Broad Tuning Research Articles

Using Interleaved Stimulation to Measure the Size and Selectivity of the Sustained Phase-Locked Neural Response to Cochlear Implant Stimulation

We measured the sustained neural response to electrical stimulation by a cochlear implant (CI). To do so, we interleaved two stimuli with frequencies F1 and F2 Hz and recorded a neural distortion response (NDR) at F2-F1 Hz. We show that, because any one time point contains only the F1 or F2 stimulus, the instantaneous nonlinearities typical of electrical artefact should not produce distortion at this frequency. However, if the stimulus is smoothed, such as by charge integration at the nerve membrane, subsequent (neural) nonlinearities can produce a component at F2-F1 Hz. We stimulated a single CI electrode with interleaved sinusoids or interleaved amplitude-modulated pulse trains such that F2 = 1.5F1, and found no evidence for an NDR when F2-F1 was between 90 and 120 Hz. However, interleaved amplitude-modulated pulse trains with F2-F1~40 Hz revealed a substantial NDR with a group delay of about 45 ms, consistent with a thalamic and/or cortical response. The NDR could be measured even from recording electrodes adjacent to the implant and at the highest pulse rates (> 4000 pps) used clinically. We then measured the selectivity of this sustained response by presenting F1 and F2 to different electrodes and at different between-electrode distances. This revealed a broad tuning that, we argue, reflects the overlap between the excitation elicited by the two electrodes. Our results also provide a glimpse of the neural nonlinearity in the auditory system, unaffected by the biomechanical cochlear nonlinearities that accompany acoustic stimulation. Several potential clinical applications of our findings are discussed.

Application of multi-objective optimization algorithm for siting and sizing of distributed generations in distribution networks

Multi-objective optimization for siting and sizing of Distributed Generations (DGs) is difficult because of the highly non-linear interactions of a large number of variables. Furthermore, effective optimization algorithms are often highly problem-dependent and need broad tuning, which limits their applicability to the real world. To address this issue, in this study, Multi-Objective Differential Evolution (MODE) algorithms have been proposed for siting and sizing of DGs. The site and size of DGs play a vital role in the minimization of real power losses and enhancement of voltage profile in distribution systems. This study intends to attain the technical, economic, and environmental benefits of DGs. Hence, the Objective Functions such as minimization of power losses, voltage deviation, energy cost, emissions while generating power, and enhancement of the Voltage Stability Index have been considered. The simulations of two different multi-objective operational cases have been carried out on IEEE 33 bus system, IEEE 69 bus system, and Tamil Nadu Generation and Distribution Corporation Limited, as a real part of 62 bus Indian Utility System. The simulation results of MODE have shown its superior performance and have ensured the economic and environmental benefits of integrating DGs.

Watt-level, high wall plug efficiency, continuous-wave room temperature quantum cascade laser emitting at 7.7µm.

In this article, a InP based strain-balanced In0.58Ga0.42As/In0.47Al0.53As quantum cascade laser emitting at 7.7µm is reported. The active region is based on a slightly-diagonal bound to continuum design with 50 cascade stages and a low voltage defect Δinj of 96 meV. By optimizing the active region and waveguide structure, the waveguide loss αw of 1.18cm-1 are obtained, which contribute to a high wall-plug efficiency (WPE) of 9.08% and low threshold current of only 1.09 kA/cm2 in continuous-wave(CW) operation at 293K. The maximum single facet output power of 1.17W in CW operation and 2.3W in pulsed operation are measured at 293K. The narrow ridge and buried ridge structure epi-side-down-mounted on the diamond heatsink improved the heat dissipation of the device. A beam of pure zero order mode and a broad external-cavity tuning range from 7.16µm to 8.16µm are also achieved.

Refractive Guide Switching a Regenerative Amplifier Free-Electron Laser for High Peak and Average Power Hard X Rays.

