Optical Power Density Research Articles

Temperature characteristics of Rb-N2 single-beam magnetometer with different buffer gas pressures

We demonstrate the temperature characteristics of Rb-N2 single-beam spin-exchange relaxation-free (SERF) magnetometer with different buffer gas pressures. An optimization model is proposed to reveal that the optimal condition is determined by cell temperature and gas pressure simultaneously, where the magnetometer response is maximized. Three Rb-N2 vapor cells under different gas pressures are constructed with interior volume of 3 × 3 × 3 mm3, and a single-beam zero-field modulation magnetometer is carried out. For each individual vapor cell, the optical power density and cell temperature are optimized to obtain the maximum response signal. The unique optimal condition including cell temperature and gas pressure is determined with 1.55 amg at 148 ℃, which verifies the proposed optimization model. This study is significative for optimizing the miniature vapor cell parameters and contributes to further improving the performance of SERF magnetometer.

3.2: Elevating the Light Output Power Density of Scaling‐down AlGaN Ultraviolet‐C Micro‐LED

Ultraviolet band‐C (UV‐C) micro‐light‐emitting diodes (Micro‐LEDs) with high optical power density are increasingly demanded in the utilization of sterilization, solar‐blind communications, and neuroscience for the robust structure and adjustable emission wavelength. In this work, AlGaN UV‐C Micro‐LEDs are fabricated and characterized in 5×5, 10×10, 20×20, 30×30, 50×50, 80×80, and 100×100 µm2. With pixel size scaling down, the smaller devices have the potential to emit more considerable light output power (LOP) density at the same injected current density. This LOP density sizing effect implies higher luminescence efficiencies on small‐sized UV‐C Micro‐LEDs, which could be widely adopted by the industry.

Combination of porous covalent triazine frameworks with spinel for highly improved photothermal catalytic oxidation of toluene

The synergistic effect of adsorption and photothermal can be used to develop new efficient pollutant treatment technologies. Here, porous covalent triazine frameworks modified CuO@CoMn2O4 catalyst (PCTF-CCM) was constructed and applied to the full spectrum driven photothermal degradation of toluene. Toluene conversion and CO2 yield over PCTF-CCM under simulated solar irradiation (optical power density=350 mW/cm2) could be reached 100% and 98% with 40 min. Oxygen vacancies play an important role in thermal and photothermal catalysis by enhancing the adsorption of pollutant gases and the mobility of oxygen. The porous covalent triazine frameworks enriched the concentration of pollutants on the surface of catalyst and significantly improved the water resistant capability, leading to the performance of photothermal catalytic oxidation of toluene highly improved. The designed system extends the range and service life of catalyst in practical applications and provides a new idea for the design of adsorption-photothermal catalyst.

Parameter optimisation of miniaturised SERF magnetometer below relaxation rate saturation region

The miniaturised spin-exchange relaxation-free (SERF) magnetometer has promising applications in biomagnetic measurements owing to its ultrasensitive magnetic field measurement capacity. In this paper, we propose a single-beam parameter-optimised SERF magnetometer that operates below the relaxation rate saturation region. Based on the relaxation rate model under zero-field resonance, the relationship between the magnetometer optimal working point and multiphysical parameters, including the optical power density, cell temperature, and nitrogen density, was studied. The experimental results verified the accuracy of the law deduced from the model. By adjusting the parameters according to our experimental results, a sensitivity of 17 fT/Hz1/2 (@31.5Hz) and bandwidth of 110 Hz were obtained optimally in a 5.5 cm3 magnetometer. The superiority of the optimised parameters in the miniaturised SERF magnetometer is verified. We found that the relaxation rate saturation region results from light absorption saturation, which leads to a performance decline of the SERF magnetometer. This relaxation rate saturation phenomenon widely exists in miniaturised SERF magnetometers, and the study of this phenomenon is helpful for further applications in arrayed integration of biomagnetic measurement.

Wirelessly Operated, Implantable Flexible Optoelectrical Probes for Optogenetics Neural Stimulation

Optogenetic methods with high temporal and spatial specificity have rapidly become a powerful technique in neuroscience. However, implantable fibers coupled with external laser sources as the traditional stimulated approach, constrain the free movement of experimental animals. In this study, we introduced a wireless-operated miniaturized implantable system with superior biocompatibility. The injectable dual-channel flexible neural probe with blue and orange light-emitting diodes (LEDs) was controlled using a customized circuit module powered by a rechargeable lithium battery. For excitatory and inhibitory neurons, the optical power densities of the blue and orange LEDs transferred to the flexible substrate can reach 25 and 18 mW/mm2, respectively, under an injection current of 5 mA. Moreover, the 2.4 GHz Bluetooth wireless communication protocol integrated into the circuit module enables LEDs to be remotely operated using mobile devices at a distance as far as 22.5 m. The movement traces of behaving animals after stimulation with optical signals can be recorded using an accelerometer and feedback to the terminal through Bluetooth chips. The proposed system provides an effective route for evaluating the functions and analyzing the structures of complicated animals.

