Showing papers on "Laser published in 1998"

Laser beam propagation through random media

Abstract: Random Processes and Random Fields Optical Turbulence in the Atmosphere Free-Space Propagation of Gaussian-Beam Waves Classical Theory of Optical Wave Propagation Line-of-Sight Propagation - Weak Fluctuation Theory, Part 1 Line-of-Sight Propagation - Weak Fluctuation Theory, Part 2 Propagation Through Random Phase Screens Laser Satellite Communication Systems Propagation Through Complex Paraxial ABCD Optical Systems Doublepassage Problems - Laser Radar Systems Line-of-Sight Propagation - Strong Fluctuation Theory Appendices - Special Functions Integral Table Tables of Beam Statistics Mathematica Programmes.

Room-temperature ultraviolet laser emission from self-assembled ZnO microcrystallite thin films

Abstract: Room-temperature ultraviolet (UV) laser emission of ZnO microcrystallite thin films is reported. The hexagonal ZnO microcrystallites are grown by laser molecular beam epitaxy. They are self-assembled and parallelly arrayed on sapphire substrates. The facets of the hexagons form natural Fabry-Perot lasing cavities. The optical gain for the room-temperature UV stimulated emission is of an excitonic nature and has a peak value an order of magnitude larger than that of bulk ZnO crystal. The observation of room-temperature UV lasing from the ordered, nano-sized ZnO crystals represents an important step towards the development of nanometer photoelectronics. © 1998 American Institute of Physics.

Control of Chemical Reactions by Feedback-Optimized Phase-Shaped Femtosecond Laser Pulses

Abstract: Tailored femtosecond laser pulses from a computer-controlled pulse shaper were used to optimize the branching ratios of different organometallic photodissociation reaction channels. The optimization procedure is based on the feedback from reaction product quantities in a learning evolutionary algorithm that iteratively improves the phase of the applied femtosecond laser pulse. In the case of CpFe(CO)2Cl, it is shown that two different bond-cleaving reactions can be selected, resulting in chemically different products. At least in this case, the method works automatically and finds optimal solutions without previous knowledge of the molecular system and the experimental environment.

Optical Alignment and Spinning of Laser-Trapped Microscopic Particles

Abstract: Light-induced rotation of absorbing microscopic particles by transfer of angular momentum from light to the material raises the possibility of optically driven micromachines. The phenomenon has been observed using elliptically polarized laser beams1 or beams with helical phase structure2,3. But it is difficult to develop high power in such experiments because of overheating and unwanted axial forces, limiting the achievable rotation rates to a few hertz. This problem can in principle be overcome by using transparent particles, transferring angular momentum by a mechanism first observed by Beth in 19364, when he reported a tiny torque developed in a quartz ‘wave-plate’ owing to the change in polarization of transmitted light. Here we show that an optical torque can be induced on microscopic birefringent particles of calcite held by optical tweezers5. Depending on the polarization of the incident beam, the particles either become aligned with the plane of polarization (and thus can be rotated through specified angles) or spin with constant rotation frequency. Because these microscopic particles are transparent, they can be held in three-dimensional optical traps at very high power without heating, leading to rotation rates of over 350 Hz.

Electromagnetically induced transparency

Abstract: The subject of electromagnetically induced transparency (EIT) is reviewed in this paper. Emphasis is placed on the experimental work reported in this field since 1990. Theoretical work is also covered, although it is not intended to review all the very numerous recent theoretical treatments on this topic. The basic physical ideas behind EIT are elucidated. The relation of EIT to other processes involving laser-induced atomic coherence, such as coherent population trapping, coherent adiabatic population transfer and lasing without inversion, is discussed. Experimental work is described covering the following topics: EIT with pulsed and continuous-wave sources, lasing without inversion, pulse propagation in a laser dressed medium and EIT in nonlinear optical processes. A full set of references and a bibliography are included.

Femtosecond Optical Breakdown in Dielectrics

Abstract: We report measurements of the optical breakdown threshold and ablation depth in dielectrics with different band gaps for laser pulse durations ranging from 5 ps to 5 fs at a carrier wavelength of 780 nm. For t, 100 fs, the dominant channel for free electron generation is found to be either impact or multiphoton ionization (MPI) depending on the size of the band gap. The observed MPI rates are substantially lower than those predicted by the Keldysh theory. We demonstrate that sub-10-fs laser pulses open up the way to reversible nonperturbative nonlinear optics (at intensities greater than 10 14 Wycm 2 slightly below damage threshold) and to nanometer-precision laser ablation (slightly above threshold) in dielectric materials. [S0031-9007(98)05969-9]

Microstructuring of silicon with femtosecond laser pulses

Abstract: We report that silicon surfaces develop an array of sharp conical spikes when irradiated with 500 laser pulses of 100-fs duration, 10-kJ/m2 fluence in 500-Torr SF6 or Cl2. The spikes are up to 40-μm tall, and taper to about 1-μm diam at the tip. Irradiation of silicon surfaces in N2, Ne, or vacuum creates structured surfaces, but does not create sharp conical spikes.

