Showing papers in "Laser Physics in 2001"

Single atoms bound in orbit by single photons

Abstract: Two recent experiments are discussed which demonstrate real-time trapping and monitoring of single atoms within a high-finesse optical resonator. A single atom moving within the resonator generates large variations in the transmission of a weak probe laser, which are recorded in real time. These cavity QED signals are used to trigger ON a confining potential to trap the atom. In the first experiment, the confining potential is provided by the single-photon forces associated with the cavity QED interaction. An inversion algorithm allows individual atom trajectories to be reconstructed from the record of cavity transmission, and reveals single atoms bound in orbit by the mechanical forces associated with single photons. In a second experiment, an additional classical standing-wave dipole trap provides the trapping, enabling a 28 ms trap lifetime for single strongly coupled atoms.

Bose-Einstein Condensation of Trapped Atomic Gases

Abstract: This article reviews recent investigations on the phenomenon of Bose-Einstein condensation of dilute gases. Since the experimental observation of quantum degeneracy in atomic gases, the research activity in the field of coherent matter-waves literally exploded. The present topical review aims to give an introduction into the thermodynamics of Bose-Einstein condensation, a general overview over experimental techniques and investigations, and a theoretical foundation for the description of bosonic many-body quantum systems.

Spectroscopy of the gases interacting with intense femtosecond laser pulses

Abstract: As a result of focusing ultrafast Ti: Sapphire laser pulses in a gas, a plasma column is created. The spectrum of the radiation from the excited species in the column might be used as a new spectroscopic source free of plasma continuum. Furthermore, from the photoemission spectra, valuable information on the mechanism of interaction of strong laser pulses with atoms and molecules could be obtained.

Potential of ceramic YAG lasers

Abstract: Optical properties including optical absorption, emission spectra and fluorescence lifetime have been investigated for polycrystalline Nd-doped Y 3 Al 5 O 12 (Nd:YAG) ceramics. Very similar results were obtained with that of Nd:YAG single crystals. Highly efficient laser oscillation at 1064 nm has been obtained with this kind of ceramic. Using 1 W LD end-pumping scheme, with 883 mW pumping, 499 mW 1064 nm cw laser output has been obtained corresponding to an optical to optical efficiency of 56.5%. High power cw Nd:YAG ceramic rod laser was also demonstrated using virtual point source (VPS) system. The maximum output power of 84 W with slope efficiency of 36.3% was obtained at 1064 nm. The future of Nd:YAG ceramic laser was also discussed.

High-harmonic generation by a bichromatic bicircular laser field

Abstract: The generation of circularly polarized high-order harmonics is considered by a bichromatic laser field whose two components with frequencies ω and 2ω are circularly polarized in the same plane, but rotate in opposite directions. Both exact and approximate (obtained by using the saddle-point method) results for the harmonic spectra are presented. The effect of the ratio of the intensities of the two components on the harmonic spectrum is investigated. For the case where the high-frequency field has twice the intensity of the low-frequency field, the possibility of generating a train of attosecond pulses is pointed out that consists of circularly polarized harmonics all having the same helicity.

Tetragonal vanadates YVO4 and GdVO4-new efficient X(3)-active crystals for Raman laser converters

Abstract: High-order Stokes and anti-Stokes picosecond generation in tetragonal YVO 4 and GdVO 4 host crystals for lasing trivalent lanthanides (Ln 3+ ) has been observed for the first time. All measured stimulated Raman scattering (SRS) wavelengths in the visible and near infrared (NIR) are identified and attributed to the SRS-active vibration modes of these vanadates.

Dynamics of deep short pulse laser drilling : Ablative stages and light propagation

Abstract: Dynamics of ablation rate and laser light propagation under deep laser drilling of chemical vapor deposited (CVD) diamond by picosecond (300 ps) and nanosecond (9 ns) pulses at 1078 nm wavelength has been studied. Based upon the ablation rate behavior, the drilling process consists of four stages: (1) ablation in a shallow crater (depth-to-diameter ratio LID 7) under low and quite stable ablation rate, and (4) quick reduction of the ablation rate down to zero and the ablation process stopping. In the frame of this general scheme the absolute value of the ablation rate substantially depends on pulse energy and pulsewidth. The rise of laser fluence at the channel bottom at the channel deepening (L/D < 20) was revealed in the experiments. It has been shown that strong reduction of the ablation rate at LID = 1-7 can be explained neither by the energy losses due to beam divergence nor by strengthening of plasma shielding effect. Strong bending of the experimental channels, which occurs under a certain orientation of the radiation polarization relating to the film surface, was also observed and discussed.