We present an x-ray regenerative amplifier free-electron laser design capable of producing fully coherent hard x-ray pulses across a broad tuning range at a high steady state repetition rate. The scheme leverages a strong undulator taper and an apertured diamond output-coupling cavity crystal to produce both high peak and average spectral brightness radiation that is 2 to 3 orders of magnitude greater than conventional single-pass self-amplified spontaneous emission free-electron laser amplifiers. Refractive guiding in the postsaturation regime is found to play a key role in passively controlling the stored cavity power. The scheme is explored both analytically and numerically in the context of the Linac Coherent Light Source II High Energy upgrade.

Straining and Tuning Atomic Layer Nanoelectromechanical Resonators via Comb‐Drive MEMS Actuators

AbstractBroad frequency tuning is an essential attribute desired in resonant nano/microelectromechanical systems (NEMS/MEMS) and their many applications. Endowed with ultrahigh intrinsic strain limits, combined with other unconventional properties, atomically thin 2D crystalline materials are excellent candidates for building highly tunable resonant NEMS. Here a heterogeneous integration approach is demonstrated to enable on‐chip, continuous, and broad frequency tuning in 2D NEMS resonators by directly controlling strain via voltage‐controlled silicon‐on‐insulator (SOI) comb‐drive MEMS actuators. By varying the comb‐drive actuation voltage, resonance frequency of the 2D NEMS can be tuned as large as 75% continuously with precise control. The comb‐drive actuation‐enabled direct straining and tuning also yield quality (Q) factor boost up to twofold. It is validated that this technique is readily applicable to straining and tuning representative 2D NEMS in various leading materials: graphene, molybdenum disulfide (MoS2), and hexagonal boron nitride (h‐BN). This study demonstrates that additively integrating 2D resonators atop mainstream SOI MEMS enables a versatile platform, and opens new possibilities for voltage control and broad tuning of 2D NEMS on chip.

Spectroscopic properties of Er:BZMT ceramics for laser emission

We report on the characterization and analysis of the spectroscopic properties of an Er3+-doped Ba(Zr,Mg,Ta)O3 (Er:BZMT) transparent ceramic showing a disordered perovskite structure. The Judd-Ofelt model was applied to estimate the radiative lifetimes and branching ratios of the 1.5 and 3 µm emission transitions, which are potential for laser operation. The experimental fluorescence lifetimes of the transitions of 4I13/2 → 4I15/2 and 4I11/2 → 4I13/2 were recorded to calculate the radiative quantum efficiencies. According to the analysis, significant non-radiative relaxation processes from the 4I11/2 multiplets should exist in the Er:BZMT system, which makes this material more suitable for laser operation at 1.6 µm. Finally, the gain cross-section for the potential 1.6 µm laser emission was calculated, showing a broad tuning range from less than 1.6 µm to around 1.7 µm. This result also indicates that femtosecond level laser pulses are possible using Er:BZMT as laser gain medium.

A Switched Tunable Inductor Based on Magnetic Flux Linkage Modification

An on-chip switched tunable inductor based on magnetic flux linkage modification is proposed. The magnetic flux linkage was adjusted through an auxiliary coil that was connected to a flip-flop in parallel with a MOS switch. The inductance was monotonically tuned by applying MOS gate voltages, which had a broad tuning range of 1.32–2.03 nH (42.4%) at 8 GHz. A high quality factor of 61.6 at 8 GHz was achieved because the flip-flop provided negative resistance.

Broadly and finely tunable hybrid silicon laser with nanosecond-scale switching speed.

We demonstrate a hybrid silicon tunable laser with wide tunability and rapid switching speed for applications in sensing and optical networks. By implementing an optimized carrier injection phase shifter design, the filters of the silicon laser cavity may be efficiently controlled, enabling both fine and broad wavelength tuning across a 56 nm range, in addition to a rapid 10 ns switching time. The laser emits up to 10 dBm output power, and the linewidth is near 200 kHz. The fast wavelength switching demonstrated here may be employed in data center and access networks, while the potential for rapid wavelength sweeping is attractive for optical sensing and imaging applications.

