Enhancement of third-order nonlinear optical susceptibilities incompressively strained quantum wells under the population inversioncondition

Third-order optical nonlinear susceptibilities χ⁽³⁾ in compressively strained and nonstrained InGaAs-InGaAsP quantum wells (QW's) under the population inversion condition are discussed. The small effective mass of compressively strained QW's increases the contribution of the carrier density pulsation effect and the carrier heating effect of χ⁽³⁾. The hole burning effect is also increased due to the decrease of the carrier-carrier scattering rate. The calculation including these effects shows an enhancement of factor 3 due to 0.8% compressive strain. The values of χ⁽³⁾ are experimentally estimated from the data of nondegenerate four-wave mixing in λ/4-shifted distributed feedback lasers. χ⁽³⁾ in 0.8% compressively strained QW's is three times larger than that in nonstrained QW's with the same linear gain     

Ultrafast optical nonlinearity by virtual charge polarization inDC-biased quantum well structures

An optical nonlinearity due to virtual charge polarization in a quantum-well structure biased by a DC electric field is proposed and discussed. The switching time of the nonlinearity is expected to be extremely short, on the order of 100 fs. An effective degenerative four-wave χ³ parameter is estimated to be 1.0×10 ^-9 [esu] for a graded gap AlGaAs quantum well and a detuning energy of pump light, 35 meV. The nonlinearity is observable and quite useful for designing an all-optical and ultrafast optical gate     

Optical Kerr shutter performance of a solution of organic nonlinearoptical material

An optical Kerr shutter with much higher efficiency than the conventional CS₂ liquid is demonstrated, utilizing a solution of π-conjugated monomer material. Third-order nonlinear optical susceptibility χ⁽³⁾ of the solution is evaluated by the efficiency of the Kerr shutter operation. The χ⁽³⁾ value of the solution depends on the molecular density, the molecular orientation behavior, and the dielectric constant of the solvent. To utilize these effects most efficiently, 4'-dimethylamino-N-methyl-4-stilbazolium methylsulfate is dissolved in formamide to the saturation concentration. This results in χ⁽³⁾ =3.0×10^-12 esu and a Kerr shutter transmission of four times that of CS₂     

Hot carrier relaxation in InP and GaAs on a subpicosecond time scale

Hot carrier relaxation in InP and GaAs is investigated in a comparative study using three different time resolved optical techniques. At carrier densities between 10¹⁷ to 10¹⁸ cm⁻³ in both materials the relaxation is strongly influenced by carrier-carrier-scattering. Internal thermalization of electrons and holes occurs on a femtosecond time scale comparable to the duration of the excitation. The temperatures of electrons and holes, however, remain different on a time scale of picoseconds.     

Investigation of transient nonlinear optical mechanisms using avariable pulselength laser

We show a variable pulselength laser to be extremely useful when investigating the dynamics and parameters of optomechanical nonlinarities in optical materials. Pulselength variability allows observation of the transient nonlinear behavior using nanosecond pulses and determination of the underlying physical mechanisms using subnanosecond pulses. It also enables observation of the transition response between the two time regimes where intrinsic polarizabilities compete with optomechanical effects. We examine the transient nonlinear scattering properties of an optical power limiter, platinum ethynyl, and compare it to carbon disulfide using degenerate four-wave mixing. Intensity, polarization, and pulselength control allow us to measure acoustic scattering and decay parameters, χ⁽³⁾ magnitudes, and the transient laser-induced grating behavior     

Impact of amplified spontaneous emission on carrier density formeasurement of optical nonlinearities in GaAs/AlGaAs multiple quantumwells

It is shown that amplified spontaneous emission (ASE) can affect the measurement of nonlinear optical properties in semiconductors through its reduction of the excess carrier density. When the optical excitation area is large, lateral ASE can reduce the carriers within a much shorter period of time than the normal carrier lifetime. Just after ASE is over, the time-integrated surface photoluminescence signal may be used as a measure of the carrier density, which is the carrier density experienced by an ns probe in ps pump experiments. When the excited carrier density is at 10²⁰ cm^-3, surface ASE is also possible and is observed, even with small spot sizes     

Specialty optical fibre for stabilising and enhancing electro-opticeffect induced by poling

A special fibre has been demonstrated that increases the lifetime of the electro-optic coefficient induced by poling. Furthermore, a higher increase in χ⁽³⁾ was observed with UV-poling (~3.4 times) than thermal poling (~2 times)     

Improved second-harmonic generation by selective Yb ion doping in anew nonlinear optical crystal YCa4O(BO3)3

