Inhomogeneous Intensity Distribution Research Articles

Spatial hole burning and spectral stability of a quantum-dot laser

The inhomogeneous intensity distribution of the optical model along the axis of a semiconductor quantum-dot laser results in spatial hole burning. The influence of this phenomenon on the stability of the multifrequency emission spectrum is studied when the optical transition of the quantum dots is characterized by considerable homogeneous broadening. The results of two models—in which inhomogeneous broadening is disregarded and taken into account—regarding the stability of the radiation spectrum under the influence of slight variation of the spectral loss dependence in the resonator are compared. Inhomogeneous distribution of the charge carriers (spatial hole burning) is found to be a critical factor in determining the form and stability of the spectrum.

Mismatch between baseline diffusion-weighted images and apparent diffusion coefficient maps predicts favourable penumbra: a retrospective study

BackgroundHyperintense diffusion-weighted images (DWIs) were thought to represent infarct, whereas hypointense apparent diffusion coefficient (ADC) maps could represent both infarct and penumbra. The mechanism and implication of the inhomogeneous intensity distribution of DWI infarct are unknown. This study aimed to investigate whether imaging characteristics of baseline DWIs and ADC maps could guide decision making of thrombolysis. Methods70 consecutive patients with confirmed ischaemia and ictus within 9 h from four centres in China were prospectively recruited. On the basis of our previous work to approximate infarct and penumbra from baseline ADC maps, high-DWI regions were determined as voxels within the approximated infarct core and penumbra as areas with high DWI intensity (greater than average of the mean and maximum). Within the high-DWI regions, the proportion of approximated penumbra voxels in terms of ADC intensity was calculated and denoted as rDWI–ADC. Good clinical response was based on the responder analysis. Receiver operating characteristic analysis was used to derive the optimum threshold of rDWI–ADC to maximise the Youden index. Patients with rDWI–ADC greater than or equal to the optimum threshold were deemed to have DWI–ADC mismatch. We tested the hypothesis that patients with DWI–ADC mismatch would have a better clinical response from thrombolysis than those without the mismatch. The Institutional Review Boards of the four centres approved the study, and written informed consent was obtained from all patients. Findings20 (29%) patients had DWI–ADC mismatch. Thrombolysis was significantly associated with good clinical response in patients with DWI–ADC mismatch compared with no thrombolysis (odds ratio [OR] 35; p=0·002). Thrombolysis based on DWI-ADC mismatch yields a substantially better clinical response than that based on perfusion–diffusion mismatch (OR 8·3; p=0·04). InterpretationDWI–ADC mismatch could be used to identify patients purely from baseline imaging who are likely to benefit from intravenous thrombolysis. FundingChinese National Program on Key Basic Research Project (2013CB733800 and 2013CB733803), Chinese National Science and Technology Pillar Program during the Twelfth Five-Year Plan Period (2011BAI08B09), Shenzhen Key Technical Development Grant (CXZZ20140610151856719), and Shenzhen Basic Research Grant (JCYJ20140414170821262).

Hanle detection for optical clocks.

Considering the strong inhomogeneous spatial polarization and intensity distribution of spontaneous decay fluorescence due to the Hanle effect, we propose and demonstrate a universe Hanle detection configuration of electron-shelving method for optical clocks. Experimental results from Ca atomic beam optical frequency standard with electron-shelving method show that a designed Hanle detection geometry with optimized magnetic field direction, detection laser beam propagation and polarization direction, and detector position can improve the fluorescence collection rate by more than one order of magnitude comparing with that of inefficient geometry. With the fixed 423 nm fluorescence, the improved 657 nm optical frequency standard signal intensity is presented. The potential application of the Hanle detection geometry designed for facilitating the fluorescence collection for optical lattice clock with a limited solid angle of the fluorescence collection has been discussed. The Hanle detection geometry is also effective for ion detection in ion optical clock and quantum information experiments. Besides, a cylinder fluorescence collection structure is designed to increase the solid angle of the fluorescence collection in Ca atomic beam optical frequency standard.

