Showing papers in "Journal of Modern Optics in 2006"

Coherent imaging with pseudo-thermal incoherent light

Abstract: We investigate experimentally fundamental properties of coherent ghost imaging using spatially incoherent beams generated from a pseudo-thermal source. A complementarity between the coherence of the beams and the correlation between them is demonstrated by showing a complementarity between ghost diffraction and ordinary diffraction patterns. In order for the ghost imaging scheme to work it is therefore crucial to have incoherent beams. The visibility of the information is shown for the ghost image to become better as the object size relative to the speckle size is decreased, and therefore a remarkable tradeoff between resolution and visibility exists. The experimental conclusions are backed up by both theory and numerical simulations.

Self-compression of ultra-short laser pulses down to one optical cycle by filamentation

Abstract: Theoretical studies of filamentation of ultra-short near-IR laser pulses propagating in a noble gas predict near single-cycle pulses with the intensity being clamped to the field ionization threshold. Experimental results show that this method is carrier envelope offset phase preserving and provides a very simple source for generating few-cycle intense laser pulses. This suggests a very simple design for the generation of ultra-short, sub-femtosecond XUV optical pulses.

Slow light in paraffin-coated Rb vapour cells

Abstract: Preliminary results from an experimental study of slow light in anti-relaxation-coated Rb vapour cells are presented, and the construction and testing of such cells are described. The slow ground state decoherence rate allowed by coated cell walls leads to a dual-structured electromagnetically induced transparency (EIT) spectrum with a very narrow (< 100 Hz) transparency peak on top of a broad pedestal. Such dual-structured EIT permits optical probe pulses to propagate with greatly reduced group velocity on two time scales. Ongoing efforts to optimize the pulse delay in such coated cell systems are discussed.

Quantum-orbit theory of high-order atomic processes in intense laser fields

Abstract: The quantum-orbit formalism of high-order atomic processes in a strong laser field is presented starting from the strong-field approximation generalized to include higher-order effects. It is shown how to apply the quantum-orbit theory to various processes such as high-order above-threshold ionization and detachment, high-order harmonic generation, laser-assisted X-ray–atom scattering, laser-assisted electron–ion recombination, laser-assisted electron–atom scattering and non-sequential double ionization. Particular attention is devoted to high-order above-threshold ionization by few-cycle laser pulses. The results obtained using the strong-field approximation and the theory of quantum orbits are compared with the ab initio solution of the time-dependent Schrodinger equation. It is shown that Coulomb effects are important for low-energy electron spectra.

Atom guiding along high order Laguerre–Gaussian light beams formed by spatial light modulation

Abstract: A spatial light modulator (SLM) has been used to create high quality Laguerre–Gaussian (LG) light beams, which have been used to study the guiding of cold rubidium atoms. The SLM allows real-time variation of the hollow guiding beam and permits direct comparison of the guided atom fluxes for different LG modes with minimal adjustment of the other optical components. It is demonstrated that, by increasing the azimuthal index l of the Laguerre–Gaussian beam, the radiation pressure pushing the trapped atoms may be reduced while maintaining the same guided flux. This is the first comparative study of hollow beam atom guiding, and further demonstrates the versatility of the SLM for studies in atom optics.

Entanglement and electron correlation in quantum chemistry calculations

Abstract: Electron–electron correlation in quantum chemistry calculations can be analysed in terms of entanglement between electrons. Two exactly solvable models: two fixed spin-1/2 particles and two-electron two-site Hubbard model are used to define and discuss the entanglement as a function of the system parameters. Ab initio configuration interaction calculation for entanglement is presented for the H2 molecule. Qualitatively, entanglement and electron–electron correlation have similar behaviour. Thus, entanglement might be used as an alternative measure of electron correlation in quantum chemistry calculations.

Modelling photo-detectors in quantum optics

Abstract: Photo-detection plays a fundamental role in experimental quantum optics and is of particular importance in the emerging field of linear optics quantum computing. Present theoretical treatment of photo-detectors is highly idealized and fails to consider many important physical effects. We present a physically motivated model for photo-detectors which accommodates for the effects of finite resolution, bandwidth and efficiency, as well as dark counts and dead-time. We apply our model to two simple well-known applications, which illustrates the significance of these characteristics.

Control of high-order harmonic emission using attosecond pulse trains

Abstract: We show that attosecond pulse trains are a natural tool to control strong field processes such as high-order harmonic generation. Coherently combining an attosecond pulse train with an IR driving field, we predict and experimentally confirm enhancement and spectral narrowing of the harmonic yield at photon energies around 90 eV. The use of an attosecond pulse train to seed the harmonic generation process replaces tunneling ionization with a single-photon ionization step, therefore permitting the manipulation of the time–frequency properties of high-order harmonic generation already at the single-atom level.

