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In this article, several designs of the active region of the THz radiation source are considered, taking into account grown super-multiperiod AlGaAs/GaAs superlattices. For the proposed designs, the principal device characteristics are computed: energy band diagram, gain spectrum, and transport characteristics. Based on the calculation results, the authors proposed an optimal design of the active region of a tunable THz radiation source. [ABSTRACT FROM AUTHOR]

Patterned light from a laser can induce rapid self-folding of pre-strained polymer sheets. Black ink coated on the sheet absorbs the light, which converts the photon energy into thermal energy that heats the sheet locally; the temperature of the sheet is highest at the surface where the light impinges on the sheet and decreases through the sheet thickness. The gradient of temperature induces a gradient of strain relaxation through the depth of the sheet, which causes folding within seconds of irradiation. The pattern of laser light that irradiates the compositionally homogeneous two-dimensional (2D) substrate dictates the resulting three-dimensional (3D) shape. Unlike most approaches to self-folding, the methodology described here requires no patterning of pre-defined hinges. It opens up the possibility of using a patterning technique that is inherently 2D to form 3D shapes. The use of lasers also enables systematic control of key process parameters such as power, intensity, and the pattern of light (i.e., beam width and shape). The rate of folding and folding angle measured with respect to these parameters provide an indirect quantification of heat loss in the sample and thereby identify the threshold power and power intensity that must be delivered to the hinge for folding to occur. [ABSTRACT FROM AUTHOR]

In this work, the resolving limit of maskless direct laser writing is overcome by cooperative manipulation from nonlinear reverse saturation absorption and thermal diffusion, where the nonlinear reverse saturation absorption can induce the formation of below diffraction-limited energy absorption spot, and the thermal diffusion manipulation can make the heat quantity at the central region of energy absorption spot propagate along the thin film thickness direction. The temperature at the central region of energy absorption spot transiently reaches up to melting point and realizes nanolithography. The sample "glass substrate/AgInSbTe" is prepared, where AgInSbTe is taken as nonlinear reverse saturation absorption thin film. The below diffractionlimited energy absorption spot is simulated theoretically and verified experimentally by near-field spot scanning method. The "glass substrate/Al/AgInSbTe" sample is prepared, where the Al is used as thermal conductive layer to manipulate the thermal diffusion channel because the thermal diffusivity coefficient of Al is much larger than that of AgInSbTe. The direct laser writing is conducted by a setup with a laser wavelength of 650 nm and a converging lens of NA = 0:85, the lithographic marks with a size of about 100 nm are obtained, and the size is only about 1/10 the incident focused spot. The experimental results indicate that the cooperative manipulation from nonlinear reverse saturation absorption and thermal diffusion is a good method to realize nanolithography in maskless direct laser writing with visible light. [ABSTRACT FROM AUTHOR]

This paper details infra-red pump and probe studies on nitric oxide conducted with two continuous wave quantum cascade lasers both operating around 5 μm. The pump laser prepares a velocity selected population in a chosen rotational quantum state of the v = 1 level which is subsequently probed using a second laser tuned to a rotational transition within the v = 2 ← v = 1 hot band. The rapid frequency scan of the probe (with respect to the molecular collision rate) in combination with the velocity selective pumping allows observation of marked rapid passage signatures in the transient absorption profiles from the polarized vibrationally excited sample. These coherent transient signals are influenced by the underlying hyperfine structure of the pump and probe transitions, the sample pressure, and the coherent properties of the lasers. Pulsed pump and probe studies show that the transient absorption signals decay within 1 μs at 50 mTorr total pressure, reflecting both the polarization and population dephasing times of the vibrationally excited sample. The experimental observations are supported by simulation based upon solving the optical Bloch equations for a two level system. [ABSTRACT FROM AUTHOR]

We investigate the effect of mid-infrared (MIR) pumping on the transport properties of GaAs/(Al,Ga)As terahertz (THz) quantum lasers (TQLs), which rely on quantum coherence effects of intersubband transitions. Aiming at THz lasing at elevated temperatures, we extend the concept of THz gain with and without population inversion of a single, MIR-pumped, electrically driven THz stage proposed by Waldmueller et al. [Phys. Rev. Lett. 99, 117401 (2007)] to an entire TQL. However, experiments using a CO2 as well as a free-electron laser and numerical simulations show that this resonant MIR pumping causes a negative differential conductivity (NDC) in addition to the NDC caused by sequential tunneling. Lasing of these TQLs is prevented by the formation of electric-field domains below the resonance field strength for gain of each single THz stage. [ABSTRACT FROM AUTHOR]

A white SnCl2 phosphor is synthesized by the chemical etching of Sn shots in an aqueous HCl solution. The optical properties of this phosphor are investigated using diffuse reflectance, photoluminescence (PL) analysis, PL excitation spectroscopy, and PL lifetime measurements. The SnCl2 phosphor exhibits PL in the blue (∼460 nm) and red spectral regions (∼600 nm) under ultraviolet excitation (≥4.1 eV). The differences in the PL features observed using Nd:YAG (λ = 266 nm) and He-Cd (λ = 325 nm) lasers as excitation light sources are interpreted using the configurational-coordinate model, taking into account the band-gap energy (∼3.8 eV) of SnCl2 and the excited-state energies of the Sn2+ ions. A reliable energy level diagram for Sn2+ ions in the SnCl2 host is also proposed. [ABSTRACT FROM AUTHOR]