AlGaN/GaN‐Based Optoelectronic Synaptic Devices for Neuromorphic Computing

AbstractNeuromorphic computing promises to overcome the Von Neumann bottleneck of traditional computers. Optoelectronic synaptic devices are greatly desired given their potential applications in neuromorphic computing. In this work, optoelectronic synaptic devices with long‐term memory are fabricated. The devices are based on a GaN/AlGaN/AlN/GaN heterojunction and a SiNx charge trapping layer. Synaptic functionalities including excitatory postsynaptic current, paired pulse facilitation, and transition from short‐term memory to long‐term memory are realized. The retention time of long‐term memory may be more than 10 years, demonstrating the reliable charge storage of the devices. Logic functions are also realized by synergizing the photogating and electrical gating of the devices. The responsivity and specific detectivity are 2.64 × 105 A W−1 and 1.79 × 1016 Jones at the optical power density of 1.4 mW cm−2, respectively. In addition, the devices have a reconfigurable switch of 1000 cycles with working temperature up to 200 °C.

Efficient label-free in vivo photoacoustic imaging of melanoma cells using a condensed NIR-I spectral window

In this paper, we propose an efficient label-free in vivo photoacoustic (PA) imaging of melanoma using a condensed near infrared-I (NIR-I) supercontinuum light source. Although NIR-II spectral window is advantageous such as longer penetration depth compared to the NIR-I region, supercontinuum light sources emitting both NIR-I and NIR-II region could lower the efficiency to target melanoma because of low optical power density in the melanoma’s absorption spectra. To exploit efficient in vivo PA imaging of melanoma, we demonstrated the light source emitting from visible (532–600 nm) to NIR-I (600–1000 nm) by optimizing stimulated Raman scattering induced supercontinuum generation. The melanoma’s structure is successfully differentiated from blood vessels at a high pulse energy of 2.5 µJ and a flexible pulse repetition rate (PRR) of 5–50 kHz. The proposed light source with the microjoules energies and tens of kHz of PRR can potentially accelerate clinical trials such as early diagnosis of melanoma.

Flexible Ag-Coated Carbon Nanofibers/Bacterial Cellulose Films for Electromagnetic Interface Shielding and Multifunctional Thermal Applications

Flexible electromagnetic interference (EMI) shielding materials are in urgent need in the flexible and wearable electronics fields. In this study, multifunctional flexible silver-coated carbon nanofibers/bacterial cellulose (Ag/CNFs/BC) films were fabricated by vacuum filtration equipment using bacterial cellulose as a flexible substrate. Silver-coated carbon nanofibers were prepared by the chemical deposition method, and the silver was uniformly coated on the surface of carbon nanofibers with scanning electron microscopy (SEM) observation. Compared with the carbon nanofibers/bacterial cellulose (CNFs/BC) films, the EMI performance of Ag/CNFs/BC films was greatly enhanced because of the improvement of conductivity up to 4184 S/m. The EMI shielding effectiveness was highly enhanced over 55 dB in the X-band (8–12 GHz) under a relatively low thickness of 34 μm for the Ag/CNFs60/BC film. Moreover, the Ag/CNFs60/BC film also showed great multifunctional thermal applications. A Joule heating of over 110 °C at 3 V direct current and a photothermal conversion over 60 °C under an optical power density of 100 mW/cm2 were achieved. These results indicated that the Ag/CNFs/BC films with nanomaterials have huge application potential as flexible wearable self-heating products with excellent electromagnetic shielding performances.

Pyro-Phototronic Effect Enhanced Pyramid Structured p-Si/n-ZnO Nanowires Heterojunction Photodetector.

