Low Modulation Depths Research Articles

Unlocking Giant Third‐Order Optical Nonlinearity in (MA)2CuX4 through Introducing Jahn‐Teller Distortion

AbstractNonlinear absorption coefficient and modulation depth stand as pivotal properties of nonlinear optical (NLO) materials, while the existing NLO materials exhibit limitations such as low nonlinear absorption coefficients and/or small modulation depths, thereby severely impeding their practical application. Here we unveil that introducing Jahn–Teller distortion in a Mott‐Hubbard system, (MA)2CuX4 (MA=methylammonium; X=Cl, Br) affords the simultaneous attainment of a giant nonlinear absorption coefficient and substantial modulation depth. The optimized compound, (MA)2CuCl4, demonstrates a nonlinear absorption coefficient of (1.5±0.08)×105 cm GW−1, a modulation depth of 60 %, and a relatively low optical limiting threshold of 1.22×10−5 J cm−2. These outstanding attributes surpass those of most reported NLO materials. Our investigation reveals that a more pronounced distortion of the [CuX6]4− octahedron emerges as a crucial factor in augmenting optical nonlinearity. Mechanism study involving structural and spectral characterization along with theoretical calculations indicates a correlation between the compelling performance and the Mott‐Hubbard band structure of the materials, coupled with the Jahn–Teller distortion‐induced d‐d transition. This study not only introduces a promising category of high‐performance NLO materials but also provides novel insights into enhancing the performance of such materials.

Unlocking Giant Third-Order Optical Nonlinearity in (MA)2CuX4 through Introducing Jahn-Teller Distortion.

Nonlinear absorption coefficient and modulation depth stand as pivotal properties of nonlinear optical (NLO) materials, while the existing NLO materials exhibit limitations such as low nonlinear absorption coefficients and/or small modulation depths, thereby severely impeding their practical application. Here we unveil that introducing Jahn-Teller distortion in a Mott-Hubbard system, (MA)2CuX4 (MA=methylammonium; X=Cl, Br) affords the simultaneous attainment of a giant nonlinear absorption coefficient and substantial modulation depth. The optimized compound, (MA)2CuCl4, demonstrates a nonlinear absorption coefficient of (1.5±0.08)×105 cm GW-1, a modulation depth of 60 %, and a relatively low optical limiting threshold of 1.22×10-5 J cm-2. These outstanding attributes surpass those of most reported NLO materials. Our investigation reveals that a more pronounced distortion of the [CuX6]4- octahedron emerges as a crucial factor in augmenting optical nonlinearity. Mechanism study involving structural and spectral characterization along with theoretical calculations indicates a correlation between the compelling performance and the Mott-Hubbard band structure of the materials, coupled with the Jahn-Teller distortion-induced d-d transition. This study not only introduces a promising category of high-performance NLO materials but also provides novel insights into enhancing the performance of such materials.

Tunable multistate terahertz switch based on multilayered graphene metamaterial

We proposed plasmonic effect-based narrow band tunable terahertz switches consisting of multilayered graphene metamaterial. Though several terahertz optical switches based on metamaterials were previously reported, these switches had complicated fabrication processes, limited tunability, and low modulation depths. We designed and simulated ingenious four- and eight-state terahertz optical switch designs that can be functional for multimode communication or imaging using the finite-difference time-domain simulation technique. The plasmonic bright modes and transparency regions of these structures were adjusted by varying the chemical potential of patterned graphene layers via applying voltage in different layers. The structures exhibited high modulation depth and modulation degree of frequency, low insertion loss, high spectral contrast ratio, narrow bandwidth, and high polarization sensitivity. Moreover, our proposed simple fabrication process will make these structures more feasible compared to previously reported terahertz switches. The calculated modulation depths were 98.81% and 98.71%, and the maximum modulation degree of frequencies were $$\sim$$ 61% and $$\sim$$ 29.1% for four- and eight-state terahertz switches, respectively. The maximum transmittance in transparency regions between bright modes and the spectral contrast ratio were enumerated to be 95.9% and $$\sim$$ 96%, respectively. The maximum insertion losses were quite low with values of 0.22 dB and 0.33 dB for four- and eight-state terahertz switches, respectively. Our findings will be beneficial in the development of ultra-thin graphene-based multistate photonic devices for digital switching and sensing in the terahertz regime.

