Showing papers on "Phase (waves) published in 2014"

Volta potential phase plate for in-focus phase contrast transmission electron microscopy

Abstract: We describe a phase plate for transmission electron microscopy taking advantage of a hitherto-unknown phenomenon, namely a beam-induced Volta potential on the surface of a continuous thin film. The Volta potential is negative, indicating that it is not caused by beam-induced electrostatic charging. The film must be heated to ∼200 °C to prevent contamination and enable the Volta potential effect. The phase shift is created “on the fly” by the central diffraction beam eliminating the need for precise phase plate alignment. Images acquired with the Volta phase plate (VPP) show higher contrast and unlike Zernike phase plate images no fringing artifacts. Following installation into the microscope, the VPP has an initial settling time of about a week after which the phase shift behavior becomes stable. The VPP has a long service life and has been used for more than 6 mo without noticeable degradation in performance. The mechanism underlying the VPP is the same as the one responsible for the degradation over time of the performance of thin-film Zernike phase plates, but in the VPP it is used in a constructive way. The exact physics and/or chemistry behind the process causing the Volta potential are not fully understood, but experimental evidence suggests that radiation-induced surface modification combined with a chemical equilibrium between the surface and residual gases in the vacuum play an important role.

Non-reciprocal phase shift induced by an effective magnetic flux for light

Abstract: Photons are neutral particles that do not interact directly with a magnetic field. However, recent theoretical work has shown that an effective magnetic field for photons can exist if the phase of light changes with its direction of propagation. This direction-dependent phase indicates the presence of an effective magnetic field, as shown experimentally for electrons in the Aharonov–Bohm experiment. Here, we replicate this experiment using photons. To create this effective magnetic field we construct an on-chip silicon-based Ramsey-type interferometer. This interferometer has been traditionally used to probe the phase of atomic states and here we apply it to probe the phase of photonic states. We experimentally observe an effective magnetic flux between 0 and 2π corresponding to a non-reciprocal 2π phase shift with an interferometer length of 8.35 mm and an interference-fringe extinction ratio of 2.4 dB. This non-reciprocal phase is comparable to those of common monolithically integrated magneto-optical materials.

Singular-phase nanooptics: towards label-free single molecule detection

Abstract: Non-trivial topology of phase is crucial for many important physics phenomena such as, for example, the Aharonov-Bohm effect 1 and the Berry phase 2. Light phase allows one to create "twisted" photons 3, 4 , vortex knots 5, dislocations 6 which has led to an emerging field of singular optics relying on abrupt phase changes 7. Here we demonstrate the feasibility of singular visible-light nanooptics which exploits the benefits of both plasmonic field enhancement and non-trivial topology of light phase. We show that properly designed plasmonic nanomaterials exhibit topologically protected singular phase behaviour which can be employed to radically improve sensitivity of detectors based on plasmon resonances. By using reversible hydrogenation of graphene 8 and a streptavidin-biotin test 9, we demonstrate areal mass sensitivity at a level of femto-grams per mm2 and detection of individual biomolecules, respectively. Our proof-of-concept results offer a way towards simple and scalable single-molecular label-free biosensing technologies.

A time-dependent order parameter for ultrafast photoinduced phase transitions

Abstract: Strongly correlated electron systems often exhibit very strong interactions between structural and electronic degrees of freedom that lead to complex and interesting phase diagrams. For technological applications of these materials it is important to learn how to drive transitions from one phase to another. A key question here is the ultimate speed of such phase transitions, and to understand how a phase transition evolves in the time domain. Here we apply time-resolved X-ray diffraction to directly measure the changes in long-range order during ultrafast melting of the charge and orbitally ordered phase in a perovskite manganite. We find that although the actual change in crystal symmetry associated with this transition occurs over different timescales characteristic of the many electronic and vibrational coordinates of the system, the dynamics of the phase transformation can be well described using a single time-dependent 'order parameter' that depends exclusively on the electronic excitation.