The emergence of nanomaterials has brought about the development of miniature photodetectors into a new stage, and ZnO nanomaterials are currently one of the most popular research objects. Here, the performance of a photodetector consisting of micropyramid structured p-Si/n-ZnO NWs heterojunction constructed by an anisotropic chemical etching and hydrothermal method is optimized by using the pyro-phototronic effect, and the photoresponses of the device to 405 and 648 nm lasers are investigated. The results show that, with the introduction of pyro-phototronic effect, the photoresponsivity Rpyro increases to 208 times that of Rphoto when the wavelength is 405 nm and the optical power density is 0.0693 mW/cm2. Moreover, with the increase of the chopper frequency, the photocurrent increases by more than 3 times, and the photoresponsivity is also increased by a factor of 4.5, making it possible to detect ultrafast pulsed light. In addition, in order to increase the current collection efficiency, a thin film Al layer was deposited as the back electrode on the basis of the device, and the photocurrent and photoresponsivity are significantly improved. Finally, the coupling between the pyro-phototronic effect and the piezo-phototronic effect is analyzed by applying compressive strain to the photodetector. When the compressive strain is -1.02%, the photocurrent decreases by 31.4% and the photoresponsivity decreases by 27.9% due to the opposite direction between laser illumination induced pyroelectric polarization charges and compressive strain induced piezoelectric polarization charges. This work not only demonstrates the great potential of pyro-phototronic effect in enhancing the silicon-based heterojunction photodetectors for high-performance photodetection and ultrafast pulsed light detection but also provides assistance for a better understanding of the coupling mechanism between pyro-phototronic and piezo-phototronic effects.

Laser-driven NIR light source based on MgO:Cr3+,Ni2+ phosphor-in-glass film for NIR spectroscopy application

A laser-driven NIR light source achieves effective NIR output optical power density under higher incident optical power density than NIR pc-LED, opening up a new avenue for the exploration of NIR spectroscopy technology.

Bright white electroluminescence from polycrystalline dysprosium-doped yttrium gallium garnet nanofilms fabricated by atomic layer deposition on silicon.

Bright white emission is obtained under electrical excitation from dysprosium doped Y3Ga5O12 garnet (YGG:Dy) nanofilms fabricated by atomic layer deposition on silicon substrates. Electroluminescence (EL) composed of yellow (580 nm) and blue (482 and 492 nm) emission corresponds to the CIE chromaticity coordinates of (0.3568, 0.3807) and a CCT of ∼4700 K and can be used for lighting and displays. The crystallization and micro-morphology of polycrystalline YGG:Dy nanolaminates are explored by adjusting the annealing temperature, Y/Ga ratio, Ga2O3 interlayer thickness and Dy2O3 dopant cycle. The near-stoichiometric device annealed at 1000 °C presents optimal EL performance with the maximum external quantum efficiency and the optical power density reaching 6.35% and 18.13 mW cm-2, respectively. The EL decay time is estimated to be 273.05 μs, with a large excitation section of 8.33 × 10-15 cm2. The conduction mechanism is confirmed to be the Poole-Frenkel mode under operation electric fields and the impact-excitation of Dy3+ ions by energetic electrons results in emission. Bright white emission from Si-based YGG:Dy devices can provide a new route to developing integrated light sources and display applications.

Markov Noise in Atomic Spin Gyroscopes: Analysis and Suppression Based on Allan Deviation

Atomic spin gyroscopes (ASGs) based on Spin-Exchange Relaxation-Free co-magnetometer have ultra-high rotation sensitivity and low gyro drift. However, the noise characteristics of ASGs have not been comprehensively studied. In this paper, the primary noise in ASGs which weakens the gyro’s long-term stability is analyzed and identified with Allan deviation. We discovered for the first time that the slow convergence rate of the spin-coupled ensemble brings a long correlation time of the gyro output and the Markov noise introduced by this effect is the primary noise that impacts the long-term stability of ASGs. Allan deviation is used to assess the characteristics of Markov noise and its impact on long-term stability. The correlation time is adjusted by changing the pumping power density to suppress the Markov noise. Comparing the Allan deviations under different correlation time, it’s concluded that the Markov noise caused by long correlation time restricts the improvement of the long-term stability of ASGs. Moreover, Markov noise at low frequencies is suppressed by increasing the pumped optical power density, enhancing the rotation sensitivity of ASGs. A relatively low gyro drift 5.76×10-3 °/h@100s is acquired after the suppression of the Markov noise. Different from traditional amplitude-frequency response analysis, this paper focuses on the time correlation of noise, which provides a new idea for the investigation of ASG’s noise characteristics.

