Showing papers on "Laser published in 1999"

Light speed reduction to 17 metres per second in an ultracold atomic gas

Abstract: Techniques that use quantum interference effects are being actively investigated to manipulate the optical properties of quantum systems1. One such example is electromagnetically induced transparency, a quantum effect that permits the propagation of light pulses through an otherwise opaque medium2,3,4,5. Here we report an experimental demonstration of electromagnetically induced transparency in an ultracold gas of sodium atoms, in which the optical pulses propagate at twenty million times slower than the speed of light in a vacuum. The gas is cooled to nanokelvin temperatures by laser and evaporative cooling6,7,8,9,10. The quantum interference controlling the optical properties of the medium is set up by a ‘coupling’ laser beam propagating at a right angle to the pulsed ‘probe’ beam. At nanokelvin temperatures, the variation of refractive index with probe frequency can be made very steep. In conjunction with the high atomic density, this results in the exceptionally low light speeds observed. By cooling the cloud below the transition temperature for Bose–Einstein condensation11,12,13 (causing a macroscopic population of alkali atoms in the quantum ground state of the confining potential), we observe even lower pulse propagation velocities (17?m?s−1) owing to the increased atom density. We report an inferred nonlinear refractive index of 0.18?cm2?W−1 and find that the system shows exceptionally large optical nonlinearities, which are of potential fundamental and technological interest for quantum optics.

Two-Dimensional Photonic Band-Gap Defect Mode Laser

Abstract: A laser cavity formed from a single defect in a two-dimensional photonic crystal is demonstrated. The optical microcavity consists of a half wavelength–thick waveguide for vertical confinement and a two-dimensional photonic crystal mirror for lateral localization. A defect in the photonic crystal is introduced to trap photons inside a volume of 2.5 cubic half-wavelengths, approximately 0.03 cubic micrometers. The laser is fabricated in the indium gallium arsenic phosphide material system, and optical gain is provided by strained quantum wells designed for a peak emission wavelength of 1.55 micrometers at room temperature. Pulsed lasing action has been observed at a wavelength of 1.5 micrometers from optically pumped devices with a substrate temperature of 143 kelvin.

Ultrasensitive Chemical Analysis by Raman Spectroscopy

Abstract: In the Raman effect, incident light is inelastically scattered from a sample and shifted in frequency by the energy of its characteristic molecular vibrations. Since its discovery in 1927, the effect has attracted attention from a basic research point of view as well as a powerful spectroscopic technique with many practical applications. The advent of laser light sources with monochromatic photons at high flux densities was a milestone in the history of Raman spectroscopy and resulted in dramatically improved scattering signals (for a general overview of modern Raman spectroscopy, see refs 1-5). In addition to this so-called spontaneous or incoherent Raman scattering, the development of lasers also opened the field of stimulated or coherent Raman spectroscopies, in which molecular vibrations are coherently excited. Whereas the intensity of spontaneous Raman scattering depends linearly on the number of probed molecules, the coherent Raman signal is proportional to the square of this number (for an overview, see refs 6 and 7). Coherent Raman techniques can provide interesting new opportunities such as vibrational imaging of biological samples,8 but they have not yet advanced the field of ultrasensitive trace detection. Therefore, in the following article, we shall focus on the spontaneous Raman effect, in the following simply called Raman scattering. Today, laser photons over a wide range of frequencies from the near-ultraviolet to the near-infrared region are used in Raman scattering studies, allowing selection of optimum excitation conditions for each sample. By choosing wavelengths which excite appropriate electronic transitions, resonance Raman studies of selected components of a sample or parts of a molecule can be performed.9 In the past few years, the range of excitation wavelengths has been extended to the near-infrared (NIR) region, in which background fluorescence is reduced and photoinduced degradation from the sample is diminished. High-intensity NIR diode lasers are easily available, making this region attractive for compact, low cost Raman instrumentation. Further, the development of low noise, high quantum efficiency multichannel detectors (chargecoupled device (CCD) arrays), combined with highthroughput single-stage spectrographs used in combination with holographic laser rejection filters, has led to high-sensitivity Raman spectrometers (for an overview on state-of-the-art NIR Raman systems, see ref 10). As we shall show in section 2, the nearinfrared region also has special importance for ultrasensitive Raman spectroscopy at the singlemolecule level. As with optical spectroscopy, the Raman effect can be applied noninvasively under ambient conditions in almost every environment. Measuring a Raman spectrum does not require special sample preparation techniques, in contrast with infrared absorption spectroscopy. Optical fiber probes for bringing excitation laser light to the sample and transporting scattered light to the spectrograph enable remote detection of Raman signals. Furthermore, the spatial and temporal resolution of Raman scattering are determined by the spot size and pulse length, respectively, of the excitation laser. By using a confocal microscope, Raman signals from femtoliter volumes (∼1 μm3) can by observed, enabling spatially resolved measurements in chromosomes and cells.11 Techniques such as multichannel Hadamard transform Raman microscopy12,13 or confocal scanning Fourier transform Raman microscopy14 allow generation of high-resolution Raman images of a sample. Recently, Raman spectroscopy was performed using near-field optical microscopy.15-17 Such techniques overcome the diffraction limit and allow volumes significantly smaller than the cube of the wavelength to be investigated. In the time domain, Raman spectra can be measured on the picosecond time scale, providing information on short-lived species such as excited 2957 Chem. Rev. 1999, 99, 2957−2975