Controllable transmission and total reflection through an impedance-matched acoustic metasurface

Abstract: A general design paradigm for a novel type of acoustic metasurface is proposed by introducing periodically repeated supercells on a rigid thin plate, where each supercell contains multiple cut-through slits that are filled with materials possessing different refractive indices but the same impedance as that of the host medium. When the wavelength of the incident wave is smaller than the periodicity, the direction of the transmitted wave with nearly unity transmittance can be chosen by engineering the phase discontinuities along the transverse direction. When the wavelength is larger than the periodicity, even though the metasurface is impedance matched to the host medium, most of the incident energy is reflected back and the remaining portion is converted into a surface-bound mode. We show that both the transmitted wave control and the high reflection with the surface mode excitation can be interpreted by a unified analytic model based on mode-coupling theory. Our general design principle not only supplies the functionalities of reflection-type acoustic metasurfaces, but also exhibits unprecedented flexibility and efficiency in various domains of wave manipulation for possible applications in fields like refracting, collimating, focusing or absorbing wave energy.

Optical magnetic mirrors without metals

Abstract: The reflection of an optical wave from metal, arising from strong interactions between the optical electric field and the free carriers of the metal, is accompanied by a phase reversal of the reflected electric field. A far less common route to achieving high reflectivity exploits strong interactions between the material and the optical magnetic field to produce a “magnetic mirror” that does not reverse the phase of the reflected electric field. At optical frequencies, the magnetic properties required for strong interaction can be achieved only by using artificially tailored materials. Here, we experimentally demonstrate, for the first time to the best of our knowledge, the magnetic mirror behavior of a low-loss all-dielectric metasurface at infrared optical frequencies through direct measurements of the phase and amplitude of the reflected optical wave. The enhanced absorption and emission of transverse-electric dipoles placed close to magnetic mirrors can lead to exciting new advances in sensors, photodetectors, and light sources.

Joint estimation of phase and phase diffusion for quantum metrology.

Abstract: Phase estimation, at the heart of many quantum metrology and communication schemes, can be strongly affected by noise, whose amplitude may not be known, or might be subject to drift. Here we investigate the joint estimation of a phase shift and the amplitude of phase diffusion at the quantum limit. For several relevant instances, this multiparameter estimation problem can be effectively reshaped as a two-dimensional Hilbert space model, encompassing the description of an interferometer phase probed with relevant quantum states--split single-photons, coherent states or N00N states. For these cases, we obtain a trade-off bound on the statistical variances for the joint estimation of phase and phase diffusion, as well as optimum measurement schemes. We use this bound to quantify the effectiveness of an actual experimental set-up for joint parameter estimation for polarimetry. We conclude by discussing the form of the trade-off relations for more general states and measurements.

Optical Magnetic Mirrors without Metals

Abstract: The reflection of an optical wave from a metal, arising from strong interactions between the optical electric field and the free carriers of the metal, is accompanied by a phase reversal of the reflected electric field. A far less common route to achieve high reflectivity exploits strong interactions between the material and the optical magnetic field to produce a magnetic mirror which does not reverse the phase of the reflected electric field. At optical frequencies, the magnetic properties required for strong interaction can only be achieved through the use of artificially tailored materials. Here we experimentally demonstrate, for the first time, the magnetic mirror behavior of a low-loss, all-dielectric metasurface at infrared optical frequencies through direct measurements of the phase and amplitude of the reflected optical wave. The enhanced absorption and emission of transverse electric dipoles placed very close to magnetic mirrors can lead to exciting new advances in sensors, photodetectors, and light sources.

Optical Huygens' Metasurfaces with Independent Control of the Magnitude and Phase of the Local Reflection Coefficients

Abstract: Implementation of abrupt phase discontinuities along a surface has been the theme of recent research on electromagnetic metasurfaces. Simple functionalities such as reflecting, refracting, or focusing plane waves have been demonstrated with devices featuring phase discontinuities, but optical surfaces allowing independent magnitude and phase control on the scattered waves have yet to emerge. In this paper, we propose the first true optical Huygens’ surface, which explicitly utilizes orthogonal electric and magnetic responses to realize total control on an optical surface’s local reflection coefficients. This extends the functionality of metasurfaces to an unprecedented level. We first demonstrate that a nanorod gap-surface plasmon resonator can act as a Huygens’ source. Thereafter, by properly tuning and rotating these resonators, we realize arbitrary reflection optical metasurfaces—surfaces for which the local reflection coefficients can be independently tailored in both magnitude and phase. We demonstrate the versatility of this approach through designs of a metasurface that asymmetrically reflects two copolarized beams and a Dolph-Tschebyscheff optical reflectarray.

