Showing papers in "Journal of Modern Optics in 2008"

Strong field approximation for systems with Coulomb interaction

Abstract: A theory describing above-threshold ionization of atoms and ions in a strong electromagnetic field is presented. It is based on the widely known strong field approximation and incorporates the Coulomb interaction between the photoelectron and the nucleus using the method of complex classical trajectories. A central result of the theory is the Coulomb-corrected ionization amplitude whose evaluation requires little extra numerical effort. By comparing our predictions with the results of ab initio numerical solutions for two examples we show that the new theory provides a significant improvement of the Coulomb-free strong field approximation. For the case of above-threshold ionization in elliptically polarized fields a comparison with available experimental data is also presented.

Adaptive optics and vision

Abstract: It has long been recognised that the optical quality of the human eye is far from diffraction limited. This affects our visual acuity and severely limits the resolution at which images of the living retina can be obtained. Adaptive optics is a technique that can correct for the eye's aberrations and provide diffraction limited resolution. The origins of the technique lie in astronomy, but it was successfully adapted to the human eye just over 10 years ago. Since then there have been rapid developments in the field of adaptive optics and vision science. In vivo images of the retina can now be routinely achieved with unprecedented resolution. Sophisticated experiments can be performed to gain a deeper knowledge of the interaction of neural retinal architecture and visual perception. This article presents the theory behind adaptive optics for the human eye and reviews the developments in this field to date.

Structural, optical and electrical properties of CdZnS thin films

Abstract: The thin films of Cd1-x Zn x S (x = 0, 0.2, 0.4, 0.6, 0.8 and 1) have been prepared by the vacuum evaporation method using a mechanically alloyed mixture of CdS and ZnS. The structural, optical and electrical properties have been investigated through the X-ray diffractometer, spectrophotometer and Keithley electrometer. The X-ray diffraction patterns of these films show that films are polycrystalline in nature having preferential orientation along the (002) plane. In the absorption spectra of these films, absorption edge shifts towards lower wavelength with the increase of Zn concentration. The energy band gap has been determined using these spectra. It is found that the energy band gap increases with increasing Zn concentration. The electrical conductivity of so prepared thin films has been determined using a I–V characteristic curve for these films. The result indicates that the electrical conductivity decreases with increasing Zn content and increases with temperature. An effort has also been made to o...

Quantum theory of optomechanical cooling

Abstract: We review the quantum theory of cooling of a mechanical oscillator subject to the radiation pressure force due to light circulating inside a driven optical cavity. Such optomechanical setups have been used recently in a series of experiments by various groups to cool mechanical oscillators (such as cantilevers) by factors reaching 10 5 , and they may soon go to the ground state of mechanical motion. We emphasize the importance of the sideband-resolved regime for ground state cooling, where the cavity ring-down rate is smaller than the mechanical frequency. Moreover, we illustrate the strong coupling regime, where the cooling rate exceeds the cavity ring-down rate and where the driven cavity resonance and the mechanical oscillation hybridize. Keywords: cavity QED, optomechanics, micromechanics, sideband cooling, radiation pressure

Analytical Capabilities of Coherent Anti-Stokes Raman Scattering Microspectroscopy.

Abstract: Nonlinear Raman scattering is an emerging spectroscopy technique for non-invasive microscopic imaging. It can produce a fluorescence background free vibrational spectrum from a microscopic volume of a sample providing chemically specific information about its molecular composition. We analyze the ability of nonlinear Raman microspectroscopy to detect low concentrated molecular species and evaluate its applicability to study complex solutions.

Quantum encoder and decoder for practical quantum key distribution using a planar lightwave circuit

Abstract: To launch quantum key distribution (QKD) into the commercial market, it is important to develop a system that is simpler and more reliable using current technology. This report proposes quantum encoders and decoders using a passive planar lightwave circuit (PLC) that is useful for implementing optical-fiber-based QKD systems. Our encoders and decoders are based on an asymmetric Mach–Zehnder interferometer and allow us to prepare and analyze various photonic time-bin qubits reliably. The system can be stable and polarization-insensitive merely by stabilizing and controlling the device temperature. Our PLC-based devices enables us to simplify the QKD system and increase its reliability.

