Masamitsu Hirai

Improved recombination lifetime of photoexcited carriers in GaAs single quantum well heterostructures confined by GaAs/AlAs short‐period superlattices

Photoluminescence (PL) decay time measurements at 77 and 300 K are reported from 6.1 nm GaAs single quantum well heterostructures (SQWH’s) confined by GaAs/AlAs short‐period superlattices (SPS’s) or ternary AlGaAs alloys with similar Al content, prepared by molecular beam epitaxy. The SQW PL intensity exhibits a single exponential decay with a time constant of 1.6 ns for SQWH’s confined by SPS’s and 0.3 ns for SQWH’s confined by AlGaAs alloys at 77 K. From comparison of the decay rates in both types of the sample, it is found that the radiative recombination efficiency is improved by a factor of about 6 in SPS confined SQWH’s. This higher efficiency is attributed to the improved heterointerfaces in addition to the enhanced radiative recombination rate due to the increased overlap of electron and hole wave functions in the narrow SQW.

The M-Center Luminescence in KCl and KBr Crystals

The M -center luminescence in KCl and KBr crystals has been investigated extensively over a temperature range from about 4°K to 370°K. From the temperature dependence of the band width of the luminescence, vibrational frequencies influencing the M center in the excited states are estimated as 3.9×10 12 cps and 3.8×10 12 cps, respectively, in KCl and KBr crystals. By stimulating the crystal with M 4 light almost the same luminescence as that by M 1 light is observed in KCl. New absorption bands, M L , being associated with the M center and located at 360 mµ, 308 mµ, 250 mµ, and 400 mµ, 325 mµ, 275 mµ in KCl and KBr crystals, respectively, have been found, which may be analogous to the L bands of the F center. By comparing with the quantum yield of the F -center luminescence, the quantum yield of the M -center luminescence is estimated as 0.37±0.05 in KCl crystals at liquid nitrogen temperature. From the temperature dependence of the M -center luminescence intensity the energy gap between the first excited ...

Hot luminescence and non-radiative transition of F centers in KCl and NaCl crystals

Abstract The hot luminescence from F centers in KCl has been measured at LNT and LHeT. At both temperatures the intensity shows exponential dependence on wave number shift up to 6000 cm-1 from the excitation energy, while the degree of polarization depends only weakly on the wave number shift in the same region. The possibility of the non-radiative transitions due to level crossing of ground and excited states has been studied in F centers in KCl and NaCl, by using photocalorimetric spectroscopy (PCS). The results show that the ratio of the non-radiative and radiative transition probabilities depends on photon energy of excitation and that level crossing is dominant for the non-radiative process in NaCl:F.

Photo-induced absorption chance in a-AS2S3 films at 80K

Absorption change in the spectral range from ~1.5eV to ~6.0eV in a-As2S3 films with various thickness between 10.2TM and 797TM has been investigated by illuminating them with light peaking around 4eV with a band width of ~ 0.3eV. All experiments are carried out at 80K. After the illumination, absorption around 5.5eV decreases, while that around 3eV increases. Namely, the absorption band becomes flat after the illumination. The flattening is independent of the film thickness, and seems to be the origin of the edge shift reported in previous papers. In addition to above, the flattening is quite similar to that resulting from temperature rise of the films. A possible reason for the flattening is discussed.

Picosecond studies of cascade relaxation processes among the stacking-fault exciton states in layered BiI3 single crystals

Abstract The luminescence decay processes of the three ground-state excitons localized at a stacking fault with quasi two-dimensional symmetry in layered BiI 3 crystals were investigated in the picosecond time domain. The characteristic time behaviour of these luminescences were quantitatively analyzed on the basis of the cascade relaxation processes among these exciton states. Lifetimes of 0.2, 0.40 and 0.80 ns for the exciton states in descending order of energy were obtained. The anomalous intensity ratios of the luminescence lines are discussed in terms of efficient cascade processes and k -selective radiation processes.

Formation of color centers in anion-doped crystals

Abstract Point defects such as F, X − 2 , H centers and so on are produced in nominally pure alkali halide crystals under irradiation by ionizing radiation. These defects are the products of purely electronic processes within the ions of the matrix. On the other hand, a small concentration of halogen or chalcogen anions as impurities in the crystal, acts as a catalyst or a depressor for the formation of defects. The functions of anions for defect formation, mainly in alkali halide crystals, are described in this paper.

Picosecond pulse sources by cerenkov light generated from fused silica and water

Characteristics of the picosecond Cerenkov light pulse generated from fused silica in a shape of cone and from water in a conical vessel have been investigated in the wavelength range from 230 nm to 500 nm. The pulse durations are estimated to be 27.0±4.0 ps for fused silica and 30.0±7.0 ps for water. It is shown that the present Cerenkov light pulse can be used for measuring the luminescent decay time in the picosecond range in conventional materials such as PPO and Tl+ monomers in KI crystals at room temperature.

Dynamics of excitons sinking into an enlarged well in GaAs/AlAs short-period superlattice

Abstract We have studied the dynamics of excitons using picosecond luminescence measurements of the GaAs/AlAs superlattice (L B =1.2 nm and L Z =3.4 nm) with an enlarged well (L Z =6.1 nm), which is intentionally introduced. The lateral transfer and the vertical transport are observed.

Luminescent decay of the relaxed excitons localized at iodine dimers in NaCl:I and KCl:I

Abstract The luminescent decay of the relaxed excitons localized at iodine dimers in NaCl : I and KCl : I has been investigated. Their temperature dependence is explained by the combination of spin-lattice relaxation processes in the triplet sublevels and thermal conversion to the singlet state. Selective population distribution in the triplet sublevels is suggested.

UV absorption shape between 3.5eV and 5.6eV in very thin a-As2S3 films at 80K

Optical absorption spectra for a-As2S3 films with thickness from 65nm to 797nm have been measured at 80K after annealing at 463K for 2 hours. Net absorption coefficient (a) at the energies (Es) from 2.0eV to 5.6eV has been deduced by correcting the interference effect. Although a below 3.5ev followed the linear relation of (αE)1/2 vs. E, the relation of (αE)vs.seems linear above 3.5eV. Condisering the density of the valence state observed, a stepwise conduction state is suggested to explain the both relation.