Search

Precise control of highly anisotropic reactive-ion-beam-etching (RIBE) for GaAs/AlGaAs-based two-dimensional photonic crystals (2DPCs) is investigated in terms of the substrate temperature, T s , ion accelerating voltage, V i , and Cl 2 gas pressure, p . T s is shown to influence the shape of the sidewall, while the balance of the physical etching dominated by the value of V i and the chemical etching dominated by the value of p is essential for keeping smooth and vertical sidewalls of 100-nm-scale air-holes. 2DPC air-hole patterns are defined by an electron beam (EB) lithography machine and air-holes are dry-etched with the EB resist as an etching mask. The optimized balance between the V i and p for 0.5–1.0-μm-deep air-holes results in the high-rate-etching regime given at V i = 500 V and p = 8 × 10 −4 Torr using a 650-nm-thick resist mask, while the optimized balance for 50-nm-scale fine air-holes results in the low-rate-etching regime given at V i = 330 V and p = 5 × 10 −4 Torr using a 350-nm-thick resist mask. In particular, the latter condition is essential for fabricating topology-optimized 2DPC air-hole arrays with the minimum air-hole-size of 50 nm or less. These process conditions definitely contribute to excellent measured transmission spectra in good agreement with the calculated one in a near-infrared range.

We report successful studies on improved key nano-fabrication processes for AlGaAs-based air-bridge-type two-dimensional photonic-crystal (2DPC) slab waveguides. The first topic is proximity-effect correction in electron beam lithography for the definition of a uniform and precise periodic/non-periodic 2DPC waveguide pattern. The second topic is optimization of the selective wet-etching condition in an air-bridge fabrication process. The resultant nano-fabrication processes have been proved to be quite effective for the realization of 2DPC slab waveguides such as modified and topology-optimized bend waveguides having broad-band and high-transmittance spectra in excellent agreement with the calculated ones.

We have successfully fabricated InAs quantum dots (QDs) embedded in a line-defect waveguide in an air-bridge type GaAs photonic-crystal slab (PCS) and observed laser action from optical-pumping. This lasing is found to occur without any optical cavity, such as a set of Fabry-Perot mirrors. Comparison of the observed transmittance spectrum with the calculated band dispersion of the triple-lines defect mode enables us to specify the lasing wavelength as that at the band edge. From this fact, it follows that the distributed feedback mechanism at the band edge with an infinitely small group velocity is responsible for the present lasing.

We have experimentally demonstrated two- dimensional (2-D) photonic crystal (PC) waveguides (WGs) such as straight, bent, Y-branch, directional coupler (DC), and quantum dots (QDs)-based nonlinear optical (NLO) WGs using a GaAs air-bridge membrane with triangular-lattice line-defect WGs. These WGs are necessary for a 2-D PC-based symmetric Mach-Zehnder (PC-SMZ) type ultrafast all-optical switch. Experimental results on the clear observation of the optical interference at the Y-junction DC, nonlinearity-induced phase shift at the NLO waveguide, and transmission spectra in the total PC-SMZ WG configuration will be addressed, as well as measured results on each WG element.

The oxidation process applied to the fabrication of an oxide-cladding two-dimensional AlGaAs-based photonic crystal waveguide with tapered ridge waveguides is thoroughly investigated. Combining an extremely small amount of oxidation with the tapered ridge waveguides yielded the self-aligned selective oxidation without any masking layers. This tapered ridge waveguide with partial oxidation functions like a spot size converter that reduces propagation loss and, more importantly, lessens the significant strain induced by oxidation. On the other hand, satisfactory agreement between transmission measurements of fabricated samples and three-dimensional finite-difference time-domain calculations demonstrates the high quality of the photonic crystal slab waveguide.

