Excitation Photon Energy Dependence Research Articles

Silver photodiffusion into amorphous Ge chalcogenides

Silver photodiffusion into amorphous chalcogenides involves the movement of ions controlled by a UV-visible light illumination, and has potential application to memory devices. Understanding the kinetics of this phenomenon will expand the range of possible applications. Herein, we report the excitation photon energy dependence of the silver photodiffusion kinetics in Ag/amorphous Ge20S80/Si substrate stacks, probed by neutron reflectivity using four light-emitting diodes with different peak wavelengths. Time-dependent changes were clearly observed in all three of the Ag/Ag-doped reaction/chalcogenide host layers, in terms of layer thickness, scattering length density, and roughness. Silver photodiffusion effectively occurred when the excitation photon energy was greater than the optical gap of the chalcogenide host material. Excitation of lone-pair electrons to anti-bonding states at the chalcogenide layer therefore appears to play a crucial role in triggering silver photodiffusion.

Ultrafast spectroscopy of shift-current in ferroelectric semiconductor Sn2P2S6

We report sub-picosecond photocarrier dynamics observed via emitted terahertz waves in a ferroelectric semiconductor Sn2P2S6. Excitation photon energy, intensity and polarization dependences of the photocarrier dynamics testify that the ultrafast photocurrent originates from the shift-current under interband excitation. The photocurrent excitation spectrum shows a quantitative agreement with that derived from first-principles calculations with the Berry connection integrated, showing the shift-current to be a sensitive feature in ferroelectrics. The terahertz emission spectroscopy is a powerful tool to identify the intricate carrier dynamics under pulsed photoexcitation.

Half-metallicity of the ferrimagnet Mn2VAl revealed by resonant inelastic soft x-ray scattering in a magnetic field

Detailed information on the electronic states of both V and Mn $3d$ electrons in the ferrimagnet $\mathrm{M}{\mathrm{n}}_{2}\mathrm{VAl}$ is obtained by bulk sensitive resonant inelastic soft x-ray scattering (SX-RIXS) excited with circularly polarized light under an external magnetic field. The results under the V $L$-edge excitation have revealed the negligible partial density of states (PDOS) of the V $3d$ states around the Fermi energy as well as their rather localized character. Under the Mn $L$-edge excitation, on the other hand, the spectra are dominated by fluorescence with clear magnetic circular dichroism with noticeable excitation photon energy dependence. Compared with the theoretical prediction of the RIXS spectra based on the density-functional-theory band structure calculation, an itinerant, spin-dependent character of the Mn $3d$ states and decays of the Mn $2p$ core states are confirmed in consistence with the half-metallicity of the Mn $3d$ states.

Observation of Efficient Upconversion Photoluminescence of Single-Walled Carbon Nanotubes

Because of their potential applications in optoelectronics and bioimaging, photoluminescence (PL) properties of single-walled carbon nanotubes (SWNTs) have attracted much attention over the past decade [1-3]. Here, we report a novel aspect of the PL properties of SWNTs: availability of near-infrared PL (~950-1000 nm) by excitation with longer wavelength light (>1100-1200 nm), namely, upconversion PL. On the basis of the observations of temperature and excitation photon energy dependence, the observed phenomenon is attributed to the efficient phonon-assisted exciton upconversion processes. Furthermore, we demonstrate that strong enhancement of the upconversion PL occurs with additional chemical treatment on SWNTs. Materials exhibiting efficient upconversion PL in the near infrared range have been strongly desired for the observation of deep inside of highly scattering biological media. Our findings may thus open the door to the application of SWNTs in upconversion imaging of unprecedentedly deep inside of biological tissues. [1] Y. Miyauchi, J. Mater. Chem. C 1, 6499 (2013). [2] Y. Miyauchi, M. Iwamura, S. Mouri, T. Kawazoe, M. Ohtsu, and K. Matsuda, Nature Photon. 7, 715 (2013). [3] M. Iwamura, N. Akizuki, Y. Miyauchi, S. Mouri, J. Shaver, Z. Gao, L. Cognet, B. Lounis and K. Matsuda, ACS Nano, asap (2014).

