Excitonic Nonlinearity Research Articles

Quantum dot lattice and enhanced excitonic optical nonlinearity

We propose a concept of quantum dot lattice for enhancing the excitonic optical nonlinearity, especially its figures of merit. The most important aspect of the quantum dot lattice for enhancing the optical nonlinearity is the enlarged exciton coherence length due to the cooperation of a large number of quantum dots. The optical nonlinearity of the quantum dot lattice composed of typical semiconductor materials is estimated and an enhancement in the figures of merit of several orders of magnitude over the ever known values is predicted.

Picosecond absorption recovery of 100 nm GaAs/AlGaAs MQW wires

We have studied narrow multiple quantum-well (MQW) wires using conventional absorption spectroscopy and time-resolved absorption measurements. Wires down to 130 nm were fabricated from MQW's using focused ion-beam lithography and electron cyclotron-resonance chlorine-plasma etching. In this structure, the photoexcited carriers diffuse toward sidewalls and recombine on the sidewall surface. We confirmed from the absorption spectrum of MQW wires that the exciton peak is preserved even in wires 130 nm wide. We show that the excitonic absorption recovery of MQW wires is reduced to 11 ps, preserving excitonic optical nonlinearity.

Carrier-induced optical nonlinear effects in semiconductor quantum well wire structure

Carrier-induced optical nonlinearities in a GaAs quantum well wire structure are calculated within the framework of the effective mass approximation to clarify the nonlinear features of the one-dimensional (1D) system. In the calculation, the excitonic nonlinearity due to phase-space-filling, band-gap renormalization, and Coulomb screening effects is estimated on the basis of an effective 1D Coulomb potential. The 1D potential is derived, taking wire cross section into account. The change in the optical absorption for the band-to-band transition is evaluated, considering the band-filling effect and the changes in the Sommerfeld factor caused by the screening effects. The numerical results show that a blue shift of the excitonic absorption peak occurs at the absorption edge because the blue shift caused by the screening effect exceeds the red shift due to the band-gap shrinkage. In the spectral region above the band edge, where the band-to-band transition determines the optical properties, a decrease in optical absorption caused by band filling is compensated by increasing absorption, which stems from recovery of the Sommerfeld factor because of the Coulomb screening. The effect of the Coulomb interaction in a quantum wire laser is also discussed in relation to the gain spectrum in the high excitation regime. These results show that exact inclusion of the Coulomb interaction is crucial in estimating both linear and nonlinear optical properties in the 1D system.

Picosecond excitonic absorption recovery of 100 nm GaAs/AlGaAs narrow multiple quantum-well wires

We report the time-resolved absorption measurement of narrow multiple quantum-well (MQW) wires to investigate their fast recoveries from excitonic absorption bleaching. Wires down to 130 nm were fabricated from MQWs using focused ion beam lithography and electron cyclotron-resonance chlorine-plasma etching. In this structure, the photoexcited carriers diffuse toward the sidewalls and recombine on the surface of the sidewalls. We show that the strong optical nonlinearity of excitons is preserved, even in wires of 130 nm width, and having a fast recovery time in the picosecond region. We also briefly discuss the possibility of making quantum wires which have a faster recovery time and larger optical nonlinearity.

Large excitonic nonlinearity in InAs quantum sheets

We measure the nonlinearity of the excitonic absorption in isolated InAs planes inserted in a bulk-like GaAs matrix. In cw pump and probe experiments absorption changes as high as 37% are observed at a pumping intensity of 80 W/cm2. The corresponding saturation intensity is 400 W/cm2. The nonlinear absorption cross section is determined to be 1×10−12 cm2, which is in excellent agreement with the theoretical prediction.

Strong excitonic nonlinearity in a P-I-N photodiode incorporating narrow asymmetric coupled quantum wells

Strong excitonic nonlinearity in the photoconductive response of a P-I-N photodiode incorporating narrow asymmetric coupled quantum wells has been observed at 78 K. When the P-I-N photodiode is overbiased, the heavyhole energy levels in the two coupled quantum wells are moved toward the resonance by increasing the laser intensity. Also, both the light-hole and the heavy-hole excitonic transitions undergo intensity-dependent shifts. Both these effects indicate intrinsic change of bias due to redistribution of photogenerated carriers and, therefore, the existence of an intrinsic feedback mechanism. The magnitude of the blue shift of the heavy-hole excitonic transitions significantly increases when the laser intensity is changed from 9.2 to ~270 mW/cm(2).