On the validity of the strong field approximation and simple man's theory

Abstract: Few-cycle above-threshold ionization spectra of atomic hydrogen, calculated via the numerical solution of the time-dependent Schrodinger equation (TDSE), are compared with those predicted by the strong field approximation. Good agreement is obtained for the energetic, rescattered photoelectrons whereas the low-energy part of the electron spectra differ significantly. The latter disagreement is shown to originate from the long-range character of the Coulomb potential. In the second part of the paper a novel quantum distribution function is introduced, which facilitates a direct comparison of the classical electron orbits used in simple man's theory with the exact numerical TDSE result. It is shown that well localized electron wave packets emerge, oriented along the simple man's classical trajectories as the energy resolution in the quantum distribution function is reduced.

Implementation of sub-Rayleigh-resolution lithography using an N-photon absorber

Abstract: A nonlinear optical, phase-shifted-grating method for improving the resolution of feature sizes is implemented experimentally using an N-photon lithographic material. For the recording medium, we used poly(methyl-methacrylate) (PMMA), which is a UV lithographic material that can be excited by multi-photon absorption in the visible region. We achieved a two-fold enhancement of the resolution over the standard Rayleigh limit of half of the wavelength.

Computation of the optical trapping force on small particles illuminated with a focused light beam using a FDTD method

Abstract: According to the electromagnetic momentum interpretation due to Minkowski, the optical trapping force is determined by momentum transfer. The computation details related to computing the forces of optical radiation pressure on small particles using the scattered field three-dimensional (3D) grid finite difference time domain (FDTD) algorithm are presented. The technique is based on propagating the focused electromagnetic fields through the grid and determining the changes in the optical energy flow with and without the trapped object in the system. The Richards–Wolf vector field equations are applied to the scattered FDTD approach to specify an incident focused beam. We show computational results for a high refractive index particle. These results are in agreement with published experiments and are similar to other computational methods. Compared with some other calculation results using the FDTD method, our results are more consistent with the results measured.

Ultrafast dynamics and carrier-envelope phase sensitivity of multiphoton photoemission from metal surfaces

Abstract: Subcycle dynamics of multiphoton-induced photoelectron emission from metal surfaces is analysed using a simple phenomenological model to assess optimum conditions for direct carrier-envelope phase measurement. To gain further insight femtosecond time-resolved measurements were carried out on a polycrystalline gold surface with ultrashort laser pulses to explain the recently found, unexpectedly low carrier-envelope phase dependence of the photoemission process in this particular case. In the higher-order interferometric autocorrelation distribution additional short side wings appeared suggesting that ultrafast dynamics of hot electrons reduce the carrier-envelope phase dependence of the photoemission electron yield produced by few-cycle laser pulses. Other metals can be investigated with this simple and fast method to pave the way towards the construction of a solid-state-based, direct carrier-envelope phase detector.

Resolution in rotation measurements

Abstract: The limiting resolution in optical interferometry is set by the number of photons used, with the functional dependence determined by the state of light that is prepared. We consider the problem of measuring the rotation of a beam of light about an optical axis and show how the limiting resolution depends on the total number of quanta of orbital angular momentum carried by the light beam.

Propagation of the electric correlation matrix and the van Cittert–Zernike theorem for random electromagnetic fields

Abstract: The basic mathematical tool of the theory of random, statistically stationary electromagnetic optical fields is a 3 × 3 correlation matrix of its electric field. In this paper we derive formulae for propagation of this matrix from a plane z = 0 into the half-space z > 0, and we find an analogue for electromagnetic fields to the propagation law derived by Zernike for scalar fields.

Playing checkers: detection and eye–hand coordination in simulated prosthetic vision

Abstract: In order to assess the potential for visual inspection and eye–hand coordination without tactile feedback under conditions that may be available to future retinal prosthesis wearers, we studied the ability of sighted individuals to act upon pixelized visual information at very low resolution, equivalent to 20/2400 visual acuity. Live images from a head-mounted camera were low-pass filtered and presented in a raster of 6 × 10 circular Gaussian dots. Subjects could either freely move their gaze across the raster (free-viewing condition) or the raster position was locked to the subject's gaze by means of video-based pupil tracking (gaze-locked condition). Four normally sighted and one severely visually impaired subject with moderate nystagmus participated in a series of four experiments. Subjects' task was to count 1 to 16 white fields randomly distributed across an otherwise black checkerboard (counting task) or to place a black checker on each of the white fields (placing task). We found that all subjects ...

