Threshold Gain Research Articles

Ultrafast carrier dynamics of PbS quantum dots in glasses for light amplification

Lead sulfide quantum dots (PbS QDs) have a unique performance in the near-infrared (NIR) range. In-situ precipitation of PbS QDs in glass not only improves the stability, but also is easy to be fabricated fiber compatible with silicon photonics. In this work, we reveal that absorption cross sections of PbS QDs at 1040 nm are 4–13 × 10−16 cm2, the bi- and tri-excitons lifetime are∼60 and ∼10 ps due to the trap-assisted Auger recombination using transient absorption (TA). Furthermore, we demonstrate the broadband net gain in 1500–1600 nm in single PbS QDs embedded glass with a gain threshold of <N>= 4.1 (averaged number of excitons), and achieve the maximum gain time of ∼20 ps and gain coefficient of >120 cm−1. These results illustrate the existing obstacles to achieve the net gain in PbS QDs embedded glass, which is important for applications on fiber amplifiers, NIR laser and high-capacity telecommunication.

Sensitized photon avalanche nanothermometry in Pr3+ and Yb3+ co-doped NaYF4 colloidal nanoparticles

Photon avalanche (PA) is a highly nonlinear luminescence phenomenon that occurs in lanthanide doped materials. PA exhibits a very steep power law relationship between luminescence intensity and the optical pump power. Due to the mechanism of PA emission, even weak perturbations to the energy looping and energy distribution within excited levels of lanthanide emitters are expected to significantly modify luminescent properties. Therefore, in this work, we experimentally study the impact of temperature (from – 175 to 175 °C, with 25 °C steps) on the sensitized PA emission in NaYF4 nanoparticles co-doped with 15% of Yb3+ and 0.5% of Pr3+ ions under 852 nm pumping wavelength. Significant variations of the PA nonlinearity (S = 4.5–9), PA gain (from 50 up to 175), and PA threshold (from 100 up to 700 kW/cm2) were observed under temperature rise from – 175 to 175 °C, respectively. The relative temperature sensitivities based on luminescence intensity changes were larger than 1.5% °C–1 in the whole temperature range, reaching the maximal value of 7.5% °C–1 at 0 °C. Moreover, a new thermometric parameter was proposed, namely, the PA pump power threshold, which exhibited over 0.5% °C–1 relative sensitivities in the same wide temperature range. Owing to PA properties, the temperature sensitivity range and the corresponding relative sensitivities may be intentionally tuned by selecting the appropriate pump intensity in respect to the power dependence relationship. These studies not only provide a better understanding of fundamental processes and susceptibility of the sensitized photon avalanche emission to temperature variation, but also show the possibility of using PA materials as sensitive (nano)thermometers.

A Wideband Generalization of the Near-Field Region for Extremely Large Phased-Arrays

The narrowband and far-field assumption in conventional wireless system design leads to a mismatch with the optimal beamforming required for wideband and near-field systems. This discrepancy is exacerbated for larger apertures and bandwidths. To characterize the behavior of near-field and wideband systems, we derive the beamforming gain expression achieved by a frequency-flat phased array designed for plane-wave propagation. To determine the far-field to near-field boundary for a wideband system, we propose a frequency-selective distance metric. The proposed far-field threshold increases for frequencies away from the center frequency. The analysis results in a fundamental upper bound on the product of the array aperture and the system bandwidth. We present numerical results to illustrate how the gain threshold affects the maximum usable bandwidth for the n260 and n261 5G NR bands.

Ultra-small low-threshold mid-infrared plasmonic nanowire lasers based on n-doped GaN

An ultra-small mid-infrared plasmonic nanowire laser based on n-doped GaN metallic material is proposed and studied by the finite-difference time-domain method. In comparison with the noble metals, nGaN is found to possess superior permittivity characteristics in the mid-infrared range, beneficial for generating low-loss surface plasmon polaritons and achieving strong subwavelength optical confinement. The results show that at a wavelength of 4.2 µm, the penetration depth into the dielectric is substantially decreased from 1384 to 163 nm by replacing Au with nGaN, and the cutoff diameter of nGaN-based laser is as small as 265 nm, only 65% that of the Au-based one. To suppress the relatively large propagation loss induced by nGaN, an nGaN/Au-based laser structure is designed, whose threshold gain has been reduced by nearly half. This work may pave the way for the development of miniaturized low-consumption mid-infrared lasers.

