Showing papers by "Yuan Gao published in 2021"

All-Inorganic Quantum-Dot LEDs Based on a Phase-Stabilized α-CsPbI 3 Perovskite

Abstract: The all-inorganic nature of CsPbI3 perovskites allows to enhance stability in perovskite devices. Research efforts have led to improved stability of the black phase in CsPbI3 films; however, these strategies-including strain and doping-are based on organic-ligand-capped perovskites, which prevent perovskites from forming the close-packed quantum dot (QD) solids necessary to achieve high charge and thermal transport. We developed an inorganic ligand exchange that leads to CsPbI3 QD films with superior phase stability and increased thermal transport. The atomic-ligand-exchanged QD films, once mechanically coupled, exhibit improved phase stability, and we link this to distributing strain across the film. Operando measurements of the temperature of the LEDs indicate that KI-exchanged QD films exhibit increased thermal transport compared to controls that rely on organic ligands. The LEDs exhibit a maximum EQE of 23 % with an electroluminescence emission centered at 640 nm (FWHM: ≈31 nm). These red LEDs provide an operating half-lifetime of 10 h (luminance of 200 cd m-2 ) and an operating stability that is 6× higher than that of control devices.

Deep-Blue Perovskite Single-Mode Lasing through Efficient Vapor-Assisted Chlorination.

Abstract: Metal halide perovskites have emerged as promising candidates for solution-processed laser gain materials, with impressive performance in the green and red spectral regions. Despite exciting progress, deep-blue-an important wavelength for laser applications-remains underexplored; indeed, cavity integration and single-mode lasing from large-bandgap perovskites have yet to be achieved. Here, a vapor-assisted chlorination strategy that enables synthesis of low-dimensional CsPbCl3 thin films exhibiting deep-blue emission is reported. Using this approach, high-quality perovskite thin films having a low surface roughness (RMS ≈ 1.3 nm) and efficient charge transfer properties are achieved. These enable us to document low-threshold amplified spontaneous emission. Levering the high quality of the gain medium, vertical-cavity surface-emitting lasers with a low lasing threshold of 6.5 µJ cm-2 are fabricated. This report of deep-blue perovskite single-mode lasing showcases the prospect of increasing the range of deep-blue laser sources.

NDIR CO2 gas sensing using CMOS compatible MEMS ScAlN-based pyroelectric detector

Abstract: We demonstrate NDIR CO2 gas sensing using CMOS compatible MEMS ScAlN-based pyroelectric detectors. The ScAlN-based pyroelectric detectors are fabricated using 8-inch wafer level technology with 12 % Sc-doped AlN deposited at a temperature of ∼200 °C. Together with a blackbody thermal emitter, a 10 cm long enclosed gas channel with only inlet and outlet holes connected to tubings, and testing using 2 different reference gases (N2 and synthetic air), measurements show voltage signal drop due to CO2 gas absorption at the 4.26 μm wavelength at CO2 gas concentrations ranging from 5000 ppm down to 25 ppm. The signal change due to the CO2 gas response ranges from ∼2% at 100 ppm CO2 concentration to ∼40 % at 5000 ppm CO2 gas concentration for both CO2 gas measured in N2 and in synthetic air. CO2 gas response times are also measured for CO2 gas in N2 and in synthetic air at concentrations of 5000 ppm, 1000 ppm and 400 ppm. The gas response times measured around 2 s and lower. Introduction of humidity show some minor effect (

Quantum Dot Self-Assembly Enables Low-Threshold Lasing.