Recent progress in quantum and nonlinear optical lithography

Abstract: We review the status of the field of quantum lithography, that is, the use of quantum-mechanical effects to write lithographic features with resolution finer than that achievable according to the Rayleigh criterion. In particular, we first review the original quantum lithography proposal by Boto et al., and we then describe the status of research aimed at realizing this process.

Accurate phase–height mapping algorithm for PMP

Abstract: In phase measurement profilometry (PMP), the projector can be regarded as another camera according to the reversibility of the light path principle. The relationship of projecting spatial points to image plane of camera and projector is studied, and the phase–height mapping equation without projector distortion is obtained. The equation is then expanded to a polynomial for the convenience of calibration. Furthermore, the relation between the distortion value and the phase is investigated. Finally the phase–height mapping algorithm considering projector distortion and its polynomial expression are acquired. The accuracy of approximation is studied and compared with another two existing algorithms by computer simulation. It is revealed that the absolute error of the new algorithm expressed with quartic polynomial reaches 5.380× 10−3 mm and its standard deviation reaches 3.354× 10−4 mm under general lens distortion. The accuracy of the new algorithm is the highest among the three algorithms. In experiment, t...

Motion perception tasks as potential correlates to driving difficulty in the elderly

Abstract: Changes in the demographics indicates that the population older than 65 is on the rise because of the aging of the ‘baby boom’ generation. This aging trend and driving related accident statistics reveal the need for procedures and tests that would assess the driving ability of older adults and predict whether they would be safe or unsafe drivers. Literature shows that an attention based test called the useful field of view (UFOV) was a significant predictor of accident rates compared to any other visual function tests. The present study evaluates a qualitative trend on using motion perception tasks as a potential visual perceptual correlates in screening elderly drivers who might have difficulty in driving. Data was collected from 15 older subjects with a mean age of 71. Motion perception tasks included—speed discrimination with radial and lamellar motion, time to collision using prediction motion and estimating direction of heading. A motion index score was calculated which was indicative of performance ...

Broadband amplified spontaneous emission double-clad fibre source with central wavelengths near 2 μm

Abstract: Amplified spontaneous emission broadband light around 2 µm has been successfully generated in both double-clad Tm–silica and Tm:Ho fluoride fibres using a high-power 803 nm diode laser pump source. For the Tm–silica fibre an output power was produced greater than 40 mW and with a bandwidth (full width at half-maximum (FWHM)) of about 50 nm. For output powers greater than 90 mW, the FWHM of the output spectrum reduced to about 40 nm. The Tm:Ho fluoride fibre source produced about 2 mW output power and an FWHM of about 20 nm; this was reduced to about 10 nm as the output power was scaled up to 20 mW. The central wavelength for each system was greater than 2 µm.

The 4f-5d luminescence transitions in cerium-doped LuF3

Abstract: Emission and excitation spectra of the Ce3+ ion in LuF3 single crystal were measured at 77 K. The broad bands observed in these spectra were attributed to the parity-allowed electric-dipole 4f ← 5d transitions within Ce3+ ion. No zero-phonon lines were observed, which is indicative of a strong electron-phonon coupling in this host. It is shown that Ce3+ 5d excited configuration splits into five crystal-field components in LuF3. The influence of the crystalline environment on the position of the lowest Ce3+ 5d level is investigated. The energy of the lowest level of the 4f N −15d excited configuration was predicted for all the trivalent rare earth ions embedded in LuF3. Positions of crystal field spitting levels of 4f N −15d configuration relative to the host electronic bands were discussed.

Negatively refracting atomic vapour

Abstract: A new mechanism for realizing negative refractive index with a four-level atomic system is suggested The explicit expressions for the electric permittivity and magnetic permeability at probe frequency are presented It is shown that there is a frequency band in which the four-level photonic-resonant atomic vapour may exhibit simultaneously negative permittivity and permeability, and that such an atomic vapour may become a left-handed material (negatively refracting medium) Compared with the previous schemes to realize negative refraction within the framework of classical electromagnetic theory, the most remarkable features of the present scenario are as follows: (i) isotropic material with microscopic structure units at atomic-scale level, (ii) negative refraction in visible and infrared frequency bands, (iii) controllable manipulation by external fields and (iv) based on quantum coherence in a multilevel atomic system

Resonant energy absorption of rare-gas clusters in strong laser pulses

Abstract: The ionization dynamics of rare-gas clusters in strong femtosecond laser pulses is studied microscopically by means of classical molecular dynamics using a hierarchical tree code. Systematic investigations for laser pulses from 25 to 400 fs duration have shown a transition from field ionization to resonant excitation of a collective motion of the cluster electrons. The latter effect is associated with a strong energy absorption and can be microscopically identified by examining the phase lag of the electron oscillation with respect to the laser field. The optimal conditions depend, via the expansion time, strongly on the cluster size, which was varied from 102 up to 104 atoms.

