Resonant Excitation Conditions Research Articles

LF-mode excitation in FEL caused by stimulated scattering of operating HF-mode

Conditions for resonant excitation of the parasitic LF-mode in the presence of the HF-mode operation, are analyzed. It is shown that due to the effect of stimulated scattering of the waves the HF-mode generation can lead to parasitic excitation even in the cases, when the operating electron current is significantly smaller than the starting current for the LF-mode in the absence of the HF-mode.

Optical characteristics of 1.18-eV luminescence band complexes in n-GaAs:Sn(Si): Results of a photoluminescence study with polarized resonant excitation

Experimental values of the polarization of the low-temperature luminescence from the VGaSnGa and VGaSiGa complexes in n-GaAs under conditions of resonant excitation by polarized light propagating along the [110] or [100] crystal axis are compared with expressions obtained in the classical dipole approximation for defects with triclinic or monoclinic symmetry. It is shown that the rotator fraction in the superposition of rotator and oscillator contributions to the emission of the complexes is 17–18%. The direction of the axis of these dipoles, which matches the experimental data, is consistent with the assumption that the effect of the donor on the vacancy orbitals of a hole localized in the complex is lower than that of the Jahn-Teller effect. The resulting symmetry of the complex may be monoclinic or triclinic. In either case, deviation of the optical dipole axis of the complex from the dipole axis of an isolated VGa vacancy distorted as a result of the Jahn-Teller effect is lower for the VGaSnGa and VGaSiGa complexes than for VGaTeAs complexes. This means that the effect of the donor on the electron structure of the VGaTeAs complexes is greater than in the VGaSnGa and VGaSiGa complexes. This correlates with the difference in the donor position in these complexes.

Tandem mass spectrometric determination of polychlorodibenzo- p-dioxins and polychlorodibenzofurans in a quadrupole ion trap using multi-frequency resonant excitation

A tandem mass spectrometric analytical method is presented for the concurrent determination of tetra- to octa-chlorodibenzo- p-dioxins and tetra- to octa-chlorodibenzofurans. The method entails operation of a quadrupole ion trap mass spectrometer wherein two isolated molecular ion species are fragmented by irradiation with a band of multiple frequencies for a period of but 10 ms. Up to three co-eluting compounds have been determined individually using a software program, Toolkit, which permits successive activation of multiple scan functions customized for each compound and optimized for the resonant excitation conditions employed. Two calibration curves have been obtained for each compound over a 400-fold concentration range; one calibration curve is based on the single major fragmentation channel of COCl-loss while the other is based on multiple fragmentation channels. The CID efficiency, defined as the signal ratio of fragment ions detected from the major dissociation channels to molecular ions isolated, was 0.80–0.90 for all chlorocongeners. The calibration curves were prepared using a set of standard solutions containing labelled chlorocongeners. Optimum results were obtained when fragment ion signal intensities were normalized to those obtained from a single standard solution. In general, tandem mass spectrometric response factors display a 4-fold range with response factor decreasing with increasing chlorine substitution.

Coherent spin dynamics of excitons in quantum wells

Coherent spin dynamics of excitons in quantum wells is investigated theoretically. Resonant and nonresonant excitation conditions are considered. The single-particle hole spin-flip mechanism within the exciton is always the dominant cause for breaking the exchange coupling. The two-dimensional confinement and low temperature are the most important factors which stabilize the hole spin orientation within the exciton. The experimental observations reported previously are interpreted.

Two-beam femtosecond high-field interactions with a solid target. Surface modification and second harmonic generation under the conditions of surface electromagnetic wave excitation

Very intense interfering beams generated a high-temperature near-surface femtosecond plasma with periodically modulated parameters. The dynamics of the efficiency of diffraction by such an induced 'grating' was studied. A background-free method for measuring the duration of high-intensity laser pulses on the basis of noncollinear second-harmonic generation was demonstrated. An increase in the second-harmonic signal was observed for the first time under the conditions of resonant excitation of a surface electromagnetic wave by high-intensity radiation of femtosecond duration.

