Using a quantum well heterostructure to study the longitudinal and
transverse electric field components of a strongly focused laser beam
G. Kihara Rurimo,
M. Schardt,
S. Quabis,
S. Malzer,
Ch. Dotzler,
A. Winkler,
G. Leuchs,
G. H. Dohler,
D. Driscoll, et al.
JOURNAL OF APPLIED PHYSICS
100
(2)
023112
(2006)
| Journal
We report a method to measure the electric energy density of longitudinal and transverse electric field components of strongly focused polarized laser beams. We used a quantum well photodetector and exploited the polarization dependent optical transitions of light holes and heavy holes to probe the electric field distribution in the focal region. A comparison of the measured photocurrent spectra for radially and azimuthally polarized beams at the light and heavy hole absorption peaks provides a measure of the amount of the longitudinal electric field component. (c) 2006 American Institute of Physics.
One-way quantum key distribution: Simple upper bound on the secret key
rate
We present a simple method to obtain an upper bound on the achievable secret key rate in quantum key distribution (QKD) protocols that use only unidirectional classical communication during the public-discussion phase. This method is based on a necessary precondition for one-way secret key distillation; the legitimate users need to prove that there exists no quantum state having a symmetric extension that is compatible with the available measurements results. The main advantage of the obtained upper bound is that it can be formulated as a semidefinite program, which can be efficiently solved. We illustrate our results by analyzing two well-known qubit-based QKD protocols: the four-state protocol and the six-state protocol.
Automatic coarse registration of three-dimensional surfaces by
information theoretic selection of salient points
We describe a new method to register surface data measured by optical three-dimensional (3-D) sensors from various views of an object. With our method, complete 3-D models of objects can be generated without user interaction. Circumferential acquisition of 3-D objects is done by taking several views from different directions. To generate a complete 3-D-model, the views must be aligned with each other. This process is called registration and is commonly done interactively by searching for so-called corresponding points in the different views and by use of these points to calculate the appropriate rotation and translation. Our approach is based on automatically finding points that are eye catching or salient compared with other surface points. We derive a quantitative measure of point salience and a feature definition for free-form surfaces by introducing a concept to measure pragmatic information. Experiments confirm that our salient points can be robustly located on general free-form surfaces, even if there are no corners or edges. Furthermore, the neighborhoods of the salient points are highly distinguishable from each other. This results in a large reduction of the complexity of the subsequent geometric matching. The computing time is only a few seconds. We present results from various fields of application. (c) 2006 Optical Society of America
Sum and difference frequency generation as diagnostics for leaky eigenmodes in two-dimensional photonic crystal waveguides
AD Bristow,
JP Mondia,
HM van Driel
JOURNAL OF APPLIED PHYSICS
99
(2)
023105
(2006)
| Journal
We experimentally demonstrate how sum frequency generation (SFG) and difference frequency generation (DFG) of 150 fs pulses can be enhanced and serve as diagnostics for leaky eigenmodes in a two-dimensional GaAs photonic crystal waveguide. SFG at 795 nm is obtained in reflection from s-polarized 1900 nm and p-polarized 1360 nm pulses, with both input beams coupled to leaky eigenmodes; the SFG is enhanced by > 350x compared to that from an untextured GaAs surface. We are able to detect Drude induced subnanometer blueshifts of the SFG, corresponding to refractive index changes of <= 10(-3). DFG of 1360 nm light obtained in reflection from s-polarized 1900 nm and p-polarized 793 nm pulses displays an enhancement of > 500x via three different leaky eigenmodes. As the 793 nm beam polarization is varied from p polarized to right and left circularly polarized, the DFG remains essentially linearly polarized but with a reduced, albeit different, intensity for right and left circularly polarized 795 nm pulses. Futhermore, the plane of polarization also rotates by different amounts for the left and right circularly polarized light, demonstrating interference of the components generated by s- and p-polarized 793 nm pulses. Overall, our results demonstrate how enhanced DFG and SFG from leaky eigenmodes can be used to characterize their properties more precisely than linear optical techniques. (c) 2006 American Institute of Physics.
