Ultralow Threshold Research Articles

Ultrasensitive and ultrastretchable electrically self-healing conductors

Self-healing is a bioinspired strategy to repair damaged conductors under repetitive wear and tear, thereby largely extending the life span of electronic devices. The self-healing process often demands external triggering conditions as the practical challenges for the widespread applications. Here, a compliant conductor with electrically self-healing capability is introduced by combining ultrahigh sensitivity to minor damages and reliable recovery from ultrahigh tensile deformations. Conductive features are created in a scalable and low-cost fabrication process comprising a copper layer on top of liquid metal microcapsules. The efficient rupture of microcapsules is triggered by structural damages in the copper layer under stress conditions as a result of the strong interfacial interactions. The liquid metal is selectively filled into the damaged site for the instantaneous restoration of the metallic conductivity. The unique healing mechanism is responsive to various structural degradations including microcracks under bending conditions and severe fractures upon large stretching. The compliant conductor demonstrates high conductivity of ∼12,000 S/cm, ultrahigh stretchability of up to 1,200% strain, an ultralow threshold to activate the healing actions, instantaneous electrical recovery in microseconds, and exceptional electromechanical durability. Successful implementations in a light emitting diode (LED) matrix display and a multifunctional electronic patch demonstrate the practical suitability of the electrically self-healing conductor in flexible and stretchable electronics. The developments provide a promising approach to improving the self-healing capability of compliant conductors.

Fowler-Nordheim tunneling mechanism Graphene/GaN ultraviolet position-sensitive detector with high precision for optoelectronic demodulation applications

As an important component of optical sensors, position-sensitive detector (PSD) plays an important role in non-contact measurement systems and is widely used in many important fields. However, the preparation and application of ultraviolet (UV) PSD are still to be studied. Herein, we propose a high precision UV PSD constructed by graphene (Gr)/GaN heterojunction (active area is 5 ×5 mm2) for the first time, and Fowler-Nordheim tunneling mechanism has been used to improve device performance. Benefiting from the FNT, under a weak light power of 0.06 mW/cm2, the PSD is able to determine the position of the UV light spot through the output photocurrent difference, displays high precision (0.16 μA/mm), excellent weak light detection capability，and good linearity without power supply. Besides, the Gr/GaN show an ultralow threshold voltage (0.132 V) of FNT. When the bias is higher than 0.132 V, photogenerated carriers will tunneling the thin insulation layer by the FNT, resulting in a multiplication of the photocurrent. Therefore, under the same power densities light irradiation (0.06 mW/cm2), the device exhibits an ultrahigh responsivity of 1.88 A/W, a remarkable detectivity of 2.71 × 1013 Jones, and a high LDR of over 99.58 at 0 V bias. More importantly, as an optoelectronic demodulator the device realizes the decoding of optical signals into electrical signals, showing great potential for application in non-contact measurement systems.

Magnetically tunable ultralow threshold optical bistability by exciting LRSPR in InAs/graphene layers at the terahertz region.

With the experimental corroboration employing a transfer matrix method, an analytical observation of optical bistability using long-range surface plasmon resonance (LRSPR) through the external magnetic field is presented for a very low threshold value. The proposed analytical method has been verified with the reported experimental data provided by Liu etal. [Curr. Appl. Phys.29, 66 (2021)1567-173910.1016/j.cap.2021.06.003]. Now theoretical analysis is further extended in the proposed multilayered structure comprising an InAs layer sandwiched between two graphene layers, whose electromagnetic response at 2THz can be regulated by employing a magnetic field and may tune the optical bistability without modifying the geometry or the characteristics of the structure. The observed threshold intensity for the switch-up is 6.6615×104 W/c m 2 at 0.001T; thus, this analytical approach is able to achieve 2 orders lower threshold for magnetically tunable upswitching of the optical bistable process. This suggested magnetically adjustable optical bistable arrangement gives a possibility for the comprehension of optical logic gates, optic memory, opto-transistors, and switches at a low switching threshold due to extraordinary features of the composite layers due to local field amplification of the graphene layer.

