Excitation Laser Intensity Research Articles

Photoluminescence dynamics and reduced Auger recombination inSi1−xGex/Sisuperlattices under high-density photoexcitation

Optical gain and stimulated emission processes in Si nanostructures are controlled by the dynamics of high-density carriers. Here, we report photoluminescence (PL) dynamics and multiexciton recombination in ${\text{Si}}_{1\ensuremath{-}x}{\text{Ge}}_{x}/\text{Si}$ superlattices (SLs) under high-density excitation. Saturation of the PL intensity and rapid PL decay are observed as the excitation laser intensity is increased. These phenomena occur due to nonradiative Auger recombination of the electron-hole pairs. We find that the Auger process in ${\text{Si}}_{1\ensuremath{-}x}{\text{Ge}}_{x}/\text{Si}$ SLs is less pronounced than that in the ${\text{Si}}_{1\ensuremath{-}x}{\text{Ge}}_{x}/\text{Si}$ single quantum wells. Our results show that coupled nanostructures have an advantage in efficient light emission and the control of many-body carrier dynamics.

Nonlinear processes induced by the enhanced, evanescent field of surface plasmons excited by femtosecond laser pulses

Evanescent fields of surface plasmon polaritons (SPP) above metal surfaces can reach 1-2 orders of magnitude higher, nearly atomic field strengths in comparison to the relatively weak exciting laser fields of a femtosecond Ti:sapphire laser oscillator. We used these high plasmonic fields to study the characteristic SPP phenomena of intense field optics experimentally. It was found that both the intensity and the angular distribution of SPP emitted light depend nonlinearly on the exciting laser intensity in the higher-intensity, non-perturbative range of the interactions. These results are supported by our theory. At these strong excitations, an additional, depolarized, diffuse spectrum also appeared which can be attributed either to the fluorescence of Au, or to the non-equlibrium Planck radiation, originating from the fast cooling of the conduction electron cloud of Au excited by the femtosecond laser pulse.

Impact of laser excitation intensity on deep UV fluorescence detection in microchip electrophoresis

A high intensity 266 nm continuous wave (cw-) laser developed for material processing was utilised as an excitation source for sensitive native fluorescence detection of unlabelled compounds in MCE. This 120 mW laser was attached via an optical fibre into a commercial epifluorescence microscope. With this MCE set-up we evaluated the impact of laser power on the S/N of aromatic compounds as well as of proteins. Compared with a previous work which used a 4 mW pulsed laser for excitation, improved S/N for small aromatics and to a lesser extent for proteins could be attained. The LOD of the system was determined down to 24 ng/mL for serotonin (113 nM), 24 ng/mL for propranolol (81 nM), 80 ng/mL for tryptophan (392 nM) and 80 ng/mL for an aromatic diol (475 nM). Sensitive protein detection was obtained at concentrations of 5 microg/mL for lysocyme, trypsinogen and chymotrypsinogen (340, 208 and 195 nM, respectively). Finally, a comparison of the cw- with a pulsed 266 nm laser, operating at the same average power, showed a higher attainable sensitivity of the cw-laser. This can be attributed to fluorescence saturation and photobleaching effects of the pulsed laser at high pulse energies.

Top-hat cw laser induced thermal mirror: a complete model for material characterization

A complete theoretical model is presented for the thermal mirror technique under top-hat laser excitation. Considering the attenuation of the top-hat excitation laser intensity along the thickness of a sample due to its optical absorption coefficient, we calculate the laser-induced temperature and surface deformation profiles. A simplified theoretical model for a high absorption sample is also developed. The center intensity of a probe beam reflected from the thermal mirror at a detector plane is derived. Numerical simulation shows that the thermal mirror under the top-hat laser excitation is as sensitive as that under Gaussian laser excitation. With top-hat laser excitation, the experimental results of thermo-physical properties of opaque samples are found to be well consistent with literature values, validating the theory.

