Exciton Relaxation Processes Research Articles

EXCITON DYNAMICS OF ORTHORHOMBIC PHASE PbI2 EMBEDDED IN PbI2-PbBr2 MIXED CRYSTALS

Reflection, time-integrated and time-resolved luminescence spectra have been measured in PbI 2(1-x) Br 2x(x≥0.5) mixed crystals in order to elucidate the excitonic properties and relaxation processes of orthorhombic phase PbI 2 embedded in the mixed crystals. The crystal structure of PbI 2(1-x) Br 2x above x=0.5 is orthorhombic and an observed new reflection peak (~2.5 × 104 cm -1) above x=0.5 is due to an excitonic state of orthorhombic phase PbI 2. On the excitation energy above 2.5 × 104 cm -1, a broad luminescence band with a large Stokes shift from the reflection peak was observed. This luminescence band is attributed to a self-trapped exciton state. The decay of the luminescence has two components. Based on an atomic excitation picture, the shorter decay component (≤10 ns ) is attributed to the allowed transition from 3P1 to 1S0. On the other hand, the longer decay component (~3 μ s ) might be due to the forbidden transition from 3P0 to 1S0.

SELF-TRAPPED EXCITONS IN ORTHORHOMBIC SnBr2

Luminescence properties of SnBr 2 have been studied to reveal the photo-excited exciton relaxation process. Two types of luminescence with large Stokes shifts are found at low temperatures; the 2.2-eV luminescence band produced under the photo-excitation in the first exciton region, and the 2.5-eV luminescence band stimulated by photons with energies above the bandgap. The time-resolved photoluminescence measurements have revealed that the 2.2-eV luminescence comprises fast (1.2 μs) and slow (6.4 μs) exponential decay components, whereas the 2.5-eV luminescence shows the time dependence of I(t)∞ t-0.9. These results suggest that the former band is attributed to the radiative decay of self-trapped excitons, and the latter band would originate from tunneling recombination of holes with the STEL as in the case of lead halides.

Ultrafast exciton dynamics of J-aggregates in room temperature solution studied by third-order nonlinear optical spectroscopy and numerical simulation based on exciton theory

We report a study of the exciton dynamics in 1,1′-diethyl-3,3′-bis(sulforpropyl)-5,5′,6,6′ -tetrachlorobenzimidacarbocyanine (BIC) J-aggregates in water solution at room temperature by third-order nonlinear optical spectroscopy and numerical simulations based on exciton theory. The temporal profiles of the transient grating signals depend strongly on the excitation intensity as a result of exciton–exciton annihilation. On the other hand, the peak shift measurement gives information on the fluctuations of the transition frequency of the system. The peak shift decays with time constants of 26 and 128 fs. There is no finite peak shift on a longer time scale. The electronic state of J-aggregates is described by a Frenkel exciton Hamiltonian, and the exciton population relaxation processes is described by Redfield equations. Based on the numerical simulations, the peak shift data can only be explained even qualitatively when both exchange narrowing and exciton relaxation process are included in the model. The 128-fs component is assigned to a “hopping” time between exciton units. We confirmed that while the static disorder within an exction state that is partially delocalized due to static disorder is exchange-narrowed, the exchange narrowing of the dynamical disorder is not complete but appears as lifetime broadening, which competes with the exchange narrowing of the fluctuations. The effect of the exciton relaxation on the absorption spectrum is discussed.

Ultrafast relaxation processes of solitons, polarons, and self-trapped excitons in MX-chain

Ultrafast relaxation processes of photoexcited states of a quasi-one-dimensional halogen-bridged mixed-valence metal complex [Pt(en) 2][Pt(en) 2Cl 2](PF 6) 4 have been investigated by femtosecond transient absorption spectra measurements. It has been found that a considerable part of the neutral soliton pairs is generated directly from photogenerated charge transfer excitons. Charged soliton pairs have been found to be generated from free electron–hole pairs. The rise time and the decay time of the charged soliton absorption band are 2.2 and 6.5 ps, respectively. The polaron pairs are generated from free electron–hole pairs within time resolution.

