Nonradiative Relaxation Time Research Articles

A photothermal study on chromium doped low silica calcium aluminate glass

The thermo-optical properties, the non-radiative relaxation time, τ r, and the characteristic diffusion time, τ β, of a 0.001 mol% Cr 2O 3 doped low silica calcium aluminate glass were investigated by photothermal technique and photoacoustic spectroscopy, respectively. The results are interpreted in terms of frequency dependence predicted by the theory of Resencwaig and Gersho, and by the thermoelastic model. In the PA spectra, it is possible to identify two bands centered at 380 and 620 nm, the τ r of which is 2.18 and 1.55 ms, respectively, and τ β was found to be 0.53 and 0.49 ms, respectively, for each band.

Photoacoustic Spectroscopy to determine in vitro the non radiative relaxation time of protoporphyrin IX solution containing gold metallic nanoparticles

In order to compare the non radiative relaxation time (NRRT) between standard protoporphyrin IX (PpIX) and protoporphyrin PpIX(1) solution containing gold metallic nanoparticles, we measured the photoacoustic spectroscopy (PAS) signal phase to determine, for each solution, the NRRT by using the Rosencwaig-Gesho theory, modified to include the effect of a finite non radiative deexcitation time. A NRRT average value, obtained for each solution, is reported and compared with some NRRT of triplet states reported in the literature for molecules with the same tetrapyrrolic structure. In the study was used PpIX disodium salt (DS) solution of 25% HCL. From each solution it was obtained its optical absorption spectrum, by using a UV-Vis spectrophotometer. After this, in the maximum observed optical absorption peak (404 nm), it was obtained the Photoacoustic (PA) signal phase as a function of the light modulation frequency, from 17 to 80 Hz. Our investigations are devoted to the improvement of the thermal treatments of drugs for medical applications.

Femtosecond carrier dynamics of InxGa1−xN thin films grown on GaN (0001): Effect of carrier-defect scattering

Ultrafast carrier dynamics in ultrathin InxGa1−xN ternary alloys were investigated in detail, using femtosecond transient differential optical absorption measurements. Measurements were performed with probing wavelengths above and below the band edge of the materials. Furthermore, we performed a structural characterization by x-ray diffraction reciprocal space maps and we found that the alloys with the higher InN mole fraction (x=0.89 and x=0.43) present large lattice mismatch-strain relaxation whereas the lower InN content alloy was fully coherent with the underlying GaN (0001) layer. Our analysis showed that the observed nonradiative carrier dynamics was strongly related with the carrier-defect scattering of the materials. Our measurements in conjunction with the numerical analysis showed that when we excite these alloys with ultrashort laser pulses the background carrier concentration for In-rich InGaN samples participates to the photoexcited carrier relaxation process via carrier momentum scattering. For the higher InN content alloys (x=0.89 and 0.43) the relaxation times were short (0.4–1.4ps) whereas for the full strained alloy (x=0.07) a slower nonradiative relaxation time (∼25ps) was observed. Finally, the energy loss rate of this material (2.05±0.10meV∕fs) as well as the optical phonon lifetime (44±2fs) were extracted.

Observation of the quasidiffusive propagation of phonons in Mg2SiO4:Ho3+ using time-resolved photo-phonon spectroscopy

The energy flux of phonons produced due to the nonradiative laser-induced transitions of Ho3+ impurity ions in forsterite from the 5F5 states has been measured using a superconductor bolometer at a temperature of 2 K. The dependence of the flux on the laser wavelength, the time elapsed after the action of a laser pulse, and the phonon propagation path length is analyzed. It is found that the excitation of Ho3+ to some states leads to the diffusive propagation of emitted phonons in the spontaneous frequency decay mode (quasidiffusive mode of propagation): the time of arrival of a phonon pulse is almost a linear function of the path length, but it is several times longer than the longest ballistic time of flight (for transverse phonons). The diffusion coefficient and the nonradiative relaxation time are determined from the best fit to the experiment.