Excitonic optical nonlinearity in conjugated polymers and aromatic molecular chromophores — A theoretical approach

Excitonic optical nonlinearity in conjugated polymers and aromatic molecular chromophores — A theoretical approach

NONBOSON TREATMENT OF EXCITONIC NONLINEARITY IN OPTICALLY EXCITED MEDIA

The present article shortly reviews some recent results in the study of excitonic nonlinearity in optically excited media using a nonboson treatment for many-exciton systems. After a brief discussion of the exciton nonbosonity the closed commutation relations are given for exciton operators which hold for any exciton density and type. The nonboson treatment is then applied to the problems of intrinsic optical bistability and nonlinear polariton yielding quite interesting and new effects, e.g. new shapes of hysteresis loops of intrinsic optical bistability or anomalies of polariton dispersion.

Dependence on dimensionality of excitonic optical nonlinearity in quantum confined structures

The dependence on dimensionality of the excitonic optical nonlinearity and the figures of merit in quantum confined structures is clarified for the resonant third-order optical nonlinearity. An important parameter which determines the dimensionality dependence of the figures of merit is found to be the ratio of the homogeneous linewidth to the exciton binding energy or the ratio of the exciton coherence length to the exciton Bohr radius. It is predicted that for the III–V compound semiconductors the exciton confinement in low-dimensional structures (d=1,2 d:dimensionality) is favorable for enhancing the figures of merit, whereas for the I–VII compound semiconductors the exciton confinement does not always improve the figures of merit.

Excitonic optical nonlinearity induced by internal field screening in (211) oriented strained-layer superlattices

The nonlinear optical properties of a new class of strained-layer superlattices (intrinsic Stark effect superlattices) have been investigated. Specifically, we have compared the nonlinear transmission of Ga1−xInxAs-GaAs strained-layer superlattices grown along the (211) axis to identical superlattices grown along the (100) axis, and found that the optical nonlinearity in the (211) sample is about one order of magnitude greater than in the (100) sample. A blue shift of the exciton resonance and an increase in the exciton absorption strength in the (211) sample with increasing light intensity was observed (attributed to screening of the intrinsic Stark effect fields by photogenerated carriers), resulting in the stronger optical nonlinearity. The maximum of the nonlinear absorption index, ‖α2‖, in the (211) sample was 54 cm/W (‖Im χ3‖=0.33 esu) whereas in the (100) sample the maximum of ‖α2‖ was 6.9 cm/W (‖Im χ3‖=0.042 esu). The measured carrier recovery time in both samples was 2 ns.

Optical nonlinearities of quantum-well excitons near two-photon resonance

Optical nonlinearities of quantum-well excitons near two-photon resonance

Resonant photodiffractive effect in semi-insulating multiple quantum wells

We use semi-insulating multiple quantum wells to combine the holographic properties of the photorefractive effect with the large resonant optical nonlinearities of quantum-confined excitons. GaAs–AlGaAs multiple-quantum-well structures are made semi-insulating by proton implantation. The implant damage produces defects that are available to trap and store charge during transient holographic recording by means of coherent excitation. The advantages of charge storage and resonant optical nonlinearity combine to produce new optical devices with large sensitivities. The potential use of these devices for image processing is demonstrated by using the Franz–Keldysh effect in four-wave mixing at wavelengths near 830 nm.