Laser dynamic forming (LDF) is a novel high energy rate microfabrication technique, which makes use of the shock pressure induced by laser to generate dynamic high strain rate three dimensional (3D) forming of thin films. In LDF process, a high shock pressure accelerates the workpiece to a high velocity and deforms it into complex 3D shapes. The forming velocity of the workpiece imparted by a single laser pulse with high energy may exceed the critical forming velocity of the material, and thus causing it to fracture. This problem is more severe when 3D structure of large aspect ratio needs to be formed. To overcome this problem, multiple-pulse LDF is investigated in this study. The total laser energy is evenly distributed in different laser pulses to keep the forming velocity below the critical forming velocity of the material. The effects of the multiple-pulse LDF on the deformation behavior of ultrathin foils are investigated. The deformation depth and thickness variation distribution of the formed 3D features are characterized to reveal these effects. In addition, the effects of vacuum conditions on multiple-pulse LDF process are carried out. It is found that the bounce off of the foil can be effectively reduced by multiple-pulse LDF and the final shape could be controlled much more accurately. By extending single pulse LDF to multiple-pulse LDF, the forming capability of LDF is further enhanced, and thus enlarges the applicable range of this technique. [ABSTRACT FROM AUTHOR]

The relationship between facet stress and reliability of AlGaInAs edge-emitting lasers is unclear despite it being an important issue. We prepared two 1.3 μm AlGaInAs Fabry–Pérot buried-heterostructure (BH) lasers that were identical except that they had tensile and compressive stress at the facet. The magnitude of the facet stress was controlled to be approximately 200 MPa in both lasers. We performed three reliability tests. In forward-biased electrostatic discharge tests, which can evaluate the resistance to optical damage, the cumulative degradation ratio of the compressive stressed laser was 33% lower than that of the tensile stressed laser. This result indicates a reduction in the optical absorption due to enlargement of the energy band gap at the facet. In the long-term aging of the light output power of 8 mW at 85 °C over 5 000 h, no dependence of the facet stress on the lifetime was observed. Since a major limitation of InP-based BH lasers is the BH interface, the aging results are reasonable. In the accelerated aging of a large current of 200 mA at 85 °C over 800 h, degradation (defined as an increase in the threshold current of over 10%) was observed only in the tensile stressed laser. This degradation differed completely from that caused by optical absorption; dislocation loops covered the entire active layer at the facet. To the best of our knowledge, this is the first time such degradation has been reported. We found the degradation depends on the facet stress and that compressive stress can suppress the degradation. [ABSTRACT FROM AUTHOR]

We have investigated the mechanism of exciton-exciton annihilation in organic lanthanide complexes by measuring the excitation density dependence of photoluminescence yield in neat films of a model europium chelate. The observed luminescence efficiency reduction at high excitation intensities has been attributed to the operation of the mutual annihilation of the ligand singlet excitons. The second order interaction rate constant, the exciton diffusivity, and the singlet exciton diffusion length in this material have been determined. These results were used to evaluate the role of exciton annihilation in organic light emitting diodes, lanthanide-based organic lasers, and light up-converters. Indications concerning the optimization of the performances of such devices have also been given. [ABSTRACT FROM AUTHOR]

The gain and refractive index dynamics of dilute nitride antimonide semiconductor optical amplifiers are studied using heterodyne pump probe spectroscopy, both in forward and reverse bias regimes. In the forward biased absorption regime, both gain and refractive index relax on the same timescale indicating that both quantities are linked to the same relaxation process, interband recombination. Above transparency, in the forward biased gain regime, the gain and phase exhibit differing timescales resulting in a dynamical alpha factor that varies strongly with time. Reversed bias measurements suggest a recombination dominated absorption recovery where the recovery timescale increases with increasing reversed bias, possibly due to charge separation effects. [ABSTRACT FROM AUTHOR]

A multiple graphene layer (MGL) structure with a stack of GLs and a highly conducting bottom GL on SiC substrate pumped by optical radiation is considered as an active region of terahertz and far infrared lasers with external metal mirrors. The dynamic conductivity of the MGL structure is calculated as a function of the signal frequency, the number of GLs, and the optical pumping intensity. The utilization of optically pumped MGL structures might provide the achievement of lasing with the frequencies of about 1 THz at room temperature due to a high efficiency of pumping. [ABSTRACT FROM AUTHOR]

A comparative study of single and double pulse laser induced breakdown spectroscopy (LIBS) using the fundamental (1064 nm) and the second harmonics (532 nm) of Nd:YAG lasers is presented. The double pulse collinear configuration yields more than three hundred times signal enhancement in the singly ionized aluminum lines as compared to the single pulse LIBS spectrum. The effect of interpulse delay between the two laser pulses and the laser pulses energies ratio in the double pulse spectrum are studied. A comparison of variations of plasma parameters along the plume axis in the single and the double pulse has also been studied. [ABSTRACT FROM AUTHOR]

