Bragg-spaced Quantum Research Articles

Linear and circular polarized tunable slow light in Bragg-spaced graphene layers

The light pulse delay in Bragg-spaced graphene layers (BSGs) combined with a magnetic field is investigated theoretically. BSGs can slow down the group velocity of light more effectively than traditional Bragg-spaced quantum wells due to the large binding energy and strong dipole oscillator strength of the magnetic-exciton of graphene. The group velocity can be tuned by varying the pulse frequency, the Bragg frequency, and the magnetic field. Especially, by tuning the occupation number of the Landau level the group velocity in BSGs shows strong tunable circular dichroism. Our findings could have applications in photonic integrated circuits and quantum computation.

Ultrafast all-optical polarization QDs switch room temperature properties

All-optical switching consisting of Bragg-spaced quantum dots layers is investigated in theory. The quantum dots are applied in this switch for their spin-dependent polarization and ultrafast full recovery of the sample after passage of a circle-polarized control light. The circular dichroism and birefringence are induced, and the polarization of the signal pulse is changed, which can be applied to design the all-optical switch. This switching structure shows great advantages of lower requirement of pump light intensity, and is a promising candidate for the ultrafast all-optical switching devices at room temperature comparing to the similar quantum well structure.

Temperature tuning of the Bragg resonance of InAsP/InP Bragg-spaced quantum wells grown by MOCVD

In this article, we presented a study of InAs0.04P0.96/InP Bragg-spaced quantum wells (BSQWs), which were grown by metal organic chemical vapor deposition (MOCVD). The quantum wells were characterized by photoluminescence (PL), double-crystal x-ray diffraction (DC-XRD), and reflection spectra. We found that the BSQWs structure grown at 580°C appears to be extremely abrupt, uniform, free of misfit dislocations, and of narrow PL line width. From the reflection spectra at different temperatures, we presented a theoretical analysis of the changes in band structure for resonant and near-resonant wells, and proposed a new scheme of using the temperature to tune the Bragg resonance of Bragg spaced quantum wells.

All-optical polarization switching based on Bragg-spaced quantum dots layers

One kind of Bragg-spaced all-optical switching has been proposed in this paper, and the quantum dots ensembles are used in it as active layers. By one-dimensional photonic crystal theory and transmission matrix method, we have studied the reflectivity stop band and switching effect based on the ac Stark effect. The reflectivity stop band of this switch can be suppressed or recovered, and the circular dichroism and birefringence are induced by a circle-polarized control light, which result in a significant polarization switching effect. This switching structure shows great advantages of lower requirement of pump light intensity, larger contrast ratio than that of Bragg-spaced quantum wells with the same period, especially this switching can be used at room temperature theoretically. So we predict that there are prodigious prospects for its using in high speed optical communications as all-optical switching.

Temperature dependence of active photonic band gap in bragg-spaced quantum wells

A novel all-optical polarization switch of active photonic band gap structure based on non-resonant optical Stark effect bragg-spaced quantum wells was investigated and it could be compatible with the optical communication system. The theory is based on InGaAsP/InP Bragg-spaced quantum wells (BSQWs). Mainly through the design of the InGaAsP well layer component and InP barrier thickness to make the quantum-period cycle meet the bragg condition and the bragg frequency is equal to re-hole exciton resonance frequency. When a spectrally narrow control pulse is tuned within the forbidden gap, such BSQWs have been shown to exhibit large optical nonlinearities and ps recovery times, which can form T hz switch. However, the exciton binding energy of InGaAsP will be automatically separate at room temperature, so the effect of all-optical polarization switching of active photonic band gap bragg structure quantum wells can only be studied at low temperature. By a large number of experiments, we tested part of the material parameters of BSQWs in the temperature range 10-300K. On this basis, the InGaAsP and InP refractive index changes with wavelength, InP thermal expansion coefficient are studied and a relationship equation is established. Experimental results show that the bragg reflection spectra with temperature mainly is effected by InP refractive index changes with temperature. Our theoretical study and experiment are an instruction as a reference in the designs and experiments of future practical optical switches.