Nonradiative Relaxation Processes of Excitons inβ-ZnP2Studied by Photocalorimetric Spectroscopy

Nonradiative relaxation processes of excitons have been investigated by means of photocalorimetric (PC) spectroscopy in the exciton resonance region of β-ZnP 2 by detecting temperature increase of the crystal under excitation with a Ti:sapphire laser at 4.5 K. For singlet excitons, power of the light absorbed by the crystal is converted to that of the thermal heating of the crystal almost at a constant rate as high as about 80% in the energy range from the 1 s longitudinal exciton energy E L to the band gap energy E g . The heat generation, however, is largely suppressed in the 1 s reflection band, where the exciton luminescence is also decreased in intensity, which suggests the presence of other optical process in this region such as resonant Rayleigh scattering. From comparison of the PC spectrum with excitation spectrum for the singlet exciton luminescence, it is found that a ratio of the PC signal to the luminescence intensity increases at the exciton resonances n =1, 2, 3 and 4. This fact indicates t...

Excitons at a single localized center induced by a natural composition modulation in bulkGa0.5In0.5P

Optical properties of ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}$ grown by gas-source molecular beam epitaxy are reported. A strong optical anisotropy due to the spontaneous lateral composition modulation of Ga and In was observed in macro- and micro-photoluminescence (PL) spectra. The micro-PL study revealed that the PL band is composed of sharp lines that arise from the radiative decay of excitons trapped at local potential minima caused by the composition modulation. Anisotropy in PL rise and decay times was also observed. The detection energy dependence of the PL decay time is well explained by considering the relaxation process of excitons from shallow traps to deep traps.

Capture, relaxation, and recombination in two-dimensional quantum-dot superlattices

We present a systematic study of the exciton capture, relaxation, and recombination processes in two-dimensional quantum-dot superlattices (2D QDSL's) based on time-resolved photoluminescence measurements. Due to the formation of minibands in 2D QDSL's, the capture of excitons from the miniband into some large islands is found to be a quantum capture process. The capture time increases significantly with increasing excitation density. In addition, the excitons relax rapidly within the miniband. However, the relaxation becomes slower at high excitation densities. Furthermore, the fast recombination in the miniband indicates the drastic elongation of the exciton coherence length and the significant delocalization of the wave functions. Finally, the observation of radiative recombination over a wide energy region implies the relaxation of momentum conservation or the small exciton effective mass in 2D QDSL's. All phenomena suggest that the exciton dynamics in 2D QDSL's is governed by the exciton coherence length in the miniband.

Intrinsic luminescence in PbBr 2 crystals under one- and two-photon excitation

We studied luminescence properties in PbBr 2 crystals under one- and two-photon excitation, and found that a luminescence band at 2.56 eV is intrinsic in origin. Decay time measurements revealed the exciton relaxation process in which an exciton is decomposed into a charge-separated pair of a self-trapped electron (STEL) and a self-trapped hole (STH). The relaxation process is ascribed to strong interaction of electrons and holes with phonons.

Photoluminescence from heavy-hole and light-hole excitons split by thermal strain in CuI thin films

We have investigated photoluminescence properties of heavy-hole and light-hole excitons in 30-nm CuI thin films grown by vacuum deposition onto (0 0 0 1) Al 2O 3, (0 0 1) NaCl, and (0 0 1) KCl substrates. In the absorption spectra, the thermal-strain-induced splitting of the heavy-hole- and light-hole-exciton energies is observed. The magnitude of the splitting energy depends on substrate materials. The free-exciton photoluminescence exhibits a remarkable dependence on the excitation energy. The peak energy and intensity of the photoluminescence oscillate with the period of the longitudinal optical (LO)-phonon energy reflecting intense nonequilibrium in the relaxation process of the excitons. Furthermore, it is found that whether the exciton-splitting energy is smaller than the LO-phonon energy or not dominates the final stage of the exciton-relaxation process.

Optical sampling four-wave-mixing experiment for exciton relaxation processes

We have developed the optical sampling four-wave mixing technique applicable to measurements of the ultrafast dynamical processes in liquid and solid materials. The method is examined to reveal the exciton dynamical processes in GaAs quantum wells at low temperatures. By using the optical sampling principle, this new spectroscopic tool provides well timing-controlled two-color light pulses for excitation and detection, and also wide observable temporal range from sub-picoseconds to a few nanoseconds with femtosecond time resolution. Various transient processes such as phase relaxation, spin relaxation, and energy relaxation processes can therefore be observed rapidly in the common measurement system.