Ultra-low threshold continuous-wave quantum dot mini-BIC lasers

Highly compact lasers with ultra-low threshold and single-mode continuous wave (CW) operation have been a long sought-after component for photonic integrated circuits (PICs). Photonic bound states in the continuum (BICs), due to their excellent ability of trapping light and enhancing light-matter interaction, have been investigated in lasing configurations combining various BIC cavities and optical gain materials. However, the realization of BIC laser with a highly compact size and an ultra-low CW threshold has remained elusive. We demonstrate room temperature CW BIC lasers in the 1310 nm O-band wavelength range, by fabricating a miniaturized BIC cavity in an InAs/GaAs epitaxial quantum dot (QD) gain membrane. By enabling effective trapping of both light and carriers in all three dimensions, ultra-low threshold of 12 μW (0.052 kW cm−2) is achieved at room temperature. Single-mode lasing is also realized in cavities as small as only 5 × 5 unit cells (~2.5 × 2.5 μm2 cavity size) with a mode volume of 1.16(λ/n)3. The maximum operation temperature reaches 70 °C with a characteristic temperature of T0 ~93.9 K. With its advantages in terms of a small footprint, ultra-low power consumption, and adaptability for integration, the mini-BIC lasers offer a perspective light source for future PICs aimed at high-capacity optical communications, sensing and quantum information.

Ultralow threshold surface emitting ultraviolet lasers with semiconductor nanowires

Surface-emitting (SE) semiconductor lasers have changed our everyday life in various ways such as communication and sensing. Expanding the operation wavelength of SE semiconductor lasers to shorter ultraviolet (UV) wavelength range further broadens the applications to disinfection, medical diagnostics, phototherapy, and so on. Nonetheless, realizing SE lasers in the UV range has remained to be a challenge. Despite of the recent breakthrough in UV SE lasers with aluminum gallium nitride (AlGaN), the electrically injected AlGaN nanowire UV lasers are based on random optical cavities, whereas AlGaN UV vertical-cavity SE lasers (VCSELs) are all through optical pumping and are all with large lasing threshold power densities in the range of several hundred kW/cm2 to MW/cm2. Herein, we report ultralow threshold, SE lasing in the UV spectral range with GaN-based epitaxial nanowire photonic crystals. Lasing at 367 nm is measured, with a threshold of only around 7 kW/cm2 (~ 49 μJ/cm2), a factor of 100× reduction compared to the previously reported conventional AlGaN UV VCSELs at similar lasing wavelengths. This is also the first achievement of nanowire photonic crystal SE lasers in the UV range. Further given the excellent electrical doping that has already been established in III-nitride nanowires, this work offers a viable path for the development of the long-sought-after semiconductor UV SE lasers.

On-Chip Ultralow-Threshold Tunable CdSSe Nanobelt Lasers Excited by the Emission of Linked ZnO Nanowire.

The integration of optical waveguide and on-chip nanolasers source has been one of the trends in photonic devices. For on-chip nanolasers, the integration of nanowires and high antidamage ability are imperative. Herein, we realized the on-chip ultralow-threshold and wavelength-tunable lasing from alloyed CdSSe nanobelt chip that is excited by the emission from linked ZnO nanowires. ZnO nanowire arrays are integrated into CdSSe nanobelt chips by the dry transfer method. A one-dimensional (1D) ZnO nanowire forms high-quality optical resonators and serves as an indirect pumping light to stimulate CdSSe nanobelt chips, and then wavelength-tunable lasing is generated with the ultralow threshold of 3.88 μW. The lasing mechanism is quite different than direct excitation by nanosecond laser pulse and indirect pumping by ZnO emission. The ZnO-CdSSe blocks provide a new solution to realize nanowire lasing from linked nanowires rather than direct laser pumping and thus avoid the light direct damage under general nanosecond laser excitation.