Encoding complex fields by using a phase-only optical element

Abstract: We show that the amplitude and phase information from a two-dimensional complex field can be synthesized from a phase-only optical element with micrometric resolution. The principle of the method is based on the combination of two spatially sampled phase elements by using a low-pass filter at the Fourier plane of a 4-f optical system. The proposed encoding technique was theoretically demonstrated, as well as experimentally validated with the help of a phase-only spatial light modulator for phase encoding, a conventional CMOS camera to measure the amplitude of the complex field, and a Shack-Hartmann wavefront sensor to determine its phase.

Transport of Intensity phase imaging by intensity spectrum fitting of exponentially spaced defocus planes

Abstract: We propose an alternative method for solving the Transport of Intensity equation (TIE) from a stack of through-focus intensity images taken by a microscope or lensless imager. Our method enables quantitative phase and amplitude imaging with improved accuracy and reduced data capture, while also being computationally efficient and robust to noise. We use prior knowledge of how intensity varies with propagation in the spatial frequency domain in order to constrain a fitting algorithm [Gaussian process (GP) regression] for estimating the axial intensity derivative. Solving the problem in the frequency domain inspires an efficient measurement scheme which captures images at exponentially spaced focal steps, significantly reducing the number of images required. Low-frequency artifacts that plague traditional TIE methods can be suppressed without an excessive number of captured images. We validate our technique experimentally by recovering the phase of human cheek cells in a brightfield microscope.

A modular spiral phase plate design for orbital angular momentum generation at millimetre wavelengths

Abstract: Proof of concept measurements of a modular spiral phase plate design able to generate millimetre wavelength beams with an azimuthal mode number of l = ±10 are presented. The plate is comprised of ten single modules that interlock to create the full plate assembly, allowing improved machining accuracy compared to standard techniques. Therefore, this design could be used in millimetre wavelength systems that require the manipulation of large OAM modes. The plate was manufactured from polypropylene (index of refraction n ≈ 1.5), and was measured at 100GHz. A three dimensional field scanner was used to measure three near field surfaces behind the plate. Intensity measurements showed the expected OAM intensity ring, and phase measurements showed ten phase dislocations, implying proper functionality.

Interferometric probes of many-body localization.

Abstract: We propose a method for detecting many-body localization (MBL) in disordered spin systems. The method involves pulsed coherent spin manipulations that probe the dephasing of a given spin due to its entanglement with a set of distant spins. It allows one to distinguish the MBL phase from a noninteracting localized phase and a delocalized phase. In particular, we show that for a properly chosen pulse sequence the MBL phase exhibits a characteristic power-law decay reflecting its slow growth of entanglement. We find that this power-law decay is robust with respect to thermal and disorder averaging, provide numerical simulations supporting our results, and discuss possible experimental realizations in solid-state and cold-atom systems.

New chirp sequence radar waveform

Abstract: The general requirement in the automotive radar application is to measure the target range R and radial velocity v r simultaneously and unambiguously with high accuracy and resolution even in multitarget situations, which is a matter of the appropriate waveform design. Based on a single continuous wave chirp transmit signal, target range R and radial velocity v r cannot be measured in an unambiguous way. Therefore a so-called multiple frequency shift keying (MFSK) transmit signal was developed, which is applied to measure target range and radial velocity separately and simultaneously. In this case the radar measurement is based on a frequency and additionally on a phase measurement, which suffers from a lower estimation accuracy compared with a pure frequency measurement. This MFSK waveform can therefore be improved and outperformed by a chirp sequences waveform. Each chirp signal has in this case very short time duration T chirp . Therefore the measured beat frequency f B is dominated by target range R and is less influenced by the radial velocity v r . The range and radial velocity estimation is based on two separate frequency measurements with high accuracy in both cases. Classical chirp sequence waveforms suffer from possible ambiguities in the velocity measurement. It is the objective of this paper to modify the classical chirp sequence to get an unambiguous velocity measurement even in multitarget situations.

