Two-step Absorption Research Articles

Two-color picosecond and continuous-wave experiments on anti-Stokes and Stokes carrier-transfer phenomena inGaAs/AlxGa1−xAsandInGaP2/AlxGa1−xAsheterostructures

We present direct evidence of the two-step absorption process in anti-Stokes photoluminescence in both ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ and ${\mathrm{InGaP}}_{2}/{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterostructures using two-color picosecond and continuous-wave photoluminescence experiments. We show information about the lifetime of the defect states that participate in the two-step absorption process. As a result, we conclude that the long-lived states rather than excitons play the dominant role in the two-step absorption process. We also study the possible contribution of the two-step absorption process to Stokes carrier transfer in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ asymmetric double quantum well structures.

Anti-Stokes photoluminescence of yellow band in GaN: evidence of two-photon excitation process

Time-dependent photoluminescence of yellow band in GaN has been investigated under various excitation conditions using a Q-switched Nd : YAG laser as an excitation source. An anti-Stokes photoluminescence of the yellow band that dominates the photoluminescence (PL) spectra has been observed under the below band-gap excitation. The intensity of the anti-Stokes PL shows a non-linear dependence on the excitation intensity. Our results suggest that the energy up-conversion is via a two-step two-photon absorption process.

Dynamic process of anti-Stokes photoluminescence at a long-range-orderedGa0.5In0.5P/GaAsheterointerface

Efficient anti-Stokes photoluminescence (PL) has been observed in ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}$ and GaAs single heterostructure. PL of ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}$ was observed when photoexciting the ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ interface. The anti-Stokes PL intensity from the ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}$ layer is about 1% of the GaAs PL. The observed anti-Stokes PL exhibits a characteristic intensity dependence in the double-logarithmic plot, which reveals two straight lines having slopes of about 2 and 4. Time-resolved measurements show that the anti-Stokes PL is described by two components: the rapid decay caused by the two-step two-photon absorption of the excitation laser light and the slower decay by energy transfer of electron-hole recombination energy in the GaAs. We found that many-particle effects in the GaAs under high excitations cause the characteristic intensity dependence of the anti-Stokes PL.

Site-selective upconversion excitation of Er3+:KYF4

A spectroscopic study of the green (4S3/2 4I15/2) and blue (2P3/2 4I11/2) emissions from Er3+:KYF4 induced by a red tunable laser excitation is realized at liquid nitrogen temperature, for two Er3+ concentrations: 0.1 and 5 at.%. The emission spectrum can only be accurately interpreted by considering that it is due to two sites, whose contributions are successfully isolated using a two-step absorption process via 4I13/2 for the green emission and a three-step absorption process via 4I13/2 and 4S3/2, for the blue emission. The excited-state energy transfer prevails over the excited-state absorption at high concentration (5 at.%).

Magnetic field mediated anti-Stokes luminescence of semimagnetic ADQW structures

A giant anti-Stokes PL (ASPL) was observed in a specially designed II–VI asymmetric DQW made up of a magnetic and a nonmagnetic well by applying an external magnetic field. ASPL from the high energy well was generated by exciting the lower energy well. The ASPL intensity is of the same order of magnitude as the Stokes band. Evidence is given for a two-step absorption (TSA) process. As intermediate states of the TSA act the long living spatially indirect excitons.

Charge Compensation of Er3+ Ions in Ca1—xErxF2+x Thin Films at Low Rare-Earth Substitution Rates

The Er 3+ ions in Ca 1-x Er x F 2+x (x ≤ 0.01) thin layers grown by sublimation on Si substrates were selectively excited using a laser-induced up-conversion technique under focused excitation at liquid nitrogen temperature. By comparison with the results previously published on bulk crystals, two (Er 3+ , F i-) centers of tetragonal and trigonal symmetry and one (Er 3+ , O 2- ) center of trigonal symmetry were identified in these films. These centers exhibit noticeable green luminescence when the laser wavenumber is tuned on the excited-state transition 4 I 13/2 → 4 F 5/2 of each one of them. The emitting 4 S 3/2 level is populated through a two-step absorption process. A comparison of photoluminescence spectra of thin films and bulk crystals testifies to a lower density of aggregated centers in the films.

The slowing-down of ultrafast relaxation in C 60 films at high femtosecond pump intensities

thin film was investigated in the energy range between 1.6 eV and 3.4 eV by a pump-supercontinuum probe (PSCP) technique with a 40 fs time resolution. The relaxation rate showed pronounced spectral dependence, with a maximum at 2 eV in the region of photo-induced darkening and at 2.4 eV in the region of photo-induced bleaching. The ultrafast relaxation rate decreased with increasing pump-pulse intensity. We suggest that the observed decrease results from extraheating of the carriers in the hu and t1u bands due to internal conversion from higher excited states, which are populated by two-step photon absorption of the intense pump pulse.

