Spectral Oscillations Research Articles

Observation of Fine Structures in Laser-Driven Electron Beams Using Coherent Transition Radiation

We have measured the coherent optical transition radiation emitted by an electron beam from laser-plasma interaction. The measurement of the spectrum of the radiation reveals fine structures of the electron beam in the range 400-1000 nm. These structures are reproduced using an electron distribution from a 3D particle-in-cell simulation and are attributed to microbunching of the electron bunch due to its interaction with the laser field. When the radiator is placed closer to the interaction point, spectral oscillations have also been recorded, signature of the interference of the radiation produced by two electron bunches delayed by 74 fs. The second electron bunch duration is shown to be ultrashort to match the intensity level of the radiation. Whereas transition radiation was used at longer wavelengths in order to estimate the electron bunch length, this study focuses on the ultrashort structures of the electron beam.

Optoelectronic delay dynamics: from optical chaos communication to high purity microwave oscillators

Nonlinear delay oscillators in optics have been first intensively studied in the context of chaos and nonlinear dynamics theory in the early 80s. Ten years later, they have been independently and nearly simultaneously explored in the context of two antagonistic applications: one is concerned by broadband spread spectrum optical chaos communications and the other dealt with ultra high spectral purity microwave oscillations. We report here on recent surprising results obtained while developing nonlinear dynamical stability analysis for each application, and considering the relevant quantitative differences between the two configurations of the same oscillator architectures.

Long, low loss etched As_2S_3 chalcogenide waveguides for all-optical signal regeneration

We report on the fabrication and optical properties of etched highly nonlinear As(2)S(3) chalcogenide planar rib waveguides with lengths up to 22.5 cm and optical losses as low as 0.05 dB/cm at 1550 nm - the lowest ever reported. We demonstrate strong spectral broadening of 1.2 ps pulses, in good agreement with simulations, and find that the ratio of nonlinearity and dispersion linearizes the pulse chirp, reducing the spectral oscillations caused by self-phase modulation alone. When combined with a spectrally offset band-pass filter, this gives rise to a nonlinear transfer function suitable for all-optical regeneration of high data rate signals.

Apodized fibre Bragg grating acousto-ultrasonic sensor under arbitrary strain using dual Fabry–Perot filters

This paper describes a novel acousto-ultrasonic (AU) sensing system in which a fibre Bragg grating (FBG) is permanently built in a structure for in situ health monitoring. The AU wave detection using an FBG can be realized by a demodulation technique using an optical filter or light source narrower than the FBG spectrum. However, since the spectral wavelength shift induced from structural strain is much larger than the spectral oscillation induced from the AU wave, it is difficult for the demodulation wavelength to follow the spectral shift. For this work, we introduce a passive demodulation method regardless of the spectral shift based on an apodized FBG (AFBG) and dual fibre Fabry–Perot (FFP) filters. The free spectral range (FSR) of a high-finesse FFP filter is matched to the full-bandwidth of the AFBG without side lobes. Therefore, a part of the AFBG spectrum is always filtered by one of the multiple interferometric peaks. However, the wavelength regime near the maximum or minimum reflectivity of the AFBG provides insufficient sensitivity with respect to the wave. Therefore, another 1/4 FSR-biased FFP filter with the same FSR is simultaneously operated.

Sarin produces delayed cardiac and central autonomic changes

The aim was to evaluate the acute and delayed effects of low dose sarin exposure on cardiac autonomic and brainstem catecholaminergic function in mice. The rationale was to expand our knowledge of the cardiovascular effects of this neurotoxic, acetylcholinesterase (AChE) inhibitor. C57BL/6 male mice with telemetric arterial catheters were injected with saline or sarin (8 μg/kg, 0.05× LD 50; sc, two injections) with blood pressure (BP) measurements made at 1 and 10 weeks after sarin exposure. BP and pulse interval variability (PI) and low and high frequency spectral oscillations were measured using autoregressive spectral analysis. In situ hybridization (ISH) was used to quantify tyrosine hydroxylase (TH) mRNA expression in brainstem cardiovascular centers. Sarin had no effect on blood AChE activity, heart rate (HR) or BP. There was a biphasic response in PI variance, an early increase (+ 140%) and a delayed decrease (− 62%) at more than 2 months after sarin exposure. There were no changes in BP variance. Assuming that increased PI variance is a positive outcome, the short-term response to sarin should be protective. This is opposite for the delayed decrease in PI variance which is associated with adverse cardiovascular effects. There was an increase in TH mRNA in both locus coeruleus (0.18 ± 0.05 vs. 1.4 ± 0.2 μCi/g; control vs. sarin) and dorsal vagal complex (0.09 ± 0.06 vs. 1.17 ± 0.03 μCi/g; control vs. sarin). Results show that a dose of sarin which had no peripheral cholinergic effects caused changes in autonomic modulation, a short-term enhancement followed by a delayed impairment in heart rate variability. Sarin-induced cardiac effects suggest a controversial aspect to the use of pharmacological agents which target AChE for management of cardiovascular risk.