Reconfigurable superdirective beamshaping using a PTX-synthesis metasurface

Parity-time-reciprocal scaling (PTX)-symmetry has been recently proposed to tailor the resonance linewidth and gain threshold of non-Hermitian systems with new exhilarating applications, such as coherent perfect absorber-laser (CPAL) and exceptional point (EP)-based devices. Here, we put forward a nearly-lossless, low-index metachannel formed by PTX-symmetric metasurfaces operating at the CPAL point, supporting the undamped weakly-guided fast wave (leaky mode) and thus achieving ultradirective leaky-wave radiation. Moreover, this structure allows for a reconfigurable and tunable radiation angle as well as beamwidth determined by the reciprocally scaled gain-loss parameter. We envision that the proposed PTX-symmetric metasurfaces will shed light on the design of antennas and emitters with ultrahigh directionality, as well as emerging applications enabled by extreme material properties, such as epsilon-near-zero (ENZ) and beyond.

Identification of the difference threshold for curvature gain of redirected walking

Identification of the difference threshold for curvature gain of redirected walking

Optimization of Flow Cytometric Sorting Parameters for High-Throughput Isolation and Purification of Small Extracellular Vesicles.

Small extracellular vesicles (sEV) can be released from all cell types and carry protein, DNA, and RNA. Signaling molecules serve as indicators of the physiological and pathological state of a cell. However, there is no standard method for sEV isolation, which prevents downstream biomarker identification and drug intervention studies. In this article, we provide a detailed protocol for the isolation and purification of 50-200 nm sEV by a flow cell sorter. For this, a 50 µm nozzle and 80 psi sheath fluid pressure were selected to obtain a good sorting rate and stable side stream. Standard sized polystyrene microspheres were used to locate populations of 100, 200, and 300 nm particles. With additional optimization of the voltage, gain, and forward scatter (FSC) triggering threshold, the sEV signal could be separated from the background noise. These optimizations provide a panel of critical sort settings that enables one to obtain a representative population of sEV using FSC vs. side scatter (SSC) only. The flow cytometry-based isolation method not only allows for high-throughput analysis but also allows for synchronous classification or proteome analysis of sEV based on the biomarker expression, opening numerous downstream research applications.

Energy Efficiency Optimization in Adaptive Transmit Antenna Selection Systems With Limited Feedback

We investigate the energy efficiency (EE) of cellular downlink transmissions with limited feedback, where the channel state information (CSI) assumed to be available at the destination is quantized and fed back to the multiantenna base station for transmitting precoding purposes. Typically, the employment of antennas with poor channel quality marginally enhances system performance but substantially consumes energy resources such as circuit power. Thus, we propose an adaptive transmit antenna selection (ATAS) scheme via setting a channel gain threshold to select transmit antennas with relatively good channel condition for reducing power resource consumption. Meanwhile, the conventional single transmit antenna (STAS) scheme is considered as a baseline. In order to improve EE while considering the impact of feedback overhead, we conduct the effective EE analysis for both ATAS and STAS schemes and show that a maximal effective EE of the proposed ATAS scheme can be achieved by jointly adjusting the channel gain threshold and transmit power. As a consequence, we propose the ergodic effective EE optimization of ATAS (eEEEO-ATAS) scheme to maximize the effective EE. Numerical simulation results demonstrate that the proposed ATAS scheme performs better than the STAS scheme in terms of effective EE.

Night-to-Night Variability of Polysomnography-Derived Physiologic Endotypic Traits in Patients With Moderate to Severe OSA

Emerging data suggest that determination of physiologic endotypic traits (eg, loop gain) may enable precision medicine in OSA. Does a single-night assessment of polysomnography-derived endotypic traits provide reliable estimates in moderate to severe OSA? Two consecutive in-lab polysomnography tests from a clinical trial (n= 67; male, 69%; mean ± SD age, 61 ± 10 years; apnea-hypopnea index [AHI] 53 ± 22 events/h) were used for the reliability analysis. Endotypic traits, reflecting upper airway collapsibility (ventilation at eupneic drive [Vpassive]), upper airway dilator muscle tone (ventilation at the arousal threshold [Vactive]),loop gain (stability of ventilatory control, LG1), and arousal threshold (ArTh) were determined. Reliability was expressed as an intraclass correlation coefficient (ICC). Minimal detectable differences (MDDs) were computed to provide an estimate of maximum spontaneous variability. Further assessment across four repeated polysomnography tests was performed in a subcohort (n= 22). Reliability of endotypic traits between the two consecutive nights was moderate to good (ICC: Vpassive= 0.82, Vactive= 0.76, LG1= 0.72, ArTh= 0.83). Variability in AHI, but not in body position or in sleep stages, was associated with fluctuations in Vpassive and Vactive (r= -0.49 and r= -0.41, respectively; P< .001 for both). MDDs for single-night assessments were: Vpassive= 22, Vactive= 34, LG1= 0.17, and ArTh= 21. Multiple assessments (mean of two nights, n= 22) further reduced MDDs by approximately 20%to 30%. Endotypic trait analysis using a single standard polysomnography shows acceptable reliability and reproducibility in patients with moderate to severe OSA. The reported MDDs of endotypic traits may facilitate the quantification of relevant changes and may guide future evaluation of interventions in OSA.