Communication with Chaotic Lasers

Abstract: Recent experiments with chaotic electronic circuits have shown the possibility of communication with chaos The experimental demonstration of chaotic communication with an optical system is described An erbium-doped fiber ring laser (EDFRL) was used to produce chaotic light with a wavelength of 153 micrometers A small 10-megahertz message was embedded in the larger chaotic carrier and transmitted to a receiver system where the message was recovered from the chaos Chaotic optical waveforms can thus be used to communicate masked information at high bandwidths

Phase-Matched Generation of Coherent Soft X-rays

Abstract: Phase-matched harmonic conversion of visible laser light into soft x-rays was demonstrated. The recently developed technique of guided-wave frequency conversion was used to upshift light from 800 nanometers to the range from 17 to 32 nanometers. This process increased the coherent x-ray output by factors of 10(2) to 10(3) compared to the non-phase-matched case. This source uses a small-scale (sub-millijoule) high repetition-rate laser and will enable a wide variety of new experimental investigations in linear and nonlinear x-ray science.

High-Power Directional Emission from Microlasers with Chaotic Resonators

Abstract: High-power and highly directional semiconductor microcylinder lasers based on an optical resonator with deformed cross section are reported. In the favorable directions of the far-field, a power increase of up to three orders of magnitude over the conventional circularly symmetric lasers was obtained. A “bow-tie”–shaped resonance is responsible for the improved performance of the lasers in the higher range of deformations, in contrast to “whispering-gallery”–type modes of circular and weakly deformed lasers. This resonator design, although demonstrated here in midinfrared quantum-cascade lasers, should be applicable to any laser based on semiconductors or other high–refractive index materials.

Coherent quantum control of two-photon transitions by a femtosecond laser pulse

Abstract: Coherent quantum control1,2,3 has attracted interest as a means to influence the outcome of a quantum-mechanical interaction. In principle, the quantum system can be steered towards a desired state by its interaction with light. For example, in photoinduced transitions between atomic energy levels, quantum interference effects can lead to enhancement or cancellation of the total transition probability. The interference depends on the spectral phase distribution of the incident beam; as this phase distribution can be tuned, the outcome of the interaction can in principle be controlled. Here we demonstrate that a femtosecond laser pulse can be tailored, using ultrashort pulse-shaping4,5,6,7 techniques, to control two-photon transitions in caesium. By varying the spectral phases of the pulse components, we observe the predicted cancellation of the transitions due to destructive quantum interference; the power spectrum and energy of these ‘dark pulses’ are unchanged. We also show that the pulse shape can be modified extensively without affecting the two-photon transition probability.

High power ultrafast lasers

Abstract: In this article, we review progress in the development of high peak-power ultrafast lasers, and discuss in detail the design issues which determine the performance of these systems. Presently, lasers capable of generating terawatt peak powers with unprecedented short pulse duration can now be built on a single optical table in a small-scale laboratory, while large-scale lasers can generate peak power of over a petawatt. This progress is made possible by the use of the chirped-pulse amplification technique, combined with the use of broad-bandwidth laser materials such as Ti:sapphire, and the development of techniques for generating and propagating very short (10–30 fs) duration light pulses. We also briefly summarize some of the new scientific advances made possible by this technology, such as the generation of coherent femtosecond x-ray pulses, and the generation of MeV-energy electron beams and high-energy ions.

Diode laser absorption sensors for gas-dynamic and combustion flows.