(Invited) Study of Carrier Localization, Carrier Transportation and Carrier Recombination Processes in Blue-Emitting InGaN/GaN MQWs

InGaN/GaN MQWs grown on c-plane sapphire substrate show high internal quantum efficiency, regardless of their high threading dislocation density (108 cm-2~ 1010 cm-2) that is introduced by lattice mismatch. This high-IQE phenomenon is attributed to bandgap energy fluctuation and carrier localization in the QW layer [1].In the present work, temperature-dependent PL, confocal microscopy, and nanometer-scale time-resolved photoluminescence (TRPL) spectroscopy were employed to analyze the carrier localization, carrier transportation and carrier recombination processes in blue-emitting InGaN/GaN MQWs.The temperature-dependent PL spectra were taken from 10 K to 300 K. The decreased spectrum-integrated PL intensity with increasing temperature was observed, which was fitted with second-order Arrhenius equation, as shown in Fig. 1. The fitting indicated two non-radiative channels: the one with high activation energy (dominated at high temperature) corresponded to thermal activation of carriers out of the strongly localized states, while the one with low activation energy (dominated at low temperature) corresponded to thermal activation of carriers out of the weakly localized states. The S-shaped shift of PL peak energy and the inverse S-shaped shift of PL FWHM were also observed, as shown in Fig. 2 and Fig. 3, which were explained with carrier localization and carrier dynamics.The confocal microscope image collecting near bandedge emission showed inhomogeneous PL intensity distribution, as shown in Fig. 4. Previous analysis [2] has shown that the bright region had smaller bandgap energy than dark region did. Nanometer-scale TRPL spectra were taken at different places in the confocal microscope image. As shown in Fig. 5, TRPL spectra taken at the center of bright region and the center of dark region show single exponential decay, which are characterized by a single lifetime and corresponds to carrier localization in strongly localized state and weakly localized state, respectively. Comparing with bright region, dark region has shorter lifetime, indicating a higher probability of nonradiative recombination and carrier transportation from dark region to bright region. TRPL spectrum taken at the boundary of bright region, however, shows double exponential decay, as shown in Fig. 5, which is characterized by two lifetimes. The short lifetime corresponds to carrier transportation from weakly localized state to strongly localized states, while the long lifetime corresponds to carrier recombination at local strongly localized state.

Advanced attenuation and fluorescence measurement methods in the investigation of photodarkening and related properties of ytterbium-doped fibers

Considering the continuous development of ytterbium (Yb)-doped fibers for fiber lasers and amplifiers, thorough characterization of the features and quality of such fibers is necessary now more than ever. In particular, the evaluation of the very strong core attenuation that results from Yb absorption and the losses accompanying photodarkening (PD) processes in Yb-doped optical fibers is still of immense interest. Keeping that in mind, the potential of a method using pinhole fibers will be demonstrated here. Moreover, the investigation of fluorescence properties of Yb3+ is also important; it can be affected by phenomena such as reabsorption, inhomogeneous pump intensity distribution, amplified spontaneous emission, lasing, and, lastly, PD. As these phenomena occur while measuring the fluorescence properties of Yb3+ in optical fibers, they can distort the fluorescence spectra and fluorescence decay significantly. We suggest a twice-perpendicular measurement method and strict time management to avoid these interfering phenomena during the fluorescence measurements.

Correlation between structure and photoluminescence properties in InGaN epilayers with thicknesses below and above critical thickness

The layer strain and its relaxation effects on the photoluminescence (PL) of InGaN layers are studied using confocal microscopy. The relaxation imposed structural changes are studied by X-ray diffraction (XRD) reciprocal space mapping and atomic force microscopy. Initial layer relaxation generated misfit dislocations were observed by confocal microscopy as intersecting parallel lines of lower PL intensity. The splitting of the PL spectrum into several PL bands indicated an onset of changes in the layer structure, which were confirmed by XRD measurements. The PL bands were attributed to two sub-layers of the sample: A relaxed upper sub-layer and a strained sub-layer underneath. Bright spots, approximately 250 nm in diameter, were observed on the background of the inhomogeneous PL intensity distribution due to fluctuations of In content. The bright spots correspond to column-like structures with relaxed lattice, In content as in the initial strained layer, and lower density of nonradiative recombination centers than that in the surrounding background.

Unintentional annealing of the active layer in the growth of InGaN/GaN quantum well light-emitting diode structures

The effects of increasing p-type layer thickness on the photoluminescence (PL) of InGaN/GaN quantum well (QW) light-emitting diode (LED) structures are studied using confocal microscopy. Due to composition fluctuations in the QWs, spatially inhomogeneous PL intensity distribution is observed in all the LED structures with different p-type layer thicknesses. Meanwhile, the spectral peak position and the difference in PL intensity between the bright and dark areas exhibit nonmonotonous behavior. Such behavior is attributed to the concurrent effects of thermal annealing during the high-temperature overgrowth of p-type layer, the variation of the quantum-confined Stark effect (QCSE) due to the increase of p-type layer thickness, and carrier delocalization. The PL intensity increases with increasing p-type thickness when the thickness is smaller than a certain value and decreases beyond this thickness.

