All-optical Transistor Action Research Articles

Efficient All-Optical Transistor Action in Short-Length Multimode Optical Fibers

The quest for realizing an all-optical transistor while fulfilling desired criteria has been around since long time. Various schemes have been put forward over the years to achieve it; however, almost all techniques make use of at least two different laser sources. Optical event horizon principle has also been employed to see transistor action in glass fibers using two different frequency sources. In this paper, we report an optical transistor action in short-length, tailor-made multimode optical fibers through a single pump only. The higher order mode properties of the fiber and intermodal nonlinear interaction (mainly cross-phase modulation) are used to achieve efficient transistor action. Both the soliton and a weak dispersive pulse are excited in $ LP_{02}$ and $ LP_{01}$ modes of the fiber, respectively, with a certain temporal delay set by the sign of the differential group delay between the modes. We believe that the scheme has potential to improve system performance owing to the fact that only a single-wavelength pump source is required as opposed to the optical-transistor devices available till date that essentially require two-color pumping.

Off-resonant activation of optical emission

Recent reports have identified a three-wave optically parametric mechanism for the active enhancement of fluorescence using off-resonant radiation. In this Letter it is shown by numerical simulation that the output of a laser system optically pumped just below threshold, can be strongly enhanced by this mechanism, using an ancillary beam of moderate intensity. The electrodynamics and kinetics of the nonlinear optical mechanism are analyzed, model calculations performed, and the output is illustrated graphically. The response demonstrates a novel method for achieving all-optical transistor action.

Coherent all-optical switching by resonant quantum-dot distributions in photonic band-gap waveguides

We study the detailed propagative characteristics of optical pulses in photonic band-gap (PBG) waveguides, coupled near resonantly to inhomogeneously broadened distributions of quantum dots. The line centers of the quantum-dot (QD) distributions are placed near a sharp discontinuity in the local electromagnetic density of states. Using finite-difference time-domain (FDTD) simulations of optical pulse dynamics and independent QD susceptibilities associated with resonance fluorescence, we demonstrate subpicosecond switching from pulse absorption to pulse amplification using steady-state optical holding and gate fields with power levels on the order of 1 milliwatt. In the case of collective response of QDs within the periodic dielectric microstructure, the gate power level is reduced to 200 microwatt for room temperature operation. In principle, this enables 200 Gbits per second optical information processing at wavelengths near 1.5 microns in various wavelength channels. The allowed pulse bandwidth in a given waveguide channel exceeds 0.5 THz allowing switching of subpicosecond laser pulses without pulse distortion. The switching contrast from absorption to gain is governed by the QD oscillator strength and dipole dephasing time scale. We consider dephasing time scales ranging from nanoseconds (low-temperature operation) to one picosecond (room-temperature operation). This all-optical transistor action is based on simple Markovian models of single-dot and collective-dot inversion and switching by coherent resonant pumping near the photon density of states discontinuity. The structured electromagnetic vacuum is provided by two-mode waveguide architectures in which one waveguide mode has a cutoff that occurs, with very large Purcell factor, near the QDs resonance, while the other waveguide mode exhibits nearly linear dispersion for fast optical propagation and modulation. Unlike optical switching based on Kerr nonlinearities in an optical cavity resonator, switching power levels and switching speeds for our QD device are not inversely proportional to cavity quality factors.

Resonance fluorescence in photonic band gap waveguide architectures: Engineering the vacuum for all-optical switching

We describe the spectral characteristics of the radiation scattered by two-level atoms (quantum dots) driven by a strong external field, and coupled to a photonic crystal radiation reservoir. We show that in the presence of strong variations with the frequency of the photonic reservoir density of states, the atomic, Mollow, sideband components of the scattered intensity can be strongly modified. Consequently, a weak optical probe field experiences a substantial differential gain in response to slight variations in the intensity of an optical driving field. We suggest that these effects may be of relevance to all-optical transistor action in photonic crystals. Using a specific photonic crystal heterostructure, we suggest that an all-optical microtransistor based on photonic crystals may operate at less than $100\phantom{\rule{0.3em}{0ex}}\text{nW}$ switching threshold power. Collective $N$-atom effects substantially enhance this optical switching effect. Near the switching threshold intensity, collective effects are manifest in the ${N}^{2}$ scaling of the intensity spectrum (reminiscent of superradiance). Above and below this critical region, the gain spectrum widens (linearly with $N$). This correspondingly reduces the switching time scales of the atomic system in response to external fields. Furthermore, the quantum degree of second-order coherence exhibits unusual features. Scattered photons display a variable degree of antibunching as function of driving laser field intensity and the photonic density of states discontinuity. We analyze the effects of the inhomogeneous atomic line broadening on the amplification process. We show, using suitable photonic density of states engineering, that it is possible to select a narrow spectral range around the central frequency of the atomic frequency distribution over which amplification and switching occur. This is done either by spectral decoupling of the active elements from the electromagnetic field (through the introduction of band gaps at specified spectral locations) or through incoherent pumping to selectively saturate atoms outside the spectral region chosen for amplification.