An AlGaAs-based near-infrared 2-D photonic crystal (PC) with an air-bridge structure featuring defect waveguides has been developed. For the sample without defect waveguides, measurements of the optical transmission characteristics in the wavelength range from 850 nm to 1100 nm showed a deep attenuation due to a bandgap with 30-35 dB attenuation and transmittance of nearly 100% for the guided modes. Optical propagation properties of defect waveguides were obtained by two methods: measurements of transmission spectra and plan-view observations of the optical beam trace along the waveguide with an infrared-vidicon camera. 3-D finite-difference time-domain simulations for the band structure and transmission spectra in the air-bridge slab with and without defect waveguides have revealed the appearance of four defect propagation modes specific to the defect waveguide, between two slab modes for the defect-free photonic crystal slab. These defect modes were experimentally identified in the measured transmission spectra.

Photonic crystals having two-dimensional periodicity on a length scale of 320–450 nm are fabricated by electron beam lithography in combination with high-aspect-ratio dry etching. To achieve three-dimensional control of the optical properties, three kinds of dielectric waveguide structures based on AlGaAs heterostructures, that is, semiconductor-clad, air-bridge, and oxide-clad structures, are investigated. The ability to guide light through such photonic crystal devices is demonstrated by optical transmission measurements. Clear photonic band gap effects resulting in 30 dB attenuation of the transmitted light can be observed in the stop-band regions. The measured results are in good agreement with theoretical band-structure calculations and with numerically computed transmission spectra obtained by the finite-difference time-domain method.

The colloidal crystallization of fine particles is a phenomenon of great interest in connection with the research on the fabrication of photonic crystals. This paper describes a newly proposed method, in which self-arrangement and self-assembly of particles are generated under radiation force, i.e. gradient and scattering forces, and capillary force, respectively. The hexagonal structure in the Fourier-transformed patterns of 2D colloidal crystals shows that the triangular lattice structure is successfully fabricated by the proposed method. This paper is concerned with the principle of newly proposed method, capturing and self-arranging particles by radiation pressure, self-assembling particles by the desiccation process, the enlarging of colloid crystal, and the verification of the triangular lattice structure.

Selective wet etching is a technique that allows fabrication of freely suspended thin slabs in epitaxial heterostructures. This technique, together with high-aspect ratio dry etching, is used to create two-dimensional (2D) photonic crystals for near-infrared wavelengths in thin suspended AlGaAs membranes. The active photonic crystal areas contain hexagonal arrays of cylindrical holes completely perforating the membrane. For an appropriate selection of the slab thickness and the lattice parameter, dispersion relations for the guided modes are explicitly computed by solving the Maxwell equations in a geometry containing a 2D photonic crystal defined in the core layer of a dielectric waveguide structure. Measurements of the spectral transmittance in air-bridge samples show a narrow 30-dB-deep stop band for TM polarized incident light. The spectral position of the stop band shows a good agreement with theoretical band structure calculations when also the ‘quasi-guided’ resonant modes outside the light cone are taken into account.

An observation of light propagation in a two-dimensional photonic crystal-based (2D–PC) optical waveguide is reported. Bent waveguides with a bending angle of 60° embedded in a 2D–PC triangular lattice with air columns were fabricated on a molecular-beam epitaxially grown AlGaAs/GaAs structure. The light-propagation characteristics were examined by observing scattered light from PC regions with incident light in the 850–950 nm range under an optical microscope using a charge coupled device camera. The incident light was strongly guided for wavelengths corresponding to the photonic band gap.

In the last decade, photonic crystals have attracted an increasing interest, mainly because the optical field, i.e., radiation field and light propagation characteristics can be thereby controlled in principle. This is important not only in the basic science such as physics, but also in developing new devices in optoelectronics. For the latter, it is expected, for example, that using a variety of novel unique features of photonic crystals, an extremely-miniaturized planar light circuit will be developed in future. In this review article, the present status of researches in this field is highlighted. First, several pronounced features of two- and three-dimensional photonic crystals are summarized. Second, some aspects of recent development of the crystals are presented with special concern to two-dimensional ones. Third, examples of controlling optical field are presented, which include laser action, enhancement of second-harmonic generation signal, photonic crystal waveguides, and light delay component.