Dynamics of anti-Stokes photoluminescence in type-IIAlxGa1−xAs−GaInP2heterostructures: The important role of long-lived carriers near the interface

We have studied the anti-Stokes photoluminescence (ASPL) of a type-II ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{A}\mathrm{s}\ensuremath{-}\mathrm{G}\mathrm{a}\mathrm{I}\mathrm{n}\mathrm{P}}_{2}$ heterojunction. We have found that the ASPL can appear in both layers adjacent to the heterojunction when the excitation photon energy is higher than the energy of below-band-gap (BBG) luminescence. The intensity of ${\mathrm{GaInP}}_{2}$-related ASPL shows an almost linear dependence on the excitation intensity. Time-resolved photoluminescence experiments reveal that the ASPL can be sustained as long as the BBG luminescence remains. Our results suggest that the energy up conversion for the ASPL is via a two-step two-photon absorption process involving localized, long-lived carriers near the type-II interface.

Red to blue up-conversion in Y 2SiO 5:Tm 3+

Avalanche up-conversion and two-step absorption are reported in Y 2SiO 5:Tm 3+. Absorption of red laser light produces intense blue fluorescence. Steady-state and temporal measurements are used to investigate the up-conversion mechanisms.

Laser photosynthesis of previtamin D: new effects of high-intensity picosecond irradiation

An investigation was made of the influence of the intensity of picosecond (τp ≈ 23 ps) laser pulses on the kinetics of the photosynthesis of previtamin D3. An increase of the intensity from 1012 to 1014 W m-2 resulted in significant changes in the photosynthesis kinetics: the photoreaction rate fell and so did the efficiency of the cis—trans isomerisation of previtamin D. An analysis showed that two-step absorption made only a small contribution to these effects and that they were mainly associated with the excitation of nonequilibrium conformers of the previtamin molecule.

A Three-Color, Solid-State, Three-Dimensional Display

A three-color, solid-state, volumetric display based on two-step, two-frequency upconversion in rare earth-doped heavy metal fluoride glass is described. The device uses infrared laser beams that intersect inside a transparent volume of active optical material to address red, green, and blue voxels by sequential two-step resonant absorption. Three-dimensional wire-frame images, surface areas, and solids are drawn by scanning the point of intersection of the lasers around inside of the material. The prototype device is driven with laser diodes, uses conventional focusing optics and mechanical scanners, and is bright enough to be seen in ambient room lighting conditions. QuickTime movie of the three-dimensional display.

Room-temperature green upconversion fluorescence of an Er^3+-doped laser liquid

Room-temperature green upconversion fluorescence of an Er(3+)-doped POCl(3)-SnCl(4) solution is achieved when the solution is pumped with an 800-nm GaAs laser diode. The upconversion emission at 550 nm is determined to be the result of a two-step absorption process. Numerical analysis reveals that laser systems that employ this erbium-doped laser liquid should be realizable with threshold pump powers as low as 45 mW.

Photon-gated photoconductivity of Pr^3+:YAG

We have observed photon-gated photoconductivity in Pr:YAG, providing clear evidence for photoionization of the trivalent praseodymium ion in the solid state. Photoionization occurs by two-step absorption from (3)H(4)(1) ? (1)D(2)(1) followed by an excited-state absorption from (1)D(2) to the lowest component of the 4f5d configuration. The excitation spectrum of the photocurrent shows sharp (1)D(2) spectral features, providing unambiguous assignment of the photocurrent.

Low-temperature anti-Stokes luminescence mediated by disorder in semiconductor quantum-well structures.

In low-temperature photoluminescence experiments on II-VI semiconductor quantum wells we find an energy transfer from confined quantum-well states to above-barrier states. The observed anti-Stokes barrier luminescence exhibits a characteristic intensity dependence showing that this transfer is caused by a two-step absorption process involving localized or impurity bound-exciton states in the quantum well. Time-resolved photoluminescence experiments show that the photon for the second intraband absorption step can be provided by the quantum-well luminescence, i.e., the anti-Stokes barrier luminescence is a direct consequence of photon recycling.