In vitro determination of the non radiative relaxation time of triplet state in protoporphyrin IX

In this work we used Photoacoustic Spectroscopy (PAS) to determine in vitro the non radiative relaxation time (NRRT) of protoporphyrin IX (PpIX) triplet state. In order to obtain this characteristic time, we have used PpIX disodium salt (DS) solutions of 25 % HCl. From each solution it was obtained its optical absorption spectrum, by using PAS, ranging from 300 to 800 nm. In the maximum absorption peak, at 410nm, we obtain the photoacoustic (PA) signal, amplitude and phase, as a function of the light modulation frequency, from 17 to 80 Hz. From the PA signal phase it was possible to determine, for each solution, the NRRT by using the Rosencwaig-Gesho theory, modified to include the effect of a finite nonradiative deexitation time. A NRRT average value, obtained from these solutions, is reported and compared with some NRRT of triplet states reported in the literature for molecules with the same tetrapyrrolic stricture.

Growth of GaNxAs1−x atomic monolayers and their insertion in the vicinity of GaInAs quantum wells

The deposition of N-rich GaNAs atomic monolayers was investigated. Such layers were successfully grown while exposing a GaAs surface to a nitrogen plasma source during a growth interruption at 400 °C in a molecular beam epitaxy reactor. N accumulation was confirmed and evaluated by secondary ion mass spectroscopy. This process is compatible with regrowth, as in situ monitored by reflection high-energy electron diffraction. The crystal shows good structural quality, as displayed by transmission electron microscopy, that reveals that the accumulation occurred within 1 nm. In a series of samples, two of these ultrathin GaNAs layers were inserted in GaAs barriers, on each side of a GaInAs quantum well (QW). A drastic effect of the N-rich layers on the QW photoluminescence (PL) intensity was observed, as well as on the carrier recombination dynamics, with a strong influence of the spacer thickness between the QW and the N-rich layers. A time-resolved PL analysis of these samples evidenced nonradiative relaxation times in the range of a few ps. This very short carrier lifetime is attributed to the presence of nonradiative centres related to the N-rich layers close to the QW, and can be used to design ultrafast optical devices.

Nonradiative relaxation times in diagonal transition Si/SiGe quantum cascade structures

Here, we explore experimentally and theoretically the possibility to prolong the upper hole state nonradiative lifetime of Si/SiGe quantum cascade (QC) structures by using a spatially indirect diagonal transition between two SiGe quantum well ground states. With the recent observation of well resolved midinfrared electroluminescence from heavy hole intersubband transitions in Si/SiGe valence-band QC structures, a Si-based QC laser seems no longer to be out of reach. A long carrier lifetime and maybe population inversion, however, appear to be impossible for structure designs with a vertical intersubband transition studied so far. This is due to the nonresonant behavior of deformation potential scattering dominant in unipolar SiGe. We report on calculations of the band structure using a six-band k⋅p model and of hole deformation potential scattering that predict significantly increased nonradiative lifetimes for large barrier thickness, reaching about 20 ps for 35 Å Si barrier layer width. Electroluminesence measurements of a series of QC structures with varied barrier width reveal comparable efficiencies and the deduced lifetimes confirm our model calculations.

Enhancement of room-temperature photoluminescence in InAs quantum dots

We report pronounced enhancement of room-temperature photoluminescence up to 80-fold induced by proton implantation and the rapid thermal annealing process in a multilayer InAs/GaAs quantum-dot structure. This effect is studied by a combination of material methods and resulted from both proton passivation and carrier capture enhancement effects. The maximum photoluminescence peak shift is about 23 meV, resulting from the intermixing of quantum dots. Linear dependence behavior as observed for both the nonradiative recombination time and carrier relaxation time on the ion-implantation dose. Maximum enhancement of the photoluminescence is observed for a proton implantation dose of 1.0×1014 cm−2 followed by rapid thermal annealing at 700 °C. These effects will be useful for quantum dot optoelectronic devices.

