Glaucoma is a progressive optic neuropathy, characterized by the selective loss of retinal ganglion cells (RGCs). Therefore, monitoring the change of number or morphology of RGC is essential for the early detection as well as investigation of pathophysiology of glaucoma. Since RGC layer is transparent and hyporeflective, the direct optical visualization of RGCs has not been successful so far. Therefore, glaucoma evaluation mostly depends on indirect diagnostic methods such as the evaluation of optic disc morphology or retinal nerve fiber layer thickness measurement by optical coherence tomography.
We have previously demonstrated single photoreceptor cell imaging with differential interference contrast (DIC) microscopy. Herein, we successfully visualized single RGC using DIC microscopy. Since RGC layer is much less reflective than photoreceptor layer, various techniques including the control of light wavelength and bandwidth using a tunable band pass filter were adopted to reduce the chromatic aberration in z-axis for higher and clearer resolution. To verify that the imaged cells were the RGCs, the flat-mounted retina of Sprague-Dawley rat, in which the RGCs were retrogradely labeled with fluorescence, was observed by both fluorescence and DIC microscopies for direct comparison. We have confirmed that the cell images obtained by fluorescence microscopy were perfectly matched with cell images by DIC microscopy.
As conclusion, we have visualized single RGC with DIC microscopy, and confirmed with fluorescence microscopy.
In this report, we present a novel approach to exploit the nonlinear response of terahertz (THz) field allowing the observation of ionization phenomenon in a single metal nano island. Because it is not easy to access such high power THz source to generate field over the threshold of the materials, fundamental studies on nonlinear terahertz waves and their applications in spectroscopy have been limited thus far. We are able to overcome this limitation through the use of a metallic nano island embedded in a slot antenna which strongly confines the terahertz electric field driving the system into a highly nonlinear regime. The structure, composed of a nano slot antenna and a nano island located at the center, highly confines THz electromagnetic field at the center of the structure, resulting in huge field enhancement by orders of magnitude at a specific frequency. Electrons on a metallic surface experience a ponderomotive force in a highly confined and enhanced THz electric field directed towards the weak field area by a field gradient. As a result, the accelerated electrons acquire enough energy to ionize ambient carbon atoms. It has to be stressed that it is the first time to observe the ionization of atoms induced by the enhanced terahertz radiation.
In this report, a novel type of highly sensitive small molecule sensing tool has been employed to detect residual pesticide molecules including e. g. methomyl using terahertz (THz) time-domain spectroscopy (TDS) system with nano-slotantenna array. Enhance THz wave by the nano-slot-antenna array induces strong THz field enhancement around nano antenna and thus increases an absorption cross section leading to the detection sensitivity upto ppm level even in solution state. Measured spectrums in transmission and reflection show an excellent performance in both sensitivity and selectivity. We also tested the performance of our nano-antenna array in reflection imaging geometry to simply detect the contained residual pesticide at the real fruit surface as it is, without any extraction or sampling preprocess. The clear difference in the obtained THz reflection image distinguishes the stained area with methomyl from the bare area. Our observation can offer the possibility for further application as a prompt and an accurate small molecule monitoring tool in real time. A quantitative analysis tool for such small molecule can be also developed by this method.
In this study, we successfully generated the large bandwidth of supercontinuum spectra through hollow fibers filled with DNA. Also, by observing that spectra bandwidth was the widest in the order of the hollow core fiber filled with DNA modified by copper ion, the hollow core fiber with only DNA, and the bulk hollow core fiber, we demonstrated that DNA material modified with copper ions can further enhance the spectral bandwidth of supercontinuum. As a result, we anticipate that the SCG as a broadband light source can be used in analytical methods to demonstrate a wide range of biological and environmental questions.
Graphene is a promising material for vapor sensor applications because of its potential to be functionalized for specific chemical gases. In this work, we present a graphene gas sensor that uses single-stranded DNA (ssDNA) molecules as its sensing agent. We investigate the characteristics of graphene field effect transistors (FETs) coated with different ssDNAs. The sensitivity and recovery rate for a specific gas are modified according to the differences in the DNA molecules’ Guanine (G) and Cytosine (C) content. ssDNA-functionalized devices show a higher recovery rate compared to bare graphene devices. Pattern analysis of a 2-by-2 sensor array composed of graphene devices functionalized with different-sequence ssDNA enables identification of NH<sub>3</sub>, NO<sub>2</sub>, CO, SO<sub>2</sub> using Principle Component Analysis (PCA).
The current study describes metal ion sensing with double crossover DNAs (DX<sub>1</sub> and DX<sub>2</sub>), artificially designed as a platform of doping. The sample for sensing is prepared by a facile annealing method to grow the DXs lattice on a silicon/silicon oxide. Adding and incubating metal ion solution with the sensor substrate into the micro-tube lead the optical property change. Photoluminescence (PL) is employed for detecting the concentration of metal ion in the specimen. We investigated PL emission for sensor application with the divalent copper. In the range from 400 to 650 nm, the PL features of samples provide significantly different peak positions with excitation and emission detection. Metal ions contribute to modify the optical characteristics of DX with structural and functional change, which results from the intercalation of them into hydrogen bonding positioned at the center of double helix. The PL intensity is decreased gradually after doping copper ion in the DX tile on the substrate.