Radiation modes and roughness loss in high index-contrast waveguides
Christopher G. Poulton,
Christian Koos,
Masafumi Fujii,
Andreas Pfrang,
Thomas Schimmel,
Juerg Leuthold,
Wolfgang Freude
IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS
12
(6)
1306-1321
(2006)
| Journal
We predict the scattering loss in rectangular high index-contrast waveguides, using a new variation of the classical approach of coupled-mode theory. The loss predicted by this three-dimensional (3-D) model is considerably larger than that calculated using previous treatments that approximate the true 3-D radiation modes with their two-dimensional counterparts. The 3-D radiation modes of the ideal waveguide are expanded in a series of cylindrical harmonics, and the coupling between the guided and radiation modes due to the sidewall perturbation is computed. The waveguide attenuation can then be calculated semianalytically. It is found that the dominant loss mechanism is radiation rather than reflection, and that the transverse electric polarization exhibits much larger attenuation than transverse magnetic polarization. The method also gives simple rules that can be used in the design of low-loss optical waveguides. The structural properties of sidewall roughness of an InGaAs/InP pedestal waveguide are measured using atomic force microscopy, and the measured attenuation is found to compare well with that predicted by the model.
Photonic-crystal fibers
Philip St. J. Russell
JOURNAL OF LIGHTWAVE TECHNOLOGY
24
(12)
4729-4749
(2006)
| Journal
The history, fabrication, theory, numerical modeling, optical properties, guidance mechanisms, and applications of photonic-crystal fibers are reviewed.
Entanglement verification for quantum-key-distribution systems with an
underlying bipartite qubit-mode structure
We consider entanglement detection for quantum-key-distribution systems that use two signal states and continuous-variable measurements. This problem can be formulated as a separability problem in a qubit-mode system. To verify entanglement, we introduce an object that combines the covariance matrix of the mode with the density matrix of the qubit. We derive necessary separability criteria for this scenario. These criteria can be readily evaluated using semidefinite programming and we apply them to the specific quantum key distribution protocol.
Experimentally realizable quantum comparison of coherent states and its
applications
When comparing quantum states to each other, it is possible to obtain an unambiguous answer, indicating that the states are definitely different, already after a single measurement. In this paper we investigate comparison of coherent states, which is the simplest example of quantum state comparison for continuous variables. The method we present has a high success probability, and is experimentally feasible to realize as the only required components are beam splitters and photon detectors. An easily realizable method for quantum state comparison could be important for real applications. As examples of such applications we present a "lock and key" scheme and a simple scheme for quantum public key distribution.
Application of frequency combs in the measurement of the refractive
index of air
J. Zhang,
Z. H. Lu,
B. Menegozzi,
L. J. Wang
REVIEW OF SCIENTIFIC INSTRUMENTS
77
(8)
083104
(2006)
| Journal
We report a new method in the precision measurement of the refractive index of air using a highly unbalanced Michelson interferometer with a femtosecond optical frequency comb as the light source. Standard dry air is filled into a 30 m multipass cell, serving as the long arm of the interferometer, while a short arm acts as the reference path. Both time and frequency domain interferograms are recorded to measure the refractive index of air. The deviation of our experimental results with Edlen's formula is 1.4x10(-9) at 800 nm. Our experiment has a standard error of 5.2x10(-9) at fixed parameters (pressure and temperature). This is achieved by putting the multipass cell into a temperature-stabilized box, and also by locking the interferometer path length with a He-Ne laser. We achieved a temperature stabilization of 0.8 mK for 25 h. This corresponds to 0.4 mu m multipass cell length change. The locking of the He-Ne interferometer enables us to achieve 7 nm path-length change outside the multipass cell. Combined with accurate measurement of temperature and pressure, we were able to achieve an accuracy of 7.7x10(-9). (c) 2006 American Institute of Physics.