Abstract: Recent advances in room-temperature, near-IR and visible diode laser sources for tele-communication, high-speed computer networks, and optical data storage applications are enabling a new generation of gas-dynamic and combustion-flow sensors based on laser absorption spectroscopy. In addition to conventional species concentration and density measurements, spectroscopic techniques for temperature, velocity, pressure and mass flux have been demonstrated in laboratory, industrial and technical flows. Combined with fibreoptic distribution networks and ultrasensitive detection strategies, compact and portable sensors are now appearing for a variety of applications. In many cases, the superior spectroscopic quality of the new laser sources compared with earlier cryogenic, mid-IR devices is allowing increased sensitivity of trace species measurements, high-precision spectroscopy of major gas constituents, and stable, autonomous measurement systems. The purpose of this article is to review recent progress in this field and suggest likely directions for future research and development. The various laser-source technologies are briefly reviewed as they relate to sensor applications. Basic theory for laser absorption measurements of gas-dynamic properties is reviewed and special detection strategies for the weak near-IR and visible absorption spectra are described. Typical sensor configurations are described and compared for various application scenarios, ranging from laboratory research to automated field and airborne packages. Recent applications of gas-dynamic sensors for air flows and fluxes of trace atmospheric species are presented. Applications of gas-dynamic and combustion sensors to research and development of high-speed flows aeropropulsion engines, and combustion emissions monitoring are presented in detail, along with emerging flow control systems based on these new sensors. Finally, technology in nonlinear frequency conversion, UV laser materials, room-temperature mid-IR materials and broadly tunable multisection devices is reviewed to suggest new sensor possibilities.

Ultrahigh‐Intensity Lasers: Physics of the Extreme on a Tabletop

Abstract: Over the past ten years, laser intensities have increased by more than four orders of magnitude to reach enormous intensities of 1020 W/cm2. The field strength at these intensities is on the order of a teravolt per centimeter, or a hundred times the Coulombic field binding the ground state electron in the hydrogen atom. The electrons driven by such a field are relativistic, with an oscillatory energy of 10 MeV. At these intensities, the light pressure, P = I/c, is extreme, on the order of giga‐ to terabars. The laser interacting with matter—solid, gas, plasma—generates high‐order harmonics of the incident beam up to the 3 nm wavelength range, energetic ions or electrons with mega‐electron‐volt energies (figure 1), gigagauss magnetic fields and violent accelerations of 1021 g (g is Earth's gravity). Finally, the interaction of an ultraintense beam with superrelativistic particles can produce fields approaching the critical field in which an electron gains in one Compton wavelength an energy equal to twice ...

Ultraviolet lasing in resonators formed by scattering in semiconductor polycrystalline films

Abstract: A semiconductor laser whose cavities are “self-formed” due to strong optical scattering in highly disordered gain media is demonstrated. The lasers are made of zinc oxide polycrystalline films grown on amorphous fused silica substrates. Lasing occurs at an ultraviolet wavelength of ∼380 nm under optical pumping. Actual images of the microscopic laser cavities formed by multiple scattering have been captured. These results suggest the possibility of using disordered semiconductor microstructures as alternative sources of coherent light emission.

Physics of Solid-State Laser Materials

Abstract: 1. Introduction.- 1.1 Solid-State Laser Operation and Design Parameters.- 1.2 Material Requirements for Laser Hosts and Active Ions.- 1.3 Material Preparation and Optical Quality.- References.- 2. Electronic Energy Levels.- 2.1 Free-Ion Energy Levels.- 2.2 Elements of Group Theory.- 2.3 Crystal-Field Splitting of Energy Levels.- References.- 3. Radiative Transitions.- 3.1 The Photon Field.- 3.2 Selection Rules.- 3.3 Properties of Spectral Lines.- 3.4 Nonlinear Optical Properties.- References.- 4. Electron-Phonon Interactions.- 4.1 The Phonon Field.- 4.2 Weak Coupling: Radiationless Transitions.- 4.3 Weak Coupling: Vibronic Transitions.- 4.4 Weak Coupling: Spectral Linewidth and Line Position.- 4.5 Example: Spectral Properties of SrTiO3: Cr3+.- 4.6 Strong Coupling.- 4.7 Jahn-Teller Effect.- References.- 5. Ion-Ion Interaction.- 5.1 Exchange-Coupled Ion Pairs.- 5.2 Nonradiative Energy Transfer: Single-Step Process.- 5.3 Phonon-Assisted Energy Transfer.- 5.4 Nonradiative Energy Transfer: Multistep Process.- 5.5 Connection with Experiment: Rate Equation Analysis.- References.- 6. Al2O3: Cr3+ Laser Crystals.- 6.1 Energy Levels of Cr3+.- 6.2 Crystal-Field Splitting.- 6.3 Spin-Orbit Splitting and Selection Rules.- 6.4 Strong-Field Laser Materials.- References.- 7. Transition-Metal-Ion Laser Materials.- 7.1 Broad-Band Cr3+ Laser Materials: Alexandrite.- 7.2 Spectral Properties of Cr3+ in Different Hosts and Their Laser Characteristics.- 7.3 Transition-Metal Ions and Host Crystals.- 7.4 Laser Materials Based on Ti3+ Ions.- 7.5 Laser Materials Based on Ions with 3d2 Configurations.- 7.6 Laser Materials Based on Ions with 3d3 Through 3d8Configurations.- References.- 8. Y3A15012: Nd3+ Laser Crystals.- 8.1 Energy Levels of Nd3+.- 8.2 Crystal-Field Splitting.- 8.3 Radiative Transitions: Judd-Ofelt Theory.- 8.4 Example: Y3A15O12:Nd3+.- References.- 9. Rare-Earth-Ion Laser Materials.- 9.1 Nd3+ Lasers.- 9.2 Other Trivalent Lanthanide Lasers.- References.- 10. Miscellaneous Laser Materials.- 10.1 Other Rare-Earth-Ion Lasers.- 10.2 Nonlinear Optical Lasers.- 10.3 Color-Center Lasers.- 10.4 Other Solid-State Lasers.- References.

