Keunhan Park

Study of the surface and bulk polaritons with a negative index metamaterial

This work investigates the impact of surface and bulk polaritons on the optical properties, especially the reflectance, of a negative index metamaterial (NIM) sandwiched between different dielectric layers. Regime maps are developed to describe the polariton dispersion relations and to help understand the effect of the NIM layer thickness on the polariton resonance frequencies. It is shown that polaritons exist for both p and s polarizations in the same frequency region where the refractive index is negative; beyond this region, surface polaritons exist for p polarization only. For an NIM layer, the dispersion curves of a surface polariton and a bulk polariton are smoothly connected, suggesting that a surface mode can be converted into a bulk mode, and vice versa. In an attenuated total reflection configuration, the width of the gap between the prism and the NIM layer has a strong influence on the location and the magnitude of the reflectance minimum. Furthermore, surface polaritons may exist at a single boundary and enhance the energy transmission via photon tunneling. The results demonstrate that NIMs may be used to effectively

Radiative Heat Transfer Analysis in Plasmonic Nanofluids for Direct Solar Thermal Absorption

The present study reports a novel concept of a direct solar thermal collector that harnesses the localized surface plasmon of metallic nanoparticles suspended in water. At the plasmon resonance frequency, the absorption and scattering from the nanoparticle can be greatly enhanced via the coupling of the incident radiation with the collective motion of electrons in metal. However, the surface plasmon induces strong absorption with a sharp peak due to its resonant nature, which is not desirable for broad-band solar absorption. In order to achieve the broad-band absorption, we propose a direct solar thermal collector that has four types of gold-nanoshell particles blended in the aquatic solution. Numerical simulations based on the Monte Carlo algorithm and finite element analysis have shown that the use of blended plasmonic nanofluids can significantly enhance the solar collector efficiency with an extremely low particle concentration (e.g., approximately 70% for a 0.05% particle volume fraction). The low particle concentration ensures that nanoparticles do not significantly alter the flow characteristics of nanofluids inside the solar collector. The results obtained from this study will facilitate the development of highly efficient solar thermal collectors using plasmonic nanofluids. [DOI: 10.1115/1.4005756]

Topography imaging with a heated atomic force microscope cantilever in tapping mode

This article describes tapping mode atomic force microscopy (AFM) using a heated AFM cantilever. The electrical and thermal responses of the cantilever were investigated while the cantilever oscillated in free space or was in intermittent contact with a surface. The cantilever oscillates at its mechanical resonant frequency, 70.36 kHz, which is much faster than its thermal time constant of 300 micros, and so the cantilever operates in thermal steady state. The thermal impedance between the cantilever heater and the sample was measured through the cantilever temperature signal. Topographical imaging was performed on silicon calibration gratings of height 20 and 100 nm. The obtained topography sensitivity is as high as 200 microVnm and the resolution is as good as 0.5 nmHz(1/2), depending on the cantilever power. The cantilever heating power ranges 0-7 mW, which corresponds to a temperature range of 25-700 degrees C. The imaging was performed entirely using the cantilever thermal signal and no laser or other optics was required. As in conventional AFM, the tapping mode operation demonstrated here can suppress imaging artifacts and enable imaging of soft samples.

Frequency-Dependent Electrical and Thermal Response of Heated Atomic Force Microscope Cantilevers

This paper investigates the electrical and thermal response of the heated atomic force microscope (AFM) cantilevers in the frequency range from 10 Hz to 1 MHz. Spectrum analysis of the cantilever voltage response to periodic heating distinguishes different thermal behaviors of the cantilever in the frequency domain: the cantilever voltage at low frequencies is modulated by higher-order harmonics, and at high frequencies it oscillates with 1-omega only. A simple model facilitates the understanding of complicated electrical and thermal behaviors in the cantilever, thus, it is possible to determine the cantilever temperature. The calculation predicts that temperature oscillation is restricted to the heater region when the cantilever is operated at about 10 kHz, suggesting that the periodic-heating operation of the cantilever may be employed for highly sensitive thermal metrology

Experimental Investigation on the Heat Transfer Between a Heated Microcantilever and a Substrate

This work describes the heat transfer process from a heated microcantilever to a substrate. A platinum-resistance thermometer with a 140 nm width was fabricated on a SiO 2 -coated silicon substrate. The temperature coefficient of resistance estimated from the measurement was 7X10- 4 K -1 , about one-fifth of the bulk value of platinum. The temperature distribution on the substrate was obtained from the thermometer reading, as the cantilever raster scanned the substrate. Comparison between the measurement and calculation reveals that up to 75% of the cantilever power is directly transferred to the substrate through the air gap. From the force-displacement experiment, the effective tip-specimen contact thermal conductance was estimated to be around 40 nW/K. The findings from this study should help understand the thermal interaction between the heated cantilever and the substrate, which is essential to many nanoscale technologies using heated cantilevers.