Design Method for Circularly Polarized Fabry–Perot Cavity Antennas

Abstract: A new class of circularly polarized (CP) Fabry-Perot cavity antennas is introduced that maintain the simplicity of a linearly polarized primary feed and a single cavity structure. The proposed antennas employ a double-sided partially reflective surface (PRS), which allows independent control of the magnitude and phase responses for the reflection and transmission coefficients. In conjunction with an anisotropic high-impedance surface (HIS) ground plane, this arrangement allows for the first time a single cavity antenna to produce a specified gain in CP from a linearly polarized primary source. A design procedure for this class of antennas is introduced. The method exploits a simple ray optics model to calculate the magnitude and phase of the electric field in the cavity upon plane wave excitation. Based on this model, analytical expressions are derived, which enforce the resonance condition for both polarizations at a predetermined PRS reflectivity (and hence predetermined antenna gain) together with a 90 ° differential phase between them. The validity of the concept is confirmed by means of an example entailing an antenna with gain of approximately 21 dB at 15 GHz. Full-wave simulation results and experimental testing on a fabricated prototype are presented and agree well with the theoretical predictions.

Anomalous reflection phase of graphene plasmons and its influence on resonators

Abstract: The phase picked up by a graphene plasmon upon scattering by an abrupt edge is commonly assumed to be $\ensuremath{-}\ensuremath{\pi}$. Here, it is demonstrated that for high plasmon momenta this reflection phase is $\ensuremath{\approx}$$\ensuremath{-}3\ensuremath{\pi}/4$, virtually independent on either chemical potential, wavelength, or dielectric substrate. This nontrivial phase arises from a complex excitation of highly evanescent modes close to the edge, which are required to satisfy the continuity of electric and magnetic fields. A similar result for the reflection phase is expected for other two-dimensional systems supporting highly confined plasmons (very thin metal films, topological insulators, transition polaritonic layers, etc.). The knowledge of the reflection phase, combined with the phase picked up by the plasmon upon propagation, allows for the estimation of resonator properties from the dispersion relation of plasmons in the infinite monolayer.

Limitations to the determination of a Laguerre–Gauss spectrum via projective, phase-flattening measurement

Abstract: One of the most widely used techniques for measuring the orbital angular momentum (OAM) components of a light beam is to flatten the spiral phase front of a mode, in order to couple it to a single-mode optical fiber (SMOF). This method, however, suffers from an efficiency that depends on the OAM of the initial mode and on the presence of higher-order radial modes. The reason is that once the phase has been flattened, the field retains its ringed intensity pattern and is therefore a nontrivial superposition of purely radial modes, of which only the fundamental one couples to a SMOF. In this paper, we study the efficiency of this technique both theoretically and experimentally. We find that even for low values of the OAM, a large amount of light can fall outside the fundamental mode of the fiber, and we quantify the losses as functions of the waist of the coupling beam of the OAM and radial indices. Our results can be used as a tool to remove the efficiency bias where fair-sampling loopholes are not a concern. However, we hope that our study will encourage the development of better detection methods of the OAM content of a beam of light.

Ge-on-Si and Ge-on-SOI thermo-optic phase shifters for the mid-infrared

Abstract: Germanium-on-silicon thermo-optic phase shifters are demonstrated in the 5 μm wavelength range. Basic phase shifters require 700 mW of power for a 2π phase shift. The required power is brought down to 80 mW by complete undercut using focused ion beam. Finally an efficient thermo-optic phase shifter is demonstrated on the germanium on SOI platform. A tuning power (for a 2π phase shift) of 105 mW is achieved for a Ge-on-SOI structure which is lowered to 16 mW for a free standing phase shifter.

Stretched cell cycle model for proliferating lymphocytes

Abstract: Stochastic variation in cell cycle time is a consistent feature of otherwise similar cells within a growing population. Classic studies concluded that the bulk of the variation occurs in the G1 phase, and many mathematical models assume a constant time for traversing the S/G2/M phases. By direct observation of transgenic fluorescent fusion proteins that report the onset of S phase, we establish that dividing B and T lymphocytes spend a near-fixed proportion of total division time in S/G2/M phases, and this proportion is correlated between sibling cells. This result is inconsistent with models that assume independent times for consecutive phases. Instead, we propose a stretching model for dividing lymphocytes where all parts of the cell cycle are proportional to total division time. Data fitting based on a stretched cell cycle model can significantly improve estimates of cell cycle parameters drawn from DNA labeling data used to monitor immune cell dynamics.

