Showing papers on "Coherence (physics) published in 2001"

Optical Coherence Tomography of the Human Retina

Abstract: Objective: To demonstrate optical coherence tomography for high-resolution, noninvasive imaging of the human retina. Optical coherence tomography is a new imaging technique analogous to ultrasound B scan that can provide cross-sectional images of the retina with micrometer-scale resolution. Design: Survey optical coherence tomographic examination of the retina, including the macula and optic nerve head in normal human subjects. Settings Research laboratory. Participants: Convenience sample of normal human subjects. Main Outcome Measures: Correlation of optical coherence retinal tomographs with known normal retinal anatomy. Results: Optical coherence tomographs can discriminate the cross-sectional morphologic features of the fovea and optic disc, the layered structure of the retina, and normal anatomic variations in retinal and retinal nerve fiber layer thicknesses with 10- μm depth resolution. Conclusion: Optical coherence tomography is a potentially useful technique for high depth resolution, cross-sectional examination of the fundus.

Fabry - Perot interference in a nanotube electron waveguide

Abstract: The behaviour of traditional electronic devices can be understood in terms of the classical diffusive motion of electrons. As the size of a device becomes comparable to the electron coherence length, however, quantum interference between electron waves becomes increasingly important, leading to dramatic changes in device properties. This classical-to-quantum transition in device behaviour suggests the possibility for nanometer-sized electronic elements that make use of quantum coherence. Molecular electronic devices are promising candidates for realizing such device elements because the electronic motion in molecules is inherently quantum mechanical and it can be modified by well defined chemistry. Here we describe an example of a coherent molecular electronic device whose behaviour is explicitly dependent on quantum interference between propagating electron waves-a Fabry-Perot electron resonator based on individual single-walled carbon nanotubes with near-perfect ohmic contacts to electrodes. In these devices, the nanotubes act as coherent electron waveguides, with the resonant cavity formed between the two nanotube-electrode interfaces. We use a theoretical model based on the multichannel Landauer-Buttiker formalism to analyse the device characteristics and find that coupling between the two propagating modes of the nanotubes caused by electron scattering at the nanotube-electrode interfaces is important.

Coherence radar and spectral radar —new tools for dermatological diagnosis

Abstract: "Coherence radar," an optical 3-D sensor based on short coherence interferometry, is used to measure skin surface topology. This method is called optical coherence profilometry (OCP) and it may be a useful tool for medical diagnosis in dermatology because different medical conditions show distinct alterations of the skin surface. The measuring uncertainty is less than 2 μm. The measuring time is about 4 s. in vivo 3-D mapping of naked skin was performed without preparation. For clinical application, a fiber optical implementation was introduced. Spectral radar is an optical sensor for the acquisition of skin morphology based on OCT techniques. The scattering amplitude a(z) along one vertical axis from the surface into the bulk can be measured within one exposure. No reference arm scanning is necessary. The theory of the sensor, including the dynamic range, is discussed and in vivo measurements of human skin by a fiber optical implementation of the sensor are demonstrated.

Josephson junction arrays with Bose-Einstein condensates.

Abstract: We report on the direct observation of an oscillating atomic current in a one-dimensional array of Josephson junctions realized with an atomic Bose-Einstein condensate. The array is created by a laser standing wave, with the condensates trapped in the valleys of the periodic potential and weakly coupled by the interwell barriers. The coherence of multiple tunneling between adjacent wells is continuously probed by atomic interference. The square of the small-amplitude oscillation frequency is proportional to the microscopic tunneling rate of each condensate through the barriers and provides a direct measurement of the Josephson critical current as a function of the intermediate barrier heights. Our superfluid array may allow investigation of phenomena so far inaccessible to superconducting Josephson junctions and lays a bridge between the condensate dynamics and the physics of discrete nonlinear media.

Two-dimensional phase unwrapping with use of statistical models for cost functions in nonlinear optimization

Abstract: Interferometric radar techniques often necessitate two-dimensional (2-D) phase unwrapping, defined here as the estimation of unambiguous phase data from a 2-D array known only modulo 2pi rad. We develop a maximum a posteriori probability (MAP) estimation approach for this problem, and we derive an algorithm that approximately maximizes the conditional probability of its phase-unwrapped solution given observable quantities such as wrapped phase, image intensity, and interferogram coherence. Examining topographic and differential interferometry separately, we derive simple, working models for the joint statistics of the estimated and the observed signals. We use generalized, nonlinear cost functions to reflect these probability relationships, and we employ nonlinear network-flow techniques to approximate MAP solutions. We apply our algorithm both to a topographic interferogram exhibiting rough terrain and layover and to a differential interferogram measuring the deformation from a large earthquake. The MAP solutions are complete and are more accurate than those of other tested algorithms.