Spectroscopy and cw laser action, magnetooptics and nonlinear optical frequency conversion in Ln3+ doped and undoped Bi4Ge3O12 and Bi4Si3O12 crystals

Abstract: Detailed spectroscopic, stimulated emission (SE), and X (2) - and X (3) -nonlinear optical properties of cubic piezoelectric Bi 4 Ge 3 O 12 and Bi 4 Si 3 O 12 compounds undoped and doped with trivalent lanthanides (Ln 3+ ) were investigated. Particular attention was paid to bismuth germanate crystals doped with Nd 3+ ions. Their absorption and luminescence intensity characteristics, including the spectroscopic quality parameter X Nd , peak and effective cross sections for inter-Stark laser transitions of the 4 F 3/2 → 4 I 1 1/2 and 4 F 3/2 → 4 I 13/2 channels, were determined. Continuous-wave (cw) laser action into two near-infrared intermanifold transitions 4 F 3/2 → 4 I 11/2 and 4 F 3/2 → 4 I 13/2 this crystal with GaAlAs-laser-diode pumping was achieved. Magnetooptical polarization effects have been studied. For the first time, also pulsed laser action of Tm 3+ and Yb 3+ ions in Bi 4 Ge 3 O 12 with Xe-flashlamp excitation was obtained. All the observed SE transitions for Nd 3+ and Yb 3+ ions in bismuth germanate were identified. Nonlinear X (2) - and X (3) -effects in acentric Bi 4 Ge 3 O 12 and Bi 4 Si 3 O 12 crystals as a high-order stimulated Raman scattering (SRS), second harmonic generation (SHG), and sum-frequency generation of the pump and SRS beams produced by picosecond Nd 3+ :Y 3 Al 5 O 12 laser excitation were investigated. All observed multiple Stokes and anti-Stokes (up to 40-th order) components were analyzed and attributed to the translation T'(Bi 3+ ) vibration modes of these crystals. The room-temperature (RT) steady-state Raman gain coefficients for these new X (3) -active media were estimated to 0.8 cm GW -1 . Therefore, we classify the undoped and Ln 3+ -ions doped Bi 4 Ge 3 O 12 and Bi 4 Si 3 O 12 crystals as a promising materials for multifunctional optical laser devices, including self-SRS laser shifters.

Nonground state condensates of ultracold trapped atoms

Abstract: The population dynamics of a trapped Bose-Einstein condensate, subject to the action of an oscillatory field, is studied. This field produces a modulation of the trapping potential with the frequency close to the transition frequency between the ground state and an excited energy level. Unusual critical effects are found exhibiting sharp qualitative changes in the population dynamics. An effective averaged system is constructed explicitly illustrating the occurrence of critical phenomena. The related critical indices are calculated.

Femtosecond frequency combs stabilized with a He-Ne/CH4 laser: Toward a femtosecond optical clock

Abstract: A concept of high-precision optical frequency measurements involving the use of femtosecond frequency combs with a bandwidths exceeding the frequency of a reference laser source is discussed. This relation between the frequency of the reference laser source and the bandwidth of the frequency comb allows the difference frequency of two modes in a femtosecond frequency comb to be phase-locked to the reference laser. The radio-frequency interval between two adjacent modes in the femtosecond frequency comb is also stabilized in this case, allowing high-precision frequency measurements to be performed in a very simple way. A laser system allowing this principle of high-precision frequency measurements to be implemented includes a mode-locked Ti:sapphire laser, a methane-stabilized He-Ne laser, and a tapered fiber. Special measures have been taken to improve stability characteristics of the laser system. The process of spectral broadening of femtosecond pulse trains in holey and tapered fibers is considered in terms of the Zakharov equation as a result of four-wave mixing of phase-locked equidistant spectral components. The main physical factors influencing the phase distribution and coherence properties of broadly spanning frequency combs produced with the use of this technique are discussed. Phase shifts and spectral distortions arising due to dispersion effects, modulation instabilities, and shock waves of pulse envelopes are explored, and general recipes to reduce the influence of these effects by means of fiber optics are discussed.

