Nanowatt-Level Photoactivated Gas Sensor Based on Fully-Integrated Visible MicroLED and Plasmonic Nanomaterials.

Photoactivated gas sensors that are fully integrated with micro light-emitting diodes (µLED) have shown great potential to substitute conventional micro/nano-electromechanical (M/NEMS) gas sensors owing to their low power consumption, high mechanical stability, and mass-producibility. Previous photoactivated gas sensors mostly have utilized ultra-violet (UV) light (250-400 nm) for activating high-bandgap metal oxides, although energy conversion efficiencies of gallium nitride (GaN) LEDs are maximized in the blue range (430-470 nm). This study presents a more advanced monolithic photoactivated gas sensor based on a nanowatt-level, ultra-low-power blue (λpeak  = 435 nm) µLED platform (µLP). To promote the blue light absorbance of the sensing material, plasmonic silver (Ag) nanoparticles (NPs) are uniformly coated on porous indium oxide (In2 O3 ) thin films. By the plasmonic effect, Ag NPs absorb the blue light and spontaneously transfer excited hot electrons to the surface of In2 O3 . Consequently, high external quantum efficiency (EQE, ≈17.3%) and sensor response (ΔR/R0 (%) = 1319%) to 1 ppm NO2 gas can be achieved with a small power consumption of 63 nW. Therefore, it is highly expected to realize various practical applications of mobile gas sensors such as personal environmental monitoring devices, smart factories, farms, and home appliances.

Spectrally encoded dual-mode interferometry with orthogonal scanning.

Spectrally encoded confocal microscopy (SECM) is a high-speed reflectance confocal microscopy technique. Here, we present a method to integrate optical coherence tomography (OCT) and SECM for complementary imaging by adding orthogonal scanning to the SECM configuration. The co-registration of SECM and OCT is automatic, as all system components are shared in the same order, eliminating the need for additional optical alignment. The proposed multimode imaging system is compact and cost-effective while providing the benefits of imaging aiming and guidance. Furthermore, speckle noise can be suppressed by averaging the speckles generated by shifting the spectral-encoded field in the direction of dispersion. Using a near infrared (NIR) card and a biological sample, we demonstrated the capability of the proposed system by showing SECM imaging at depths of interest guided by the OCT in real time and speckle noise reduction. Interfaced multimodal imaging of SECM and OCT was implemented at a speed of approximately 7 frames/s using fast-switching technology and GPU processing.

Spiral-phase-objective for a compact spiral-phase-contrast microscopy.

Spiral-phase-contrast imaging, which utilizes a spiral phase optical element, has proven to be effective in enhancing various aspects of imaging, such as edge contrast and shadow imaging. Typically, the implementation of spiral-phase-contrast imaging requires the formation of a Fourier plane through a 4f optical configuration in addition to an existing optical microscope. In this study, we present what we believe to be a novel single spiral-phase-objective, integrating a spiral phase plate, which can be easily and simply applied to a standard microscope, such as a conventional objective. Using a new hybrid design approach that combines ray-tracing and field-tracing simulations, we theoretically realized a well-defined and high-quality vortex beam through the spiral-phase-objective. The spiral-phase-objective was designed to have conditions that are practically manufacturable while providing predictable performance. To evaluate its capabilities, we utilized the designed spiral-phase-objective to investigate isotropic spiral phase contrast and anisotropic shadow imaging through field-tracing simulations, and explored the variation of edge contrast caused by changes in the thickness of the imaging object.

Wavelength-tunable spiral-phase-contrast imaging.

Wavelength-tunable spiral-phase-contrast (SPC) imaging was experimentally accomplished in the visible wavelengths spanning a broad bandwidth of ∼200 nm based on a single off-axis spiral phase mirror (OSPM). By the rotation of an OSPM, which was designed with an integer orbital angular momentum (OAM) of l = 1 at a wavelength of 561 nm and incidence angle of 45°, high-quality SPC imaging was obtained at different wavelengths. For the comparison with wavelength-tunable SPC imaging using an OSPM, SPC imaging using a spiral phase plate (manufactured to generate an OAM of l = 1 at 561 nm) was performed at three wavelengths (473, 561, and 660 nm), resulting in clear differences. Theoretically, based on field tracing simulations, high-quality wavelength-tunable SPC imaging could be demonstrated in a very broad bandwidth of ∼400 nm, which is beyond the bandwidth of ∼200 nm obtained experimentally. This technique contribute to developing high-performance wavelength-tunable SPC imaging by simply integrating an OSPM into the current optical imaging technologies.

