Characterization of an uncooled mid-infrared Schottky photodetector based on a 2D Au/PtSi/p-Si nanohole array at a higher light modulation frequency.

In this study, an uncooled 2D nanohole array PtSi/p-Si Schottky mid-infrared (MIR) photodetector, which is essential for on-chip Si-based low-barrier MIR detectors, is presented. Room temperature operation introduces susceptibility to thermal noise and can impact stability. Through modulation frequency and reverse bias optimization, the stability improved by 7 times at 170 Hz and -3.5V, respectively. The effective light detection and stability were confirmed through ON/OFF response measurements over a longer time. The wavelength-dependent responsivity, measured with a tunable MIR laser, confirmed the responsiveness of the device in the MIR region of 2.5 µm to 4.0 µm, with a maximum specific detectivity (D*) of 2.0×103 c m H z 1/2 W -1 at 3.0 µm; this result shows its potential applicability for noninvasive human lipid monitoring. Overall, this study focuses on the crucial role of signal analysis optimization in enhancing the performance of MIR photodetectors at room temperature.

Spiral Chiral Metamaterial Structure Shape for Optical Activity Improvements.

We report on a spiral structure suitable for obtaining a large optical response. We constructed a structural mechanics model of the shape of the planar spiral structure when deformed and verified the effectiveness of the model. As a verification structure, we fabricated a large-scale spiral structure that operates in the GHz band by laser processing. Based on the GHz radio wave experiments, a more uniform deformation structure exhibited a higher cross-polarization component. This result suggests that uniform deformation structures can improve circular dichroism. Since large-scale devices enable speedy prototype verification, the obtained knowledge can be exported to miniaturized-scale devices, such as MEMS terahertz metamaterials.

Electromechanically reconfigurable plasmonic photodetector with a distinct shift in resonant wavelength.

Plasmonic photodetectors have received increasing attention because their detection properties can be designed by tailoring their metal structures on surfaces without using any additional components. Reconfiguration of the plasmonic resonant state in a photodetector is relevant for various applications, including investigating in situ adaptive detection property changes, depending on the situation, and performing single-pixel spectroscopy in geometrically limited regions. However, the spectral responsivity change with conventional reconfiguration methods is relatively small. Here, we propose a plasmonic photodetector that reconfigures its spectral responsivity with electromechanical deformation instead of bias tuning. The photodetector consists of a gold plasmonic grating formed on an n-type silicon cantilever, and the spectral responsivity is reconfigured by electromechanically scanning at an incident angle to the grating on the cantilever. The photodetector exhibits peak shifts in spectral responsivity in a wavelength range from 1250 to 1310 nm after electromechanical reconfiguration. Finally, for potential future applications, we demonstrate near-infrared spectroscopy using the photodetector. This photodetector has the potential to be adopted as a near-infrared spectrometer in industrial silicon imaging systems because its structure enables subbandgap photodetection on silicon by a Schottky junction.

Plasmonic mid-infrared photodetector with narrow trenches for reconstructive spectroscopy.

Reconstructive spectroscopy in the mid-infrared (MIR) range is an attractive method for miniaturizing MIR spectrometers. Although detectors with a sharp responsivity spectrum and a high dynamic range are required, there remains room for improvement. This article reports on a set of MIR photodetectors that satisfy these requirements with aluminum-covered narrow trench gratings. We experimentally found the optimal grating design, by which the responsivity changed 0.75% of the maximum value per nanometer of wavelength, and the minimum/maximum ratio was 8.7% in the wavelength range of 2.5-3.7 µm. A detector set of this optimal design was capable of accurate and robust reconstruction against measurement errors. This simple design structure based on standard materials will assist in realizing miniature MIR spectrometers.

Room-temperature plasmonic mid-infrared photodetector based on PtSi/p-Si low Schottky-barrier junction.

In this study, a low Schottky-barrier photodetector with a plasmonic assist using a two-dimensional (2D) nanohole array was demonstrated, which receives mid-infrared (MIR) light at room temperature. In the structural design, it was confirmed that the 2D nanohole-array photodetector has high absorbance in the MIR region using rigorous coupled-wave analysis. The result showed that the nanoholes formed in p-type silicon (p-Si), platinum silicide (PtSi), to form Schottky barriers, and gold (Au), for photocurrent extraction, had high absorbance in the MIR region along with the Fabry-Perot resonance mode toward the depth of the nanohole. The 2D nanohole array, with Au/PtSi/p-Si layers, has high absorbance for illuminating MIR light near 3.46 µm from the backside. The current-voltage characteristics indicated a low Schottky barrier of 0.32 eV, confirming the photoresponsive potential in the MIR photodetection. The photocurrent response to the modulation signal was obtained at room temperature. In addition, signal processing through transimpedance and lock-in amplifiers enabled us to obtain characteristics with high linearity for light intensities in milliwatts. Light acquisition for 2.5-3.8-µm-long MIR wavelength became possible, and applications in gas sensing, including vibrational absorption bands of alkane groups, are expected.

