High-contrast Switching Research Articles

Dynamics of all-optical switching in polymethine dye molecules

All-optical switching has been analyzed in the recently reported Polymethine dye (2-[2-[3-[(1,3-dihydro-3,3-dimethyl-1-phenyl-2H-indol-2-ylidene)ethylidne]-2-phenyl-1-cyclohexene-1-yl] ethenyl]-3,3-dimethyl-1-phenylindolium perchlorate) (PD3) [O.V. Przhonska, D.J. Hagan, E. Novikov, R. Lepkowicz, E.W.V. Stryland, M.V. Bondar, Y.L. Slominsky, A.D. Kachkovski, Chem. Phys. 273 (2001) 235.] that exhibits large excited-state absorption, using the rate equation approach, to achieve high contrast and fast switching. The transmission of a cw probe laser beam ( I p) at 532 nm through PD3 dissolved in (i) ethanol and (ii) polyurethane acrylate (PUA), is switched by a pulsed pump laser beam at 532 and 650 nm, respectively, which excite molecules from the ground state. The theoretical results show good agreement with the reported experimental results for case (i). The switching characteristics have been shown to be sensitive to variation in concentration, pump pulse width (Δ t), peak pumping intensity ( I m0′), absorption cross-section of the excited-state at 532 nm and lifetime of excited-state ( τ 1). The same switching contrast can be achieved at relatively lower pump powers at 650 nm for case (ii). It is shown that there is an optimum value of concentration at which maximum modulation can be achieved. We can get 96% modulation of I p at I m0′=1 GW/cm 2 at 650 nm, with Δ t=30 ps and concentration of 0.14 mM in PUA, resulting in switch off and on time of 95 ps and 18 ns, respectively. The results have also been used to design all-optical NOT and the universal NOR and NAND logic gates with multiple pump laser pulses, which are the basic building blocks of computing circuits.

Dynamics of all-optical switching in <inline-formula><math display="inline" overflow="scroll"><msub><mrow><mrow><mstyle mathvariant="normal"><mtext>C</mtext></mstyle></mrow></mrow><mrow><mn>6</mn><mn>0</mn></mrow></msub></math></inline-formula> and its application to optical logic gates

The dynamics of all-optical switching in C60 has been theoretically analyzed based on the principle of nonlinear intensity-induced excited-state absorption to achieve high contrast and fast switching. The transmission of a cw probe laser beam at 885 nm corresponding to the peak absorption of the S1 state through C60 in toluene is switched by a pulsed pump laser beam at 532 nm that excites molecules from the ground state. Switching characteristics are shown to be sensitive to the variation of pump pulse width, peak pumping intensity, thickness of the medium, intersystem crossing time between the S1 and T1 states and the absorption cross section of the S1 state at a probe wavelength, and their effect are analyzed in detail. It is shown that the transmission of the probe beam can be completely switched off (100% modulation) by the pump beam at a pump intensity of 100 MW/cm2 and a pulse width 1.5 ns, with switch on and off times of 2.5 and 7.5 ns, respectively. The results are used to design all-optical NOT and the universal NOR and NAND logic gates with multiple pump laser pulses, which are the basic building blocks of computing circuits.

High-contrast all-optical bistable switching in photonic crystal microcavities

We present a bistable photonic crystal configuration consisting of a waveguide sided coupled to a single-mode cavity with instantaneous Kerr nonlinearity. We show that such a configuration can generate extremely high contrast between the bistable states in its transmission with low input power. We also provide an analytic theory that can completely account for the entire transient switching dynamics, as revealed by finite difference time domain simulations.

Wavelength conversion of 1.53-microm-wavelength picosecond pulses in an ion-implanted multiple-quantum-well all-optical switch.

An ultrafast high-contrast all-optical switch produced from a metal-organic vapor phase epitaxy-grown wafer incorporating a 50-period InGaAsP/InGaAsP multiple-quantum-well (MQW) saturable absorber (SA) and a distributed Bragg reflector is described. Postgrowth implantation with 4-MeV nitrogen ions reduces the MQW free-carrier lifetime, and hence the switch recovery time, to 5.2 ps. Incorporation of the MQW SA in an optical cavity results in switching contrast ratios greater than 10 dB. The all-optical switch is used to perform wavelength conversion of 2-ps pulses.

