Pranay K. Sen

Theory of optical nutation in direct-gap semiconductors due to ultrashort resonant laser irradiation

An analytical investigation of optical nutation in direct-gap semiconductors such as GaAs, InSb, and Hg/sub 1-x/Cd/sub x/Te based on the time-dependent perturbation technique is discussed. The crystals are considered to be irradiated by short pulsed moderate power near-resonant lasers producing significant density of optically-induced free electron-hole pairs. Incoherent dephasing mechanisms have been introduced phenomenologically into the coherent radiation-semiconductor interaction model. The theory, on application to the case of a specific crystal such as Hg/sub 1-x/Cd/sub x/Te with x=0.18 irradiated by a pulsed 10.6 mu m CO/sub 2/ laser, manifests distinctly the occurrence of ringing behavior in the transmitted intensity, transient dispersion, and absorption even in the absence of any theoretical averaging or the consideration of the effective density of states. >

Optical nutation in GaAs/GaxA11−xAs quantum well structures

Abstract The possibility of occurrence of the coherent optical transient effect known as optical nutation has been analytically established in the semiconductor quantum well (QW) structure, namely GaAs/GaxA11−xAs most extensively used in optical electronics. Ultra-short-pulse low-intensity band-to-band excitation of electrons to the 1s Wannier-Mott exciton state of the crystal has been considered to play an important role in the coherent radiation—QW interaction. Numerical estimations of the complex optical susceptibility and the transmitted intensity under the transient regime reveal ringing behaviour confirming the occurrence of optical nutation in III-V semiconducting QW structures.

Effect of Shell and Shell Thickness on Photoluminescence (PL) of a CdSe/ZnS Core – Shell Quantum Dot

The Photoluminescence (PL) intensity is theoretically calculated for a CdSe/ZnS Core – Shell Quantum Dot incorporating WKB approximation. Coating CdSe bare quantum dots with a ZnS layer indicates an enhancement in the PL intensity remarkably with a red-shift in spectra. The PL intensity increases with initial shell growth and then it starts to decrease if outer shell growths continue. Smaller Core – Shell Quantum Dots shows greater PL intensity with more red-sift in PL spectra.

Effect of transient susceptibility on femtosecond pulse propagation in semiconductor quantum well waveguide

Ultra fast pulse evolution in a semiconductor quantum well structure (QWS) is theoretically analyzed. The polarization induced in the medium due to an incident Gaussian electromagnetic beam has been obtained using the semiconductor Bloch equations. The non-linear Schrödinger equation is used to analyze the effect of the induced polarization on the pulse. An interesting manifestation of the intensity dependence of the refractive index in non-linear media occurs through self-phase modulation (SPM), a phenomenon that leads to spectral broadening of the optical pulses. In doing the miniaturization of the device, we use semiconductor nanostructures in which the non-linearity is very large as compared to their bulk counterparts. Consequently, the phenomenon of SPM becomes significant at lower length scale leading to the limitations of the device. Numerical analysis was performed for a 150 fs Ti:sapphire laser radiation propagating along the transverse plane of a GaAs/AlGaAs QWS with realistic material parameters reveals asymmetric spectral broadening of the pulse due to SPM. The results agree qualitatively well with those available in the literature.

Spin Relaxation in Mn doped CdTe/ZnTe QDs

The spin relaxation times of the electron have been calculated for phonon assisted transitions between the Zeeman sublevels arising as a result of magnetic impurity doping and applied magnetic field in Mn doped CdTe/ZnTe quantum dots. Taking into account effective mass approximation with four band k.p perturbation theory and incorporating the sp-d exchange interaction, the energies of the split states have been obtained and analyzed with magnetic field and effective concentration. The spin relaxation times have been found to be considerably longer with higher dopant concentration in high magnetic fields at very low temperature.