Third-order optical non-linearities of CuCl-doped glasses in a near resonance region

Third-order optical non-linearities of CuCl microcrystallites with sizes of 6.8-11.0 nm in glasses were investigated. Absolute values of third-order non-linear susceptibilities, |χ (3) (-ω 1 : ω 1 , -ω 1 , ω 1 )|, were measured in the resonance region by a degenerate four-wave mixing method and |χ (3) (-ω 2 : ω 1 , -ω 1 , ω 2 )| were measured in the near resonance region by a non-degenerate four-wave mixing method. |χ (3) (-ω 1 : ω 1 , - ω 1 , ω 1 )| was strongly dependent on the frequency, ω 1 , of incident beam, while |χ (3) (-ω 2 : ω 1 , - ω 1 , ω 2 )| was less dependent on the frequency, ω 2 , of incident beam under the resonant excitation condition. In the non-degenerate four-wave mixing configuration in which the pumping frequency is resonant with the confined excitons, the figure of merit, |χ (3) |/α, was enhanced to more than an order of magnitude compared with that in the degenerate four-wave mixing configuration in a near resonance region.

Optical spectroscopy in (Zn,Cd)Se-ZnSe graded-index separate-confinement heterostructures.

We report a detailed examination of the electronic structure and of the thermal transport in a graded-index separate-confinement heterostructure based on (Zn,Cd)Se wide-band-gap II-VI semiconductors, and designed in the view of a blue-green light emission device. The band offsets and strain state of the heterostructure are obtained from 2-K photoreflectance measurements. The temperature dependence of the photoluminescence spectra taken both in a resonant in-well excitation condition and in an above-barrier excitation condition has enabled us to quantify the mechanisms responsible for the photoluminescence thermal quenching. This has been done in the context of a sophisticated model that includes several nonradiative processes. In the 10--70 K temperature range, the photoluminescence intensity is found to be ruled by a nonradiative detrapping towards interfacial defects, whilst the thermal escape effect is responsible for the photoluminescence quenching at higher temperatures. In the case of an above barrier excitation condition, the contribution of the carriers diffusion from the barriers to the well leads to an increase of the quantum-well photoluminescence, the intensity of which exhibits a maximum around 50 K. \textcopyright{} 1996 The American Physical Society.

Optical properties and thermal transport of carriers in (Zn,Cd)Se-ZnSe heterostructures

We report a detailed optical study of ZnSe-based graded index separate confinement heterostructures. These structures were grown by metalorganic vapor phase epitaxy and are composed of either one or two Zn0.79Cd0.21Se central well(s) embedded between two ZnCdSe barriers which cadmium composition varies linearly from 5% near the wells to 0% at the end of the barriers. 2K photoreflectance and reflectivity experiments allow the observation of excitonic transitions involving the third electron and heavy hole confined states. The temperature dependence of the photoluminescence lines under in-well resonant excitation conditions (Eexc = 2.661 eV) shows that the thermal quenching of the photoluminescence line is ruled by nonradiative recombinations on defects localized at the heterointerfaces at low temperature and by the thermal escape of the minority carriers at higher temperatures. Under above-barrier excitation conditions (Eexc=3.814 eV), the temperature dependence of the photoluminescence line from the well shows a strong influence of the mechanism of diffusion of the carriers from the barriers to the well.

Conversion between polarization states at the sinusoidal boundary of a uniaxial crystal.

The conversion between polarization states upon reflection on the weakly corrugated interface between uniaxial crystals and isotropic media is studied by means of a model based on the Rayleigh hypothesis. We deal with sinusoidal gratings with the wave vector of the incident light being perpendicular to the grating grooves. Two situations are considered: (i) incidences from an isotropic, lossless dielectric onto a crystal and (ii) from a crystal onto a metal. The results show that in the first case total reflection plays an important role in polarization conversion, even for flat interfaces, where nearly 54% of s-to-p conversion can be attained, and that a weak corrugation can increase this value slightly. On the other hand, for metal-crystal interfaces illuminated under conditions of resonant excitation of surface plasmons, corrugation has a much stronger effect in the change of polarization of the incident wave. It is shown that the efficiency of conversion between ordinary and extraordinary modes can increase more than 250 times when the value of the groove height-to-period ratio is varied from 0 to 0.2. This effect is accompanied by a strong absorption in the metal and by narrow dips in the curves of copolarized efficiencies as functions of the angle of incidence.