Random laser action in semiconductor powder

Abstract: We report the first observation of random laser action with coherent feedback in semiconductor powder. Since the scattering mean free path is less than the emission wavelength, recurrent light scattering arises and provides coherent feedback for lasing. Discrete lasing modes have been observed above the threshold. The dependence of the lasing threshold intensity on the excitation volume agrees with the random laser theory. Laser emission from the powder could be observed in all directions. This observation also provides direct evidence for the existence of recurrent scattering of light.

Airborne laser scanning—an introduction and overview

Abstract: This tutorial paper gives an introduction and overview of various topics related to airborne laser scanning (ALS) as used to measure range to and reflectance of objects on the earth surface. After a short introduction, the basic principles of laser, the two main classes, i.e., pulse and continuous-wave lasers, and relations with respect to time-of-flight, range, resolution, and precision are presented. The main laser components and the role of the laser wavelength, including eye safety considerations, are explained. Different scanning mechanisms and the integration of laser with GPS and INS for position and orientation determination are presented. The data processing chain for producing digital terrain and surface models is outlined. Finally, a short overview of applications is given.

Three-Dimensional Vibrational Imaging by Coherent Anti-Stokes Raman Scattering

Abstract: Confocal [1] and multiphoton [2] fluorescence microscopy have become powerful techniques for threedimensional imaging of chemical and biological samples, especially for live cells. This coincides with developments of various natural and artificial fluorescent probes for cellular constituents [3]. For chemical species or cellular components that either do not fluoresce or cannot tolerate labeling, infrared microscopy and spontaneous Raman microscopy can be used as contrast mechanisms based on vibrational properties. Conventional infrared microscopy is limited to low spatial resolution because of the long wavelength of light used. High resolution Raman microscopy of biological samples has been demonstrated with a confocal microscope [4]. However, the intrinsically weak Raman signal necessitates high laser powers (typically .10 mW) and is often overwhelmed by the fluorescence background of the sample. A multiphoton microscopy based on coherent anti-Stokes Raman scattering (CARS) [5] was put forward as an alternative way of providing vibrational contrast [6]. However, the sensitivity of CARS microscopy was limited by the nonresonant background signal, and high resolution three-dimensional sectioning was not achieved. In the study reported here, we demonstrate CARS microscopy in the chemically interesting vibrational spectral region around 3000 cm21 with high spatial resolution and three-dimensional sectioning capability. Most importantly, the use of near-infrared laser pulses generated by a Ti:sapphire laser s,855 nmd and an optical parametric oscillator/amplifier s,1.2 mmd allows a significant improvement in signal to background ratio in CARS detection. Unlike spontaneous Raman microscopy, the highly sensitive CARS microscopy requires only a moderate average power for excitation s,0.1 mWd, tolerable by most biological samples. CARS spectroscopy has been extensively used as a spectroscopic tool for chemical analyses in the condensed and gas phases [7]. In doing CARS spectroscopy, a pump laser and a Stokes laser beam, with center frequencies of np and nS , respectively, are spatially overlapped. The CARS signal at 2np 2 nS is generated in a direction determined by the phase-matching conditions. When the frequency difference np 2 nS coincides with the frequency of a molecular vibration of the sample, the CARS

In vivo ultrahigh-resolution optical coherence tomography.

