Optical Power Density Research Articles

Classification and Summary of Photocatalytic Chemical Reactions Driven by Triplet‐Triplet Annihilation Upconversion

AbstractTriplet‐triplet annihilation upconversion (TTA‐UC) technology could convert low‐energy light into high‐energy light, and it is a very promising one of the upconversion technologies due to its non‐coherent excitation light, low required excitation optical power density and sensitizer/annihilator flexible adjustability. The application of TTA‐UC into photocatalysis could not only broaden the range of solar energy spectrum utilization, but also bring mild reaction conditions and higher product yields via avoiding the side reaction. However, the detailed catalytic mechanism of TTA‐UC is unclear. Therefore, in this review, we summarized and classified TTA‐UC photocatalytic chemical reactions in terms of mechanism.

Size-dependent competitive effect between surface recombination and self-heat on efficiency droop for 250 nm AlGaN-based DUV LEDs.

In this work, electrical and optical performances for 250 nm AlGaN-based flip-chip deep ultraviolet light emitting diodes (DUV LEDs) with different chip sizes are studied. Reduced chip size helps increase the light extraction efficiency (LEE) with the cost of increased surface nonradiative recombination. Nevertheless, a thin p-Al0.67-0Ga0.33-1N layer of 10 nm can manage current distribution while suppressing surface recombination and reducing light absorption simultaneously, which results in the increased optical power density. Thanks to the better current management and reduced optical self-absorption effect, the reduced Joule heating effect suppresses the thermal droop of the optical power density for a small DUV LED chip. We also find that the p-Al0.67-0Ga0.33-1N layer thickness shows very significant impact on device resistance especially for the small DUV LED chip, such that the device resistance has a remarkable increase when the p-Al0.67-0Ga0.33-1N layer is thickened to 100 nm.

Enhancing optical output power efficiency in nitride‐based green micro light‐emitting diodes by sidewall ion implantation

AbstractThough micro‐LED (light‐emitting diode) displays are considered the emerging display technology, the micron‐scale LED chip size encounters severe efficiency degradation that may impact the power budget of the displays. This work proposes an ion implantation method to deliberately create a high‐resistivity sidewall in the InGaN/GaN green LED. Our study demonstrates that ion implantation suppresses reverse leakage current due to the mitigation of sidewall defects. For an LED mesa size of 10 × 10 μm2, optical output power density is improved by 36.2% compared to the device without implantation. Compared to a larger 100 × 100 μm2 device without implantation, we achieve only 21.3% degradation of output power density under 10 A/cm2 injection for a 10 × 10 μm2 LED with implantation. In addition, the ion implantation method can lower the wavelength shift by reducing light emission from the region near the sidewall (where the amount of Indium [In] clustering differs from the mesa region). The results show promise in addressing the efficiency challenges of micro‐LED displays by selective ion implantation.

Improved Process Stability and Light Detection in Phototransistors via Inverted MoS2/a‐IGZO Heterojunction Integration

The integration of molybdenum disulfide (MoS2) with other semiconductors to form heterojunction phototransistors demonstrates potential for optoelectronic applications due to their strong interaction with light and tunable bandgaps. Conventionally, bottom‐gate phototransistors based on MoS2 and amorphous indium–gallium–zinc oxide (a‐IGZO) are fabricated by deposition of the a‐IGZO film on the gate dielectric, followed by deposition of the MoS2 film, resulting in a MoS2/a‐IGZO heterojunction. In this study, an inverted MoS2/a‐IGZO (or denoted as a‐IGZO/MoS2) heterojunction integration for bottom‐gate phototransistors, where the MoS2 film is deposited on the gate dielectric first, followed by the deposition of the a‐IGZO film, is presented. This configuration is designed to enhance the processability and electrical properties of the phototransistor. Under visible light exposure with a low optical power density of 5.3 μW cm−2, the a‐IGZO/MoS2 heterojunction phototransistor demonstrates significantly improved photoresponsivity (1.6, 2.8, and 17 A W−1 at 635, 520, and 405 nm wavelengths, respectively) compared to the MoS2 single‐channel phototransistor. Time‐dependent photoresponse measurements reveal that the a‐IGZO/MoS2 heterojunction phototransistor responds more quickly to light changes and generates a tenfold‐greater photocurrent than that of the MoS2 single‐channel phototransistor.

