In recent years, optoelectronic devices are implemented based on natural DNA with enhanced performance and efficiency. In this study, we present stimulus pulse-dependent responses in natural DNA biopolymer devices. The device consists of a simple sandwich structure and the resistivity can be manipulated with respect to voltage operation. We characterize the stimulus pulse-dependent responses, where the synaptic plasticity will be presented. To further explore dynamics of resistive states, the effect of incorporating a photo-responsive material on the light-triggered electrical characteristics will be discussed. Our results reveal natural DNA biopolymer shows great promise for the development of synaptic devices for neuromorphic circuitry.
Electromagnetic induced transparency (EIT) is a quantum phenomenon, featuring strong dispersion at the transparent window in an absorption spectral region. In recent years, EIT-like behaviors have also been discussed in artificial nanostructures, where a transparent window emerges in an otherwise high reflection band. Up to date, many demonstrations are implemented based on all-dielectric metamaterials as the loss can be reduced compared to their metallic counterparts. However, studies are mostly presented based on high-refractive-index dielectrics, in which the choice of materials is limited at optical frequencies. In this study, we present a new strategy that enables handedness-dependent EIT in a lower refractive index dielectric material (n~1.5), which can be more widely implemented in polymer-based fabrication platforms. In the first part of the study, we numerically present the evolution of EIT response in a helix structure from high to low refractive indices. As the refractive index decreases from 3.5 to 1.5, the resonances are less pronounced and the EIT behavior cannot be maintained. Therefore, we show that by properly tailoring the geometrical parameters, the EIT response may emerge again without increasing the refractive index. In the second part of the study, we characterize the effect of substrate on the handedness-dependent EIT response of the helix structure. We show that the EIT performance is severely degraded since the dielectric helix has a refractive index close to the glass substrate. To resolve the issue, we present a rod-supported structure to effectively retrieve the EIT response. As EIT-based devices are widely used for sensors and nonlinear optics, our design which can be implemented on a polymer-based platform may broaden the horizon of applications in sensors and optoelectronics devices.
Chiral metamaterials have attracted great interests in recent years owing to fascinating properties such as negative refraction, strong optical activity, and circular dichroism, which can be applied in many optoelectronic devices. Helix is especially suitable for studying chiral responses as the helical geometry well resembles the feature of circularly polarized light. In this study, we use a helix structure as a model system to demonstrate the general response of circularly polarized light in a three-dimensional dielectric helix nanostructure. The optical characterization is performed based on finitedifference time-domain (FDTD) method and our results show that the helix structure, consisting of dielectric helices arranged in a square lattice, exhibits multiple resonant peaks. The retrieved effective parameters from the complex transmittance and reflectance will be presented and the dispersion characteristics will be discussed based on various geometric parameters. The resonant frequencies can be tuned by structural parameters, and negative permeability can be achieved when the resonances are adjacent in frequencies. Depending on the geometrical arrangements, we will demonstrate the unique optical properties in an anisotropic helix nanostructure. Our analysis yields physical insight into the interaction of circularly polarized light with a three-dimensional chiral nanostructure, and provides design guidelines towards the implementation of all-dielectric photonic metamaterials.
Photonic amorphous structure (PAS) has attracted increasing research attention due to their interesting characteristics,
such as noniridescent structural colors and isotropic photonic band gap. In this work, we present PAS with different
characteristic lengths and analyze their structural and topological properties. First, a Fourier spectral method was used to
solve Cahn-Hilliard equation and generate a spinodal binary phase structure. By changing the time of the evolution of
phase field, mobility, and standard deviation, the characteristic length of amorphous structures can be adjusted. We
present the numerical analysis based on finite-difference time-domain (FDTD) method to characterize the density of
state (DOS) of PAS based on different time of the evolution of phase field. The corresponding spatial Fourier spectrum
of PAS is calculated to examine the characteristic length, and the photonic band gap properties will be discussed in
association with the characteristic length. These results are crucial for design of new optical materials display devices
base on dielectric amorphous photonic structures.
Hyperbolic metamaterial (HMM) has attracted considerable attention owing to several exotic optical properties,
including negative refraction, enhanced spontaneous emission, and subwavelength imaging. The hyperbolic dispersion of
HMMs increases photonic density of states in a broad bandwidth, leading to enhancement of spontaneous emission.