Raman-like light scattering from acoustic phonons in photonic crystal
fiber
P Dainese,
PSJ Russell,
GS Wiederhecker,
N Joly,
HL Fragnito,
V Laude,
A Khelif
Raman and Brillouin scattering are normally quite distinct processes that take place when light is resonantly scattered by, respectively, optical and acoustic phonons. We show how few-GHz acoustic phonons acquire many of the same characteristics as optical phonons when they are tightly trapped, transversely and close to modal cut-off, inside the wavelength-scale core of an air-glass photonic crystal fiber (PCF). The result is an optical scattering effect that closely resembles Raman scattering, though at much lower frequencies. We use photoacoustic techniques to probe the effect experimentally and finite element modelling to explain the results. We also show by numerical modelling that the cladding structure supports two phononic band gaps that contribute to the confinement of sound in the core. (c) 2006 Optical Society of America
Photonic sensing based on variation of propagation properties of
photonic crystal fibres
John H. Rothwell,
Donal A. Flavin,
William N. MacPherson,
Julian D. C. Jones,
Jonathan C. Knight,
Philip St. J. Russell
We report on a low-coherence interferometric scheme for the measurement of the strain and temperature dependences of group delay and dispersion in short, index-guiding, 'endlessly-single-mode' photonic crystal fibre elements in the 840 nm and 1550 nm regions. Based on the measurements, we propose two schemes for simultaneous strain and temperature measurement using a single unmodified PCF element, without a requirement for any compensating components, and we project the measurement accuracies of these schemes.
Experimental investigation of a modified NOLM for phase-encoded signal
regeneration
K. Cvecek,
G. Onishchukov,
K. Sponsel,
A. G. Striegler,
B. Schmauss,
G. Leuchs
We experimentally investigate the amplitude and phase transfer characteristics of a modified nonlinear optical loop mirror (NOLM) with a directional attenuator (DA-NOLM) optimized for differential phase-shift keying signal regeneration. The results show that the phase relation is preserved in the setup and thus the DA-NOLM is suitable for amplitude regeneration of phase-shift-keyed signals.
Absolute calibration in grazing incidence interferometry via rotational
averaging
Klaus Mantel,
Juergen Lamprecht,
Norbert Lindlein,
Johannes Schwider
Interferometry in grazing incidence can be used to test cylindrical mantle surfaces. The absolute accuracy of the resulting surface profiles is limited by systematic wavefront aberrations caused in the interferometer, in particular due to an inversion of the test wavefront in an interferometer using diffractive beam splitters. For cylindrical specimens, a calibration method using four positions has therefore been investigated. This test is combined with another method of optical metrology: the rotational averaging procedure. The implementation for grazing incidence is described and measurement results for hollow cylinders are presented. The gain in accuracy is demonstrated. (c) 2006 Optical Society of America.
Nonunity gain quantum nondemolition measurements based on measurement
and repreparation
Jessica Schneider,
Oliver Gloeckl,
Gerd Leuchs,
Ulrik L. Andersen
We demonstrate experimentally a nonunity gain quantum nondemolition measurement based on a simple homodyne measurement and recreation strategy. Although the output state is an amplified version of the input state, the device meets standard criteria for QND measurements: the transfer coefficient was measured to 1,78, and the conditional variance was measured to 0.66. (c) 2006 Optical Society of America.
Upper bound on the secret key rate distillable from effective quantum
correlations with imperfect detectors
We provide a simple method to obtain an upper bound on the secret key rate that is particularly suited to analyze practical realizations of quantum key distribution protocols with imperfect devices. We consider the so-called trusted device scenario where Eve cannot modify the actual detection devices employed by Alice and Bob. The upper bound obtained is based on the available measurements results, but it includes the effect of the noise and losses present in the detectors of the legitimate users.
Spin lifetimes and strain-controlled spin precession of drifting
electrons in GaAs
We study the transport of spin-polarized electrons in n-GaAs using spatially resolved continuous-wave Faraday rotation. From the measured steady-state distribution, we determine spin relaxation times under drift conditions and, in the presence of strain, the induced spin splitting from the observed spin precession. Controlled variation of strain along [110] allows us to deduce the deformation potential causing this effect, while strain along [100] has no effect. The electric-field dependence of the spin lifetime is explained quantitatively in terms of an increase of the electron temperature.