Second-harmonic generation (SHG) properties of the hybrid-films of a clay and [Δ-Ru (4,7-diphenyl-1,10-phenanthroline) 3 ](ClO 4 ) 2 (Δ-Ru(dpp) 3 ), which was found to be stable enough over a very long time, were studied. The ratio of the SHG signal intensity between Z-and Y-Type films of Δ-Ru(dpp) 3 has been compared. Furthermore, the SHG coefficient d 33 at 1.06 μm is determined as 30 pm/V for the Y-Type hybrid-film of Δ-Ru(dpp) 3 .

The temperature dependence of the optical homogeneous width was investigated for mesoporphyrin and its aggregate doped in polyvinylalcohol from 1.8 to 50 K by means of persistent spectral hole burning and accumulated photon echo. Because the exponent of the temperature dependence changes around 10 K, there exist two dephasing mechanisms that are characteristic for amorphous polymers. At lower temperatures, phonon-assisted tunneling of two-level systems (TLS) is dominant. On the other hand, the Raman process of low-frequency phonons is dominant at higher temperatures.

X-Ray crystallographic analyses have been performed for the crystals of α,α′-dibenzylidenecycloalkanone (DBCA) derivatives which preferentially crystallize in non-centrosymmetric space groups and become second-order nonlinear optical crystals. We examined the obtained structures while paying attention to the intermolecular short contacts common to their structure. As a result, relatively short C–H···O contacts between the cycloalkanone rings were found for 8 out of 10 DBCA derivatives. The C–H···O distances for all cyclopentanone derivatives are almost the same as the sum of the van der Waals radius of carbon and that of oxygen; also, the geometrical characteristics found from the C–H···O contacts are consistent with the C–H···O hydrogen bond. The derivatives form a non-centrosymmetric one-dimensional chain with this contact. Their non-centrosymmetric crystal structures are led not only by their molecular shape, but also by this C–H···O interaction.

Charge-transfer (CT) interactions of tetrathiafulvalene (TTF)-based Langmuir–Blodgett (LB) films with non-centrosymmetrical structure were examined from the viewpoint of nonlinear optics. Five kinds of LB films of CT complexes were prepared using TTF-based electron donors and acceptors of 7,7,8,8-tetracyanoquinodimethane and p -benzoquinone derivatives. The difference of the dipole moments between ground and excited states of these LB films was evaluated from the measurements of electroabsorption spectra.

Polar phonons have been investigated in the hexagonal BaTiO 3 over a wide temperature range by hyper-Raman scattering. The lowest-frequency polar TO-phonon with A 2u -species exhibits a soft phonon behavior in the prototype hexagonal phase and shows a splitting into two peaks below the hexagonal-to-orthorhombic phase transition temperature T a =222 K. The splitting of the polar soft phonon is caused by an accidental coupling to the non-polar E 2g -phonon. Having this in mind, we have estimated the static dielectric constant e c along the c -axis, from the hyper-Raman spectrum of the polar soft phonon in terms of the Kramers-Kronig relations. The result shows a small bump at T a and a monotonous increase below 200 K, which reproduces well the anomalous temperature-dependent behavior of the reported dielectric constant. The saturating trend of the latter as the temperature approaches 0 K, is also found to be primarily caused by the above accidental coupling, not by quantum effect.

Two-dimensional photonic crystals of triangular and square lattices both composed of parallel air rods with a lattice constant of 170 \ensuremath{\mu}m were fabricated and their transmittances in the far-infrared region were measured to examine to what extent they quantitatively correspond to the calculated photonic band structures. In addition to an opaque region due to the band gap, an uncoupled mode, which cannot be excited by an external electromagnetic wave, has been clearly observed in both lattices. The calculated results agree very closely with the observed features. Furthermore, other regions with very low transmittance have been unexpectedly found to exist for both lattices. Tentatively, their manifestation is explained only qualitatively as arising from a poor coupling due to wave-vector mismatch, between the relevant band and that of incident wave. The present result implies that information on transmittance alone does not suffice for properly understanding the photonic band structure.