Single‐beam pumped coherent anti‐Stokes Raman scattering on carbon nanotubes

AbstractGiant optical fields locally generated in nanoscale metal structures through the local excitation of surface plasmons underlie a variety of nonlinear optical processes. Using silver and gold rough metallic films, reproducible enhancements of the Raman signal on various materials including carbon nanotubes have been demonstrated. In addition, an abnormal anti‐Stokes Raman emission, reminiscent of Coherent anti‐Stokes Raman Scattering (CARS), is accurately observed on nanometric thin films. We demonstrate that under a tight focusing of the excitation light, a CARS‐type emission, resulting from a wave mixing process between the incident laser light and Stokes Raman light generated by a SERS mechanism, is produced. The results which corroborate the CARS emission are the following: a square relation between the CARS signal intensity and the film thickness; a square relationship between the CARS signal intensity and the exciting laser intensity; a dependence of the CARS intensity on the numerical aperture (NA) of the microscope objective used for the detection of the anti‐Stokes signal; a polarization ratio in the anti‐Stokes side always greater in comparison with that measured for an usual Stokes Raman signal. We demonstrate also that this mechanism may contribute to a clear identification of the state of the observed nanotubes, i.e. in isolated or bundled forms, even when tubes are in powders. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonlinear Confocal Microscopy for High-Resolution Measurement

We demonstrate cell imaging with a new confocal nonlinear optical microscope using a low-power cw laser. The confocal nonlinear optical microscope, employing degenerate four-wave mixing geometry, can detect the fine structure of submicron objects with nanoscale contents such as biological cells. The optical signal, which is given by the third-order susceptibility tensor, is confined to the focal region of the focusing incident beam, because the absorption of the object is dependent on the third power of the excitation laser intensity. We have observed a thylakoid membrane in a chloroplast by scanning the tensor element inside.

Nature and dynamics of photoexcited states in an electroluminescent poly(phenylene vinylene-co-fluorenylene vinylene)-based π-conjugated polymer

This paper describes the optical properties of an electroluminescent poly(phenylene vinylene-co-fluorenylene vinylene) (BPPPV-PF)-based π-conjugated polymer using absorption, photoluminescence (PL), time-resolved photoluminescence (TRPL), continuous wave (CW) and transient-photoinduced absorption (PA) spectroscopic techniques. The TRPL decay spectra of BPPPV-PF in chloroform and film exhibit single exponential decay with PL lifetimes of 0.8 ns and 0.2 ns, respectively. The CW-PA spectrum exhibits a single well-defined band centred at 1.5 eV, which is assigned to the T–T* transition of the triplet excitons. The monomolecular lifetime ( τ) of the triplet excitons was estimated from the intensity dependence of CW-PA and found to be ∼1.2 ms at 80 K. The temperature dependence of the CW-PA signal at 1.5 eV was studied for a temperature range from 80 K to 298 K. It was observed that the PA signal at 1.5 eV was persistent to relatively high temperatures, which may be due to the bulky side chain of the polymer and morphology of the film. The relaxation process of this triplet excitation was studied by measuring the transient decay of the PA signal for various temperatures. The transient-PA signal shows monomolecular recombination process at all recorded temperatures and the temperature dependence of monomolecular lifetime ( τ) was studied. It was found that the monomolecular lifetime ( τ) is dependent on temperature but independent of the laser excitation intensity. The obtained results are analyzed based on the molecular structure of the PPV-PF copolymer.

Temperature and excitation intensity tuned photoluminescence in Tl4GaIn3S8 layered single crystals

AbstractPhotoluminescence spectra of Tl4GaIn3S8 layered crystals grown by Bridgman method have been studied in the wavelength region of 500–780 nm and in the temperature range of 26–130 K with extrinsic excitation source (λexc = 532 nm), and at T = 26 K with intrinsic excitation source (λexc = 406 nm). Three emission bands A, B and C centered at 514 nm (2.41 eV), 588 nm (2.11 eV) and 686 nm (1.81 eV), respectively, were observed for extrinsic excitation process. Variations in emission spectra have been studied as a function of excitation laser intensity in the 0.9‐183.0 mW cm–2 range for extrinsic excitation at T = 26 and 50 K. Radiative transitions from the donor levels located at 0.03 and 0.01 eV below the bottom of the conduction band to the acceptor levels located at 0.81 and 0.19 eV above the top of the valence band were proposed to be responsible for the observed A‐ and C‐bands. The anomalous temperature dependence of the B‐band peak energy was explained by configurational coordinate model. From X‐ray powder diffraction and energy dispersive spectroscopic analysis, the monoclinic unit cell parameters and compositional parameters of Tl4GaIn3S8 crystals were determined, respectively. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarization resolved luminescence in asymmetric n-type GaAs∕AlGaAs resonant tunneling diodes

We have investigated the polarized emission from a n-type GaAs∕AlGaAs resonant tunneling diode under magnetic field. The GaAs contact layer emission shows a large constant negative circular polarization. A similar result is observed for the quantum well, but only when electrons are injected from the substrate, while for inverted biases, the polarization tends to become positive for small voltages and large laser excitation intensities. We believe that the quantum well polarization may be associated to the partial thermalization of minority carriers on the well subbands and is thus critically dependent on the bias-controlled density of carriers accumulated in the well.