FDTD-modelling of dispersive nonlinear ring resonators: Accuracy studies
and experiments
Christian Koos,
Masafumi Fujii,
Christopher G. Poulton,
Ralf Steingrueber,
Juerg Leuthold,
Wolfgang Freude
The accuracy of nonlinear finite-difference time-domain (FDTD) methods is investigated by modeling nonlinear optical interaction in a ring resonator. We have developed a parallelized 3-D FDTD algorithm which incorporates material dispersion, chi((3))-nonlinearities and stair-casing error correction. The results of this implementation are compared and experiments, and intrinsic errors of the FDTD algorithn, Or', from geometrical uncertainties arising from,the falif. icition tolerances of the device. A series of progressively less c0n0ex FDTD models is investigated, omitting material dispersior abandoning the stair-casing error correction, and approximating the structure by a 2-D effective index model. We compare the results of the different algorithms and give guidelines as to which degree of complexity is needed in order to obtain reliable simulation results in the linear and the nonlinear regime. In both cases, incorporating stair-casing error correction and material dispersion into a 2-D effective index model turns out to be computationally much cheaper and more effective than performing a fully three-dimensional simulation without these features.
Efficiency of coherent-state quantum cryptography in the presence of
loss: Influence of realistic error correction
We investigate the performance of a continuous-variable quantum key distribution scheme in a practical setting. More specifically, we take a nonideal error reconciliation procedure into account. The quantum channel connecting the two honest parties is assumed to be lossy but noiseless. Secret key rates are given for the case that the measurement outcomes are postselected or a reverse reconciliation scheme is applied. The reverse reconciliation scheme loses its initial advantage in the practical setting. If one combines postselection with reverse reconciliation, however, much of this advantage can be recovered.
Diamagnetic shift of disorder-localized excitons in narrow GaAs/AlGaAs
quantum wells
M. Erdmann,
C. Ropers,
M. Wenderoth,
R. G. Ulbrich,
S. Malzer,
G. H. Doehler
A correlation between the diamagnetic shift and transition energy of disorder-localized excitons is observed in magnetomicrophotoluminescence (mu PL) on narrow GaAs/Al0.3Ga0.7As quantum wells (QW's). The QW's were grown by molecular-beam epitaxy without growth interruption at the interfaces. mu PL spectra were obtained in a confocal setup with the magnetic field applied normal to the QW plane. The lowest-energy exciton states have the smallest diamagnetic coefficients; the exciton diamagnetic shift in the localized exciton tail of the QW emission spectra increases by a factor of 2 as a function of transition energy. The positive correlation between diamagnetic shift and emission energy reveals exciton localization by short-range correlated interface disorder.
Probing semiconductor gap states with resonant tunneling
S Loth,
M Wenderoth,
L Winking,
RG Ulbrich,
S Malzer,
GH Dohler
Tunneling transport through the depletion layer under a GaAs {110} surface is studied with a low temperature scanning tunneling microscope (STM). The observed negative differential conductivity is due to a resonant enhancement of the tunneling probability through the depletion layer mediated by individual shallow acceptors. The STM experiment probes, for appropriate bias voltages, evanescent states in the GaAs band gap. Energetically and spatially resolved spectra show that the pronounced anisotropic contrast pattern of shallow acceptors occurs exclusively for this specific transport channel. Our findings suggest that the complex band structure causes the observed anisotropies connected with the zinc blende symmetry.
Calibration for cylindrical specimens in grazing-incidence
interferometry via integration of difference measurements
Klaus Mantel,
Juergen Lamprecht,
Norbert Lindlein,
Johannes Schwider
Cylindrical specimens may be tested advantageously by using grazing-incidence interferometry. A multiple positions test in combination with rotational averaging has recently been used to separate the surface deviations of the specimen from the interferometric aberrations. To reduce the measuring time and to check whether the results are reliable, a second procedure is now investigated, which uses the principle of the multiple positions test to determine quantities proportional to the difference quotients of the surface deviations. After numerical integration, the results can be compared with those obtained previously by rotational averaging. The measurement principle is described, and calibration results are presented. (c) 2006 Optical Society of America.
Quasi-absolute measurement of aspheres with a combined diffractive
optical element as reference
Frank Simon,
Gufran Khan,
Klaus Mantel,
Norbert Lindlein,
Johannes Schwider
We have already reported a method for the quasi-absolute test of rotationally symmetric aspheres by means of combined diffractive optical elements (combo-DOEs). The combo-DOEs carry the information for the ideal shape of an aspheric surface under test as well as a spherical wave for the measurement at the cat's eye position. An experimental demonstration of the procedure is given. Measurements with two different designs of combo-DOEs have been conducted, and their relative advantages and disadvantages are discussed. (c) 2006 Optical Society of America.