Indium selenide film: a promising saturable absorber in 3- to 4-μm band for mid-infrared pulsed laser

Abstract Indium selenide (InSe) film, an emerging two-dimensional chalcogenide semiconductor, has recently attracted growing interests in optoelectronics. However, its nonlinear characteristics and application potentials in mid-infrared (IR) region remain open, which is a very attractive but undeveloped spectral region currently. In this work, it is demonstrated that InSe film possesses excellent nonlinear absorption properties in 3- to 4-μm band. Saturable absorption measurements of InSe film at 2.8 and 3.5 μm show very low saturation energy fluences and moderate modulation depths. Pump–probe measurements at 3 and 4 μm indicate that InSe film has ultrafast responses in mid-IR region. Furthermore, the application of InSe film in mid-IR pulsed laser is demonstrated, and stable Q-switching operation of fiber laser at 2.8 μm is realized. These results show that InSe film is a promising saturable absorber for mid-IR pulsed laser.

Temporal-pitch sensitivity in electric hearing with amplitude modulation and inserted pulses with short inter-pulse intervals.

Listeners with cochlear implants (CIs) typically show poor sensitivity to the temporal-envelope pitch of high-rate pulse trains. Sensitivity to interaural time differences improves when adding pulses with short inter-pulse intervals (SIPIs) to high-rate pulse trains. In the current study, monaural temporal-pitch sensitivity with SIPI pulses was investigated for six CI listeners. Amplitude-modulated single-electrode stimuli, representing the coding of the fundamental frequency (F0) in the envelope of a high-rate carrier, were used. Two SIPI-insertion approaches, five modulation depths, two typical speech-F0s, and two carrier rates were tested. SIPI pulses were inserted either in every amplitude-modulation period (full-rate SIPI) to support the F0 cue or in every other amplitude-modulation period (half-rate SIPI) to circumvent a potential rate limitation at higher F0s. The results demonstrate that full-rate SIPI pulses improve temporal-pitch sensitivity across F0s and particularly at low modulation depths where envelope-pitch cues are weak. The half-rate SIPI pulses did not circumvent the limitation and further increased variability across listeners. Further, no effect of the carrier rate was found. Thus, the SIPI approach appears to be a promising approach to enhance CI listeners' access to temporal-envelope pitch cues at pulse rates used clinically.

A Tale of Two Tantalum Borides as Potential Saturable Absorbers for Q-Switched Fiber Lasers

In this paper, we analyze the performance of two tantalum-based boride (TaB and TaB 2 ) microparticles as potential saturable absorbers for high-power fiber lasers. Both materials are ultrahigh temperature ceramics with melting points above 3000 °C, but with different crystalline structures: TaB has an orthorhombic structure (nearly isotropic), whereas TaB 2 has a hexagonal structure (uniaxial, anisotropic). Despite their different crystalline structures, the microparticles have a similar low fluence attenuation (between 2.3 and 2.60 dB/μm) and modulation depths (around 2.0 dB/μm), but remarkable different saturation fluences: TaB has a saturation fluence of 160 μJ/cm 2 , whereas TaB 2 has a saturation fluence of 110 μJ/cm 2 . The measured damage thresholds are 112 and 10 6 mJ/cm 2 /pulse for TaB and TaB 2 , respectively. When incorporated to a fiber laser, the materials produce pulses with durations of 345 ns, lower than those reported by our group in previous papers. The results show that the materials can find potential applications in high-power Q-switched lasers.

Towards increasing timing sensitivity in electric hearing

Sensitivity of bilateral cochlear-implant (CI) listeners to interaural time differences (ITDs) in electric pulse trains is degraded compared to normal-hearing (NH) listeners presented with ITDs in pure tones. This degradation manifests both as an elevated ITD threshold and upper perceptual limit of stimulation rates. Similar limitations were observed for temporal pitch, despite the difficulty to disentangle temporal and place pitch cues in NH listeners. We tested the hypothesis that ITD and monaural rate-pitch sensitivity of CI listeners at high rates of electric stimulation can be improved by introducing extra pulses with short inter-pulse intervals (SIPIs) at low rates in amplitude modulated high-rate pulse trains. Results show that SIPIs significantly improved ITD and monaural rate-pitch sensitivity at low modulation depths. These similar improvements suggest a possible overlapping benefit of SIPIs for CI listeners in everyday environments requiring high rates for encoding speech. Considering the documented effects of neural deprivation, its potential reversal by training, and the potential for NH-like timing sensitivity as observed in exceptional CI listeners, we began to investigate the effects of vision-induced ITD training in CI listeners. Preliminary results will be discussed in the light of the timing sensitivity limitations in electric hearing.