Generation of wavelength-tunable optical vortices using an off-axis spiral phase mirror: publisher's note.

This publisher's note contains corrections to Opt. Lett.46, 4216 (2021)OPLEDP0146-959210.1364/OL.432413.

Generation of wavelength-tunable optical vortices using an off-axis spiral phase mirror.

Wavelength-tunable optical vortices with a topological charge equal to l=1 of orbital angular momentum (OAM) were experimentally realized using a single off-axis spiral phase mirror (OSPM) with lasers of various visible-light wavelengths. Using an OSPM designed for 561 nm and an incidence angle of 45°, circular doughnut-shaped l=1 optical vortices were obtained at 561, 473, and 660 nm by rotating the OSPM to modify the laser incidence angle. Wavelength-tunable l=1 optical vortices were obtained at the respective incidence angles of 45°, 53.4°, and 33.7°, because the effective geometrical thickness of the OSPM, which determines the order of OAM, was identical at each wavelength. This flexible OSPM which operates over a wide wavelength range will provide continuously wavelength-tunable optical vortices for applications in the fields of advanced optics and photonics in which optical vortices with wide wavelength tunability are in demand.

Estimation and compensation of phase errors induced by axial bulk motion of a sample in wavelength-sweeping parallel Fourier domain OCT.

We report a new, to the best of our knowledge, approach to correct image blurring due to the axial bulk motion of a sample in wavelength-sweeping Fourier domain parallel optical coherence tomography (OCT). This approach can estimate phase errors changing rapidly in time through direct measurements of the apparent axial shift during the sampling interval using common phase changes in parallel detection without additional hardware. To demonstrate the performance of the proposed algorithm, a single reflection and scattering sample were imaged with wavelength-sweeping parallel OCT implemented by scanning a spectrally dispersed line-field along the line direction. In addition, we quantitatively demonstrated that even a small axial movement of the sample could cause serious image blur at a high nonlinear degree of movement.

Design, Fabrication, and Performance Evaluation of Portable and Large-Area Blackbody System.

In this study, a portable and large-area blackbody system was developed following a series of processes including design, computational analysis, fabrication, and experimental analysis and evaluation. The blackbody system was designed to be lightweight (5 kg), and its temperature could exceed the ambient temperature by up to 15 °C under operation. A carbon-fiber-based heat source was used to achieve a uniform temperature distribution. A heat shield fabricated from an insulation material was embedded at the opposite side of the heating element to minimize heat loss. A prototype of the blackbody system was fabricated based on the design and transient coupled electro-thermal simulation results. The operation performance of this system, such as the thermal response, signal transfer function, and noise equivalent temperature difference, was evaluated by employing an infrared imaging system. In addition, emissivity was measured during operation. The results of this study show that the developed portable and large-area blackbody system can be expected to serve as a reliable reference source for the calibration of aerial infrared images for the application of aerial infrared techniques to remote sensing.

Bragg scattering from a millimeter-scale periodic structure with extremely small aspect ratios.

The periodic structure on the optical surface affects the beam shape and its propagation. As the size of the optical elements becomes larger and its shape becomes complicated, the quantitative analysis of the effect of the periodic structure on the optical surface becomes indispensable given that it is very difficult to completely eliminate the microscopic periodic structures. Herein, we have experimentally investigated Bragg scattering from an optical surface with extremely small aspect ratios (~10-5) and groove densities (0.5 lines/mm). We observed the period of the constructive interference formed due to the propagation of the 0th, 1st, and -1st beam modes caused by Bragg scattering. When the periodic structure has a modulation depth of ± 50 nm, the intensity increase of constructive interference between the beam modes formed by Bragg scattering was > 10 times greater than the intensity of a flat surface at the propagation distance at which constructive interference was most pronounced. This study is envisaged to open new avenues for the quantification of the effect of periodic structures based on the observation of the interference on the beam profile formed by Bragg scattering during the beam propagation.