Effect of annealing on the optical properties of the ion beam sputtered NiO thin film

Effect of annealing on optical characteristics of Nickel oxide thin films deposited by ion beam sputtering technique from a Ni target in a mixture of oxygen and argon gas on to a glass substrate has been studied. The deposited films were characterized in as deposited state(S1) and after annealing(S2) at temp of 523u2005K. Crystalline properties of films were investigated using X-ray diffraction technique from which we found that both S1 and S2 shows the polycrystalline nature with preferential growth along (111) plane. The transmittance of the S2 films was decreased. The surface morphology of the film was studied using atomic force microscopy (AFM). The nonlinear optical properties of the films were obtained using z-scan technique which reveals that the nonlinear absorption coefficient of S2 films is larger than that of S1 samples. Improved nonlinearity suggests the utility of the grown films for optoelectronic device application.

Effect of oxygen partial pressure on the structural and optical properties of ion beam sputtered TiO2 thin films

We report the effect of oxygen partial pressure on the structural, electronic and nonlinear optical properties of ion beam sputtered TiO2 thin films deposited on glass substrate at 40% of oxygen (S1) and 20% of oxygen (S2) partial pressure. XRD data shows the crystalline nature of S1 film while the film S2 was amorphous in nature. The energy band gap of the thin films calculated from their UV-Vis spectra was found to be 3.63 eV (S1) and 3.56 eV (S2). The decrease in the band gap with decrease in oxygen partial pressure may be attributed to the amorphous nature of the film. The nonlinear refractive indices for both the films were obtained from the closed aperture Z-scan experiment performed using a cw He-Ne laser source operating at 632.8 nm and were found to be 17.6×10-9 m2/W and -5.64×10-9 m2/W for S1and S2 films, respectively. The reversal in the sign of the nonlinear refractive index may also be ascribed to the crystallinity of the grown films.

Effect of dispersion and nonlinearity on intense femtosecond pulse propagation in GaAs/AlGaAs waveguide structure

Ultra fast pulse evolution in a semiconductor quantum well structure (QWS) is theoretically analyzed using split-step Fourier method. The induced polarization in the medium has been obtained using Semiconductor Bloch equations. The non-linear Schrödinger equation is numerically solved by taking into account the induced polarization. So far, the pulse shape studies are based on the group velocity dispersion (GVD), third-order dispersion (TOD) and self phase modulation (SPM) effects. However, at high excitation intensity the group velocity also becomes intensity dependent. In the present analysis, the interplay of GVD, TOD and SPM leading to the change in pulse shape is studied giving due consideration to the intensity dependent group velocity of the medium. The result of numerical analysis made for a GaAs/AlGaAs QWS manifests significant influence of intensity dependent group velocity on the pulse shape.

Pulse propagation in quantum wells with multiple valence subband structure

Using the Luttinger formulations, we have analyzed the propagation characteristics of an ultrashort (femtosecond) coherent radiation pulse in the transverse plane of a direct gap semiconductor quantum well waveguide structure (QWWS). The semiconductor QWWS is considered to possess degenerate valence subbands near the centre of the Brillouin zone. The photoinduced resonant transitions below the band edge to both 1s and 2s excitonic states from the light hole (lh) and heavy hole (hh) subbands have been considered. The cross-over betwen the lh and hh bands occurs in case of transverse plane where the light holes are found to play more dominant role than the heavy holes. We have analyzed the transient pulse propagation characteristics like pulse break-up and optical nutation. The transverse electric mode propagation is also studied in the QWWS assuming the same to behave as a three-layer asymmetric planar waveguide. Numerical analysis made for GaAs/AlGaAs QWWS duly shined by a 150 fs Ti:Sapphire laser shows good qualitative agreement of the present results with those available in literature.

Pump-probe absorption spectra of small quantum dots

Density matrix approach has been employed to analyze the pump-probe spectroscopic absorption spectra of small semiconductor quantum dots (QDs) under strong confinement regime (SCR) with sizes smaller than the bulk exciton Bohr radius such that the Columbic interaction energy becomes negligible in comparison to the confinement energy. The average time rate of absorption has been obtained by incorporating the radiative and nonradiative decay processes as well as the inhomogeneous broadening arising due to nonuniform QD sizes. The analytical results are obtained for QDs duly irradiated by a strong near resonant pump and broadband weak probe. Numerical estimations have been made for (i) isolated QDs and (ii) QD-arrays of GaAs and CdS. The results agree very well with the available experimental observations in CdS QDs. The results in case of GaAs QDs can lead one to experimentally estimate absorption/gain spectra in the important III-V semiconducting mesoscopic structures.