Photoluminescence asymmetry with quantum state preparation

We show that the number of photons in a strongly coupled exciton–photon system is asymmetric with the detuning of the modes when, in the spontaneous emission regime, the two modes are entangled. As changing the detuning is easy in semiconductor microcavities–where on the other hand the nature of the strong-coupling in terms of single-particle effects is not yet resolved–we propose this effect as a test of the quantum character of microcavity polaritons.     

Exciton-Boson Formalism in the Theory of Laser-Excited Semiconductors and Its Application in Coherent Four-Wave-Mixing Spectroscopy

By the use of a bosonization transformation and group-theoretical arguments, the Hamiltonian of an electron–hole–photon system in a laser-excited direct two-band semiconductor is transcribed into that of an exciton–photon system with the particle spins rigorously taken into consideration. It is shown that the third-order optical nonlinearities in the spectral region below the band edge have their microscopic origin in two-exciton correlations, which are expressed in terms of the effective exciton–exciton and anharmonic exciton–photon interactions. The dependence of the interparticle interactions on the spin states of quasiparticles is behind the polarization dependence of the semiconductor nonlinear optical response. On the example of the system of heavy hole excitons in quantum wells, grown from compounds with the zinc blende type of symmetry, it is demonstrated that the effective exciton–exciton interaction in two-exciton states with nonzero total spin is repulsive, while in zero-spin states it is attractive, which may result in the biexciton formation. The derived Heisenberg equations of motion for the exciton and biexciton operators form the basis for a theoretical study of the coherent four-wave-mixing in GaAs and ZnSe quantum wells. It is readily apparent from the equations that in different polarization configurations the coherent four-wave-mixing is generated by different ingredients of two-exciton Coulomb correlations: in the co-circular configuration, it is the interexciton repulsion, in the cross-linear configuration, the formation of the biexciton and its coupling to excitons, and in the collinear configuration, both of them jointly. The obtained expressions for the time-resolved and frequency-resolved four-wave-mixing signals adequately describe the main characteristics and various details of wave mixing phenomena, including a biexciton signature in the appropriate polarization configurations. Results of the work clarify the microscopic mechanism of the polarization dependence in coherent four-wave-mixing spectroscopy in semiconductor quantum wells.     

Diamagnetism of microcavity polaritons induced by spin-dependent polariton–polariton interactions

Diamagnetism of condensed microcavity polaritons in a vertically applied magnetic field is theoretically studied by using the density of free energy of polaritons. The magnetic dependence of polariton–polariton interactions and spin polarization degree of polaritons are derived, and are used to show the diamagnetic behavior of the polariton spin polarization, which is discussed for GaAs-based microcavities. We show that for strong magnetic field the spin polarization of the polaritons is paramagnetic as usual, while around positive exciton–photon detuning and special Rabi splitting, the spin polarization of the polaritons could be diamagnetic. In addition, weak magnetic field and high polariton density are beneficial to observe the polariton diamagnetism.     

Quantum Monte Carlo simulation of exciton–exciton scattering in a GaAs/AlGaAs quantum well

We present a computer simulation of exciton–exciton scattering in a quantum well. Specifically, we use quantum Monte Carlo techniques to study the bound and continuum states of two excitons in a 10 nm wide GaAs/Al_0.3Ga_0.7As quantum well. From these bound and continuum states we extract the momentum-dependent phase shifts for s-wave scattering. A surprising finding of this work is that a commonly studied effective-mass model for excitons in a 10 nm quantum well actually supports two bound biexciton states. The second, weakly bound state may dramatically enhance exciton–exciton interactions. We also fit our results to a hard-disk model and indicate directions for future work.     

Optical properties of MnAl films

The optical properties of MnAl films of different compositions, deposited at different substrate temperatures and in the thickness range 25 to 90 nm, are reported. The reflectance and transmittance are measured in the wavelength range 320–900 nm for near normal incidence of light. These measurements are used to calculate the optical constants namely, refractive index (n) and extinction coefficient (k). The wavelength and thickness dependence of these optical constants are reported. The optical measurements for films with higher Mn content and grown at higher substrate temperature reveal that n and k values are constant over a wide wavelength range (500–900 nm) showing high reflectance in the visible and near infrared region.     

The effect of the relative position of the bandgap with respect to laser wavelength on the behaviour of DFWM in semiconductor-doped glasses

Degenerated four-wave mixing (DFWM) was realised in five commercial semiconductor-doped glasses (Corning CS3–66, CS3–67, CS3–68, CS3–69, CS3–70). The reflectivity obtained for each glass seems very dependent on the energy excess (E=hv–E _g) and relatively independant on l, the product of the absorption coefficient () by the sample thickness (l). The decay time of the DFWM signal was measured for each glass. The relaxation was found to be different when the photon energy is under or over the gap.     

