Steady-state Excitation Conditions Research Articles

Photoluminescence study of anatase TiO2 photocatalysts at the pico- and nanosecond timescales.

We studied the photoluminescence decay kinetics of three nanosized anatase TiO2 photocatalysts (particle diameter: 7, 25, or 200nm) at the pico- and nanosecond timescales for elucidating the origin of the luminescence. Luminescence spectra from these photocatalysts obtained under steady-state excitation conditions comprised green luminescence that decayed on the picosecond timescale and red luminescence that persisted at the nanosecond timescale. Among the photocatalysts with different sizes, there were marked differences in the rate of luminescence decay at the picosecond timescale (<600ps), although the spectral shapes were comparable. The similarity in the spectral shape indicated that self-trapped excitons (STEs) directly populated in the bulk of the particle by light excitation emit the luminescence in a picosecond timescale, and the difference in the rate of luminescence decay originated from the quenching at the particle surface. Furthermore, we theoretically considered excitation light intensity dependence on the quantum yield of the luminescence and found that the quenching reaction was not limited by the diffusion of the STEs but by the reaction at the particle surface. Both the spectral shape and time-evolution of the red luminescence from the deep trapped excitons in the nanosecond timescale varied among the photocatalysts, suggesting that the trap sites in different photocatalysts have different characteristics with respect to luminescence. Therefore, the relation between trap states and photocatalytic activity will be elucidated from the red luminescence study.

Ultrafast Spectroscopy of Plasmonic Titanium Nitride Nanoparticle Lattices

Titanium nitride (TiN) is an advantageous plasmonic material for optoelectronic applications that require resilience to extreme irradiation or temperatures. Although TiN is optically similar to noble metals at near-infrared wavelengths under steady-state excitation conditions, their photoexcited properties are distinct at ultrafast time scales. This paper describes the differences in optical properties between coupled TiN nanoparticles in 2D arrays that support surface lattice resonances (SLRs) and TiN nanoparticle arrays that support only localized surface plasmons (LSPs). Compared to symmetric photoinduced peak broadening in the LSPs, we found that SLRs show asymmetric broadening at ps-time scales in transient absorption measurements. Furthermore, TiN nanoparticle arrays were robust and withstood pump fluences exceeding 50 mJ/cm2 for over half an hour with little to no change in bleach wavelength.

Embedded Acoustic Black Holes for semi-passive broadband vibration attenuation in thin-walled structures

We explore the use of structure-embedded Acoustic Black Holes (ABH) to design thin-walled structural components exhibiting broadband vibration attenuation characteristics. The ABH is a geometric taper with a power-law profile fully integrated into the structural component and able to induce a smooth and progressive decrease of both the velocity and the wavelength of flexural waves. Previous studies have shown these characteristics to be critical to enable highly efficient vibration attenuation systems. The performance of ABH thin-walled structures is numerically and experimentally evaluated under both transient and steady state excitation conditions. Both numerical and experimental results suggest that the proposed approach enables highly efficient and broadband vibration attenuation performance.

Continuous wave lasing from individual GaAs-AlGaAs core-shell nanowires

We demonstrate single-mode continuous wave (cw) lasing from individual GaAs-AlGaAs core-shell nanowires (NWs) subject to optical excitation. By comparing the s-shaped input-output characteristics of cw-excited NW lasers with those obtained under pulsed excitation at 4 K, we observe a ∼4× lower equivalent pump power for cw-excitation and a lower minimum lasing linewidth of ∼200 μeV, indicative of the steady-state excitation conditions that result in improved temporal coherence of the emission. Analysis of the NW cavity length dependence of the mode characteristics reveals a clear inverse scaling behavior, with the spacing of Fabry–Perot modes corresponding to a group refractive index of ∼8. Remarkably, when subject to cw excitation heating of the NW-lasers is found to be negligible, as verified by a constant lasing linewidth as well as the absence of red-shifted lasing peak emission at high excitation levels.