High-speed phase- and group-delay scanning with a grating-based phase control delay line

Abstract: A rapid-scanning optical delay line that employs phase control has several advantages, including high speed, high duty cycle, phase- and group-delay independence, and group-velocity dispersion compensation, over existing optical delay methods for interferometric optical ranging applications. We discuss the grating-based phase-control delay line and its applications to interferometric optical ranging and measurement techniques such as optical coherence domain reflectometry and optical coherence tomography. The system performs optical ranging over an axial range of 3 mm with a scanning rate of 6 m/s and a repetition rate of 2 kHz. The device is especially well suited for applications such as optical coherence tomography that require high-speed, repetitive, linear delay line scanning with a high duty cycle.

Exciton rabi oscillation in a single quantum dot

Abstract: A spectroscopic method, which enables characterization of a single isolated quantum dot and a quantum wave function interferometry, is applied to an exciton discrete excited state in an InGaAs quantum dot. Long coherence of zero-dimensional excitonic states made possible the observation of coherent population flopping in a 0D excitonic two-level system in a time-domain interferometric measurement. Corresponding energy splitting is also manifested in an energy-domain measurement.

High-speed optical coherence domain reflectometry

Abstract: We describe a high-speed optical coherence domain reflectometer. Scan speeds of 40 mm/s are achieved with a dynamic range of >90 dB and a spatial resolution of 17 μm. Two applications are presented: the noninvasive measurement of anterior eye structure in a rabbit in vivo and the characterization of reflections and interelement spacing in a multielement lens.

Two-dimensional Fourier transform electronic spectroscopy

Abstract: Two-dimensional Fourier transform electronic spectra of the cyanine dye IR144 in methanol are used to explore new aspects of optical 2D spectroscopy on a femtosecond timescale The experiments reported here are pulse sequence and coherence pathway analogs of the two-dimensional magnetic resonance techniques known as COSY (correlated spectroscopy) and NOESY (nuclear Overhauser effect spectroscopy) Noncollinear three pulse scattering allows selection of electronic coherence pathways by choice of phase matching geometry, temporal pulse order, and Fourier transform variables Signal fields and delays between excitation pulses are measured by spectral interferometry Separate real (absorptive) and imaginary (dispersive) 2D spectra are generated by measuring the signal field at the sample exit, performing a 2D scan that equally weights rephasing and nonrephasing coherence pathways, and phasing the 2D spectra against spectrally resolved pump–probe signals A 3D signal propagation function is used to correct the 2D spectra for excitation pulse propagation and signal pulse generation inside the sample At relaxation times greater than all solvent and vibrational relaxation timescales, the experimental 2D electronic spectra can be predicted from linear spectroscopic measurements without any adjustable parameters The 2D correlation spectra verify recent computational predictions of a negative region above the diagonal, a displacement of the 2D peak off the diagonal, and a narrowing of the 2D cross-width below the vibrational linewidth The negative region arises from 4-level four-wave mixing processes with negative transition dipole products, the displacement off the diagonal arises from a dynamic Stokes shift during signal radiation, and the narrow 2D cross-width indicates femtosecond freezing of vibrational motion

Phase-Coherent Optical Pulse Synthesis from Separate Femtosecond Lasers

Abstract: We generated a coherently synthesized optical pulse from two independent mode-locked femtosecond lasers, providing a route to extend the coherent bandwidth available for ultrafast science. The two separate lasers (one centered at 760 nanometers wavelength, the other at 810 nanometers) are tightly synchronized and phase-locked. Coherence between the two lasers is demonstrated via spectral interferometry and second-order field cross-correlation. Measurements reveal a coherently synthesized pulse that has a temporally narrower second-order autocorrelation width and that exhibits a larger amplitude than the individual laser outputs. This work represents a new and flexible approach to the synthesis of coherent light.

Array-enhanced coherence resonance: nontrivial effects of heterogeneity and spatial independence of noise.

Abstract: We demonstrate the effect of coherence resonance in a heterogeneous array of coupled Fitz Hugh--Nagumo neurons. It is shown that coupling of such elements leads to a significantly stronger coherence compared to that of a single element. We report nontrivial effects of parameter heterogeneity and spatial independence of noise on array-enhanced coherence resonance; especially, we find that (i) the coherence increases as spatial correlation of the noise decreases, and (ii) inhomogeneity in the parameters of the array enhances the coherence. Our results have the implication that generic heterogeneity and background noise can play a constructive role to enhance the time precision of firing in neural systems.

