Large Spectral Bandwidth Research Articles

A seismic passive imaging step beyond SPAC and ReMi

The basic property of passive imaging is that, given any two points, one of them can be taken as the source of the waves and the other as the recording station. This property can be derived from the statistical self-alignment of the rays along the vector joining the two points, and applies also to nondiffuse wavefields like seismic tremor. It provides a statistical basis for the use of the stationary phase integral, allowing passive interferometry under the mild constraint of mechanical homogeneity at a local scale. Combined with the tremor’s large spectral bandwidth, it allows one to recover the local Green’s function from spatial correlation. Furthermore, combining this property also with the azimuthal isotropy of either the wavefield or the array, and using the statistical mode as the estimator, provides a new technique to measure the local velocity dispersion in the subsoil. This technique exploits the potential of spatial autocorrelation (SPAC) and refraction microtremor (ReMi), allowing one (1) to use sparse small-aperture arrays with simple geometry, (2) to dispense with the fitting of Bessel functions, and (3) to measure, in a few minutes, the local (phase and group) wave velocity as a function of frequency of potentially all the wave-propagation modes — body and surface — and not just of the one prevailing at each frequency.

Growth and spectral analysis of stacked quantum dots for broadband superluminescent diodes

Stacks of different quantum dot (QD) structures were adopted and characterized for broadband superluminescent diodes (SLDs). Each QD structure in the stacked QD structures exhibited a unique peak in the photoluminescence (PL) spectrum, showing ground-state peaks of ∼1272, ∼1185 and ∼1090 nm for InGaAs-capped InAs QDs, InAs QDs, and InGaAs QDs, respectively. Two SLD structures, one with the stack of InGaAs-capped InAs QDs, InAs QDs, and InGaAs QDs and the other with the stack of InGaAs-capped InAs QDs and InAs QDs, were grown and fabricated into devices. The SLDs with a stack of three different QD structures and with a stack of two different QD structures showed spectral bandwidths of 173.6 nm and 188.2 nm, respectively, in the electroluminescence (EL) measurements. The larger spectral bandwidth of the SLD with three different QD structures is thought to be due to the addition of the short-wavelength-emitting InGaAs QD structure. We analyzed the sub-peaks in the EL spectra of the SLD structures and suggested the stacking of QD structures to get broadband SLDs.

Optimization of Quantum-Dot Molecular Beam Epitaxy for Broad Spectral Bandwidth Devices

The optimization of the key growth parameters for broad spectral bandwidth devices based on quantum dots is reported. A combination of atomic force microscopy, photoluminescence of test samples, and optoelectronic characterization of superluminescent diodes (SLDs) is used to optimize the growth conditions to obtain high-quality devices with large spectral bandwidth, radiative efficiency (due to a reduced defective-dot density), and thus output power. The defective-dot density is highlighted as being responsible for the degradation of device performance. An SLD device with 160 nm of bandwidth centered at 1230 nm is demonstrated.

Selective excitation of resonance-enhanced multiphoton-ionization photoelectron spectroscopy via a cubic phase modulation

Resonantly enhanced multiphoton ionization photoelectron spectroscopy (REMPI-PS) has proven to be a well-established tool to study the excited-state structure. However, femtosecond-induced REMPI-PS suffers from poor selectivity between neighboring excited states due to the large spectral bandwidth. In this paper, we present a feasible scheme to realize the selective excitation of (2+1) REMPI-PS in sodium atoms. We show that, by shaping the femtosecond laser pulse with a cubic phase modulation, a high-resolution (2+1) REMPI-PS is obtained, and the (2+1) REMPI-PS signal from one excited state is enhanced while that from the other excited state is effectively suppressed. Furthermore, we explain the physical control mechanism of the selective excitation of (2+1) REMPI-PS by considering the two-photon power spectrum.

Achieving high-resolution photoelectron spectroscopy from a broadband femtosecond laser pulse

Femtosecond-induced resonance-enhanced multiphoton-ionization photoelectron spectroscopy (REMPI-PS) suffers from poor spectral resolution due to the large laser spectral bandwidth. In this paper, we present a scheme to achieve high-resolution REMPI-PS in the potassium atom by shaping the ultrashort femtosecond laser pulse. Our results show that the REMPI-PS bandwidth can be narrowed by several tens of times with a cubic spectral phase modulation, and thus high-resolution REMPI-PS and the fine structure of the energy-level diagram for multiple excited states can be obtained in spite of the broadband femtosecond laser pulse. Furthermore, we explain the physical control mechanism of the narrowband REMPI-PS by considering the two-photon power spectrum.