Generation of a nonlinear two-mode Stark shift through the adiabatic elimination method

Abstract: In this contribution a three-level atom in interaction with a two-mode quantized electromagnetic field, initially prepared in an entangled two-mode coherent state is considered. Through the elimination of an intermediate level by using the adiabatic elimination method a nonlinear Stark shift is introduced. Exact solution of the wave function in the Schrodinger picture is obtained. Some statistical aspects through the effective two-level atom interacting with the mode through multiphotons with the Stark shift are presented. The results are employed to perform a careful investigation of the temporal evolution of the atomic inversion, entropy squeezing. It has been shown that the system is sensitive to any change in the parameter representing the Stark shift. General conclusions reached are illustrated by numerical results.

Quantum dense coding via cavity decay

Abstract: We investigate a scheme for implementing quantum dense coding via cavity decay and linear optics devices. Our scheme combines two distinct advantages: the atomic qubit serves as a stationary bit and the photonic qubit as a flying bit, thus it is suitable for long distant quantum communication.

Application of vertical cavity surface emitting lasers in self-oscillating atomic clocks

Abstract: We discuss practical aspects of building atomic clocks consisting of an optoelectronic oscillator stabilized with the electromagnetically induced transparency resonance in a 87Rb atomic vapour cell, where a vertical cavity surface emitting laser (VCSEL) is used as a light source. In particular, we show how the variable microwave power injection into the VCSEL, its optical injection-locking, and the phase stability of the optical-microwave link influences the stability of the clocks.

Using quantum interferometers to make measurements at the Heisenberg limit

Abstract: In recent work we proposed a quantum interferometer and showed how it could be used to significantly enhance the resolution that could be achieved in measurement schemes. In this paper, we outline a detailed scheme on how these quantum interferometers could be implemented. We also analyze the effects of dissipation and of imperfect detectors and show that this scheme is remarkably robust to both. This suggests that quantum interferometers may provide a promising route for implementing sub-shot-noise limited measurements in the laboratory.

An optimal photon counting polarimeter

Abstract: We present the experimental results for a method used to perform polarimetry on ensembles of single photons. Our setup is based on a measurement method known to be optimal for estimating the state of two-level systems. The setup has no moving parts and is sensitive to weak sources (emitting single photons) of light as it relies on photon counting, and has potential applications in both classical polarization measurements and quantum communication scenarios. In our implementation, we are able to reconstruct the Stokes parameters of pure polarization states with an average fidelity of 99.9%.

Information hiding based on double-random phase encoding technology

Abstract: This paper proposes a method for information hiding based on doubled-random phase encoding technology. In this method, data is split into real and imaginary compartments. The real part of the encoded data is embedded into a large enough host image, together with the imaginary part through grey level superposition. The composed image is not subject to severe degradation compared to the original host image. During the decryption process, the composed image is decrypted directly without the use of the original host image. Factors affecting the quality of the composed image as well as the reconstructed image are discussed. It is shown how optimum results are achieved by adjusting the superposition weight of the definitive host and hiding images.

Optimizing chromatic aberration calibration using a novel genetic algorithm

Abstract: Advances in digitalized image optics has increased the importance of chromatic aberration. The axial and lateral chromatic aberrations of an optical lens depends on the choice of optical glass. Based on statistics from glass companies worldwide, more than 300 optical glasses have been developed for commercial purposes. However, the complexity of optical systems makes it extremely difficult to obtain the right solution to eliminate small chromatic aberration. Even the damped least-squares technique, which is a ray-tracing-based method, is limited owing to its inability to identify an enhanced optical system configuration. Alternatively, this study instead attempts to eliminate even negligible axial and lateral colour aberration by using algorithms involving the theories of geometric optics in triplet lens, binary and real encoding, multiple dynamic crossover and random gene mutation techniques.

Hybrid-matrix algorithm for rigorous coupled-wave analysis of multilayered diffraction gratings

Abstract: This paper presents an alternative approach called the hybrid-matrix or H-matrix algorithm for the stable and efficient implementation of the rigorous coupled-wave analysis of multilayered diffraction gratings. The H matrix is superior to both the T and R matrices because of its numerical stability across the whole range of large and small layer thicknesses. It is also more concise and simple than the S matrix in obviating the need to invoke auxiliary reference layers or two consecutive layers. For high efficiency, the stack H-matrix recursive algorithm is derived in terms of eigen-submatrices directly without any auxiliary layer matrix. Compared with the enhanced transmittance matrix (partial-solution) approach, the enhanced H-submatrix approach achieves 30% higher computation efficiency and requires less memory.