Power Of Excitation Beam Research Articles

Parametric investigations on the saturation intensity of Coumarin 102 for stimulated emission depletion application.

Stimulated emission depletion (STED) microscopy performed using continuous-wave (CW) lasers has been investigated and developed by Willig etal. (Nature Methods, 2007, 4(11):915) for nearly a decade. Kuang etal. (Review of Scientific Instruments, 2010, 81:053709) developed the CW STED microscopy technique with 405 nm excitation and 532 nm depletion beams. In their research, Coumarin 102 dye was adopted and was found to be depletable. In this study, a parametric investigation of the depletion of Coumarin 102 dye is carried out experimentally. The influence of the excitation and depletion beam intensities and dye concentrations on the depletion efficiency are studied in detail. The results indicate the following: (1) The highest depletion occurs for the 100 μM Coumarin 102 solution, with a 1.4 μW excitation beam and a 115.3 mW depletion beam. (2) The minimum saturation intensity (Is) of STED, that is 13 MWcm-2 , is observed when the Coumarin 102 solution concentration is 10 μM. (3) Is values calculated directly from the depletion power derived with the cross-sectional area due to the full-width-at-half-maximum (FWHM) of the depletion beam show poor accuracy, where Is may be overestimated. Thus, a correction factor for the cross-sectional area is proposed. We also find that Is is not exactly constant for a fixed excitation beam power and dye concentration. This trend indicates that the conventional suppression function η(x)=e- ln (2)ISTED(x)/Is derived from picosecond STED may cause errors in evaluating the depletion process in CW STED microscopy.

Environment effects on the nonlinear absorption properties of Methylene blue under different power of excitation beam

Abstract In this paper, the nonlinear optical absorption properties of Methylene blue solutions were studied by open aperture Z-scan technique under a continuous-wave laser irradiation with various input powers. The experimental results show that the mechanism of nonlinear absorption and the calculated coefficients depend highly on the molecular surrounding media properties and the nature of intermolecular interactions. From the experimental results, the contributions of various intermolecular interactions were determined as well. Exploiting these results, it is possible to determine the effective media polarity parameters on the nonlinear responses of optical materials. According to the experimental results, by increasing of excitation beam power, the strong hydrogen bonds with high contributions increase the third order nonlinear responses of Methylene blue molecules.

Dependence on the incident light power of the internal electric fields in a GaAs p-i-n solar cell according to bright photoreflectance spectroscopy

Bright photoreflectance (BPR) spectroscopy at room temperature is used to examine the internal electric fields in a GaAs p-i-n solar cell for their dependence on the incident light power. Electric fields are observed at 30 µW and 100 µW of incident light. With increasing power, the strengths of the two electric fields are reduced due to the photovoltage effect. The electric field observed at 30 µW is assigned to the p-i interface, which is close to the surface. The other electric field is due to the i-n interface because the incident light penetrates deeper as the light power is increased. The electric field strength of 35.6 kV/cm at the p-i interface is lower than that of 42.9 kV/cm at the i-n interface at 500 µW of light power because the photovoltage effect is proportional to the number of photo-generated carriers, which is reduced as the distance from the surface increases. When the incident light power is similar to the excitation beam power, the electric fields at the p-i interface are saturated.

Light-Induced Mid-Infrared Emission of Liquid Carbon Tetrachloride and Benzene

Light-induced infrared emission spectroscopy (LIRES) is a novel technique that permits to receive high-quality spectra in the mid-infrared region. Low-intensity visible light connected to a highly sensitive FTIR spectrometer is more advantageous for studying any samples, including biological samples without any damage. This technique permits obtaining unique information on the molecule structure via vibrational excitation fundamental frequencies, overtones, and combination modes. It also enables a direct observation of vibrational radiation transitions in vibrationally excited molecules as well as the channels of vibration energy redistribution, which is not allowed with any other method. In this work, the LIRES is being tested as a technique for studying of vibrationally-excited molecules of carbon tetrachloride and benzene in the liquid phase. On the other hand, using transparent liquids, we had tried to understand some of the physical phenomena that can drive emission in mid-IR. The characteristics of the infrared emission of both liquid species produced by different wavelength radiation from various types of light systems (100 - watt Xe-lamp and Nd:YAG laser; lambda = 1064 nm (8 mW) and lambda = 532 nm (4 mW)) are presented. We demonstrated that the IR-signal, as well as spectral properties of carbon tetrachloride and benzene, was dependent on the wavelength and power of excitation beam. Results obtained with different light sources show that the visible light produces a nonlinear IR-emission signal in transparent liquids. We believe that the visible light is the source of the nonlinear response and is producing the vibration excitation as well as photostimulated transformations of the molecules possessing the high activity for the nonlinear response.