Crystalline phase distribution of Dy_2(MoO_4)_3 in glass induced by 250 kHz femtosecond laser irradiation

Spatial precipitation of Dy2(MoO4)3 crystal in the glass is achieved by using 800 nm, 250 kHz femtosecond laser. Micro-Raman spectra show that multiple crystalline phases of Dy2(MoO4)3 can be formed in femtosecond laser-modified region. Their distributions depend mainly on femtosecond laser-induced temperature field, which is asymmetric along the light propagation direction. This phenomenon results from an inhomogeneous intensity distribution of the incident pulse due to both of self-focusing effect and spherical aberration effect. Furthermore, the EPMA mapping demonstrates that the O element concentration is reduced in the center of the modified region, while the Mo element one increases. The composition change is according strongly with the phase transformation of Dy2(MoO4)3 crystal. The present study implies that the asymmetry of the temperature field is an important factor to influence the crystal precipitation.

Full-field unsymmetrical beam shaping for decreasing and homogenizing the thermal deformation of optical element in a beam control system

We propose and demonstrate the full-field unsymmetrical beam shaping for decreasing and homogenizing the thermal deformation of optical element in a beam control system. The transformation of square dark hollow beam with unsymmetrical and inhomogeneous intensity distribution into square dark hollow beam with homogeneous intensity distribution is chosen to prove the validity of the technique. Dual deformable mirrors (DMs) based on the stochastic parallel gradient descent (SPGD) controller are used to redistribute the intensity of input beam and generate homogeneous square dark hollow beam with near-diffraction-limited performance. The SPGD algorithm adaptively optimizes the coefficients of Lukosz-Zernike polynomials to form the phase distributions for dual DMs. Based on the finite element method, the thermal deformations of CaF(2) half transparent and half reflecting mirror irradiated by high power laser beam before and after beam shaping are numerically simulated and compared. The thermal deformations of the mirror irradiated by the laser beam with different powers and the influences of thermal deformation on beam quality are also numerically studied. Results show that full-field beam shaping can greatly decrease and homogenize the thermal deformation of the mirror in the beam control system. The strehl ratios of the high power laser beams passing through the beam control system can be greatly improved by the full-field beam shaping. The technique presented in this paper can provide effective guidance for optimum design of high power laser cavity and beam shaping system.

Spatially‐resolved photoluminescence study of high indium content InGaN LED structures

AbstractSpatially and spectrally resolved photoluminescence (PL) of green light‐emitting diode (LED) structure with InGaN multiple quantum wells was studied using confocal and scanning near field optical microscopy techniques. Inhomogeneous distribution of PL intensity on the scale of 200 nm has been observed. The PL bands peaked at approximately 492 nm and 502 nm in bright and dark areas, respectively. The spot‐to‐spot variation of the peak position of the main PL band was in the range of ∼30 nm for the bright and ∼50 nm for the dark areas. The highest PL intensity in the bright areas was observed in the spots with PL band peaked at ∼492 nm, while emission intensity in dark areas had no correlation with the PL band spectral position. The inhomogeneous PL intensity distribution is explained by inhomogeneous distribution of nonradiative recombination centers, in contrast to a fairly constant tail states density (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

3D multiple optical trapping of Au-nanoparticles and prokaryote E. coli using intra-cavity generated non-circular beam of inhomogeneous intensity

We report 3D multiple trapping of dielectric polystyrene (PS) beads and gold nano-particles (GNPs) in single beam optical tweezers system using an asymmetric beam of inhomogeneous intensity distribution. This special kind of beam of quasi-TEM11 profile was generated from intra-cavity CW-laser source operating at 532 nm. Multiple trapping of both the low refractive index rod-like Escherichia coli bacteria and 253 nm plasmonic GNPs dispersed in 1.025 μm PS beads which were homogenized in de-ionized water was realized utilizing this spatial beam. Laser-GNPs interaction rendered the enhancement of local surface plasmon resonance field around GNPs causing long-range aggregation of PS beads. The multiple trapping of plasmonic GNPs by the present simple method might find applications for micro- and nano-connectors, underlying physical processes in light-matter interaction assays for inter-particle force analysis, cancer diagnostic and photothermolysis, surface-enhanced Raman scattering (SERS) spectroscopy, and surface plasmon based biological and chemical sensors.