A Flexible Multifunctional Sensor Based on an AgNW@ZnONR Composite Material.

A multifunctional sensor comprising flexible and transparent ultraviolet (UV) photodetectors (PDs) with strain gauges based on Ag nanowire (AgNW)@ZnO nanorods (ZnONRs) was fabricated using a cost-effective, simple, and efficient method. High-aspect ratio silver nanowires were synthesized using the polyol method. An AgNW@ZnONR composite was formed via the hydrothermal method to ensure the multifunctional capability of the flexible sensors. After refining the process parameters, the size of the ZnO nanorods was decreased to fabricate pliable multifunctional sensors using AgNW@ZnONRs. At a deposition of 0.207 g of AgNW@ZnONRs, the sensor achieves its maximum switching ratio and fastest response time under conditions of 2000 μW/cm2 UV optical power density. With a ton (rise time) of 2.7 s and a toff (fall time) of 2.3 s, the ratio of Ion to Ioff current is 1151. Additionally, the sensor's maximum optical current value correlates linearly with UV light's power density. The maximum response current increased from 222.5 pA to 588.1 pA, an increase of 164.3%, when the bending angle was increased from 15° to 90° for the sensor with a deposition of 0.276 g of AgNW@ZnONRs. There was no degradation in the response of the sensors after 10,000 bending cycles, as they have excellent stability and repeatability, which means they can meet the requirements of wearable sensor applications. Therefore, there is great potential for the practical application of multifunctional AgNW@ZnONRs in flexible sensors.

Perovskite polycrystal for optically tuning terahertz interference fringes

We optically tuned the terahertz interference fringe using a perovskite polycrystal. Specifically, we used CH3NH3PbI3 polycrystals with different morphologies that were annealed at different temperatures. The largest modulations depth (23 %) was obtained for the CH3NH3PbI3 polycrystal drop-coated at 130 °C; this was also greater than that of a spin-coated CH3NH3PbI3 thin film. We demonstrated that the combination of a drop-coated CH3NH3PbI3 polycrystal with a metasurface allows modulation of terahertz waves. Using the interference formed by partial-through measurement and the shift of the periodic peak in the terahertz time-domain spectroscopy system, we experimentally verified that the perovskite polycrystal can redshift the terahertz-band interference fringes by 50 GHz by changing the optical power density of the external excitation light source. This provides a new idea and possibility for the development of novel terahertz modulation devices.

High-Performance CsPbI3 Quantum Dot Photodetector with a Vertical Structure Based on the Frenkel-Poole Emission Effect.

The selection of photoactive materials and the design of device structures are critical to the photoelectronic performance of photodetectors. This study reports on a vertically structured photodetector device with rapid, stable, and efficient photoelectric performance across the UV-visible broadband range based on the Si++/SiO2/Au/single-layer graphene/CsPbI3 quantum dots (QDs) configuration. In this specific device structure, a relatively high conductivity Si++/SiO2 wafer was used as the substrate, a CsPbI3 QD film with high light absorption was used as the photoactive layer, and a monolayer graphene with high conductivity was inserted between the substrate and the CsPbI3 QD film to form a heterojunction with the QD film. Based on the Frenkel-Poole emission effect arising from the high trap state density within the SiO2 layer, the device exhibited excellent photoelectric performances. Especially at a wavelength of 365 nm, a photocurrent responsivity of 2319 A/W, a specific detectivity of 1.15 × 1014 Jones, an external quantum efficiency of 7883%, and an on/off time of 39/36 ms at a Si++ terminal voltage of -80 V and an optical power density of 84.03 nW/cm2 can be achieved.