Dual-band terahertz switch with stretchable Bloch-mode metasurface

A polydimethylsiloxane (PDMS)-based stretchable metadevice for dual-band switching of terahertz radiation is experimentally demonstrated. The metasurface can efficiently excite dipole resonance of the metal structure and the surface Bloch mode generated by the periodic lattice substrate. In the tensile deformation operation, these two resonant modes show significant frequency shift sensitivity characteristics, which provides a feasible solution for the realization of dual-band terahertz switches. A transmittance modulation depth of 90% is achieved by the dipole resonance, with a frequency shift of 0.14 THz. The other transmittance modulation depth of 65% is achieved by the surface Bloch mode, with a frequency shift of 0.4 THz. The broad tuning of 0.4 THz is attributed to the surface mode is period-sensitive. This approach provides a promising method for broad frequency tuning of stretchable metasurfaces.

Frequency response areas of neurons in the mouse inferior colliculus. III. Time-domain responses: Constancy, dynamics, and precision in relation to spectral resolution, and perception in the time domain.

The auditory midbrain (central nucleus of inferior colliculus, ICC) receives multiple brainstem projections and recodes auditory information for perception in higher centers. Many neural response characteristics are represented in gradients (maps) in the three-dimensional ICC space. Map overlap suggests that neurons, depending on their ICC location, encode information in several domains simultaneously by different aspects of their responses. Thus, interdependence of coding, e.g. in spectral and temporal domains, seems to be a general ICC principle. Studies on covariation of response properties and possible impact on sound perception are, however, rare. Here, we evaluated tone-evoked single neuron activity from the mouse ICC and compared shapes of excitatory frequency-response areas (including strength and shape of inhibition within and around the excitatory area; classes I, II, III) with types of temporal response patterns and first-spike response latencies. Analyses showed covariation of sharpness of frequency tuning with constancy and precision of responding to tone onsets. Highest precision (first-spike latency jitter < 1 ms) and stable phasic responses throughout frequency-response areas were the quality mainly of class III neurons with broad frequency tuning, least influenced by inhibition. Class II neurons with narrow frequency tuning and dominating inhibitory influence were unsuitable for time domain coding with high precision. The ICC center seems specialized rather for high spectral resolution (class II presence), lateral parts for constantly precise responding to sound onsets (class III presence). Further, the variation of tone-response latencies in the frequency-response areas of individual neurons with phasic, tonic, phasic-tonic, or pauser responses gave rise to the definition of a core area, which represented a time window of about 20 ms from tone onset for tone-onset responding of the whole ICC. This time window corresponds to the roughly 20 ms shortest time interval that was found critical in several auditory perceptual tasks in humans and mice.

Manipulation of the Endocochlear Potential Reveals Two Distinct Types of Cochlear Nonlinearity

The mammalian hearing organ, the cochlea, contains an active amplifier to boost the vibrational response to low level sounds. Hallmarks of this active process are sharp location-dependent frequency tuning and compressive nonlinearity over a wide stimulus range. The amplifier relies on outer hair cell (OHC)-generated forces driven in part by the endocochlear potential, the ∼+80 mV potential maintained in scala media, generated by the stria vascularis. We transiently eliminated the endocochlear potential in vivo by an intravenous injection of furosemide and measured the vibrations of different layers in the cochlea’s organ of Corti using optical coherence tomography. Distortion product otoacoustic emissions were also monitored. After furosemide injection, the vibrations of the basilar membrane lost the best frequency (BF) peak and showed broad tuning similar to a passive cochlea. The intra-organ of Corti vibrations measured in the region of the OHCs lost the BF peak and showed low-pass responses but retained nonlinearity. This strongly suggests that OHC electromotility was operating and being driven by nonlinear OHC current. Thus, although electromotility is presumably necessary to produce a healthy BF peak, the mere presence of electromotility is not sufficient. The BF peak recovered nearly fully within 2 h, along with the recovery of odd-order distortion product otoacoustic emissions. The recovery pattern suggests that physical shifts in operating condition are a critical step in the recovery process.