Luminescence of black silicon

Room temperature visible and near-infrared photoluminescence from black silicon has been observed. The black silicon is manufactured by shining femtosecond laser pulses on silicon wafers in air, which were later annealed in vacuum. The photoluminescence is quenched above 120 K due to thermalization and competing nonradiative recombination of the carriers. The photoluminescence intensity at 10K depends sublinearly on the excitation laser intensity confirming band tail recombination at the defect sites.

Exciton dephasing and multiexciton recombinations in a single carbon nanotube

We studied the temperature and excitation intensity dependence of exciton luminescence in individual single-walled carbon nanotubes (SWNTs) using single nanotube photoluminescence (PL) spectroscopy. The linear temperature dependence of the PL linewidth in a single SWNT implies that the exciton dephasing is dominated by the interaction between the exciton and the phonon mode with very low energy under lower-excitation conditions. Saturation of the PL intensity and broadening of the PL linewidth in a single SWNT occur simultaneously with an increase in excitation laser intensity. Our findings show that the rapid exciton-exciton annihilation through multiparticle Auger recombination broadens the homogeneous PL linewidth.

Dynamics of electron–hole plasmas and localized excitons in highly excited GaN-based ternary alloys

We have studied photoluminescence (PL) spectra of GaN crystals and InGaN ternary alloys at low temperatures as a function of the femtosecond laser excitation intensity. With an increase of the intensity, the broad PL due to electron–hole plasmas (EHP) appears below the biexciton PL in the GaN sample. On the other hand, the broad EHP PL appears above the localized exciton PL in the InGaN sample. The intensity dependence of PL properties of InGaN crystals is completely different from that of GaN crystals. The effect of alloy disorder on PL processes in ternary alloys is discussed.

Below and above bandgap excited photoluminescence inTl4InGa3S8 layered single crystals

Photoluminescence (PL) spectra ofTl4InGa3S8 layered single crystals grown by the Bridgman method have been studied in the550–710 nm wavelength and 80–300 K temperature ranges with below bandgap excitation(λexc = 532 nm),and in the 420–600 nm wavelength and 30–300 K temperature ranges with above bandgap excitation(λexc = 325 nm). The broad emission bands centered at 580 nm (2.14 eV) and 496 nm (2.49 eV) were observed atT = 80 and 30 K for below and above bandgap excitation processes, respectively. Variations inemission spectra have been studied as a function of excitation laser intensity in the10.3–429.7 mW cm−2 range for below bandgap excitation. Radiative transitions from the donor levels located at0.02 and 0.37 eV below the bottom of the conduction band to the deep acceptor levellocated at 0.20 eV above the top of the valence band were proposed to be responsible forthe observed PL bands. From x-ray powder diffraction and EDS analysis, themonoclinic unit cell parameters and compositional parameters, respectively, weredetermined.

Dependence of the Er2O3 selective emission on the intensity of laser thermal excitation

The dependence of the selective emission (SE) spectra of erbium oxide (Er2O3) in the visible and near-IR spectral ranges on the laser excitation intensity at a wavelength of 10.6 μ m is experimentally studied. The intensity ratio for the Er3+ electronic and vibronic transitions in the SE spectra is varied with an increase in the laser intensity to 10 kW/cm2. The mechanism for the multiphonon fluctuation excitation of electronic states and a possibility for the SE application in the observation of the thermo-photo-laser effect are discussed.

“Single-beam pumped” coherent anti-Stokes Raman scattering on carbon nanotubes thin films excited through surface plasmons

In this paper, we propose an interpretation for an abnormal anti-Stokes Raman emission observed on nanometric thin films of different materials and in particular carbon nanotubes. We demonstrate that under a tight-focusing of the excitation light, a coherent anti-stokes Raman scattering (CARS) emission is produced, resulting from a wave mixing process between the incident laser light ( ω l) and Stokes Raman light ( ω s) generated by a surface enhanced Raman scattering (SERS) mechanism. Although the Stokes/anti-Stokes intensity ratio has been explained differently, we present here the results which corroborate the CARS emission. They can be summarized as follows: (i) a square relationship between the CARS signal intensity and the film thickness; (ii) a square relationship between the CARS signal intensity and the exciting laser intensity; (iii) a dependence of the CARS intensity on the numerical aperture (NA) of the microscope objective used for the detection of the anti-Stokes emission. Such effects are not specific to carbon nanotubes and have been observed with other materials accommodated in similar conditions on rough metallic surfaces acting as SERS supports.

