Tadashi Itoh

Size-dependent radiative decay time of confined excitons in CuCl microcrystals

Abstract Stationary and picosecond time-resolved luminescence spectra of confined excitons have been studied in nm-size CuCl microcrystals embedded in a NaCl matrix. Size-selective resonant excitation and the analysis including strong reabsorption effect make us possible to obtain the radiative decay time of the confined excitons. The decay time is found to be inversely proportional to the volume of the microcrystals. The size- and temperature-dependent radiative decay time is quantitatively interpreted on the basis of rapid radiative decay of the confined, coherent excitons in microcrystals as proposed by Hanamura.

Measurement of the photonic de broglie wavelength of entangled photon pairs generated by spontaneous parametric down-conversion.

Using a basic Mach-Zehnder interferometer, we demonstrate experimentally the measurement of the photonic de Broglie wavelength of entangled photon pairs (biphotons) generated by spontaneous parametric down-conversion. The observed interference manifests the concept of the photonic de Broglie wavelength. We also discuss the phase uncertainty obtained from the experiment.

Generation of ultraviolet entangled photons in a semiconductor

Entanglement is one of the key features of quantum information and communications technology. The method that has been used most frequently to generate highly entangled pairs of photons is parametric down-conversion. Short-wavelength entangled photons are desirable for generating further entanglement between three or four photons, but it is difficult to use parametric down-conversion to generate suitably energetic entangled photon pairs. One method that is expected to be applicable for the generation of such photons is resonant hyper-parametric scattering (RHPS): a pair of entangled photons is generated in a semiconductor via an electronically resonant third-order nonlinear optical process. Semiconductor-based sources of entangled photons would also be advantageous for practical quantum technologies, but attempts to generate entangled photons in semiconductors have not yet been successful. Here we report experimental evidence for the generation of ultraviolet entangled photon pairs by means of biexciton resonant RHPS in a single crystal of the semiconductor CuCl. We anticipate that our results will open the way to the generation of entangled photons by current injection, analogous to current-driven single photon sources.

Ce/sup 3+/-activated fluoride crystals as prospective active media for widely tunable ultraviolet ultrafast lasers with direct 10-ns pumping

New possibilities have been investigated for recently developed solid-state tunable ultraviolet (UV) laser materials such as Ce/sup 3+/ ion-activated LuLiF/sub 4/ (LLF) and LiCaAlF/sub 6/ (LiCAF). With their broad-gain width, demonstrated reliability, and high efficiency, they are attractive for ultrashort pulse generation and amplification. To prove that, we have demonstrated UV picosecond-pulse amplification using Ce:LLF. For such new laser materials, we proposed a passive self-injection seeding scheme for the direct generation of short-pulse trains, which does not require CW operation capability or an external short-pulse seeding laser, Using this simple scheme, a UV sub-nanosecond pulse train is directly and passively generated from Ce:LLF pumped by a standard 10-ns KrF excimer laser, and Ce:LiCAF pumped by the fourth harmonic of a conventional 10-ns Q-switched Nd:YAG laser. >

Naturally formed ZnCdSe quantum dots on ZnSe (110) surfaces

We successfully realized ZnCdSe quantum dots on a cleavage-induced ZnSe (110) surface by depositing a ZnSe/ZnCdSe/ZnSe heterostructure under growth conditions that cannot lead to layer-by-layer growth of ZnSe. This growth mode introduces surface roughness to the newly deposited ZnSe layer, and ZnCdSe quantum dots are then formed. Cathodoluminescence and microphotoluminescence measurements demonstrate the formation of quantum dots.

Interface effects on the properties of confined excitons in CuCl microcrystals

Abstract Optical properties of confined excitons were studied in nanometer-size CuCl microcrystals prepared by three different methods; by annealing in crystalline NaCl and in glass matrices, and by gas evaporation. Interface effects concerning the shape of microcrystals, the effect of finite barrier height and the photo-generated exciton trapping centers were discussed.

Size-dependent homogeneous broadening of confined excitons in CuCl microcrystals

In nm-size CuCl microcrystals embedded in NaCl matrices, resonant luminescence spectra of the Z 3 confined excitons were measured under size-selective excitation among the exciton absorption band, inhomogeneously broadened due to the size-distribution of the microcrystals. The temperature- and size-dependences of the homogeneous line width of the confined exciton absorption with the lowest energy could be derived from the bandwidth of the exciton resonant luminescence. Based on the analysis similar to that for the line width of the free excitons in a bulk crystal, it is found that with decreasing microcrystal size the short-range deformation-type interaction of confined excitons with LA phonon becomes more and more important for the broadening, mainly because of the localization of the vibrational motion, while the Frohlich-type interaction with Lo phonons does not change much on account of its long-range interaction

Ce 3+ :LuLiF 4 as a broadband ultravioletamplification medium

Ce(3+):LuLiF(4) is shown to have a broad gain spectrum, which is attractive for short-pulse applications. A large small-signal gain over 6 dB/cm and a sufficiently large saturation fluence of 50 mJ/cm(2) were observed for 10-ns probe pulses under KrF excimer laser pumping. In a confocal four-pass configuration, 17-dB amplification was demonstrated for 325-nm picosecond pulses, and 20-dB gain was demonstrated for cw laser light.

Quantum confinement of excitons and their relaxation processes in CuCl microcrystals

Abstract Semiconductor microcrystals of CuCl with different mean sizes of an order of nm are grown inside transparent matrices of NaCl. Since the effective Bohr radius of excitons in CuCl is about 0.7 nm, the excitons in such microcrystals keep their bulk-like character even under the strong influence of confinement effect and the size-quantization occurs mainly in their translational motion. The blue shift and the splitting of the exciton levels are found with a decrease of the crystal size. Although there exists some size distribution of the microcrystals, exciton luminescence under the size-selective excitation with picosecond laser light and the analysis including strong reabsorption effects makes possible the determination of the quantum efficiency of exciton radiative decay. The size-dependent radiative decay time of excitons is found to be inversely proportional to the volume of the microcrystals as long as the energy separation between the size-quantized sublevels is larger than the thermal energy. This fact is quantitatively interpreted as the result of the change in the coherence volume of excitons whose motion is restricted by the microcrystal size.

Resonant Raman Scattering under Two-Photon Excitation of Excitonic Molecules in CdS

Two-photon-resonant Raman scattering under the giant two-photon excitation of excitonic molecule has been observed from various scattering directions, and two types of Raman scattering, M T and M M are found. The transverse and mixed modes of the \(A(\varGamma_{5})\) exciton state are left behind in the M T and M M Raman processes, respectively. These Raman scatterings reflect the characteristic dispersions of their respective exciton states, and conform to the geometrical selection rules derived on the assumption that the excitonic molecule state has the \(\varGamma_{1}\)-symmetry. A non-linear absorption peak due to the giant two-photon absorption is found at 2.5506 eV (λ=486.10 nm) with the use of intense probe light. The binding energy of excitonic molecule is found to be 4.4 meV for two \(A(\varGamma_{6})\) triplet excitons.