Influence of Self-Heating Effect on Interface Trap Generation in Highly Flexible Single-Crystalline Si Nanomembrane Transistors.

We analyze the interface trap states generated by the self-heating effect in flexible single-crystalline Si nanomembrane (sc-Si NM) transistors. Despite the excellent device performance (Subthreshold swing: ~61 mV/dec, Ion/off: ~109, Nit: ~5 × 1010 cm-2, µeff: ~250 cm²/V·s) and mechanical flexibility (RB,min ═ 1 mm) of sc-Si NM transistors on a polymer substrate, they are vulnerable to thermal reliability issues due to the poor thermal conductivity (κ < 1 W/m·K) of the polymer substrate. Understanding the detailed mechanism driving heat-related device degradation is key to improving device reliability, life expectancy, and overall device performance. Thus, a charge pumping method was employed to systematically analyze the device degradation caused by the self-heating effect. This enabled the interface trap density to be investigated for the flexible sc-Si NM transistors on a polymer substrate after a bias stress. For comparison, a heat spreading layer (HSL) made using a 1-µm thick Ag film (κ~400 W/m·K) was integrated into the sc-Si NM device to mitigate the self-heating effect. The results showed that the interface trap density was proportional to the self-heating effect. This facilitated the fundamental understanding of the self-heating effect of flexible sc-Si NM transistors, opening a robust route to realizing high performance flexible devices using sc-Si NM.

Gold nanorods-conjugated TiO2 nanoclusters for the synergistic combination of phototherapeutic treatments of cancer cells.

BACKGROUND: Recently, a combination of photodynamic therapy (PDT) and photothermal therapy (PTT) to generate reactive oxygen species (ROS) and heat to kill cancer cells, respectively has attracted considerable attention because it gives synergistic effects on the cancer treatment by utilizing the radiation of nontoxic low-energy photons such as long wavelength visible light and near IR (NIR) penetrating into subcutaneous region. For the effective combination of the phototherapies, various organic photosensitizer-conjugated gold nanocomplexes have been developed, but they have still some disadvantages due to photobleaching and unnecessary energy transfer of the organic photosensitizers. RESULTS: In this study, we fabricated novel inorganic phototherapeutic nanocomplexes (Au NR-TiO2 NCs) by conjugating gold nanorods (Au NRs) with defective TiO2 nanoparticle clusters (d-TiO2 NP clusters) and characterized their optical and photothermal properties. They were observed to absorb a broad range of visible light and near IR (NIR) from 500 to 1000 nm, exhibiting the generation of ROS as well as the photothermal effect for the simultaneous application of PDT and PTT. The resultant combination of PDT and PTT treatments of HeLa cells incubated with the nanocomplexes caused a synergistic increase in the cell death compared to the single treatment. CONCLUSION: The higher efficacy of cell death by the combination of PDT and PTT treatments with the nanocomplexes is likely attributed to the increases of ROS generation from the TiO2 NCs with the aid of local surface plasma resonance (LSPR)-induced hot electrons and heat generation from Au NRs, suggesting that Au NR-TiO2 NCs are promising nanomaterials for the in vivo combinatorial phototherapy of cancer.

3D Defect Localization on Exothermic Faults within Multi-Layered Structures Using Lock-In Thermography: An Experimental and Numerical Approach.