Numerical study and design of high-efficiency p-In0.1Ga0.9N/i-GaN/n-GaN heterojunction photodiode

This paper reports the performance of a proposed GaN heterojunction p-i-n photodiode with a p-type In0.1Ga0.9N layer. The simulation results show that, compared with the GaN homojunction p-i-n structure, the GaN heterojunction p-i-n structure performs better on the basis of such simulations as photocurrent density and responsivity. The effects of reverse bias voltage, light intensity and i-GaN absorbing layer thickness are explored. The peak responsivity and the cutoff frequency are found to increase remarkably with increasing reverse bias voltage. The degradation of device responsivity and cutoff frequency at high optical excitation power density are also investigated. By increasing the i-GaN absorbing layer thickness, responsivity increases but cutoff frequency decreases. The maximum responsivity is 0.3 A/W at 0.363 μm under an illumination intensity of 105 W/cm2 and an applied reverse bias voltage of −2 V, while the highest cutoff frequency 12 GHz is achieved at an applied reverse bias voltage of −14 V.

A flexible bilayer p-NiO/n-ZnO films with photodetecting properties in self power mode

We report highly efficient flexible p-NiO/n-ZnO heterojunction UV photodetectors. NiO and ZnO thin films were grown on plastic polyethylene terephthalate (PET) substrates by dc magnetron sputtering for constructing high quality interfaces. The diode has shown an excellent rectification ratio i.e. 106 under dark mode and observed more than 80% transparency in the visible region. Investigated structural, compositional, and optical properties of the film to correlate the UV photodetector transport characteristics. Further, UV illumination devices exhibit an excellent responsivity of 0.24 A W−1 with a detectivity of 1.25 × 1011 jones and the highest external quantum efficiency of 83.14% achieved at −5 V of bias. The device shows the fastest speed of response with rise/fall times of 86.10 ms/106.60 ms, accomplished at −1 V with the lowest optical power density. The photodiode has shown incredible characteristics that are mainly attributed to the built-in potential and the transport mechanism at the interface.

Thermal analysis and package optimization for the multi-wavelength laser bar module

To meet the demand of the laser application market for semiconductor laser function diversification, this paper proposes a handheld multi-wavelength laser stack packaging module. The 808 nm, 915 nm, and 980 nm semiconductor laser bars are selected for the integrated assembly of the multi-wavelength module. However, with the continuous increase of the optical power density of the integrated packaging module, thermal management has become a key factor affecting the performance and life of the device. In this paper, the finite element analysis method is used to analyze the influence of the arrangement of three laser stripe chips in the stacked packaging structure on the heat dissipation of the multi-wavelength module. The simulation results show that the heat dissipation efficiency of the multi-wavelength module can be improved by 13.74 % by changing the arrangement of chips. A back heat dissipation packaging structure with higher heat dissipation efficiency is proposed, and the influence of chip arrangement and working mode on the thermal characteristics of the device in the new packaging structure is simulated and analyzed. According to the simulation results, a chip arrangement that can make the heat dissipation effect of the multi-wavelength module achieve the best is determined.

A polarization mismatched p-GaN/p-Al0.25Ga0.75N/p-GaN structure to improve the hole injection for GaN based micro-LED with secondary etched mesa

A low hole injection efficiency for InGaN/GaN micro-light-emitting diodes (μLEDs) has become one of the main bottlenecks affecting the improvement of the external quantum efficiency (EQE) and the optical power. In this work, we propose and fabricate a polarization mismatched p-GaN/p-Al0.25Ga0.75N/p-GaN structure for 445 nm GaN-based μLEDs with the size of 40 × 40 μm2, which serves as the hole injection layer. The polarization-induced electric field in the p-GaN/p-Al0.25Ga0.75N/p-GaN structure provides holes with more energy and can facilitate the non-equilibrium holes to transport into the active region for radiative recombination. Meanwhile, a secondary etched mesa for μLEDs is also designed, which can effectively keep the holes apart from the defected region of the mesa sidewalls, and the surface nonradiative recombination can be suppressed. Therefore, the proposed μLED with the secondary etched mesa and the p-GaN/p-Al0.25Ga0.75N/p-GaN structure has the enhanced EQE and the improved optical power density when compared with the μLED without such designs.

Millimeter-Wave Permittivity Variations of an HR Silicon Substrate from the Photoconductive Effect.

The photoinduced microwave complex permittivity of a highly resistive single-crystal silicon wafer was extracted from a bistatic free-space characterization test bench operating in the 26.5–40 GHz frequency band under CW optical illumination at wavelengths of 806 and 971 nm. Significant variations in the real and imaginary parts of the substrate’s permittivity induced by direct photoconductivity are reported, with an optical power density dependence, in agreement with the theoretical predictions. These experimental results open the route to ultrafast system reconfiguration of microwave devices in integrated technology by an external EMI-protected and contactless control with unprecedented performance.