Arrays of partially selective chemical sensors have been the focus of extensive research over the past decades because of their potential for widespread application in ambient air monitoring, health and safety, and biomedical diagnostics. Especially, vapor sensor arrays based on functionalized nanomaterials have shown great promise with their high sensitivity by dimensionality and outstanding electronic properties. Here, we introduce experiments where individual vapors and mixtures of them are examined by different chemical sensor arrays. The collected data from those sensor arrays are further analyzed by a principal component analysis (PCA) and targeted vapors are recognized based on prepared database.
Graphene is a promising material for its exceptional electrical and mechanical properties. Starting with the initial demonstration of isolating a single graphene sheet from graphite, much progress has been made in realizing graphene based devices for diverse applications. Here, we introduce an experiment in which the electrical properties of graphene are modified by coating different-sequence single-stranded deoxyribonucleic acid (ssDNA) molecules. We fabricated a graphene-field effect transistor (FET) by transferring CVD graphene on copper foil onto a Si/SiO2 wafer. A passivation layer opened up windows on the surface of the graphene to enable interaction with liquid buffers. ssDNA molecules with different base sequences were coated onto the active graphene channels. We observed a variation in the Dirac voltage of the ssDNA-coated graphene FETs according to the ssDNA base sequences. Electrical control of the graphene FET is obtained via gating effect of the deposited ssDNAs. We conduct a systematic study of this ssDNAinduced gating effect with different base sequences, concentrations, and lengths of molecules, leading to extraction of characteristic parameters of the graphene FET accordingly.
In this report, we present a new type of non-contact detection method for glucose molecule using nano antenna array based bio sensing chip that operates at terahertz frequency range (0.5 – 2.5 THz). Localized and hugely enhanced transmitted terahertz field by nano antenna array in the sensing chip induced enhancement of absorption coefficient of glucose molecule that enables us to detect even very small volume of molecules. Nano antenna based terahertz sensing chip can be expected to offer accurate identification of glucose level as a non-invasive and painless sensing tool with high sensitivity.
Ultrafast optical microscopy (UOM) combines a typical optical microscope and femtosecond (fs) lasers that produce
high intensity, ultrashort pulses at high repetition rates over a broad wavelength range. This enables us new types of
imaging modalities, including scanning optical pump-probe microscopy, which varies the pump and probe positions
relatively on the sample and ultrafast optical wide field microscopy, which is capable of rapidly acquiring wide field
images at different time delays, that is measurable nearly any sample in a non-contact manner with high spatial and
temporal resolution simultaneously. We directly tracked carriers in space and time throughout a NW by varying the
focused position of a strong optical “pump” pulse along the Si core-shell nanowires (NWs) axis while probing the
resulting changes in carrier density with a weaker “probe” pulse at one end of the NW. The resulting time-dependent
dynamics reveals the influence of oxide layer encapsulation on surface state passivation in core-shell NWs, as well as the
presence of strong acoustic phonon oscillations, observed here for the first time in single NWs. Time-resolved wide field
images of the photoinduced changes in transmission for a patterned semiconductor thin film and a single silicon
nanowire after optical excitation are also captured in real time using a two dimensional smart pixel array detector. Our
experiments enable us to extract several fundamental parameters in these samples, including the diffusion current,
surface recombination velocity, diffusion coefficients, and diffusion velocities, without the influence of contacts.
We show that accumulation of charges at the metal edges via light-induced currents creates large horizontal electric
field, which in effect attracts the incoming light. The enhanced field is fully propagating towards the far-field because no
cut-off exists. With the amplitude enhancement in the range of 1,000, the intensity enhancement of 10<sup>6</sup>, and the
nonlinear enhancement of 10<sup>12</sup>, this structure can be an excellent launching pad for inducing broad-band nonlinearity,
small signal detection in astronomy or biology, and for surface enhanced Raman scattering.
We study the dielectric constant dependent diffraction phenomena of single slit apertures, both theoretically and
experimentally. We experimentally simulate perfect metal and real metal cases by investigating subwavelength
diffraction by a single slit, both in nano-optical and in terahertz regimes, keeping the slit-width/wavelength ratio
approximately the same for both of frequency regimes. The wave-front in optical regime separates itself into forward
propagating beam and surface-bound 90-degree diffracted wave, i.e., surface plasmon polaritons; while the separation of
modes is not observed in terahertz regime.
Terahertz transmission filters have been manufactured by perforating metal surface structures with various geometric shapes which all support near-unity transmission at specific frequencies determined by geometric shape, symmetry, polarization, and lattice constant. Our results show that the structures specifically designed by the shape resonance are extremely versatile, dependable, easy to control and easy to make the multifunctional filters.
We present that size of Ge nanoparticle can be controlled by changing the angle between ultrafast laser polarization and crystal axis using ultrafast laser irradiation. The nanoparticle size dependence on the laser polarization with respect to the Ge crystal axis exhibits a sinusoidal function with a minimum size at (100) axis. Moreover, the measurement of transient reflection reveals the presence of large anisotropies in both its amplitude and its relaxation dynamics with a minimum at (100) crystal axis. This implies that the observed anisotropic dependence of nanostructure size of Ge is followed by a different carrier density as well as its relaxation process depending on the orientation of Ge crystal axis only at near and above threshold fluence.