Dongmin Wu

Terahertz plasmonic high pass filter

Metamaterials, which contain engineered subwavelength microstructures, can be designed to have positive or negative e and μ at desired frequencies. In this letter, we demonstrate a metamaterial which has a “plasmonic” response to electromagnetic waves in the terahertz (THz) range. The sharp change of reflection and transmission at this plasma frequency makes the structure a high pass filter. The reflection response is characterized by Fourier transform infrared spectroscopy, and a plasma frequency at 0.7 THz is observed, which agrees with the theoretical calculation. The metamaterial is a two-dimensional cubic lattice consisting of thin metal wires, having wire diameter of 30 μm, lattice constant of 120 μm, and wire length of 1 mm. The microstereolithography technique is employed to fabricate the high-aspect-ratio lattice.

Super-Resolution Imaging by Random Adsorbed Molecule Probes

Single molecule localization (SML) is a powerful tool to measure the position and trajectory of molecules in numerous systems, with nanometer accuracy. This technique has been recently utilized to overcome the diffraction limit in optical imaging. So far, super-resolution imaging by SML was demonstrated using photoactivable or photoswitchable fluorophores, as well as diffusive fluorophore probes in solution. All these methods, however, rely on special fluorophore or object properties. In this Letter, we propose and demonstrate a new super-resolution technique attainable for a bio/dielectric structure on a metal substrate. A sub-diffraction-limited image is obtained by randomly adsorbed fluorescent probe molecules on a liquid-solid interface, while the metal substrate, quenching the unwanted fluorescent signal, provides a significantly enhanced imaging contrast. As this approach does not use specific stain techniques, it can be readily applied to general dielectric objects, such as nanopatterned photoresist, inorganic nanowires, subcellular structures, etc.

Room temperature GaN/AlGaN self-mixing terahertz detector enhanced by resonant antennas

This letter focuses on the fabrication and characterization of a terahertz detector integrated with a group of low pass filters and resonant antennas. The detector operates as a self-mixer on GaN/AlGaN high electron mobility transistor (HEMT). At room temperature, a strong dc photocurrent is produced with the aid of the antennas and filters. The responsivity of our HEMT device is estimated to be 53 mA/W and a noise equivalent power of 1 nW/Hz can be achieved at 300 K. In addition, the sensor properties of a similar HEMT detector without filter are tested as a comparison.

Influence of carrier concentration on piezoelectric potential in a bent ZnO nanorod

The influence of carriers on the piezoelectric potential in a bent ZnO nanorod is investigated using finite difference method. The distributions of carriers and the electrical potential in the nanorod are obtained. The results shows that the positive piezoelectric potential in stretched side of the bent nanorod is significantly screened by the carriers and the negative potential in compressed side is well preserved when considering a moderate carrier concentration of 1×1017 cm−3. The calculation results agree with the experimental results that only negative pulses are observed in the nanogenerator experiment using the as-grown ZnO nanorods. Further investigation shows that when the carrier concentration is 1×1018 cm−3, the piezoelectric potential in the nanorod is almost completely screened by the redistributed carriers.

Adhesion force of polymeric three-dimensional microstructures fabricated by microstereolithography

The adhesion between microstructures represents a great challenge in reliability of polymeric three-dimensional structures fabricated by microstereolithography (μSL). During the evaporative releasing, the capillary force of the solvent causes the deformation and adhesion of the fabricated beams. We present a method to determine the adhesion force of polymeric microstructures fabricated by μSL. The test structures with parallel beams were fabricated and released from the liquid resin via evaporation. By measuring the relationship between the adhesion length and the geometry of the beams, the adhesion force between two 1,6-hexanediol diacrylate (HDDA) polymeric parallel beams is determined as γ=72±5 mN/m. This simple method and the determined adhesion force provide a key in designing reliable polymeric microelectromechanical systems in preventing the stiction problem.

Negative group velocity of surface plasmons on thin metallic films

By tailoring the dispersion curve of surface plasmons (SPs) of a thin metallic film surrounded by dielectric half-spaces, it is shown that the group velocity of the symmetric mode is always positive, while the group velocity of the anti-symmetric mode can be negative. Consequently, the forward and backward propagation of SPs, in which the energy flow is respectively parallel or antiparallel to the wave vector, can be realized. The physical origin of the intriguing backward SPs is given. Furthermore, schemes for the negative refraction and imaging of SPs are proposed by incorporating two plasmon modes with group velocities of opposite signs.

Guided-mode resonance excitation on multimode planar periodic waveguide

Planar periodic waveguides are applied to design numerous passive guided-mode resonance (GMR) elements. Key to achieving these devices with desired spectral properties is placement and manipulation of the resonance peaks. It has been shown that, the resonance locations closely track the dispersion curves of the leaky modes. In this paper, taking Bragg reflection due to periodicity and coupling between different modes into account, we investigate the dispersion relations of leaky modes in multimode planar periodic waveguides, both for s-polarized (TE mode) and p-polarized (TM mode) incident waves. Exploiting the characteristics of leaky mode dispersions in full range of the first Brillouin zone, we offer the ways to construct some novel optical elements, for example, polarization independent and angular tolerant GMR filters.

Tunable plasmonic wires at terahertz frequencies

We employed micro-electro-mechanical system (MEMS) techniques to fabricate parallel sub-wavelength thin-wire structures of metals on elastomeric matrices. From the transmission measurement by Fourier Transform Infrared Spectroscopy, we observed the depressed plasma frequencies of these thin-wire structures at terahertz (THz) ranges. Furthermore, the behavior of depressed plasma frequencies is very sensitive to the polarization of the applied field. The reasons that these engineered materials exhibit unprecedented properties not observed in nature can be interpreted by two factors: the diluted electron densities and the enhancement of electron mass. In addition, the plasma frequencies are readily tunable over a broad frequency range by extending the elastomeric matrices to change their periodicity. These novel properties of tunable and polarization-dependant plasma frequencies at THz ranges promise abundant striking applications in THz optics.

Dispersion relation of guided-mode resonances in multimode grating waveguide structures

It is well known that the location of guided-mode resonance (GMR) in grating waveguide structures closely tracks the leaky mode dispersion curves. In this paper, taking Bragg reflection due to periodicity and interaction between different modes into account, we first present a schematic diagram of the dispersion relations of leaky modes in multimode grating waveguide structures, both for s-polarized (TE mode) and p-polarized (TM mode) incident waves. Due to the perturbation of the grating layer, the interaction between different resonance modes (transverse standing waves) is inevitable. This transverse interference will result in the non-Bragg nature resonance band gaps in the dispersion curves. Exploiting the characteristics of leaky mode dispersions over the full range of the first Brillouin zone, we hoped we could gain some insight into the relationship among the mode interactions, band gaps, and their benefits to optical elements utilizing the GMR effect in grating waveguide structures. Finally, a specific structure is analyzed.

Calligraphy on self-assembled monolayer of supramolecules

Information storage utilizing the bi-stable state of organic molecules has been a hot topic in molecular electronics, due to the promising future of single molecule devices. We demonstrated in this paper nanometer-scale pattern generation on the self-assembled monolayer of [2]rotaxane. The organic molecules are covalently bonded to the Au(111) surface of the substrate through the di-thiol groups at one end of the molecules. Electric bias was applied between the Au substrate and the conductive tip of a scanning probe microscope to stimulate the pattern generation. The line width and apparent height of the pattern is related to the bias voltage and there exists a threshold, below which no pattern can be generated.