Performance of combined 〈1 0 0〉–〈1 1 0〉 ZnTe crystals in an amplified THz time-domain spectrometer

In this work we report on the performance of combined index-matched ZnTe 〈1 1 0〉–〈1 0 0〉 THz emitters and detectors for reduction of etalon effects in a nonlinear crystal-based THz time-domain spectrometer, driven by an amplified femtosecond laser. We demonstrate that the application of a combined electrooptic detector crystal reduces the strength of the first etalon and the average spectral dynamic range oscillations by a factor of two. We also show that even in a simple emitter configuration the emitter-related etalon is significantly reduced due to the transient dielectric effects of carriers created by two-photon absorption in the emitter.

A filtering procedure for systematic removal of pump-perturbed polarization artifacts

In time-dependent optical spectra, fast dynamics of narrow transitions give rise to artifacts consisting of spectral oscillations. In contrast with spectrally-integrated experiments, the interpretation of spectrally-resolved pump-probe experiments is thus far from straightforward in regimes where such coherent effects dominate, i.e., when the timescales of pump-induced processes are shorter than the medium dephasing times. Analyzing the two-dimensional Fourier transform of the time-dependent spectrum, we identify the components responsible for these artifacts and we introduce a simple Fourier-filtering procedure for their systematic removal. This technique is theoretically demonstrated on the time-dependent spectrum of a transition undergoing a frequency shift. The technique is also shown to have the additional benefit of efficient noise filtering of time-dependent spectra.

Photoluminescence and edge‐incident wavelength modulation transmittance spectroscopy characterizations of InGaN/GaN multiple‐quantum‐well structures

AbstractA novel edge‐incident wavelength modulation transmittance spectroscopy (WMTS) technique combined with conventional photoluminescence (PL) measurements is developed to investigate the optical properties of two InGaN/GaN multiple‐quantum‐well (MQW) structures in the temperature range between 15 and 300 K. The S‐shaped PL shift with temperature and the InGaN A1 (LO) phonon replicas were observed in temperature‐dependent PL measurements. It can be explained by the localization of self‐organized InN clusters within the InGaN layers. Besides, we for the first time use the edge‐incident WMTS technique to eliminate the multireflection interference effect, which in the conventional transmittance or absorption measurements usually causes spectral oscillation to make the optical features difficult to identify. The excitonic transitions and absorption edge of GaN layers have been revealed clearly. Compared to an envelope‐function calculation taking into account the quantum confined Stark effect (QCSE), the origins of these spectral features are assigned. The parameters that describe the temperature dependence of the excitonic transition energies are also evaluated. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A computational model of bottlenose dolphin sonar: Feature‐extracting method

A process of echo‐image formation in bottlenose dolphin perception was investigated in our experiments, in which animals were trained to differentiate passive actual echoes recorded beforehand purposely, as well as echo‐like simulated impulses. The results suggest that dolphin, processing echoes within critical interval of time (∼0.2 ms), utilizes the string of three independent, hierarchically organized features (echoes subjective characteristics) being defined by different scales of spectral density oscillation, and energy. Herewith, dolphin is capable to assess features average value over a series of echoes. Distinguishing stimuli, dolphin estimates successively features values from senior to minor, terminating the process at the feature, which contains detectable differences in compared stimuli (the distinctive feature). A procedure of subsequent identification of the reference stimulus can be described by the following decision rule: If the dolphin utilizes some feature as the distinctive one, then in order to preserve the image of the reference stimulus in the animal’s perception, it is necessary and sufficient to preserve the same values of the distinctive feature and all higher ones in order of hierarchy. These data were formalized mathematically and used in our computational model, comparative testing of which has revealed critical capabilities comparable with those of bottlenose dolphins.

Origin of spectral modal noise in fiber-coupled spectrographs

We describe the origin of spectral modal noise in a fiber-coupled spectrograph. Prior investigations of this phenomenon have considered optical fibers with large cores that support thousands of optical modes and used a statistical treatment to analyze spectral noise that arises from interference between the modes. We examine spectral modal noise for a fiber-coupled spectrograph with a smaller-diameter fiber core, optimized for adaptively corrected telescopes, that supports only a few propagating modes for visible and near-infrared light. Simulations to predict the minimum amount of fiber stretch required to shift spectral oscillations by a half-period are confirmed by experiment.