Energy pathways in nanoscale thermal radiation

We show in this letter that when nanoscale radiation between two parallel plates is considered, the Poynting vector is decoupled for each parallel wavevector component (β) due to the nature of thermal emission, as manifested by the fluctuation-dissipation theorem. The streamlines calculated by tracing the Poynting vector vividly demonstrate that the spectral radiative energy travels in infinite directions along curved lines. Depending on the β value, the energy pathway may exhibit considerable lateral shift. This letter elucidates the fundamental characteristics of nanoscale thermal radiation that is important for applications, such as near-field optical sensors and energy conversion devices.

Low temperature characterization of heated microcantilevers

This article describes the electrical and thermal behaviors of heated atomic force microscope cantilevers under steady- and periodic-heating operation at low temperatures and in vacuum. The cantilever resistance drastically increases as temperature decreases below 150 K, providing a large and negative temperature coefficient of resistance of −0.023 K−1 at 100 K. Under steady heating, the cantilever heater can be heated above 300 K even when its environment is at 77 K. Electrical and thermal transfer functions are derived to depict the electrical and thermal cantilever responses under periodic heating and to extract cantilever thermophysical properties. The calculation of in-phase and out-of-phase temperatures along the cantilever reveals that its response becomes out of phase and restricted to the heater region at high frequencies. These results enable the use of heated cantilevers in cryogenic applications as a localized heat source and a sensitive thermal metrology tool.

Design analysis of doped-silicon surface plasmon resonance immunosensors in mid-infrared range

This paper reports the design analysis of a microfabricatable mid-infrared (mid-IR) surface plasmon resonance (SPR) sensor platform. The proposed platform has periodic heavily doped profiles implanted into intrinsic silicon and a thin gold layer deposited on top, making a physically flat grating SPR coupler. A rigorous coupled-wave analysis was conducted to prove the design feasibility, characterize the sensors performance, and determine geometric parameters of the heavily doped profiles. Finite element analysis (FEA) was also employed to compute the electromagnetic field distributions at the plasmon resonance. Obtained results reveal that the proposed structure can excite the SPR on the normal incidence of mid-IR light, resulting in a large probing depth that will facilitate the study of larger analytes. Furthermore, the whole structure can be microfabricated with well-established batch protocols, providing tunability in the SPR excitation wavelength for specific biosensing needs with a low manufacturing cost. When the SPR sensor is to be used in a Fourier-transform infrared (FTIR) spectroscopy platform, its detection sensitivity and limit of detection are estimated to be 3022 nm/RIU and ~70 pg/mm(2), respectively, at a sample layer thickness of 100 nm. The design analysis performed in the present study will allow the fabrication of a tunable, disposable mid-IR SPR sensor that combines advantages of conventional prism and metallic grating SPR sensors.

Surface and magnetic polaritons on two-dimensional nanoslab-aligned multilayer structure

The present study theoretically investigates the radiative properties of a two-dimensional (2-D) multilayer structure that has a dielectric spacer between a metallic substrate and square cross-sectional metallic gratings. Differently from the one-dimensional metallic strips coated on a dielectric spacer atop an opaque metallic film [Opt. Express 16, 11328 (2008)], the 2-D metallic gratings can support the localized surface plasmon in addition to the propagating surface plasmon along the metal-dielectric interface. Moreover, the presence of a dielectric spacer also allows the excitation of magnetic polaritons. Underlying mechanisms of the surface and magnetic polartions on the proposed structure are elucidated by employing the 2-D rigorous coupled-wave analysis. The results obtained in this study will advance our fundamental understanding of light-matter interaction at the nanometer scale and will facilitate the development of engineered nanostructures for real-world applications, such as thermophotovoltaic and photovoltaic devices.

Note: Precision viscosity measurement using suspended microchannel resonators

We report the characterization of a suspended microchannel resonator (SMR) for viscosity measurements in a low viscosity regime (<10 mPa s) using two measurement schemes. First, the quality factor (Q-factor) of the SMR was characterized with glycerol-water mixtures. The measured Q-factor at 20 °C exhibits a bilinear behavior with the sensitivity of 1281 (mPa s)(-1) for a lower (1-4 mPa s) and 355 (mPa s)(-1) for a higher viscosity range (4-8 mPa s), respectively. The second scheme is the vibration amplitude monitoring of the SMR running in a closed loop feedback. When compared in terms of the measurement time, the amplitude-based measurement takes only 0.1 ~ 1 ms while the Q-factor-based measurement takes ~30 s. However, the viscosity resolution of the Q-factor-based measurement is at least three times better than the amplitude-based measurement. By comparing the Q-factors of heavy water and 9.65 wt.% glycerol-water mixture that have very similar viscosities but different densities, we confirmed that the SMR can measure the dynamic viscosity without the density correction. The obtained results demonstrate that the SMR can measure the fluid viscosity with high precision and even real-time monitoring of the viscosity change is possible with the amplitude-based measurement scheme.