Micro-electronic devices are increasingly incorporating miniature multi-layered integrated architectures. However, the localization of faults in three-dimensional structure remains challenging. This study involved the experimental and numerical estimation of the depth of a thermally active heating source buried in multi-layered silicon wafer architecture by using both phase information from an infrared microscopy and finite element simulation. Infrared images were acquired and real-time processed by a lock-in method. It is well known that the lock-in method can increasingly improve detection performance by enhancing the spatial and thermal resolution of measurements. Operational principle of the lock-in method is discussed, and it is represented that phase shift of the thermal emission from a silicon wafer stacked heat source chip (SSHSC) specimen can provide good metrics for the depth of the heat source buried in SSHSCs. Depth was also estimated by analyzing the transient thermal responses using the coupled electro-thermal simulations. Furthermore, the effects of the volumetric heat source configuration mimicking the 3D through silicon via integration package were investigated. Both the infrared microscopic imaging with the lock-in method and FE simulation were potentially useful for 3D isolation of exothermic faults and their depth estimation for multi-layered structures, especially in packaged semiconductors.

Laser Scanning Confocal Thermoreflectance Microscope for the Backside Thermal Imaging of Microelectronic Devices.

In this paper, we report on a confocal thermoreflectance imaging system that can examine the thermal characteristics of microelectronic devices by penetrating the backside of a device through the substrate. In this system, the local reflectivity variations due to heat generation in the device are measured point by point by a laser scanning confocal microscope capable of eliminating out-of-focus reflections and the thermoreflectance is extracted via Fourier-domain signal processing. In comparison to the conventional widefield thermoreflectance microscope, the proposed laser scanning confocal thermoreflectance microscope improves the thermoreflectance sensitivity by ~23 times and the spatial resolution by ~25% in backside thermoreflectance measurements.

Quantitative temperature measurement of multi-layered semiconductor devices using spectroscopic thermoreflectance microscopy.

Thermoreflectance microscopy is essential in understanding the unpredictable local heating generation that occurs during microelectronic device operation. However, temperature measurements of multi-layered semiconductor devices represent a challenge because the thermoreflectance coefficient is quite small and is dramatically changed by the optical interference inside transparent layers of the device. Therefore, we propose a spectroscopic thermoreflectance microscopy system using a systematic approach for improving the quantitative temperature measurement of multi-layered semiconductor devices. We demonstrate the quantitative measurement of the temperature profile for physical defects on thin-film polycrystalline silicon resistors via thermoreflectance coefficient calibration and effective coefficient κ estimation.

Experimental approach to the fundamental limit of the extinction coefficients of ultra-smooth and highly spherical gold nanoparticles.

The theoretical extinction coefficients of gold nanoparticles (AuNPs) have been mainly verified by the analytical solving of the Maxwell equation for an ideal sphere, which was firstly founded by Mie (generally referred to as Mie theory). However, in principle, it has not been directly feasible with experimental verification especially for relatively large AuNPs (i.e., >40 nm), as conventionally proposed synthetic methods have inevitably resulted in a polygonal shaped, non-ideal Au nanosphere. Here, mono-crystalline, ultra-smooth, and highly spherical AuNPs of 40-100 nm were prepared by the procedure reported in our recent work (ACS Nano, 2013, 7, 11064). The extinction coefficients of the ideally spherical AuNPs of 40-100 nm were empirically extracted using the Beer-Lambert law, and were then compared with the theoretical limits obtained by the analytical and numerical methods. The obtained extinction coefficients of the ideally spherical AuNPs herein agree much more closely with the theoretical limits, compared with those of the faceted or polygonal shaped AuNPs. In addition, in order to further elucidate the importance of being spherical, we systematically compared our ideally spherical AuNPs with the polygonal counterparts; effectively addressing the role of the surface morphology on the spectral responses in both theoretical and experimental manners.

High-speed thermoreflectance microscopy using charge-coupled device-based Fourier-domain filtering.

We present a Fourier-domain filtering method for charge-coupled device (CCD)-based thermoreflectance microscopy to improve the thermal imaging speed while maintaining high thermal sensitivity. The time-varying reflected light distribution from the surface of bias-modulated microresistor was recorded by a CCD camera in free-run mode and converted to the frequency domain using the fast Fourier transform (FFT) for all pixels of the CCD. After frequency peak filtering followed by inverse FFT, a thermoreflectance image was obtained. The imaging results of the proposed method were quantitatively compared with those of the conventional four-bucket method, showing that the Fourier-domain filtering method can provide thermal imaging 24-42 times faster than the four-bucket method, depending on the required thermal sensitivity.

