Induced Absorption Feature Research Articles

Picosecond kinetic confirmation of overlapping Ca cluster and MNa absorption bands in UV grade CaF2

The ArF laser induced absorption feature between 450 and 700 nm observed in the bulk of mono crystalline calcium fluoride (CaF2) upon prolonged 193 nm irradiation is shown to have two origins: The first band at 530 nm is revealed by a transient femtosecond (fs) laser transparency spectrum and attributed to Ca clusters (colloid). The second band at 600 nm originates from MNa centers as identified by their laser induced fluorescence excitation spectrum, emission wavelength, and fluorescence lifetime. The analysis of the absorption feature by fs pump-probe techniques benefits from the different lifetimes of the transient transparency (excited states of the Ca clusters) and the fluorescing MNa centers of about 5 ps and 22 ns, respectively.

γ-Ray induced color centers in pure and Yb doped LiYF4 and LiLuF4 single crystals

c-Ray irradiation was used to carry out a comparative study of the induced optical absorption phenomena and color center creation in the ultraviolet and visible spectral regions. The F-center absorption band at 315 and 330 nm in LiLuF4 and LiYF4, respectively, is the dominating induced absorption feature. The amplitude of the induced absorption is reduced by more than a factor of 3 in Yb-doped crystals compared to the undoped ones. Comparison was made with other fluorides, such as CaF2 ,K Y 3F10 and BaY2F8. In Yb-doped CaF2, LiLuF4, LiYF4, BaY2F8 and KY3F10 we observed arising of two possible types of Yb 2+ centers after c-ray irradiation. For the entire materials exclude KY3F10 we found Yb 2+ centers related to Yb 3+ , as an effect of recharging one of Yb 3+ ion from pair, while for KY3F10 we found mainly Yb 2+ centers related to isolated Yb 3+ ions, as an effect of Compton electron capturing by isolated Yb 3+ ion.

Color centers in LiCaAlF6 single crystals and their suppression by doping

LiCaAlF 6 (LiCAF) single crystals pure and doped with MgF2 and BaF2 were successfully grown by the Czochralski technique. Optical absorption measurements in the UV/Visible spectral regions following x-ray irradiation (radiation induced absorption) were performed in order to investigate the radiation damage of the crystals. The F-center absorption band at 262 nm is the dominating induced absorption feature. The amplitude of the F-absorption band is suppressed by more than a factor of 3 in Mg-doped LiCAF compared to undoped, while for Ba-doped LiCAF, almost no variation is obtained. In Mg-doped crystals the optimum doping concentration is of about 0.2 mol % of MgF2 in the melt.

X-ray induced color centres in pure and doped LiYF 4 AND LiLuF 4 single crystals

YLiF 4 and LuLiF 4 pure and doped with Yb 3+ ion were grown by the Czochralski technique under CF 4 atmosphere. X-ray irradiation was used to carry out a comparative study of the induced optical absorption phenomena and color center creation in the ultraviolet and visible spectral regions. The F-center absorption band at 315 and 340 v nm in LLF and YLF, respectively, is the dominating induced absorption feature. The amplitude of the induced absorption is reduced by more than a factor of 10 in Yb-doped crystals compared to the undoped ones.

The Linear Absorption and Pump−Probe Spectra of Cylindrical Molecular Aggregates

We study the optical response of Frenkel excitons in molecular J aggregates with a cylindrical geometry. Such aggregates have recently been prepared for a class of cyanine dyes and are akin to the rod- and ring-shaped light-harvesting systems found in certain bacteria. The linear absorption spectrum exhibits two lines with perpendicular polarization that are separated by a “ring energy scale”, which is set by the circumference of the cylinder and the intermolecular transfer interaction in the circumferential direction. On the other hand, the pump−probe spectrum shows bleaching and induced absorption features that are separated by a much smaller energy scale, which is determined by an effective Pauli gap imposed by the length of the cylinder and the transfer interaction in its longitudinal direction. We show that this can be well-understood from the approximate separation of the set of two-exciton states, into classes of inter-ring and intra-ring two-exciton states. Our calculations show that the experimental linear absorption spectrum may be used to estimate the cylinder circumference, while the pump−probe spectrum yields information on the length of the cylinder or on the delocalization length of the excitons in its longitudinal direction. We apply this method to cylindrical aggregates of cyanine dyes.

Charge-carrier effects on the optical properties of poly(p-phenylene vinylene)

We present measurements of the effects of electrically injected charge carriers on the optical properties of a soluble, poly(p-phenylene vinylene) derivative. Pulsed electrical excitation was used to acquire the modulated optical characteristics of single-carrier polymer diodes for photon energies from 0.5 to 3.1 eV and for average injected charge-carrier densities up to about $1\ifmmode\times\else\texttimes\fi{}{10}^{18}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}.$ For injected carrier densities below about $5\ifmmode\times\else\texttimes\fi{}{10}^{17}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ the modulation of the optical properties is primarily due to electric-field-induced effects. At higher densities, the effects of the injected charge carriers dominate. In the near-absorption-edge spectral region, we find that there is substantial charge-carrier-induced bleaching of the lowest-energy optical transitions. This implies that charge-carrier effects in this spectral region are the result of state filling and/or exciton screening. In the spectral region from 0.5 to 1.8 eV, in the energy gap of the polymer, we find no significant effects due to the injected charge carriers. The widely reported induced absorption features at about 0.6 and 1.6 eV that have been attributed to polarons were not observed. We find no significant polaron absorption over this spectral range in this polymer.