Decaying-dressed-state analysis of a coherently driven three-level Λ system

Abstract: We introduce decaying-dressed states for a three-level Λ system driven at one atomic transition and probed at the adjacent one. These states allow for a simple interpretation and classification of various coherent effects in such a system.

Free-electron lasers

Abstract: This is a tutorial review of the basic principles and present status of free-electron lasers (FELs) with particular emphasis on high-gain FELs such as FLASH in Hamburg. With their unprecedented intensity and a pulse duration of a few 10 fs, these novel accelerator-based radiation sources in the ultraviolet and X-ray regime will open new fields in physics, chemistry, biology and material science. Assuming that the reader is unfamiliar with FELs or particle accelerators in general, this tutorial covers the basic concepts of particle accelerators and the generation of synchrotron radiation in some detail, and finally describes FELs in the low-gain and high-gain regime.

Effects of higher-order aberrations on contrast acuity as a function of light level

Abstract: The effects of higher-order ocular aberrations on functional visual performance were assessed for seven young healthy subjects, as a function of light level. Contrast acuity thresholds were measured for a functional vision test with and without correction of higher-order aberrations and at a number of different pupil sizes in three of the subjects investigated. A visual simulator incorporating an adaptive optics (AO) system was used for this purpose. The results show that when light level is decreased, the drop in neural contrast sensitivity limits the impact that increased optical degradations have on vision. The expected AO benefit for functional vision is given for scotopic, mesopic and photopic regimes.

High harmonic generation from aligned molecules–amplitude and polarization

Abstract: While high harmonic generation from atoms is relatively well understood, the ability to align gas-phase molecules opens an opportunity to more deeply understand the underlying physics Many assumptions, such as the single active electron approximation, neglect of the Coulomb potential, the strong field approximation, and the assumption of plane waves, are being challenged by new experimental observations We study high harmonic emission from aligned molecules such as N2, O2 and CO2 We present experimental measurements of the amplitude of the emission as a function of molecular angle, as well as the polarization state

Application of fast-light in gravitational wave detection with interferometers and resonators

Abstract: In this paper, we study several designs for interferometric gravitational wave detectors, and the potential for enhancing their performance with a fast-light medium. First, we explore the effect of such a medium on designs similar to those already planned for Advanced LIGO. Then we review the zero-area Sagnac interferometer for GW detection, comparing its properties against the more conventional GW detector based on a Michelson interferometer. We next describe a modified version of such a detector where the Sagnac interferometer is replaced by a zero-area Sagnac ring resonator fed by an external laser. We then consider a GW detector based on an active, zero-area Sagnac ring resonator, where a gain medium is present inside the cavity. Finally, we show that if a medium with negative dispersion, which yields the fast-light effect, is also present inside this detector, then its sensitivity to GW strain is enhanced by the inverse of the group index of the dispersive medium. We describe conditions under which t...

Scintillation index of higher-order cos-Gaussian, cosh-Gaussian and annular beams

Abstract: In this paper, via the generalized beam formulation, we evaluate the scintillation index for higher-order general beams propagating through the weak atmospheric turbulence. The investigated higher-order beam types are cos-Gaussian, cosh-Gaussian and annular beams. The scintillation indices of these beams are plotted with respect to propagation length, source size and order of the beam. According to our graphical outputs, higher-order beams have less on-axis scintillation index than zero-order beams at longer propagation distances. The on-axis scintillation exhibits a slight increase when the order of even-ordered beams is made higher. Moreover, our study on the source size variation shows that, for most of the source size range, cos-Gaussian beams have a lower scintillation index than the other beams.

