Group Velocity Dispersion of Ring Cavities with Lateral and Angular Dispersive Elements

Abstract

A general formalism for the calculation of group velocity dispersion of laser cavities containing elements with angular or lateral dispersion is developed. This formalism is applied to the cavity of a ring laser with one prism and to a cavity with two antiparallel prisms. The calculated results are in agreement with experimental findings for such lasers.     

An Extended Model of Double Optical Resonance

Abstract

A ladder-type model of double optical resonance, extended by allowing some internal structure for the middle level, is considered and its classical analogue is given. Ionization and photoelectron energy spectra from such a model with singlet, doublet and triplet middle-level structures are evaluated numerically for a wide range of laser pulse intensities and durations. The topics of most interest are peak narrowing by extremely strong upper-level ionization damping, reversal of the peak-height asymmetry by increasing lower-resonance strength at non-zero upper-resonance detuning, and number-of-peak reduction and flattening of the spectrum by shortening of the atom-photon interaction time. When applied to natural barium, this model gives results in good agreement with experiment.     

Observation of sub-kilohertz resonance in Rf-optical double resonance experiment in rare earth ions in solids

Abstract

We have observed kilohertz and sub-kilohertz resonance structures in RF-optical double resonance experiments of rare-earth-doped solids, when the frequency of the RF field is scanned across the hyperfine transitions while monitoring the resonant optical absorption of a CW laser. The effect is observed only when the laser spectral width is broad compared to the hyperfine structure. The observed line widths are apparently free of the inhomogeneous widths of hyperfine levels and the line shape has peculiar double peak structure. The effect is modelled with a resonance involving three atomic levels interacting with three electromagnetic fields, two optical and one RF, in a triangular or “delta’ configuration. While the ordinary optical-RF two-field resonance is limited by spin inhomogeneous width, the simultaneous excitation of three coupled transitions leads to narrow and highly nonlinear resonance structures that are not averaged by the inhomogeneous distribution of hyperfine transition.     

Doppler-shifted cyclotron resonance with helicons in aluminum

Doppler-shifted cyclotron resonance (DSCR) has been observed with helicon waves propagating along the [001], [110], and [111] axes of pure aluminum crystals. Strong quantum oscillations are observed in all propagation directions, the periods of the quantum oscillations agreeing with those of the long-period de Haas-van Alphen oscillations. The dispersion relations of the helicons as well as the frequencies and field values of the DSCR surface impedance anomalies have been measured and compared to the values calculated from two current models for the aluminum Fermi surface. The nonlocal dispersion forB [001] follows closely the theoretical results, while the nonlocal effect forB [110] is weaker than expected. A helicon window and a negative nonlocal dispersion are observed forB [111], due to the resonance of the electron states on the second-zone cap region. Except for those appearing below the helicon window, all the field-frequency relations of the experimental DSCR anomalies agree with those calculated from the predicted resonance zones at the Fermi surface.Work supported by the National Science Foundation.     

Ultra-flattened negative dispersion for residual dispersion compensation using soft glass equiangular spiral photonic crystal fiber

Abstract

We present a numerical investigation of an equiangular spiral photonic crystal fibre (ES-PCF) in soft glass for negative flattened dispersion and ultra-high birefringence. An accurate numerical approach based on finite element method is used for the simulation of the proposed structure. It is demonstrated that it is possible to obtain average negative dispersion of –526.99 ps/nm/km over 1.05–1.70 μm wavelength range with dispersion variation of 3.7 ps/nm/km. The proposed ES-PCF also offers high birefringence of 0.0226 at the excitation wavelength of 1.55 μm. The results here show that the idea of using the proposed fibre can be potential means of effectively directing for residual dispersion compensation, fibre sensor design, long distance data transmission system and so forth.     

The Effect of Continuum-continuum Transitions on Laser-induced Continuum Structures

Abstract

Laser induced continuum structures (LICS) can be produced by a strong laser embedding an excited atomic bound state into a flat atomic continuum, leading to a tunable resonance of adjustable width that can be probed by a second laser. If this second laser is of similar intensity to the embedding laser then the distinction between the two becomes artificial and saturation effects become important. In this paper the effect on LICS of transitions caused by both lasers between the original atomic continuum and a second atomic continuum are studied in the Markoff approximation by means of Laplace transform methods and allowing for the ionization of each atomic state by both laser fields. Formal results correct to all orders are also given in terms of a T-matrix approach. Numerical calculations are presented showing the effects on the LICS resonant structures of continuum-continuum coupling processes. Significant changes to the Fano profiles (second laser weak) and to coherence holes (second laser strong) occur. Analytical results are also given.     