Bulk and contact-sensitized photocarrier generation in single layer
organic devices
Debdutta Ray,
Meghan P. Patankar,
Gottfried H. Doehler,
K. L. Narasimhan
JOURNAL OF APPLIED PHYSICS
100
(11)
113727
(2006)
| Journal
In this paper, we report on the photoelectronic properties of N,N-'-diphenyl-N,N-'-bis(3-methylphenyl)-(1,1(')-biphenyl)-4,4(')-diamine (TPD) studied in sandwich geometry. In particular, we have obtained from both forward and reverse bias measurements the mu tau product for holes in TPD. mu is the hole mobility and tau is the carrier trapping time. The mu tau product is a measure of the electronic quality of the material and allows a quantitative comparison of different samples. We have carried out numerical simulations to understand the photocurrent in these structures. We show that in reverse bias, the photocurrent (PC) is due to carrier generation in the bulk of the sample. The carrier generation is governed by field assisted exciton dissociation at electric fields greater than 10(6) V/cm. At lower fields the generation of carriers occurs spontaneously in the bulk of the sample. In forward bias, the photocurrent is due to exciton dissociation at the indium tin oxide contact. We also obtain a mu tau product for holes from forward bias PC measurements which is in agreement with the value obtained from reverse bias measurements. Based on our experiments, we demonstrate that TPD in a sandwich structure is a good candidate for cheap large area visible blind ultraviolet detector arrays. (c) 2006 American Institute of Physics.
Reduction of guided acoustic wave Brillouin scattering in photonic
crystal fibers
D. Elser,
U. L. Andersen,
A. Korn,
O. Gloeckl,
S. Lorenz,
Ch. Marquardt,
G. Leuchs
Guided acoustic wave Brillouin scattering (GAWBS) generates phase and polarization noise of light propagating in glass fibers. This excess noise affects the performance of various experiments operating at the quantum noise limit. We experimentally demonstrate the reduction of GAWBS noise in a photonic crystal fiber in a broad frequency range by tailoring the acoustic modes using the photonic also as a phononic crystal. We compare the noise spectrum to the one of a standard fiber and observe a tenfold noise reduction in the frequency range up to 200 MHz. Based on our measurement results as well as on numerical simulations, we establish a model for the reduction of GAWBS noise in photonic crystal fibers.
Stimulated Brillouin scattering from multi-GHz-guided acoustic phonons
in nanostructured photonic crystal fibres
P. Dainese,
P. St. J. Russell,
N. Joly,
J. C. Knight,
G. S. Wiederhecker,
H. L. Fragnito,
V. Laude,
A. Khelif
Wavelength-scale periodic microstructuring dramatically alters the optical properties of materials. An example is glass photonic crystal fibre(1) ( PCF), which guides light by means of a lattice of hollow micro/nanochannels running axially along its length. In this letter, we explore stimulated Brillouin scattering in PCFs with subwavelength-scale solid silica glass cores. The large refractive-index difference between air and glass allows much tighter confinement of light than is possible in all-solid single-mode glass optical fibres made using conventional techniques. When the silica-air PCF has a core diameter of around 70% of the vacuum wavelength of the launched laser light, we find that the spontaneous Brillouin signal develops a highly unusual multi-peaked spectrum with Stokes frequency shifts in the 10-GHz range. We attribute these peaks to several families of guided acoustic modes each with different proportions of longitudinal and shear strain, strongly localized to the core(2,3). At the same time, the threshold power for stimulated Brillouin scattering(4) increases fivefold. The results show that Brillouin scattering is strongly affected by nanoscale microstructuring, opening new opportunities for controlling light-sound interactions in optical fibres.
Spectrally smooth supercontinuum from 350 nm to 3 mu m in sub-centimeter
lengths of soft-glass photonic crystal fibers.
FG Omenetto,
NA Wolchover,
MR Wehner,
M Ross,
A Efimov,
AJ Taylor,
VVRK Kumar,
AK George,
JC Knight, et al.