Influence of beam intensity profile on the aerodynamic particle size distributions generated by femtosecond laser ablation

AbstractThe dependence of nanoparticle size distributions on laser intensity profile was determined during infrared femtosecond laser ablation of silver targets in air. Laser parameters were adjusted to ablate at the same peak fluence with spatially homogeneous (flat-top) and inhomogeneous (Gaussian) intensity distributions formed by diffractive optical elements. Aerodynamic particle size was measured online by an electric low-pressure cascade impactor. Narrower size distributions were detected for the flat-top intensity profile in the fluence range from 0.6 to 4.4 J/cm2, while the Gaussian beam produced broad and bimodal distributions. The aerodynamic number frequency of the primary nanoparticulate fraction (40 nm) was equal to the number frequency of the submicron agglomerate fraction (200 nm) at laser fluence of 1 J/cm2. The Feret diameter of primary particles was 80 nm. Geometrical interpretation of the irradiated spots at the corresponding laser fluence regimes explains the formation of bimodal (submicron and nanoparticulate) size distribution in the case of Gaussian beams. The bimodality is attributed to different thermalization pathways during laser ablation.

Characteristics of surface-enhanced Raman scattering and surface-enhanced fluorescence using a single and a double layer gold nanostructure

This paper reports the characteristics of surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) using a unique SERS-active substrate comprised of a single layer and a double layer of two-dimensional (2D) gold nanostructure. Colloidal gold nanoparticles were immobilized on a glass substrate and a multi-purpose experimental setup was adopted to obtain surface plasmon resonance (SPR), SERS and SEF on a single platform. Inhomogeneous intensity distribution was observed in correlated images of SPR and SERS. Several laser lines were used as excitation sources for further SERS measurements. Higher SERS intensities were observed with longer wavelength excitations at the same spatial position. Fluorescence measurements were carried out using 514 nm line and SEF images were obtained using the same sample. Fluorescence emissions were found to be enhanced in the presence of 2D gold nanostructure. A series of SERS spectra were recorded by conducting ensemble SERS measurements at a periodic interval of 2 microm, crossing bare substrates, the single layer and the double layer of gold nanostructure. The double layer provides higher enhancement in SERS than that of the single layer. Polarization-selective SERS measurements obtained at the single layer and double layer showed a clear difference in their dispersions. SERS intensities of the analytes adsorbed at the single layer were fitted well with cos(4)theta dependence; however, for the double layer, the relationship was quite uncertain.

Optical formation of relief holograms from photopolymerizing composites with nonpolymerization-capable additives

The process of optical formation of relief holograms from photopolymerizing composites with nonpolymerization-capable additives is experimentally investigated. The conditions for inhomogeneous optical displacement of these components onto the surface of the photopolymer are determined. It is shown that using butyl alcohol as a nonpolymerization-capable component not only makes it possible to efficiently create polymeric relief by an inhomogeneous intensity distribution even with low contrast but also makes it possible to optimize the diffraction efficiency of relief polymeric holograms by the concentration of this additive.

Improvement of planar laser diagnostics by the application of a beam homogenizer

For planar laser diagnostics, a most uniform beam profile is highly desirable for two reasons: first, subsequent corrections for an inhomogeneous intensity distribution are time consuming and prevent on-line engineering assessment and second, temporal fluctuations cannot be corrected anyway. However, in general for combustion and flow diagnostics pulsed laser sources are used to achieve a high temporal resolution which typically possess a rather poor beam quality compared to continuously emitting laser sources. And, pulse to pulse fluctuations of the beam profile directly increase the noise in single-shot measurements. In this contribution we show the application of a micro-lens array based beam homogenizer whereby an almost homogeneous illumination of the region of in interest is achieved. This enables the on-line evaluation of the measured data without subsequent corrections. Thus a general advance of laser techniques towards engineering practice is achieved. Additionally, statistical fluctuations of the beam profile are strongly reduced by the homogenizer what directly improves the local standard deviation of the measurement. These benefits are demonstrated by means of planar laser-induced fluorescence (LIF) experiments.

Fluorescence Polarization Spectroscopy at Combined High-Aperture Excitation and Detection: Application to One-Photon-Excitation Fluorescence Microscopy