Investigation on photogenerated radicals of some carboxylates in solution and in aqueous dispersions of AgBr

Abstract

The photogenerated free radicals from formate, oxalate and acetate in aqueous solutions without and with dispered AgBr grains under illuminations of 355 nm laser light, natural light and monochromatic red, green or blue light were detected by electron spin resonance with spin-trap DMPO. The results showed that: (a) ·CO₂^? radicals were produced in the formate solution by all of these illuminations; (b) the signals of ·CO₂^? radicals were greatly intensified when an AgBr dispersion was present in the formate solution; (c) the signals of ·CO₂^? radicals in the oxalate solution were also detected and intensified when an AgBr dispersion was present, but only after illumination by the 355 nm laser; (d) ·CH₃ radicals, instead of ·CO₂^?, were weakly detected in the acetate solution, but only in the presence of AgBr under illumination by the 355 nm laser. The photochemical behaviour of these carboxylates, particularly formate and oxalate, perhaps implies their capability for trapping photogenerated holes and their potential for acting as a hole converter to an effective electron carrier.     

Letter Phase-mismatching effects in the internal second-harmonic generation in InGaAs quantum-well laser diodes

Abstract

We have characterized the role of phase-mismatching in the spectral distribution of the internal second-harmonic generation (ISHG) in the CW operation of high-power InGaAs quantum-well laser diodes emitting at around 960 nm. The observed pairs of narrow blue-green peaks of ISHG have symmetrical spectral positions with respect to the central peak of the pure second harmonic at ～480 nm, suggesting that their generation in the laser waveguide is most probably mediated by a mechanism of reciprocal cancellation of the respective phase-mismatch vectors. The relatively strong emission of the ISHG radiation at the output mirror facet indicates that this second-order nonlinear optical process is enhanced by waveguiding in the laser active region.     

Negative effective mass,optical multistability and Fano line-shape control via mode tunnelling in double cavity optomechanical system

ABSTRACT

We theoretically investigate the optical and mechanical properties of a double cavity optomechanical system with one stationary and two harmonically bound mirrors. We show that it is possible for the mechanical mirrors in this system to possess negative effective mass. Working within the strong coupling and the resolved sideband regime, we show that the displacement of the middle resonator is multistable under certain constrains. We also point to the existence of optomechanically induced absorption (OMIA) and Fano resonance. Owing to the negative effective mass, our scheme can be exploited in the study of quantum optomechanical metamaterials.     

Collective modes in the two-dimensional electron liquid near the Wigner phase transition

We have investigated the properties of the collective modes of the two dimensional electron liquid in regions of phase space near the Wigner phase transition. We used a new method which we have recently developed for dealing with the strongly correlated electron liquid. We find that the dynamical correlations cause the dispersion curve of the 2D plasmon to pass through a maximum, leading to negative dispersion for largerq values and a much reduced value for the plasmon cut-offq _c. It was of interest to discover, on comparing the plasmon dispersion curve with the corresponding curve for the phonon in the Wigner crystal, that the negative dispersion of the plasmon duplicated the turnover of the phonon dispersion curve at the edge of the Brillouin zone,q=1.2G, and that the two curves were in close agreement for allq less than the plasmon cut-off. A new collective mode in the liquid phase on the highq side of the single-particle excitation spectrum was also found to match up with the phonon dispersion curve nearqG.Australian Research Council Fellow.     

Model of the nonlinear operation of a laser with a gaussian mirror

Abstract

In this paper a semi-classical model of the nonlinear operation of a laser is extended to describe a laser with a Gaussian mirror. Modified nonlinear self-consistent equations including transverse and longitudinal field dependence, gain saturation, spatial hole burning and nonlinear dispersion effects are derived. With the help of an energy theorem an approximate solution for steady-state single mode operation is presented, which relates the small signal gain to the output power and the laser system characteristic parameters. The laser output power characteristics are presented for various cavity configurations, especially, revealing an influence of the Gaussian mirror parameter on the power efficiency of the laser.     

Study of dispersion compensation in femtosecond lasers

Abstract

We present a formal analysis of dispersion compensation in femtosecond lasers and provide a detailed examination of the relevant features in Ti: sapphire and Cr: LiSAF lasers. It is shown that the essential of dispersion compensation up to the fourth order consists of searching for the solutions of a group of three linear equations. With the use of a prism pair, complete compensation of both the second- and the third-order dispersion corresponds to solving two of the three equations with the prism separation and the intraprism beam path length as variables. Owing to the limitations on the practically achievable prism separation and the intraprism path length in a laser set-up, only some of these solutions are physically meaningful; hence both zero second- and third-order cavity dispersion may be realized merely within a certain specific spectral range. The fourth-order dispersion, however, cannot be completely compensated for in this case unless a third control variable is introduced. It is also shown that the dominant parameter in determining the spectral region where dispersion is minimized is the difference between the ratio of the third-order dispersion to the second-order dispersion of the laser rod and that of the selected prism pair material. This also explains the major differences between Ti: sapphire and Cr: LiSAF lasers in terms of dispersion compensation.     

Disappearance of Trapped Yb+

Abstract

The storage time of Yb⁺ ions trapped in an r.f. trap was determined by the rf resonance absorption method in the presence of buffer gas, without and with laser radiation of a frequency equal to the transition from the S_1/2 ground state to the excited P_1/2 state. The storage time of the ions with the buffer gas (presure between 1 × 10^?4 and 4·2 × 10^?4 Pa) was several hours without the laser radiation; this period was shortened to several hundred seconds by irradiation with a 500 nW laser source. This phenomenon shows that the trapped Yb⁺ ions disappeared from the trap as a result of irradiation with the resonance laser light.     