Abstract: Perovskite quantum dots (QDs) are of interest for solution-processed lasers; however, their short Auger lifetime has limited lasing operation principally to the femtosecond temporal regime the photoexcitation levels to achieve optical gain threshold are up to two orders of magnitude higher in the nanosecond regime than in the femtosecond. Here the authors report QD superlattices in which the gain medium facilitates excitonic delocalization to decrease Auger recombination and in which the macroscopic dimensions of the structures provide the optical feedback required for lasing. The authors develope a self-assembly strategy that relies on sodiumd-an assembly director that passivates the surface of the QDs and induces self-assembly to form ordered three-dimensional cubic structures. A density functional theory model that accounts for the attraction forces between QDs allows to explain self-assembly and superlattice formation. Compared to conventional organic-ligand-passivated QDs, sodium enables higher attractive forces, ultimately leading to the formation of micron-length scale structures and the optical faceting required for feedback. Simultaneously, the decreased inter-dot distance enabled by the new ligand enhances exciton delocalization among QDs, as demonstrated by the dynamically red-shifted photoluminescence. These structures function as the lasing cavity and the gain medium, enabling nanosecond-sustained lasing with a threshold of 25 µJ cm-2 .

Non-diffracting and self-accelerating Bessel beams with on-demand tailored intensity profiles along arbitrary trajectories.

Abstract: Owing to their robustness against diffraction, Bessel beams (BBs) offer special advantages in various applications. To enhance their applicability, we present a method to generate self-accelerating zeroth-order BBs along predefined trajectories with tunable z direction intensity profiles. The character of tunable z direction intensity profiles in non-diffracting self-accelerating BBs potentially can attract interest in the regimes of particle manipulation, microfabrication, and free-space optical interconnects.

Linear Electro-Optic Modulation in Highly Polarizable Organic Perovskites

Abstract: Electrical-to-optical signal conversion is widely employed in information technology and is implemented using on-chip optical modulators. State-of-the-art modulator technologies are incompatible with silicon manufacturing techniques: inorganic nonlinear crystals such as LiNbO3 are integrated with silicon photonic chips only using complex approaches, and hybrid silicon-LiNbO3 optical modulators show either low bandwidth or high operating voltage. Organic perovskites are solution-processed materials readily integrated with silicon photonics; and organic molecules embedded within the perovskite scaffold allow in principle for high polarizability. However, it is found that the large molecules required for high polarizability also require an increase of the size of the perovskite cavity: specifically, using the highly polarizable DR2+ (R = H, F, Cl) in the A site necessitates the exploration of new X-site options. Only by introducing BF4- as the X-site molecule is it possible to synthesize (DCl)(NH4 )(BF4 )3 , a material exhibiting a linear EO coefficient of 20 pm V-1 , which is 10 times higher than that of metal halide perovskites and is a 1.5 fold enhancement compared to reported organic perovskites. The EO response of the organic perovskite approaches that of LiNbO3 (reff ≈ 30 pm V-1 ) and highlights the promise of rationally designed organic perovskites for use in efficient EO modulators.

A 310-nA Quiescent Current 3-fs-FoM Fully Integrated Capacitorless Time-Domain LDO With Event-Driven Charge Pump and Feedforward Transient Enhancement

Abstract: In this article, a fully integrated capacitorless low-dropout regulator (LDO) is presented for Internet-of-Things (IoT) edge sensor application. To achieve sub-1-V operation and fast transient response with low quiescent current, the conventional operational transconductance amplifier (OTA)-based error amplifier (EA) is replaced with oscillator-based voltage-to-time converter and time-domain signal processing, including time-domain edge-based frequency comparator (FC) and event-driven voltage mode charge pump (CP). Compared with the conventional phase frequency detector (PFD), the proposed clock-edge-based FC achieved more than six times power reduction. Event-driven CP is adopted to drive analog power transistor and the transient response is enhanced by feedforward capacitor $C_{\mathrm {FD}}$ and coarse–fine CP control. To further reduce the power consumption, multi-voltage domain and clock frequency optimization are implemented. A prototype chip is fabricated in a standard 65-nm CMOS process. The design only consumes 310-nA quiescent current while achieving 0.5–1.2-V input range, $1.0\times 10^{6}$ load dynamic range, and 3-fs figure of merit (FoM).

Electro-Optic Modulation Using Metal-Free Perovskites.