High-power, Yb-fiber-laser-pumped, picosecond parametric source tunable across 752–860 nm

We report a stable, high-power source of picosecond pulses in the near-infrared based on intracavity second harmonic generation (SHG) of a MgO:PPLN optical parametric oscillator synchronously pumped at 81 MHz by a mode-locked Yb-fiber laser. By exploiting the large spectral acceptance bandwidth for Type I (oo→e) SHG in β-BaB2O4 and a 5 mm crystal, we have generated picosecond pulses over 752-860 nm spectral range under minimal angle tuning, with as much as 3.5 W of output power at 778 nm and >2 W over 73% of the tuning range, in good beam quality with TEM00 spatial profile and M2<1.4. The SHG output pulses have durations of 15.2 ps, with a spectral bandwidth of ∼3.4 nm at 784 nm. In addition, the oscillator simultaneously provides a signal power of >1 W over 1505-1721 nm (25 THz) and idler power >1.8 W over 2787-3630 nm (25 THz), corresponding to a total (signal plus idler) tuning range of 1059 nm. The SHG, signal, and idler output exhibit passive long-term power stability better than 1.6%, 1.3%, and 1.6% rms, respectively, over 14 h.

Application of a Bayesian Method to Absorption Spectral-Line Finding in Simulated ASKAP Data

AbstractThe large spectral bandwidth and wide field of view of the Australian SKA Pathfinder radio telescope will open up a completely new parameter space for large extragalactic HI surveys. Here we focus on identifying and parametrising HI absorption lines which occur in the line of sight towards strong radio continuum sources. We have developed a method for simultaneously finding and fitting HI absorption lines in radio data by using multi-nested sampling, a Bayesian Monte Carlo algorithm. The method is tested on a simulated ASKAP data cube, and is shown to be reliable at detecting absorption lines in low signal-to-noise data without the need to smooth or alter the data. Estimation of the local Bayesian evidence statistic provides a quantitative criterion for assigning significance to a detection and selecting between competing analytical line-profile models.

Sputtered hydrogenated amorphous silicon thin films for distributed Bragg reflectors and long wavelength vertical cavity surface emitting lasers applications

In this work, we report a study of hydrogenated amorphous silicon (a-SiH) films deposited by radio frequency magnetron sputtering for application in Vertical Cavity Surface Emitting Lasers (VCSEL) elaboration. The influence of the hydrogen dilution in the plasma during the deposition on the optical and surface properties is investigated. After selection of the deposition parameters, a-SiH films have been combined with amorphous silicon nitride (a-SiN x) films to provide high reflectivity Bragg reflectors. Distributed Bragg reflector (DBR) based on these quarter wavelength thick dielectric layers have been realized and characterized by optical measurements and compared with theoretical calculations based on the transfer matrix method. A maximum reflectivity of 99.2% at 1.6 μm and a large spectral bandwidth of 700 nm have been reached with only four and a half periods of a-SiH/a-SiN x deposited on a glass substrate. Residual absorption at 1.55 μm has been measured to be as low as 60 cm −1 with a-SiH layers, compared with 400 cm −1 loss with amorphous silicon without hydrogenation step. Finally, DBR comprising six a-SiH/a-SiN x periods have been included in an InP-based VCSEL. Laser emission is demonstrated at room temperature in continuous wave operation with a photopumping experiment.

Decoherence in Attosecond Photoionization

The creation of superpositions of hole states via single-photon ionization using attosecond extreme-ultraviolet pulses is studied with the time-dependent configuration-interaction singles (TDCIS) method. Specifically, the degree of coherence between hole states in atomic xenon is investigated. We find that interchannel coupling not only affects the hole populations, but it also enhances the entanglement between the photoelectron and the remaining ion, thereby reducing the coherence within the ion. As a consequence, even if the spectral bandwidth of the ionizing pulse exceeds the energy splittings among the hole states involved, perfectly coherent hole wave packets cannot be formed. For sufficiently large spectral bandwidth, the coherence can only be increased by increasing the mean photon energy.