Ocular monochromatic aberration statistics in a large emmetropic population

Abstract: The primary objective of this study was to explore the statistics of ocular higher-order aberrations in a nearly emmetropic population. The wavefront aberration of 393 eyes of 218 subjects was obtained under natural conditions using the WaveLight WaveFront Analyser. The spherical equivalent of eyes ranged between +0.75 and − 1.25 D, with the astigmatism being less than 0.75 D. Mean age was 33.0±4.8 years. Analysis was performed for a 6 mm pupil. A significant dispersion in all Zernike coefficients was found. Population average values of Zernike coefficients were almost zero, with the exemption of primary spherical ( µm) and oblique trefoil ( ) aberrations. Mean higher-order RMS error was 0.26 µm, corresponding to an equivalent defocus of 0.20 D. An increase in higher-order RMS with age (at pre-presbyopic range) was found, which was accompanied by changes in coma-like aberrations, while spherical aberration remained unchanged. Emmetropic eyes appear to have on average lower individual and combined higher-o...

Dynamic measurement of accommodative responses while viewing stereoscopic images

Abstract: Using video refraction accommodative and convergence dynamic responses were measured to stepped changes in convergence stimuli with unchanged accommodative stimuli (conflicting stereoscopic image) and compared with responses to non-conflicting target stimuli. Three targets were used that varied in their spatial frequency components. An accommodative transient overshoot was evident in four out of seven subjects for only conflicting stimuli. One showed accommodative and convergence oscillation probably due to difficulty in fusing the stereoscopic target when it had a higher spatial component, however, this oscillation diminished when the target was spatial low-pass filtered. We hypothesise that transient responses to step stimuli is initiated by convergence-driven accommodation and subsequently followed by slower fine-control of accommodation modulated by the amount of blur. Inter-subject differences in convergence-driven accommodation may also be a factor to consider. For stereoscopic stimuli, it is proposed that the increase in blur immediately after the onset of the accommodative response inhibits cessation of the response.

Funnel ratchets in biology at low Reynolds number: choanotaxis

Abstract: We present an example of a ratchet phenomena in biology consisting of swimming bacteria in an enclosed structure separated by a wall of funnels. The funnels serve to concentrate bacteria that are motile, in spite of the fact that the motion occurs at low Reynolds numbers and would be expected to be time reversible invariant. We discuss some of the basic ideas which underly the phenomena including spatial and temporal coherence of the motion of bacteria and interaction of the bacteria with surfaces.

Field profiles of coupled surface plasmon-polaritons

Abstract: We present a study of the electromagnetic field profiles for coupled surface plasmon-polariton systems. Results for both the symmetrically-clad thin metal film structure and the metal-clad dielectric cavity are given. We also consider an asymmetrically-clad thin metal-film structure and show that such a structure may also support coupled SPP modes under appropriate conditions. We describe our method for calculating the field profiles in detail. In contrast to previous methods our approach does not require the introduction of an input field, it allows straightforward computation of the field profiles associated with the optical modes of multilayer planar structures.

Radial gradient index intraocular lens: a theoretical model

Abstract: A model for a gradient index intraocular lens (IOL) is presented. The distribution of the refractive index is Gaussian, varying radially in the equatorial plane and with distance from the optic axis in the sagittal plane. An algorithm for adjustment of the optimal gradient index distribution for individual parameters of a pseudophakic eye is also described. The performance, in terms of spherical aberration and resultant image quality that can be provided with the gradient index IOL, is compared with that provided by a commercially available IOL (Tecnis Z9000).

Velocity map imaging of atomic and molecular processes at the free electron laser in Hamburg (FLASH)

Abstract: Velocity map imaging was implemented at the free electron laser in Hamburg to image atomic and molecular photoionization processes at a photon energy of 45.55 eV. High quality momentum distributions were recorded for a range of rare gases (He, Ne, Ar, Kr and Xe) and small molecules (H2, D2, O2, N2, CO2). This proof-of-principle experiment illustrates the potential for using velocity map imaging in order to study non-linear ionization and/or dissociation processes.