Elastic backscattering spectroscopic microscopy

The spectral properties of elastic light-scattering signals have been shown to provide a wealth of information on nanostructures and microstructures. We present elastic backscattering spectroscopic microscopy that allows simultaneous acquisition of microscopic images and backscattering spectra at each pixel. Within a single homogeneous micrometer-scale particle we observe two distinct and highly localized spectral oscillation features that arise from different optical paths: (1) surface waves (e.g., the ripple structure) and (2) a not previously reported anomalous ripple structure that is due to the interference of waves scattered from front and back surfaces at the particle's center. We also demonstrate that the spectroscopic data can provide nanoscale structural information beyond what conventional microscopy reveals.

THE RELATIONSHIP OF HRV TO SLEEP EEG AND SLEEP RHYTHM

Previous studies have shown that there exists a cycle of NREM (non-rapid eye movement)–REM (rapid eye movement) during normal human sleep, and heart rate variability (HRV) has a close relationship to sleep stages and sleep cycle. This article reports the relationship between the electroencephalographic activity and the HRV spectral power in several specific frequency bands. The authors discovered that relationships do exist between HRV and electroencephalogram (EEG) during sleep. In particular, it was found that, prior to the changes of EEG, the changes of HRV usually indicate the shift of sleep stages. HRV frequency analysis indicates that the very-low-frequency components of HRV are closely related to sleep EEG. Results show that the rhythm of the spectral power oscillations in some specific frequency bands of HRV is almost the same as the sleep cycle, which reflects the rhythm of sleep to a certain extent.

Investigation of self-phase modulation based optical regeneration in single mode As2Se3 chalcogenide glass fiber

We investigate the feasibility of all-optical regeneration based on self-phase modulation in single mode As2Se3 chalcogenide fiber. By combining the chalcogenide fiber with a bandpass filter, we achieve a near step-like power transfer function with no pulse distortion. The device is shown to operate with 5.8 ps duration pulses, thus demonstrating the feasibility of this device operating with high bit-rate data signals. These results are achieved with pulse peak powers <10 W in a fully passive device, including only 2.8 m of chalcogenide fiber. We obtain an excellent agreement between theory and experiment and show that both the high nonlinearity of the chalcogenide glass along with its high normal dispersion near 1550 nm enables a significant device length reduction in comparison with silica-based devices, without compromise on the performance. We find that even for only a few meters of fiber, the large normal dispersion of the chalcogenide glass inhibits spectral oscillations that would appear with self-phase modulation alone. We measure the two photon absorption attenuation coefficient and find that it advantageously affects the device transfer function.

Optical Stark effect in a quantum dot: ultrafast control of single exciton polarizations.

We report the first experimental study of the optical Stark effect in single semiconductor quantum dots (QD). For below band gap excitation, two-color pump-probe spectra show dispersive line shapes caused by a light-induced blueshift of the excitonic resonance. The line shape depends strongly on the excitation field strength and is determined by the pump-induced phase shift of the coherent QD polarization. Transient spectral oscillations can be understood as rotations of the QD polarization phase with negligible population change. Ultrafast control of the QD polarization is demonstrated.

Characterization of supported cylinder-planar germanium waveguide sensors with synchrotron infrared radiation.

Cylinder-planar Ge waveguides are being developed as evanescent-wave sensors for chemical microanalysis. The only non-planar surface is a cylinder section having a 300-mm radius of curvature. This confers a symmetric taper, allowing for direct coupling into and out of the waveguide's 1-mm(2) end faces while obtaining multiple reflections at the central <30-microm-thick sensing region. Ray-optic calculations indicate that the propagation angle at the central minimum has a strong nonlinear dependence on both angle and vertical position of the input ray. This results in rather inefficient coupling of input light into the off-axis modes that are most useful for evanescent-wave absorption spectroscopy. Mode-specific performance of the cylinder-planar waveguides has also been investigated experimentally. As compared to a blackbody source, the much greater brightness of synchrotron-generated infrared (IR) radiation allows a similar total energy throughput, but restricted to a smaller fraction of the allowed waveguide modes. However, such angle-selective excitation results in a strong oscillatory interference pattern in the transmission spectra. These spectral oscillations are the principal technical limitation on using synchrotron radiation to measure evanescent-wave absorption spectra with the thin waveguides.

Spectral oscillations of RR intervals in sleep apnoea/hypopnoea syndrome patients.