Theoretical Analysis of Continuous-Wave Mid-Infrared Optical Vortex Source Generated by Singly Resonant Optical Parametric Oscillator

Due to the important application in the study of vibrational circular dichroism and helical dichroism of chiral molecules, the tunable vortex beam at mid-infrared region has attracted increasing attention. Based on orbital angular momentum (OAM) conservation in nonlinear interactions, the vortex pumped singly resonant optical parametric oscillator (SRO) is recognized as a versatile source of coherent vortex radiation providing high power and broad wavelength coverage from a single device. However, the low parametric gain and high oscillation threshold under continuous wave (cw) pumping has so far been the most challenging factor in generating cw tunable vortex beams. To predict the output characteristic of vortex pumped SRO, a theoretical model describing the vortex pumped SRO is needed. In this study, the theoretical model describing the vortex pumped SRO is set up under collimated Gaussian beam approximation. Output characteristics of different SROs are simulated numerically. By proper selection of pump scheme (such as double-pass pumping scheme), the vortex pumped mid-infrared SRO can oscillate at a relatively low pump power. By controlling the gain (mode overlap ratio between the pump and resonant wave in the nonlinear crystal) and loss (employing a spot-defect mirror with different defect size as the output coupler) of the resonant signal mode in the SRO, the OAM of the pump beam can be directionally transferred to a specific down converted beam. The transfer mechanism of the OAM among the pump light and the down-converted beams and factors affecting the transfer are studied. Our study provides the guidelines for the design and optimization of vortex pumped SRO under cw operation.

Spherical Bragg resonators for lasing applications: a theoretical approach.

This work considers a perfect 3D omnidirectional photonic crystal; Spherical Bragg Resonators (SBR), for lasing applications. We use the recursive transfer matrix method to study scattering in an Er3+ doped SBR. We find the threshold gain factor for lasing by scanning poles and zeros of the S-matrix in the complex frequency plane. For a six Si/SiO2 bilayer SBR, the threshold gain factor corresponds to a dopant density of Er3+ of 5.63 × 1020ions/cm3. We believe, our work is the first theoretical demonstration of the ability to engineer optical amplification and threshold gain for lasing in SBRs.

The effects of spatial configuration on relative translation gain thresholds in redirected walking

In this study, we explore how spatial configurations can be reflected in determining the threshold range of Relative Translation Gains (RTGs), a translation gain-based Redirected Walking (RDW) technique that scales the user’s movement in Virtual Reality (VR) in different ratios for width and depth. While previous works have shown that various cognitive factors influence the RDW threshold, studies investigating the impact of environmental aspects on the RDW threshold with regard to the user’s visual perception were lacking. Therefore, we examined the effect of spatial configurations on the RTG threshold by analyzing the participant’s responses and gaze distribution data in two user studies. The first study concerned the size of the virtual room and the existence of objects within it, and the second study focused on the combined impact of room size and spatial layout. Our results show that room size, the existence of objects, and spatial layout all significantly affect the RTG threshold range. Based on our findings, we propose virtual space rescaling guidelines to increase the range of adjustable movable space with RTGs for developers: placing distractors in the room, setting the perceived movable space to be larger than the adjusted movable space for an empty room, and avoiding the placement of object clusters in the center of the room. Our findings can be used to adaptively rescale VR users’ space according to the target virtual space’s configuration with a unified coordinate system that enables the utilization of physical objects in a virtual scene.

Throughput Optimization in Adaptive Transmit Antenna Selection Systems With Limited Feedback