Excitonic Energy Shifts in CdMnTe/CdMgTe Quantum Wells under Resonant Excitation in Presence of 2D Carrier Gas

Systematic studies of amplitude and energy changes of excitonic lines in a strong excitation regime were carried out by a pump-probe method. The series of samples containing quantum wells with well width from 80 A to 140 A was investigated. One 80 A sample was n-doped with iodine, the rest of the quantum wells were intentionally undoped but contained a 2D gas of free holes. Its density could be varied by changing an intensity of additional illumination. The resonant creation of high population of e1hh1 excitons causes the energetic blue shift of the line due to interactions between excitons. The blue shift did not depend on the concentration of 2D gas of carriers whereas it did depend on the power of excitation beam. Model calculations of absorption show qualitative agreement with the experimental data.

Combined laser calorimetry and surface thermal lens technique for optical coating characterization

A combined laser calorimetry (LC) and surface thermal lens (STL) technique with a continuous-wave (cw) modulated excitation is developed for characterizing the absorption properties of optical coatings. LC and STL signals are simultaneously obtained with highly reflective coating samples irradiated at 1064 nm. With a modulated irradiation power of 2.6 W, the limits of absorptance detection for the LC and STL signals are approximately 1.2×10-6 and 0.5×10-6, respectively. The influences of different excitation parameters (modulation frequency, excitation-beam power and irradiation time) on the absorptance measurements are experimentally analyzed. The applicability of the combined LC and STL technique to the investigation of the laser-induced absorption changes of optical coatings in an absolute absorptance scale is demonstrated.

Control of Photon Polarization in GaAs/AlAs Single Quantum Dot Emission

Polarization encoded single photons and photon pairs are expected to play a major role in future quantum communication schemes [1]. Therefore, control of polarization properties of single quantum dot emission is of a great value. We study magnetic field induced transition between anisotropy controlled and Zeeman controlled emission from individual GaAs/AlAs quantum dot (QD). We demonstrate the utility of these studies, involving polarized photon correlation measurements, for determination of the anisotropic exchange splitting of excitonic states in quantum dots. The sample was mounted directly on a microscope objective [2] and placed inside a pumped helium cryostat. Continuous wave excitation was performed with use of tunable Ti:Al2O3 laser at wavelength of 718 nm assuring excitation below the energy gap of the barrier material. The typical power of excitation beam was 100μW over the spot of a diameter of about 1μm. Temperature was kept constant at 1.8K. Photoluminescence was excited and collected through the same microscope objective. Correlation measurements were performed in a Hanbury-Brown and Twiss setup. The signal after polarization and spectral filtering was detected using two avalanche photodiodes. Polarization resolved crosscorrelations between biexciton (XX) and exciton (X) photons emitted subsequently in a radiative cascade were measured in magnetic field ranging from 0 to 0.5 T, applied in Faraday configuration. Polarization correlation of photon pairs in parallel linear polarizations appeared as a bunching peak in histograms of correlated counts in the absence of magnetic field. No polarization correlation in circular basis was observed at B = 0T. This is expected for QDs exhibiting in-plane anisotropy, since intermediate X state of the cascade is split by energy of electron-hole exchange interaction into two components emitting in orthogonal linear polarizations. Application of magnetic field transforms Anisotropic Exchange Splitting (AES) into Zeeman splitting of the X level, reducing thus the influence of the anisotropy. In a sufficiently high magnetic field pure exciton eigenstates of angular momentum M = +/1 are observed [3]. Expected conversion of linearly polarized excitonic states to circularly polarized ones is demonstrated by increase of the degree of circular polarization correlation with increasing magnetic field. We apply a simple model proposed by Besombes et al. [4] to describe the progressive change of the polarization correlation with increasing field. The model allows calculating of the degree of polarization correlation as a function of Zeeman splitting to AES ratio. By fitting the model to the experimental data, we were able to determine the anisotropic exchange splitting AES value of order of tens of μeV. This method of the AES determination can be used even in cases when direct measurement by polarization resolved microphotoluminescence is not sensitive enough.

