Nonlinear plasmonics is a growing field since the power threshold for observable nonlinear light emission of new frequencies can be lowered greatly due to dramatic electromagnetic field confinement. Along another research line, 2D electron gases (2DEGs) formed at interfaces of oxides have been drawing broader attention globally because the metallic constituents can be eliminated, and hence the inherent huge loss associated with uses of metals in plasmonic applications can be circumvented. Once the nonlinear materials are proximal to 2DEGs and surface plasmon polaritons (SPPs) are excited, the electromagnetic field can be strengthened several orders in magnitude. Consequently, the nonlinear processes can take place at a quite low incident light power. Considering much greater dispersion of SPPs, the second order nonlinear processes can be easily realized in terms of meeting phase matching conditions. In this paper, nonuniform 2DEG formed at the interface of a Z-cut Fe doped LiNbO3 (LN) slab and an indium-tin-oxide (ITO) thin film was analyzed with semi-classical Thomas Fermi screening model, and dispersion of index of refraction was given accordingly. A laser beam at 532 nm and a white light source illuminated the slab from the opposite directions collinearly and a remarkable light emission redistribution was observed with a continuous spectrum of short visible light peaked around 437 nm. Several confirming experimental results with ITO coated Y-cut slabs are presented and phase grating mediated SPP excitation is proposed to explain the related findings, suggesting that second order nonlinear processes strengthened by SPPs are behind the light emission redistribution.
Two dimensional electron gases (2DEGs) formed at interfaces of two oxides have been drawing growing attentions for their intriguing magnetic, 2D superconducting and optical properties. To investigate optically 2DEG formed at LiNbO3/indium-tin-oxide interface, the power of the very first reflection beam was monitored under illumination of one (two) laser beam(s). It was found the very first reflection can be reduced to as low as 1.13% from the original 12.9%, pointing unambiguously to a subwavelength coupling and corresponding to conservatively estimated exponential gain coefficient of -78525 cm-1 by taking half a wavelength as the coupling range, since the 1st reflection is dictated by what occurs in that range. Such high exponential gain coefficient, far beyond the reach of conventional photorefractive theoretical framework, is consistent with a physical picture of 2DEG supported interface plasmon polaritons. Such dramatic reflection reduction and corresponding high exponential gain coefficient are highly valuable in designing nanometric photonic devices, such as waveguides, attenuators (amplifiers), modulators and sensors, which are compatible to photonic circuits nowadays. In addition, such a material system is promising for nonlinear plasmonic applications.
With submicrometer thick photoconducting, semiconductor ZnSe thin films as interlayers between ITO glass and thin film of C60 doped mixture of polymer poly[N-vinylcarbazole] (PVK) and nematic liquid crystal (LC) 4,4’-npentylcyanobiphenyl (5CB), an updatable holographic recording medium was fabricated. When two laser beams were overlapped in the holographic recording medium, 2D diffraction patterns were seen, along with several interesting observations. The frequent sign changing of energy transferring between the two transmitted laser beams and large dynamic change in different diffraction orders implied complex processes of electric charge generation, transportation and compensation in the interfaces and within composite polymer film. Electrostatic modification based surface grating formation was proposed to explain all the findings.
With ZnSe thin films as aligning layers in fabricating liquid crystal (LC) panel with pentylcyanobiphenyl doped with C60, the response time in writing holograms was shortened to milliseconds. When two laser beams were overlapped in an LC panel, 2D diffraction patterns were observed, along with exponential gain coefficient highly LC and ZnSe thickness dependent. In addition, energy transferring in subwavelength scale through surface grating was evident. By using a hybrid LC panel, it was found the energy transferring direction was voltage polarity and thickness dependent. Electrostatic modification based surface plasmon polariton excitation was proposed to explain all the findings
Employing fast response, Cu-doped (K0.5Na0.5)0.2(Sr0.75 Ba0.25)0.9Nb2O6 (KNSBN) crystals in modifying ITO coating to
support visible surface plasmon polaritons (SPPs) to strengthen nonlinearity in KNSBN, we found: (1) a 2 dimensional
diffraction pattern was observed with only two writing beams; (2) The reflectivity on the very first surface was changed
2.3%, equivalent to 0.023 refractive index change; (3) 3.0% energy transferring to the reflection on the first surface was
measured, resulting unambiguously from energy transferring in subwavelength scale. All these results are consist with
our theoretical consideration based on phase grating mediation SPP excitation, which is promising in designing photonic
devices.
We have studied the photorefractive properties of a composite consisting of the polymer PVK doped with 4,4'- pentylcyanobiphenyl and C60. The material is homogeneous, stable, and does not require a plasticizer agent. Net gain coefficients over 100 cm-1 are observed. Measurements of orientational and electro-optic responses indicate that the orientational response is large, but markedly inhibited. We also report dramatic gain enhancement in two-beam coupling and novel angle and electric field dependencies in this composite.
We demonstrate the weakness of the real-time holographic associative memory implementation in addressing accuracy and describe a novel method to implement associative memory with accurate addressing. In this method, the memory images and their background images are stored in two common holographic memory systems, and the partial image and its background image are simultaneously used to address the two holographic memory systems. The experimental results have proved the system feasible. In addition, we describe a method of associative memory with accurate addressing in a common real-time holographic memory system using combinatorial memory images and combinatorial addressing images.
The mutually pumped phase conjugate reflection induced by a self-pumped phase conjugate reflection of another mutually coherent beam has been demonstrated in a photorefractive Cu- doped KNSBN crystal. The dependence of the mutually pumped phase conjugate reflectivity on the intensity ratio of the two incident beams and on the incident position of the signal beam was measured. We obtain phase conjugate reflectivities of greater than 90% at 488 nm.
We propose and demonstrate a novel method to improve the addressing accuracy of the holographic associative memory. The experimental results are given.
We describe an all-optical real-time holographic associative memory using doped lithium niobate (LiNbO3) crystals as hologram recording and phase-conjugate mirror elements, respectively, and we present the preliminary experimental results for associative storage. It is indicated that the system has high angular selectivity, spatial resolution, and a high degree of cross-talk suppression. The discussion is also given.
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