Ultrafast magneto-photocurrents in GaAs: Separation of surface and bulk contributions

We induce ultrafast magneto-photocurrents in a GaAs crystal employing interband excitation with femtosecond laser pulses at room temperature and non-invasively separate surface and bulk contributions to the overall current response. The separation between the different symmetry contributions is achieved by measuring the simultaneously emitted terahertz radiation for different sample orientations. Excitation intensity and photon energy dependences of the magneto-photocurrents for linearly and circularly polarized excitations reveal an involvement of different microscopic origins, one of which is the inverse spin Hall effect. Our experiments are important for a better understanding of the complex momentum-space carrier dynamics in magnetic fields.

Observation of Two Peculiar Types of Electronic Dispersive Structures in Thallium Selenide Studied by Angle-Resolved Photoemission Spectroscopy

Two peculiar types of electronic dispersive structures, irreproducible in bulk band calculations, have been observed in quasi-one-dimensional TlSe studied by angle-resolved photoemission spectroscopy. One is of the cross type centered at the Γ point and extending along the Γ–H–T direction parallel to the chains, which are formed along the crystallographic c-axis. The other is of the pyramid type with truncated pyramids regularly arranged along the Γ–N direction perpendicular to the chains. To an extent given by the obtained excitation photon energy dependence, the above electronic structures can be assigned to high-lying surface conduction bands resulting from secondary electron emission.

The observation of unoccupied quantum-well states in Bi thin film grown on Si(111) by two-photon photoemission spectroscopy

We have performed angle-resolved two-photon photoemission (AR-2PPE) measurements of Bi thin film with the thickness of 12 bilayers grown on Si(111) surface. From the excitation photon-energy dependence of AR-2PPE spectra, we have obtained the energy dispersion of unoccupied electronic states in the energy range between 2.0 and 3.0 eV above the EF. The observed electronic states are attributed to the unoccupied quantum-well states due to quantization of the 6 p-derived bulk bands along the Γ-T direction.

Excitation photon energy dependence of the relaxation processes of the photoexcited states in a quasi-one-dimensional halogen bridged Pt complex

Excitation photon energy dependence of the relaxation processes of the photoexcited states in a quasi-one-dimensional halogenbridged platinum complex has been investigated by femto second transient absorption measurements. In a Pt complex, [Pt(en)2][Pt(en)2I2](SO4)2.6H2O (Pt-I-SO4), a photo-induced absorption (PA) band caused by self trapped excitons (STE’s) has been observed when the excitation photon energy is close to the peak energy, 1.44 eV, of the absorption band of the one photon allowed charge transfer (CT) exciton with odd parity. In addition, a long lived PA bands caused by charged solitons (CS’s) has been observed. The intensity of these long lived CS absorption band shows quadratic excitation power dependence. This shows that CS’s pairs are not generated from odd-CT-excitons but from the higher energy two photon excited states. When the excitation photon energy is close to a half of the of the even CT- exciton energy, 1.84 eV, PA bands caused by STE’s and CS’s have been observed. These states are generated from the two photon excited even CT-excitons. When the excitation photon energy is much higher than the energy of the odd CT-exciton, PA bands caused by CS’s and polarons have been observed. The intensities of these PA bands show linear excitation power dependences. Photo-generated free electron hole pairs are considered to relax into CS’s and polarons.

Ultrafast electron and lattice dynamics at potassium-covered Cu(111) surfaces

Electron and coherent phonon dynamics at potassium-covered Cu(111) surfaces have been studied by using femtosecond time-resolved second-harmonic generation (TRSHG). At the coverages from 0.22 to 0.35 monolayer (ML), TRSHG traces show the oscillatory component with a frequency of $3.05\ifmmode\pm\else\textpm\fi{}0.05\text{ }\text{THz}$. The amplitude of this component decreases as coverage increases higher than $\ensuremath{\sim}0.35$ ML, whereas another oscillating component with a frequency of $1.26\ifmmode\pm\else\textpm\fi{}0.03\text{ }\text{THz}$ grows. Both components are ascribed to K-Cu stretching motion. The spectral changes with coverage suggest that the overlayer structure varies with lateral compression. The fast transient peak in TRSHG traces at around zero delay changes its sign from negative to positive when coverage exceeds $\ensuremath{\sim}0.22$ ML. Since the quantum-well state (QWS) is partly filled at around this coverage, electrons in the QWS are principally responsible for the transition of the electronic response. Furthermore, the excitation photon energy dependence of TRSHG traces indicates that the excitation of substrate $d$-band electrons, giving rise to rapid charge fluctuations in the QWS, generates the coherent K-Cu stretching vibration. Consequently, the QWS plays a major role in the electronic and nuclear dynamics induced by pump pulses at $h\ensuremath{\nu}=2.2\text{ }\text{eV}$.