Abstract: Ultrahigh-resolution optical coherence tomography (OCT) by use of state of the art broad-bandwidth femtosecond laser technology is demonstrated and applied to in vivo subcellular imaging. Imaging is performed with a Kerr-lens mode-locked Ti:sapphire laser with double-chirped mirrors that emits sub-two-cycle pulses with bandwidths of up to 350 nm, centered at 800 nm. Longitudinal resolutions of ~1mum and transverse resolution of 3mum, with a 110-dB dynamic range, are achieved in biological tissue. To overcome depth-of-field limitations we perform zone focusing and image fusion to construct a tomogram with high transverse resolution throughout the image depth. To our knowledge this is the highest longitudinal resolution demonstrated to date for in vivo OCT imaging.

Airborne laser scanning: basic relations and formulas

Abstract: An overview of basic relations and formulas concerning airborne laser scanning is given. They are divided into two main parts, the first treating lasers and laser ranging, and the second one referring to airborne laser scanning. A separate discussion is devoted to the accuracy of 3D positioning and the factors influencing it. Examples are given for most relations, using typical values for ALS and assuming an airplane platform. The relations refer mostly to pulse lasers, but CW lasers are also treated. Different scan patterns, especially parallel lines, are treated. Due to the complexity of the relations, some formulas represent approximations or are based on assumptions like constant flying speed, vertical scan, etc.

Carrier-envelope offset phase control: A novel concept for absolute optical frequency measurement and ultrashort pulse generation

Abstract: The shortest pulses periodically emitted directly from a mode-locked Ti:sapphire laser are approaching the two-optical-cycle range. In this region, the phase of the optical carrier with respect to the pulse envelope becomes important in nonlinear optical processes such as high-harmonic generation. Because there are no locking mechanisms between envelope and carrier inside a laser, their relative phase offset experiences random fluctuations. Here, we propose several novel methods to measure and to stabilize this carrier-envelope offset (CEO) phase with sub-femtosecond uncertainty. The stabilization methods are an important prerequisite for attosecond pulse generation schemes. Short and highly periodic pulses of a two-cycle laser correspond to an extremely wide frequency comb of equally spaced lines, which can be used for absolute frequency measurements. Using the proposed phase-measurement methods, it will be possible to phase-coherently link any unknown optical frequency within the comb spectrum to a primary microwave standard. Experimental studies using a sub-6-fs Ti:sapphire laser suggesting the feasibility of carrier-envelope phase control are presented.

Near-Field Fluorescence Microscopy Based on Two-Photon Excitation with Metal Tips

Abstract: We present a new scheme for near-field fluorescence imaging using a metal tip illuminated with femtosecond laser pulses of proper polarization. The strongly enhanced electric field at the metal tip ( $\ensuremath{\approx}15\mathrm{nm}$ end diameter) results in a localized excitation source for molecular fluorescence. Excitation of the sample via two-photon absorption provides good image contrast due to the quadratic intensity dependence. The spatial resolution is shown to be better than that of the conventional aperture technique. We used the technique to image fragments of photosynthetic membranes, as well as $J$-aggregates with spatial resolutions on the order of 20 nm.