Coherence preservation and beam flatness of a single-bounce multilayer monochromator (beamline ID19—ESRF)

Abstract Larger spectral bandwidth and higher photon flux density are the major advantages of multilayer monochromators over crystal-based devices. Especially for synchrotron-based hard X-ray microimaging applications the increased photon flux density is important in order to achieve high contrast and resolution in space and/or time. However, the modifications on the beam profile induced by reflection on a multilayer are a drawback which can seriously harm the performance of such a monochromator. A recent study [A. Rack, T. Weitkamp, M. Riotte, D. Grigoriev, T. Rack, L. Helfen, T. Baumbach, R. Dietsch, T. Holz, M. Kramer, F. Siewert, M. Meduňa, P. Cloetens, E. Ziegler, J. Synchrotron Radiat. 17 (2010) 496–510] has shown that the modifications in terms of beam flatness and coherence preservation can be influenced via the material composition of the multilayer coating. The present article extends this knowledge by studying further material compositions used on a daily basis for hard X-ray monochromatization at the beamline ID19 of the European Synchrotron Radiation Facility.

Parameter-tolerant binary gratings

A normalized modal analysis of binary gratings under normal TE incidence involving the most condensed set of optogeometrical parameters gives a complete solution to the problem of canceling the 0(th) transmitted order in phase masks of a low-to-high refractive index ratio down to 0.5 with a large tolerance on the corrugation duty cycle or a large spectral bandwidth. The solution is presented in the form of single normalized 3D charts which shed light on the fulfillment of the 0(th)-order cancellation condition: balanced excitation and π-phase difference between two grating modes. Examples of tolerant gratings are given.

Widely tunable optical delay generator

We propose and demonstrate a method for quasi storage of light based on periodic spectral filtering realized in the time domain by amplitude modulation using frequency-to-time conversion. The delay can be tuned in a wide range by changing the frequency of an electrical modulation signal. In our experiments, the delay of single 2.5 ps pulses varied by 66 pulse widths. The technique works equally well for more complex optical data packets. Contrary to known approaches, the method has a very large spectral bandwidth and can be implemented by either fiber or integrated solutions using existing technologies. Because of the large bandwidth, fractional delays up to several tens of thousands of pulse widths can be achieved potentially for subpicosecond pulses, which is a tremendous value regarding the implementation simplicity.

Ultrabroadband photodetection based on graphene ink

We report photodetection in a very large spectral bandwidth, which encompasses ultraviolet,visible and near infrared, using graphene inks or graphene inks functionalized with eithergold or silver nanoparticles, or gold nanoparticles further encapsulated with bovine serumalbumin deposited on interdigitated electrodes fabricated on a silicon dioxide/silicon substrate.In contrast to gold-functionalized graphene inks, which have responsivities better than1 mA W − 1 at a 0.1 V bias over the huge bandwidth extending from 215 to 2500 nm, Ag-functionalizedinks show at least a four-fold increased responsivity, with a record value of13.7 mA W − 1 in near infrared.

Designing of transparency windows by quantum interference of wave packets

It has been shown that control over the design of single- or multimode-transparency windows can be achieved by control-probe quantum interferometry using two delayed phase-locked ultrashort pulses. We have investigated how single- or multimode-transparency windows can be designed by eliminating the absorption to a set of odd or even vibrational levels of a molecule covered by the spectral bandwidth of the pulses. Suppression of absorption can be achieved by controlling the quantum interference of two vibrational wave packets excited by these two pulses, leading to destructive interference of transition amplitudes for one set of vibrational levels even or odd. Simuntaneously the interference becomes constructive for the other set of odd or even vibrational levels, leading to absorption to these levels. Thus, a set of transparency windows is designed in between two absorption peaks at two consecutive odd (even) vibrational levels by eliminating absorption at even (odd) vibrational levels between two absorption peaks. If the spectral bandwidth of the pulse [full width at half maximum (FWHM)] is less than the energy difference between two consecutive absorption peaks, the pulse will be transmitted without absorption through the transparency window. However, for pulses with a larger spectral bandwidth (FWHM), a set of broad transparency windows will be generated and different bands of pulse frequencies will be transmitted through these windows. We have shown that coherent control of the design of transparency windows can be achieved by controlling pulse duration, carrier frequency, delay, and the phase difference of two ultrashort pulses.

Comparative study of multilayers used in monochromators for synchrotron-based coherent hard X-ray imaging

A systematic study is presented in which multilayers of different composition (W/Si, Mo/Si, Pd/B(4)C), periodicity (from 2.5 to 5.5 nm) and number of layers have been characterized. In particular, the intrinsic quality (roughness and reflectivity) as well as the performance (homogeneity and coherence of the outgoing beam) as a monochromator for synchrotron radiation hard X-ray micro-imaging are investigated. The results indicate that the material composition is the dominating factor for the performance. By helping scientists and engineers specify the design parameters of multilayer monochromators, these results can contribute to a better exploitation of the advantages of multilayer monochromators over crystal-based devices; i.e. larger spectral bandwidth and high photon flux density, which are particularly useful for synchrotron-based micro-radiography and -tomography.

