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Spontaneous Raman spectroscopy (SR) is a versatile method for analysis and visualization of ferroelectric crystal structures, including domain walls. Nevertheless, the necessary acquisition time makes SR impractical for in situ analysis and large scale imaging. In this work, we introduce broadband coherent anti-Stokes Raman spectroscopy (B-CARS) as a high-speed alternative to conventional Raman techniques and demonstrate its benefits for ferroelectric domain wall analysis. Using the example of poled lithium niobate, we compare the spectral output of both techniques in terms of domain wall signatures and imaging capabilities. We extract the Raman-like resonant part of the coherent anti-Stokes signal via a Kramers–Kronig-based phase retrieval algorithm and compare the raw and phase-retrieved signals to SR characteristics. Finally, we propose a mechanism for the observed domain wall signal strength that resembles a Čerenkov-like behavior, in close analogy to domain wall signatures obtained by second-harmonic generation imaging. We, thus, lay here the foundations for future investigations on other poled ferroelectric crystals using B-CARS.

The lasing properties of quantum well structures, where the cavity is defined in the plane of the wells by circular Bragg reflectors are investigated. Diffraction of the in-plane lasing modes into the vertical direction by the circular distributed Bragg reflector (DBR) allows the simultaneous measurement of near-field emission patterns and emission spectra, allowing unambiguous assignment of azimuthal quantum numbers to the lasing modes. The radial quantum number is determined by fitting the lasing spectrum to theory. Lasing is shown to occur in modes whose wave vector is mainly radial, confined by the circular DBR structure, rather than in whispering gallery type modes which are mainly azimuthal.

We demonstrate the existence of an unusual and useful high-brightness guided mode of a multimode AlGaAs/GaAs heterostructure compound slab waveguide. This mode has a narrow near-field single lobe confined to the low-index regions of the waveguide. This mode was selectively probed by optically exciting quantum wells optimally placed in the waveguide. By pumping in a stripe geometry, lasing is observed above a threshold of 80 kW/cm2 indicating efficient lasing in the highest-order waveguide mode. The near-field emission pattern of the waveguide was imaged to provide a direct measurement of the intensity profile of the higher-order mode.

Photoluminescence lifetimes have been measured at room temperature as a function of carrier density in ZnCdSe/ZnSSe quantum wells. We show that, at low carrier density (5×109–5×1010 cm−2), nonradiative recombination dominates, while radiative recombination becomes more dominant as the carrier density is increased from 5×1010 to 5×1011 cm−2. Above ∼5×1011 cm−2, band filling effects are shown to produce a saturation of the lifetimes. A simple rate equation model approach can be used to describe the carrier density dependence of the photoluminescence decay data obtained on a wide range of samples. A representative band-to-band recombination coefficient of 8×10−4 cm2 s−1 and a Shockley–Read–Hall rate of 7.3×107 s−1 were determined for one of the better samples studied. We believe that the excellent quality of our samples has allowed for the radiative recombination coefficient to be characterized accurately at room temperature.

We analyze the transverse mode structure of selectively oxidized vertical-cavity surface-emitting semiconductor lasers as a function of the aperture size and injection current. Measurements of the spectral splittings, the near and far field sizes and the response to pulsed operation demonstrate that a parasitic thermal lens can be a dominant factor in determining the transverse resonator properties of small (

We have studied the gain characteristics of gain‐guided quantum well II–VI laser diodes by measuring the amplified spontaneous emission spectra under several pulsed conditions. The temperature rise during one current pulse affects the gain characteristics and the L‐I characteristics. The net modal gain at constant peak current increases with the pulse width. The peak gain for long pulses shows a superlinear dependence on injection current. In this case, the L‐I curve is very steep above threshold and sometimes shows an internal quantum efficiency of more than unity. This leads to an underestimation of the internal cavity loss giving a value inconsistent with the one obtained from the gain spectra. With short pulse currents (

In situ optical reflectometry at the wavelength of 488 nm was employed to control the growth of MgZnSSe/ZnSSe Bragg mirror stacks for the blue‐green spectral region. 10‐ and 20‐period layer structures of MgZnSSe/ZnSSe were grown on GaAs (100) epilayers by molecular beam epitaxy. A room‐temperature peak reflectance of 86% was obtained for the 20‐period structure at the central wavelength of 474 nm. The results show that, in general, in situ optical monitoring of growth is a viable and simple method for real‐time layer thickness control of MgZnSSe/ZnSSe quarter‐wave stacks.