Complications of Carbon Dioxide Laser Resurfacing. An Evaluation of 500 Patients

Abstract: background.Cutaneous laser resurfacing with high-energy, pulsed and scanned carbon dioxide (CO2) lasers has become popularized for the treatment of a variety of cutaneous indications, but potential complications and side effects remain a large concern. Despite the recent boom in cutaneous laser resu

Multifocal multiphoton microscopy

Abstract: We present a real-time, direct-view multiphoton excitation fluorescence microscope that provides three-dimensional imaging at high resolution. Using a rotating microlens disk, we split the near-infrared light of a mode-locked titanium:sapphire laser into an array of beams that are transformed into an array of high-aperture foci at the object. We typically scan at 225 frames per second and image the fluorescence with a camera that reads out the images at video rate. For 1.4 aperture oil and 1.2 water immersion lenses at 780-nm excitation we obtained axial resolutions of 0.84 and 1.4 µm, respectively, which are similar to that of a single-beam two-photon microscope. Compared with the latter setup, our system represents a 40–100-fold increase in efficiency, or imaging speed. Moreover, it permits the observation with the eye of high-resolution two-photon images of (live) samples.

GaAs/AlxGa1−xAs quantum cascade lasers

Abstract: A unipolar injection quantum cascade (QC) laser grown in an AlGaAs/GaAs material system by molecular beam epitaxy, is reported. The active material is a 30 period sequence of injectors/active regions made from Al0.33Ga0.67As/GaAs-coupled quantum wells. For this device a special waveguide design, which complies with a GaAs heavily doped substrate and very short Al0.90Ga0.10As cladding layers, has been optimized. At a heat-sink temperature of 77 K, the laser emission wavelength is 9.4 μm with peak optical power exceeding 70 mW and the threshold current density is 7.3 kA/cm2. The maximum operating temperature is 140 K. This work experimentally demonstrates the general validity of QC laser principles by showing laser action in a heterostructure material different from the one used until now.

Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy

Abstract: We consider several highly sensitive techniques commonly used in detection of atomic and molecular absorptions. Their basic operating principles and corresponding performances are summarized and compared. We then present our latest results on the ultrasensitive detection of molecular overtone transitions to illustrate the principle and application of the cavity-enhanced frequency-modulation (FM) spectroscopy. An external cavity is used to enhance the molecular response to the light field, and an FM technique is applied for shotnoise-limited signal recovery. A perfect match between the FM sideband frequency and the cavity free spectral range makes the detection process insensitive to the laser-frequency noise relative to the cavity, and, at the same time, overcomes the cavity bandwidth limit. Working with a 1.064-mm Nd:YAG laser, we obtained sub-Doppler overtone resonances of C2HD, C2H2, and CO2 molecules. A detection sensitivity of 5 3 10 213 of

Frequency-stabilized diode laser with the Zeeman shift in an atomic vapor

Abstract: We demonstrate a robust method of stabilizing a diode laser frequency to an atomic transition. This technique employs the Zeeman shift to generate an antisymmetric signal about a Doppler-broadened atomic resonance, and therefore offers a large recapture range as well as high stability. The frequency of a 780-nm diode laser, stabilized to such a signal in Rb, drifted less than 0.5 MHz peak–peak (1 part in 109) in 38 h. This tunable frequency lock can be constructed inexpensively, requires little laser power, rarely loses lock, and can be extended to other wavelengths by use of different atomic species.