Optical coherence topography based on a two-dimensional smart detector array

Abstract: A low-coherence reflectometer based on a conventional Michelson interferometer and a novel silicon detector chip with a two-dimensional array of pixels that allows parallel heterodyne detection is presented. We demonstrate acquisition of three-dimensional images with more than 100,000 voxels per scan at a sensitivity of -58 dB and a rate of 6 Hz.

Trapped condensates of atoms with dipole interactions

Abstract: We discuss in detail properties of trapped atomic condensates with anisotropic dipole interactions. A practical procedure for constructing anisotropic low-energy pseudopotentials is proposed and justified by the agreement with results of numerical multichannel calculations. The time dependent variational method is adapted to reveal several interesting features observed in numerical solutions of condensate wave function. Collective low-energy shape oscillations and their stability inside electric fields are investigated. Our results shed new light into macroscopic coherence properties of interacting quantum degenerate atomic gases.

Self phase-modulated Kerr-lens mode-locked Cr:forsterite laser source for optical coherence tomography

Abstract: An all-solid-state Kerr-lens mode-locked Cr:forsterite laser operating at 1.28 μm is demonstrated as a short-coherence-length, high-average-power source for optical coherence tomographic (OCT) imaging. We achieve ultrahigh resolution by spectrally broadening the laser pulses, using self-phase modulation in a dispersion-shifted single-mode fiber. OCT imaging with a resolution of 6 μm and a dynamic range of 115 dB is achieved.

Generalized forward–backward initial value representation for the calculation of correlation functions in complex systems

Abstract: The capability of two different, recently proposed semiclassical (SC) forward–backward (FB) initial value representations (IVR) to describe quantum interference and coherence effects is investigated. It is shown that depending on the way the observable under consideration is represented by unitary operators one can obtain rather different results. Although the FB-IVR based on an integral representation as a rule is capable of describing quantum interference, a closer analysis reveals that it depends on the observable under consideration if all interference that can be described semiclassically is actually included in the calculation. To overcome this problem a new, generalized FB-IVR method (GFB-IVR) is proposed, which combines the capability of the SC-IVR to describe quantum interference effects independent of the observable and the better convergence properties of the FB-IVR. The performance of this new approach is studied in some detail. In particular, it is shown that the GFB-IVR can describe both the...

Phase control of group velocity: From subluminal to superluminal light propagation

Abstract: We show that the group velocity of a weak pulse can be manipulated by controlling the phases of two weak optical fields applied to a V-shaped three-level system. Such control can even cause the probe propagation to change from subluminal to superluminal. We consider two schemes: in the first, the excited states are coupled by decay-induced coherence, which is an inherent property of the medium, and in the second, quantum coherence is created by coupling the excited states to each other by a strong microwave field. We also discuss the group velocity reduction experienced by a single weak propagating probe due to decay-induced coherence.

Expansion of a Coherent Array of Bose-Einstein Condensates

Abstract: We investigate the properties of a coherent array containing about 200 Bose-Einstein condensates produced in a far detuned 1D optical lattice. The density profile of the gas, imaged after releasing the trap, provides information about the coherence of the ground-state wave function. The measured atomic distribution is characterized by interference peaks. The time evolution of the peaks, their relative population, as well as the radial size of the expanding cloud are in good agreement with the predictions of theory. The 2D nature of the trapped condensates and the conditions required to observe the effects of coherence are also discussed.

Partial Coherence Effects on the Imaging of Small Crystals using Coherent X-ray Diffraction

Abstract: Recent achievements in experimental and computational methods have opened up the possibility of measuring and inverting the diffraction pattern from a single-crystalline particle on the nanometre scale. In this paper, a theoretical approach to the scattering of purely coherent and partially coherent x-ray radiation by such particles is discussed in detail. Test calculations based on the iterative algorithms proposed initially by Gerchberg and Saxton and generalized by Fienup are applied to reconstruct the shape of the scattering crystals. It is demonstrated that partially coherent radiation produces a small area of high intensity in the reconstructed image of the particle.