Ultrafast optical evidence for resonant electron capture in quantum wells.

The dynamics of carrier capture in GaAs quantum wells have been measured with femtosecond resolution using pump and probe experiments under resonant excitation conditions. The electron capture mechanism at room temperature shows a resonance with a time as short as 650 fs for a well thickness of 58 \AA{}. This gives strong evidence for the importance of the quantum-mechanical LO-phonon-assisted capture mechanism predicted by theoretical calculations.

Ultrafast spectroscopy of conducting polymers

We have used various experimental techniques in the femtosecond and picosecond time domains to study the ultrafast time response, the optical nonlinearities and the character of the fundamental photoexcitations in conducting polymers with (polyacetylene) and without (polythiophene, polydiacetylene) degenerate ground states. Using the technique of photoinduced absorption (PA) we found that photoexcitations in conducting polymers become self trapped in about 300 fs. They form charged and neutral solitons in polyacetylene and polarons and singlet or triplet excitons in other polymers. Due to the existence of disorder and inhomogeneity in conducting polymers thin films, the dynamics of the PA signal, which is due to recombination, is in the form of a power law t −α (α < 1) or stretched exponential exp [−( t/τ) β] (β < 1). The decays are sensitive to high pressure showing that the recombination is dominated by 3D defects in the chains, which act as recombination centers. We have also measured the magnitude and dynamical response of the third order nonlinear optical coefficient χ( 3) in conducting polymers thin films,using the femtosecond degenerate four wave mixing (DFWM) technique. χ( 3) was in the range of 10 −11 to 10 −8 esu depending on the resonance excitation conditions. Also the dynamical response of χ( 3) for resonant and non-resonant conditions is categorically different. We found, however, that the dephasing time of the photoexcitations (< 50 fs) does not depend on resonance excitation conditions.

Evanescent wave coupling of whispering gallery modes of a dielectric cylinder

By developing a direct correspondence between a given whispering gallery mode of a dielectric cylinder and a mode of an equivalent waveguide ring resonator, the evanescent wave coupling characteristics of such modes can be simply modelled. This also allows the conditions for resonant excitation of a single whispering gallery mode to be determined. The model needs a coupled mode theory which accurately describes the coupling between a straight waveguide and a curved waveguide to be developed. This theory is checked against BPM results.

Excitation profiles of Raman scattering and electron-phonon coupling in semiconductor nanocrystals

The Raman scattering spectra of the semiconductor nanocrystals embedded in glass matrices are studied at the resonance excitation conditions. It is shown that the quantum-sized transitions with the angular momenta 1=0,2 only contribute to the resonance Raman enhancement. The excitation Raman profile reveals the peaks not only within the first absorption band 1=0, n=1, but also in the region of the transitions with 1=2, n=1 and 1=0, n=2 in accordance with the theoretical analysis. The size dependence of electron-phonon coupling for the LO phonons and the elastic vibrations and its influence on the resonance Raman enhancement are discussed.

Laser photodissociation probe for ion tomography studies in a quadrupole ion-trap mass spectrometer

Ion tomography studies based on spatially resolved photodissociation of benzoyl cation (C 6H 5CO +) in an ion-trap mass spectrometer are presented. Changes in radially averaged parent ion distributions are observed as a function of applied RF potential during photodissociation and under conditions of resonant excitation with a supplementary ac voltage. This work also demonstrates stabilization of ion trajectories by helium buffer gas when the trap is used in the mass-selective instability mode of operation.