We demonstrate a method for isolating the vibrational predissociation spectra of different structural isomers of mass-selected cluster ions based on a population-labeling double resonance scheme. This involves a variation on the “ion dip” approach and is carried out with three stages of mass selection in order to separate the fragment ion signals arising from a fixed-frequency population-monitoring laser and those generated by a scanned laser that removes population of species resonant in the course of the scan. We demonstrate the method on the Ar-tagged NO2-·H2O cluster, where we identify the spectral patterns arising from two isomers. One of these structures features accommodation of the water molecule in a double H-bond arrangement, while in the other, H2O attaches in a single ionic H-bond motif where the nominally free OH group is oriented toward the N atom of NO2-. Transitions derived from both the NO2- and H2O constituents are observed for both isomers, allowing us to gauge the distortions suffered by both the ion and solvent molecules in the different hydration arrangements. [ABSTRACT FROM AUTHOR]

The transverse laser-induced thermoelectric voltages in the tilted La2-xSrxCuO4 (LSCO) thin films are observed for the first time. The detected signals are demonstrated to originate from the anisotropy of thermoelectric power and depend on the Sr-doping level drastically. The largest voltage is observed in the LSCO films at x=0.15 under the irradiation of different lasers with the wavelength in the spectrum range from infrared to ultraviolet. The dependence of the signals on Sr-doping content is associated with a two dimensional charge transport behavior in LSCO films near the optimum doping content. [ABSTRACT FROM AUTHOR]

Core-shell colloids composed of a dielectric core surrounded by a metal shell show geometric cavity resonances with optical properties that are distinctly different than those of the collective plasmon modes of the metal shell. We use finite-difference time domain calculations on silica colloids with a core diameter of 456 nm, surrounded by a 38 nm thick Au shell, to study the temporal evolution of the mode field intensity inside the cavity upon pulsed excitation. Calculations using Mie theory and the T-matrix method are used to analytically determine the dipolar cavity resonance spectrum, which is found superimposed on the broad collective dipolar plasmonic resonance modes. We characterize resonance wavelength and linewidth in terms of a geometric mode confined inside the cavity. Cavity linewidth can be controlled by metal shell thickness and quality factors Q>150 are observed. Due to the small cavity mode volume V=0.2(λ/n)3, a Purcell factor as high as P=54 is calculated. Introducing shape anisotropy lifts the cavity mode degeneracy, yielding blue- and redshifted longitudinal and transverse resonant modes, respectively. The relatively large volume over which the field enhancement is observed in these spherical and anisotropic metal shell cavities, combined with cavity quality factors that are much higher than that of the collective plasmonic modes, makes them attractive for application in nanoscale light sources, sensors, or lasers. [ABSTRACT FROM AUTHOR]

The polarization-dependent optical characteristics of violet InGaN laser diodes, such as band diagrams, emission wavelength, and threshold current, under different operation temperatures have been investigated numerically. Specifically, the normal and reversed polarizations are presented when the laser diodes with wurtzite structure are grown along Ga-face and N-face orientations, respectively. The simulation results show that the lowest threshold current is obtained for the double-quantum-well laser diode with normal polarization, while it is obtained for the single-quantum-well laser diode with reversed polarization. The main physical explanation for the phenomenon is due to effectively reduced electron leakage current, increased hole current density, and reduced Shockley–Read–Hall recombination rate within the active region as the idea of reversed polarization is considered. [ABSTRACT FROM AUTHOR]

We compared the thermal performance of mid-IR GaInAs/AlInAs quantum cascade lasers (QCLs) having identical gain medium, but different heat sinking configurations. By using a two-dimensional anisotropic thermal model, we have calculated the temperature profiles and the heat flow patterns of ridge waveguide QCLs, either buried or planarized, and mounted substrate-side or epilayer-side down. Device planarization with Y2O3:Si3N4 dielectric layers gives an ∼7% reduction of the device thermal resistance with respect to InP buried heterostructures. If this planarization is combined with thick gold electroplating and epilayer-side mounting of the device, the thermal resistance is reduced by ∼34% and ∼50%, respectively, with respect to conventional ridge waveguide structures. [ABSTRACT FROM AUTHOR]

Characterization of a vertical cavity surface emitting laser source for a frequency reference that uses coherent population trapping with atomic rubidium is reported. The frequency reference requires a low-noise laser that operates in a single transverse mode at 794.7 nm. We show that biasing the laser at low output power and modulating it at higher frequency than its low relaxation oscillation frequency are necessary to obtain symmetric modulation sidebands and improve the long-term performance of the frequency reference. [ABSTRACT FROM AUTHOR]

We studied the polarized laser emission from an anisotropic one-dimensional (1D) birefringent photonic crystal (PC) laser. An active medium layer, which consisted of an epoxy resin doped with fluorescent dye, was sandwiched between two anisotropic 1D PC films. It was shown that efficient laser emissions were generated by optical pumping at relatively low lasing thresholds. The wavelengths of the emitted lasers were 611 and 618 nm, which correspond to the two split eigenmodes at the low-energy band edges that are due to the anisotropy of the PCs. We also demonstrated that the polarization of the lasing emission can be controlled by adjusting the birefringence of the anisotropic PCs. [ABSTRACT FROM AUTHOR]

