M. Idrish Miah

Determination of multiphoton absorption coefficient of cadmium iodide using nonlinear transmittance and photoluminescence methods

Abstract Photoluminescence (PL) measurements of cadmium iodide were carried out at the liquid nitrogen temperature (80 K) under pulsed laser excitation. A broad-band luminescence spectrum with its maximum emission around 2.38 eV was observed by the two-photon pumping with radiation from a ruby laser. The two-photon absorption (2PA) coefficient determined by the nonlinear transmittance (NLT) method was found to be 3.9×10 −2 cm/MW. The two-photon and three-photon comparative luminescence measurements were used to estimate the three-photon absorption (3PA) coefficient. The value was found to be 4.5×10 −2 cm 3 / GW 2 .

Stimulated photoluminescence and optical limiting in CdI2

High quality single crystal of cadmium iodide (CdI2) grown from the melt by the refining method was studied under ruby laser excitation. The low temperature (LNT) emission spectrum of the crystal was recorded by using a spectrofluorometer. The spectrum appeared a structureless emission with its peak in the green spectral region. Analysis of photoluminescence (PL) measurements in the crystal showed a quadratic dependence of the PL peak intensity on incident laser power. This might be explained with the existence of self-trapped excitons in the crystal. In the desired energy ranges of interest, the emission intensity of CdI2 was found to depend on the square of the laser power, indicating the biphotonic process of two-photon excitation, i.e., the nonlinear response is due to a second-order process, or two-photon absorption (2PA) process. The optical limiting properties of CdI2 were studied using 7 ns pulses from the laser. This investigation leads to the conclusion that apart from the 2PA reverse saturable absorption (RSA) is another mechanism for optical limiting in CdI2.

Drift-diffusion crossover and the intrinsic spin diffusion lengths in semiconductors

We study the propagation of electron spin density polarization and spin currents in n-doped semiconductors within the two-component drift-diffusion model in an applied electric field (E). The drift and diffusion contributions to the spin currents are examined, which shows how the spin current could be enhanced. We find that there is a crossover field (Ex), where the drift and diffusion contribute equally to the spin current in the downstream direction. This suggests a possible way to identify whether the process for a given E would be in the drift or diffusion regime. We derive the expression for Ex and show that the intrinsic spin diffusion length in a semiconductor can be calculated directly from Ex. The results will be useful in obtaining transport properties of the carriers’ spin in semiconductors. This investigation, however, highlights the need for further experiments to be conducted to measure Ex in semiconductors.

Size- and temperature-dependent second-order optical effects in copper-doped cadmium iodide nanocrystals

Size and temperature dependences of second-order optical effects in Cu-doped cadmium iodide nanocrystals are studied. A large enhancement of photoinduced second harmonic generation (PSHG) with decreasing thickness of the nanocrystals or temperature is observed. The PSHG is also found to be enhanced with increasing Cu doping. However, due to the formation of the metallic agglomerates, the PSHG for the crystals with high Cu contents decreases. The results are presented by discussing photoinduced kinetics of the electron-phonon anharmonic interaction and nanosized quantum confined effect, the crucial roles played in the observed effects.

Cu-doping effects in Cdl2 layered nanostructures: The role of photoinduced electron-phonon anharmonic interactions

The Cu-doping effects in CdI2 layered nanocrystals were studied experimentally. We used a photostimulated second harmonic generation (PSHG) in order to investigate the nanoconfined effects. It was found that the PSHG increased with increasing Cu-doping within a low-density limit. For the higher Cu content the PSHG output signal was decreased to the value observed for the undoped CdI2 crystal. The above results suggest that the Cu-metallic clusters play a crucial role in the processes responsible for the observed nonlinear optical effects. The large enhancement of the PSHG observed in the Cu-doped thin nanocrystals confirms the fact that the Cu-impurities of low concentrations favor stronger local photoinduced anharmonic electron-phonon interactions.

Spin Kinetics in Low-Dimensional Semiconductor Systems

ABSTRACT Spin kinetics in low-dimensional semiconductor systems was investigated spectroscopically. In the structures, owing to the quantum confinement, the degeneracy of the heavy-hole (HH) and light-hole valence bands was removed. Semiconductor systems were pumped for governing transitions from the HH valence band to the conduction band for the generation of the conduction-band-electron spins, and a maximum ∼80% initial spin polarization was obtained in the systems at liquid helium temperature. Distinct spin oscillations and polarization decay were also observed. Spin kinetics of the drifting electrons was studied as a function of the external magnetic field as well as that of the system temperature in the exact Voigt configuration.