However, the out-coupling of light from HMMs is difficult due to the evanescent character of the high-k modes at the
surface. In this study, we implement the full-field numerical calculations based on finite-difference time-domain (FDTD)
method to characterize the optical properties of nanowire HMMs embedded in a grating structure. We first examined the
power spectrum of the nanowire HMMs. The Purcell factor and the light enhancement are also analyzed. Furthermore, to
examine the out-coupling of light by virtue of the periodic structure, the Purcell factor and enhancement of light
extraction efficiency of the hybrid structure will be examined and discussed. The analysis result is important toward
engineering highly-efficient photonic devices based on HMMs.
Gyroid is a type of three-dimensional chiral structures and has been found in many insect species. Besides the photonic crystal properties exhibited by gyroid structures, the chirality and gyroid network morphology also provide unique opportunities for manipulating propagation of light. In this work, we present studies based on finite-difference time domain (FDTD) method for analyzing the dispersion relation characteristics of dielectric single gyroid (SG) metamaterials. The band structures, transmission spectrum, dispersion surfaces, equifrequency contours (EFCs) of SG metamaterials are examined. Some interesting wave guiding characteristics, such as negative refraction and collimation, are presented and discussed. We also show how these optical properties are predicted by analyzing the EFCs at different frequencies. These results are crucial for the design of functional devices at optical frequencies based on dielectric single gyroid metamaterials.
Gyroid is a type of three-dimensional chiral structures, which have attracted much research attention recently. A
dielectric single gyroid (SG) can be a candidate for providing new means of guiding light because it has been shown to
exhibit complete photonic band gaps. Owing to the chiral nature, the SG metamaterials may exhibit circular polarization-dependent
properties, leading to new types of polarization-sensitive devices. In this work, we present studies based on
finite-difference time-domain (FDTD) method for analyzing the polarization-dependent characteristics of dielectric SG.
We show that the operation frequency of SG metamaterials can be advanced from microwave to visible region by
varying its material, lattice constant and volume fraction. The corresponding band structures, transmission spectra for
right circularly polarized (RCP) light and left circularly polarized (LCP) light, and circular dichroism (CD) indices are
examined. According to our analysis, a circular polarization gap is found in the visible region. In particular, the
correlation between the volume fraction of dielectric SG and the frequency range of circular polarization band gaps is
also investigated. These results are crucial for the design of functional polarization-sensitive devices at the visible
wavelength based on dielectric single gyroid metamaterials.
Deoxyribonucleic acid (DNA), as one kind of biopolymer, has recently emerged as an attractive optical material, showing promise in making versatile optoelectronic devices. In the present study, we report the fabrication and characterization of DNA biopolymer nanocomposite with tunable conductivities and the application in bistable memory device. DNA nanocomposite consisting of DNA biopolymer and silver nanoparticles is synthesized using a phototriggered method. The nanocomposite exhibits tunable conductivities when exposed to UV light under different periods of time. The electrical conductivity is suggested to be dependent on the quantity and the distribution of silver nanoparticles formed in DNA biopolymer. In addition, a memory device based on DNA biopolymer nanocomposite is demonstrated. The operation of different conductivity states can be adjusted by the concentration of nanoparticles. The device shows bistability of current, and presents a stable write-read-erase cycle. Detailed performance of the DNA-based memory device will be presented and discussed.
Recently, excellent solar cell device performances have been achieved with solution-processed small-molecule donor materials. Small molecules have well defined structures and thus allow better control of self-assembly in the solid state. However, the easy formation of H-type aggregates and lack of strong interactions between nanodomains could limit charge transport, device performance, and long-term stability. We have recently explored the synthesis of ring-protected small molecules (with rings surrounding the center of the molecules), studied the intermolecular interactions in solution and solid state, and conducted preliminary solar cell device fabrications. It has been found that the molecules behave very differently from conventional flat small molecules in both solution and solid states. Proton NMR study of solutions of different concentrations revealed the presence of strong intermolecular interactions as a result of absence or shortage of open-ended alkyl side chains; however, such strong interactions do not lead to precipitation of the molecules even at high concentrations. Excellent films are routinely obtained from the neat small molecules despite the much reduced number of solubilizing groups. The New findings strongly suggest that ring protection is an effective strategy to avoid Haggregation and maintain strong pi-pi interactions simultaneously. Such materials are expected to form head-tail selfassemblies that will open new possibilities for small molecule organic materials. Conceptually, thin films of such materials are potentially more isotropic in charge transport than conventional small molecule and polymer films, a property desirable for photovoltaics and some other optoelectronic applications.