Fabry–Perot Oscillation and Resonance Energy Transfer: Mechanism for Ultralow-Threshold Optically and Electrically Driven Random Laser in Quasi-2D Ruddlesden–Popper Perovskites

The recently emerged metal-halide hybrid perovskite (MHP) possesses superb optoelectronic features, which have obtained great attention in solid-state lighting, photodetection, and photovoltaic applications. Because of its excellent external quantum efficiency, MHP has promising potential for the manifestation of ultralow threshold optically pumped laser. However, the demonstration of an electrically driven laser remains a challenge because of the vulnerable degradation of perovskite, limited exciton binding energy (Eb), intensity quenching, and efficiency drop by nonradiative recombinations. In this work, based on the paradigm of integration of Fabry-Perot (F-P) oscillation and resonance energy transfer, we observed an ultralow-threshold (∼250 μWcm-2) optically pumped random laser from moisture-insensitive mixed dimensional quasi-2D Ruddlesden-Popper phase perovskite microplates. Particularly, we demonstrated an electrically driven multimode laser with a threshold of ∼60 mAcm-2 from quasi-2D RPP by judicious combination of a perovskite/hole transport layer (HTL) and an electron transport layer (ETL) having suitable band alignment and thickness. Additionally, we showed the tunability of lasing modes and color by driving an external electric potential. Performing finite difference time domain (FDTD) simulations, we confirmed the presence of F-P feedback resonance, the light trapping effect at perovskite/ETL, and resonance energy transfer contributing to laser action. Our discovery of an electrically driven laser from MHP opens a useful avenue for developing future optoelectronics.

Enhanced Mechanical Property of Polyamide-6/Graphite Sheet Composites with Segregated 3D Network Binary Structure for High Thermal Conductivity.

Segregated conductive polymer composites exhibit excellent electrical properties with a low percolation threshold. However, the mechanical properties of the segregated conductive polymer composites were always poor because the conductive fillers at the interfaces hinder polymer chain diffusion and thus lead to weak interfacial interaction between the conductive fillers and the polymer matrix. In this paper, polyamide-6 and polyamide-612 microspheres were synthesized via the in situ anionic ring opening of caprolactam and laurolactam. Segregated graphite sheets/polyamide-6(GS/PA6) and polyamide-612(PA612) composites with good mechanical properties were realized via high-pressure solid-phase compression molding. The microstructures of the composite samples were observed by scanning electron microscopy, which showed that the formation of a GS-conductive network at the PA6 granule interfaces in the segregated conductive structures and the adopting of PA612 considerably improved the interfacial adhesion of the composites. A superior impact strength of 5.1 kJ/m2 was achieved with 50 wt% PA612 loading owing to improvements in the interface compatibility between PA6 and GS. The composites possessed an ultralow percolation threshold, which was ascribed to the segregated network structure being successfully constructed inside the material. As for GS/PA6 composites, the combination of segregated GS-conductive networks achieved an ultralow percolation of 2.8 vol%. The percolation of 80PA6/20PA612-GS composites was slightly higher, measuring up to 3.2 vol%. Moreover, the thermal conductivity of the 80PA6/20PA612-GS composites increased from 0.26 to around 0.5 W/(m·K), which was 1.9 times larger than the pure polyamide.

Near-Full Current Dynamic Range THz Quantum Cascade Laser Frequency Comb.

The present study proposes a terahertz quantum cascade laser frequency comb (THz QCL FC) with a semi-insulated surface plasma waveguide characterized by a low threshold current density, high power and a wide current dynamic range. The gain dispersion value and the nonlinear susceptibility were optimized based on the combination of a hybrid bound-to-continuum active region with a semi-insulated surface plasmon waveguide. Without any extra dispersion compensator, stable frequency comb operation within a current dynamic range of more than 97% of the total was revealed by the intermode beat note map. Additionally, a total comb spectral emission of about 300 GHz centered around 4.6 THz was achieved for a 3 mm long and 150 µm wide device. At 10 K, a maximum output power of 22 mW was obtained with an ultra-low threshold current density of 64.4 A·cm-2.