Coal-based carbon quantum dots-sensitized TiO2 NRs/PTTh heterostructure for self-powered UV detection

In recent years, TiO2-based UV photodetectors (UVPDs) have gained widespread attention owing to their abundant raw materials and low synthesis cost. However, the low responsivity, slow response speed and high-power consumption limit the application of these UVPDs. In this study, p-n heterojunction self-powered UVPDs consisting of n-type coal-based carbon quantum dot-sensitized TiO2 nanorod arrays (TiO2 NRs:C-CQDs) and p-type poly(2, 2′:5′, 2′'-trithiophene) (PTTh) are reported. The structure, morphology and photoresponse properties of the TiO2 NRs:C-CQDs/PTTh heterojunctions are methodically investigated. These results indicate that the sensitization of C-CQDs has a significant impact on the electrical and optical properties of p-n heterojunction UVPDs. The performance study results of UVPDs at different optical power densities show that the device is able to accurately detect UV light of specific wavelengths, and the device achieved a maximum responsivity (R) (1.45 mA/cm2) and a high detectivity (D*) (4.94 × 1010 Jones) when the light intensity is 0.35 mW/cm2, while the rise and fall times shortened to 91.2 ms and 3.19 ms, respectively.These indicate that TiO2 NRs:C-CQDs/PTTh achieves the high performance self-powered UV light detection. This work offers theoretical support and technical route for TiO2 based self-powered UVPDs.

Development of high-power VCSEL emitter of size 30mm x 30mm, scalable in two dimensions and applicable to Si wafer annealing

Two-dimensionally scalable vertical cavity surface emitting laser (VCSEL) emitter of size 30mm×30mm was designed and fabricated for practical applications in the rapid heating processes of large-area targets. Nine emitters were arranged in a 3×3 array for an experimental VCSEL module, which was used for annealing of a 4” Si wafer. Each emitter was composed of 120 chips of size 1.26mm×1.26mm, and each chip was composed of 770 VCSEL diodes of 30μm in diameter, emitting a continuous beam with wavelength of 940nm and maximum optical power density of 47.6W/cm2. Since VCSELs generated a significant amount of Joule heat during operation, an effective cooling system was required for reliable thermal management of the emitter. A water cooling block of 126 microchannel flow passages with the heat transfer area-to-volume ratio approximately 1270m2/m3 was designed and fabricated for each emitter, resulting in the thermal resistance of 0.012K/W, equivalent to the overall heat transfer coefficient of 92,000W/m2K at water flow rate of 1L/min, which was the minimum value for the normal operation of the emitter. By comparing the experimental images on the 4” Si wafer irradiated by the single emitter and by the module with the numerical predictions based on the ray-tracing method, the laser beam emitted from a single VCSEL diode of 30μm in diameter was estimated to have the mode of TEM01* (doughnut profile, multimode) and a divergence angle of 20∘. These were two factors that affected the irradiation uniformity on the target. In addition, to validate the applicability of the high-power VCSEL system to anneal large-area targets, the heating process of 4” Si wafers using the module of nine emitters was experimentally carried out at atmospheric pressure, and was compared with numerical predictions.

Broadband photodetector based on SnTe nanofilm/n-Ge heterostructure

Combining novel two-dimensional materials with traditional semiconductors to form heterostructures for photoelectric detection have attracted great attention due to their excellent photoelectric properties. In this study, we reported the formation of a heterostructure comprising of tin telluride (SnTe) and germanium (Ge) by a simple and efficient one-step magnetron sputtering technique. A photodetector was fabricated by sputtering a nanofilm of SnTe on to a pre-masked n-Ge substrate. J–V measurements obtained from the SnTe/n-Ge photodetector demonstrated diode and photovoltaic characteristics in the visible to near-infrared (NIR) band (i.e. 400–2050 nm). Under NIR illumination at 850 nm with an optical power density of 13.81 mW cm−2, the SnTe/n-Ge photodetector exhibited a small open-circuit voltage of 0.05 V. It also attained a high responsivity (R) and detectivity (D*) of 617.34 mA W−1 (at bias voltage of −0.5 V) and 2.33 × 1011 cmHz1/2W−1 (at zero bias), respectively. Therefore, SnTe nanofilm/n-Ge heterostructure is highly suitable for used as low-power broadband photodetector due to its excellent performances and simple device configuration.