Photon Beats from a Single Semiconductor Quantum Dot

Abstract: Single-photon interference is observed on the ultranarrow long-term stable exciton resonance of an individual semiconductor quantum dot. This interference is related to the fine-structure splitting and allows direct conclusions about the coherence properties of the exciton. When selectively addressing a particular dot by quasiresonant phonon-assisted excitation, despite a rapid orientation relaxation on a 1-ps time scale, coherence is partly maintained. No significant further decoherence occurs when the ground state is reached until the exciton recombines radiatively $(\ensuremath{\approx}300\mathrm{ps})$.

Waveguide for Bose-Einstein condensates

Abstract: We report on the creation of Bose-Einstein condensates of ${}^{87}\mathrm{Rb}$ in a specially designed hybrid, optical dipole and magnetic trap. This trap naturally allows the coherent transfer of matter waves into a pure optical dipole potential waveguide based on a doughnut beam. Specifically, we present studies of the coherence of the ensemble in the hybrid trap and during the evolution in the waveguide by means of an autocorrelation interferometer scheme. We observe a mean-field dominated acceleration in the waveguide on a much longer time scale than in the free three-dimensional expansion.

Semiclassical description of quantum coherence effects and their quenching: A forward–backward initial value representation study

Abstract: The forward–backward (FB) version of the semiclassical (SC) initial value representation (IVR) is used to study quantum coherence effects in the time-dependent probability distribution of an anharmonic vibrational coordinate and its quenching when coupled to a thermal bath. It is shown that the FB-IVR accurately reproduces the detailed quantum coherent structure in the weak coupling regime, and also describes how this coherence is quenched with an increase of the system–bath coupling and/or the bath temperature. Comparisons are made with other approximations and the physical implications are discussed.

Coherence imaging by use of a Newton rings sampling function

Abstract: We show that, with suitable optics in the arm of a Michelson interferometer, orthogonal galvo-scanning mirrors build a sampling function in the form of Newton rings when the two interferometer arms are matched. Using a low-coherence source, one can obtain transversal depth-resolved images. A fast display procedure using a storage oscilloscope was devised based on this method.

Thermal vs Quantum Decoherence in Double Well Trapped Bose-Einstein Condensates

Abstract: The quantum and thermal fluctuations of the phase are investigated in a cold Bose gas confined by a double well trap. The coherence of the system is discussed in terms of the visibility of interference fringes in both momentum and coordinate space. The visibility is calculated at zero as well as at finite temperature. The thermal fluctuations are shown to affect significantly the transition from the coherent to the incoherent regime even at very low temperatures. The coherence of an array of multiple condensates is also discussed.

Enhanced depth resolution in terahertz imaging using phase-shift interferometry

Abstract: We describe an imaging technique for few-cycle optical pulses. An object to be imaged is placed at the focus of a lens in one arm of a Michaelson interferometer. This introduces a phase shift of approximately π between the two arms of the interferometer, via the Gouy phase shift. The resulting destructive interference provides a nearly background-free measurement, and a dramatic enhancement in depth resolution. We demonstrate this using single-cycle pulses of terahertz radiation, and show that it is possible to resolve features thinner than 2% of the coherence length of the radiation. This technique could have important applications in low-coherence optical tomographic measurements.

Differential phase measurements in low-coherence interferometry without 2pi ambiguity.

Abstract: Quantitative phase measurements by low-coherence interferometry and optical coherence tomography are restricted by the well-known 2π ambiguity to path-length differences smaller than λ/2. We present a method that overcomes this ambiguity. Introducing a slight dispersion imbalance between reference and sample arms of the interferometer causes the short and long wavelengths of the source spectrum to separate within the interferometric signal. This causes the phase slope to vary within the signal. The phase-difference function between two adjacent sample beam components is calculated by subtraction of their phase functions obtained from phase-sensitive interferometric signal recording. Because of the dispersive effect, the phase difference varies across the interferometric signal. The slope of that phase difference is proportional to the optical path difference, without 2π ambiguity.

Near-Field Coherent Spectroscopy and Microscopy of a Quantum Dot System

Abstract: We combined coherent nonlinear optical spectroscopy with nano-electron volt energy resolution and low-temperature near-field microscopy with subwavelength resolution (