Pump−Probe Spectroscopy and the Exciton Delocalization Length in Molecular Aggregates

Using numerical simulations, we investigate to what extent the pump−probe spectrum can be used as a tool to determine the exciton delocalization length in disordered molecular (J) aggregates. We compare the delocalization length obtained through heuristic arguments from the spectral separation between the bleaching and the one- to two-exciton induced absorption features in this spectrum to the delocalization length obtained from the participation ratio at the J band center. We find that up to a certain saturation length these two delocalization lengths are indeed proportional. In the case of long-pulse two-color pump−probe spectra, the slope of this linear scaling is insensitive to the pump frequency; it is sensitive to the width of the probe pulse. Beyond the saturation length, which is determined by the homogeneous line width of the exciton transitions, both the two-color and the short-pulse pump−probe spectra saturate and no longer can be used as a measure for the delocalization length. The latter applies to the B850 ring of bacterial antenna complexes.

Optical transient bleaching/absorption of surface-oxidized CuInS 2 nanocrystals

nanocrystals in a polymer film have been studied with the picosecond pump-probe technique. The oxidized CuInS2 nanocrystals have an additional absorption band with a peak at 1.03 eV, which is bleached under picosecond excitation. Rapid (∼50 ps) trapping into midgap surface states results in long-lived (≫300 ps) bleaching and induced absorption features. A schematical energy-level diagram for oxidized CuInS2 nanocrystals is given based on the experimental results.

Electronic structure and photoexcited-carrier dynamics in nanometer-size CdSe clusters

We use transient optical hole burning and photoluminescence to investigate the static and dynamic electronic properties of 32-\AA{} CdSe quantum dots. We observe a number of discrete electronic transitions, resolve LO-phonon progressions, and obtain homogeneous linewidths and electron--LO-phonon couplings. We find that the band-gap luminescence is not from the exciton state, but from a surface trapped state. Rapid (\ensuremath{\sim}160 fs) trapping into these surface states results in long-lived (\ensuremath{\sim}10--100 ns) bleach and induced-absorption features in pump-probe experiments.

Biexcitons in semiconductor quantum dots.

Theoretical and experimental results are reported which provide the first evidence for biexciton states in semiconductor quantum dots. The theory predicts an increasing biexciton binding energy with decreasing dot size. Unlike bulk semiconductors, quantum dots have excited biexciton states which are stable. These biexciton states are observed as pronounced induced absorption features on the high-energy side of the bleached exciton resonances in femtosecond and nanosecond pump-probe experiments of quantum dots in glass matrices.

Ultrafast optical nonlinearities in II–VI compounds

We review our experimental and theoretical results on the femtosecond laser excited CdS and CdSe bulk and quantum dots. Optical Stark effect in bulk CdS is reported where the electron-hole continuum states in addition to the bound-exciton states are blue shifted. The nonlinear optical properties of CdSe microcrystallites embedded in glass matrices are investigated using femtosecond and nanosecond laser pulses. Spectral hole-buring is observed which is accompanied by an induced absorption feature on the high-energy side. A 25±10 fs dephasing time is obtained from these measurements. It is concluded that Coulomb effects are important in quantum dots. They give rise to a ground state two-pair resonance on the low energy side of the one-pair state and higher energy two-pair resonances which are spectrally located between the one-pair states. Single-beam saturation experiments show that the intensities required for bleaching are larger for smaller crystallites.

Femtosecond optical nonlinearities of CdSe quantum dots

Femtosecond differential absorption measurements of the quantum-confined transitions in CdSe microcrystallites are reported. Spectral hole burning is observed, which is accompanied by an induced absorption feature on the high-energy side. The spectral position of the burned hole depends on the excitation wavelength. For excitation on the low-energy side of the lowest quantum-confined transition, a slight shift of the hole towards the line center is observed. The hole width increases with pump intensity and the magnitude of the induced transparency saturates at the highest excitation level. The results are consistently explained by bleaching of one-pair states and induced absorption caused by the photoexcited two electron-hole pair states. It is concluded that the presence of one electron in the excited state prevents further absorption of photons at the pair-transition energy and accounts for the major portion of the bleaching of the transition.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Absorption Spectra of Hydrogen-Halide—Rare Gas Mixtures

The null-gap regions of the 1–0 bands of HCl and HBr have been investigated with a grating spectrograph of very high resolution. The experiments were performed using an absorption tube of 90 cm length and a hydrogen-halide pressure of 25 cm Hg. The hydrogen halide was pressurized with argon, krypton, or xenon of density 2 to 5 amagats. The sensitivity of the apparatus was such that absorption features 10—4 the intensity of the P(1) line of HCl could be observed with ease. The absorption features observed are complex, relatively sharp, and totally different when argon, krypton, or xenon is used as the pressurizing gas. It is suggested that the hydrogen-halide molecule forms a molecular complex with the rare-gas atoms used as pressurizing agents. The discrete induced absorption features have an intensity considerably less than 10—3 of the intensity of the P(1) line of HCl per amagat density of pressurizing gas. In HBr the ``induced absorption features'' have approximately two to three times the relative intensity observed in HCl.