Photoacoustic spectroscopy applied to the study of clay soils

The optical absorption spectra and mineralogical composition of soil samples were characterized using photoacoustic spectroscopy and x-ray diffraction. The photoacoustic spectra observed showed transition bands associated with Fe3+ ions in octahedral or tetrahedral symmetry. From the phase behavior, both the nonradiative relaxation time τ and the characteristic thermal diffusion time τβ were determined. The x-ray diffraction analysis showed that kaolinite is a major crystalline phase (86 wt %) followed by minor quantities of anatase, gibbsite, and quartz. Rietveld refinements showed that the Fe3+ cations partially substitute for Al3+ cations in octahedral sites of the kaolinite structure.

Soil characterisation using X-ray diffraction, photoacoustic spectroscopy and electron paramagnetic resonance

The optical absorption spectra and chemical composition of soil samples were characterised using photoacoustic spectroscopy (PAS), electron paramagnetic resonance (EPR), X-ray diffraction and X-ray fluorescence. From fluorescence results, the chemical components were identified and an Fe mass concentration varying between 4% and 10% was determined. Besides that, the observed photoacoustic technique (PA) spectra showed transition bands associated with Fe 3+ ions. From the phase behaviour, both the nonradiative relaxation time τ and the characteristic thermal diffusion time τ β were determined. The X-ray diffraction analysis showed that kaolinite is a major crystalline phase (86% in mass) followed by minor quantities of anatase, gibbsite and quartz. Rietveld refinements showed that the Fe 3+ cations partially substitute for Al 3+ cations in the octahedral sites of the kaolinite structure. EPR measurements were performed in order to determine the crystalline environment of Fe ions; the observed profiles indicate that Fe sites are embedded in a distorted cubic crystalline field.

Characterization of the double sulfites Cu2SO3·MSO3·2H2O (M = Cu, Fe, Mn or Cd) by photothermal techniques

Structural, thermal and optical properties as well as the non-radiative relaxation time τR and the characteristic diffusion time τβ of the synthesized double sulfites Cu2SO3 ·MSO3·2H2O (M = Cu, Fe, Mn or Cd) were investigated by photoacoustic spectroscopy. The positions of the bands for each compound were identified upon gaussian deconvolution and by Kramers–Kronig analysis, and the d–d and intervalence transitions were observed. The detection of two absorbing bands with low and high intensities is in agreement with previous results on reflectance for Cu2SO3CuSO3·2H2O. A slight shift is observed in the peak positions and in the measured thermal diffusivity, capacity and conductivity for different Ms and the thermal conductivity decreases with the ionic radius of the M(II) cations.

Optical properties of LiGaTiO 4: Fe 3+

Emission, excitation and photoacoustic absorption data are presented. The radiative decay time at room temperature and 77 K are measured. The nonradiative relaxation time and the characteristic thermal-diffusion time are obtained. The near IR fluorescence band observed in this iron doped compound is assigned to the symmetry and spin-forbidden 4T 1( 4G) → 6A 1( 6S) transitions of tetrahedrally coordinated Fe 3+ ions.

Photoacoustic Phase Study on the Relaxation Processes of Nd(DBM)3

Measurement are reported on the phase of the photoacoustic signal of Nd-Dibenzoylmethide (Nd(DBM)3) sample for a range of chopping frequencies between 56HZ-722HZ. By applying to theoretical models, the utility of phase data to evaluate the optical absorption coefficient and nonradiative relaxation time of solid state complexes are discussed. And modified theoretical models have been established.

Room temperature intra-band lasing in quantum dot arrays placed in high photon density cavities—a theoretical study

Intersubband (intra-band) transitions are very attractive forlong wavelength lasers due to the high degree of tailoring possible in the emission spectra. In general, if intra-band population inversion is to be created in a conduction band quantum well by carrier injection at the barrier energy, it is necessary that the electron non-radiative intra-band energy relaxation times are long. Additionally, the extraction time for the electron from the lower state should be short. In a bipolar device studied here, this means the bandedge electron-hole recombination times should be short.The use of sub-two-dimensional (2D) structures (quantum dots) allows us to increase the intra-band energy relaxation time from about a picosecond for bulk or quasi-2D systems to several hundred picoseconds at room temperatures. Also, by placing these structures in a cavity with a high photon number, it is possible to decrease the bandedge electron-hole recombination times through stimulated emission. Our studies show that strong population inversion and lasing under d.c. conditions is possible at room temperature in such systems.