Crystal Growth and Spectroscopic Investigation of $Yb^ {2+}$ - Doped Fluoride Crystals

Abstract: In this presentation we report on the preparation and spectroscopic properties of Yb 2+ -doped MgF 2 , KMgF 3 , LiCaAlF 6 , and LiSrAlF 6 with respect to the realization of a tunable solid state laser based on a interconfigurational transition ofYb 2+ . All materials exhibit broadband emission in the short-wavelength region due to a 5d-4f transition. The peak emission wavelengths and bandwidths at room temperature are in particular 485 nm and 4510 cm -1 for Yb 2+ : MgF 2 , 400 nm and 3890 cm -1 for Yb 2+ : KMgF 3 , 393 nm and 3260 cm -1 for Yb 2+ : LiCaAlF 6 , 440 nm and 5180 cm -1 for Yb 2+ : LiCaAlF 6 , respectively. The emission can be excited in the energy level of the 4f 13 5d configuration. An energy level scheme for Yb 2+ : MgF 2 in the strong field assignment is proposed. The room temperature decay time of the Yb 2+ emission is 52 its in MgF 2 , 80 μs in KMgF 3 , 5.4 μs in LiCaAlF 6 and 9.9 μs in LiSrAlF 6 , respectively.

Scaling of gamma-ray transitions induced in nuclear spin isomers by X-rays

Abstract: Because of the high density of energy storage and the large cross section for its release there is considerable significance of the triggering of induced gamma emission from nuclear spin isomers to efforts to develop intense sources of short-wavelength radiation. The work reported here describes the current experimental focus and results recently obtained.

Stationary states of Bose-Einstein condensates in single- and multi-well trapping potentials

Abstract: The stationary solutions of the Gross-Pitaevskii equation can be divided in two classes: those which reduce, in the limit of vanishing nonlinearity, to the eigenfunctions of the associated Schrodinger equation and those which do not have linear counterpart. Analytical and numeri- cal results support an existence condition for the solutions of the first class in terms of the ratio between their proper frequency and the corresponding linear eigenvalue. For one-dimensional con- fined systems, we show that solutions without linear counterpart do exist in presence of a multi-well external potential. These solutions, which in the limit of strong nonlinearity have the form of chains of dark or bright solitons located near the extrema of the potential, represent macroscopically ex- cited states of a Bose-Einstein condensate and are in principle experimentally observable.

Giant optical responses in microcavity-fractal composites

Abstract: Optical properties of fractal nanostructured composite materials are considered. The fractal geometry results in localization of plasmon excitations in the hot spots, where the local field can exceed the applied field by several orders of magnitude. The high local fields of the localized fractal modes result in dramatic enhancement of optical responses, making feasible the surface-enhanced nonlinear spectroscopy of single molecules and nanocrystals. The field enhancement becomes especially large when fractals are placed inside a microcavity. A theory for the enhanced Raman and hyper-Raman surface-enhanced scattering in microcavity-fractal composites is developed.

Raman Gain Properties of Optical Fibers with a High Ge-Doped Silica Core and Standard Optical Fibers

Abstract: Raman gain characteristics are measured for single-mode optical fibers with a high content of ger- manium and for commercially available optical fibers with shifted dispersion and a large effective core area. Optical fibers of these types were studied as active media for Raman fiber amplifiers at the wavelengths of 1.5 and 1.3 μ m. The maximum Raman gain was achieved for a fiber with a high content of germanium and was equal to 23.3 dB/(km W). 1.407- and 1.229 μ m Raman lasers based on phosphosilicate fibers were employed as pump sources for Raman amplifiers.

