystals

Femtosecond laser writing has been used to create 1D and 2D diffracting microstructures in Nd³⁺:MgO co-doped LiNbO₃ crystals. The main characteristics of the diffracting device, such as its polarization dependence and diffraction efficiency have been investigated, showing that first order diffraction efficiencies in excess of 35% can be achieved. We have demonstrated that continuous wave laser oscillation from Nd³⁺ ions at 1.06 μm can be generated through the integrated diffraction gratings, thus leading to non-collinear intracavity laser propagation. The laser performance, in terms of laser thresholds and slope efficiencies, is reported and explained. The achievement of a controlled non-collinear propagation is an interesting feature in multi-frequency lasers, optical communication and optical switching. PACS 42.55.Rz; 42.70.Hj; 42.25.Fx     

Surface relief diffraction gratings written on <Emphasis Type="BoldItalic">β</Emphasis> -BaB<Subscript>2</Subscript>O<Subscript>4</Subscript> crystal by femtosecond pulses

Surface relief diffraction gratings were written at the entrance surface of a β-BaB₂O₄ (BBO) crystal under irradiation with femtosecond laser pulses. Probe–beam diffraction and atomic force microscopy (AFM) were employed to characterize the diffraction properties and the microstructures of the gratings. The maximal first-order diffraction efficiency of these gratings is approximately 63%. The surface periodic structure of the material induces noncollinear propagation of the fundamental input beam in the crystal. Using this diffractive optical element, efficient generation of multiple second-harmonic beams was achieved. PACS 42.79.Dj; 42.40.Lx; 42.65.–k     

Line shape parameters study of the 6p7p (<Superscript>1</Superscript>P<Subscript>1</Subscript>, <Superscript>3</Superscript>D<Subscript>1</Subscript> and <Superscript>3</Superscript>P<Subscript>1</Subscript>): Autoionizing resonances in barium

We report a systematic line shape analysis study of the 6p7p configuration based ¹P₁, ³D₁ and ³P₁ autoionizing resonances in barium using a Nd:YAG pumped dye laser system in conjunction with a thermionic diode ion detector. The even parity isolated autoionizing resonances have been approached via four intermediate states 6 snp ¹P₁ (6 ≤n ≤8) and 5d6p ¹P₁. A comparison of the Fano parameters of the resonance profiles reveals that the width of an autoionizing resonance is independent of the excitation path while the line profile parameter changes with the selection of different intermediate states.