The theoretical investigation of all-optical polarization switching based on InGaAs(P) Bragg-spaced quantum wells

The all-optical polarization switch adopting InGaAsP Bragg-spaced quantum wells (BSQWs) is investigated theoretically because it can be compatible with the optical communication system. The theoretical analysis is based on the transfer matrix approach which provides formalism for studying the optical response of the InGaAsP BSQWs. With this theoretical model we calculate the performance characteristics of the switch, which has a high contrast ratio (~31.1dB), a small insert loss (~13dB), and a small switching energy (~30 M W / cm(2) ). The theory can be used as a basis of experimental research of all-optical spin-dependent polarization switching in BSQWs.

Mechanism of all-optical spin-dependent polarization switching in Bragg-spaced quantum wells

The authors outline a microscopic theory of pump-induced anisotropy in the optical response of Bragg-spaced quantum wells (BSQWs). Their theory explains the manipulation of the band structure of the BSQWs by the pump through the microscopic interactions between excitons in the quantum wells. They apply their theory to understand the mechanism of an all-optical polarization switch implemented on a BSQW structure. They trace the relation between the strongly spin-dependent exciton-exciton interactions and the switching signal. Reasonably good agreement is found between their theoretical results and experimental data.

Ultrafast all-optical polarization switching in Bragg-spaced quantum wells at 80K

A polarization switch is demonstrated in Bragg-spaced quantum wells at 80K that exhibits a 0.6THz optical bandwidth with a contrast ratio greater than 30dB, a throughput of ∼4%, a switching fluence of 8μJ∕cm2, and a pulse-width-limited picosecond response time. In this device, switching is achieved by using large spin-dependent nonlinearities induced by a circularly polarized control pulse to alter the polarization state of a linearly polarized signal pulse.

Tunable slow light in Bragg-spaced quantum wells

The group velocity of light is continuously varied in the intermediate band of a Bragg-spaced quantum well structure by tuning the pulse frequency. Delays of 0–0.4bit, without significant pulse distortion, are measured. The high group index is found to lead to large Fresnel reflection coupling losses and Fabry-Pérot fringing. Antireflection (AR) coatings deposited on both sides of the Bragg-spaced quantum well structure are shown to improve coupling of light into the intermediate band but to be sensitive to small errors (∼1%) in the AR coating layer thicknesses.

Resonant photonic band gap structures realized from molecular-beam-epitaxially grown InGaAs∕GaAs Bragg-spaced quantum wells

We present a comprehensive study of the growth and fabrication of Bragg-spaced quantum wells, a type of resonant photonic band gap structure. To begin, we considered the impact of disorder and drift in the periodicity of the quantum wells on the formation of the resonant photonic band gap. We found that steady decrease in the periodicity greater than a few percent leads to collapse of the resonant photonic band gap, while random disorder in the quantum well periodicity of several percent leads to extra peaks in the resonant photonic band gap due to coupling to “intermediate band” states. Next, we optimized the growth of low x (x⩽0.06) InxGa1−xAs∕GaAs quantum wells, the building block of Bragg-spaced quantum well structures. Growth parameters optimized include growth rate, modulation of substrate temperature for barrier/quantum well, and V/III flux ratio. Fast growth of quantum wells was achieved with some of the narrowest heavy-hole exciton linewidths (0.37meV) reported to date for quantum wells of these widths. Using the optimized InGaAs∕GaAs quantum wells as a building block, we grew near-ideal N=210 Bragg-spaced quantum well structures. By monitoring growth rates during growth with reflection high energy electron diffraction and correcting drift by adjusting cell temperature, drift and disorder in periodicity were kept to less than 1%. We see no fundamental barriers to growing much longer structures such as N=1000 periods or longer.

All-optical spin-dependent polarization switching in Bragg-spaced quantum well structures

All-optical polarization switching is demonstrated in a resonant photonic band-gap structure consisting of Bragg-spaced quantum wells (BSQWs). The switch takes advantage of the large spin-dependent optical nonlinearities and the ultrafast recovery present in BSQWs to produce large throughputs (greater than 40%), high contrast ratios (greater than 40 dB), and large optical bandwidths (∼0.6THz), where both switching time and sample recovery time are control-pulse-width limited.

Distortionless light pulse delay in quantum-well Bragg structures

We describe a reflection scheme that allows Bragg-spaced semiconductor quantum wells to be used to trap, store, and release light. We study the temporal and spectral distortion of delayed light pulses and show that this geometry allows multibit delays and offers a high degree of distortion compensation.