MORPHOLOGY CONTROL OF LIQUID CRYSTALLINE COMPOSITE GELS BASED ON MOLECULAR SELF-ASSEMBLING KINETICS

Liquid crystalline composite gels consisting of a low molecular weight gelator and a low molecular weight liquid crystal were prepared by two types of gelation method: (1) a continuous cooling method and (2) an isothermal gelating method, which provide different molecular self-assembling kinetics of the low molecular weight gelator in gelation processes. Optical microscopic and atomic force microscopic studies revealed that numerous fine strands of the one-dimensionally assembled low molecular weight gelators were formed in the composite gels in the case of the continuous cooling method as well as that of the isothermal gelating method. However, the thinner strands were more homogeneously dispersed in the composite gel prepared via the isothermal gelation at an appropriate temperature than those of the continuous cooling method. This difference on the dispersion state of the strands was shown, by polarizing optical microscopic observation, to have a significant influence on the molecular alignment state of the low-molecular-weight liquid crystal in theliquid crystalline composite gel. The electro-optical response, the light scattering-transmitting switching, of the liquid crystalline composite gel to an applied electric field was drastically dependent on the morphology of the gelators. High-contrast light switching was achieved for the composite prepared by the isothermal gelation. The response time of the electro-optical switching was less than 100 w s under 30 V rms .

Design aspects of low power polymer/OLED passive-matrix displays

Polymer and small molecule organic LED technologies offer many attractive properties for use in a display. The highly efficient light generation at low supply voltages and the limited thickness of the display are advantageous for mobile applications. Furthermore, the large viewing angle, high contrast and fast switching speed give excellent picture quality for text and video mode operation. Optimal uniformity of the passive-matrix display is achieved with current-driven operation. This reduces the influence of material degradation as well as voltage drops across the connection leads. Amplitude and pulse width modulation can be used to obtain grey levels. This presentation discusses design aspects of small-size, full-colour passive-driven polymer LED matrix displays. Consequences of multiplexing, colour sub-pixelation, aperture and display parasitics are analysed and requirements for the RGB materials are formulated to obtain low power dissipation of the module. The efficiency of red polymers, that are available up to now, is too low. This leads to an unbalanced current density in comparison with green and blue which is unfavourable for the display power consumption.

All-optical switching based on cross-phase modulation in microstructure fiber

In this letter, we demonstrate all-optical switching of picosecond pulses between the output ports of a microstructure-fiber-based polarization Sagnac interferometer. High contrast switching of 2.6-ps FWHM signal pulses due to cross-phase modulation induced by 4.9-ps pump pulses in a 5.8-m long microstructure fiber is achieved at wavelengths near 1550 and 780 nm. The spectral and temporal behavior of the switching device are investigated and compared with numerical simulations based on coupled-wave theory.

Electrochemical and electrochromic properties of poly(ether naphthalimide)s and related model compounds

The electrochemical and electrochromic properties of poly(ether naphthalimide)s were studied and compared with related model compounds. The electroactive sites within the poly(ether naphthalimide)s were identified to be ketone and imide groups. The stepwise coloration process, colorless–red–dark blue corresponding to the neutral, radical anion and dianion species, was found to be related to the first reduction of the imide group and the second reduction of the ketone group. Based on this new polyimide a high-contrast switch between colorless and red or colorless and dark blue can be achieved and controlled electrically.

Multiplexing performance of a FLC with high spontaneous polarization in the bookshelf structure

The bistability and multiplexing performance of a Ferroelectric Liquid Crystal with high spontaneous polarization in the bookshelf geometry has been investigated. Such an approach allows for a high contrast switching. The bistability sticking effect is found to be predominant in the static properties but is negligible in the multiplexing regime.

Monolithic waveguide smart pixels for high-contrast and high-gain all-optical switching

We demonstrate a novel and simple smart pixel which combines free space optics for the input signal and waveguide configuration for the output beam. Our monolithic smart pixels consist of a threshold switch with high photoconductive gain and a Franz–Keldysh waveguide modulator. The switch provides sufficiently high current to control directly the output beam via the modulator. A high switching contrast of ≳17 dB and a fan-out ≳230 at an optical output power of ≳130 μW has been achieved with our first monolithic smart pixels, which are still far from being optimized.