Branching ratios, cross sections, and radiative lifetimes of rare earth ions in solids: Application to Tm3+ and Ho3+ ions in LiYF4

Abstract: The measurement of branching ratios, cross sections and radiative lifetimes for rare earth ions in solids is considered The methods are applied to Tm and Ho in YLF as a test case De-activation rates for electric dipole and magnetic dipole emission are calculated for many of the lower lying manifolds in Tm:YLF and Ho:YLF in the context of the Judd-Ofelt theory to determine radiative lifetimes Measured values for the branching ratios as well as the absorption and emission cross sections are also presented for many of the excited state manifolds From these measurements, a methodology is developed to extract measured values for the radiative lifetimes These results are compared with the Judd-Ofelt theory as a guide for consistency and for determining the accuracy of the Judd-Ofelt theory in predicting branching ratios and radiative lifetimes The parameters generated by the methods covered here have potential applications for more accurate modeling of Tm:Ho laser systems

An approach for recording and readout beyond the diffraction limit with an Sb thin film

Abstract: A technique for recording and retrieving small marks beyond the optical diffraction limit was proposed. The basic experiment with this technique was also carried out at a constant linear velocity of 2.0 m/s, rotating a disk with a multi-layered structure of Sb and GeSbTe, which were separated by a thin film of SiN. By use of the optically nonlinear property of the Sb thin film, carrier to noise ratio of more than 10 dB was obtained from recorded marks of 90 nm, using an optical system with the laser wavelength of 686 nm and a numerical aperture of 0.6.

Laser ablation and desorption

Abstract: Introduction to laser desorption and ablation, J.C. Miller mechanisms of laser ablation, R.F. Haglund low fluence laser desorption and plume formation from wide bandgap crystalline materials, J.T. Dickinson lasers, optics and thermal considerations in ablation experiments, C. Grigoropoulos gas dynamics and the characterization S. Schleberger, S. Speller and W. Heiland surface modification with lasers, Z. Ball and R. Sauerbrey chemical analysis by laser ablation, R.E. Russo matrix-assisted laser desorption and ionization, J.A Carroll and R.C. Beavis phyiscal mechanisms governing the ablation of biological tissue, G. Edwards growth and doping of compound semiconductor films by pulsed laser ablation, D.H. Lowndes laser ablation in optical components and thin films, M. Reichling industrial applications of laser ablation, R.F. Haglund.

A review of ultrashort pulsed laser ablation of materials

Abstract: The use of ultrashort pulsed lasers in materials processing is an emerging technology. These lasers have the capability to ablate materials precisely with little or no collateral damage, even with materials that are impervious to laser energy from conventional pulsed lasers. The extreme intensities and short timescale at which ultrashort pulsed lasers operate differentiate them from other lasers. The means of ultrashort pulsed laser generation is discussed; included are a survey of pulse compressor techniques with solid state lasers and a brief discussion of excimer-dye lasers. This is followed by a discussion of specific examples of ultrashort pulsed machining of specific materials, along with mechanistic details. Optical breakdown mechanisms, including electron avalanche ionization and multiphoton absorption are discussed. It is shown that as pulse width increases and intensity decreases, laser damage becomes a stochastic process in which the ultrashort pulsed, high intensity light causes optical breakd...

Cavity ring-down spectroscopy

Abstract: Cavity ring-down spectroscopy (CRDS) is a laser-based absorption spectroscopy technique that is starting to find extensive application as a consequence of the very high sensitivity of the method compared with more traditional absorption spectroscopy techniques. We describe the experimental implementation of CRDS and its application to a number of areas of research including laser diagnostics of hostile environments, reaction kinetics and spectroscopy, with particular emphasis on our ongoing studies of the fast (sub-nanosecond) predissociation of electronically excited states of small molecules and radicals.

Cavity enhanced absorption and cavity enhanced magnetic rotation spectroscopy

Abstract: It is experimentally demonstrated that a narrow band continuous wave (cw) light source can be used in combination with a high-finesse optically stable cavity to perform sensitive, high-resolution direct absorption and optical rotation spectroscopy in an amazingly simple experimental setup, using ideas from the field of cavity ring down spectroscopy Light from a scanning narrow band cw laser is coupled into the cavity via accidental coincidences of the laser frequency with the frequency of one of the multitude of modes of the cavity The absorption and polarization rotation information is extracted from a measurement of the time-integrated light intensity leaking out of the cavity as a function of laser wavelength

Long-period fibre grating fabrication with focused CO2 laser pulses

Abstract: A new, simple method of fabricating long-period fibre gratings by direct exposure of the fibre to focused 10.6 /spl mu/m wavelength CO/sub 2/ laser pulses is presented. No ultraviolet exposure is used. Hydrogen loading is found to enhance the writing sensitivity.