In recent years, deoxyribonucleic acid (DNA) biopolymers have attracted much research attention and been considered as a promising material when being employed in many optoelectronic devices. Since performance of many DNA biopolymer-based devices relies on carrier transport, it is crucial to study the carrier mobility of these DNA-surfactant complexes for practical implement. In this work, we present hole mobility characterization of cetyltrimethylammonium (CTMA)-modified DNA biopolymer by using space-charge-limited current (SCLC) method. Devices were fabricated using a sandwich structure with a buffer layer of MoO3 to enhance hole injection and achieve ohmic contact between the anode and the DNA layer. Current-voltage (I-V) curves of the devices were analyzed. A trap-free SCLC behavior can ultimately be achieved and a quadratic dependence in I-V curve was observed. With increasing electric field, a positive field-dependent mobility was demonstrated. The correlation between mobility and temperature was also investigated and a positive relation was found. The characterization results can be further utilized for DNA-based device design and applications.
Deoxyribonucleic acid (DNA) biopolymers have shown promise to be utilized in optoelectronic devices owing to several unique features of DNA molecules. In this study, we present the fabrication of DNA-Au nanoparticles (Au NPs) nanocomposite and incorporate it in organic light-emitting devices (OLEDs). DNA biopolymer attributes to a high lowest unoccupied molecular orbital (LUMO) level for electron blocking, whereas Au NPs are the hole traps to retard hole injection. We evaluate the performance of DNA-Au NPs nanocomposite OLEDs comprised of different concentrations of Au NPs. The results indicate that the utilization of DNA-Au NPs nanocomposite gives rise to higher luminance and higher current efficiency compared to the DNA-based device without Au NPs.
Research on chiral metamaterials has drawn much attention in recent years. By virtue of chirality, for example, it has been shown that chiral metamaterials can achieve negative refractive index without great energy dissipation. In this report, we applied effective parameter retrieval technique to study the material properties of helical metamaterial. The retrieval procedure yields electromagnetic parameters through employing finite-difference time-domain (FDTD) method under periodic boundary condition. We numerically obtain several electromagnetic parameters of the structure and show that the resonance properties and the index of refraction of the helical metamaterial have strong relationship with its circular dichroism. The optical properties of the structure are also discussed, which provides general design guidelines for engineering functional chiral metamaterials.
Helix photonic metamaterials are attractive to many applications due to the unique properties of strong circular dichroism
and gyrotropy. In this study, the optical properties of metallic helix metamaterial were systematically investigated. Such
metamaterial is composed of three-dimensional metallic helical nanowires arranged in a two-dimensional array. 3D
finite-difference time-domain (FDTD) method was adopted for simulating the spectral response under the excitation of
circularly polarized light. We show that the spectral responses were correlated to the dimensions of the helix structures.
Generally, the resonance wavelengths as well as optical properties were determined by the geometrical parameters and
the composed materials of the structures. When the dimension scaled down, electromagnetic interactions between helices
are pronounced, which consequently affect the optical responses of the structures. The dependency between structure
dimension and the corresponding optical properties were discussed and presented in this report.
Metallic nanorod array metamaterials, consisting of nanowires arranged in a two-dimensional array, have exhibited many
unique features and attracted much attention recently. Owing to the sensitive nature of the plasmon resonances to
changes in geometrical parameters of nanorod arrays, significant shift in resonance wavelengths along with variances in
field distribution have been observed. In this study, we characterize the distribution of electric fields and the energy flow
in the metallic nanorod metamaterial by finite-difference time-domain (FDTD) method. We show that the direction of
energy flow is strongly correlated to the geometrical parameters of nanorod arrays and the wavelength. We estimated the
energy flow along a plasmonic waveguide and analyzed the field distribution in a unit cell corresponding to different
geometrical parameters and excitation wavelength. The results show that the dominant direction of energy flow is related
to the geometrical parameters and the excitation conditions. The reported phenomena for metallic nanorod metamaterials
may find numerous applications for guiding structures and sensors.