We have studied the substituent effect of bis(benzylidene)-cycloalkanones (BBCAs) on the second-order nonlinear properties. As the result of strong donor introduction, the effective second-harmonic generation (SHG) coefficient of 2,6-bis(4′-dimethylamino-benzylidene)cyclohexanone (DMABCH) was found to be 5.3 times as large as that of m-nitroaniline at 1.06 μm. The cutoff wavelength is determined to be 530 nm. The space group symmetry of DMABCH is observed to be mm2, as is the case for BBCAs without strong donor. As for chiral effect, the SHG coefficients of chiral BBCAs are as large as that of m-nitroaniline.

We fabricated a two-dimensional photonic lattice composed of a triangular array of circular air rods with the lattice constant of 170 \ensuremath{\mu}m. The comparison of the observed transmission spectra in the far-infrared region with the calculated transmittance and band structure gave clear evidence for the existence of an uncoupled mode which cannot be excited by an external plane wave. The wave function of the uncoupled mode is antisymmetric under a relevant mirror reflection defined on the triangular lattice, which implies that the above conclusion is consistent with the earlier observation on the other bands in two-dimensional lattices by Robertson et al.

We confirm the existence of a biexciton in ZnSe-based quantum wells using transient four-wave-mixing spectroscopy. It is found that a new spectral line appears below the heavy-hole-related exciton resonance, corresponding to the exciton-biexciton transition. Through the analysis, we have determined the dephasing time of the biexciton, as well as the biexciton binding energy.

A directional coupler (DC) with a coupling-strength control defect (CCD) based on a two-dimensional photonic-crystal slab waveguide is numerically and experimentally studied. The structure consists of two parallel-coupled single-defect waveguides separated by a triple-line air-hole array, with some missing air holes in the middle of the triple line. Using a two-dimensional finite-difference time-domain method, a 50% coupler was designed and almost 50% coupling performance was experimentally confirmed at a wavelength of ∼1.3 μm with a coupling length of about 7 μm, about 1/3 in length as compared to the DC without the CCD.

This chapter describes a selection of advanced optical components including the underlying physical principles, production techniques and already existing or possible future applications.

We elucidated the temperature dependence of the Debye-Waller factor of tetrasodium 5,10,15,20-tetra(4-carbonatophenyl) porphyrin and mesoporphyrin dispersed in polyvinylalcohol by measuring the temperature variation of the area of a zero-phonon hole burned at 78 K. In addition, we obtained the Huang-Rhys factor for both samples by assuming that one-phonon approximation holds rigorously. We found that the Huang-Rhys factor, or equivalently the linear electron-phonon coupling in mesoporphyrin-doped polyvinylalcohol is extremely small. The reason of this weak coupling is discussed.

By a hyper-Raman spectroscopy, it is found that a polar soft optic phonon with A2u symmetry exists in the hexagonal BaTiO3 polymorphic to perovskite BaTiO3, independently of another non-polar E2u optic phonon responsible for a phase transition at Ta = 222 K. The frequency is monotonously soft from 670 K to Ta, and thereby a virtual characteristic temperature of ferroelectric instability is deduced as 40K. Further, the magnitude of the TO-LO splitting of the relevant mode is found to be very large as is the case for perovskite BaTiO3 and SrTiO3. The remarkable temperature dependence of the dielectric constant ec can be well accounted for from the lattice-dynamical point of view, including an anomalous bump at Ta. Implication of the present findings is discussed.

Reflectance spectra at Brewster‐angle of incidence in ZnSe/ZnS superlattices are investigated together with a ZnSe thin film for comparison. The spectra for p polarization show a distinct peak profile at the energy of the fundamental excitonic transition, with a great reduction of the nonresonant background reflectance. The dependence of the reflectance spectra on the resonance width, as well as the incident angle, is numerically investigated and found to be consistent with the experimental results.