Micro Water Flow Measurement Using a Temperature-Compensated MEMS Piezoresistive Cantilever.

In this study, we propose a microelectromechanical system (MEMS) force sensor for microflow measurements. The sensor is equipped with a flow sensing piezoresistive cantilever and a dummy piezoresistive cantilever, which acts as a temperature reference. Since the dummy cantilever is also in the form of a thin cantilever, the temperature environment of the dummy sensor is almost identical to that of the sensing cantilever. The temperature compensation effect was measured, and the piezoresistive cantilever was combined with a gasket jig to enable the direct implementation of the piezoresistive cantilever in a flow tube. The sensor device stably measured flow rates from 20 µL/s to 400 µL/s in a silicon tube with a 2-mm inner diameter without being disturbed by temperature fluctuations.

Au Nanospirals Transferred onto PDMS Film Exhibiting Circular Dichroism at Visible Wavelengths.

We propose a thin, single-layered circular dichroic filter with Au nanospiral structures on a polydimethylsiloxane (PDMS) thin film that has strong circular dichroism at visible wavelengths. Au nanospiral structures with a diameter of 70 nm were fabricated by cryogenic glancing angle deposition on a substrate with a nanodot array template patterned with the block copolymer PS-PDMS. The Au nanospiral structures were transferred onto a transparent and flexible PDMS thin film to fabricate a thin, single-layered circular dichroic filter. The filter had a very large circular dichroism peak of -830 mdeg at 630 nm. The results show that the Au nanospiral structures transferred onto PDMS thin film exhibit large circular dichroism at visible wavelengths.

Extension of the Measurable Wavelength Range for a Near-Infrared Spectrometer Using a Plasmonic Au Grating on a Si Substrate.

In this paper, we proposed near-infrared spectroscopy based on a Si photodetector equipped with a gold grating and extended the measurable wavelength range to cover 1200-1600 nm by improving a spectrum derivation procedure. In the spectrum derivation, photocurrent data during alteration of the incidence angle of the measured light were converted using a responsivity matrix R, which determines the spectroscopic characteristics of the photodetector device. A generalized inverse matrix of R was used to obtain the spectrum and to fit a situation where multiple surface plasmon resonance (SPR) peaks appeared in the scanning range. When light composed of two wavelengths, 1250 nm and 1450 nm, was irradiated, the two wavelengths were distinctively discriminated using the improved method.

Electrical detection SPR sensor with grating coupled backside illumination.

A current detection surface plasmon resonance (SPR) sensor with an Au grating on an n-Si wafer was proposed. SPR excitation light is illuminated from the backside of the device and diffracted by the grating. Since the diffraction provides matching conditions, SPR can be coupled to the Au/analyte interface. Since the coupled SPR excites free electrons on the Au surface, the SPR can be detected as a current signal by a Schottky barrier diode formed on the Au/n-Si interface. The obtained angular current spectrum showed clear agreement with SPR coupling theory, thereby confirming that the sample on the Au surface can be electrically detected using the proposed sensor.

Compact Surface Plasmon Resonance System with Au/Si Schottky Barrier.

Ethanol concentration was quantified by the use of a compact surface plasmon resonance (SPR) system, which electrically detects hot electrons via a Schottky barrier. Although it is well known that SPR can be used as bio/chemical sensors, implementation is not necessarily practical, due to the size and cost impediments associated with a system with variable wavelength or angle of incidence. However, scanning capability is not a prerequisite if the objective is to use SPR in a sensor. It is possible to build a small, inexpensive SPR sensor if the optics have no moving parts and a Schottky barrier is used for electrical current detection in place of a photodetector. This article reports on the design and performance of such a novel SPR sensor, and its application for quantifying ethanol concentration. As the concentration of ethanol is increased, the change in the angle dependence of the SPR current is observed. This change can be understood as a superposition of contributions of SPR coupled with the +3rd- and -3rd-order diffraction. Moreover, real-time monitoring of ethanol concentration was demonstrated using the proposed SPR system.

NIR spectrometer using a Schottky photodetector enhanced by grating-based SPR.