Bimetallic Ag–Au alloy nanocubes for SERS based sensitive detection of explosive molecules

We have fabricated Ag–Au alloy nanocubes using the galvanic replacement of silver nanocubes by aqueous HAuCl4 and investigated their morphological, structural, compositional and optical properties. The inter-diffusion between silver and gold at 100 °C leads to the formation of Ag–Au alloy nanocubes with hollow interiors. A broad tuning of the surface plasmon resonance (SPR) wavelength from 624 nm to 920 nm is obtained with the varying volume of HAuCl4. When excited at wavelength 785 nm, the bimetallic Ag–Au nanocubes with pinholes exhibit two-fold Raman intensity enhancement compared to pristine Ag nanocubes. The surface-enhanced Raman spectroscopy (SERS) substrate prepared with Ag–Au alloy nanocubes shows high-intensity enhancement factor of 1.9 × 107 for 11.2 wt% Au content. The SERS-active Ag–Au alloy nanocubes substrates were exploited for the detection of two explosive molecules; p-nitrobenzoic acid (PNBA) and picric acid (PA). Remarkable detection sensitivity and ultra-low detection limit of 1.7 × 10–14 M for PNBA and 4.1 × 10–11 M for PA were obtained, demonstrating the very high SERS detection capabilities of the as-prepared substrate.

Prefrontal cortex exhibits multidimensional dynamic encoding during decision-making.

Recent work has suggested that prefrontal cortex (PFC) plays a key role in context-dependent perceptual decision-making. Here we address that role using a new method for identifying task-relevant dimensions of neural population activity. Specifically, we show that PFC has a multi-dimensional code for context, decisions, and both relevant and irrelevant sensory information. Moreover, these representations evolve in time, with an early linear accumulation phase followed by a phase with rotational dynamics. We identify the dimensions of neural activity associated with these phases, and show that they do not arise from distinct populations, but of a single population with broad tuning characteristics. Finally, we use model-based decoding to show that the transition from linear to rotational dynamics coincides with a plateau in decoding accuracy, revealing that rotational dynamics in PFC preserve sensory choice information for the duration of the stimulus integration period.

Development of a transition-edge sensor bilayer process providing new modalities for critical temperature control

Transition-edge sensors (TESs) are thermal detectors in which a superconducting film that is electrically biased in the superconducting-to-normal transition is used as a thermometer. In most TESs, the film is a superconductor-normal metal bilayer where the two materials and their thicknesses are chosen to achieve various specifications including the transition temperature Tc . Traditionally, the materials in the bilayer are deposited in sequence without breaking vacuum in order to achieve a clean, uniform bilayer interface at the wafer-scale. This approach leads to constraints in material properties, fabrication techniques and, ultimately, TES designs. To overcome these constraints, we have developed a bilayer fabrication process that allows the layers to be deposited and patterned separately with an exposure to atmosphere between the deposition steps. We demonstrate better than 6% transition-temperature uniformity across a 7.6 cm (3 in) substrate and present satisfactory spectra from TES x-ray detectors fabricated in this fashion. We show how the new hybrid additive-subtractive TES fabrication process creates new design possibilities, including broad tuning of Tc across a substrate with a single bilayer thickness.

FBG-referenced interrogating system using a double-ring erbium-doped fiber laser for high power and broadband.

In this study, a double-ring erbium-doped fiber (EDF) laser with an optical switch and a fiber Bragg grating (FBG)-referenced interrogating system was developed and demonstrated. This double-ring configuration can achieve high power amplified spontaneous emission, enabling laser oscillation even within the L-band. The output range and signal-to-noise ratio (SNR) of the double-ring EDF laser were measured to be 1512-1610 nm and 55 dB. In addition, the interrogating system using FBGs for reference resulted in precision improvement of ∼25 pm over those achieved in previous studies, reaching a precision of about 7 pm. The high power, broad tuning range, and sufficiently high precision of the proposed interrogating system make it promising for use in FBG-based sensing applications.

Band-edge-enhanced tunable random laser using a polymer-stabilised cholesteric liquid crystal

ABSTRACT Electrically tunable random laser action has been demonstrated in a cavity-free structure. The scattering structure was shaped by the treatment of AC, and a polymer network structure is introduced to fix it. The fluorescence of the laser dye is scattered internally and selectively amplified at the band-edge of the photonic crystal, due to the concentrate of the photonic density of state. As the electric field increased, the deformation of the polymer network causes the cholesteric liquid crystal pitch to form a gradient distribution, resulting in the widening of the bandgap, and the random laser wavelength changes accordingly. This random laser with a broad tuning range (∼100 nm) opens new possibilities for the design of highly efficient incoherent light source in a large area with a thickness of micron level.