Narrow absorption lines induced by electron shelving

Abstract

We investigate the absorption spectrum of a laser-driven V system in which one of the upper levels is metastable. If the Rabi frequency of the laser driving the metastable transition is sufficiently small we find an extremely narrow peak in the stationary absorption spectrum of a weak probe on the strong transition. We give analytical expressions approximating the stationary absorption spectrum. The absorption spectrum of a two-level system is investigated for the case where an upper limit for the time separation of successive emissions is given. The absorption spectrum then exhibits a delta-function-like peak and using this we then show that the origin of the narrow peak in the absorption spectrum can be traced back to electron shelving, i.e. to the existence of light and dark periods in the resonance fluorescence emitted on the strong transition.     

Experimental study of organic zero-gap conductor α-(BEDT-TTF)2I3

Abstract

A zero-gap state with a Dirac cone type energy dispersion was discovered in the organic conductor α-(BEDT-TTF)₂I₃ under high hydrostatic pressures. This is the first two-dimensional (2D) zero-gap state discovered in bulk crystals with a layered structure. In contrast to the case of graphene, the Dirac cone in this system is highly anisotropic. The present system, therefore, provides a new type of massless Dirac fermion system with anisotropic Fermi velocity. This system exhibits remarkable transport phenomena characteristic to electrons on the Dirac cone type energy structure. The carrier density, written as n∝T², is a characteristic feature of the 2D zero-gap structure. On the other hand, the resistivity per layer (sheet resistance R_S) is given as R_S=h/e² and is independent of temperature. The effect of a magnetic field on samples in the zero-gap system was examined. The difference between zero-gap conductors and conventional conductors is the appearance of a Landau level called the zero mode at the contact points when a magnetic field is applied normal to the conductive layer. Zero-mode Landau carriers give rise to strong negative out-of-plane magnetoresistance.     

Voltage-controlled optical precursors in quantum dot molecules

Abstract

Optical precursors generated in a quantum-dot-molecule system by an incident square-modulated pulse are theoretically investigated. Voltage-controlled electron tunneling couples two quantum dots instead of a laser field, and a narrow transparency window appears with normal dispersion. The main pulse is delayed due to slow-light effect and the precursor pulses are obtained. We examine the linear susceptibility and find that its one part exhibits normal dispersion and the other one presents anomalous dispersion. Competition between the two parts leads to normal dispersion for the linear susceptibility, which contributes to the above results. Voltage-controlled precursors may be useful to design novel optical devices for generating precursors.     

Controlling linewidth of resonance fluorescence in the V-configuration of a four-level atom via quantum interference

Abstract

The interaction of a four-level atom consisting of one excited state, two upper levels and one ground state is considered. We use one field to drive the transition between the excited state and the ground state, and simultaneously apply another field to couple two transitions from the upper-level doublet to the ground state. We investigate how to control the central linewidth of resonance fluorescence spectrum of the driven transition via quantum interference between two transitions from the upper doublet to the ground state of the atom. In the case of orthogonal dipole moments for the upper doublet and the ground state the widths of the central line and the sidebands of the resonance fluorescence spectrum are totally controlled by the spontaneous emission rate of the upper doublet when the applying field is sufficiently strong. When the dipole moments are parallel, the central linewidth can be much narrower than the linewidths of spontaneous emission of the excited state and of the upper doublet due to the quantum interference. The smaller the upper doublet splitting, the narrower the central linewidth. We also notice that when the upper levels are degenerate the resonance fluorescence spectrum may depend on the initial condition because of the quantum interference.     

Optical Double-resonance Spectra with Time-dependent Pulses

Abstract

The optical double-resonance spectrum for a three-level atom interacting with two monochromatic laser pulses with arbitrary temporal profiles is calculated by use of Light's perturbation theory. Unlike some earlier work, the theory is not restricted to exact resonance of the pump laser, or to pulse areas which are exact multiples of 2π. A variety of pulse profiles are investigated, and the spectra are found to depend strongly on the detuning of the pump laser.     

Resonance Fluorescence in a Squeezed Vacuum

Abstract

Fluorescence from a coherently driven two-level atom that is damped by a squeezed vacuum is studied. We show that the mean atomic polarization depends on the relative phases of the squeezed vacuum and the coherent driving field. The fluorescent spectrum is calculated and shows several modifications over the spectrum for normal resonance fluorescence. In particular, the central peak of the Mollow triplet has a linewidth that depends on the phase of the driving field. For strong squeezing this peak can either be much narrower or much broader than the natural linewidth of the atom.     

Light-driven Optical Switch,Based on Excited-state Absorption in Activated Dielectric Crystals

Abstract

We show that despite the generally held view that excited-state absorption (ESA) is a totally negative phenomenon in laser physics, in some cases it has useful applications in laser technology.