UV electromagnetic irradiation sensing by GQDs sensitized ZnO/GaN heterostructure for wearable dosimetry

Sensing of Ultraviolet (UV) Electromagnetic Irradiations (EIs) for wearable dosimetry is promulgated universally by reserving their place in precise calibration for the controlled exposure of UV-EIs for the betterment of humankind. In other words, the controlled exposure of incident optical power density (OPD) of UV-EIs found advantageous and numerous noble healthcare applications such as beta-endorphin molecule (provide feel-good factor to the brain) level augmentation, adequate vitamin D (physical strength) level formation, in skin treatment like eczema and dermatitis, sensing important biomolecules like Uric Acid (responsible for critical disease related with kidney and heart). Moreover, the controlled exposure of OPDs also significantly impacts UV disinfection technologies, which provide a defensive shield against critical respiratory syndrome coronavirus 2 (SARS-CoV-2), which belongs to the COVID-19 pandemic. Therefore, by understanding the importance of limited exposure to these vital UV-EIs, the present study showcased the GQDs-sensitized ZnO/GaN heterostructured UV sensor utilized to explore the impact of UV-EIs OPDs on their performance. This study helps to develop and utilize the UV sensor-based wearable dosimetry for in-house diagnostics of critical healthcare parameters. This report also divulged an interesting core mechanism (band bending, tunneling through narrowed hole injection under increased negative bias) involved in affecting the performance of the UV-EIs sensor by a function of growing OPDs with the help of a suitable band diagram. The impact of increasing OPDs on fabricated UV-EIs sensors can be well understood by the fact that, by varying the OPDs up to ∼550%, the Gain (G), responsivity (R), external quantum efficiency (EQE) and noise equivalent power (NEP) significantly increases up to (156.7 to 332.4) ∼300%, (118 A W−1 to 3200 A W−1) 2700%, (∼870% to 12 × 103%) 1400% and (1.3 pWHz−1/2 to 50 fWHz−1/2) 10,000% respectively at an applied bias of −6 V. Furthermore, the time-correlated transient photoresponse is also dramatically improved with increasing OPDs, wherein the increment in rise and decay time is estimated as (159 ms to 7.86 ms) ∼2000% and (68.7 ms to 12.4 ms) ∼500%, respectively.

Ultrahigh stability photovoltaic performance of M-Mo6+ (M=Fe3+, Co2+, Ni2+) co-doped BiVO4 films

BiVO4 has been widely studied in photovoltaic conversion. However, as a paraelectric material, BiVO4 lacks an internal electric field for efficient carrier separation, resulting in low photocurrent and limited practical use. Enhancing the photovoltaic performance of thin films through simple methods is challenging. In this paper, M-Mo6+ (M=Fe3+, Co2+, Ni2+) ions were used to co-doped BiVO4 films, which were then composited with NiO to construct composite film structures. The NiO/Bi0.985Co0.015V0.985Mo0.015O4 film achieved a photocurrent density (Jsc) of 2.33 mA/cm2 and an open-circuit voltage (Voc) of 0.39 V. The film also demonstrated a specific detection rate of 2.67 × 1011 jones at an optical power density of 165 mW/cm2 and maintained 1.94 × 1011 jones at 103.7 mW/cm2. At an optical power density of approximately 140 mW/cm2, the specific detection rate begins to decrease. The photovoltaic conversion performance of the film exhibits high stability over a long period of time (＞10,000 s). The thin films maintain good photovoltaic performance at high temperatures. This work provides an approach for the application of BiVO4 in photovoltaics.