Simple and Effective Modulation of Diode Lasers

Abstract: Diode lasers have recently found wide application in various fields of modern physics, such as spectroscopy, metrology, and atomic physics. Because of their small size, low cost, and high spectral characteristics, the use of such lasers has substantially simplified the conduct of experiments on atomic optics and laser cooling of atoms. When working with alkali metals featuring a hyperfine splitting of their ground state, of principal importance is the use of a two-frequency laser radiation. In that case, radiation at one of the frequencies exerts a force action on the atom in hand by exciting it from one of the hyperfine-structure sublevels of the lower state, while radiation at the other frequency provides for a cyclic character of the interaction between the atom and the laser field. The frequencies of both lasers here must be sufficiently stable in time in order that radiation should be in resonance with the appropriate transitions and the frequency difference be equal to the hyperfine splitting of the ground state of the atom

A theoretical model of the linear thermo-optical response of an absorbing particle immersed in a liquid

Abstract: This paper is devoted to a theoretical analysis of the linear optoacoustic effect in an inhomogeneous medium consisting of an absorbing spherical particle in an immersion (nonabsorbing) liquid irradiated with laser pulses. It is shown that the problem of determining the optoacoustic response in an inhomogeneous medium can be separated into three sequential steps: determination of the distribution of the light flux in a medium, calculation of the temperature field distribution related to the absorption of optical energy, and the solution of the acoustic problem of thermo-optical excitation of sound by thermal sources. The cases of thermally large (the length of heat diffusion during the time of laser action is much less than the size of a particle) and thermally small particles are analyzed. It is demonstrated that, in the case of a thermally large particle, the time profile of the pressure of the acoustic signal strongly depends on the relation between the coefficient of light absorption by a particle and its radius, as well as on the duration of the laser pulse. In the case of a thermally small particle, the pressure pulse follows the derivative of the temporal envelope of the laser pulse intensity.

Synchronization of Shilnikov chaos in a CO2 laser with feedback

Abstract: We offer the first type of control of a wildly intermittent chaos that is resistant to standard control methods. Under an external periodic action, the frequency of every unstable periodic orbit embedded within the chaotic attractor can be locked to the frequency of the parameter modulation displaying either complete or partial synchronization. The phase synchronization of Shilnikov chaos was observed at small modulation amplitudes.

Recent developments in diode-pumped ultrashort pulse solide-state lasers

Abstract: In general, two groups of solid-state laser media for diode-pumped ultrashort pulse generation can be distinguished: (i) Transition metal ion doped laser crystals like Cr 3+ :LiSAF, Cr 3+ :LiCAF, Cr 3+ :LiSGaF and others out of the group of colquiriites; Cr 3+ :alexandrite, Cr 4+ :forsterite, Cr 4+ :YAG, Co 2+ :MgF 2 , Cr 2+ :ZnSe and other II-VI host crystals; other dopants in similar media like Fe 2+ , Ti 2+ , Co 2+ in CdS, CdSe, CdMnTe, etc. Such broad-band laser media allow, or potentially allow, very short pulse durations in the fs-regime down to around 10 fs spanning a wide wavelength range from ∼0.5 to 5 μm. (ii) If other laser parameters should be optimized like efficiency or average power other media are the materials of choice at the expense of somewhat longer pulse durations in the regime of 100 fs to ∼1 ps. These are rare earth doped laser media like Nd 3+ , Tm 3+ , Ho 3+ , Er 3+ in various crystals like YAG, YLF vanadate and others or in different glasses. If a good compromise for bandwidth and hence pulse duration and crystalline host properties like heat conduction should be achieved mixed garnets like GSAG:YSGG and related ones have proven to be successful. For high average powers and efficiency Yb 3+ :YAG or Yb 3+ :S-FAP are the best candidates. Various femtosecond lasers other than Ti:S have been recently realized. 12 fs out of a Cr 3+ :LiSAF laser being diode-pumped is the best value for shortest pulses so far, although at very little power of 6.2 mW. Pursuing the goal of high average powers in a direct diode-pumped fashion, 16.2 W average power at τ p = 730 fs out of a modelocked Yb:YAG thin disk laser established an impressing new best value. The optimally efficient Yb system operating in a thin disk laser or in a transversely pumped slab laser open an avenue up into the kW regime even for ultrashort pulse lasers.

Entanglement degree of a three-level atom interacting with pair-coherent states with a nonlinear medium

Abstract: It has been become known that a quantum entangled state plays important role in fields of quantum information theory, such as quantum teleportation and quantum computation. Research on quantifying entangled states has been carried out using several measures. In this paper, we will adopt this method using quantum mutual entropy to measure the degree of entanglement (DE) in the time development of a three-level atom interacting with two-mode pair-coherent states in a Kerr-like medium. The exact results are employed to perform a careful investigation of the temporal evolution of the entropy. A factorization of the initial density operator is assumed, with the privileged field mode being in pair-coherent states. We invoke the mathematical notion of maximum variation of a function to construct a measure for entropy fluctuations. It is shown that when the individual modes of the field are far detuned from the intermediate atomic level; the dynamical Stark shift induced by the Kerr-like medium. The results show that, the Kerr effect yields the superstructure of atomic Rabi oscillation. A possible experimental test of a new effect is proposed. The general conclusions reached are illustrated by numerical results.