Very small aperture lasers utilizing single rectangular apertures with geometrically resonant surface plasmons are experimentally characterized. Bimetal layers of tungsten and silver were employed to realize narrow apertures in a focused ion beam etching process. The tungsten overlayer acts as a mask suppressing aperture opening induced by the tail of a gallium ion beam. We observed strong enhancements in the optical output of narrow apertures (<45 nm) fabricated by single-pass mode, which we attribute to the geometrical resonance of surface plasmons within the apertures. Wider apertures (160 nm) fabricated in raster mode did not exhibit this enhancement, consistent with the explanation of geometrical resonance. [ABSTRACT FROM AUTHOR]

Ge-doped crystalline quartz has been examined for its thermally stimulated luminescence and has been found to exhibit TL-glow peaks at 100, 200, and 310 °C. While the peaks at 100 and 310 °C have already been noticed in conventionally grown quartz, the new peak at 200 °C, observed in the present studies, appears to be due to the presence of Ge in quartz lattice. The radiation dependence of this peak upon irradiation at 300 K by high energy electrons (1.75 MeV) has been presented and the results have been compared and discussed in terms of the hydroxyl defects in natural, cultured, and Ge-doped cultured quartz. [ABSTRACT FROM AUTHOR]

Measurements for laser parameters (i.e., small signal gain, g0, and saturation energy density, Es) have been carried out in a transversely excited N2 laser with a variable electrode gap separation, using an arrangement based on an oscillator-amplifier laser system. Our measurements indicate that the measured g0 and Es parameters depend on the electrode gap separation, and thereby the relevant characteristic curves showing this dependency, and consequently the g0 and Es dependencies on the E/p value or electron temperature in the laser amplifier are introduced. It was also found experimentally that in the present system with the corona preionization, the product of the optimum N2 gas pressure and the gap separation is a constant value of 54 Torr cm, where it is valid only for a system operating in a rather small range of electrode gap separation where the laser output power is maximized. Therefore, we show that the pd product is playing an important role for optimized performances of transversely excited N2 lasers. At last, the g0 and Es measurements across the amplifier discharge width have shown that the g0 parameter is not uniformly distributed within the discharge electrode gap separation, while the Es parameter has a minimum at the gap center with a symmetrical distribution of the output energy density. [ABSTRACT FROM AUTHOR]

Simultaneous self-mode locking and self-Q switching in a diode-pumped intracavity frequency doubled Cr4+:Nd3+:YAG/KTP green laser operating at 532 nm is presented. Nearly 100% modulation depth for self-mode-locked green pulses has been achieved and a maximum average output power of 132 mW was obtained at an incident pump power of 5.72 W. By using a hyperbolic secant function method, considering the Gaussian spatial distribution of the intracavity photon density and the influences of continuous pump rate, as well as the stimulated radiation lifetime of the active medium and the excited-state lifetime of the saturable absorber, a rate equation model was introduced to reconstruct the Q-switched and mode-locked lasers. The theoretical results are in good agreement with the experimental results, and the width of the self-mode-locked green pulse was estimated to be about 350 ps. [ABSTRACT FROM AUTHOR]

Ultrashort optical pulses are used to excite and probe picosecond acoustic pulses in a sample consisting of an opaque material upon which ice is continuously deposited from the vapor phase at ∼110 K. By analysis of the ultrasonic propagation and reflection inside the submicron ice film and taking into account the scattering of the probe light by the acoustic waves, the thickness, sound velocity, refractive index, ultrasonic attenuation, and photoelastic constant of the ice film are derived. This method should find applications for the in situ monitoring of thin transparent films during growth. [ABSTRACT FROM AUTHOR]

Optical measurements of the cadmium plasma produced by the fundamental, second, and third harmonics of a Nd:YAG laser are reported. The excitation temperature and ionic temperature have been determined from the Boltzmann plot and Saha equation, whereas the number density is estimated from the Stark broadened profile of the spectral lines. The variations in the excitation temperature and number density with the ambient air pressure as well as with the laser irradiance have been studied. Besides, the spatial distributions of the temperature and number density have been investigated. [ABSTRACT FROM AUTHOR]