A note on the measurement of phase space observables with an eight-port homodyne detector

Abstract: It is well known that the Husimi Q-function of the signal field can actually be measured by the eight-port homodyne detection technique, provided that the reference beam (used for homodyne detection) is a very strong coherent field so that it can be treated classically [see e.g. Leonhardt, U.; Paul, H. Phys. Rev. A 1993, 47, R2460–R2463]. Using recent rigorous results on the quantum theory of homodyne detection observables [Kiukas, J.; Lahti, P. J. Mod. Opt., in press (see arXiv:0706.4436v1 [quant-ph])], we show that any phase space observable, and not only the Q-function, can be obtained as a high amplitude limit of the signal observable actually measured by an eight-port homodyne detector. The proof of this fact does not involve any classicality assumption.

A model-based approach to video-based eye tracking

Abstract: Video-based eye tracking typically relies on tracking the pupil and a first-surface corneal reflection (CR) of an illumination source. The positional difference between these two features is used to determine the observer's eye-in-head orientation. With the use of head-mounted eye trackers, this positional difference is unavoidably affected by relative movements between the eye tracking camera and the subject's eye. Video-based trackers also suffer from problems resulting in poor CR detection, such as spurious reflections being mistaken as the desired CR. We approach these problems by modelling how these features—pupil and CR—are affected by different relative movements of the eye. Optical relationships between the offset of the apparent pupil centre and that of the CR are derived. An experiment was conducted with five observers to support these derivations. Solutions to the aforementioned problems using these offset relationships are provided. The first application compensates for movements of an eye tra...

Spectral changes in electromagnetic stochastic beams propagating through turbulent atmosphere

Abstract: It is known that the spectrum of light can change on propagation, even in free space. Such changes are due to its coherence properties. We determine how the spectrum of a stochastic electromagnetic beam of any state of coherence changes on propagation in atmospheric turbulence. We illustrate the results by examining changes in the spectrum of an electromagnetic Gaussian Schell-model beam whose spectrum consists of a single spectral line with a Gaussian profile, which propagates through atmospheric turbulence, with Kolmogorov power spectrum. The results are of interest for optical communications through the atmosphere and for laser radar systems.

Mechanical Stresses Produced by a Light Beam

Abstract: A circularly polarized electromagnetic beam is considered, which is absorbed by a plane, and the mechanical stress produced in the plane by the beam is calculated. It is shown that the central part of the beam produces a torque at the central region of the plane due to the spin of the beam, and the wall of the beam produces an additional torque due to orbital angular momentum of the beam. The total torque acting on the plane equals twice the power of the beam divided by the frequency. This fact contradicts the standard electrodynamics, which predicts the torque equals power of the beam divided by frequency, and means the electrodynamics, as well as the whole classical field theory, is incomplete. Introducing the spin tensor corrects the electrodynamics.

Two-mode squeezed number state as a two-variable Hermite-polynomial excitation on the squeezed vacuum

Abstract: Based on the two-mode squeezing operator which is the quantum version of the symplectic transformation, we find that the corresponding squeezed two-mode number state is just a two-variable Hermite polynomial excited state, which possesses well-behaved features, e.g. its Wigner function is a direct product of two Laguerre polynomials; its Husimi function, a Gaussian broaden version of the Wigner function, is related to two-variable Hermite polynomials. Moreover, its quantum statistical features, such as squeezing properties and the inter-mode photon bunching, are discussed.

Zernike representation and Strehl ratio of optical systems with variable numerical aperture

Abstract: We consider optical systems with variable numerical aperture (NA) on the level of the Zernike coefficients of the correspondingly scalable pupil function. We thus present formulas for the Zernike coefficients and their first two derivatives as a function of the scaling factor e ≤ 1, and we apply this to the Strehl ratio and its derivatives of NA-reduced optical systems. The formulas for the Zernike coefficients of NA-reduced optical systems are also useful for the forward calculation of point-spread functions and aberration retrieval within the Extended Nijboer–Zernike (ENZ) formalism for optical systems with reduced NA or systems that have a central obstruction. Thus, we retrieve a Gaussian, comatic pupil function on an annular set from the intensity point-spread function in the focal region under high-NA conditions.