A Bragg reflector consisting of a 25‐period MnZnSSe/ZnSSE Bragg stack is reported. The II–VI semiconductor structure was grown by molecular beam epitaxy on a GaAs (100) epilayer. Structural characterization of the Bragg reflector was performed with double crystal x‐ray diffraction and transmission electron microscopy. These studies indicated that the epitaxial II–VI structure, whose total thickness is about 2150 nm, remains pseudomorphic with the GaAs substrate. The Bragg stack has a maximum reflectance of 81% at 468 nm. This result shows that fabrication of high reflectance mirrors from epitaxial ZnSe‐based II–VI compounds is possible in spite of relatively small refractive index differences between constituent II–VI layers.

We report the use of laser diode amplifiers, whose facets have unequal mirror reflectivities, as asymmetric Fabry–Perot modulators. Due to the presence of optical gain in these devices we observe modulation of reflected light with both large absolute modulation depth and high contrast ratio.

The saturation of excitonic absorption in strained InGaAs/AlGaAs quantum wells is systematically measured as a function of strain. By comparison with an unstrained GaAs/AlGaAs quantum well sample a reduction by a factor of up to 9 in the saturation carrier density is observed in strained samples with indium concentrations of 10% and 15%. Very low saturation densities, as low as 0.82×1017 cm−3, are reported for the InGaAs/AlGaAs quantum wells with an indium concentration of 15%. The reduction in the saturation density is attributed to the change in the valence band density of states and the fact that these samples were designed to be fully strained. A novel method of measuring the absorption without antireflection coatings is described.

The effect of strain on the optical nonlinearities and operation of an all‐optical asymmetric Fabry–Perot etalon is investigated. A high reflectivity modulation of 60% is reported with a contrast ratio of 12.2:1 and insertion loss of 1.87 dB. High contrast is achieved through absorption matching requiring a thick active layer. The effect of a thick structure on the strain reduced saturation carrier density is measured. The saturation density is calculated to be a factor of 2.5 less than in a similar GaAs modulator, showing thicker strained devices still display the advantages of thinner structures.

The effect of strain, strain relief, and barrier design on the carrier lifetime in InGaAs/(Al)GaAs multiple‐quantum‐well samples is investigated. Carriers lifetimes are measured in samples with varying amount of strain, due to increasing indium concentration in the wells, as well as in samples subject to strain relief, with thick barriers and GaAs barriers. Lifetimes of the order of 0.5 ns are measured. The lifetime is sensitive to the presence of indium in the wells but remarkably insensitive to the indium concentration, the strain in the samples, and the barrier composition.

We report bistable operation of a strained‐layer InGaAs/GaAs asymmetric Fabry–Perot optical modulator configured as a self‐electro‐optic effect device (SEED) operating in reflection mode. Bistable loops are observed from 949 to 962 nm with switching powers down to submicrowatt levels. The contrast ratio between on and off states is as large as 5:1 (7 dB) and the device will hold in either state indefinitely. A 600‐μm‐diam device has a switching time of 20 μs for 2.1 fJ μm−2 switching energy. Large optical latch arrays are envisaged using this device.

We have determined the depth of ion implant damage in semiconductor materials by nonlinear optical measurements. The carrier lifetime in ion‐implanted mesas was measured by the pump‐probe technique, and the carrier diffusion coefficient in unetched material was measured by degenerate four‐wave mixing. An effective depth of damage within which the carriers experience fast recombination is then determined by modeling of the carrier dynamics in the mesa structure.