Purcell effect in photonic crystal microcavities embedding InAs/InP quantum wires

The spontaneous emission rate and Purcell factor of self-assembled quantum wires embedded in photonic crystal micro-cavities are measured at 80 K by using micro-photoluminescence, under transient and steady state excitation conditions. The Purcell factors fall in the range 1.1 - 2 despite the theoretical prediction of ≈15.5 for the figure of merit. We explain this difference by introducing a polarization dependence on the cavity orientation, parallel or perpendicular with respect to the wire axis, plus spectral and spatial detuning factors for the emitters and the cavity modes, taking in account the finite size of the quantum wires.

Characterization of Photoinduced Isomerization and Back-Isomerization of the Cyanine Dye Cy5 by Fluorescence Correlation Spectroscopy

Cy5 is one of a few commercially available dyes in the near-infrared wavelength range. In this study, the fluorescence fluctuations of Cy5 have been investigated under steady-state excitation conditions by fluorescence correlation spectroscopy (FCS). The fluctuations in fluorescence are compatible with and can be used to characterize the photoinduced isomerization and back-isomerization, as well as the transitions between the singlet and triplet states of the dye. By employing a simple kinetic model, the rate constants of these processes can be determined. The model was used over a broad range of experimental conditions, where the influence on the isomerization properties of solvent viscosity, polarity, and temperature, excitation intensity and wavelength, and the presence of different side groups was investigated. We propose FCS as a useful and simple complementary approach to study isomerization processes of cyanine dyes yielding information about the rates of both the photoinduced isomerization and the...

Radiative lifetimes, quasi-Fermi-levels, and carrier densities in GaAs-(Ga,Al)As quantum-well photoluminescence under steady-state excitation conditions

A quantum-mechanical calculation of the carrier densities, electron and hole quasi-Fermi-levels, and various radiative decay times in GaAs-(Ga,Al)As quantum wells is performed, under steady-state excitation conditions, as functions of the cw laser intensity, temperature, well widths, and acceptor distribution in the well. We consider the radiative recombination of electrons with free holes and with holes bound at neutral acceptors. Our calculations---which have no free parameters---are in quantitative agreement in the intermediate laser-intensity regime at T=300 K with the results by Ding et al. [Appl. Phys. Lett. 60, 2051 (1992)], who obtained the carrier density for multiple asymmetric coupled quantum wells through a fitting procedure that reproduced the total experimental photoluminescence intensity. Results for the carrier-dependent e-h recombination decay time are in good agreement with experimental data by Bongiovanni and Staehli [Phys. Rev. B 46, 9861 (1992)].

Steady-state vs. pulsed excitation recombination kinetics: Simulations and experiments on pores, powders and random media

Abstract The non-classical kinetics of exciton recombination in restricted geometries provides the foundation for a new experimental technique of probing the exciton dynamics and the sample topology. The phosphorescence and delayed flourescence decays exhibit a dramatic dependence on the duration of the excitation. The comparison of pulsed and steady-state excitation provides information on the local topology of the sample and on the average hopping time of the exciton and the exciton diffusion length. This is possible because the distribution of the exciton population is non-Poissonian under steady-state excitation conditions. In addition, the pulse-created distribution also loses its Poissonian character with time. The experimental systems are: 1) isotopic mixed naphthalene crystals above and below percolation; 2) naphthalene crystalline powder; 3) naphthalene embedded into porous glass. Except for the mixed crystals above the percolation concentration, all samples exhibit non-classical effects. The interpretation is aided by Monte Carlo simulations.

Steady-State vs. Pulsed Excitation Recombination Kinetics: Simulations and Experiments on Pores, Powders and Random Media

Abstract The non-classical kinetics of exciton recombination in restricted geometries provides the foundation for a new experimental technique of probing the exciton dynamics and the sample topology. The phosphorescence and delayed fluorescence decays exhibit a dramatic dependence on the duration of the excitation. The comparison of pulsed and steady-state excitation provides information on the local topology of the sample and on the average hopping time of the exciton and the exciton diffusion length. This is possible because the distribution of the exciton population is non-Poissonian under steady-state excitation conditions. In addition, the pulse-created distribution also loses its Poissonian character with time. The experimental systems are: 1) Isotopic mixed naphthalene crystals above and below percolation; 2) Naphthalene crystalline powder; 3) Naphthalene embedded into porous glass. Except for the mixed crystals above the percolation concentration, all samples exhibit the non-classical effects. The...