We describe experiments characterizing a new nonlinear optical crystal, YCa₄O(BO₃)₃ (YCOB). This crystal has a number of advantages over other commonly available nonlinear optical crystals. It has a higher nonlinear coefficient than KDP, can be fabricated to large sizes (~3-in diameter, 8-in length), and has a high damage threshold. Moreover, this new nonlinear optical crystal is nonhygroscopic, has good optical quality and mechanical properties, allowing easy optical polishing. This crystal, YCa₄ O(BO₃)₃, commonly termed YCOB, is one of a family of new nonlinear crystals, the oxyborates, that include RECa₄O(BO₃)₃ (RE=La³⁺, La ³⁺, Y³⁺, Sm³⁺, Gd³⁺, Er³⁺, and Nd³⁺). In this paper, we also successfully demonstrate a technique for improving the nonlinear optical properties of this crystal. This technique, ion substitution, has previously had limited success with other crystal hosts. However, the inclusion of yttrium in YCOB provides the opportunity to exploit this technique. Yb³⁺, which has larger mass, but approximately the same atomic size as Y³⁺ can be substituted into the crystal structure without introducing stress and nonuniformities. A systematic investigation of the linear and nonlinear characteristics of several crystals doped with various levels of Yb demonstrate that selective substitution of Yb in YCa₄O(BO₃)₃ improves the second-harmonic conversion efficiency by increasing the optical nonlinearity     

Interband nonlinear optical generation in presence of intersubbandlight in asymmetric quantum wells

The interband sum frequency generation process due to three-wave interaction of interband and intersubband coupling lights has been investigated in a semiconductor quantum well using the perturbational density matrix approach. The origin of the nonlinear process lies in the second-order susceptibility χ_c2-h1⁽²⁾ arising due to the optical transition between the second conduction subband and the first heavy hole state. Both the sign and the magnitude of the second-order susceptibility of the well may be controlled by the carrier density level and the frequency of the intersubband field     

Optical frequency conversions in nonlinear medium with periodicallymodulated linear and nonlinear optical parameters

A new method considering both linear and nonlinear optical parameter modulation and propagation loss is proposed to treat second-order nonlinear optical interactions in nonlinear media having periodic structures. Mathematical expressions are derived for difference frequency generation (DFG) and compared with those for second-harmonic generation (SHG). Wavelength conversions around 1.55 μm, which are very interesting in optical communication systems, are studied for semiconductor DFG devices with periodic structures. Semiconductor (e.g., AlGaAs) DFG devices of standard device lengths (several mm) and pump light intensities (10⁵ kW/cm²) are shown capable of practical level conversion efficiencies (~10%) and extremely wide bandwidths (⩾100 nm). Effects of propagation loss in DFG devices are also examined     

Observation of highly nondegenerate four-wave mixing in 1.5 μmtraveling-wave semiconductor optical amplifiers and estimation ofnonlinear gain coefficient

Nondegenerate four-wave mixing is measured in the 1.5 μm traveling-wave semiconductor optical amplifier medium as a function of the pump-probe detuning frequency ranging from a few GHz to 400 GHz. It is found that two different sources are responsible for the four-wave mixing: the carrier density modulation and the nonlinear gain effect. The latter is clearly observed when the detuning frequency increases above 100 GHz. The nonlinear gain coefficient ϵ, which induces a gain grating through the pump-probe beating, is estimated to be 1.75×10^-23 m³. The change in the real refractive index associated with the nonlinear gain effect, which generates an index grating, is negligibly small. The relaxation time involved in the nonlinear gain effect is found to be less than 0.3 ps. These results support the role of the spectral hole burning rather than the carrier heating in the nonlinear gain effect     

Asymmetric two-step GaAlAs quantum well for cascaded second-orderprocesses

An asymmetric two-step GaAlAs quantum-well suitable for applications that employ the cascading second-order nonlinear effect is proposed. The existence of a large χ_zzz⁽²⁾ (d ₃₃) second-order susceptibility tensor based on transitions between the light hole and the conduction band at a wavelength of 1.55 μm is demonstrated. The expression for χ_zzz⁽²⁾ is obtained using the density matrix formalism and the envelope function approximation including the valence band-mixing model. The magnitude of the χ_zzz⁽²⁾ is estimated to be of the same order as that of the bulk GaAs material and χ_xzx ⁽²⁾     

Optical phase conjugation and nonlinear optical bandpass filtercharacteristics in CdSSe microcrystallite-doped glass

Experimental measurements of the backward CW four-wave mixing response in semiconductor-doped glass in a regime important to applications are described. In particular, it is shown that lowering the operating temperature to 125 K yields a χ³≈2×10 ^-4 ESU. Aberration correction through optical phase conjugation is demonstrated with a phase conjugate reflectivity of 0.3% along with a wide-angle field-of-view response. In addition, the nearly-degenerate four-wave mixing response of importance to tunable bandpass optical filters is reported     

Design of a multi-petawatt optical parametric chirped pulseamplifier for the iodine laser ASTERIX IV

A theoretical design is presented for the generation of peak powers up to ~5 PW and focused intensities higher than 10²³ Wcm^-2 with pulse duration ~20 fs using an optical parametric chirped pulse amplifier (OPCPA) pumped by the high-power iodine laser Asterix IV. These values represent an improvement to the existing Asterix laser by a factor of ~1000 and at least LOO 000 for the output peak power and focused intensity, respectively. The OPCPA system consists of two LBO preamplifiers and a KDP power amplifier and is pumped by the laser third-harmonic output (438 nm, 500 ps, 500 J). Numerical simulations show that saturation within the OPCPA can be effectively used to counteract the bandwidth narrowing effects brought about by the temporal variation of the pump pulse intensity. The results demonstrate that the OPCPA represents a powerful technique for the generation of ultrahigh powers with iodine lasers at a level currently not achievable with any other approach, and proves that the iodine system is particularly suitable as an OPCPA pump source     