Binocular Shack–Hartmann sensor for the human eye

Abstract: During steady-state fixation the aberrations of the human eye display dynamic behaviour. It has been suggested that the fluctuations in focus are correlated between both eyes. However, nothing is known about the dynamic correlation between the aberrations other than focus. We have developed an open-view binocular Shack–Hartmann sensor which measures the ocular wavefront aberrations simultaneously in both eyes at 25 Hz. A single sensor and laser source are used to reduce system cost and complexity. Speckle is reduced using a rotating diffuser in a plane conjugate to the retinae. We measured the wavefront dynamics of two subjects and decomposed the data into Zernike aberration terms up to and including fifth radial order. Coherence function analysis was used to determine the correlation between aberrations in the frequency domain. The correlations were dependent upon subject, frequency and aberration type.

Ionisation of H2 in intense ultrashort laser pulses: parallel versus perpendicular orientation

Abstract: A theoretical comparison of the electronic excitation and ionisation behaviour of molecular hydrogen oriented either parallel or perpendicular to a linear polarised laser pulse is performed. The investigation is based on a non-perturbative treatment that solves the full time-dependent Schrodinger equation of both correlated electrons within the fixed-nuclei approximation and the dipole approximation. Results are shown for two different laser pulse lengths and intensities as well as for a large variety of photon frequencies starting in the one- and reaching into the six-photon regime. In order to investigate the influence of the intrinsic diatomic two-centre problem even further, two values of the internuclear separation and a newly developed atomic model are considered.

3D-vortex labyrinths in the near field of solid-state microchip laser

Abstract: The usage of vortex-labyrinth fields and Talbot lattices as optical dipole traps for neutral atoms is considered for the wavelength of trapping radiation in the range 0.98–2.79 µm. The square vortex lattices generated in high Fresnel number solid-state microchip lasers are studied as a possible realization. The distribution of light field is obtained via a nonstationary computational model based on Maxwell–Bloch equations for a class-B laser, discrete Fox–Lee map with relaxation of inversion and a static model based on superposition of copropagating Gaussian beams. The spatial patterns obtained numerically and observed experimentally previously are interpreted as nonlinear superposition of vortices with helicoidal phase dislocations. The distribution of light field is approximated analytically by a sum of array of vortex lines and an additional parabolic subtrap. The separable optical trapping potential is proposed with similar intensity distribution. The factorization of the macroscopic wavefunction has ...

Generation of entanglement outside of the light cone

Abstract: The Feynman propagator has nonzero values outside of the forward light cone. That does not allow messages to be transmitted faster than the speed of light, but it is shown here that it does allow entanglement and mutual information to be generated at space-like separated points. These effects can be interpreted as being due to the propagation of virtual photons outside of the light cone or as a transfer of pre-existing entanglement from the quantum vacuum. The differences between these two interpretations are discussed.

Electronic speckle pattern interferometry based on spatial fringe analysis method using two cameras

Abstract: A new speckle measurement method is proposed by applying a spatial phase shifting method to multi-camera technology in order to perform a high resolution, high speed, and large deformation measurement. It is confirmed that the alignment of optical elements in this method is easier than the ordinary multi-camera methods because the optical system uses only two cameras. The validity of principle of the method is discussed by the results of experiments. It is shown that measurement precision of this method is about 1/50 wavelength in a small deformation measurement. Furthermore, the method is improved for a large deformation measurement method by accumulating the results of the small continuous deformation measurement. The optimum sampling process of the large deformation of an object is proposed in order to detect the phase map of the large deformation. It is confirmed that the large deformation can be precisely measured by this method.

Laser tweezers for atomic solitons

Abstract: We describe controllable and precise laser tweezers for Bose–Einstein condensates of ultracold atomic gases. In our configuration, a laser beam is used to locally modify the sign of the scattering length in the vicinity of a trapped BEC. The induced attractive interactions between atoms allow us to extract and transport a controllable number of atoms. We analyze, through numerical simulations, the number of emitted atoms as a function of the width and intensity of the outcoupling beam. We also study different configurations of our system, as the use of moving beams. The main advantage of using the control laser beam to modify the nonlinear interactions in comparison to the usual way of inducing optical forces, i.e. through linear trapping potentials, is to improve the controllability of the outcoupled solitary wave-packet, which opens new possibilities for engineering macroscopic quantum states.