The effects of some parameters of the propagation constant for heterojunction constructions on the optical modes

Abstract: Some parameters of step index optical heterojunction lasers and waveguides are discussed and the basic components of propagating modes are derived. Equivalent refractive index method which is useful for mode analysis of the active region of a laser and a slab waveguide is introduced. The operation of semiconductor lasers and waveguides are strongly affected by materials from which they are made. Active regions of the laser and the waveguide are sandwiched between the cladding layers, which have a slightly lower refractive index than the refraction index of active region. The layers act as a slab waveguide confining the photons closely around the active region through action of the total internal reflection. The optical power does not propagate outwards from the active region. With the well controlled waveguiding to confine the light to active region where the electrons and holes recombine most strongly, the diode lasers and Bragg lasers have achieved their preeminence as signal sources for communication system. In the active region it is possible to consider the energy exchange between the optical fields and the polarization currents. So, the active region provides a stable platform for the electronic interaction with the changes in optical power. Especially in quantum lasers made from III-V semiconductor for the investigation of quantum phenomena, this work based on the electromagnetic field and its parameters and the study of the confinement of the carriers and the electric field for the semiconductor quantum well can also be used confidently.

An all-solid-state sub-40-fs self-starting Cr4+ : Forsterite laser with holey-fiber beam delivery and chirp control for coherence-domain and nonlinear-optical biomedical applications

Abstract: An all-solid-state sub-40-fs self-starting Cr 4+ : forsterite laser with a wavelength tunable within the range of 1.21-1.29 μm adapted in its power, temporal, and spectral parameters for high-resolution optical coherence tomography (OCT) and nonlinear-optical tissue imaging is presented. Stable self-starting mode locking in the created laser is achieved both with and without semiconductor saturable-absorber mirrors, while the double-pulse regime of lasing permits time-resolved measurements on biological systems to be performed. Frequency doubling is implemented through second-harmonic generation in a 2.5-mm-long DCDA crystal, ensuring group-velocity matching for femtosecond pulses produced by the Cr: forsterite laser. A set of holey fibers with a period of the photonic-crystal structure of the cladding ranging from 468 nm to 32 μm ensure the tunability of the photonic band gap of the cladding within a broad spectral range, providing different waveguiding regimes for Cr 4+ : forsterite laser radiation.

Multiple Stokes and anti-stokes picosecond generation, cw laser action at wavelengths of two stimulated-emission channels 4F3/2→ 4I11/2 and 4F3/2→ 4I13/2, and nanosecond self-SRS lasing in undoped and nd3+-doped tetragonal PbMoO4 crystals

Abstract: Spectroscopic, laser, and x (3) -nonlinear optical properties of undoped and Nd 3+ -doped tetragonal PbMoO 4 were investigated. For the first time cw lasing in the two 4 F 3/2 → 4 I 11/2 and 4 F 3/2 → 4 I 13/2 channels, and picosecond high-order Stokes and anti-Stokes generation in this molybdate were excited. All observed laser and scattering wavelengths were identified.

A new type of frequency chain and its application to optical frequency metrology

Abstract: We have demonstrated the usefulness of the frequency comb generated by a mode-locked femtosecond laser to measure large frequency differences. The development towards the measurement of larger and larger frequency intervals culminates in the single laser femtosecond frequency chain which greatly simplifies the task of measuring optical frequencies. We have used this new technique to measure the frequency of the cesium D 1 line and the 1S-2S transition in atomic hydrogen with unprecedented accuracy.