We present a near-infrared (NIR) spectrum measurement method using a Schottky photodetector enhanced by surface plasmon resonance (SPR). An Au grating was fabricated on an n-type silicon wafer to form a Schottky barrier and act as an SPR coupler. The resulting photodetector provides wavelength-selective photodetection depending on the SPR coupling angle. A matrix was pre-calculated to describe this characteristic. The spectrum was obtained from this matrix and the measured photocurrents at various SPR coupling angles. Light with single and multiple wavelengths was tested. Comparative measurements showed that our method is able to detect spectra with a wavelength resolution comparable to that of a commercial spectrometer.

Enantiomeric switching of chiral metamaterial for terahertz polarization modulation employing vertically deformable MEMS spirals.

Active modulation of the polarization states of terahertz light is indispensable for polarization-sensitive spectroscopy, having important applications such as non-contact Hall measurements, vibrational circular dichroism measurements and anisotropy imaging. In the terahertz region, the lack of a polarization modulator similar to a photoelastic modulator in the visible range hampers expansion of such spectroscopy. A terahertz chiral metamaterial has a huge optical activity unavailable in nature; nevertheless, its modulation is still challenging. Here we demonstrate a handedness-switchable chiral metamaterial for polarization modulation employing vertically deformable Micro Electro Mechanical Systems. Vertical deformation of a planar spiral by a pneumatic force creates a three-dimensional spiral. Enantiomeric switching is realized by selecting the deformation direction, where the polarity of the optical activity is altered while maintaining the spectral shape. A polarization rotation as high as 28° is experimentally observed, thus providing a practical and compact polarization modulator for the terahertz range.

Out-of-plane actuation with a sub-micron initial gap for reconfigurable terahertz micro-electro-mechanical systems metamaterials.

We propose a reconfigurable terahertz (THz) metamaterial that can control the transmittance by out-of-plane actuation with changing the sub-micron gap distance between electrically coupled metamaterial elements. By using the out-of-plane actuation, it was possible to avoid contact between the coupled metamaterial elements across the small initial gap during the adjustment of the gap size. THz spectroscopy was performed during actuation, and the transmission dip frequency was confirmed to be tunable from 0.82 to 0.92 THz for one linear polarization state and from 0.80 to 0.91 THz for the other linear polarization; the two polarizations were orthogonal. The proposed approach will contribute to the development of tunable metamaterials based on structural deformations.

Measurement method for light transmittance of layered metamaterials.

We propose a method to measure light transmittance of layered metamaterials by placing the metamaterials directly on a Si photodiode. Our measurement method enables the direct detection of transmitted light that appears as an evanescent wave in natural materials. Here, we report the transmittance measurements of a typical metamaterial using this method. The metamaterial was composed of Ag/Al(2)O(3) layers and was fabricated by direct evaporation on the Si photodiode. The measured transmittance agrees with the simulated transmittance. Our results confirmed that this measurement method can determine the transmittance properties of metamaterials and that it is applicable to other types of metamaterials.

Ratiometric optical temperature sensor using two fluorescent dyes dissolved in an ionic liquid encapsulated by Parylene film.

A temperature sensor that uses temperature-sensitive fluorescent dyes is developed. The droplet sensor has a diameter of 40 µm and uses 1 g/L of Rhodamine B (RhB) and 0.5 g/L of Rhodamine 110 (Rh110), which are fluorescent dyes that are dissolved in an ionic liquid (1-ethyl-3-methylimidazolium ethyl sulfate) to function as temperature indicators. This ionic liquid is encapsulated using vacuum Parylene film deposition (which is known as the Parylene-on-liquid-deposition (PoLD) method). The droplet is sealed by the chemically stable and impermeable Parylene film, which prevents the dye from interacting with the molecules in the solution and keeps the volume and concentration of the fluorescent material fixed. The two fluorescent dyes enable the temperature to be measured ratiometrically such that the droplet sensor can be used in various applications, such as the wireless temperature measurement of microregions. The sensor can measure the temperature of such microregions with an accuracy of 1.9 °C, a precision of 3.7 °C, and a fluorescence intensity change sensitivity of 1.0%/K. The sensor can measure temperatures at different sensor depths in water, ranging from 0 to 850 µm. The droplet sensor is fabricated using microelectromechanical system (MEMS) technology and is highly applicable to lab-on-a-chip devices.

Electrochemical impedance measurement of a carbon nanotube probe electrode.