The Type IIP Supernova 2012aw in M95: hydrodynamical modelling of the photospheric phase from accurate spectrophotometric monitoring

Abstract: We present an extensive optical and near-infrared photometric and spectroscopic campaign of the type IIP supernova SN 2012aw. The dataset densely covers the evolution of SN 2012aw shortly after the explosion up to the end of the photospheric phase, with two additional photometric observations collected during the nebular phase, to fit the radioactive tail and estimate the $^{56}$Ni mass. Also included in our analysis is the already published \textit{Swift} UV data, therefore providing a complete view of the ultraviolet-optical-infrared evolution of the photospheric phase. On the basis of our dataset, we estimate all the relevant physical parameters of SN 2012aw with our radiation-hydrodynamics code: envelope mass $M_{env} \sim 20 M_\odot$, progenitor radius $R \sim 3 \times 10^{13}$ cm ($ \sim 430 R_\odot$), explosion energy $E \sim 1.5$ foe, and initial $^{56}$Ni mass $\sim 0.06$ $M_\odot$. These mass and radius values are reasonably well supported by independent evolutionary models of the progenitor, and may suggest a progenitor mass higher than the observational limit of $16.5 \pm 1.5 M_\odot$ of the Type IIP events.

Generation of high-order Laguerre–Gaussian modes by means of spiral phase plates

Abstract: Spiral phase plates for the generation of Laguerre-Gaussian (LG) beam with non-null radial index were designed and fabricated by electron beam lithography on polymethylmethacrylate over glass substrates. The optical response of these phase optical elements was theoretically considered and experimentally measured, and the purity of the experimental beams was investigated in terms of LG modes contributions. The far-field intensity pattern was compared with theoretical models and numerical simulations, whereas interferometric analyses confirmed the expected phase features of the generated beams. The high quality of the output beams confirms the applicability of these phase plates for the generation of high-order LG beams.

Design and evaluation of GaN-based dual-phase interleaved MHz critical mode PFC converter

Abstract: This paper presents the design consideration and performance evaluation of gallium nitride (GaN) high electron mobility transistor (HEMT) based dual-phase interleaved MHz critical conduction mode (CRM) power factor correction (PFC) converter. A 1.2kW 1-3MHz interleaved boost PFC converter prototype is built with 97.9% peak efficiency and 120W/in3 power density. The significant impact of MHz frequency is demonstrated as dramatically size reduction of boost inductor and electro-magnetic interference (EMI) filter. Several inductor designs are discussed. The corner frequency of EMI filter is pushed to several hundreds of kHz. Finally, the limitation of conventional boost PFC converter is discussed as high conduction loss on diode rectifier bridge and high switching loss caused by valley switching, which is negligible in other low frequency PFC converter but significant in MHz PFC converter. The totem-pole bridgeless PFC converter is introduced to further improve the efficiency with no rectifier bridge and zero-voltage switching (ZVS) extension strategy.

Complex differential variance algorithm for optical coherence tomography angiography.

Abstract: We describe a complex differential variance (CDV) algorithm for optical coherence tomography based angiography. The algorithm exploits both the intensity and phase changes of optical coherence tomography (OCT) signals from flowing blood to achieve high vascular contrast, and also intrinsically reject undesirable phase signals originating from small displacement axial bulk tissue motion and instrument synchronization errors. We present this algorithm within a broader discussion of the properties of OCT signal dynamics. The performance of the algorithm is compared against two other existing algorithms using both phantom measurements and in vivo data. We show that the algorithm provides better contrast for a given number of measurements and equivalent spatial averaging.

Phase regeneration of DPSK signals in a silicon waveguide with reverse-biased p-i-n junction.

