Hayato Kamioka

Third-order optical nonlinearity originating from room-temperature exciton in layered compounds LaCuOS and LaCuOSe

We have studied the third-order optical nonlinearity (χ(3)) of epitaxial thin films of layered compounds LaCuOS and LaCuOSe at room temperature by a spectrally resolved degenerative four-wave mixing technique with femtosecond time resolution. The χ(3) values in both films are sharply resonant to optical absorption bands in the ultraviolet (UV) light region due to room-temperature exciton. The peak values are evaluated to be as large as 2–4×10−9 esu with a fast time response of 250–300 fs. These findings indicate that LaCuOS and LaCuOSe are promising materials for emerging optical nonlinear devices that operate in the UV light region compatible for GaN-based lasers.

Interstitial oxygen molecules in amorphous SiO2. III. Measurements of dissolution kinetics, diffusion coefficient, and solubility by infrared photoluminescence

Concentration changes of interstitial oxygen molecules (O2) in amorphous SiO2(a-SiO2) thermally annealed in oxygen atmosphere were examined by the O2 photoluminescence at 1272 nm excited with 765-nm light of titanium sapphire laser. This highly sensitive technique allows the time- and temperature-dependent concentration changes of interstitial O2 due to their incorporation from an oxygen atmosphere to be directly measured. The data provide the dissolution rate, the diffusion coefficient, and the solubility of interstitial O2 in a-SiO2 and are able to exclude interferences from other forms of mobile oxygen species in a-SiO2. These observations confirm that O2 molecules are incorporated into a-SiO2 without separating into monoatomic species, diffuse in a-SiO2 without extensive interaction with the a-SiO2 network, and play a primary role in the thermal oxidation of silicon.

Photoluminescence of CuInS2-based semiconductor quantum dots; Its origin and the effect of ZnS coating

The origin of the size-dependent photoluminescence (PL) of CuInS2-ZnS alloyed quantum dots and the effect of a ZnS thin layer coating on PL properties were studied. The PL emission band shifted to the shorter wavelength region with a decrease in the crystal size. A time-resolved measurement indicated that the PL lifetime for the lower energy component of the emission band was longer than that of the higher energy component. Although this is a characteristic feature of the donor-acceptor pair (DAP) recombination, the size dependent shift of the band was too large to be attributed to DAP recombination. The emission was attributed to an electronic transition from the quantum confined conduction band to an acceptor level. By coating with a ZnS thin layer the PL quantum yield increased up to 31%.

Interstitial oxygen molecules in amorphous SiO2. II. The influence of common dopants (SiOH, SiF, and SiCl groups) and fictive temperature on the decay of singlet photoluminescence

Time decay of photoluminescence due to interstitial oxygen molecules (O2) in synthetic amorphous SiO2(a-SiO2) was studied by varying the fictive temperature and the concentrations of common dopants (SiOH, SiCl, and SiF groups). The decay constant is insensitive to the fictive temperature, but strongly depends on the type of dopants: it is reduced by the nonradiative decay via an energy transfer from O2 to the vibrational modes of the dopants. The increases in the nonradiative decay rate due to SiOH, SiF, and SiCl groups are strong, slight, and negligible, respectively, which correlates with their vibrational energies. The quantum yield decreases by ∼20% as the SiOH content increases from 1017 to 1020cm−3. The deviation from the single exponential decay is due to the shape variation in the a-SiO2 network cages that surround O2, and to the distance distribution between O2 and SiOH groups.

Quantum beat between two excitonic levels split by spin-orbit interactions in the oxychalcogenide LaCuOS

Degenerate four-wave mixing signals excited by femtosecond laser pulses were measured in LaCuOCh (Ch = S and Se) at 4 K. The signals for LaCuOS exhibit a beat structure with a period of 480 fs just at the exciton peak energy, indicating that the lowest exciton states are split by 9 meV, but no beat structure was observed in LaCuOSe, regardless of the lasers energy. The spin--orbit interaction of the Ch ion accompanied by the hybridization of Cu 3d orbitals causes splitting of the exciton levels. Furthermore, the contribution of Ch p orbitals in the valence band maximum is larger in LaCuOSe owing to increased covalency in the Cu--Ch bond when S is replaced with Se.