Assembly and Function of the tRNA-Modifying GTPase MnmE Adsorbed to Surface Functionalized Bioactive Glass

Protein adsorption onto solid surfaces is a common phenomenon in tissue engineering related applications, and considerable progress was achieved in this field. However, there are still unanswered questions or contradictory opinions concerning details of the protein's structure, conformational changes, or aggregation once adsorbed onto solid surfaces. Electron paramagnetic resonance (EPR) spectroscopy and site-directed spin labeling (SDSL) were employed in this work to investigate the conformational changes and dynamics of the tRNA-modifying dimeric protein MnmE from E. coli, an ortholog of the human GTPBP3, upon adsorption on bioactive glass mimicking the composition of the classical 45S5 Bioglass. In addition, prior to protein attachment, the bioactive glass surface was modified with the protein coupling agent glutaraldehyde. Continuous wave EPR spectra of different spin labeled MnmE mutants were recorded to assess the dynamics of the attached spin labels before and after protein adsorption. The area of the continuous wave (cw)-EPR absorption spectrum was further used to determine the amount of the attached protein. Double electron-electron resonance (DEER) experiments were conducted to measure distances between the spin labels before and after adsorption. The results revealed that the contact regions between MnmE and the bioactive glass surface are located at the G domains and at the N-terminal domains. The low modulation depths of all DEER time traces recorded for the adsorbed single MnmE mutants, corroborated with the DEER measurements performed on MnmE double mutants, show that the adsorption process leads to dissociation of the dimer and alters the tertiary structure of MnmE, thereby abolishing its functionality. However, glutaraldehyde reduces the aggressiveness of the adsorption process and improves the stability of the protein attachment.

Neurometric amplitude‐modulation detection threshold in the guinea‐pig ventral cochlear nucleus

Amplitude modulation (AM) is a pervasive feature of natural sounds. Neural detection and processing of modulation cues is behaviourally important across species. Although most ecologically relevant sounds are not fully modulated, physiological studies have usually concentrated on fully modulated (100% modulation depth) signals. Psychoacoustic experiments mainly operate at low modulation depths, around detection threshold (∼5% AM). We presented sinusoidal amplitude-modulated tones, systematically varying modulation depth between zero and 100%, at a range of modulation frequencies, to anaesthetised guinea-pigs while recording spikes from neurons in the ventral cochlear nucleus (VCN). The cochlear nucleus is the site of the first synapse in the central auditory system. At this locus significant signal processing occurs with respect to representation of AM signals. Spike trains were analysed in terms of the vector strength of spike synchrony to the amplitude envelope. Neurons showed either low-pass or band-pass temporal modulation transfer functions, with the proportion of band-pass responses increasing with increasing sound level. The proportion of units showing a band-pass response varies with unit type: sustained chopper (CS) > transient chopper (CT) > primary-like (PL). Spike synchrony increased with increasing modulation depth. At the lowest modulation depth (6%), significant spike synchrony was only observed near to the unit's best modulation frequency for all unit types tested. Modulation tuning therefore became sharper with decreasing modulation depth. AM detection threshold was calculated for each individual unit as a function of modulation frequency. Chopper units have significantly better AM detection thresholds than do primary-like units. AM detection threshold is significantly worse at 40 dB vs. 10 dB above pure-tone spike rate threshold. Mean modulation detection thresholds for sounds 10 dB above pure-tone spike rate threshold at best modulation frequency are (95% CI) 11.6% (10.0-13.1) for PL units, 9.8% (8.2-11.5) for CT units, and 10.8% (8.4-13.2) for CS units. The most sensitive guinea-pig VCN single unit AM detection thresholds are similar to human psychophysical performance (∼3% AM), while the mean neurometric thresholds approach whole animal behavioural performance (∼10% AM).

A New Space-Vector PWM With Optimal Switching Selection for Multilevel Coupled Inductor Inverters

Multilevel space vector pulsewidth modulation (SVPWM) control is shown to be a very useful tool for coupled inductor inverters as six independent pulsewidth modulation (PWM) signals are required, and there is additional complexity of meeting the performance requirements for the coupled inductor while balancing the winding common-mode dc current and generating high-quality multilevel PWM output voltages. A new multilevel SVPWM technique with a five-segment switching sequence is described, where half-wave symmetrical PWM voltage waveforms are used to balance the inductor common-mode dc voltages and also to avoid all possible switching states with a high winding current ripple. The proposed SVPWM is shown to have better inverter performance, compared with traditional carrier-based and the original multilevel SVPWM schemes at low modulation depths. Inverter operation with the proposed SVPWM is validated through simulation and experimental results.