Detailed understanding of the thermal processes in biological targets undergoing laser irradiation continues to be a challenging problem. For example, the contemporary pulsed dye laser (PDL) delivers a complex pulse format which presents specific challenges for theoretical understanding and further development. Numerical methods allow for adequate description of the thermal processes, but are lacking for clarifying the effects of the laser parameters. The purpose of this work is to derive a simplified analytical model that can guide the development of future laser designs. A mathematical model of heating and cooling processes in tissue is developed. Exact analytical solutions of the model are found when applied to specific temporal and spatial profiles of heat sources. Solutions are reduced to simple algebraic expressions. An algorithm is presented for approximating realistic cases of laser heating of skin structures by heat sources of the type found to have exact solutions. The simple algebraic expressions are used to provide insight into realistic laser irradiation cases. The model is compared with experiments on purpura threshold radiant exposure for PDL. These include data from four independent groups over a period of 20 years. Two of the data sets are taken from previously published articles. Two more data sets were collected from two groups of patients that were treated with two PDLs (585 and 595 nm) on normal buttocks skin. Laser pulse durations were varied between 0.5 and 40 ms; radiant exposures were varied between 3 and 20 J/cm2. Treatment sites were evaluated 0.5, 1, and 24 hours later to determine purpuric threshold. The analytical model is in excellent agreement with a wide range of experimental data for purpura threshold radiant exposure. The data collected by independent research groups over the last 20 years with PDLs with wavelengths ranged from 577 to 595 nm were described accurately by this model. The simple analytical model provides an accurate description of a wide range of experimental data. The model can be used to guide the development of future laser designs and help refine laser parameters. [ABSTRACT FROM AUTHOR]

The pinholelike defects often observed in multistack InAs quantum dot structures on GaAs have been investigated comprehensively. Due to the high surface stress of InAs quantum dots, the overgrowth of GaAs and InGaAs capping layers on InAs quantum dots is far from conformal and leads to the formation of these defects. Growth interruptions during GaAs spacer layer formation and thermal annealing after the GaAs growth are employed to completely eliminate the pinholelike defects in multistack quantum dot structures. Ridge-waveguide 1.32 μm InAs quantum dot lasers prepared by this method exhibit internal quantum efficiency as high as 62%. [ABSTRACT FROM AUTHOR]

Limitations are discussed on the use in quantum dot (QD) lasers of the conventional method of determining internal loss coefficient αint and internal quantum efficiency ηint from a measured plot of the reciprocal slope efficiency versus the cavity length L. The limitations are imposed by the L-dependence of αint and ηint themselves. The effect of internal loss is quantitatively analyzed, which originates from the dependence of αint on the carrier density, with the latter being L-dependent. In short cavities, a plot of the reciprocal slope efficiency versus L can significantly deviate from a straight line, thus limiting the practicality of the standard procedure. For L longer than several hundred μm, the limitations are strong in a single-QD-layer laser and moderate in a multiple-QD-layer laser. [ABSTRACT FROM AUTHOR]

The phenomenon of electronic orbital angular momentum L uncoupled from its internuclear axis has been observed in the sodium dimer using high-resolution cw optical-optical double-resonance spectroscopy. When L uncoupling occurs, the degeneracy of Λ doubling is removed. In our experiment, the intermediate B 1Πu state of Na2 is excited from the thermally populated ground X 1Σg+ state by a single-line Ar+ laser. Then, a single-mode dye laser is used to probe the Rydberg states from the intermediate state. The signals are detected by monitoring the UV fluorescence from the triplet gerade states back to the a 3Σu+ state via collision energy transfer. Under our experimental resolution, the splitting of Λ doubling in the 5 1Δg state of Na2 can be measured. A total of 136 rovibronic levels with e/f parities have been assigned to the 5 1Δg state. The Λ-splitting constants deduced from these data are q0=0.376(90)×10-4 cm-1, qv=0.114(6)×10-4 cm-1, and μ=0.76(33)×10-8 cm-1. In general, the Λ splitting of the Δ states is considerably smaller than that of the Π states. However, the first-order splitting constants q0 and qv reported here are larger than those in the B 1Πu state. This is due to the L uncoupling of the Rydberg states. [ABSTRACT FROM AUTHOR]

The phenomenon of electronic orbital angular momentum L uncoupled from its internuclear axis has been observed in the sodium dimer using high-resolution cw optical-optical double-resonance spectroscopy. When L uncoupling occurs, the degeneracy of Λ doubling is removed. In our experiment, the intermediate B 1Πu state of Na2 is excited from the thermally populated ground X 1Σg+ state by a single-line Ar+ laser. Then, a single-mode dye laser is used to probe the Rydberg states from the intermediate state. The signals are detected by monitoring the UV fluorescence from the triplet gerade states back to the a 3Σu+ state via collision energy transfer. Under our experimental resolution, the splitting of Λ doubling in the 5 1Δg state of Na2 can be measured. A total of 136 rovibronic levels with e/f parities have been assigned to the 5 1Δg state. The Λ-splitting constants deduced from these data are q0=0.376(90)×10-4 cm-1, qv=0.114(6)×10-4 cm-1, and μ=0.76(33)×10-8 cm-1. In general, the Λ splitting of the Δ states is considerably smaller than that of the Π states. However, the first-order splitting constants q0 and qv reported here are larger than those in the B 1Πu state. This is due to the L uncoupling of the Rydberg states. [ABSTRACT FROM AUTHOR]

The improvement of the laser performances of an unprocessed 5.6 at. % Nd3+:LaB3O6 cleavage microchip has been reported. Pumped by a Ti:sapphire laser at 871 nm, quasi-cw output laser power up to 395 mW around 1060 nm wavelength with slope efficiency near 52% has been obtained in an end-pumped plano-plano resonator. The influence of the ratio between the cavity mode and pump beam area on the laser performances and laser spectra at various output powers has been investigated. The experimental results have shown that the cleavage technique can become a practical and effective method to obtain the microchip laser medium easily and directly. [ABSTRACT FROM AUTHOR]