Critical behavior of hyper-Rayleigh (second-harmonic) scattering was investigated for the first time for a new type of second-order displacive phase transition at Ta = 222 K in the hexagonal barium titanate, which is neither ferroelectric nor proper-ferroelastic in nature. It is found that the scattering intensity in a 90° scattering configuration increases strikingly in such a manner as I2ω ∼ (T-Ta)−2, the square of the generalized susceptibility also observed, as Ta is approached in the centrosymmetric phase, only for those polarizations enabling one to observe the relevant nonpolar soft phonon, while that for other polarizations the 90 intensity remains almost constant. These aspects newly found can be well accounted for with a model that residual point defects acting as strongly localizing “field” should induce microclusters in their vicinity such that development of a locally condensed order-parameter, the square of which I2ω is proportinal to, follows the generalized susceptibility.

Two kinds of novel spectroscopic methods of two-photon resonant second-harmonic generation (SHG) and Brewster-angle reflection were investigated for exploring the electronic structure in strained ZnSe/GaAs thin films. A line splitting into two, which is caused by in-plane tensile strain, in the excitation spectrum of the normally forbidden SHG signal was observed in resonance with the 2P exciton, whereas three lines in resonance were observed with the 1S exciton polaritons. In the Brewster-angle reflection spectrum, on the other hand, only one broad line with the width presumably broadened inhomogeneously, was observed in the 1S exciton region. Next, Brewster-angle reflection spectroscopy was also investigated in a few samples of ZnSe/ZnS quantum wells. For those, two split lines were distinctly observed in the 1S exciton region, as it should be. Comparison between those spectral lines and the one-photon excitation spectrum of luminescence indicates that they are consistent. Thus, it is concluded that both methods should serve as a simple and novel tool for characterizing II–VI thin films.

We have studied the electronic structure of strained-layer ZnSe/ZnS superlattices by using two kinds of two-photon spectroscopy: two-photon absorption spectroscopy and resonant second-harmonic generation. The exciton binding energy is found to exhibit a blue shift as compared to the bulk value due to the exciton confinement effect. Based on the observed subband energies, we derive a conduction-band offset of ∼ 70 meV and a valence-band offset of ∼ 800 meV.

Sixty-degree-bent defect waveguides (DWGs) in an AlGaAs-based air-bridge-type two-dimensional photonic crystal (2D PC) slab were fabricated, and the resulting optical transmission spectra and the plan-view image of the optical beam propagating along the bent DWG were measured in the wavelength range between 850 and 1100 nm. Using the two output ports of the stripe waveguides, one aligned to and another offset from the output edge of the bent DWG, the guided mode specific to the DWG and the slab mode in the defect-free PC slab were experimentally observed with the same sample. These results were verified using the calculated band structure and transmission spectra.

A series of 3-benzylidene-D-camphor derivatives has been synthesized in order to examine the second-harmonic generation (SHG) efficiency along with the transparent wavelength region. Through reflection powder measurement, 3-cinnamylidene-D-camphor having a cutoff wavelength of 350 nm, shows the efficiency 10 times as large as that of urea.

A detailed investigation was undertaken from both theoretical and experimental perspectives of straight defect waveguides in an AlGaAs-based air-bridge-type two-dimensional photonic crystal slab. Optical propagation properties were obtained by two methods: measurement of transmission spectra at wavelengths ranging from 850 to 1100 nm, and with plan-view observations of the optical beam trace along the waveguide measured with an infrared-vidicon camera. Three-dimensional finite-difference time-domain simulations for the band structure and transmission spectra in the air-bridge slab with and without defect waveguides resulted in the appearance of defect propagation modes specific to the defect waveguide, between two slab modes for the defect-free photonic crystal slab. These defect modes were experimentally identified in the measured transmission spectra.

We have performed hyper-Raman spectroscopy in the prototype phase of hexagonal barium titanate above a structural phase transition at Ta = 221.5 K. First we report, on the basis of detailed measurements on the profile, that a silent soft phonon with E2u symmetry responsible for the phase transition, behaves ideally concerning the frequency ω0 and the scattering efficiency S or the fluctuations of order parameter Q 2, according to classical Landau theory, with both ωo −2 and Q 2 changing as proportional to (T – T a)−1 with temperature. Thus, we have for the first time established that the phase transition is a most typical one induced by a silent soft phonon. Second, we report the characteristics of several other phonon modes in the hyper-Raman spectra, which resemble considerably those of other related crystals composed of TiO6 octahedra such as SrTiO3, cubic BaTiO3 and TiO2. In particular, the absolute scattering efficiencies of the lowest lying TO and the highest frequency LO phonons are estima...