Photothermal Spectroscopic Measurements by Dual Sampling Method in Intermittent-Contact-Mode Atomic Force Microscopy

We have proposed a dual sampling method in intermittent-contact-mode atomic force microscopy with a piezoresistive cantilever for photothermal (PT) measurements. This method realizes a sensitive detection of thermal expansion in a sample by excluding crosstalk from photoabsorption signals generated at the piezoresistive sensor on the cantilever. We have confirmed the feasibility of this method through the excitation photon energy dependence of the PT signal measured on Si and GaAs.

Photon energy and carrier density dependence of spin dynamics in bulk CdTe crystal at room temperature

Excitation photon energy and carrier density dependence of spin dynamics in bulk CdTe crystal was studied by time resolved pump-probe reflectivity technique at room temperature. The results show that spin relaxation time decreases monotonously. While with increasing excitation carrier density, the time constants increases initially then decreases after reaching a maximum value. Our experimental results reveal that both D’yakonov–Perel’ [M. I. D’yakonov and V. I. Perel’, Sov. Phys. JETP 38, 177 (1974)] and Elliot–Yafet [R. J. Elliott, Phys. Rev. 96, 266 (1954); Y. Yafet, Solid State Phys. 14, 1 (1963)] mechanisms dominate the spin relaxation process in CdTe crystal.

Excitation photon energy dependence of photo-induced phase transition in (EDO-TTF)2PF6

The conducting charge transfer complex (EDO-TTF)2PF6 has two types of charge transfer bands in the low temperature insulator phase: CT1 at 0.56 eV and CT2 at 1.38 eV. We excited these bands independently with a tunable ultrashort pulse laser and studied the difference of the photo-induced phases by measuring change in reflectivity spectrum over a wide photon energy range. As a result, we found that both the photo-induced phases by CT1 and CT2 excitation are the same except for their photo-conversion efficiencies

Exciton luminescence spectra of CuCl films and their change by photo-irradiation

We have observed luminescence spectra of vapor-deposited CuCl films on rutile TiO 2 under photo-irradiation with picosecond light pulses at 4.6 K. Decrease in the Z 3 -exciton and I 1 luminescence intensities and an appearance of new exciton luminescence are found to be induced by the photo-irradiation. From the excitation photon energy dependence of the luminescence spectra, it is found that the Z 3 -exciton luminescence is resolved into two components: one is attributed to the optical phonon Raman lines and the other to the free Z 3 -exciton luminescence. The intensity of the Raman lines resonantly increases at the Z 3 -exciton band resulting in the optical phonon structures in the excitation spectrum. The excitation spectra of the new exciton and I 1 luminescence are similar to the absorption spectrum of CuCl, which suggest that the resonant Raman process observed in the Z 3 -exciton luminescence hardly contributes to the generation of the excitons associated with the new exciton and I 1 luminescence.

Angular distribution measurements for spin-orbit-state-resolved S2pphotoelectrons ofSF6in the shape-resonance region

The angular distributions of the spin-orbit state resolved S $2p$ photoelectrons of ${\mathrm{SF}}_{6}$ have been measured in the vicinity of the shape resonances. Relative partial photoionization cross sections of ${\mathrm{SF}}_{6}$ into the (S ${2p}_{3/2}{)}^{\ensuremath{-}1}$ and (S ${2p}_{1/2}{)}^{\ensuremath{-}1}$ states, and the asymmetry parameters of the S ${2p}_{3/2}$ and S ${2p}_{1/2}$ photoelectrons are presented. Enhancement of the photoionization cross section into the (S ${2p}_{3/2}{)}^{\ensuremath{-}1}$ state at the (S ${2p}_{1/2}{)}^{\ensuremath{-}1}$ ${2t}_{2g}$ resonance has been observed. The excitation photon energy dependence of the asymmetry parameter shows a broad dip at about the photon energies of the resonances. Continuum multiple-scattering calculation of the asymmetry parameters for the (S ${2p}_{3/2}{)}^{\ensuremath{-}1}$ and (S ${2p}_{1/2}{)}^{\ensuremath{-}1}$ states of ${\mathrm{SF}}_{6}$ reproduces this behavior.