All-optical transistor action with bistable switching in a photonic crystal cross-waveguide geometry.

We demonstrate all-optical switching action in a nonlinear photonic crystal cross-waveguide geometry with instantaneous Kerr nonlinearity, in which the transmission of a signal can be reversibly switched on and off by a control input. Our geometry accomplishes both spatial and spectral separation between the signal and the control in the nonlinear regime. The device occupies a small footprint of a few micrometers squared and requires only a few milliwatts of power at a 10-Gbit/s switching rate by use of Kerr nonlinearity in AlGaAs below half the electronic bandgap. We also show that the switching dynamics, as revealed by both coupled-mode theory and finite-difference time domain simulations, exhibits collective behavior that can be exploited to generate high-contrast logic levels and all-optical memory.

Photonic bandgap materials: towards an all-optical micro-transistor

We describe all-optical transistor action in photonic bandgap (PBG) materials doped with active atoms and analyse the advantages of this system over other all-optical transistor proposals. In the presence of a PBG material, a coherent laser beam with the frequency slightly detuned from the resonant atomic transition frequency can drive a collection of two-level atoms to an almost totally inverted state, a phenomenon strictly forbidden in ordinary vacuum. By varying the laser field intensity in the neighbourhood of a threshold value, it is possible to drive the atomic system through a transition from states in which the atoms populate preferentially the ground level to almost totally inverted states. In this process, the atomic system switches from a passive medium (highly absorptive) to a active medium (highly amplifying). The large differential gain exhibited by the atomic medium is very robust with respect to nonradiative relaxation and dephasing mechanisms. The switching action in a PBG material is not associated with operation near a narrow cavity resonance with conventional trade-off between switching time and switching threshold intensity. Rather it is associated with an abrupt discontinuity in the engineered broad-band electromagnetic density of states of the PBG material. We demonstrate all-optical transistor action in PBG materials by analysing the absorption spectrum of a second probe laser beam and we show that the probe beam experiences a substantial differential gain by slight intensity modulations in the control laser field. Under certain conditions, the fluctuations in the number of totally inverted atoms that contribute to the amplification process are strongly diminished (the statistics of the excited atoms becomes sub-Poissonian), which, in turn, determines a very low-noise regime of amplification.

Photonic bandgap formation and tunability in certain self-organizing systems

We describe the microfabrication and band structure of large scale three-dimensional (3D) photonic bandgap (PBG) materials based on self-organizing templates. The simplest of these templates is an fcc lattice of close-packed, weakly sintered spheres. Other templates include hcp and hexagonal AB/sub 2/ self-organizing photonic crystals. These photonic crystals may be converted into PEG materials by partially infiltrating the template with high refractive index semiconductors such as Si, Ge, or GaP and subsequently removing the template. The resulting "inverse opal" structure exhibits both a photonic pseudogap and a complete (3D) PBG in the near visible spectrum, spanning up to 15% of the gap center frequency. The local density of states (LDOS) for photons exhibits considerable variation from point to point in coordinate space and reveals large spectral gaps even in the absence of a PEG in the total density of states. These gaps in the LDOS may lead to novel effects in quantum and nonlinear optics when active atoms or molecules are placed within the PBG material. These effects include anomalous, low threshold nonlinear response, collective atomic switching, and low-threshold all-optical transistor action. When an optically birefringent nematic liquid crystal is infiltrated into the void regions of the "inverse" opal PBG material, the resulting composite material exhibits a completely tunable PBG. In particular, the 3D PBG can be completely opened or closed by applying an electric field which rotates the axis of the nematic molecules relative to the inverse opal backbone.

?(2) cascading phenomena and their applications to all-optical signal processing, mode-locking, pulse compression and solitons

Cascading is the process by which the exchange of energy between optical beams interacting via second order nonlinearities (χ(2)) leads to various effects such as nonlinear phase shifts, the generation of new beams, all-optical transistor action, the formation of soliton-like (solitary) waves, etc. Here we review the fundamentals of the processes and discuss experimental verification of the effects and various related applications.

All-optical modulation via nonlinear cascading in type II second-harmonic generation

Utilizing a type II interaction for second-harmonic generation in a crystal of potassium titanyl phosphate, we experimentally demonstrate the all-optical action of a light modulator with both signal and output at the same optical wavelength. This modulator is controlled by the intensity of the injected signal, a characteristic that makes it a suitable candidate for all-optical transistor action and ultrafast analog processing in transparent networks for telecommunications.

All-optical transistor action by polarisationrotation during type-II phase-matchedsecond harmonic generation

Intensity dependent polarisation rotation of the field at the fundamental wavelength in a type-II phase-matched frequency-doubling crystal is used for optical signal amplification. A gain of more than 21 dB was experimentally obtained using KTP for the nonlinear material.