Variation of carrier lifetime with optical excitation power density in micro-LEDs

GaN-based micro-LEDs are applied to visible light communication due to their high modulation bandwidth with reduced chip size. It requires a deep understanding of recombination processes and their impact on the bandwidth, which is mainly determined by the carrier lifetime. We employed confocal time-resolved photoluminescence (TRPL) to characterize the variation of carrier lifetime with optical excitation power density on micro-LEDs. We observed an initial increase followed by a sudden decrease within the power density range of 96.7 kW/cm2 to 546 kW/cm2 on a blue micro-LED with a chip size of 80 µm. We attribute this phenomenon to increased optical excitation power, gradually saturating the defect-dominated non-radiative recombination centers, with radiative recombination processes gradually taking over. We compared the power density at the inflection point for different regions on the sample and the samples with different sizes and sidewall structures. The power density for the lifetime inflection point at the center of the sample is smaller than that at the edge. We also find that the value is smaller for the sample with a chip size of 40 µm which prompts fewer total defects. The power density for sudden lifetime drop on samples with inclined sidewall structures is also smaller than those with vertical sidewall structures. Furthermore, we find that the excitation power density corresponding to the highest luminous efficiency is higher than that corresponding to the sudden drop at the beginning of the lifetime. This opens up possibilities for simultaneously achieving high modulation bandwidth and high efficiency between the two inflection points.

"Nonperturbative Nonlinearities": Perhaps Less than Meets the Eye.

We address challenges in characterizing changes in permittivity and refractive index beyond standard perturbative methods with special attention given to transparent conductive oxides (TCOs). We unveil a realistic limit to permittivity changes under high optical power densities. Our study covers both slow and ultrafast nonlinearities, demonstrating that all nonlinearities induce refractive index changes accurately described by a simple curve with saturation electric field (or irradiance) and maximum change of permittivity at saturation. Our model, grounded in material properties, like oscillator strength and characteristic times, offers a robust framework for understanding and predicting nonlinear optical phenomena in TCOs and other materials. We differentiate between the significance of higher-order nonlinear susceptibilities in ultrafast and slow nonlinear scenarios. We aim to provide valuable insights for researchers exploring strong light-matter interaction.

Design optimization of a modified Uni-Traveling- carrier photodiode for high optical power operation

We systematically investigated the design optimization of an InGaAs/InP modified Uni-Traveling-Carrier Photodiode (MUTC-PD) under high optical power input conditions. Based on internally fabricated experimental devices for a baseline design, we first achieved an excellent match between our experimental and simulation results under low optical power density of 1.4 mW/μm2, demonstrating the accuracy of our simulation model. Based on this model we showed that as optical power injection increases, the intrinsic bandwidth of the MUTC-PD degradation is the primary limitation on its high-speed response. We quantitatively analyzed the fundamental carrier transport physics and key design parameters affecting intrinsic bandwidth degradation under these conditions. In particular cliff layer doping concentration was found to be a very sensitive parameter affecting the MUTC-PD Optical Electrical (OE) intrinsic bandwidth. Based on detailed simulated results, we realized an optimized device design that is capable of achieving a 3 dB bandwidth of approximately 230 GHz at 10 mW/μm2 input optical power density for the first time.

Standard Response Calculation of Electric Dipole Polarizability and Specific Optical Rotation Power Densities.

A time-dependent method has been developed to solve the standard response equation for the calculation of dynamic molecular property densities, endowed with the characteristic of being origin-invariant, entirely in the atomic orbital basis at both HF and DFT level of theory. The method has been tuned in particular for the calculation of origin-independent electric dipole polarizability density and specific rotation power density. Some demonstrations are given for the hexabenzocoronene molecule and the Tröger's base.

Revisiting optical absorption power density of inhomogeneous plane waves

We review the analytical expressions for the complex Poynting’s vector in the case of arbitrary plane-waves in a lossy isotropic medium. We demonstrate how these expressions can be used to recover the optical absorption power density Q, considering the divergence of the time-averaged Poynting vector. This quantity, proportional to the imaginary part of the dielectric function and the field intensity, i.e. Q∝|E|2ɛ”, is usually established for harmonic fields, using the Poynting’s identity. The derivation from the complex Poynting vector expression is more direct for TE-polarized homogeneous waves, but the derivation encompasses the other cases like inhomogeneous TM plane waves. As an application, the optical absorption profile Q(x) within 1D multilayers is detailed using matrix transfer method for both TE and TM plane waves, including the evanescent case.