Nuclear fusion from explosions of femtosecond laser-heated deuterium clusters

Abstract: As a form of matter intermediate between molecules and bulk solids, atomic clusters have been much studied1 Light-induced processes in clusters can lead to photo-fragmentation2,3 and Coulombic fission4, producing atom and ion fragments with a few electronvolts (eV) of energy However, recent studies of thephotoionization of atomic clusters with high intensity (>1016 W cm−2) femtosecond laser pulses have shown that these interactions can be far more energetic5,6,7,8,9,10,11,12,13—excitation of large atomic clusters can produce a superheated microplasma that ejects ions with kinetic energies up to 1 MeV (ref 10) This phenomenon suggests that through irradiation of deuterium clusters, it would be possible to create plasmas with sufficient average ion energy for substantial nuclear fusion Here we report the observation of nuclear fusion from the explosions of deuterium clusters heated with a compact, high-repetition-rate table-top laser We achieve an efficiency of about 105 fusion neutrons per joule of incident laser energy, which approaches the efficiency of large-scale laser-driven fusion experiments Our results should facilitate a range of fusion experiments using small-scale lasers, and may ultimately lead to the development of a table-top neutron source, which could potentially find wide application in materials studies

Coherent two-dimensional lasing action in surface-emitting laser with triangular-lattice photonic crystal structure

Abstract: Lasing action of a surface-emitting laser with a two-dimensional photonic crystal structure is investigated. The photonic crystal has a triangular-lattice structure composed of InP and air holes, which is integrated with an InGaAsP/InP multiple-quantum-well active layer by a wafer fusion technique. Uniform two-dimensional lasing oscillation based on the coupling of light propagating in six equivalent Γ−X directions is successfully observed, where the wavelength of the active layer is designed to match the folded (second-order) Γ point of the Γ−X direction. The very narrow divergence angle of far field pattern and/or the lasing spectrum, which is considered to reflect the two-dimensional stop band, also indicate that the lasing oscillation occurs coherently.

Visible lasers with subhertz linewidths

Abstract: We report a visible laser with a subhertz linewidth for use in precision spectroscopy and as a local oscillator for an optical frequency standard. The laser derives its stability from a well-isolated, high-finesse, Fabry-P\'erot cavity. For a 563 nm laser beam locked to our stable cavity, we measure a linewidth of 0.6 Hz for averaging times up to 32 s. The fractional frequency instability for the light locked to the cavity is typically $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ at 1 s. Both the linewidth and fractional frequency instability are approximately an order of magnitude less than previously published results for stabilized lasers.

Laser-induced plasma formation in water at nanosecond to femtosecond time scales: calculation of thresholds, absorption coefficients, and energy density

Abstract: The generation of plasmas in water by high-power laser pulses was investigated for pulse durations between 100 ns and 100 fs on the basis of a rate equation for the free electron density. The rate equation was numerically solved to calculate the evolution of the electron density during the laser pulse and to determine the absorption coefficient and energy density of the plasma. For nanosecond laser pulses, the generation of free electrons in distilled water is initiated by multiphoton ionization but then dominated by cascade ionization. For shorter laser pulses, multiphoton ionization gains ever more importance, and collision and recombination losses during breakdown diminish. The corresponding changes in the evolution of the free carrier density explain the reduction of the energy threshold for breakdown and of the plasma energy density observed with decreasing laser pulse duration. By solving the rate equation, we could also explain the complex pulse duration dependence of plasma transmission found in previous experiments. Good quantitative agreement was found between calculated and measured values for the breakdown threshold, plasma absorption coefficient, and plasma energy density.

Laser photothermal melting and fragmentation of gold nanorods: Energy and laser pulse-width dependence

Abstract: We studied the shape transformation (by use of TEM and optical absorption spectroscopy) of gold nanorods in micellar solution by exposure to laser pulses of different pulse width (100 fs and 7 ns) and different energies (μJ to mJ) at 800 nm, where the longitudinal surface plasmon oscillation of the nanorods absorb At moderate energies, the femtosecond irradiation melts the nanorods to near spherical particles of comparable volumes while the nanosecond pulses fragment them to smaller near-spherical particles At high energies, fragmentation is also observed for the femtosecond irradiation A mechanism involving the rate of energy deposition as compared to the rate of electron−phonon and phonon−phonon relaxation processes is proposed to determine the final fate of the laser-exposed nanorods, ie, melting or fragmentation

Petawatt laser pulses.

Abstract: We have developed a hybrid Ti:sapphire–Nd:glass laser system that produces more than 1500??TW (1.5??PW) of peak power. The system produces 660??J of power in a compressed 440±20 fs pulse by use of 94-cm master diffraction gratings. Focusing to an irradiance of >7×1020 W/cm2 is achieved by use of a Cassegrainian focusing system employing a plasma mirror.