Effects of temporal properties on compound action potentials in response to amplitude-modulated electric pulse trains in guinea pigs

The electrically evoked compound action potential (ECAP) of the auditory nerve in response to amplitude-modulated pulse trains varies over time, but the response amplitudes are not linearly proportional to the level of stimulus pulses. At least two mechanisms could contribute to the deviations of the ECAP response pattern from that of the stimulus envelope. The first mechanism is time-invariant or stationary that reflects the non-linear growth of response amplitude with changes in stimulus level that is evident in the response to single pulses. This can be considered a time-invariant or stationary effect. The second mechanism is time-variant or non-stationary and reflects neural refractoriness and adaptation. The purpose of this study was to characterize the auditory nerve responses to amplitude-modulated pulse trains and also to evaluate the extent to which the stationary and non-stationary effects may contribute to those responses. ECAP amplitudes were predicted from single-pulse growth functions of the auditory nerve to account for time-invariant effects. Linear regression was performed on the measured vs. predicted ECAP amplitudes to quantify the discrepancies between the two datasets, thereby separating the influence of non-linear growth from time-varying effects on ECAP amplitudes. The results demonstrated a bandpass function of the modulated response amplitudes, with a low-cutoff modulation frequency at 300 Hz and a high-cutoff modulation frequency at 800 Hz, depending on the carrier pulse rate. The relative contribution of the temporal effects on ECAP amplitudes is greatest at low stimulus levels and low modulation depths.

Carrier-dependent temporal processing in an auditory interneuron

Signal processing in the auditory interneuron Omega Neuron 1 (ON1) of the cricket Teleogryllus oceanicus was compared at high- and low-carrier frequencies in three different experimental paradigms. First, integration time, which corresponds to the time it takes for a neuron to reach threshold when stimulated at the minimum effective intensity, was found to be significantly shorter at high-carrier frequency than at low-carrier frequency. Second, phase locking to sinusoidally amplitude modulated signals was more efficient at high frequency, especially at high modulation rates and low modulation depths. Finally, we examined the efficiency with which ON1 detects gaps in a constant tone. As reflected by the decrease in firing rate in the vicinity of the gap, ON1 is better at detecting gaps at low-carrier frequency. Following a gap, firing rate increases beyond the pre-gap level. This "rebound" phenomenon is similar for low- and high-carrier frequencies.

Parallel detection of low modulation depth signals: application to picosecond ultrasonics

We demonstrate parallel detection of laser ultrasonic signals above 50 GHz, where the signals are encoded as small modulations on a large dc background signal. The measurement problem addressed here is generic for many situations, particularly pump/probe experiments, and this paper discusses the problems of moving from single point to parallel detection and practical solutions. This is achieved with a commercial detector array, custom interface electronics and a carefully selected phase stepping algorithm. The parallel detection of Brillouin oscillations illustrates the very low modulation depths that can be measured with this technique. Noise performance and projected improvements for future custom detectors are also considered.

Dispersion Management of Electrically Precompensated RZ Single-Sideband Signals at 10 Gb/s Without Inline Dispersion Compensation

An alternative approach is presented to minimize the impact of fiber nonlinearities in optical single-sideband systems at 10 Gb/s using electrical dispersion precompensation by adding optical postcompensation while maintaining low accumulated dispersion. Numerical simulation is used to show that the dependence of the system performance on the dispersion compensation scheme resembles a pseudolinear transmission regime, although it is not considered as such in the strictest sense. A launched power increase of 3 dB is achieved when compared to full electrically precompensated systems. A feasible implementation of the optical transmitter is considered, which imposes a maximum transmitted modulation depth. It is shown that low modulation depths result in an asymmetric optimum dispersion compensation map.

Response adaptation to broadband sounds in primary auditory cortex of the awake ferret

Driven by previous reports of adaptation to persistent stimuli in other brain regions, we investigated adaptive effects in the Primary Auditory Cortex of awake non-behaving ferrets ( Mustela putorius furo). Electrophysiological data was obtained in response to the presentation of auditory gratings with a structured spectro-temporal envelope of varying bandwidth which had repeated transitions between low and high modulation depths. The responses were analyzed in terms of the evoked spike rates and in terms of the degree of phase locking to the modulation. We found two populations of cells, both of which showed adaptation in the traditional sense. For one population, we also found a second order of adaptation – i.e., adaptation of the adaptation. This suggests the existence of at least two coding strategies which differ in the weight placed on sensory context.