Experimental data are presented on the bias dependence of the photocurrent of short-wave infrared n+-on-p Hg1-xCdxTe (MCT) photodiodes responding to an attenuated Nd: yttrium lithium fluoride laser, and the results are interpreted using the sweep-out theory. At high photon injection level, it is observed that the photocurrent is linearly dependent at reverse bias before saturating, and a high reverse bias is required to recover the linear response of the photocurrent to the irradiance. The conventional photocurrent model fails to explain our observations. The bias dependence is attributed to recombination of the photogenerated carriers instead of their being swept out of the depletion region, as a result of the partial cancellation of the built-in field by the spatial distribution of the high density of photogenerated electrons and holes in this region. An interpretation based upon the photoconductivity of MCT photoconductors is proposed and found to be in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

The study of degradation process in high-power laser diodes, in particular, high-power laser bars, has become increasingly important as the output power of these devices continues to rise. We present a “by-emitter” degradation analysis technique, which examines degradation processes at both the bar and emitter levels. This technique focuses on understanding the dynamic mechanisms by which packaging-induced strain and operating conditions lead to the formation of defects and subsequent emitter and bar degradations. In the example presented, we examine a highly compressively strained bar, where thermally induced current runaway is found to be an important factor in the bar degradation and eventual device failure. [ABSTRACT FROM AUTHOR]

We used time-gated optical emission spectroscopy to investigate the characteristics of aluminum plumes and their vacuum expansion after femtosecond laser ablation at different fluences. The prominent feature is the presence of two main classes of species in the plume: very fast Al atoms and ions preceding the plume bulk essentially constituted of much slower Al nanoparticles expanding with a ten times smaller average velocity. Atomic force microscopy of deposited Al nanoparticles evidenced an average size of about 10 nm with a pretty narrow size distribution. These results and the peculiar feature of nanoparticle formation during femtosecond laser irradiation of matter were very satisfactorily interpreted and reproduced by molecular-dynamics simulation of the process. Finally, the analysis of the dependence on laser fluence of the ablation process showed an initial logarithmic increase of ablation yield, up to about 500 mJ/cm2, followed by a sudden and very steep increase at higher fluences. According to our numerical calculations, this latter feature can be ascribed to the increase of the overheated material volume due to electron heat diffusion. [ABSTRACT FROM AUTHOR]

The advances in laser technology have made available very short and intense laser pulses which can be used to seed a high-gain single-pass free-electron laser (FEL) amplifier. With these seed pulses, a regime of the FEL interaction where the radiation evolution is simultaneously dominated by nonlinear effects (saturation) and time-dependent effects (slippage) can be explored. This regime is characterized by the propagation of a solitary wavelike pulse where the power of the optical wave grows quadratically with time, its pulse length decreases and the spectral bandwidth increases. We analyze the interplay between the field and particle dynamics of this propagation regime which was studied before and termed super-radiance. Furthermore we analyze the properties of the strong higher-order harmonic emission from this wave and its behavior when propagating in a cascade FEL. The super-radiant pulse is indeed capable of passing through the stages of a cascade FEL and to regenerate itself at the wavelength of the higher-order harmonic. The optical pulse obtained is shorter than a cooperation length and is strongly chirped in frequency, thus allowing further longitudinal compression down to the attosecond time scale. [ABSTRACT FROM AUTHOR]

We report on a detailed parametric study of the extremely efficient supersonic chemical oxygen-iodine laser recently developed in our laboratory [V. Rybalkin, A. Katz, B. D. Barmashenko, and S. Rosenwaks, Appl. Phys. Lett. 85, 5851 (2004)]. At the early stage of operation, 40.0% efficiency was measured for 1 s followed by a sustained 35.5% chemical efficiency for 20 s. The power and spatial distributions of the gain and temperature across the flow were measured for different supersonic nozzles with both staggered and nonstaggered iodine injection holes, different injection locations along the flow and nozzle throat heights. The effects of the partial pressure of O2 and the residence time of the flow in the generator, as well as the heating of the nozzle, are discussed and shown to be crucial in attaining this high efficiency. By carefully studying and optimizing the operation of the chemical generator, 0.73 yield of singlet oxygen was obtained for conditions corresponding to the highest efficiency. [ABSTRACT FROM AUTHOR]

Recent advances in optical microscopy, such as total internal reflection and confocal scanning laser techniques, now permit the direct three-dimensional tracking of large numbers of colloidal particles both near and far from interfaces. A novel application of this technology, currently being developed by one of the authors under the name of diffusing colloidal probe microscopy (DCPM), is to use colloidal particles as probes of the energetic characteristics of a surface. A major theoretical challenge in implementing DCPM is to obtain the potential energy of a single particle in the external field created by the surface, from the measured particle trajectories in a dense colloidal system. In this paper we develop an approach based on an inversion of density-functional theory (DFT), where we calculate the single-particle-surface potential from the experimentally measured equilibrium density profile in a nondilute colloidal fluid. The underlying DFT formulation is based on the recent work of Zhou and Ruckenstein [Zhou and Ruckenstein, J. Chem. Phys. 112, 8079 (2000)]. For model hard-sphere and Lennard-Jones systems, using Monte Carlo simulation to provide the “experimental” density profiles, we found that the inversion procedure reproduces the true particle-surface-potential energy to an accuracy within typical DCPM experimental limitations (∼0.1kT) at low to moderate colloidal densities. The choice of DFT closures also significantly affects the accuracy. [ABSTRACT FROM AUTHOR]