Strong increase of the derivative of the carrier-induced indexchange of semiconductor lasers at low injected carrier density

The carrier-induced index change of a semiconductor laser was measured for injected carrier density ranging from 3 × 10¹⁶ cm^-3 to 2 × 10¹⁸ cm^-3. A strong nonlinear behavior between index change and carrier density is observed. The derivative of the index change versus carrier density at low carrier density can be 15 times larger than the derivative of the index change at high carrier density     

Carrier-induced change in refractive index of InP, GaAs and InGaAsP

The change in refractive index Δn produced by injection of free carriers in InP, GaAs, and InGaAsP is theoretically estimated. Bandfilling (Burstein-Moss effect), bandgap shrinkage, and free-carrier absorption (plasma effect) are included. Carrier concentrations of 10¹⁶/cm³ to 10¹⁹/cm ³ and photon energies of 0.8 to 2.0 eV are considered. Predictions for Δn are in reasonably good agreement with the limited experimental data available. Refractive index changes as large as 10^-2 are predicted for carrier concentrations of 10 ⁸/cm³ suggested that low-loss optical phase modulators and switches using carrier injection are feasible in these materials     

Fabrication of planar optical waveguides in LiB3O5 by 2 MeV He+ ion implantation

The fabrication of planar optical waveguides in LiB₃O ₅ is discussed. Using 2-MeV ⁴He⁺ implantation with a dose of 1.5×10¹⁶ ions/cm² at 300 K, the refractive indexes of a 0.2-μm-thick layer 5.1 μm below the crystal surface are reduced to form optical barrier guides. For this ion dose the maximum change from the bulk values of refractive index at a wavelength of 0.488 μm are 1.5%, 5.25%, and 4% for n_x, n_y, and n_z, respectively. The refractive indexes of the guiding region change by less than 0.02% from the bulk values. The dose dependence of the optical barrier height has been measured. A threshold ion dose of about 0.75×10¹⁶ ions/cm² is required to form a refractive index barrier and ion doses higher than about 2.5×10¹⁶ ions/cm². saturate the refractive index decrease. Waveguide propagation losses for annealed single energy implants of dose 1.5×10¹⁶ ions/cm² are dominated by tunneling and are estimated to be ~8.9 dB/cm for the z-cut waveguides used. Multiple energy implants broaden the optical barrier, and losses of <4 dB/cm have been observed     

Time Evolution of the Screening of Piezoelectric Fields in InGaN Quantum Wells

We have measured the time response of the emission spectra of In _0.07Ga_0.93N quantum wells with widths of 2, 3, and 4nm in GaN following pulsed optical excitation. We observe a blue shift of the emission peak during the excitation and a subsequent red shift as the carriers recombine in the 3- and 4-nm wells, and a negligible shift for the 2-nm well. Using a comprehensive theory we are able to fit both the time evolution of the peak emission energy and the integrated emission intensity. The shift of the emission peak (by about 17 meV) arises from the balancing of the change in screening of the internal piezoelectric field as the carrier density changes and bandgap renormalization. We have projected the calculations to quantify the degree of screening at typical threshold carrier densities. At transparency we estimate carrier densities of 4.3times10¹⁶ m^-2 and 4.8times10¹⁶ m^-2 for the 4- and 3-nm wells, respectively, which reduce the internal piezoelectric field in the well to 0.97times10⁸ (4 nm) and 1.03times10 ⁸ (3 nm) Vmiddotm^-1 compared with the unscreened value of about 1.23times10⁸ Vmiddotm^-1. Thus, a substantial field remains in these wells under laser conditions. We find that this partially screened field is beneficial in reducing the threshold current compared with that of a square well for modal gains up to about 150 cm^-1     

Resonantly enhanced nonlinearity using stimulated down-pumping inneodymium doped twin-core fibre

Preliminary observations and measurements of increased relaxation speeds for optical switching in resonantly enhanced nonlinear systems are demonstrated. The effect is observed using a neodymium-doped twin-core fibre interferometer, and uses stimulated down-pumping of the ⁴F_3/2 metastable state in neodymium. The relaxation time has been reduced from ~390 μs to less than 120 μs, and it is shown that the reduction is dependent on the power of the down-pumping source     

Noise performance of InGaAs-InP quantum-well infraredphotodetectors

Dark current noise measurements were carried out between 10 and 10 ⁴ Hz at T=80 K on two InGaAs-InP quantum-well infrared photodetectors (QWIPs) designed for 8-μm infrared (IR) detection. Using the measured noise data, we have calculated the thermal generation rate, bias-dependant gain, electron trapping probability, and electron diffusion length. The calculated thermal generation rate (~7×10 ²² cm^-3·s^-1) is similar to AlGaAs-GaAs QWIPs with similar peak wavelengths, but the gain is 50× larger, indicating improved transport and carrier lifetime are obtained in the binary InP barriers