Hybrid assembly of polymeric nanofiber network for robust and electronically conductive hydrogels

Electroconductive hydrogels have been applied in implantable bioelectronics, tissue engineering platforms, soft actuators, and other emerging technologies. However, achieving high conductivity and mechanical robustness remains challenging. Here we report an approach to fabricating electroconductive hydrogels based on the hybrid assembly of polymeric nanofiber networks. In these hydrogels, conducting polymers self-organize into highly connected three dimensional nanostructures with an ultralow threshold (~1 wt%) for electrical percolation, assisted by templating effects from aramid nanofibers, to achieve high electronic conductivity and structural robustness without sacrificing porosity or water content. We show that a hydrogel composed of polypyrrole, aramid nanofibers and polyvinyl alcohol achieves conductivity of ~80 S cm−1, mechanical strength of ~9.4 MPa and stretchability of ~36%. We show that patterned conductive nanofiber hydrogels can be used as electrodes and interconnects with favorable electrochemical impedance and charge injection capacity for electrophysiological applications. In addition, we demonstrate that cardiomyocytes cultured on soft and conductive nanofiber hydrogel substrates exhibit spontaneous and synchronous beating, suggesting opportunities for the development of advanced implantable devices and tissue engineering technologies.

Ultralow-threshold quasi-CW lasing from FAPbBr3 perovskite first-order DFB laser

Metal halide perovskites are emerging materials for integrated photonics. Here we report a quasi-CW pumped ultra-low ASE/lasing threshold formamidinium lead bromide (FAPbBr3) laser. The laser achieved stable lasing at 555 nm with a full width at half maximum (FWHM) of 0.6 nm, showing a low lasing threshold of 22.6 μJ cm−2 under 3.5 nanosecond quasi-CW excitation at room temperature. The material also showed an ultra-low ASE threshold of 46 μJ cm−2 under the same pumping condition. Through polymer doping, we showed that the material’s performance can be improved by increasing bimolecular recombination rate with reduced grain size.

Stable continuous-wave lasing from discrete cesium lead bromide quantum dots embedded in a microcavity.

All-inorganic cesium lead bromide (CsPbBr3) quantum dots (QDs) with high photoluminescence (PL) quantum efficiency have been reported as ideal gain materials for high-performance lasers. Nevertheless, isolated CsPbBr3 QDs have not achieved lasing emission (LE) due to finite absorption cross-section. Here, we demonstrate continuous-wave lasing of isolated CsPbBr3 QDs embedded in a microcavity. Distributed Bragg reflectors (DBRs), together with isolated CsPbBr3 QDs in a polymer matrix, are introduced to construct a vertical-cavity surface-emitting laser (VCSEL), which exhibits stable single-mode lasing emissions with an ultra-low threshold of 8.8 W cm-2 and a high Q factor of 1787. Such perovskite-based microcavity structures sustain highly stable excitons at room temperature and can provide an excellent experimental platform to further study the single-particle nano-lasers and quantum physics frontiers such as exciton-polariton condensation, single-photon emission, and optical quantum communication.

Ultralow percolation threshold biodegradable PLA/PBS/MWCNTs with segregated conductive networks for high‐performance electromagnetic interference shielding applications

AbstractThe formation of dense and complete conductive networks in the electromagnetic interference (EMI) shielding composite is the basis for its excellent EMI shielding performance. In this work, biodegradable poly (lactic acid)/poly (butylene succinate)/multi‐walled carbon nanotubes (PLA/PBS/MWCNTs) nanocomposites with segregated structures were successfully prepared via melt blending. Due to the successful preparation of segregated structures and the fact that MWCNTs were mainly dispersed in the PBS phase, an ultralow percolation value of 0.071 vol% was achieved in biodegradable PLA/PBS/MWCNTs nanocomposites. When the MWCNTs content is 0.499 vol%, the electrical conductivity of the PLA/PBS/MWCNTs nanocomposites with segregated structures is around 7.15 × 10−3 S/m, which is about 6 orders of magnitude higher than that of the PLA/PBS/MWCNTs nanocomposites with normal structures. When the MWCNTs content increased to 2.0 wt%, the average EMI shielding effectiveness (SE) of segregated structures remained stable at 27.56 dB, which can effectively block 99.82% of the microwave radiation. Furthermore, as suggested in the EMI shielding analysis, the EMI shielding of PLA/PBS/MWCNTs nanocomposites is mainly through absorption shielding, so there will be no secondary environmental pollution. This study provides a practical and universal method to prepare biodegradable conductive polymer composites with ultralow percolation threshold and excellent EMI SE.