Spatio-Temporal Control in Multiphoton Fluorescence Laser-Scanning Microscopy

Confocal microscopy, one of the modern day fluorescence laser-scanning microscopic techniques, is suffered from having sufficient out-of-focus signal. On the other hand, in multiphoton microscopy ultrafast laser pulses are commonly used to circumvent low multiphoton absorption cross-sections of common fluorophores; due to broad overlapping two-photon absorption (TPA) spectra of fluorophores and large spectral bandwidth of a short pulse, simultaneous excitation of many fluorophores is common demanding selective excitation of individual fluorophores if required.

Aperiodic W/B 4C multilayer systems for X-ray optics: Quantitative determination of layer thickness by HAADF-STEM and X-ray reflectivity

X-ray microanalysis of materials by synchrotron radiation has generated an increasing demand for advanced multilayer based X-ray optical elements that can be used in beam-lines as monochromators of small or large spectral bandwidth, or in components for shaping high-intensity X-ray beams with highest reflectivities. Modern thin film technologies allow the fabrication of appropriate multilayer systems with reproducible control of layer thicknesses and resulting excellent reflectivity properties that are well adjusted to the requirements. Aperiodic multilayer systems combine the advantage of large bandwidths and decent reflectivity values. Their development necessitates quantitative control of the processing and of the resulting microstructures on the nanometer scale. We have investigated aperiodic multilayer systems consisting of W/B 4C bi-layers on silicon, with single layer thicknesses ranging from about 1 nm to 2 nm. The microstructure and the bi-layer and single layer thickness are characterized quantitatively on cross-sectional samples by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The thickness of bi-layers is determined by geometric phase analysis and that of single layers by evaluation of STEM intensity line profiles. Comparisons with nominal thickness values and with those derived from X-ray reflectivity scans show very good agreement.

Coherence function control of Quantum Dot Superluminescent Light Emitting Diodes by frequency selective optical feedback

Low coherent light interferometry requires broad bandwidth light sources to achieve high axial resolution. Here, Superluminescent Light Emitting Diodes (SLDs) utilizing Quantum Dot (QD) gain materials are promising devices as they unify large spectral bandwidths with sufficient power at desired emission wavelengths. However, frequently a dip occurs in the optical spectrum that translates into high side lobes in the coherence function thereby reducing axial resolution and image quality. We apply the experimental technique of frequency selective feedback to shape the optical spectrum of the QD-SLD, hence optimizing the coherence properties. For well-selected feedback parameters, a strong reduction of the parasitic side lobes by a factor of 3.5 was achieved accompanied by a power increase of 40% and an improvement of 10% in the coherence length. The experimental results are in excellent agreement with simulations that even indicate potential for further optimizations.

Long-period-fiber-grating-based filter configuration enabling arbitrary linear filtering characteristics

The filtering scheme proposed here is based on transmission through a dual long-period-fiber-grating (LPFG) configuration and enables implementation of arbitrary spectral transfer functions using available inverse-scattering design algorithms, such as those widely used for fiber Bragg gratings (FBGs) operating in reflection. Besides the important technical advantage of operation in transmission, the proposed device can reach large spectral bandwidths that would be extremely challenging to reach by, e.g., FBG devices. The proposed concept is demonstrated by designing and fabricating a LPFG-based filter for synthesis of transform-limited 1.5-ps-long square-like pulses.

Fabry–Pérot filter cavities for wide-spaced frequency combs with large spectral bandwidth

We use low-finesse Fabry–Perot cavities in series to generate frequency combs with a large mode spacing in a way that allows its application to a large optical bandwidth. The attenuation of laser modes closest to the pass bands of the cavity exceeds 70 dB for a filter ratio of m=20 relative to the resonant modes centered within the pass bands. We also identify the best cavity geometry to suppress spurious transmission of higher order transversal modes. Such a thinned out frequency comb can be used to calibrate traditional spectrographs for precision astronomy. In the time domain mode filtering generates a pulse train with a multiplied repetition rate. High-fidelity filtering, as described here, implies small variations of the pulse energies.