Ultrafast optical nonlinearities of type II AlxGa1−xAs/AlAs multiple quantum wells

We present experimental studies of the transient optical nonlinearities near the direct band gap of type II AlxGa1−xAs/AlAs multiple quantum wells. The response of the optical nonlinearity at the band-gap excitonic transition is critically dependent on laser photon energy. A complete recovery of the bleached absorption in the lower part of the excitonic resonance with a picosecond time constant at room temperature occurs due to an enhanced collision broadening of the excitonic resonance after the fast spatial Γ–X transfer. Our results demonstrate that the intrinsic response of optical devices based on exciton nonlinearities can be much faster in type II structures as compared to type I structures.<squeeze;2p>

Excitonic optical nonlinearity in quantum-confined CuCl-doped borosilicate glass

The nonlinear refractive index n2 is reported for quantum-confined CuCl microcrystallites in borosilicate glass near the Z3 excitonic resonance. Induced index changes were estimated using Kramers–Kronig analyses of absorption bleaching data from 370 to 388 nm. The nonlinearity was found to increase with increasing particle radius over the range 22 to 34 Å, in agreement with theoretical predictions.

Wave propagation in a nonlinear periodic medium.

The propagation of electronic and electromagnetic waves in a periodic, nonlinear medium is described in terms of a discrete dynamical system represented by a universal, area-preserving map of a plane onto itself. The map is characterized by two control parameters: the wave vector k and the current density j. The dynamics of this Hamiltonian map is very complex admitting periodic, quasiperiodic, and chaotic orbits bifurcating and resonating at various points of the two-dimensional parameter space (k,j). The analysis of this dynamical system is based on the pattern of strong resonances and then applied to the problems of electromagnetic-wave propagation through a superlattice characterized by a strong excitonic nonlinearity and the ballistic transport of electrons in spatially periodic media.

Resonant photodiffractive four-wave mixing in semi-insulating GaAs/AlGaAs quantum wells

We have performed photodiffractive four-wave mixing in semi-insulating multiple GaAs/AlGaAs quantum wells at a wavelength of 0.83 microm. The quantum wells were made semi-insulating by proton implantation, which introduces defects that are available to trap and store charge during holographic recording. The experiments demonstrate how photodiffractive behavior using the large resonant nonlinearities of quantum-confined excitons yields highly sensitive material for optical image processing. When pump powers of 1 mW/cm(2) are used, the measured sensitivity is 2 orders of magnitude greater than that of bulk, nonresonant photorefractive semiconductors.

X-ray detection using the quantum-well exciton nonlinearity

Two related schemes for detecting X-rays based on (1) the scattering and (2) the reflection of a pulsed optical probe beam by carriers produced by the absorption of a single energetic X-ray photon are described. The detection method is based on the strong carrier density dependence of the (complex) index of refraction of the exciton absorption lines in a GaAs/AlGaAs multiple-quantum-well structure. These schemes can be used to construct a class of very sensitive gated X-ray detectors with micron spatial resolution and temporal resolution in the 10-30-ps regime.

Optical nonlinearities of quasi two-dimensional excitions under various excitation conditions

Abstract Excitonic nonlinearities under nonresonant, quasi-stationary excitation conditions in a GaAs/AlGaAs multiple quantum well structure are investigated. By detuning the pump laser below the lowest heavy hole resonance, we find the first hint of the AC Stark effect in this excitation regime. In another series of experiments we study the light hole to heavy hole relaxation dynamics under ps excitation. Long transition times, exceeding 500 ps, are observed.

Nonresonant excitonic optical nonlinearity in semiconductors.

The nonresonant excitonic optical nonlinearity is analyzed theoretically, based on the boson formalism for the many-exciton theory. The theory presented in this paper treats both the mutual interaction between excitons and the anharmonic interaction of excitons with the radiation field on an equal basis. It is pointed out that the effect of the latter anharmonicity on the third-order optical susceptibility is of crucial importance for materials with small exciton binding energy, such as GaAs and other III-V semiconductors.

A simplified analysis of the optical bistability of multiple quantum well etalons

An analysis of static room-temperature-operation characteristics of a reflection-mode semiconductor multiple-quantum well etalon by a simplified two-level system formula of optical nonlinear properties and a computer simulation is presented. In the calculation for a GaAs/AlGaAs multiple-quantum-well etalon using the parameters reported in experimental measurement, it is shown that the very large excitonic optical nonlinearity is not effectively utilized for the optical bistable operation. This occurs because the effective positive-feedback mechanism which increases the etalon internal light intensity is suppressed when a hardly saturable background interband absorption tail is larger than a few thousand cm/sup -1/ at a wavelength where the large excitonic dispersive nonlinearity is observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>