Sensitivity of resonant excitation and photoluminescence excitation measurements to exciton localization effects in GaAs/AlGaAs quantum wells

Low temperature (2 K) photoluminescence (PL), PL excitation (PLE), and resonant excitation (RE) measurements on a very high quality, 150-Å-wide GaAs/Al0.3Ga0.7As multiquantum well structure are reported, with a focus on exciton localization effects. The PL spectra show evidence of effective submonolayer well width fluctuations. Excitons observed via RE are found to be predominantly localized; i.e., they decay from the same effective well-width region in which they are formed, while both localized and delocalized excitons are observed in PLE measurements. Delocalized excitons are free to diffuse to differing effective well-width regions which are energetically accessible. These results are supported by time-resolved PL measurements, which show essentially identical time-responses for differing effective well-width regions under resonant excitation conditions, thereby demonstrating the localized nature of resonantly created excitons.

Ultrafast adiabatic following in semiconductors.

The full semiconductor Bloch equations are solved to analyze the ultrafast exciton dynamics in semiconductors. Transient adiabatic following is obtained for large detunings and arbitrary intensities, leading to exciton Stark shift, bleaching, and recovery on the time scale of the pump pulse. The many-body Coulomb effects strongly influence the semiconductor response for resonant excitation conditions, almost doubling the effective Rabi frequency of the applied field

Enhancement of the yield of infrared laser dissociation of molecules near a surface with a periodic relief

A considerable enhancement of the yield and a reduction of the threshold of infrared multiphoton dissociation of SiH4 molecules is predicted theoretically and confirmed experimentally near the surface of a copper grating with a periodic relief. The effect is due to an increase in the local fields as a result of diffraction of bounded light beams by the rough surface under conditions of resonant excitation of surface electromagnetic waves.

Thermal lensing and permanent refractive index changes in rare-earth-doped glasses

The characteristics of thermal lensing were measured for Eu3+- and Nd3+-doped phosphate glasses under resonant and nonresonant excitation conditions. The relative strengths and response times of the laser-induced lenses were determined and intepreted in terms of the thermal properties of the glass hosts for nonresonant excitation and the coupling of the rare-earth ions to the local vibrations for resonant excitation. Significant enhancement of the strength of thermal lensing was observed for resonant excitation, and this effect was much greater for the Eu-doped glass than for the Nd-doped glass. The response times for thermal lensing were also different for resonant and nonresonant excitation. In addition, a permanent change in the refractive index of Δn = 1.23 × 10−5 was observed for the Eu-doped glass and not for the Nd-doped glass. These results are discussed in terms of the different distributions of vibrational modes generated by the different types of laser excitation of these two rare-earth ions.

Sublimation of low-temperature matrices containing a vibrationally excited component

A dual mass spectrometer was used to investigate the sublimation of solid Ar–SF6 mixtures exposed to CO2 laser radiation (τ ~ 0.2 µsec, Φ ~ 0.1 J/cm2, T0 = 8–20 K). Under conditions of resonant vibrational excitation of the molecular impurity, the only observed sublimation product was argon. The dependence of the argon yield on the laser radiation energy was consistent with the activation law although, when the frequency departed from resonance, not only the exponential term but also, which was particularly important, the preexponential factor decreased. Hence, under the conditions studied, the evaporation process was essentially non thermal. It may be postulated that elementary events involving direct transfer of the intramolecular vibration energy to the translational degrees of freedom of the emitted particles play an important role in this process.

Transient Characteristics of Luminescence from GaAs/Ga0.6Al0.4As Multi-Quantum-Well Structure under Resonant Excitation

The transient characteristics of photoluminescence from the exciton band in a multi-quantum well have been examined under a resonant excitation condition of the exciton by using a time-correlated single-photon counting method. We have found that the delay, rise and decay times of the luminescence depend on the wavelength of excitation and observation, and that the dependences disappear with increasing temperature from 20 to 50 K. These results could be well explained in terms of exciton relaxation through localized levels created by a fluctuation of the width of the multi-quantum well.