Photo-induced charge state conversion of Eu2+ in Ca2ZnSi2O7

Eu2+ doped sorosilicate Ca2ZnSi2O7 (melilite) exhibits a broad band emission peaking at ∼600 nm (∼2 eV) due to the electric dipole allowed transition of 4f65d1 to 4f7 of Eu2+ by an excitation with blue light (460 nm). Strong O2− ligand field with low symmetry due to the layered tetragonal crystallographic structure of the melilite may play a dominant role in lowering the emission band energy to ∼2 eV. In addition, line emissions attributable to the transitions from D50 to F7J of Eu3+ ions are detected by an excitation with deep UV light with sub-360 nm wavelengths. This is due to the formation of transient Eu3+ ions via charge transfer from Eu2+ to the matrix. The lifetime of the transient Eu3+ ion is found to be 58 ms by a pump-probe measurement, in which UV pulse laser and green continuous wave laser are employed as pump and probe lights, respectively. Based on these results, the energy diagram of Eu2+ in Ca2ZnSi2O7 is proposed.

Magnetic and Electronic Properties of Valence-Controlled Ni–Fe Cyanide

Magnetics and electronic properties has been investigated for the Prussian blue type cyano-bridged transition metal compound, Na 0.72-δ Ni[Fe(CN) 6 ] 0.68 ·5.1H 2 O (0.0 ≤δ≤0.58), as a function of the hole concentration δ of the d -electron system. Mother compound (δ= 0) takes Ni 2+ ( t 2g 6 e g 2 : S = 1)–Fe 2+ ( t 2g 6 : S = 0) configuration, and is paramagnetic down to zero temperature. Holes are selectively introduced on the Fe site. Ferromagnetic transition appears at δ= 0.26, and the transition temperature T C increases from 11 K at δ= 0.26 to 21 K at δ=0.58. We further systematically investigated the absorption spectra in the infrared–violet region against δ. We compared these magnetic and electronic properties of the valence-controlled Ni–Fe compound with those of an isostructural valence-controlled Co–Fe compound.

Carrier formation dynamics of a small-molecular organic photovoltaic

We investigated carrier formation dynamics in a small-molecular bulk heterojunction solar cell, 2,5-di-(2-ethylhexyl)-3,6-bis-(5″-n-hexy-[2,2′,5′,2″]terthiophen-5-yl)-pyrrolo[3,4-c]pyrrolo-1,4-dione/[6,6]-phenyl C71-butyric acid methyl ester, with low bandgap (Egap≈1.5 eV). The photoinduced absorption (PIA) spectra of the blend film were decomposed into three PIAs, i.e., those due to donor exciton (D*), acceptor exciton (A*), and mobile carrier (D+). The analysis revealed carrier conversion from D* with a conversion time of ∼1.3 ps.

Carrier Formation Dynamics of Organic Photovoltaics as Investigated by Time-Resolved Spectroscopy

Bulk heterojunction (BHJ) based on a donor (D) polymer and an acceptor (A) fullerene derivative is a promising organic photovoltaics (OPV). In order to improve the incident photon-to-current efficiency (IPCE) of the BHJ solar cell, a comprehensive understanding of the ultrafast dynamics of excited species, such as singlet exciton (D*), interfacial charge-transfer (CT) state, and carrier (D

Molecular mixing in donor and acceptor domains as investigated by scanning transmission X-ray microscopy

The nanolevel molecular structure of a bulk heterojunction (BHJ) with a donor (D) polymer and acceptor (A) fullerene derivative is indispensable for true comprehension of highly efficient organic photovoltaic devices. Here, we performed scanning transmission X-ray microscopy of a poly(9,9-dioctylfluorene-co-bithiophene) (F8T2)/[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) blend film with periodic nanostructure. The spatially resolved carbon K-edge absorption spectra revealed that the nanostructure consists of two types of domains with considerable molecular mixing. The fullerene mass fraction is 71 ± 1 and 33 ± 2 wt % for the PC71BM- and F8T2-rich domains, respectively.