Reconfigurable Low-Threshold All-Optical Nonlinear Activation Functions Based on an Add-Drop Silicon Microring Resonator

The realization of optical nonlinear activation functions (NAFs) is essential for integrated optical neural networks (ONNs). Here, we propose and experimentally demonstrate a photonic method to implement reconfigurable and low-threshold all-optical NAFs based on a compact and high-Q add-drop microring resonator (MRR) on silicon. In the experiment, four different NAFs including softplus, radial basis, clamped ReLU, and sigmoid functions are realized by exploiting the thermo-optical (TO) effect of the MRR. The threshold to implement NAFs is as low as 0.08 mW. As a demonstration, a handwritten digit classification benchmark task is simulated based on a convolutional neural network (CNN) using the obtained activation functions, where an accuracy of 98% is realized. Thanks to the unique advantages of ultra-compact footprint and ultralow threshold, the proposed nonlinear unit is promising to be widely used in large-scale integrated ONNs.

GaAs on (001) Si templates for near-infrared InP quantum dot lasers

We investigated the effects of thermal cycle annealing (TCA) at high temperatures on the defect density and morphology of GaAs epilayers grown on (001) Si substrates. Several types of TCA combined with dislocation filter layers (DFLs) were introduced to reduce the defect densities in the GaAs grown on Si substrates. Plan-view transmission electron microscopy shows that the defect density of a 2.7 μm-thick GaAs/Si template with optimized TCA and DFLs is 1.4 × 107 cm−2, lower than a 1.1 μm-thick GaAs/Si template by a factor of 40. The surface roughness of the optimized GaAs/Si template is 1.3 nm after insertion of the DFLs. Additionally, optically pumped InP quantum dot micro-disk lasers (MDLs) were fabricated on these GaAs/Si templates to evaluate the template quality. Room-temperature continuous-wave lasing of 1.5 μm-diameter MDLs was observed, with ultralow lasing thresholds ranging from 0.5 to 2 μW.

All-optical switch with ultrahigh switching efficiency and ultralow threshold energy based on a one-dimensional PT-symmetric ring optical waveguide network

In this work, we propose an all-optical switch based on one-dimensional (1D) parity-time-symmetric (PT-symmetric) ring optical waveguide network (PTSROWN). By use of the resonant coupling effect of the PT-symmetric structure, the all-optical switch based on our designed 1D PTSROWN generates an ultrahigh transmissivity of 1.42×1018 at ON state, which breaks the bottleneck of the transmissivity of 1 at ON state produced by the all-optical switches made of dielectric optical waveguide networks, photonic crystals, microring resonators, and meanwhile, creates an ultralow transmissivity of 1.01×10−18 at OFF state. It causes our designing all-optical switch reproducing extraordinary large switch efficiency of 1.41×1036, which is 10 orders of magnitude greater than the optimal results reported yet. On the other hand, the super resonant coupling effects can also make the all-optical switch generate ultrastrong photonic localization at ON state, and then drastically reduce the threshold energy. In addition, we have deeply optimized the PT-symmetric optical waveguide network and obtain better extremum spontaneous PT-symmetric breaking points. On this condition, our designing all-optical switch can produce an ultralow threshold energy of 4.3×10−34 J, which is 12 orders of magnitude smaller than the best reported results. This not only provides a better choice for developing all-optical switching devices with better performance, but also possesses a broad application prospect in designing high-efficiency optical amplifiers, optical energy saver devices, etc.