A Dynamic Voltage Restorer (DVR) With Selective Harmonic Compensation at Medium Voltage Level

Dynamic voltage restorers (DVRs) are now becoming more established in industry to reduce the impact of voltage sags to sensitive loads. However, DVRs spend most of their time in standby mode, since voltage sags occur very infrequently, and hence their utilization is low. In principle, it would be advantageous if the series-connected inverter of a DVR could also be used to compensate for any steady-state load voltage harmonics, since this would increase the power quality "value-added" benefits to the grid system. However, before this can be done, consideration must be given to the control of steady-state power through the DVR, the increased losses, and the low modulation depths at which the scheme must operate to achieve acceptable harmonic compensation performance. This paper presents a selective harmonic feedback control strategy that can be easily added to medium-voltage DVR systems to provide voltage harmonic compensation capabilities with minimal effect on the sag compensation performance of the basic DVR. The proposed controller has been experimentally verified on a medium-voltage (10 kV) three-phase DVR prototype under a range of conditions, including distorted supply voltages, nonlinear loads, and operation during distorted voltage sags.

Photorefractive grating dynamics in Bi 12SiO 20 using optical pulses

A numerical simulation of the photorefractive grating dynamics using high intensity pulses of nanosecond length and under an external electric field has been made. The band transport model with one level of recombination and only one type of charge carrier at high modulation depth was solved. Some unusual effects on the recording curves of the space charge field were found. One of these effects was the increment showed by the amplitude of the space charge field after the illumination pulse was turned off. Other surprising effect found, was the appearance of damped mode oscillations on the space charge field at both low and high modulation depths, before attaining the quasi-steady state. In order to accomplish a complete description of grating dynamics, the grating dark decay and the optical erasure have also been calculated. An excellent agreement has been found between our calculations for the diffraction efficiency in Bi 12SiO 20 (BSO) and published experimental data.

Effects of light modulation on grating phase shifts in photorefractive recording

Phase mismatches with regard to light of photorefractive gratings have been investigated for arbitrary modulation depths of the illuminating pattern. The time evolution of the phases during recording under an applied field as well as the steady state values have been calculated for various harmonics. A powerful numerical method has been used to solve the band transport model equations. A clear correlation between the oscillations of the phase shift and these for the grating amplitude have been found for low modulation depths m. They both are markedly damped for high m. The steady-state phase mismatch for the fundamental grating increases with m at variance with the linear prediction. This increase together with the superlinear behaviour for the amplitude gives rise to a marked enhancement of the photorefractive coupling coefficients with regard to the linear values.

Optimization of selective erasure in photorefractive memories

A theoretical analysis for the optimization of the selective-erasure process in holographic photorefractive memories is presented. It is based on the solution of the standard material equations under a linear approximation (low modulation depths). Specific expressions for the optimum phase shifts and erasure rates are obtained. The approach includes all transport processes and so applies to photovoltaic materials such as LiNbO3. The different behavior with regard to nonphotovoltaic materials is discussed. Some additional strategies to improve the overall erasure process are proposed.

Quantitative millimetre wave spectroscopy. part II: Determination of working conditions in an open Fabry-Perot cavity

A confocal Fabry-Perot frequency modulated cavity spectrometer of quality factor 1.25 × 10 5 operating inside a chamber maintained at ambient temperature and pressure of 1 Pa to 1 KPa was employed for spectrometric measurements in the region of 72 GHz and 140 to 160 GHz. The spectrometer used a spatial filter to suppress unwanted, non-axial modes. The solid state microwave source frequency was derived from a phase-locked frequency synthesizer and detection was by a liquid helium cooled bolometer. Transitions in acrylonitrile, formaldehyde, and sulphur dioxide were studied demonstrating parts per million sensitivity for these species in atmospheric samples, whilst carbonyl sulphide samples were detected at sub-parts per million concentration. The effect of pressure on line intensities was studied in order to determine the optimum operating regime. It was found that the technique was not restricted to the 5–50 Pa region characteristic of centimetric wave spectroscopy, but was able also to function in the 0.1 to 1 KPa regime. Furthermore the intensities in this latter region were found to be not critically dependent on sample pressure. A treatment of the effect of pressure and depth of frequency modulation on absorption signals was carried out and the resulting theory applied to the observed intensity-pressure relationships. There was good quantitative agreement between the frequency modulation depth and cavity response characteristics and qualitative agreement between the pressure, frequency modulation and spectral line intensity characteristics. It became clear that the possibility of power saturation, coupled with the non-uniform power distribution within the cavity, was affecting the fits of theoretical curves to the observed data, and that taking this into account produced marked improvement in the fits. Nonetheless the treatment permitted some practically useful conclusions: at low modulation depths and pressures, the sharp spectral absorption peak makes identification of the target species easy, but extraction of quantitative information more difficult, as the intensity will depend critically on the power level in the cavity. At pressures in the 100 Pa region however, the signal obtained is maximal, power broadening minimal and comparative intensity measurements possible over a range of sample species and concentrations.