Abstract: Phase regeneration of differential phase-shift keying (DPSK) signals is demonstrated using a silicon waveguide as nonlinear medium for the first time. A p-i-n junction across the waveguide enables decreasing the nonlinear losses introduced by free-carrier absorption (FCA), thus allowing phase-sensitive extinction ratios as high as 20 dB to be reached under continuous-wave (CW) pumping operation. Furthermore the regeneration properties are investigated under dynamic operation for a 10-Gb/s DPSK signal degraded by phase noise, showing receiver sensitivity improvements above 14 dB. Different phase noise frequencies and amplitudes are examined, resulting in an improvement of the performance of the regenerated signal in all the considered cases.

Love number can be hard to measure

Abstract: The waveform phase for a neutron star binary can be split into point-particle terms and finite-size terms (characterized by the Love number) that account for equation of state effects. The latter first enter at 5 post-Newtonian (PN) order (i.e. proportional to the tenth power of the orbital velocity), but the former are only known completely to 3.5 PN order, with higher order terms only known to leading-order in the mass-ratio. We here find that not including point-particle terms at 4PN order to leading- and first-order in the mass ratio in the template model can severely deteriorate our ability to measure the equation of state. This problem can be solved if one uses numerical waveforms once their own systematic errors are under control.

Hybridized resonances to design tunable binary phase metasurface unit cells

Abstract: The recent concept of metasurfaces is a powerful tool to shape waves by governing precisely the phase response of each constituting element through its resonance properties. While most efforts are devoted to realize reconfigurable metasurfaces that allow such complete phase control, for many applications a binary one is sufficient. Here, we propose and demonstrate through experiments and simulations a binary state tunable phase reflector based on the concept of hybridized resonators as unit cell for a possible metasurface. The concept presents the great advantages to be very general, scalable to all frequency domains and above all very robust to fluctuations induced by the tunable mechanism, as we prove it at microwave frequencies using electronically tunable patch reflectors.

Input Modeling with Phase-Type Distributions and Markov Models

Abstract: Containing a summary of several recent results on Markov-based input modeling in a coherent notation, this book introduces and compares algorithms for parameter fitting and gives an overview of available software tools in the area. Due to progress made in recent years with respect to new algorithms to generate PH distributions and Markovian arrival processes from measured data, the models outlined are useful alternatives to other distributions or stochastic processes used for input modeling. Graduate students and researchers in applied probability, operations research and computer science along with practitioners using simulation or analytical models for performance analysis and capacity planning will find the unified notation and up-to-date results presented useful. Input modeling is the key step in model based system analysis to adequately describe the load of a system using stochastic models. The goal of input modeling is to find a stochastic model to describe a sequence ofmeasurements from a real system to model for example the inter-arrival times of packets in a computer network or failure times of components in a manufacturing plant. Typical application areas are performance and dependability analysis of computer systems, communication networks, logistics or manufacturing systems but also the analysis of biological or chemical reaction networks and similar problems. Often the measured values have a high variability and are correlated. Its been known for a long time that Markov based models like phase type distributions or Markovian arrival processes are very general and allow one to capture even complex behaviors. However, the parameterization of these models results often in a complex and non-linear optimization problem. Only recently, several new results about the modeling capabilities of Markov based models and algorithms to fit the parameters of those models have been published.

Tailoring Reflections From Thin Composite Metamirrors

Abstract: We propose an effective route to fully control the phase of plane waves reflected from electrically (optically) thin composite sheets. This becomes possible using engineered artificial full-reflection layers (metamirrors) formed by arrays of electrically small resonant bi-anisotropic particles. In this scenario, fully reflecting mirrors do not contain any continuous ground plane, but only arrays of small particles. Bi-anisotropic omega coupling is required to get asymmetric response in reflection phase for plane waves incident from the opposite sides of the composite mirror. It is shown that with this concept one can independently tailor the phase of electromagnetic waves reflected from both sides of the mirror array.

Broadband nanoelectromechanical phase shifting of light on a chip

Abstract: We demonstrate an optomechanical phase shifter. By electrostatically deflecting the nanofabricated mechanical structure, the effective index of a nearby waveguide is changed and the resulting phase shift is measured using an integrated Mach-Zehnder interferometer. Comparing to thermo-optical phase shifters, our device does not consume power in static operation and also it can operate over large frequency, wavelength, and power ranges. Operation in the MHz range and sub-μs pulses is demonstrated.