We investigate the performance of a downlink wireless system with a multiantenna base station under the effect of limited channel feedback. In order to inactivate transmit antennas with poor channel quality and, thus, reduce the feedback overhead, a channel gain threshold-based multiantenna selection (CGT-MAS) scheme is proposed without optimization (NOCGT-MAS) and the corresponding single-antenna scheme (NOCGT-SAS) is considered as a baseline. Considering that the number of selected antennas affects both data transmission rate and the feedback overhead, an effective throughput defined as the difference between the data transmission rate and feedback rate is employed for evaluating the system performance. We conduct an effective throughput analysis for both the NOCGT-SAS and NOCGT-MAS schemes and show that an optimal channel gain threshold exists to maximize the effective throughput of the proposed NOCGT-MAS scheme. As a consequence, we propose the optimal instantaneous CGT-MAS algorithm (OICGT-MAS) to maximize the instantaneous effective throughput. Besides, we derive a closed-form expression of an average effective throughput for the CGT-MAS scheme and propose an optimal average channel gain threshold-based multiantenna selection algorithm (OACGT-MAS) for maximizing the derived average effective throughput. Numerical results demonstrate that the effective throughput of the proposed NOCGT-MAS is substantially better than that of conventional NOCGT-SAS. Moreover, although the OICGT-MAS scheme has a slight advantage over OACGT-MAS in terms of the effective throughput, it has a much higher computational complexity than the latter method.

Spectral singularities and threshold gain of a slab laser under illumination of a focused Gaussian beam

Spectral singularities of an infinite planar slab containing homogeneous optically active material at the focal plane of a thin lens are investigated. The field distribution of the beam behind the lens is Gaussian that we approximate by a plane wave concentrated near the optical axis at the focal plane of the lens, such that its phase and amplitude are distance-dependent. The consequences of this configuration are explored by determining the threshold gain of the active medium and tuning the resonance frequencies related to spectral singularities. We find that the spectral singularities and the threshold gain, besides that, vary with distance from the center of the Gaussian beam; also they change with the relative aperture of the focusing lens. Numerical studies show that for the central rays of the beam compared to the side rays, the resonances related to the spectral singularities occur at lower wavelengths (higher frequencies) with the highest threshold gain. Also, for the ray at a fixed distance from the center of the beam, increasing the relative aperture of the illuminating system leads to the same results. Numerical results confirm the theoretical findings.

Colloidal Quantum Dot Infrared Lasers Featuring Sub-Single-Exciton Threshold and Very High Gain.

The use of colloidal quantum dots (CQDs) as a gain medium in infrared laser devices has been underpinned by the need for high pumping intensities, very short gain lifetimes, and low gain coefficients. Here, PbS/PbSSe core/alloyed-shell CQDs are employed as an infrared gain medium that results in highly suppressed Auger recombination with a lifetime of 485ps, lowering the amplified spontaneous emission (ASE) threshold down to 300µJcm-2 , and showing a record high net modal gain coefficient of 2180cm-1 . By doping these engineered core/shell CQDs up to nearly filling the first excited state, a significant reduction of optical gain threshold is demonstrated, measured by transient absorption, to an average-exciton population-per-dot 〈Nth 〉g of 0.45due to bleaching of the ground state absorption. This in turn have led to a fivefold reduction in ASE threshold at 〈Nth 〉ASE =0.70excitons-per-dot, associated with a gain lifetime of 280ps. Finally, these heterostructured QDs are used to achieve near-infrared lasing at 1670nm at a pump fluences corresponding to sub-single-exciton-per-dot threshold (〈Nth 〉Las =0.87). This work brings infrared CQD lasing thresholds on par to their visible counterparts, and paves the way toward solution-processed infrared laser diodes.

Optical gain in CdS/ZnSe/ZnSSe heterostructures with type II band offsets

A resonance-periodic gain heterostructure containing 8 ZnSe/CdS/ZnSe quantum wells (QWs) with type II band offsets was grown by metalorganic vapor phase epitaxy (MOVPE). Based on this structure, a semiconductor laser with a microcavity and longitudinal pumping by laser diode radiation with a wavelength of 440 nm was implemented. We obtained a peak power of 0.32 W at a wavelength of 525 nm with a slope efficiency of 10 %. The threshold gain was estimated at ∼ 104 cm−1. The calculation of the gain spectra at different pump levels is provided, which showed that at a concentration of nonequilibrium carriers of 1.5∙1013 cm−2, the gain can exceed 5000 cm−1.

P124 Differences in OSA pathophysiological traits between mandibular advancement therapy responders versus non-responses and the influence of obesity