Erratum: "Simple, low-noise piezo driver with feed-forward for broad tuning of external cavity diode lasers" [Rev. Sci. Instrum. 89, 023102 (2018)

Erratum: "Simple, low-noise piezo driver with feed-forward for broad tuning of external cavity diode lasers" [Rev. Sci. Instrum. 89, 023102 (2018)

Passively Q-switched thulium fluoride fiber laser operating in S-band region using N-doped graphene saturable absorber

A passively Q-switched S-band fiber laser is proposed and demonstrated using a thulium-doped fluoride fiber as laser gain medium. The proposed design possesses significant advantages including broader tuning wavelength range as compared to previously reported Q-switched S-band lasers. The laser cavity is configured in a simple forward-pumped ring design with a 1400 nm single upconversion pumping scheme. Q-switching is achieved using a polyvinyl alcohol/nitrogen-doped graphene nanosheet (PVA–N-Gns)-based thin-film saturable absorber. Stable Q-switched pulses are obtained at a center wavelength of 1504.7 nm over a pump power range from 101.8 to 280.6 mW. The generated pulses have a maximum pulse repetition rate of 36.85 kHz and minimum pulse width of 4.8 μs at the pump power of 280.6 mW. The maximum pulse output power of 1.30 mW indicates a laser efficiency of approximately 0.63% while a signal-to-noise ratio of 57 dB shows pulse stability. The ability of the proposed cavity design to produce broad wavelength tunability range from 1450 to 1510 nm make it highly suitable for various application in the S and S + band regions.

Broadband Light Amplitude Tuning Characteristics of SnSe2 Coated Microfiber

Broadband light amplitude tuning characteristics are demonstrated in a 2-D tin diselenide (SnSe2) nanosheets coated microfiber (MF) structure (SnSe2CMF). Due to the strong absorption characteristics of SnSe2 in broadband wavelengths, the transmitted optical power (TOP) of SnSe2CMF can be tuned by external pump lights with different wavelengths (405, 532, and 660 nm) over a broadband spectrum (1520–1620 nm). Five wavelengths are extracted in order to investigate the tuning performance. The TOP of SnSe2CMF increases with the pump light power and a maximum tuning efficiency of ∼0.3074 dB/mW is achieved. In terms of tuning speed, the minimum response time is ∼3.7/3.9 ms. The experimental results have qualitatively good agreement with the simulations predicted by FEM models. The demonstrated structure has the advantages of broad tuning bandwidth and high tuning efficiency. Therefore, it might be a promising candidate for building high performance broadband fiber-optic sensors and modulators.

Broadband Entrainment of Striatal Low-Threshold Spike Interneurons.

Striatal interneurons and spiny projection (SP) neurons are differentially tuned to spectral components of their input. Previous studies showed that spike responses of somatostatin/NPY-expressing low threshold spike (LTS) interneurons have broad frequency tuning, setting these cells apart from other striatal GABAergic interneurons and SP neurons. We investigated the mechanism of LTS interneuron spiking resonance and its relationship to non-spiking membrane impedance resonance, finding that abolition of impedance resonance did not alter spiking resonance. Because LTS interneurons are pacemakers whose rhythmic firing is perturbed by synaptic input, we tested the hypothesis that their spiking resonance arises from their phase resetting properties. Phase resetting curves (PRCs) were measured in LTS interneurons and SP neurons and used to make phase-oscillator models of both cell types. The models reproduced the broad tuning of LTS interneurons, and the differences from SP neurons. The spectral components of the PRC predicted each cell’s sensitivity to corresponding input frequencies. LTS interneuron PRCs contain larger high-frequency components than SP neuron PRCs, providing enhanced responses to input frequencies above the cells’ average firing rates. Thus, LTS cells can be entrained by input oscillations to which SP neurons are less responsive. These findings suggest that feedforward inhibition by LTS interneurons may regulate SP neurons’ entrainment by oscillatory afferents.