We report two-photon-resonant phenomena of hyper-Raman-scattering by optic phonons in ${\mathrm{TiO}}_{2}$. It is found that the scattering efficiency of the allowed mode in the 90\ifmmode^\circ\else\textdegree\fi{}-scattering geometry increases remarkably as twice the incident photon energy, 2\ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{\mathit{i}}$, approaches the direct forbidden energy gap from below. In addition, the configuration-forbidden LO mode in the backward-scattering geometry is found to show a sharp two-photon-resonant behavior at the band edge. The prominent polarization dependence of the phenomena is observed, reflecting the anisotropic nature of ${\mathrm{TiO}}_{2}$. Further, the absolute value of the scattering efficiency is estimated as a function of 2\ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{\mathit{i}}$. It is shown that these results can be interpreted quantitatively with a model on the basis of the electronic band structure. In the course of the analysis, we have estimated the second-lowest-direct-gap energy as 3.10 eV. Finally, we emphasize that the resonant hyper-Raman spectroscopy would be useful as a method for studying the band structure in solids.

Dynamics of quasi-two-dimensional excitons in layered compound GaSe was investigated in the femtosecond regime through time-resolved degenerate four-wave-mixing: stimulated photon-echo and transient grating methods. The mechanisms of phase, population and orientational relaxations of the excitons were considered on the basis of their temperature dependences.

Two-photon absorption spectroscopy is used as a novel tool to study residual strain in ZnSe epitaxial layers grown on GaAs substrates. Taking advantage of the polarization selection rules, we find that ZnSe layers thicker than 2 μm suffer in-plane tensile strain of the order of 10 -4 , while the epilayer thinner than 1 μm undergoes biaxial compression. The measured thickness dependence of misfit strains is compared to that obtained from X-ray diffraction.

Strong signals of both hyper-Raman scattering by optic phonons and hyper-Rayleigh scattering (SHG), the latter of which is forbidden by the usual selection rules, were found to emerge in two-photon resonance with excitation energy of the 2P excitons in a thin film sample of ZnSe on GaAs substrate. Both excitation spectra, which are found to be basically similar to each other, indicate an energy-splitting of the 2P excitons. As a result, it is found that the sample of 5 μm thickness suffer in-plane strains due to lattice misfit between ZnSe and GaAs, an order of which has been estimated thereby as e ≈ 5 × 10-4. A similar resonant hyper-Raman scattering was also found in a bulk sample of CdS, and is utilized for gaining into insight the microscopic origin of the phenomenon. On the basis of the results, it is discussed that the present phenomena, found for the first time in solids, would serve as novel spectroscopic methods for characterizing sample quality of II–VI compounds recently developed by state-of-the-art methods, through the strain-induced structure of the 2P excitons free from polariton effects.

We demonstrated a novel two-dimensional photonic crystal (PC) based Symmetric Mach Zehnder type all-optical switch (PC-SMZ) with InAs quantum dots (QDs) acting as a nonlinear phase-shift source. The 600- ?m-long PC-SMZ having integrated wavelength-selective PC-based directional couplers and other PC components exhibited a 15-ps-wide switching-window with 2-ps rise/fall time at a wavelength of 1.3 ?m. Nonlinear optical phase shift in the 500-?m-long straight PC waveguide was also achieved at sufficiently low optical-energy (e.g., ??phase shift at ~100-fJ control-pulse energy) due to the small saturation energy density of the QDs, which is enhanced in the PC waveguide, without using conventional measures such as SOAs with current-injected gain. The results pave the way to novel PC- and QD-based photonic integrated circuits including multiple PC-SMZs and other novel functional devices.