We measured and analyzed the electrochemical impedance of carbon nanotube (CNT) probe electrodes fabricated through the physical separation of insulated CNT bridges. The fabricated CNT electrodes were free-standing CNTs that were completely covered with an insulator, except for their tips. Typical dimensions of the nanoelectrodes were 1-10 nm in CNT diameter, 80-300 nm in insulator diameter, 0.5-4 µm in exposed CNT length and 1-10 µm in probe length. The electrochemical impedance at frequencies ranging from 40 Hz to 1 MHz was measured in physiological saline. The measured impedance of the CNT electrode was constant at 32 MΩ at frequencies below 1 kHz and was inversely proportional to frequency at frequencies above 10 kHz. By means of comparison with the parasitic capacitive impedance of the insulator membrane, we confirmed that the electrode was sufficiently insulated such that the measured constant impedance was given by the exposed CNT tip. Consequently, we can use the CNT electrode for highly localized electrochemical impedance measurements below 1 kHz. Considering an equivalent circuit and the nanoscopic dimensions of the CNT electrode, we demonstrated that the constant impedance was governed by diffusion impedance, whereas the solution resistance, charge-transfer resistance and double-layer capacitance were negligible.

Optical measurement of directional strain by scattering from nano-disk pairs aligned on an elastomer.

We propose a strain measurement method utilizing light scattered by a pair of nano-disks onto an elastomer sheet. Such nanoparticle pairs exhibit a scattering spectrum that is dependent on the gap distance and the incident light polarization. We utilized this behavior by forming nano-disk pairs with a diameter of 105 nm and a gap of 20-50 nm on a polydimethylsiloxane (PDMS) sheet so that the gap could be altered with strain. The axial direction of the nano-disk pair could be identified by finding the peak wavelength maximum while rotating the polarization angle. Under tensile strain, the peak wavelength decreased from 674 nm at a strain of 0% to 637 nm at a strain of 9.5%, with a peak-shift sensitivity of  - 3.4 nm/% strain. Under compression strain, the peak wavelength increased from 681 nm at a strain of 0% to 705 nm at a strain of  - 3.8%, and the peak-shift sensitivity was  - 16 nm/% strain for strains of 0 to  - 1%. These experimentally determined peak scattering wavelength positions and peak-shift directions are consistent with the simulation results. This method offers wireless measurements of in situ strain on a nanometric scale.

Mechanical impulses can control metaphase progression in a mammalian cell.

Chromosome segregation machinery is controlled by mechanochemical regulation. Tension in a mitotic spindle, which is balanced by molecular motors and polymerization-depolymerization dynamics of microtubules, is thought to be essential for determining the timing of chromosome segregation after the establishment of the kinetochore-microtubule attachments. It is not known, however, whether and how applied mechanical forces modulate the tension balance and chemically affect the molecular processes involved in chromosome segregation. Here we found that a mechanical impulse externally applied to mitotic HeLa cells alters the balance of forces within the mitotic spindle. We identified two distinct mitotic responses to the applied mechanical force that either facilitate or delay anaphase onset, depending on the direction of force and the extent of cell compression. An external mechanical impulse that physically increases tension within the mitotic spindle accelerates anaphase onset, and this is attributed to the facilitation of physical cleavage of sister chromatid cohesion. On the other hand, a decrease in tension activates the spindle assembly checkpoint, which impedes the degradation of mitotic proteins and delays the timing of chromosome segregation. Thus, the external mechanical force acts as a crucial regulator for metaphase progression, modulating the internal force balance and thereby triggering specific mechanochemical cellular reactions.

Piezoresistor-equipped fluorescence-based cantilever probe for near-field scanning.

Scanning near-field optical microscopes (SNOMs) with fluorescence-based probes are promising tools for evaluating the optical characteristics of nanoaperture devices used for biological investigations, and this article reports on the development of a microfabricated fluorescence-based SNOM probe with a piezoresistor. The piezoresistor was built into a two-legged root of a 160-microm-long cantilever. To improve the displacement sensitivity of the cantilever, the piezoresistor's doped area was shallowly formed on the cantilever surface. A fluorescent bead, 500 nm in diameter, was attached to the bottom of the cantilever end as a light-intensity-sensitive material in the visible-light range. The surface of the scanned sample was simply detected by the probe's end being displaced by contact with the sample. Measuring displacements piezoresistively is advantageous because it eliminates the noise arising from the use of the optical-lever method and is free of any disturbance in the absorption or the emission spectrum of the fluorescent material at the probe tip. The displacement sensitivity was estimated to be 6.1 x 10(-6) nm(-1), and the minimum measurable displacement was small enough for near-field measurement. This probe enabled clear scanning images of the light field near a 300 x 300 nm(2) aperture to be obtained in the near-field region where the tip-sample distance is much shorter than the light wavelength. This scanning result indicates that the piezoresistive way of tip-sample distance regulation is effective for characterizing nanoaperture optical devices.