A recent study has shown that daytime heart rate variability is reduced in obstructive sleep apnoea/hypopnoea syndrome (OSAHS) patients. In the present study, the hypothesis was that sympathovagal balance around apnoeas/hypopnoeas and nocturnal autonomic activity are altered in OSAHS patients. Frequency- and time-domain analyses of RR intervals were performed to monitor sympathovagal activity noninvasively. Fourteen untreated OSAHS patients and seven healthy subjects underwent overnight polysomnography. Low (LF) and total (TF) frequency power increased 2 min around the end of apnoeas/hypopnoeas (LF 229+/-38 ms2 TF 345+/-45 ms2) compared with undisturbed sleep (LF 106+/-18 ms2, TF 203+/-23 ms2). The increase in high frequency (HF) power was not significant. LF increase was proportionally higher than the HF increase (normalised LF (LFn) 67+/-1 units, normalised HF (HFn) 33+/-1 units) compared with undisturbed sleep (LFn 52+/-2 units, HFn 48+/-2 units). RR duration did not change around apnoeas/hypopnoeas (RR 904+/-28 ms). The LF and TF power increase was greater around arousal-inducing (LF 260+/-45 ms2 TF 390+/-65 ms2) compared with self-terminating (LF 161+/-31 ms2, TF 249+/-40 ms2) apnoeas/hypopnoeas; the LF and LFn increases were significant in both groups compared with undisturbed sleep and HF power differences were nonsignificant. RR intervals were longer around self-terminating apnoeas/hypopnoeas (RR 914+/-29 ms); the differences were not significant compared with undisturbed sleep. RR interval spectral power was not influenced by the event type. RR duration decreased (912+/-28 ms) and LF, HF and TF power increased (LF 111+/-16 ms2 , HF 62+/-6 ms , TF 173+/-21 ms2) across patients, compared with healthy controls (RR 1138+/-91 ms, LF 57+/-3 ms2, HF 35+/-3 ms2, TF 91+/-6 ms2). LFn and HFn did not change significantly. Sympathetic activity increases around apnoeas/hypopnoeas. The recurrent nocturnal fluctuations of sympathovagal balance and the overall increase of nocturnal autonomic activity may be of importance in the development of cardiovascular disease in sleep apnoea patients.

Performance of optical automatic gain control EDFA with dual-oscillating control lasers

The authors present a simple configuration for dual-oscillating lasers gain control erbium-doped fiber amplifier (EDFA) in which dual-oscillating lasers are formed by two pairs of fiber Bragg gratings (FBGs). Experimental results show that dual-oscillating lasers dramatically reduce steady and transient gain variations caused by spectral hole burning and relaxation oscillations, and the dual-oscillating lasers gain controlled EDFAs operate satisfactorily with channel’s add/drop frequency up to 43 kHz which is significantly larger than that with single-oscillating laser control. Furthermore, we present detail performance of the gain control EDFAs with different dual-oscillating control lasers. Experimental results show that the function of different dual-oscillating lasers is similar. Thus dual-oscillating lasers can be chosen with large spectral space in order not to affect the signals in optical communication window.

Theory for the nonlinear optical response of semiconductor surfaces: Application to the optical Stark effect and spectral oscillations of the Si(111)-(2×1)surface exciton

A microscopic approach is presented to compute nonlinear optical properties of semiconductor surfaces. The method uses quasiparticle wave functions and dispersions obtained from ab initio band-structure theory as an inputfor Bloch equations which describe the optical properties. Excitonic effects in the linear absorption spectra of the Si(111)-(2×1) surface are obtained by integrating equations of motion. To demonstrate the applicability of the approach for analyzing nonlinear optical properties, ultrafast light-intensity-dependent absorption changes of the surface exciton are predicted. The numerical results discuss the optical Stark effect and spectral oscillations of the surface exciton which can both be observed in pump-probe experiments.

Nanoscale phase separation in La0.7Ca0.3MnO3films: evidence for texture-driven optical anisotropy

The infrared optical absorption (0.1 eV < ℏω < 1.5 eV) in La0.7Ca0.3MnO3films on LaAlO3substrates exhibits a drastic temperature evolution of the spectral weight,evidencing an insulator-to-metal transition. Single-crystal films were found toreveal strong linear dichroism with anomalous spectral oscillations and fairly weaktemperature dependence. Starting from the concept of phase separation, wedevelop an effective medium model to account for these effects. The opticalanisotropy of the films is attributed to the texturization of the ellipsoidalinclusions of the quasimetal phase caused by a mismatch of the film and substrateand the twin texture of the latter.

Thermal emission from undoped and degenerate silicon structures and from a metallic grating structure

The thermal spectral emission properties of three types of periodic grating structure were investigated: (1) a lightly doped (intrinsic-like) silicon grating, (2) a degenerate (metallic-like) silicon grating, and (3) silicon gratings coated with nickel metal. The structural dimensions of all samples were the same, with feature sizes comparable to the measurement wavelengths (2.5–25 μm). The measurement temperatures for all samples were in the range 27–740 °C. In general, it was found that the spectral emission of the metallic-like gratings depended on the parameters of the grating material and on the structural dimensions. A spectral oscillation in the wavelength range comparable to the structural dimension was observed.