Low power electronic optical bistability in single quantum well InP/InGaAsP Fabry–Perot waveguide resonators

Optical bistability in a single quantum well waveguide resonator in the InP/InGaAsP material system is demonstrated. High contrast ratio (≳3:1) and switching at low power (≊1.5 mW) was obtained by using a single quantum well at a wavelength near the maximum nonlinear refractive index change and by depositing a reflective coating on the rear end of the waveguide. Clear optical bistability is observed in a wide wavelength range of 1480–1525 nm. Due to the low input power, cw operation of the device was possible at room temperature.

Picosecond cross-polarization beam coupling in semi-insulating Ga-doped CdTe at high fluences

We demonstrate the high-contrast polarization-rotation switching in semi-insulating CdTe:Ga using a 30-ps pump and probe pulses at a wavelength of 1.064 μm. It is found that the maximum switching contrast in the photorefractive cross-polarization beam coupling configuration is about eight times higher for CdTe:Ga compared to undoped GaAs at fluences above 10 mJ/cm2.

Polarization properties of a vertical cavity surface emitting laser using a fractional layer superlattice gain medium

We investigate the polarization properties of a vertical cavity surface emitting laser that uses an (Al0.5Ga0.5As)1/2(GaAs)1/2 fractional-layer superlattice (FLS) as an anisotropic gain medium. The anisotropy in the gain enables us to both control and switch the polarization state of the optically pumped lasing output. We obtain room-temperature lasing for wavelengths from 690 to 720 nm. The output is linearly polarized and the polarization direction is fixed, either parallel or perpendicular to the FLS layers. By tuning the cavity resonance wavelength, we demonstrate high contrast switching between two orthogonal linear polarization states in the FLS surface emitting laser.

Complete all-optical switching of visible picosecond pulses in birefringent fiber

Switching of visible picosecond pulses with a 50:1 contrast ratio is demonstrated in a time-division interferometry arrangement. The high-contrast switching of the signal is accomplished by uniform cross-phase modulation by an orthogonally polarized control pulse that walks through the signal during propagation in a strongly birefringent 10-m fiber.

Grating coupled liquid crystal waveguide

Abstract A waveguide consisting of a thin (< 1 μm) layer of nematic liquid crystal sandwiched between two parallel aligned gratings is described. Grating coupling to guided modes is demonstrated by monitoring the reflection and transmission from the cell as a function of incident angle. A reduced birefringence is seen compared to the bulk case which is explained in terms of disordered regions of liquid crystal close to the grating surfaces. The movement of guided modes under electrical bias is shown and time resolved experiments demonstrate high contrast switching with rise times as fast as 18 μs.

All optical switching of reflectivity and transmissivity by a nonlinear sandwich

A simple model system comprised of a thin nonlinear layer sandwiched between two glass plates is demonstrated. This nonlinear sandwich allows the investigation of both the transmitted and reflected beams for all-optical switching. With a thermal (negative) nonlinearity high-contrast switching is obtained within several nanoseconds. The authors also show for the first time, the avoidance of multiple thresholds and associated beam breakup for a nonsaturating nonlinearity for both the transmitted and reflected beams. This promises cascadability of all optical devices based on the nonlinear interface.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Polarisation bistability and high contrast switching in an isotrophic nonlinear cavity

A demonstration of all-optical polarisation bistability and high contrast switching in a nonlinear cavity fabricated entirely from isotropic materials is reported with the appropriate analysis. The polarisation dependence arises from the difference in phase change for each orthogonal polarisation on reflecting from a dielectric-metal interface at non-normal incidence.

Single-step switching at a nonlinear interface.

Theoretical predictions of the behavior of a light beam crossing a plane interface between a linear and nonlinear dielectric medium indicate staircased multiple threshold switching in nondiffusive media when an incident Gaussian beam instead of a plane wave is assumed. We report experimental findings showing unexpected high-contrast single-threshold switching, which may have important implications for optical signal processing. No evidence was found for the existence of bistable behavior indicated by previous experiments.

Pure phase bistability with a nonlinear slab waveguide

We describe the bistable behavior of a nonlinear triple-boundary slab waveguide. We show that, in addition to the previously reported reflected intensity bistability, this device exhibits bistability in the phase of the reflected beam. It is shown that pure phase signal bistability is possible. A system based on the Mach-Zehnder interferometric configuration is proposed to achieve a high-contrast bistable switch.