We report a 27:1 switching contrast ratio with 2.5 mW of power in an asymmetric Fabry–Perot etalon. The modulation is achieved by optical saturation of the excitonic absorption profile of a 95 A GaAs/AlGaAs multiple quantum well structure grown on a high‐reflectivity dielectric stack mirror.

We observe free excitons in absorption and emission at room temperature in Cd0.25Zn0.75Te/ZnTe multiple quantum wells. A large overlap between luminescence and absorption peaks observed at 10 K is maintained up to room temperature showing that the luminescence is predominantly excitonic. From a rate equation analysis we calculate the branching ratio for free carriers relaxing into excitons to free‐carrier radiative recombination.

We investigate numerically and experimentally the properties of a passively mode locked quantum dot semiconductor laser under the influence of cw optical injection. We demonstrate that the waveform instability at high pumping for these devices can be overcome when one mode of the device is locked to the injected master laser and additionally show spectral narrowing and tunability. Experimental and numerical analyses demonstrate that the stable locking boundaries are similar to these obtained for optical injection in CW lasers. (C) 2010 American Institute of Physics. (doi:10.1063/1.3483231)

We have investigated Cdx Zn1−xTe/ZnTe multiple quantum wells using absorption techniques. We have observed sharp excitonic features at low temperatures which strongly broaden at room temperature. The strength of the exciton‐phonon coupling is determined from linewidth analysis. The large measured coupling explains the lack of well defined exciton resonances at room temperature, an important consequence for their use as optoelectronic devices.

A novel triple layer film is described which provides single mode propagation in a magneto‐optically active waveguide of convenient thickness. All three layers are modified yttrium iron garnet (YIG) films grown by liquid phase epitaxy on a gadolinium gallium garnet substrate. The top film is the active layer, several microns thick. The middle layer is similar but it has a slightly smaller refractive index to assure single mode operation of the top layer. The bottom layer has high optical absorption to eliminate all of the higher order modes which are propagated in the triple layer and a slightly higher refractive index to prevent the confinement of any modes to the top two low loss layers. Using a praseodymium YIG film as the absorbing layer, 17 TE modes were propagated at 1.51 μm where the Pr absorption is low but only the TE0 mode was observed at 1.48 and 1.54 μm where the Pr absorption is high. Such triple layer garnet films will be useful in all magneto‐optic waveguide devices.

We demonstrate optical amplification of 10‐ps‐long pulses in a traveling‐wave type GaAs double heterostructure laser diode. A continuous train of mode‐locked pulses from a dye laser focused end on into the GaAs active region is amplified by a factor of 10 under injection by a synchronous train of current pulses and attenuated by a factor of 10 in the absence of current injection. This gives a switch/modulator with modulation depths of 100, overall gain, and measured switching speed of many GHz. A cw beam can be modulated at similar gains and speeds and since the device is capable of integration the performance of many optical functions without loss is possible.

We report photon echoes from two‐dimensional (2D) excitons in a GaAs‐AlGaAs quantum well structure at 2 K. We analyze our data with a theoretical model of the generation of photon echoes by pulses of finite length which includes phase relaxation during excitation. A line shape analysis yields the phase relaxation time of 2D excitons. We find that the phase coherence near the line center decays, together with the resonant population, within 4 ps, whereas well below line center the phase coherence is conserved for times up to 20 ps.

We have observed degenerate four‐wave mixing, on a picosecond timescale, at the lowest two‐dimensional exciton resonance line of GaAs‐Al0.25Ga0.75As layer structures, grown by molecular beam epitaxy. Mixing is observed at exciton densities as low as 109 cm−2 per layer and is strongly resonant at the heavy exciton line, where the diffraction efficiency reaches a maximum of 12% for a 2×10−7 J/cm2 pump pulse.