Ultra-Efficient Optical Gain and Lasing in MDACl2-Doped Perovskite Thin Films

Solution-processed perovskite materials have been enticing candidates for optical gain and lasing media because of their low cost, remarkable color purity, facile bandgap tunability, and high absorption cross-section. However, it is difficult, if not impossible, for them to highly amplify light with stable operations because they experience severe non-radiative emission losses due to their high density of surface and bulk defect centers and irregular composition. Here, we report that incorporating 5% of methylenediammonium dichloride (MDACl2) into mixed perovskite systems leads to a drastic reduction in the density of iodide interstitials and surface recombination losses. Subsequently, we record that MDA-treated, cavity-free thin perovskite films exhibit very photostable and ultra-low threshold amplified spontaneous emission of <6 μJ/cm2, approximately 30 times lower than the untreated films. Moreover, an ultra-high optical gain of 1000 cm–1 is successfully achieved, representing the highest reported gain for cavity-free perovskite films. These results are fully supported by extensive high-level density functional theory calculations and ultrafast transient absorption measurements. These findings will serve as a benchmark for the design and fabrication of low-threshold, high amplification perovskite media for lasers and light emission applications.

Pump tuning of a mid-infrared whispering gallery optical parametric oscillator.

Optical parametric oscillators (OPOs) constitute an important coherent, narrow-linewidth and widely tunable light source with applications in spectroscopy and many other fields. Their realizations based on whispering gallery resonators (WGRs) provide a small footprint and ultra-low thresholds, with demonstrations of tunability typically done via temperature variation. In this work, we show the pump tuning capabilities of a mm-sized WGR mid-infrared OPO made of CdSiP2. By tuning a telecom wavelength diode laser by 16 nm, we generate tunable light from 2708 to 3575 nm. Furthermore, we show controlled tuning in steps of 1 free spectral range (FSR) and the possibility of 12 GHz of continuous tuning. All these features are in good agreement with the theoretical predictions. We conclude that tuning from 2.4 to 4.9 µm is even possible, while still using commercially available near-infrared diode lasers. This work highlights the advantages of pump tuning of WGR OPOs and provides valuable insights for their precise control.

Low-threshold sapphire detector for rare event searches

Low mass nuclear recoil dark matter and coherent-elastic-neutrino-nucleus-scattering (CEνNS) searches confront similar challenges in choosing ultra-low threshold and large-mass detectors. We report experimental results from a 100 g single-crystal sapphire detector design with a diameter of 76 mm and thickness of 4 mm instrumented with transition edge sensors (TES). Sapphire is a crystal of aluminum oxide (Al2O3) and has been found to be a good candidate for light mass dark matter search experiments due to its lower atomic mass compared to other detector materials such as germanium and silicon. This new phonon-assisted sapphire detector was characterized to yield a baseline recoil energy resolution of 28.4 ± 0.4 eV. The detector is designed to be sensitive to low-energy rare interactions with an intention to investigate the low-mass region of dark matter phase-space and search for CEνNS at the reactor site.

Ultralow Threshold Lasing from a Continuous-Wave-Pumped SiNx/CsPbBr3/Ag Thin Film Mediated by the Whispering Gallery Modes of a SiO2 Microsphere

Thin-film pervoskite lasers driven by a continuous wave (CW) laser with ultralow thresholds, which is crucial for the development of on-chip electrically driven lasers, have not yet been realized owing to the low excitation power density of the CW laser. Here, we reported the CW-laser-pumped lasing from a thin film of CsPbBr3 quantum dots (QDs) sandwiched by a SiNx and a Ag thin film and mediated by the whispering gallery modes of a SiO2 microsphere. The stable photoluminescence from CsPbBr3 QDs with a quantum efficiency of ∼45% is realized by encapsulating with a thin SiNx film. Upon CW-laser pumping, lasing from the whispering gallery modes with a threshold of ∼11.6 W/cm2 is successfully demonstrated at room temperature. The strong localization of electric field achieved in the particle-on-film system, which is revealed in the numerical simulations and lifetime measurements, plays a crucial role in the realization of the ultralow threshold lasing. Our findings open a new avenue for designing photostable CW-laser-pumped pervoskite lasers.