The methods of stimulated emission pumping-hole filling spectroscopy (SEP-HFS) and stimulated emission pumping population transfer spectroscopy (SEP-PTS) were applied to the conformation-specific study of conformational isomerization in tryptamine [TRA, 3-(2-aminoethyl)indole]. These experimental methods employ stimulated emission pumping to selectively excite a fraction of the population of a single conformation of TRA to well-defined ground-state vibrational levels. This produces single conformations with well-defined internal energy, tunable over a range of energies from near the zero-point level to well above the lowest barriers to conformational isomerization. When the SEP step overcomes a barrier to isomerization, a fraction of the excited population isomerizes to form that product. By carrying out SEP excitation early in a supersonic expansion, these product molecules are subsequently cooled to their zero-point vibrational levels, where they can be detected downstream with a third tunable laser that probes the ground-state population of a particular product conformer via a unique ultraviolet transition using laser-induced fluorescence. The population transfer spectra (recorded by tuning the SEP dump laser while holding the pump and probe lasers fixed) exhibit sharp onsets that directly determine the energy thresholds for conformational isomerization in a given reactant-product conformer pair. In the absence of tunneling effects, the first observed transition in a given X-Y PTS constitutes an upper bound to the energy barrier to conformational isomerization, while the last transition not observed constitutes a lower bound. The bounds for isomerizing conformer A of tryptamine to B(688–748 cm-1), C(1)(860–1000 cm-1), C(2)(1219–1316 cm-1), D(1219–1282 cm-1), E(1219–1316 cm-1), and F(688–748 cm-1) are determined. In addition, thresholds for isomerizing from B to A(<1562 cm-1), B to F(562–688 cm-1), and out of C(2) to B(<747 cm-1) are also determined. The A→B and B→A transitions are used to place bounds on the relative energies of minima B relative to A, with B lying at least 126 cm-1 above A. The corresponding barriers have been computed using both density functional and second-order many-body perturbation theory methods in order to establish the level of theory needed to reproduce experimental results. While most of the computed barriers match experiment well, the barriers for the A–F and B–F transitions are too high by almost a factor of 2. Possible reasons for this discrepancy are discussed. [ABSTRACT FROM AUTHOR]

We present temperature-dependent measurements of the local Z parameter, which reflects the dominant recombination processes. At room temperature, Z increases with current from 1.2 at low bias up to a threshold value, Zth, of only 2.0, demonstrating the existence of significant amounts of monomolecular recombination. At elevated temperatures, Zth rises above 2.5. We calculate the laser’s band diagram and estimate 35 meV of band-gap renormalization at threshold through polarization-resolved measurements of exciton peaks in the absorption spectrum and inflection points of the absorption, spontaneous emission, and gain spectra. The small effective valence-band barrier height of 115 meV leads to hole leakage due to thermionic emission and carrier spillover into the barriers. This could explain the increase in Zth at elevated temperatures and decrease in internal quantum efficiency from 57% at 15 °C to 47% at 75 °C. We also analyze the spontaneous emission and gain spectra below and above threshold. The spontaneous emission clamped rather weakly at threshold, which also partially explains the low internal efficiency at room temperature. We present initial reliability measurements of over 100 h at 10-mW continuous-wave power from a single facet. After 50 h of life testing, the threshold current increased by 79% and the external differential quantum efficiency decreased by 39% of their respective pretest values. By comparing the local Z parameter versus current-density curves before and after life testing, we have identified an increase in monomolecular recombination as the main source of degraded device performance. [ABSTRACT FROM AUTHOR]

An analysis technique is developed for the purpose of determining which semiconductor laser parameter is responsible for degradation. The technique allows one to distinguish between the two modes of degradation: (1) a reduction in electrical carrier injection efficiency or (2) an increase in optical loss. In the case of increasing optical loss, one can determine where longitudinally along the waveguide this degraded optical loss is occurring. Experimentally, we apply the technique to an earlier generation of high-power, single-mode 14XX-nm pump lasers under various accelerated aging conditions. [ABSTRACT FROM AUTHOR]

We have investigated negative differential conductivity (NDC) and laser level population in undoped GaAs/(Al,Ga)As quantum-cascade structures using current–voltage characteristics and interband photoluminescence spectroscopy. While for both GaAs/Al0.33Ga0.67As and GaAs/Al0.45Ga0.55As structures a strong, bistable NDC is observed, a weaker NDC without bistability appears only in the GaAs/Al0.45Ga0.55As structure, which is due to the resonant coupling between injector and upper laser level. Although the bistable NDC is connected with a significant population of the laser levels, it cannot be explained by resonant coupling between electronic states in the active region. We believe that the bistable NDC is caused by an interplay of resonant coupling effects within the injector with the carrier redistribution in the vicinity of the optically active region. Furthermore, a still unidentified state, which exhibits a strong photoluminescence signal, may play an important role for the bistability. [ABSTRACT FROM AUTHOR]