Abstract: Electro-optic (EO) modulation is of interest to impart information onto an optical carrier. Inorganic crystals-most notably LiNbO3 and BaTiO3-exhibit EO modulation and good stability, but are difficult to integrate with silicon photonic technology. Solution-processed organic EO materials are readily integrated but suffer from thermal degradation at the temperatures required in operating conditions for accelerated reliability studies. Hybrid organic-inorganic metal halide perovskites have the potential to overcome these limitations; however, these have so far relied on heavy metals such as lead and cadmium. Here, we report linear EO modulation using metal-free perovskites, which maintain the crystalline features of the inorganic EO materials and incorporate the flexible functionality of organic EO chromophores. We find that, by introducing a deficiency of cations, we reduce the symmetry in the perovskite crystal and produce thereby an increased EO response. The best-engineered perovskites reported herein showcase an EO coefficient of 14 pm V-1 at a modulation frequency of 80 kHz, an order of magnitude higher than in the nondefective materials. We observe split peaks in the X-ray diffraction and neutron diffraction patterns of the defective sample, indicating that the crystalline structure has been distorted and the symmetry reduced. Density functional theory (DFT) studies link this decreased symmetry to NH4+ deficiencies. This demonstration of EO from metal-free perovskites highlights their potential in next-generation optical information transmission.

Curvilinear Poincaré vector beams

Abstract: We develop a method for completely shaping optical vector beams with controllable amplitude, phase, and polarization gradients along three-dimensional freestyle trajectories. We design theoretically and demonstrate experimentally curvilinear Poincare vector beams that exhibit high intensity gradients and accurate state of polarization prescribed along the beam trajectory.

28.5 A 0.6V/0.9V 26.6-to-119.3µW ΔΣ-Based Bio-Impedance Readout IC with 101.9dB SNR and <0.1Hz 1/f Corner

Abstract: Bio-impedance (BioZ) is an important physiological parameter in wearable healthcare sensing. Besides the inherent cardiac and respiratory information, BioZ can be also used for other emerging applications such as non-invasive blood status sensing [1]. A conventiona14-e1ectrode (4E) setup eliminates the effect of electrode-tissue impedance (ETI) at the expense of user comfort, system complexity, and cost. On the other hand, a 2-electrode (2E) setup avoids short-falls of 4E but can only capture relative changes of Bi0Z instead of its absolute value. In addition, a readout front-end (RFE) with wide dynamic range (DR) and high signal-to-noise ratio (SNR) is needed to deal with small BioZ variation (0.1$\sim10\Omega$) as well as large baseline resistance (>10k$\Omega$). A conventional RFE architecture employing an instrumentation amplifier (IA) and ADC has to trade-off between resolution, DR and noise [2, 3]. Although flicker noise in the current generator (CG) is mitigated through dynamic element matching (DEM) [2], the reference current (IREF) noise issue remains unaddressed. In [5], digital-assisted baseline cancellation and IREF correlated noise cancellation are proposed, which help eliminate IREF noise and input-dependent noise [4] due to the large signal in the current-balance instrumentation amplifier (CBIA). Nevertheless, larger noise is still observed due to the finite residual current $(I_{res})$ from the baseline cancellation.

Twin curvilinear vortex beams.

Abstract: We report on a novel curvilinear optical vortex beam named twin curvilinear vortex beams (TCVBs) with intensity and phase distribution along a pair of two- or three-dimensional curves, both of which share the same shape and the same topological charge. The TCVBs also possess the character of perfect optical vortex, namely having a size independent of topological charge. We theoretically demonstrate that a TCVB rather than a single-curve vortex beam can be created by the Fourier transform of a cylindrically polarized beam. The behavior of TCVBs generated through our method is investigated by simulation and experiment, including interference experiments for identifying the vortex property of the TCVBs. The TCVBs may find applications in optical tweezers, such as trapping low refractive index particles in the dark region between two curves and driving them moving along the curvilinear trajectory.

Radially self-accelerating Stokes vortices in nondiffracting Bessel-Poincaré beams.