Fluorescence and gain predictions in laser dye mixtures

Radiative energy transfer in three different laser dye mixtures composed of (1) dichlorofluorescein (donor) and DODC (acceptor), (2) dichlorofluorescein (donor) and RhB (acceptor), and (3) coumarine (donor) and RhB (acceptor) have been studied under steady state excitation conditions. Analytical expressions have been developed to predict steady state fluorescence and hence gain line shape of laser dye mixtures using computer simulation. The theoretical predictions derived are generally in excellent agreement with experimental results, which confirm that at the mixture concentrations needed for lasing, radiative transfer is the dominant energy transfer mechanism. The method developed is effective and practical for predicting laser gain lineshapes (and hence tunability) as well as predicting fluorescence emission spectra of dye mixtures.

Laser intensity effects in the IR multiphoton dissociation of CF2HCl and CF2CFCl

CO2 laser pulses of 2, 10, and 50 ns duration, for which the temporal profile was approximately rectangular, were used in the multiphoton dissociation of low pressure CF2HCl and CF2CFCl. Probing a region of well-defined CO2 laser intensity, laser excited fluorescence determined the yield of CF2 formed in the v=0 and in the high vibrationally excited v2=5 (Evib=3335 cm−1) levels as a function of fluence (F) and intensity (I) over a factor of 100 variation. In the dissociation of CF2HCl by pulses of a given F, increasing I by a factor of 25 (50 vs 2 ns pulse) typically increased CF2(v=0) yield by factors of 8; this I dependence is probably due to power broadening of the discrete levels. The CF2(v=0) yield from CF2CFCl was almost independent of I over this range, which may reflect the coincidence of the 1079 cm−1 R(24) laser frequency with a CF2CFCl Q branch head at 1080 cm−1. The ratio of CF2(v2=5)/CF2(v=0), which is insensitive to discrete levels effects in the excitation process, increases with I for both reactants. This ratio may be expressed as a vibrational temperature, Tv for the CF2 fragments, and varied from about 1400 to 2600 K and from 900 to 1400 K for CF2CFCl and CF2HCl reactants, respectively, as I increased from 55 MW/cm2 to 3.3 GW/cm2 for the 50 ns laser pulses. Arguments are presented relating these results to the establishment of steady-state excitation conditions and to absorption cross sections in the continuum levels.

Spectral change mechanism in the pulsed high-pressure sodium arc

The enhancement of radiative output of the pulsed high-pressure sodium discharge in the blue region of the visible spectrum is due principally to radiation from the nd2D→3p2P transitions of the sodium atom where n=4–15. Excitation of these upper energy levels results from a transient non-steady-state increase in plasma temperature to about 5500 °K in the center of the arc. This temperature decays to a value near that encountered in steady-state excitation conditions in a time comparable to the empirically optimized pulse duration and to the previously determined time for plasma temperature relaxation.

Effect of two-photon saturation on ordinary Raman scattering

The excitation profile of ordinary Raman scattering under steady-state excitation conditions and the time-resolved emission spectrum of ordinary Raman scattering under transient excitation conditions undergo considerable changes when the excitation frequency ω approaches 1 2 ω mn , where ω mn is the resonance frequency of a two-photon transition from the ground state | n to an excited state | m of a molecule. The appearance of ghost peaks, dips or dispersion-like features centred at ω ≋ 1 2 ω mn in the excitation profile and of coherence effects such as Rabi nutations with unusual time-dependence in the time-resolved spectrum are predicted.

Measurement of loss and output numerical aperture of optical fiber splices

An ensemble of optical fiber splices has been evaluated to determine how source launching conditions and length of fiber between the source, splice, and detector affect the splice loss and far field output NA. The loss of a splice is strongly dependent upon the energy distribution and NA of the beam at the input of the splice. For the range of NA's considered, the splice loss varied by 0.5 dB. The average NA at the output of the ensemble of splices was greater than its corresponding input NA by as much as 12%. The splice appears to act as a transformer converting the input energy distribution into one at the output that contains a greater amount of its energy in higher order modes. As a consequence of this, the loss of a fiber following the splice in a long transmission path is greater than the loss the fiber would exhibit if measured with steady state excitation conditions.