A Passive-Active Anti/Deicing Coating Integrating Superhydrophobicity, Thermal Insulation, and Photo/Electrothermal Conversion Effects.

Anti/deicing coatings that combine active and passive methods can utilize various energy sources to achieve anti/deicing effects. However, poor photothermal or electrothermal performance and inevitable heat loss often reduce their anti/deicing efficiency. Herein, copper sulfide loaded activated biochar (AC@CuS) as photo/electric material, polydimethylsiloxane as hydrophobic component, thermally expandable microspheres as foaming agent, and an anti/deicing coating integrating thermal insulation, superhydrophobicity, photo/electrothermal effects was successfully constructed. Benefiting from the synergistic effect of superhydrophobicity and thermal insulation, the freezing time of water droplets on the coating surface is extended from 150 to 2140 s, showing excellent passive anti-icing performance. AC@CuS exhibits photo/electrothermal effects, and porous expanded microspheres reduce heat loss, which endows the coating with desirable photo/electrothermal conversion performance. Under the conditions of 0.2 W/cm2 electric power density (EPD) and 0.1 W/cm2 optical power density (OPD), the temperature of the coating increases from 24 to 96.4 and 113 °C, respectively. Interestingly, with a coheating of 0.05 W/cm2 weaker OPD and 0.05 W/cm2 lower EPD, the ice on the coating surface can be quickly melted in 2.5 min, showing synergistic deicing performance. In addition, the WCA of the prepared coating remains above 150° after mechanical damage, rain impact, UV irradiation, chemical corrosion, and high-temperature treatment, and good superhydrophobic durability ensures the anti/deicing durability of the coating.

Effects of oxygen vacancies on the optical and electrical performances of silicon-based Er doped Ga2O3 films

Effects of oxygen vacancies on the optical and electrical properties of silicon-based Er doped Ga2O3 films are investigated. The content of oxygen vacancies can be tuned by changing the Ar:O2 flow ratios during sputtering, and the films sputtered in pure Ar are proved to possess the most oxygen vacancies by the calculations of energy band structure and carrier concentration. It is found that oxygen vacancies are involved in the process of indirect Er3+ ions related emission. With more oxygen vacancies introduced, the conductivity and energy transfer efficiency of Er doped Ga2O3 films can be enhanced simultaneously, leading to the increasing electroluminescence intensity of the light-emitting devices. The optimized silicon-based devices present the maximum optical power density of ∼2 μW cm−2.

Analytical solutions in the modeling of the endovenous laser ablation

We modeled the operative treatment of incompetent truncal veins using endovenous laser ablation (EVLA). The main concern regarding the thermoablative technique is tissue damage, which is correlated with (1) the energy provided by the laser power and (2) temperature distribution. Our objective was to accurate the two functions, namely the fluence rate and the temperature, depending on the thermoablative technique and the endovenous laser treatment (ELT). First, we considered three differential equations: diffusion, heat, and bioheat equations in the endovenous-perivenous multidomain to describe the lumen, the vein wall, the tissue pad, and the skin. Second, we examined the power source according to the Beer-Lambert law in the incident beam irradiance and the heat source as the so-called absorbed optical power density. Third, we checked out the heat transfer at the skin boundary according to Newton’s law of cooling, which stands for a Robin boundary condition. For this new model, we proposed exact solutions: applying differential equations techniques, we solved (1) a diffusion approximation of the radiative transfer equation under the considered power source, and (2) the coupled heat and bioheat equations under the considered heat source accomplished with the Robin boundary condition. Then, we graphically illustrated the fluence rate profile and discussed its time dependence and steady state. Besides, we discuss thermal damage to the vein-tissue system and present open problems.