Deoxyribonucleic acid (DNA) biopolymer has been emerging as a promising material for photonic applications. As
many optoelectronic devices rely on carrier transportation to achieve desired functionality, carrier mobility is important
for the exploitation of these biopolymer-based materials for practical implementation. In this study, we present the
mobility measurement by employing time-of-flight technique and characterize the current-voltage (I-V) properties based
on DNA-surfactant complexes. An additional NPB layer was introduced in the fabricated structure to serve as a charge
generation layer (CGL). The dependency of hole mobility with respect to the applied electric field was characterized and
a linear correlation was exhibited. Hole transport was found to be dispersive, indicating a high degree energetic disorder
in these DNA-surfactant complexes. The characterization results show promises for the employment of DNA complexes
in the applications of organic light-emitting devices and organic field-effect transistors.
Thermal imprint provides a stable and rapid approach in the fabrication of precision V-groove structures.
This paper presents a theoretical and experimental study focusing on the estimation of mold life based on
both the formation and wear mechanisms. In the experiment, BK-7 was used as the substrate, and the
mold with V-groove patterns was fabricated with glassy carbon. The formation of V-groove
microstructures on BK-7 glass substrate was implemented by a lab made thermal imprint equipment,
while the precision of imprinted pattern was measured by an optical measurement system. Additionally,
the micro-scale friction and wear theories were adopted to estimate the mold life. Finally, the prediction
model of mold life can be estimated by the relative friction and wear theories and measurement data,
which enable us to efficiently optimize the glass thermal imprint process.
DNA biopolymer has emerging as a promising material in photonic applications. In this paper, we present the
preparation and characterization of a series of DNA-surfactant complexes based on aromatic surfactants, including
vinylbenzyltrimethylammonium chloride, benzyltrimethylammonium chloride, and phenyltrimethylammonium chloride.
Fourier-transform infrared spectroscopy (FTIR) and UV-VIS spectroscopy were used to characterize the presence of
specific chemical groups in the materials. These synthesized DNA complexes show high transparency from 400nm to
1100nm. These materials can be spin casted into thin films from nm to um and the morphology was examined by SEM
and AFM. Thermal property was characterized by thermogravimetric analysis. Conductivity was examined to investigate
the effect of aromatic surfactants on the electrical properties of DNA complexes. In addition, the photoluminescence and
lasing properties for DNA-aromatic surfactants with rhodamine dyes were investigated to study the amplified
spontaneous emission where the ASE emission wavelength, lasing threshold, and gain were presented and discussed. The
results were compared with DNA complex with single chain aliphatic surfactant complex (DNA-cetyltrimethylammoniumchloride).
Recent advances in polymer materials have significantly increase the available electrooptic coefficients. This has now
stimulated the development of new designs and configurations for high frequency optical modulators. In addition, it has
opened up the field to new applications including high speed optical Digital Signal Processing. The initial areas
investigated include linear modulators, true time delays and arbitrary waveform generation. More complex devices with
multiple elements in series are now being investigated.
We present recent developments in etchless fabrication techniques for defining low-loss waveguides in polymers. Photobleached waveguides with optical propagation loss equal to the inherent loss of the core materials have been fabricated, as well as Mach-Zehnder modulators with 4.5 volt driving voltage and fiber-to-fiber insertion loss of 8 dB. In terms of new configurations, a novel linearized directional coupler modulator that has a 10 dB enhancement in the dynamic range compared to conventional Mach-Zehnder modulators is presented. We report on the design and fabrication of polymer digital optical switches with switching voltages of 7 volts and extinction ratios greater than 20 dB. Simultaneous serrodyne frequency shifting and high-frequency phase modulation in a polymer phase modulator are demonstrated in order to simplify the setup required to implement two-color heterodyne ranging. Finally, we propose implementations of optical signal processors based on polymer optical delay lines, couplers, and electrooptic modulators, and discuss their applications to optical signal processing.
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