We have observed laser action from optically-pumped InAs-quantum-dots embedded in a line-defect waveguide in an air-bridge type GaAs-photonic-crystal slab (an array of air-holes). The lasing is found to occur without any optical cavity such as a set of Fabry-Perot mirrors. Comparison of the observed transmittance spectrum with the calculated band dispersion of the W3 defect-mode enables us to specify the lasing wavelength as that at the band edge. From this fact it follows that distributed feedback mechanism at the band edge with a vanishingly small group-velocity should be responsible for the present lasing. Usefulness of this kind of compact laser in a future ultrafast planar photonic integrated circuit is discussed.

We experimentally studied enhancement of the third-order nonlinear optical phenomena, i.e., self-phase modulation due to optical Kerr effect and two-photon absorption (TPA) in a small group-velocity (V(g)) line-defect guided-mode of AlGaAs-based photonic-crystal slab waveguide. We found that the phase shift Deltaphi or nominal Kerr constant n'(2) and TPA coefficient beta were strikingly enhanced due to small V(g) as the band edge was approached, such that they were proportional to (V(g))(-2); the nonlinear refractive index n(2) is enhanced proportional to V(g)(-1). We also observed that owing to this enhancement as well as an extremely small cross-section area, the energy required for inducing pi-phase shift was very small, being of an order of a few pJ for 5 ps optical pulse and for a 0.5-mm long sample. Based on those results, we discuss the possibility of developing ultra-fast and ultra-small all-optical switches that operate due to cross-phase modulation.

The cw and time-resolved photoluminescence spectra of a [0 0 1]-(GaAs) 10 /(AlAs) 10 short-period superlattice are studied under [0 0 1] uniaxial stress. The superlattice is of the type-II and the lowest excited state is the “X z ” exciton. The stress dependence of the exciton decay time is found to be explained in terms of a thermally activated transition of excitons to the l'state. Results for the stress-induced change in the X-Γ mixing are also presented.

We have measured the intensity of Raman scattering light, which is due to optical phonons, as a function of excitation laser wavelength for the guide-mode band of ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$-based photonic-crystal-slab line-defect waveguide. As a result, we find that the Raman scattering efficiency is pronouncedly enhanced as the signal wavelength approaches the band edge. Comparison of the result with theoretical prediction reveals that the enhancement should reflect increase of the photon density of states in a divergent manner, which is characteristic of a one-dimensional system.

L'efficacite de la diffusion des modes autorises dans la geometrie de diffusion a 90° augmente de facon remarquable comme deux fois l'energie des photons incidents, 2 trωi, et approche Eg. Le resultat peut etre interprete de facon raisonable en termes d'une resonance a deux photons accompagnant la bande interdite Eg. En outre, le mode LO interdit dans la geometrie de diffusion inverse presente un comportement a deux photons, au bord de la bande. Son origine peut etre attribuee a la resonance avec des excitons due a l'interaction de Frohlich dependante de q

Y-branch defect waveguides (DWGs) in an AlGaAs-based air-bridge-type two-dimensional (2D) photonic crystal (PC) slab were fabricated, and the resulting optical transmission spectra were measured. Optical beam propagation along the waveguide was successfully observed in vidicon IR images. The measured transmission spectra at the two output ports in the Y-branch DWG were remarkably similar in shape and magnitude. From comparison between the measured and calculated transmission spectra, the observed optical beam was identified as the guided mode based on the Y-branch DWG in the 2D PC slab. The results suggest that the Y-branch DWG used in the experiment has the potential to function as an ultrasmall 3-dB beam splitter with good performance.

Multiple coherent anti-Stokes Raman scattering signals of the phase-matched SHG light were observed in $\mathrm{K}\mathrm{Nb}{\mathrm{O}}_{3}$, by using a pair of crossed beams of two near-infrared femtosecond pulses under the resonant pumping of a particular optical phonon. Angle-resolved spectroscopy reveals that the phonon grating induced by the crossed beams is responsible for the signals up to the eleventh order covering a broad wavelength region, with the intensities of the same order. A theoretical explanation for the present phenomenon is presented. The result is compared with similar but collinear ones due to vibration or rotation modes in molecular gases in literatures. The significance of the present work is discussed from the viewpoints of Raman coherence and of developing ultrafast pulses in a condensed matter.