Thermal lens studies of the reaction of iron (II) with 1,10-phenanthroline at the nanogram level.

The determination of iron(II) with 1,10-phenanthroline in aqueous solutions was carried out exemplarily by thermal lens spectrometry. The peculiarities of analytical reactions at the nanogram level of reactants can be studied using this method. Under the conditions of the competing reaction of ligand protonation, the overall stability constant for iron(II) chelate with 1,10-phenanthroline was determined at a level of n x 10(-7) mol L(-1), logbeta3 = 21.3+/-0.1. The rates of formation and dissociation of iron(II) tris-(1,10-phenanthrolinate) at a level of n x 10(-8) mol L(-1) were found to be (2.05+/-0.05) x 10(-2) min(-1) and (3.0+/-0.1) x 10(-3) min(-1), respectively. The conditions for the determination of iron(II) with 1,10-phenanthroline by thermal lensing were reconsidered, and ascorbic acid was shown to be the best reducing agent, which provided minimum and reproducible sample pretreatment. Changes in the conditions at the nanogram level improved both the selectivity and sensitivity of determination. The optimum measurement conditions for thermal lensing were determined not only by the absorption of the analyte and reagents, but also by the background absorption of the solvent. The limits of detection and quantification of iron(II) at 488.0 nm (excitation beam power 140 mW) are 1 x 10(-9) and 6 x 10(-9) mol L(-1), respectively; the reproducibility RSD for the range n x 10(-8)-n x 10(-6) mol L(-1) is 2-5%.

Phase signal of optical beam deflection from single microparticles: Theory and experiment

The optical beam deflection (OBD) signal from a single microparticulate sample was theoretically derived for the photothermal response to an intensity-modulated excitation in the transverse experimental configuration. The dependencies of phase signal on the normal and transverse offsets of the probe beam were calculated and then experimentally verified. The OBD phase signal was chosen as a means of inspecting the particle interior, since it contains information about the heat source depth. The results showed that the phase signal was independent of the excitation beam power and that the surface absorbing layer thickness could be estimated from the phase variation using the modulation frequency. The possibility of correcting beam offsets by the phase signal was also considered.

Addition de photons par transfert d'énergie et absorption dans l'état excité dans les matériaux MClF: Sm+2 (M = Ba, Sr)

Under selective pulsed excitation into the first metastable excited state 5 D 0 of the Sm +2 ion embedded in BaCIF and SrCIF crystals with different concentrations (0.1‰, 1‰, 5‰), we have observed anti-Stokes emissions originating from the 5 D 1 , and 5 D 2 , multiplets respectively located at 1 339 and 3 280 cm −1 above 5 D 0 , at 4.4 K. The analysis of the fluorescence decays shows that two mechanisms are involved: 1) an excited-state absorption from the 5 D 0 level unambiguously identified by a double excitation experiment; 2) an up-conversion by energy transfer between Sm +2 ions excited in the 5 D 0 state evidenced by the effect of the dopant concentration. The competition between the two mechanisms appears clearly when looking at the dependence of the anti-Stokes fluorescence decay profiles versus the intensity and wavelength of the excitation beam. The numerical solution of the rate equations involving both processes allows to reproduce the whole of the experimental results. Evaluating the branching ratio of the two mechanisms leads to the conclusion that 70% of the anti-Stokes fluorescence arises from the energy transfer in the BaCIF: 5‰ Sm +2 crystal at low beam intensity level, this proportion being drastically lowered when increasing the excitation beam power Excitation selective par impulsions laser dans le 1er niveau metastable 5 D 0 de Sm 2+ . Observation de fluorescences anti-Stokes issus des niveaux superieurs 5 D 1 et 5 D 2 , a 4,4 K. Analyse de l'evolution temporelle de ces emissions, mettant en evidence deux mecanismes: 1) absorption a partir de l'etat excite 5 D 0 , d'apres l'experience de double excitation; 2) addition de photons par transfert d'energie, d'apres l'effet des variations de concentration de Sm 2+ . Manifestation nette de la competition entre ces deux mecanismes lorsque varient l'intensite et la longueur d'onde de l'impulsion laser