Abstract Mandibular advancement therapy (MAS) is a recognised second-line therapy for obstructive sleep apnoea (OSA). However, MAS treatment outcomes vary and are difficult to predict. Recent studies have investigated the role of OSA endotypes to predict MAS therapy outcomes. However, whether OSA endotype predictors differ between obese and non-obese people with OSA is unknown. Thus, this study aimed to compare OSA endotypes between responders and non-responders to MAS therapy in obese and non-obese individuals. 90 people with OSA (AHI&amp;gt;10events/h) were studied. OSA was confirmed via in-laboratory polysomnography. A detailed physiology night was subsequently performed prior to MAS therapy. OSA endotypes were estimated from the detailed physiology polysomnography using a custom-designed, validated, semiautomated script. OSA endotypes were compared between responders (residual AHI&amp;lt;10events/h) and non-responders to MAS therapy. Further comparisons were made between obese and non-obese groups. Responders to MAS therapy had a less collapsible upper airway; Vpassive: 93[88,96]vs.85[62,94]%Veupnea,p=0.003); Vactive: 103[95,112]vs.98[16,104] %Veupnea,p=0.008), better pharyngeal muscle compensation Vcomp: 9.8[4.3,19.6]vs.3.3[-19.1,18.6]%Veupnea,p=0.02), lower arousal threshold (114[109,144]vs. 142 [117, 177]%Veupnea,p=0.006) and tended to also have lower loop gain (0.48 ±0.1vs. 0.42±0.1,p=0.05). . These findings were similar in obese versus non-obese individuals although loop gain was significantly lower in responders versus non-responders in the non-obese (p=0.01) but not the obese group (0.97). MAS therapy was most beneficial in people with a less collapsible upper airway, good pharyngeal muscle compensation, lower arousal threshold and loop gain at baseline. Prospective assessment of OSA endotypic traits may therefore help guide treatment decisions and improve MAS therapy outcomes.

Facet Engineering for Amplified Spontaneous Emission in Metal Halide Perovskite Nanocrystals.

Auger recombination and thermalization time are detrimental in reducing the gain threshold of optically pumped semiconductor nanocrystal (NC) lasers for future on-chip nanophotonic devices. Here, we report the design strategy of facet engineering to reduce the gain threshold of amplified spontaneous emission by manyfold in NCs of the same concentration and edge length. We achieved this hallmark result by controlling the Auger recombination rates dominated by processes involving NC volume and thermalization time to the emitting states by optimizing the number of facets from 6 (cube) to 12 (rhombic dodecahedron) and 26 (rhombicuboctahedrons) in CsPbBr3 NCs. For instance, we demonstrate a 2-fold reduction in Auger recombination rates and thermalization time with increased number of facets. The gain threshold can be further reduced ∼50% by decreasing the sample temperature to 4 K. Our systematic studies offer a new method to reduce the gain threshold that ultimately forms the basis of nanolasers.

Secrecy Throughput Optimization and Precoding Design in Adaptive Transmit Antenna Selection Systems With Limited Feedback

We analyze the secrecy performance of a wireless system with limited feedback, where the channel state information (CSI) is imperfectly known at a multi-antenna source. In order to mitigate the adverse effects of limited feedback and eavesdropper attack on secrecy transmission, we propose the quantized secrecy precoding (QSP) oriented adaptive transmit antenna selection (ATAS) scheme denoted as QSP-ATAS, where the transmit antennas with poor channel quality are inactivated by adjusting the channel gain threshold. Besides, the conventional single transmit antenna selection (STAS) scheme without secrecy precoding (NSP-STAS), where only the optimal transmit antenna has the chance to be selected for transmission, is considered as a baseline. Since that both the number of active transmit antennas and the number of quantization bits affect the quantization error, we define the effective secrecy throughput as the difference between the secrecy data rate and feedback rate to evaluate the system performance. We conduct an effective secrecy throughput analysis for both QSP-ATAS and NSP-STAS schemes and show the existence of the maximal effective secrecy throughput of proposed QSP-ATAS scheme. Furthermore, an optimization analysis of the QSP-ATAS scheme is carried out for the sake of further improving the effective secrecy throughput with regard to the channel gain threshold. Numerical simulation results demonstrate that our proposed QSP-ATAS scheme performs better than the conventional NSP-STAS scheme in terms of the effective secrecy throughput.

Stimulated Brillouin Scattering Threshold in Presence of Modulation Instability for Optical Pulse in Long Optical Fiber

A theoretical and experimental study on the stimulated Brillouin scattering (SBS) threshold for optical pulse in the presence of modulation instability (MI) in long optical fiber is presented. The effects of MI on the SBS gain and threshold are analyzed based on the coupled-wave equation. An analytic expression is obtained to calculate the SBS threshold in the presence of MI. Numerical simulation is conducted to study the effects of the repetition rate and amplified spontaneous emission (ASE) noise on the SBS threshold for optical pulses. An experiment is conducted, and the results agree well with the theoretical analysis. The results clearly reveal how MI affects the Brillouin gain and SBS threshold of optical pulses in long optical fiber. The SBS threshold is affected by the ASE noise level and the repetition rate of the optical pulse. The study is helpful for the power evaluation of interferometric fiber sensing systems and optical power transmission systems.