The conversion of light fields in photonic crystal fibers ( PCFs) capitalizes on the dramatic enhancement of several optical nonlinearities. We present here spectrally smooth, highly broadband supercontinuum radiation in a short piece of high-nonlinearity soft-glass PCF. This supercontinuum spans several optical octaves, with a spectral range extending from 350 nm to beyond 3000 nm. The selection of an appropriate propagation-length determines the spectral quality of the supercontinuum generated. Experimentally, we clearly identify two regimes of nonlinear pulse transformation: when the fiber length is much shorter than the dispersion length, soliton propagation is not important and a symmetric supercontinuum spectrum arises from almost pure self-phase modulation. For longer fiber lengths the supercontinuum is formed by the breakup of multiple Raman-shifting solitons. In both regions very broad supercontinuum radiation is produced. (c) 2006 Optical Society of America.
Non-reciprocal transmission and Schmitt trigger operation in strongly
modulated asymmetric WBGs
Masafumi Fujii,
Ayan Maitra,
Christopher Poulton,
Juerg Leuthold,
Wolfgang Freude
We investigate numerically a non-reciprocal switching behavior in strongly modulated waveguide Bragg gratings (WBGs) having a longitudinally asymmetric stopband configuration. The minimum power predicted for a stable switching operation is found to be approximately 77 mW for a realistic waveguide structure made of prospective materials; we assume in this paper a nano-strip InGaAsP/InP waveguide having longitudinally asymmetric modulation of the waveguide width. The analysis has been performed with our in-house nonlinear finite-difference time-domain (FDTD) code adapted to parallel computing. The numerical results clearly show low-threshold Schmitt trigger operation, as well as non-reciprocal transmission property where the switching threshold for one propagation direction is lower than that for the other direction. In addition, we discuss the modulation-like instability phenomena in such nonlinear periodic devices by employing both an instantaneous Kerr nonlinearity and a more involved saturable nonlinearity model. (C) 2006 Optical Society of America.
Verifying continuous-variable entanglement of intense light pulses
Three different methods have been discussed to verify continuous variable entanglement of intense light beams. We demonstrate all three methods using the same setup to facilitate the comparison. The nonlinearity used to generate entanglement is the Kerr effect in optical fibers. Due to the brightness of the entangled pulses, standard homodyne detection is not an appropriate tool for the verification. However, we show that by using large asymmetric interferometers on each beam individually, two noncommuting variables can be accessed and the presence of entanglement verified via joint measurements on the two beams. Alternatively, we witness entanglement by combining the two beams on a beam splitter that yields certain linear combinations of quadrature amplitudes which suffice to prove the presence of entanglement.
Entanglement detection typically relies on linear inequalities for mean values of certain observables (entanglement witnesses), where violation indicates entanglement. We provide a general method to improve any of these inequalities for bipartite systems via nonlinear expressions. The nonlinearities are of different orders and can be directly measured in experiments, often without any extra effort.
Atomic beam focusing with a curved magnetic mirror
H. Merimeche
JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS
39
(18)
3723-3731
(2006)
| Journal
Reflective atom optics has been experimentally realized with a continuous flux of cold atoms by using a magnetized videotape. We have studied the behaviour of a slow atomic beam with a velocity of 20 m s(-1) incident at different angles on the surface of a curved magnetic mirror. Focusing of the beam has been demonstrated and this induces an amplification up to a factor 3 of the fluorescence emitted by the atoms around the focal point. We deduced the shape of our reflector from the focal length measurements and show that the result is completely consistent with those obtained from the reflection of a red diode laser beam on the curved mirror.
Coherent-state information concentration and purification in atomic
memory
We propose a feasible method of coherent-state information concentration and purification utilizing quantum memory. The method allows us to optimally concentrate and purify information carried by many noisy copies of an unknown coherent state (randomly distributed in time) to a single copy. Thus nonclassical resources and operations can be saved, if we compare information processing with many noisy copies and a single copy with concentrated and purified information.
Distillation of squeezing from non-Gaussian quantum states
J. Heersink,
Ch. Marquardt,
R. Dong,
R. Filip,
S. Lorenz,
G. Leuchs,
U. L. Andersen
We show that single copy distillation of squeezing from continuous variable non-Gaussian states is possible using linear optics and conditional homodyne detection. A specific non-Gaussian noise source, corresponding to a random linear displacement, is investigated experimentally. Conditioning the signal on a tap measurement, we observe probabilistic recovery of squeezing.
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