Characteristics of laser induced plasma near a flat gas-liquid interface and its effect on the performance of laser induced breakdown spectroscopy (LIBS) detection

Moving the laser focus to the vicinity of the gas-liquid interface is the key point for many new enhanced and new methods to improve the quality of spectral signals in water Laser induced breakdown spectroscopy (LIBS) detection. Understanding the generation and evolution characteristics of the plasma induced by pulsed laser near the gas–liquid interface is of great significance for the establishment of evolution models and improvement of these new LIBS methods. In this paper, a set of slow horizontal flow auxiliary system is established to provide an ideal flat gas–liquid two-phase interface experimental condition. Experimental research on vertical incidence flat system was conducted using techniques such as time-resolved imaging, plasma characterization diagnosis, and spectral analysis. And the detection capabilities of the system were also tested. The characteristics and mechanisms of LIBS near the gas-liquid two-phase interface were investigated with the laser incident on the sample along the vertical direction. Simulation of the laser beam focusing process and observation of laser beam spot images show that the shift of plasma generation position relative to the focal point results from the refraction of the laser beam entering the solution from the air and the ‘interface effect’ of propagation on the vertical direction. Moreover, the plasma forms only the optical power density surpasses the breakdown threshold. In this work, plasma with smaller size, rounder shape, stronger radiation, higher temperature, and higher density can be produced when the focus position is in the liquid column 0.3 mm away from the upper interface. Simultaneously, for example, the Mg ion line at 285.213 nm, the obtained spectral intensity to signal-to-background ratio reaches the maximum value, and a better spectral signal can be obtained, which is 2–4 times of other positions, and the detection limits of the elements Na, Mg, and Ca also reach the lowest level, with 1.6–2.4 times of the detection limit of other focusing positions for Mg and 1.4–1.7 times for Ca, respectively.

Effect of turbulence-induced sediment resuspension and sedimentation on underwater wireless optical communication

In real-life marine environments, the composition and grain size of suspended sediments and the resuspension and sedimentation of sediments caused by turbulence may have a significant impact on underwater wireless optical communication (UWOC). However, to date, researchers have not conducted quantitative research on this issue. To this end, we innovatively study the effects of different compositions and grain sizes of suspended sediments on UWOC and the effects of turbulence-induced sediment resuspension and sedimentation on UWOC in this paper. Quartz and kaolin with different grain sizes are used to simulate sediments in seawater. An oscillating grid that can vary frequency and stroke is used to generate turbulence of different intensities. By comparing the turbidity and optical power density of different simulated sediments with different grain sizes, we find that the smaller the grain size of the simulated sediments, the higher the bit error rate (BER) under the same turbidity. But different simulated sediments with different grain sizes have similar effects on BER performance under the same optical power density. Therefore, turbidity can be used to characterize the changes of underwater channels, and optical power density can be used to evaluate the attenuation of light at the receiving end after transmission through the underwater channel. By continuously changing the frequency of the grid to cause the sediments to resuspend and sink, we prove that the process of turbulence-induced sediment resuspension and sedimentation can seriously affect the BER performance. The larger the frequency of the grid, the greater the turbulence intensity and the worse the BER performance. This study lays a foundation for the practical application of UWOC in mobile ocean observation networks.

Over 43%-power-efficiency GaInN-based photoelectric transducer on free-standing GaN substrate for optical wireless power transmission system

We investigated the GaInN-based photoelectric transducers (PTs) aiming at the application to optical wireless power transmission systems. A PT device structure with Ga0.9In0.1N multiple-quantum-wells (MQWs) as a light absorption layer was grown on a free-standing GaN substrate by metalorganic chemical vapor deposition and subjected to the device fabrication. The PT performance was evaluated via the two-terminal current-density vs. voltage characteristics taken under a monochromatic light illumination. The fabricated PT devices exhibited a high open-circuit voltage of approximately 2.3 V and a high shunt resistance of 41 kΩcm2, thanks to its good material qualities. In addition, its surface reflection was markedly suppressed by an adoption of a wet surface treatment and an anti-reflection coating, resulting in a high external quantum efficiency of 90% and a high short-circuit current density of 1.4 mAcm−2. Through the above investigation, a high power-conversion efficiency as great as 43.7% was achieved for the GaInN MQW PTs at a light illumination with 390 nm in wavelength and 5 mWcm−2 in optical power density.