Dependence on temperature of homogeneous broadening of InGaAs/InAs/GaAs quantum dot fundamental transitions

We report systematic temperature-dependent measurements of photoluminescence spectra in self-assembled InGaAs/InAs/GaAs quantum dots (QDs). We have studied the rise in temperature of the ground-state homogeneous linewidth.A theoretical model is presented and accounts for the phonon-assisted broadening of this transition in individual QD. We have estimated the homogeneous linewidth of an individual QD from PL spectra of self-organized InAs/GaAs QDs by isolating the PL of each individual QD and fitting the narrow line associated with self-organized QDs through a Lorentzian convoluted by a Gaussian. We have observed a strong exciton–LO–phonon coupling (γ_LO) which becomes the dominating contribution to the linewidth above the temperature of 45 K. We have also derived the activation energy (ΔE) of the exciton–LO–phonon coupling, zero temperature linewidth (Γ₀) and the exciton-LA-phonon coupling parameter (γ_Ac). We report that our values are close to the values found in the literature for single InGaAs QD and InAs QD.     

Fundamental Absorption Edge of Thin PbWO4 Films

Based on the temperature dependence of the fundamental absorption edge of thin PbWO₄ films, we studied the exciton–phonon interaction, which made it possible to interpret the absorption edge as the absorption of autolocalized excitons. The temperature dependence of the forbidden band gap is investigated.     

Renormalization Effects in the Spectrum of a Coherently Driven Exciton–Biexciton System

Starting from the total Hamiltonian of an excited exciton–biexciton system, nonresonant renormalizations in the electronic spectrum of a coherently driven direct semiconductor are considered. Stringent group-theoretical inclusion of the particle spin in the Hamiltonian allows one to account for the dependence of different renormalization effects on polarizations of the incident laser fields. On the example of circularly polarized driving and probing pulses it is shown that the kind of observed renormalization is defined by the pump-and-probe polarization geometry. Thus, the exciton optical Stark effect must appear in the case of co-circular pump-and-probe, whereas a mixing of the polariton and biexciton spectra is possible only in the case of counter-circular pump-and-probe. The polariton--biexciton dispersion renormalization may manifest itself as synchronous splittings of the exciton--polariton and biexciton spectra under resonant pumping at a frequency of the polariton--biexciton transition, or in their shifts in opposite directions under near-resonant pumping. The mechanisms of both kinds of renormalization effects are analyzed, and the dependence of their characteristics on the pump parameters and microscopic parameters of the exciton–biexciton–photon system is established. An evaluation of the characteristics shows that the effect of polariton–biexciton dispersion renormalization dominates in the spectra of semiconductors with stable biexciton formation. Results of the theoretical study provide an adequate explanation of available experimental data.     

Resonant elastic scattering of light from single quantum wells in semiconductor multilayers

A theory is developed for steady-state elastic scattering of light via quasi-2D excitons from a quantum well (QW) whose interfaces are randomly rough. The study is mainly focused on the angle dependences of radiation giving direct information about static disorder responsible for the elastic scattering. A nonlocal excitonic susceptibility is expressed in terms of random profile functions of QW interfaces. Treated is elastic scattering of light from a disordered QW in the following actual dielectric environments: (i) a uniform background, (ii) a Fabry–Perot film with rough boundaries, and (iii) a semiconductor microcavity. The cross-sections are derived analytically for scattering of linearly polarized light to the lowest (Born's) approximation with arbitrary roughness statistics. The spectral and angle dependencies of scattering intensity are analyzed numerically in the absolute-value scale with Gaussian correlation of interface roughness. The probability 10⁻² was found for the exciton-mediated scattering of a photon from a QW interface roughness whose root-mean-square height is on the level of 2×10⁻¹ nm. This probability is shown to exceed by two orders of magnitude that is typical of resonant scattering from either a single semiconductor surface or rough boundaries of a semiconductor Fabry–Perot film containing the QW. The scattering spectrum of a QW placed in a microcavity is predicted to have a doublet structure whose components are associated with the cavity exciton–polaritons.     