Optics of Nanostructured Materials

Abstract: Optics of Nanostructured Materials covers a selection of advanced research topics that deal with the optical properties of disordered materials including fractals, clusters, nanocomposites, aggregates and semiconductor nanostructures, and those of ordered artificial systems like the photonic band-gap crystals and the photonic crystal fibres. Each chapter of this book is written by leading scientists in their own field. The format is that of a review article, which first places the field in context and then gives a detailed and in-depth description of recent developments in that field. The chapters are well documented with an extensive bibliography, which will make the book valuable for advanced graduate students and researchers who are interested in learning about a given topic and getting an overview of the latest results. The first two chapters deal with photonic crystals with an emphasis on the concept of photonic band-gap and the theoretical methods used to describe the electromagnetic waves in these materials. Recent achievements in this field are described: they include a discussion of defects in artificially made 3D photonic crystals and their contribution to the transmission of the waves when absorption is accounted for. Applications of photonic crystals to waveguides and resonant cavity antennas are described. A separate chapter is devoted to the fabrication of photonic crystal fibres and a description of their intriguing and sometimes counterintuitive waveguiding properties. Two chapters are devoted to near-field optics. The first one gives an exhaustive and comprehensive presentation of both the main principles of this technique with its various configurations and the challenging aspects of the theoretical description of near-field optical phenomena. Several illustrations of these phenomena are described in detail: light confinement by phase conjugation of optical near-field, localization of surface plasmon polaritons by surface roughness and the observation of localized dipolar excitations. The second chapter deals with near-field optics in semiconductor heterostructures and nanostructures. A microscopic theory of the optical properties of semiconductor structures is presented when either detection or excitation (or both) is performed in the near-field. The excitation of a sample through an aperture placed above or on a semiconductor surface is treated in detail to illustrate the specificity of near-field versus far-field optical studies. The use of near-field techniques to study localized excitons in disordered heterostructures and quantum dots is then reviewed with an emphasis on the distortion of the radiative properties of excitons under these observation conditions and on wave packet dynamics of coherently excited excitons. Semiconductor nanostructures are described in two separate chapters reviewing the optical and electronic properties of quantum wires and quantum dots. The chapter devoted to quantum wires places a large emphasis on their optoelectronic properties and their potential device applications. A complete section describes a calculation of third-order nonlinear optical susceptibility resulting from excitonic and biexcitonic contributions. The effect of a magnetic field on the binding energy of both the excitons and the biexcitons is also presented with the details of the calculation. The chapter on semiconductor quantum dots starts with a review of the principal fabrication techniques as many approaches have been used to produce quantum dots with different charcteristics. A section on optical devices describes specific applications where quantum dots present an advantage over conventional semiconductor heterostructures. A final section describes the development of quantum logic gates and the limitations imposed by quantum coherence on their operation. The remaining six chapters of this book cover a classical description of the optical properties of various nanostructures. One chapter deals with the occurrence of light localization in three-dimensional disordered dielectrics. The theoretical model is developed completely and new aspects of the Anderson localization of electromagnetic waves in 3D random dielectric media are unravelled. A chapter on random metal-dielectric films deals with the optical properties of these films and a theoretical understanding of anomalous optical phenomena that are found in the near IR and microwave range of the EM spectrum. Through detailed computer simulations giant fluctuations of the electric and magnetic fields are found near the percolation threshold. A chapter on the optical nonlinearities in metal colloidal solutions reviews various aspects of experimental studies of metal fractal aggregates. A theoretical description of the linear and nonlinear optical properties of disordered clusters and nanocomposites is described in a separate chapter with an emphasis on photoprocesses and their enhancement in clusters. Two chapters are dedicated to the optical properties of fractal smoke or soot aggregates. One emphasizes the geometrical aspects and the structure of soot aggregates that modify the optical properties of these soot particles when they coagulate in large aerosol structures. The other one presents a theoretical approach to the calculation of the optical properties of fractal smoke assuming a simple fractal structure that is far less complex than the structure of soot aggregates found in aerosols. Daniel Oberli

Characterization of the transverse coherence of hard synchrotron radiation by intensity interferometry.

Abstract: The transverse coherence of x rays was measured with an intensity interferometer using a 120-microeV-bandwidth monochromator operating at 14.41 keV. By analyzing the transverse coherence profiles, a vertical source profile of a 25-m long undulator of SPring-8, as well as the coherence degradation by a phase object in the beam path, were quantitatively characterized.

Quantum state of an ideal propagating laser field.

Abstract: We give a quantum information-theoretic description of an ideal propagating cw laser field and reinterpret typical quantum-optical experiments in light of this. In particular, we show that, contrary to recent claims [T. Rudolph and B. C. Sanders, Phys. Rev. Lett. 87, 077903 (2001)], a conventional laser can be used for quantum teleportation with continuous variables and for generating continuous-variable entanglement. Optical coherence is not required, but phase coherence is. We also show that coherent states play a privileged role in the description of laser light.