Ultrafast dynamics of reversibly photoinduced neutral-ionic transition in quasi-one-dimensional organic crystal tetrathiafulvalene-chloranil

We report that the phase transition between neutral and ionic phases (N-I transition) in the single crystal of tetrathiafulvalene-chloranil (TTF-CA) can be triggered by pulsed laser excitation with 80 femto-second pulse width. Irradiation with the pulsed laser light of which photon energy is 1.55eV can drive the phase changes in both I-to-N and N-to-I directions. We also report the excitation photon energy dependence of the N-I domain-wall dynamics.

Photoluminescence in CVD Diamond Films

Photoluminescence (PL) measurements of CVD diamond films reveal well-defined lines at 1.68, 1.945 and 2.156 eV, in addition to broad red, green and blue bands. The 1.68 eV line is attributed to Si-related center and when accompanied by the red band, shows a strong dependence of its intensity on the excitation photon energy. The 2.156 and 1.945 eV lines are assigned to nitrogen–vacancy complexes. The absence of the 1.945 eV line for samples with low nitrogen content shows that the formation of the 2.156 eV center is more favorable than of the 1.945 eV center. Blue, green and red broad PL bands are attributed to dislocations, boron-related defect and to nitrogen, respectively. Results on intensity and excitation photon energy dependences of PL measured for CVD and shock-synthesis diamond showed that a donor–acceptor recombination model cannot explain the broad band PL in our samples.

Solvation state selective excitation in resonance Raman spectroscopy. I. Experimental study on the C=N and the C=O stretching modes of phenol blue

We have measured the excitation photon energy dependence of the resonance Raman spectra of a solvatochromic dye, phenol blue (PB) in various solvents from nonpolar to dipolar ones, including supercritical fluids of trifluoromethane and carbon dioxide. We have found that the band peak positions of the C=N stretching mode of PB appear to change with the excitation laser frequency in polar solvents. On the other hand, no dependence of the band peak positions on the excitation frequency is found in nonpolar solvents. The peak positions of the C=O stretching mode also show small excitation energy dependence in chloroform and in methanol. The extent of the excitation energy dependence is correlated with the amount of the fluctuation of the local field on PB molecule exerted by the solvent, which had been estimated from the bandwidths of the absorption and the Raman spectra of PB in solution. The IR absorption spectra of PB are measured in chloroform and carbon tetrachloride, and compared with the Raman spectra in the same solvents. The excitation energy dependence is explained by the solvation state selective excitation in the resonance Raman spectroscopy, that is, the inhomogeneity that appears in the absorption spectra of PB also appears in the resonance Raman process.

Photocarrier generation in directly synthesized trans-polyacetylene

The direct photocarrier generation edge of directly synthesized trans-polyacetylene was studied by means of electric field, temperature and excitation photon energy dependence of photocarrier generation efficiency. We found that the excited states have an energy barrier to generate the charged carrier at the absorption edge and the direct barrierless photocarrier generation starts at about 1.7 eV. These results strongly support the existence of the exciton state at the absorption edge in trans-polyacetylene.

Excitation Photon Energy Dependent Relaxation Processes of the Photoexcited States in Wide Gap MX-Chain Complexes

Abstract Photo-induced absorption (PA) spectra of [Pt (en)2] [Pt (en)2X2] (SO2)2. 6H2O (X=Cl and Br) have been measured. Large excitation photon energy dependent changes were found in the features of the PA spectra. The excitation photon energy dependences are considered to be due to the difference between the relaxation processes of the charge transfer excitons and those of the higher energy excited states. Several new PA bands have been found.

Photon-Stimulated Desorption Mechanism of Cl+ Ions from Cl/Si(111) Surface

Photon-stimulated desorption (PSD) of Cl+ ions from a Cl/Si(111) surface was studied using synchrotron radiation. The kinetic energy distribution of Cl+ ions in PSD agreed with our previous results of electron-stimulated desorption (ESD). The excitation energy threshold of Cl+ desorption also agreed within the experimental error. Hence, the desorption mechanisms in both PSD and ESD were considered to be the same. We studied the excitation (photon) energy dependence of the Cl+ yield more precisely than ESD to elucidate the excitation mechanism for ion desorption. We found that the lower threshold photon energy was 17eV. Comparison with the ultraviolet photoelectron spectroscopy (UPS) spectrum showed that this energy corresponded to the excitation from the Cl 3s state to the Fermi level. Hence, this PSD process was explained by the KF model, where the Cl 3s core hole was filled by electrons of the Cl-Si bond through the Auger-like process, and then Cl+ ions desorbed by repulsive Coulomb force from the adjacent Si+.