We report the fabrication of widely tuneable (627–702 nm) optically pumped two-dimensional distributed feedback polymer lasers that utilize a red-emission fluorene copolymer as the active gain medium. The lasers exhibit efficient, low threshold operation and emit highly directional output beams as a result of the enhanced two-dimensional photonic confinement provided by the employed resonator. Their emission and operating characteristics are described in detail. We demonstrate that the very wide spectral range (Δλ>=75 nm) over which these lasers can be systematically tuned is in very good agreement with theoretical predictions based on a simple waveguide model. In addition, we show that the lasers have long operating lifetimes τ1/2>=2×107 pulses and we discuss the impact that degradation has on the laser output characteristics. [ABSTRACT FROM AUTHOR]

Optically pumped organic polymer lasers are fabricated by spin coating a thin polymer film onto a nanopatterned SiO2 circular-grating surface-emitting distributed Bragg reflector. For certain grating parameters, we observe a peak inside the stop band that leads to lasing with a reduced threshold. An analytical model, based on the transfer-matrix method, has been developed to investigate the origin of this peak. The theoretical results are in good agreement with the experimental findings. [ABSTRACT FROM AUTHOR]

Data are presented on vertical-cavity surface-emitting lasers designed for cryogenic operation. Low-loss cavity design and high quality factor of the cavity provide the superior lasing characteristics at T=80 K. Ultralow lasing threshold current of 15 μA with external differential quantum efficiency 26% is achieved for a 5 μm aperture device, and a 31% efficiency is achieved for a 8 μm aperture device with a 30 μA threshold and 47 A/cm2 threshold current density. [ABSTRACT FROM AUTHOR]

The heating of a semiconductor laser bar imbedded between two heat spreaders has been studied theoretically and experimentally. The model included the p-cladding layer, active region, n-cladding layer, and n-substrate. Heat sources from the active region and both cladding layers were considered. An analytical relation was obtained between the temperature distribution in the laser and the bar geometry. The analytical approach revealed features that are usually missing in purely numerical modeling. It was shown that the current density across the active region is nonuniform, and that the magnitude of the nonuniformity grows when the resistance of the p-cladding layer decreases. This nonuniform current distribution can lead to higher temperatures at the edges of the laser stripe than in the middle. It was found that mutual heating of the individual lasers in the bar is controlled mainly by the overlap of the temperature fields in the heat spreaders. The theoretical results obtained without any fitting parameters show good agreement with the experimentally measured dependence of the active region temperature on pumping current. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

High gain KrF amplifier simulations require a realistic model of amplified spontaneous emission (ASE). We have recently developed an accurate three-dimensional time-dependent code to model ASE and parasitic oscillations in the Nike and Electra amplifiers. It currently includes arbitrary specular reflections at all of the walls and can be easily extended to nonspecular reflections. It can also simulate the ASE that would be seen by another amplifier stage or a camera located outside the amplifier module. The code approximates the ASE light by a discrete set of ordinates or quasi-plane waves whose propagation vectors represent all directions, but cluster preferentially around the amplifier axis, where the gain is highest. At each grid point, it updates the directed intensity by adding an analytic solution of the radiation transport equation within time increment At to the earlier intensity at a "local look-back" (LLB) point; this point is located a distance cat back along the ordinate's characteristic direction. Because the LLB does not generally lie at a grid point, interpolation is required to calculate the earlier flux. Trilinear interpolation is simple and computationally fast, but it can introduce numerical spatial diffusion in the specific intensity. This diffusion is usually tolerable, but it can be a significant limitation if one attempts to treat a problem where the operating conditions or ASE viewing position favor a narrow range of directions that are not parallel to one of the Cartesian axes. For those conditions, we use an alternative interpolation scheme based on the flux-corrected transport algorithm, which previously has been used only to treat shock wave propagation in fluids. This article describes the code in detail, then shows ASE simulations illustrating the code's capabilities and the effects of transient excitation, diffusion, and gain narrowing. [ABSTRACT FROM AUTHOR]

Purpose: The antimicrobial activity of the Nd:YAG laser was evaluated in vitro on the cariogenic oral bacteria Lactobacillus sp. Materials and Methods: Various power settings and times of exposure were used at a fixed distance of 3 cm. The cultures of Lactobacillus sp were irradiated sowed, and incubated, and bacterial colonies in the irradiated group were counted. Nonirradiated controls were analyzed following the same procedure. Results: Bacterial inhibition was found in all lased groups. Energy of 40 mJ per pulse and 20 Hz applied from a distance of 3 cm for 4 min was enough to obtain a reduction rate of 96.58% of the Lactobacillus sp. Conclusion: The bactericidal efficacy reported here is comparable to the results of other in vitro studies. Irradiation of Lactobacillus sp with the Nd:YAG laser offers the promise of future implementation in preventive dentistry. [ABSTRACT FROM AUTHOR]