Energy balance of optical breakdown in water at nanosecond to femtosecond time scales

Abstract: During optical breakdown, the energy delivered to the sample is either transmitted, reflected, scattered, or ab- sorbed. Pathways for the division of the absorbed energy are the evaporation of the focal volume, the plasma radiation, and the mechanical effects such as shock wave emission and cav- itation. The partition of laser energy between these channels during breakdown in water was investigated for four selected laser parameters typical for intraocular microsurgery ( 6-ns pulses of 1 and 10 mJ focused at an angle of 22 ,a nd30-ps pulses of 50 mJ and 1m Jfocused at 14 ,a ll at1064 nm). Scattering and reflection were found to be small compared to transmission and absorption during optical breakdown. The ratio of the shock wave energy and cavitation bubble energy was approximately constant (between 1.5:1 and 2:1). These results allowed us to perform a more comprehensive study of the influence of pulse duration ( 100 fs- 76 ns )a nd focus- ing angle (4- 32) on the energy partition by measuring only the plasma transmission and the cavitation bubble energy. The bubble energy was used as an indicator for the total amount of mechanical energy. We found that the absorption at the breakdown site first decreases strongly with decreasing pulse duration, but increases again for < 3p s. The conversion of the absorbed energy into mechanical energy is 90% with ns pulses at large focusing angles. It decreases both with de- creasing focusing angle and pulse duration (to < 15 %f or fs pulses). The disruptive character of plasma-mediated laser effects is therefore strongly reduced when ultrashort laser pulses are used.

Sub-two-cycle pulses from a Kerr-lens mode-locked Ti:sapphire laser.

Abstract: Pulses shorter than two optical cycles with bandwidths in excess of 400 nm have been generated from a Kerr-lens mode-locked Ti:sapphire laser with a repetition rate of 90 MHz and an average power of 200 mW. Low-dispersion prisms and double-chirped mirrors provide broadband controlled dispersion and high reflectivity. These pulse durations are to our knowledge the shortest ever generated directly from a laser oscillator.

Extremely low room-temperature threshold current density diode lasers using InAs dots in In0.15Ga0.85As quantum well

Abstract: The lowest room-temperature threshold current density, 26 A/cm/sup 2/, of any semiconductor diode lasers is reported for a quantum dot device with a single InAs dot layer contained within a strained In/sub 0.15/Ga/sub 0.85/As quantum well. The lasers are epitaxially grown on a GaAs substrate, and the emission wavelength is 1.25 /spl mu/m.

Pump interactions in a 100-nm bandwidth Raman amplifier

Abstract: A design for a 100-nm bandwidth Raman amplifier is presented. The amplifier is pumped with eight, 130-mW lasers with wavelengths ranging from 1416 to 1502 nm. The peak-to-peak gain ripple is 1.1 dB. A new model was developed for this design that includes pump-to-pump and signal-to-signal interactions in addition to double Rayleigh scattering and amplified spontaneous emission. An understanding of the interactions among these various effects was essential to this design. These modeling results are based on measurements of the physical characteristics of the transmission fiber.

Infrared photosensitivity in silica glasses exposed to femtosecond laser pulses

Abstract: We investigate the use of infrared femtosecond laser pulses to induce highly localized refractive-index changes in fused-silica glasses. We characterize the magnitude of the change as a function of exposure and measure index changes as large as 3x10(-3) and 5x10(-3) in pure fused silica and boron-doped silica, respectively. The potential of this technique for writing three-dimensional photonic structures in bulk glasses is demonstrated by the fabrication of a Y coupler within a sample of pure fused silica.

Accurate measurement of large optical frequency differences with a mode-locked laser

Abstract: We have used the comb of optical frequencies emitted by a mode-locked laser as a ruler to measure differences of as much as 20 THz between laser frequencies. This is to our knowledge the largest gap measured with a frequency comb, with high potential for further improvements. To check the accuracy of this approach we show that the modes are distributed uniformly in frequency space within the experimental limit of 3.0 parts in 1017. By comparison with an optical frequency comb generator we have verified that the mode separation equals the pulse repetition rate within the experimental limit of 6.0 parts in 1016.