Time-Resolved Spectroscopic Studies of Localized Excitons in CsCl:I and CsCl:Br

Luminescence spectra of excitons localized at iodine or bromine ions in CsCl have been studied in the temperature range 10–300 K. Two bands appear at 3.96 and 5.13 eV in CsCl:I, while three bands are observed at 3.84, 4.34 and 5.27 eV in CsCl:Br. The decay behaviors of these bands are measured under the single-bunch operation of synchrotron radiation. It is found that the 4.34-eV luminescence in CsCl:Br has a rise component around 1 ns just after the pulse excitation. In addition, time-resolved luminescence spectra are investigated through two different time windows. For the 3.96-eV luminescence in CsCl:I, the slow-component peak is located about 40 meV lower than the fast-component peak. For the 4.34-eV luminescence in CsCl:Br, the slow-component peak is located 210 meV lower than the fast one. These results suggest that an adiabatic potential energy surface has some local minima for the localized excitons in CsCl. The difference in the relaxation process of excitons between NaCl- and CsCl-type structure...

Ultrafast relaxation processes of excitons to the relaxed excited state in one-dimensional dimethylglyoxime platinum complexes driven by intramolecular vibration

Optical response due to the photo excited carrier or exciton dynamics in a one-dimensional (1D) dimethylglyoxime Pt complex [Pt(dmg)2] has been investigated by femtosecond pump–probe spectroscopy. Measurements were made at several excitation energies between the exciton state and the free electron hole pair state. Induced absorption was observed in the low-energy side of the exciton band at any excitation energy. The spectral shape of the induced absorption changed with time between 0.1 and 0.3 ps after instantaneous rise up. These results can be explained by the formation and thermalization processes of the relaxed excited state on the adiabatic potential surface. The unthermalized relaxed excited state is formed in a very short time (<50 fs). From the Raman spectrum, the frequency of the intermolecular stretching mode along the chain was estimated to be 80 cm−1 (T∼400 fs). These facts suggest that, in contrast to other 1D systems such as polydiacetylenes (PDA) and halogen bridged mixed valence metal complexes (MX), the formation of the relaxed excited state is caused not by the stretching vibration along the 1D chain. We propose the possibility that the triggering of the primary relaxation is caused by a high frequency (>600 cm−1) intramolecular mode which is not in the direction of the 1D chain.

Subpicosecond optical sampling spectrometer using asynchronous tunable mode-locked lasers

We have developed a subpicosecond nondegenerate pump-probe spectrometer based on the optical sampling technique using a combination of asynchronous tunable femtosecond lasers. Owing to its simple instrumentation, time evolution of subpicosecond to nanosecond excited molecular processes in solution and of exciton relaxation processes in semiconductor multiple quantum wells has been detected on an oscilloscope by an effective time base magnification of 760 000 times. Fast acquisition of the temporal profiles and ease of wavelength scanning allowed compilation of time-resolved absorption and excitation spectra. A possible extention of the spectrometer with one of the light sources being a synchrotron radiation instead of the femtosecond laser is discussed.

Photoluminescence of orthorhombic and cubic single crystals

The photoluminescence of orthorhombic and cubic crystals has been studied with the use of a deuterium lamp or synchrotron radiation as the light source. The spectra of orthorhombic samples exhibit a single band at 2.07 eV, while the spectra of cubic samples show an intense band at 4.10 eV, accompanied by some other bands depending on the sample. These two main bands are stimulated only under exciton-band excitation. No luminescence is detected when both crystals are excited with photons in the band-to-band region. The present results are compared with earlier data for and which have the orthorhombic structure. The relaxation processes of excitons and electron-hole pairs in lead halides are discussed.

Sharp photoluminescence lines of InAs quantum dot embedded in GaAs mesa

Sharp photoluminescence (PL) lines were observed in a single InAs quantum dot embedded in a micro-sized mesa of GaAs. These lines are in groups which conform to the conventional PL spectra. Power and temperature dependence of PL were measured to study the energy level structure. Clear state filling was observed in each sharp PL line by increasing the excitation power density. With the increase of the temperature, the PL intensity of these lines became gradually very weak successively from the higher energy side. These results suggest that most of the PL lines observed in our experiments come from the excited states of excitons confined in InAs quantum dots. Level filling and the relaxation process of excitons are discussed.