A problem of the one-photon-excitation fluorescence polarization spectroscopy of macroscopically isotropic media, in the case of combined high-aperture excitation and detection, is considered and described in a spherical representation. The case of inhomogeneous intensity distribution in the cross-section of the parallel beam of exciting light, which is converted by an objective lens into inhomogeneous radial distribution of the intensity of the focused exciting light, is also taken into account. The obtained formalism is adapted to the description of confocal fluorescence polarization microscopy. It is shown that the total and magic-angle-detected fluorescence decays do not solely represent the kinetic evolution of the excited-state because of the contribution of the dynamic evolution of photoselected fluorophores. The time-evolution of emission anisotropy is nonexponential. The outlined theory predicts that the total and magic-angle-detected fluorescence decays solely represent the kinetic fluorescence decay, and thereby, the emission anisotropy becomes an (multi)exponential function of time for the excitation-detection cone half-angles not higher than about 15-20 degrees. The initial values of the emission anisotropy are not modified by the application of the excitation-detection apertures if the cone half-angles do not exceed 10-15 degrees. The histograms of unpolarized fluorescence, calculated from the parallel and the perpendicular components of polarized fluorescence, detected at the excitation-detection cones wider than about 65 degrees solely represent the kinetic fluorescence decay. At such conditions, the microscope objective operates like an "integrating sphere". The calibration method, which is based on a general (symmetry adapted) formula describing fluorescence polarization experiments on macroscopically isotropic samples, is discussed. This method enables the analysis of all fluorescence polarization experiments without the necessity of considering the expressions for polarized fluorescence decays relating to a particular experimental case of interest. With this method, any commercially available microscope objective can be calibrated, and its optical properties can be precisely verified. The application of the outlined theory to different fluorescence spectroscopy techniques is indicated. The expressions derived for confocal fluorescence polarization microscopy can be employed in the numerical analysis of the data recovered from the photochemical bioimaging.

Measurement of the SBS gain of radiation in glasses

A method for determining the specific gain under conditions of stimulated Brillouin scattering (SBS) is proposed, which is based on measuring the amplification of a Stokes beam with an inhomogeneous spatial intensity distribution in a sample used as an SBS amplifier. Under the same experimental conditions, the values of the SBS gain are obtained for the new phosphate glass KGSS 0180 (g = 1.9 cm/GW) and for the GLS 22 (g = 1.4 cm/GW), K8 (g = 1.3 cm/GW), and KU2 (g = 1.5 cm/GW) glasses, which are widely used in laser systems. The SBS frequency shift of radiation with a wavelength of 1.054 μm (14.57±0.02 GHz) and the hypersonic velocity (5030 m/s) have been determined for the KGSS 0180 glass by optical heterodyning. The relaxation time of hypersound in the noted media is estimated.

Transformation of the spatial coherence of pulsed laser radiation transmitted in the nonlinear regime through a multimode graded-index fibre

A method is proposed for transformation of the spatial coherence of pulsed laser radiation upon nonlinear interaction in a multimode fibre. The specific features of the transmission of correlation properties of radiation in a graded-index fibre with regular and irregular profiles of the refractive index of the fibre core are analysed. A comparative analysis of the parameter of global degree of radiation coherence at the output of inhomogeneous waveguide and non-waveguide media is performed. It is shown that the most efficient mechanism of decorrelation of pulsed radiation in an optical fibre is fluctuations of the phase of radiation scattered by inhomogeneities of the refractive index of the fibre core induced due to nonlinear interaction with radiation with the spatially inhomogeneous intensity distribution.

Particle on surface: 3D-effects in dry laser cleaning

Previous analysis of dry laser cleaning within the frame of a one-dimensional (1D) model with homogeneous surface heating shows that this model disagrees with experiments by one–two orders of magnitude. The particle on the surface produces an inhomogeneous intensity distribution in its vicinity due to scattering and diffraction. This produces a nonstationary 3D distribution of the temperature and nonstationary 3D thermal deformations of the surface. If one uses the Mie theory for calculation of inhomogeneous intensity then in some region of the parameters, the 3D model predicts results close to the experimental ones. The next step was done when the scattering effects for radiation reflected from the surface was taken into account (so-called “particle on surface” problem). This approach yields results close to the experimental one within the wide range of parameters.

Influence of the spatial beam profile on hard tissue ablation, part II: pulse energy and energy density distribution in simple beams.

When calculating applied flux densities in practice, the beam profile of a laser is often erroneously assumed to be homogeneous. In addition, there is usually no consistency in the choice of a suitable measuring method for determining the beam diameter. This failure to observe the inhomogeneous intensity distribution within the beam cross-section, combined with the imprecise knowledge of the beam diameter, leads to flux densities being stated that represent mean values at best. The present paper gives definitions for the flux densities of simple, radially symmetrical beam cross-sections, taking the top-hat and Gaussian profiles as examples. In connection with the inhomogeneous energy distribution in the Gaussian beam, a concept of integral and local energy density is discussed, which differs from the customary definition of the energy density as a constant. Also presented are the consequences of the mathematical concepts in terms of measurement, giving particular consideration to the case where the energy density as the measured variable matches the integral energy density. The significance of the integral and local energy density for hard-tissue ablation is described, based on the practical example of the ablation of dental hard substance. The central result is that the integral flux density is directly accessible as a measured variable, while the effect on the tissue is determined by the local flux density. If the form of the beam is known, the integral flux density can be converted into the local flux density.