Anisotropy of the nonlinear optical susceptibility χ(3) in polydiacetylene single crystals

The anisotropy of ⁽³⁾ in Poly-bis(p-Toluene Sulfonate) of 2.4-hexadiyne-1.6-diol (PTS) PolyDiAcetylene (PDA) single crystals is investigated by Degenerate Four-Wave Mixing (DFWM) at a wavelength of 720 nm. The symmetries of ⁽³⁾(–;, –,) identify the non-vanishing and independent components of the tensor. For the calculation of the ⁽³⁾ elements from the measured values, the anisotropy of both the linear optical index of refraction and the absorption coefficient is included in the coupled wave approach. A magnitude of 10^–10 esu is measured for the ⁽³⁾ component if all beam polarizations are parallel to the polymer chains. The least upper bound for all other measurable components is estimated to be 10^–12 esu. The results for the intensity dependence of the DFWM signal are explained in terms of a thermooptic effect. Measurements on the orientational dependence of the DFWM signal in the samples are carried out, and the influence of the anisotropy of the linear optical parameters on the measured curves is discussed. A method for the preparation of thin layered polymer single crystals of poly-bis (4-ButoxyCarbonyle-Methylene-Urethane) of 4.6-decadiyne-1.1-diol (4-BCMU) and the first DFWM measurements on these samples are presented.     

Micropillar resonator in a magnetic field: Zero and finite temperature cases

In this work, we present a theoretical study of a quantum dot–microcavity system which includes a constant magnetic field in the growth direction of the micropillar. First, we study the zero temperature case by means of a self-consistent procedure with a trial function composed of a coherent photon field and a BCS function for the electron–hole pairs. The dependence of the ground state energy on the magnetic field and the number of polaritons is found. We show that the magnetic field can be used as a control parameter for the photon number, and we make explicit the scaling of the total energy with the number of polaritons. Next, we study this problem at finite temperatures and obtain the scaling of the critical temperature with the number of polaritons.     

Thickness dependence of temperature coefficient of resistivity of polycrystalline bismuth films

Results for the temperature coefficient of resistivity (TCR) of polycrystalline bismuth films deposited on to glass substrate are reported for the thickness range 30–300 nm. The film TCR is found to be negative for all thicknesses studied and its absolute value exhibits a maximum of 3.70×10^–3 K^–1 near 72.5 nm. The variation of charge carrier density with film thickness has been estimated from the presence of surface states. To include the thickness dependence of charge carrier density, a modified theory has been used to explain the observed behaviour of the TCR. The experimental results for the TCR of Bi films are found to be consistent with the theoretical values. The existence of the extremum is theoretically verified. From the analysis, the specularity parameter p is about 0.44 and the reflection coefficient R is 0.1.     

Temperature accelerated dielectric breakdown of PECVD low-k carbon doped silicon dioxide dielectric thin films

The dielectric breakdown strength of carbon doped silicon dioxide thin films with thickness d from 32 nm to 153 nm is determined at 25 °C, 50 °C, 100 °C, 150 °C and 200 °C, using I–V measurements with metal-insulator-semiconductor (MIS) structures. It is found that the dielectric breakdown strength, E_B, decreases with increasing temperature for a given film thickness. In addition, a film thickness dependence of breakdown is also observed, which is argued to show an inverse relation to thickness d in the form of E_B∝(d-d_c)^-n. The exponential parameter n and critical thickness limit d_c also exhibit temperature dependent behavior, suggesting a temperature accelerated electron trapping process. The activation energy for the temperature acceleration was shown to be thickness dependent, indicating a thickness dependent conduction mechanism. It is thereafter demonstrated that for relatively thick films (thickness >50 nm), the conduction mechanism is Schottky emission. For relatively thin films (thickness <50 nm), the Schottky conduction mechanism was obeyed at low field region while FN tunnelling was observed as a prevail one in the high field region. PACS 73.40.Qv     

The effects of collisional quenching on degenerate four-wave mixing

We report a theoretical and experimental investigation of the effects of collisional quenching on resonant degenerate four-wave mixing (DFWM). Using single-mode laser radiation, peak signal intensity measurements were performed on an isolated line in the A – X transition of NO. By using appropriate mixtures of N₂ and CO₂ as buffer gases, we varied the collisional quenching rate over several orders of magnitude while maintaining a fixed total collisional dephasing rate. The mixtures had approximately 100 Torr total pressure and were at room temperature. For I/I _sat approximately equal to 0.02, DFWM intensities were found to be less affected by variations in quench rate than were laser-induced fluorescence (LIF) intensities (I and I _sat are the pump laser and one-photon saturation intensities, respectively). Moreover, for I/I _sat roughly equal to 0.5, DFWM intensities were observed to be nearly independent of quench rate. The results are compared to two theoretical predictions, with good agreement observed. Both theories indicate that the minimum sensitivity of DFWM to quenching occurs near I/I _sat1.     