Two-photon excitation of alkyl-substituted magnesium phthalocyanine: radical formation via higher excited states

The photophysical properties of tetra-(tert-butyl)-phthalocyanato-magnesium (t 4 -PcMg) in solution and microheterogeneous systems (liposomes and micelles) were investigated. Radical cation formation occurs in chloroform during UV excitation in the presence of an electron acceptor (CBr 4 ). The same result is achieved by two-step absorption in the singlet manifold using pulsed excitation at λ exc =670 nm, which is of interest from the viewpoint of photon delivery through the therapeutic window of tissues. To obtain a deeper insight into the photophysics leading to radical cation formation via the higher excited singlet state, the transient spectra and singlet—singlet absorption cross-sections were determined. In addition to strong excited state absorption within the spectral range of the Q x -band, relatively large absorption cross-sections were also found in regions with low ground state absorption. The importance of these transitions for an effective two-colour excitation regime is discussed with regard to new start mechanisms for photodynamic laser tumour theraphy.

Upconversion excitation of green fluorescence in Er : YAG

Conversion of red and infrared laser light to green fluorescence is reported in the system YAG : Er3+. Cooperative energy transfer is not very efficient in YAG : Er3+ because of the short4I11/2 lifetime; however, a two-step resonant absorption process, with the second step corresponding to a4I13/2 →4F5/2 transition, uses the long lived4I13/2 state as an effective population reservoir.

Characterization of intrinsic and impurity deep levels in ZnSe and ZnO crystals by nonlinear spectroscopy

Laser induced absorption and photoluminescence are used for the investigation of band states and deep levels (DLs) in ZnSe and ZnO bulk crystals. Interband two-photon and two-step absorption processes, both via DLs as intermediate (virtual and real, correspondingly) states, take place and give information about the main parameters of the DLs. The effects are investigated depending on doping and thermal treatment which change the contamination of native defects. The results illustrate the great possibilities of nonlinear spectroscopy methods in condenced matter science and especially in local defect study.

Upconversion and relaxation of high lying states in rare-earth-doped ZBLAN bulk and fibers

Upconversion pumping of fluorescence of Nd3+-, Tm3+- and Pr3+-doped and Pr3+-Yb3+ co-doped samples of ZBLAN in bulk and fiber form was measured using cw and pulsed laser excitation. It is shown that single-laser-beam excited fluorescence is very sensitive to the resonance between the photon energy and an excited state transition frequency. Such resonance in the ESA mechanism can be associated with avalanche absorption in crystals and suggests the possibility of observing similar effects in these glasses, but it could be two-step absorption given the broad bands of glasses. It is concluded that the fiber configuration, as compared with bulk, is the best suited for achieving efficient upconversion lasers.

Oil absorption into cotton-phenolic material

The kinetics of oil absorption into cotton-phenolic ball-bearing retainer material was determined. The results fit a model of two-step absorption in which the first step can be described by capillary rise and the second step by diffusion of oil from the capillaries into the bulk phenolic resin. The capillaries are associated with the cotton threads and are on the order of several micrometers or less in radius. The model successfully describes data for two cotton-phenolics with different cloth weaves, and five nonpolar liquids (four lubricating oils and heptane) with viscosities that vary over 3 orders of magnitude. The total amount of oil absorbed at the completion of the first step (1.5 to 3.4% v/v in this work) is proportional to the volume of the sample, not its surface area, suggesting that the entire volume of the material stores oil and not just the surface region. If the material is not completely dry when submerged in oil, the capillaries are blocked and only surface absorption of oil can take place. The saturation amount of oil in the resin after diffusion is complete at about 2 to 3% v/v (not including the amount of oil stored in the capillaries). The diffusion coefficient calculated using our model is 3 × 10−12 cm2/s, which is reasonable when compared with available literature data on large and small molecules diffusing into polymers.

Blue and green upconversion in Er3+-doped fluoroindate glasses

Abstract The upconversion properties of Er3+ in fluoroindate bulk glasses (composition: 40InF3−20ZnF2−16BaF2−(20−x)SrF2−2GaF3−2NaF−xErF3 with x=1,2,3 and 4 mole %) are investigated, following 4 F 9 2 excitation with a red krypton laser. A strong green and a weaker blue luminescence is observed at room temperature corresponding to emissions from the thermally coupled 4 S 3 2 and 2 H 11 2 bands and the 2 H 9 2 level respectively. Resonant energy transfer processes involving two excited erbium ions and a two-step absorption process are proposed to explain the upconversion phenomena. The emission intensities depend on the excitation power as Pexcn with 1.5≤n≤1.7 for the green and 1.6≤n≤1.9 for the blue emission. The decay times and the relative intensities of the luminescence are also studied as a function of Er3+ concentration.