Glass transition of associated solvents studied by fluorescence measurement of doped chromophores.

The fluorescence lifetime of a triphenylmethane dye, malachite green, doped in three glass-forming associated solvents, 1-propanol, propylene glycol, and glycerol, was measured in a wide temperature range. For each sample three temperature regimes were found in the temperature dependence of the nonradiative relaxation time of malachite green. The lower crossover temperature corresponds to the calorimetric glass transition temperature, and the higher one, 30--50 K above the lower one, is attributed to the critical temperature that is predicted by mode-coupling theory. \textcopyright{} 1996 The American Physical Society.

Subpicosecond time-resolved Raman investigation of optical phonon modes in Cr-doped forsterite

The nonradiative relaxation dynamics of forsterite was investigated by studying different phonon modes using time-resolved Raman spectroscopy. We found that only the 225, 335, and 370 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ anti-Stokes Raman lines exhibit changes in their intensity at various pump-probe delay times indicating the presence of nonequilibrium phonon populations. The experiments were performed at room and liquid nitrogen temperatures. A rate equations analysis was used to describe the dynamics of the ion-lattice system. The nonradiative relaxation time and the phonon lifetimes were estimated by fitting to the experimental data.

Stimulated-Brillouin-scattering compression of pulses from an Nd : YAG laser with a short cavity and measurement of the nonradiative relaxation time of the lower active level

A short (~11 cm) cavity of an Nd : YAG laser was Q-switched by LiF : and GSGG : Cr3+ : Cr4+ : Nd3+ crystals. This resulted in stable generation of short (~2.5 ns) pulses with energies in excess of 6 mJ. Pulses with ~300 and ~150 mJ energies for the first and second harmonics, respectively, and of ~100 ps duration were obtained at the output of a stimulated-Brillouin-scattering compressor and a three-pass Nd : YAG amplifier. A study of the gain recovery after passage of a short saturating pulse through the active element yielded the upper limit (1 ns) of the nonradiative relaxation time of the 4I11/2 lower active level of the neodymium ion.

Phase Resolved Method of Photoacoustic Spectrum on Amino Acid L-Tryptophan Compounds

Abstract A discussion on using the phase shifts of the photoacoustic signal of a monophase sample for resolving its overlapping absorption peaks is presented. It is experimentally shown that the method is simple and useful. And the time delay which includes the nonradiative relaxation time, the thermal-diffusion time and the time of generating the acoustic signal is calculated. This is demonstrated using L-Tryptophan compounds.

Laser-induced photoacoustic signal phase study of stratum corneum and epidermis.

Laser-induced photoacoustic signal phase was employed for measurements of optothermal parameters of pig abdomen stratum corneum and epidermis. The non-radiative relaxation time was found to be 28 ms and the thermal diffusion time was 370 ms, yielding a thermal diffusivity value of (4.1 +/- 0.8) x 10(-4) cm2 s-1. This value compares well with other literature values for skin obtained using photothermal techniques.

Measurement of the subnanosecond, nonradiative relaxation time from excited states of Nd^3+ in a Nd:YAG crystal

We have time resolved the nonradiative relaxation from the (4)G(7/2) manifold to the upper laser level (for lambda = 1.06 microm) of Nd(3+) in a Nd:YAG crystal. This has been performed by monitoring the onset of gain for the associated (4)F(3/2)-(4)I(11/2) transitionfollowing excitation of the (4)G(7/2) pump band by a 40-psec, 0.53-micro m pump pulse. The nonradiative relaxation time was found to be 470 +/- 60 psec at room temperature and no shorter than 400 psec at 440 K.