Abstract: We theoretically propose and experimentally generate the nondiffracting Bessel–Poincare beams whose Stokes vortices radially accelerate during propagation. To this end, we design the Bessel beams whose intensity is specified to be uniformly distributed along the longitudinal direction. By superposing two such Bessel beams having different helical phases and mutually orthogonal polarizations, the synthesized vector beam is endowed with the polarization singularity that can rotate about the optical axis, while the total intensities maintain their profiles. Radially self-accelerating Stokes vortices in the resulting beam can be manipulated by adjusting the predefined parameters in the constituent beams.

Impact of the spatial coherence on self-interference digital holography*

Abstract: Owing to the unique feature that the signal and reference waves of self-interference digital holography (SIDH) contain the same spatial information from the same point of object, compared with conventional digital holography, the SIDH has the special spatial coherence properties. We present a statistical optics approach to analyzing the formation of cross-correlation image in SIDH. Our study reveals that the spatial coherence of illumination light can greatly influence the imaging characteristics of SIDH, and the impact extent of the spatial coherence depends substantially on the recording distance of hologram. The theoretical conclusions are supported well by numerical simulation and optical experiments.

CO2Gas Sensing By Cmos-Mems Scaln-Based Pyroelectric Detector Based on MID-IR Absorption

Abstract: CO 2 gas sensing responses are measured using our in-house fabricated CMOS-MEMS ScAlN-based pyroelectric detector with 12% Sc doped concentration. Leveraging on mid-IR absorption behavior of CO 2 gas caused by interaction of CO 2 with light at 4.26 µm wavelength, our ScAlN-based pyroelectric detector successfully detect CO 2 gas response ranging from CO 2 concentrations of 5000 ppm down to 200 ppm. Gas channel lengths are varied from 6 cm to 10 cm and their respective CO 2 responses are measured. Finally, with N2 as the reference gas, we calculate the gas response percentage change of CO 2 gas at different gas channel lengths and concentration.

Design of Fully Differential Energy-Efficient Inverter-Based Low-Noise Amplifier for Ultrasound Imaging

Abstract: Inverter-based low-noise amplifier (LNA) offers an elegant solution in terms of power and area efficiency, which is favorable for ultrasound receivers. Nevertheless, it imposes challenges on circuit designs regarding the biasing circuit, the limited gain, robustness over process, supply voltage and temperature (PVT) variations. This paper presents an in-depth study of various inverter-based LNAs for ultrasound receiver design. In addition, three fully differential inverter-based LNAs are designed and optimized with minimum power and area penalty in the common-mode feedback (CMFB) circuit. The LNAs are implemented in a standard 0.18 - $\mu m$ CMOS process, with the identical power consumption, simulation results show that LNA with single CMFB achieves lower distortion, i.e. a third harmonic distortion (HD3) of $\lt -51.5$ dB with 35 mVp-p input, LNA with dual CMFB obtains better noise performance, i.e. $4.85 nV / \sqrt{Hz}$ at 3 MHz.

A 13.56 MHz Active Rectifier with PMOS AC-DC Interface for Wireless Powered Medical Implants

Abstract: A 13.56 MHz active rectifier for wireless powered medical implants is presented in this paper. Only PMOS power transistors are used in the AC-DC interface in the active rectifier to prevent the inherent parasitic diode conduction that may lead to potential latch-up during the rectifier start-up period. To eliminate the reverse current introduced by the delay of comparator signal, a delay compensation loop is designed to dynamically adjust the rectifier at various DC output. A fully onchip $2 \times$ charge pump and capacitor assisted driver/sampling circuits are proposed to turn on/off the power PMOS in the rectifier. The rectifier is implemented in a standard 0.18-$\mu$ m CMOS process with a core active area of 0.4 mm $\times0.9$ mm. The measurement results show that this PMOS rectifier achieves peak power efficiency of 85% when load resistor is $500 \Omega$ and the DC output voltage range is 2V--3.5V.