Light controlled spin polarization in asymmetric n-type resonant tunneling diode

The authors have observed a strong dependence of the circular polarization degree from the quantum well emission in an asymmetric n-type GaAs∕AlAs∕AlGaAs resonant tunneling diode on both the laser excitation intensity and the applied bias voltage. The sign of the circular polarization can be reversed by increasing the light excitation intensity when the structure is biased with voltages slightly larger than the first electron resonance. The variation of polarization is associated with a large density of photogenerated holes accumulated in the quantum well, which is enhanced due to the asymmetry of the structure.

Study of molecular surface coating on the stability of maghemite nanoparticles

In this study γ-Fe 2O 3 nanoparticle, surface-coated with increasing amount of oleic acid, have been prepared while the stability against particle degradation under laser excitation intensity was investigated. Maghemite nanoparticle was obtained via oxidation of magnetite nanoparticle, the latter synthesized by co-precipitation of Fe (II) and Fe (III) ions in alkaline medium. By varying the experimental conditions of surface-coating maghemite nanoparticles with oleic acid, samples with different grafting coefficient were obtained and investigated using X-ray diffraction and different spectroscopic techniques, namely Raman, Mössbauer, and infrared. The amount of oleic acid adsorbed on the maghemite surface was estimated via the carbon content obtained from elemental analysis.

Laser-Induced Resonance Shifts of Single Molecules Self-Coupled by a Metallic Surface

The spectral properties of single molecules placed near a metallic surface are investigated at low temperatures. Because of the high quality factor of the optical resonance, a laser-induced shift of the molecular lines is evidenced for the first time. The shift dependence on the laser excitation intensity and on the dephasing rate of the transition dipole is studied. A simple theoretical model of a laser-driven molecule self-coupled by a mirror is developed to qualitatively interpret the observations.

Photobleaching Pathways in Single-Molecule FRET Experiments

To acquire accurate structural and dynamical information on complex biomolecular machines using single-molecule fluorescence resonance energy transfer (sm-FRET), a large flux of donor and acceptor photons is needed. To achieve such fluxes, one may use higher laser excitation intensity; however, this induces increased rates of photobleaching. Anti-oxidant additives have been extensively used for reducing acceptor's photobleaching. Here we focus on deciphering the initial step along the photobleaching pathway. Utilizing an array of recently developed single-molecule and ensemble spectroscopies and doubly labeled Acyl-CoA binding protein and double-stranded DNA as model systems, we study these photobleaching pathways, which place fundamental limitations on sm-FRET experiments. We find that: (i) acceptor photobleaching scales with FRET efficiency, (ii) acceptor photobleaching is enhanced under picosecond-pulsed (vs continuous-wave) excitation, and (iii) acceptor photobleaching scales with the intensity of only the short wavelength (donor) excitation laser. We infer from these findings that the main pathway for acceptor's photobleaching is through absorption of a short wavelength photon from the acceptor's first excited singlet state and that donor's photobleaching is usually not a concern. We conclude by suggesting the use of short pulses for donor excitation, among other possible remedies, for reducing acceptor's photobleaching in sm-FRET measurements.

Low-temperature visible photoluminescence and optical absorption in Tl 2In 2Se 3S semiconductor

The emission band spectra of undoped Tl 2In 2Se 3S layered crystals have been studied in the temperature range of 25–63 K and the wavelength region of 570–700 nm. A broad photoluminescence band centered at 633 nm (1.96 eV) was observed at T=25 K. Variation of emission band has been studied as a function of excitation laser intensity in the 2.7–111.4 mW cm −2 range. Radiative transitions from shallow donor level located at 0.03 eV below the bottom of conduction band to deep acceptor level located at 0.23 eV above the top of the valence band were suggested to be responsible for the observed PL band. From X-ray powder diffraction and optical absorption study, the parameters of monoclinic unit cell and the energy of indirect band gap were determined, respectively.