Superradiance regime of laser cooling of crystals and glasses doped with rare-earth ions

Abstract: The method of elimination of bosonic operators is used to theoretically analyze the possibility and optimal conditions of superradiance laser cooling of crystals and glasses doped with rare-earth ions. We consider both stationary and pulsed regimes of laser cooling of solid samples. The exact kinetic equation is derived for the stationary case, and the steady-state solution to this equation is found. This steady-state solution is then employed to estimate the temperature of laser-cooled glass doped with trivalent ytterbium. A set of kinetic equations governing the processes of laser cooling and optical superradiance in a multilevel system is derived for the pulsed regime. It is demonstrated that the efficiency of laser cooling in this regime is proportional to a factor N 2 λ 2 Kn S , where N is the number of active impurity centers, λ is the superradiance wavelength, K is the number of superradiance channels, and n S is the number of superradiance events per second.

Enhanced ionization of the two-electron molecule H2 in intense laser fields : Mechanism of the creation of doorway states

Abstract: We investigate the mechanism of enhanced ionization in the two electron molecule H 2 subjected to an ultrashort, intense laser pulse by solving exactly the time-dependent Schrodinger equation for a one-dimensional model. Results of the simulation are analyzed by using the field following three adiabatic states that are adiabatically connected with the lowest three essential electronic states of H 2 as a field changes. Ionization is enhanced when the excited ionic state H - H + is most efficiently created from the covalent ground state HH in the level dynamics prior to ionization. An analytic expression for the ionic and covalent crossing condition is also obtained in terms of the three essential states and agrees well with the numerical results. As the internuclear distance R decreases, the population of the H - H + created increases whereas the ionization rate from a pure H - H + state decreases owing to the stronger attraction by the distant nucleus. As a result, the ionization is most enhanced at intermediate internuclear R (6) range where covalent and ionic configurations cross.

Laser-assisted radiative recombination and X-ray generation

Abstract: We investigate electron-ion recombination in a powerful laser field. During this process a certain number of laser photons can be absorbed or emitted yielding a spectrum of harmonics of the form ω X = E p + nω L + I i0 + U p where E p is the initial electron energy, n the number of emitted or absorbed laser photons ω L , I i0 the ionization energy and U p the ponderomotive energy of the laser field. As it turns out, the maximum energy that can be absorbed from the laser field during the recombination is given by n max ω L = 2√2U p E p . This process is of interest for the generation of high frequency fields during the laser heating of plasmas. We shall evaluate the nonlinear recombination probabilities using the inverse form of the Keldysh-Faisal-Reiss model, taking also into account in an approximate way the Coulomb effects of the ion on the recombining electron.

Super-selective polysulfone hollow fibre membranes for gas separation: assessment of molecular orientation by Raman spectroscopy

Abstract: It has been shown that the polarized Raman scattering method previously developed for quantitative analysis of molecular orientation in polyethylene can be applied to polysulfone hollow fibre membranes The orientation parameters for the in-plane vibration of the benzene ring in the active layers of the membranes were determined for different processing conditions of the fibres The results reported here confirm the fact, revealed previously by IR spectroscopy, that the spinning of membrane fibres at high shear rates may induce anisotropy at the molecular level in the active layer

Tissue blood flux monitoring by laser speckle photography

Abstract: The time-space cross-correlation analysis of the temporal evaluation of hiospeckle patterns is shown to be a means of tissue blood microcirculation flow visualization. A map of the blood microcirculation activity in a living tissue is constructed using digital recording of the sequences of the dynamic biospeckle patterns produced when a tissue under study is illuminated by a laser beam.

Changes in transmission spectrum of human venous blood under action of low-intensity He-Ne laser

Abstract: We report experimental evidence of light-induced changes in the transmission spectrum of human venous blood under the action of low-intensity radiation from He-Ne laser. The transmission spectra of diluted and nondiluted heparinized human blood have been analyzed before, after and in the course of irradiation. When irradiating diluted blood, no changes in the blood spectrum have been detected. However, irradiating nondiluted blood, the reproducible variations of the blood transmission spectrum in the range of 640-805 nm have been observed. These changes correlate with oxygen saturation. The decreased oxygen saturation in venous blood is stabilized at the level of about 70-80% from the initial one after 5-8 procedures of laser irradiation. The decrease of oxygen saturation in venous blood after the course of treatment means that irradiation of He-Ne laser permits an addition quantity of oxygen to be involved in exchange processes. The changes of oxygen saturation in venous blood after 1-3 procedures of irradiation can be used as a method to detect an individual sensitivity to intravein laser irradiation.