Dynamics of polaritons resonantly created at the upper polariton branch

We have studied the polariton relaxation dynamics in a CdTe microcavity at low temperatures after resonant excitation into the upper polariton branch (UPB). Initially, we have set a negative exciton–cavity detuning, such that the energy difference between the two polariton branches coincides with that of an LO phonon. Our experimental results reveal a sublinear dependence of the integrated emission from the lower polariton branch (LPB) with excitation power. This evidences not only an inefficient LO phonon mediated relaxation from the UPB to the LPB but also a substantial inhibition of polariton relaxation along the LPB. After that, we have progressively reduced the negative detuning, approaching the exciton–cavity resonance. Under these conditions it is possible to observe a nonlinear emission arising from K0 LPB-states similar to that observed after nonresonant excitation. Marked oscillations are present in the time evolution traces, with a period that does not depend on excitation power or detuning.     

Ion beam mixing and sputtering of Kr-irradiated TiN films measured with RBS,RNRA, and PIXE

Thin titanium nitride films of 10–300 nm thickness were irradiated with ⁸⁴Kr ions of 80–700 keV energy and fluences ranging from 10¹⁶ cm² to 2×10¹⁷ cm². Sputter yields (Y=0.4–1.0) and mixing rates (k=0.05–0.5 nm⁴) were determined using the depth profiling methods RBS, RNRA, and PIXE. While the sputter yields agree well with the modified Sigmund theory, the energy dependence of the mixing rates cannot be explained by standard models.     

High-resolution degenerate four-wave-mixing spectroscopy of OH in a flame with a novel single-mode tunable laser

The first application of a novel single-mode tunable laser system to nonlinear spectroscopy is reported. The device uses a modeless dye laser, pumped by a single longitudinal mode (SLM) Q-switched Nd:YAG laser, as a narrow-bandwidth amplifier of the output of a SLM diode laser. The system provides pulses of 5-ns duration, 30-mJ energy and 165-MHz spectral line width tunable in the range 632–639 nm at 10-Hz repetition rate. The frequency-doubled output of the laser is used to record spectral line shapes of degenerate four wave mixing (DFWM) signals from the P₁(15) line of the A²–X² (0,0) band of OH in a methane/oxygen flame. Pressure broadening of the DFWM line shape is studied for the first time in a low-pressure flame and a pressure-broadening rate of 1.31±0.09×10^-4 cm^-1/Torr is derived from the data. Power-broadening effects are measured and compared with predictions of the standard perturbative model and of an analytical solution derived from a non-perturbative treatment of DFWM with arbitrary pump and probe intensities. PACS 42.55.Px; 42.62.Fi; 42.65.-k     

Effect of poly silicon thickness on the formation of Ni-FUSI gate by using atomic layer deposited nickel film

The effect of poly-Si thickness on silicidation of Ni film was investigated by using X-ray diffraction, auger electron spectroscopy, cross-sectional scanning transmission electron microscopy, resistivity, I–V, and C–V measurements. The poly-Si films with various thickness of 30–200 nm were deposited by LPCVD on thermally grown 50 nm thick SiO₂, followed by deposition of Ni film right after removing the native oxide. The Ni film was prepared by using atomic layer deposition with a N²-hydroxyhexafluoroisopropyl-N¹ (Bis-Ni) precursor. Rapid thermal process was then applied for a formation of fully silicide (FUSI) gate at temperature of 500 °C in N₂ ambient during 30 s. The resultant phase of Ni-silicide was strongly dependent on the thickness of poly-Si layer, continuously changing its phase from Ni-rich (Ni₃Si₂) to Si-rich (NiSi₂) with increasing the thickness of the poly-Si layer, which is believed to be responsible for the observed flat band voltage shift, ΔV_FB, in C–V curves.     

Femtosecond Time-Resolved Third-Order Nonlinear Response of 4-(N, N-Diethylamino)-β-Nitrostyrene Solutions in the Nonresonant Region—Experimental Evidence of Group-Velocity Dispersion Effec—

We measure the femtosecond time-dependent, third-order nonlinear optical response for 4-(N, N-diethylamino)-β-nitrostyrene (DEANST) dissolved in N, N-dimethylformamide (DMF) from 0.1 to 1.4 M concentrations in the nonresonant region using time-resolved degenerate four-wave mixing (DFWM) spectroscopy. It is found that the DFWM signal profile with delayed responses depends on the concentration. This concentration dependence is determined to be mainly due to the group-velocity dispersion effect of the DEANST solution at the high concentration and the nonlinear response of the DMF solvent at the low concentration, but not due to the change in third-order nonlinear response of DEANST itself.