Laser action from two-dimensional distributed feedback in photonic crystals

Abstract: We report an analysis of the operation of a new type of laser resonator with two-dimensional distributed feedback from a photonic crystal. The gain medium consists of a 2-(4-biphenylyl)-5(4-tert-butylphenyl)-1,3,4-oxadiazole host doped with Coumarin 490 and DCM and is deposited on lithographically patterned Si/SiO2 structures. Bragg reflections caused by the grating diminish the group velocity of photons along some directions of crystallographic symmetry to zero, and the resulting feedback gives rise to laser oscillations. Dispersion relations for photons were calculated analytically and are used to interpret the laser emission spectra.

Corneal refractive surgery with femtosecond lasers

Abstract: We investigated the use of ultrashort pulsed (femtosecond) laser technology in corneal refractive surgery. When compared to longer pulsewidth nanosecond or picosecond laser pulses, femtosecond laser-tissue interactions are characterized by significantly smaller and more deterministic photodisruptive energy thresholds, as well as reduced shock waves and smaller cavitation bubbles. We utilized a highly reliable all-solid-state femtosecond laser system for all studies to demonstrate practicality in real-world operating conditions. Contiguous tissue effects were achieved by scanning a 5-/spl mu/m focused laser spot below the corneal surface at pulse energies of approximately 2-4 /spl mu/J. A variety of scanning patterns was used to perform three prototype procedures in animal eyes; corneal flap cutting, keratomileusis, and intrastromal vision correction. Superior dissection and surface quality results were obtained for lamellar procedures (corneal flap cutting and keratornileusis). Preliminary in vivo studies of intrastromal vision correction suggest that consistent refractive changes can also be achieved with this method. We conclude that femtosecond laser technology may be able to perform a variety of corneal refractive procedures with high precision, offering advantages over current mechanical and laser devices and techniques.

Semiconductor saturable-absorber mirror assisted Kerr-lens mode-locked Ti:sapphire laser producing pulses in the two-cycle regime.

Abstract: Pulses of sub-6-fs duration have been obtained from a Kerr-lens mode-locked Ti:sapphire laser at a repetition rate of 100 MHz and an average power of 300 mW. Fitting an ideal sech(2) to the autocorrelation data yields a 4.8-fs pulse duration, whereas reconstruction of the pulse amplitude profile gives 5.8 fs. The pulse spectrum covers wavelengths from above 950 nm to below 630 nm, extending into the yellow beyond the gain bandwidth of Ti:sapphire. This improvement in bandwidth has been made possible by three key ingredients: carefully designed spectral shaping of the output coupling, better suppression of the dispersion oscillation of the double-chirped mirrors, and a novel broadband semiconductor saturable-absorber mirror.

Lasers Based on Semiconducting Organic Materials

Abstract: The semiconductor laser is the cornerstone of modern technology and science. It is being incorporated into an increasing number of applications, ranging from element detection through telecommunications to entertainment. This wide spread of applications is due to the spectral range of the semiconductor laser, which extends from the blue to the far IR, and the attainable output power of several tens of watts. Since light-emitting organic materials are also semiconductors, it is an obvious step to try to introduce their inherent advantages into the laser field. While the direct benefits of incorporating organic lasers into applications are very stimulating, the process of making a laser can also teach us much about its constituent material properties. Since organic materials are constantly evolving, the making of lasers incorporates valuable contributions from a variety of disciplines, extending from organic chemistry through physics to device engineering. With this in mind, this review is also intended for those not necessarily familiar with lasers. The rest of the article is organized as follows: A start is made by looking at the definition of a laser and discussing the issues associated with determining its threshold. A historical overview is then given, spanning from the 1960s to the 1990s. Section 5.1 reviews the recent progress in optically pumped lasers and Section 5.2 is devoted to electrical properties of light-emitting diodes (LEDs), which may affect the performance of future electrically pumped lasers. The figures used in this work are largely taken from data acquired by the author and his colleagues. However, the rapid progress in this field is due to a large number of research groups, as the text will show.