Spin polarization of exciton luminescence from orderedGa0.5In0.5P

Circularly polarized excitation light produces spin-polarized excitons in long-range ordered ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}$ because of a splitting at the valence-band maximum. We observed the spin-relaxation process in ordered ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}$ under resonant excitation of heavy-hole excitons. The circularly polarized exciton luminescence shows a maximum anisotropy of about 53%. The decay profile of the polarized luminescence is described by two components: the rapid decay by exciton-relaxation processes and the slower exciton recombination. The relaxation of the exciton-spin polarization obeys a decay with a time constant of 105 ps.

Ultrafast Exciton Motion in Photosynthetic Antenna Systems: The FMO-Complex

Excitation energy transfer in the Fenna−Matthews−Olson (FMO) photosynthetic antenna complex of chlorobium tepidum is investigated theoretically. On the basis of a dissipative multiexciton theory, the numerical simulations reproduce the cw-absorption and, in using the same parameters, the differential absorption of ultrafast pump−probe experiments. Exciton relaxation processes are included via a coupling to the vibrations of the protein matrix. In order take into account a delocalized protein−pigment interaction a correlation radius of the protein vibrations is introduced. The model allows for the study of the temperature dependence of optical spectra and enables one to utilize exciton relaxation data as a probe for a global-shape estimation of the spectral density of low-frequency protein vibrations. In fitting the cw-absorption measured at 5 and 107 K, the strength of the exciton-vibrational coupling, the related correlation radius and spectral density, and the inhomogeneous broadening of the exciton lev...

Localized exciton and its stimulated emission in InGaN multiple quantum wells

We have studied exciton localization and its stimulated emission in InGaN multiple quantum wells at 2K. Mobility edge was determined by the use of site-selectively excited photoluminescence (PL) measurement. A time-resolved PL spectroscopy was performed to study exciton relaxation processes. The PL decay time increased with the decrease of the detection photon energy, indicating the dynamical features of exciton localization. Under the high-density excitation, we observed the localized exciton luminescence turned into the stimulated emission just below the mobility edge. In addition, a nonlinear luminescence measurement was performed to study the detailed optical property of the stimulated emission.

Subpicosecond Pump−Supercontinuum Probe Spectroscopy of LH2 Photosynthetic Antenna Proteins at Low Temperature

Exciton relaxation and energy-transfer processes in the B850 circular aggregate of bacteriochlorophyll a molecules from the purple nonsulfur photosynthetic bacterium Rhodobacter sphaeroides were studied at temperatures below 18 K. Excitons were selectively excited by a 7 nm spectral bandwidth pump pulse resonant with the inhomogeneously broadened long-wavelength side of the B850 ground-state absorption spectrum (between 860 and 879 nm). The transient spectra were measured over the 786−924 nm spectral range using a white light continuum probe pulse. Characteristic changes of transient spectra were observed over 4 decades of time, from about 10-13 to about 10-9 s. The spectral evolution was pump wavelength-dependent, changes being least notable at far-red excitation. A simple model was put forward to interpret the data, assuming that the sample consists of an ensemble of spectrally disorded excitons, each representing a separate B850 ring. It was found that the exciton coupling and diagonal disorder play almost equally important roles in the formation of the spectral and dynamical properties of light excitations in B850 antenna. The main effects of disorder considered were the spectral shifts, splitting of the degenerate exciton levels, and redistribution of the dipole strength of the transitions. Assuming that the contribution of least disturbed excitons is largest near the peak of the ground-state absorption spectrum and greatest near the edge, most of the known spectroscopic properties of LH2 complexes can be well understood, at least qualitatively. Specifically, the rough 100 fs response time was assigned to interexciton level relaxation; the three time constants, ca. 800 fs, ca. 15 ps, and ca. 150 ps, were attributed to exciton energy transfer, most likely, between the B850 rings. The ca. 1 ns decay time is due to the finite exciton lifetime. The circular LH1 antennas likely possess similar properties.

Exciton localisation in CdSe islands buried in a quantum well of Zn 1− xCd xSe

The optical properties of CdSe/ZnSe quantum structures with nominal thickness of the CdSe layer between 1 and 4 monolayers (ML) have been studied. The transition from two- to zero-dimensional quantum confinement can be monitored by photoluminescence (PL), photoluminescence-excitation (PLE), and time-resolved PL experiments. The influence of different degrees of localisation on the relaxation process of excitons is investigated. Experimental results for the excitonic transition energies are in good agreement with theory when strain-, confinement- and alloy-fluctuation-effects are included. The observation of multiple phonon modes indicates the formation of a Zn 1− x Cd x Se alloy with varying composition as well as the accumulation of CdSe in small islands.