Fast mapping of absorbing defects in optical materials by full-field photothermal reflectance microscopy.

We report a technique for rapidly mapping absorbing defects in optical materials, which act as laser-induced damage precursors, based on full-field photothermal reflectance microscopy. An intensity-modulated pump beam heats absorbing defects in the optical sample, creating localized, modulated refractive-index variations around the defects. A probe beam then illuminates the defect sites, and the measured amplitude of the reflectance variation is used to map the distribution of defects in the medium. Measurements show that this method offers a faster defect mapping speed of about 0.03 mm(2) per minute and a detectivity of a few tens of nanometers comparable to that of conventional scanning photothermal deflection microscopy.

Label-free photothermal optical coherence microscopy to locate desired regions of interest in multiphoton imaging of volumetric specimens.

Biochip-based research is currently evolving into a three-dimensional and large-scale basis similar to the in vivo microenvironment. For the long-term live and high-resolution imaging in these specimens, nonlinear microscopy capable of label-free and multiscale imaging is becoming increasingly important. Combination with non-destructive contrast imaging will be useful for effectively locating regions of interest (ROI) in large specimens and consequently minimizing photodamage. In this study, a label-free photothermal optical coherence microscopy (OCM) serves as a new approach to locate the desired ROI within biological samples which are under investigation by multiphoton microscopy (MPM). The weak photothermal perturbation in sample by the MPM laser with reduced power was detected at the endogenous photothermal particles within the ROI using the highly sensitive phase-differentiated photothermal (PD-PT) OCM. By monitoring the temporal change of the photothermal response signal of the PD-PT OCM, the hotspot generated within the sample focused by the MPM laser was located on the ROI. Combined with automated sample movement in the x-y axis, the focal plane of MPM could be effectively navigated to the desired portion of a volumetric sample for high-resolution targeted MPM imaging. We demonstrated the feasibility of the proposed method in second harmonic generation microscopy using two phantom samples and a biological sample, a fixed insect on microscope slide, with dimensions of 4 mm wide, 4 mm long, and 1 mm thick.

Precise temperature mapping of GaN-based LEDs by quantitative infrared micro-thermography.

A method of measuring the precise temperature distribution of GaN-based light-emitting diodes (LEDs) by quantitative infrared micro-thermography is reported. To reduce the calibration error, the same measuring conditions were used for both calibration and thermal imaging; calibration was conducted on a highly emissive black-painted area on a dummy sapphire wafer loaded near the LED wafer on a thermoelectric cooler mount. We used infrared thermal radiation images of the black-painted area on the dummy wafer and an unbiased LED wafer at two different temperatures to determine the factors that degrade the accuracy of temperature measurement, i.e., the non-uniform response of the instrument, superimposed offset radiation, reflected radiation, and emissivity map of the LED surface. By correcting these factors from the measured infrared thermal radiation images of biased LEDs, we determined a precise absolute temperature image. Consequently, we could observe from where the local self-heat emerges and how it distributes on the emitting area of the LEDs. The experimental results demonstrated that highly localized self-heating and a remarkable temperature gradient, which are detrimental to LED performance and reliability, arise near the p-contact edge of the LED surface at high injection levels owing to the current crowding effect.

Wafer-scale, highly uniform, and well-arrayed suspended nanostructures for enhancing the performance of electronic devices.

Suspended nanostructures play an important role in enhancing the performance of a diverse group of nanodevices. However, realizing a good arrangement and suspension for nanostructures of various shapes remains a significant challenge. Herein, a rapid and simple method for fabricating wafer-scale, highly uniform, well-arrayed suspended nanostructures via nanowelding lithography is reported. Suspended nanostructures with various shapes (nanowires, nanoholes, nanomesh, and nanofilms) and materials (gold, silver, and palladium metals) were employed to demonstrate the applicability of our method. Moreover, gas sensors and thermoacoustic speakers with suspended nanowires outperformed those with unsuspended nanostructures. The proposed method is expected to help advance the development of future nanodevices based on suspended nanostructures.