Taejoon Kouh

Diffraction effects in optical interferometric displacement detection in nanoelectromechanical systems

Optical interferometric displacement detection techniques have recently found use in the study of nanoelectromechanical systems (NEMS). Here, we study the effectiveness of these techniques as the relevant NEMS dimensions are reduced beyond the optical wavelength used. We first demonstrate that optical cavities formed in the sacrificial gaps of subwavelength NEMS enable enhanced displacement detection sensitivity. In a second set of measurements, we show that the displacement sensitivity of conventional path-stabilized Michelson interferometry degrades rapidly beyond the diffraction limit. Both experiments are consistent with numerical models.

Optical knife-edge technique for nanomechanical displacement detection

We describe an optical knife-edge technique for nanomechanical displacement detection. Here, one carefully focuses a laser spot on a moving edge and monitors the reflected power as the edge is displaced sideways. To demonstrate nanomechanical displacement detection using the knife-edge technique, we have measured in-plane resonances of nanometer scale doubly clamped beams. The obtained displacement sensitivity is in the ∼1pm∕Hz range—in close agreement with a simple analytical model.

Analysis of optical interferometric displacement detection in nanoelectromechanical systems

Optical interferometry has found recent use in the detection of nanometer scale displacements of nanoelectromechanical systems (NEMS). At the reduced length scale of NEMS, these measurements are strongly affected by the diffraction of light. Here, we present a rigorous numerical model of optical interferometric displacement detection in NEMS. Our model combines finite element methods with Fourier optics to determine the electromagnetic field in the near-field region of the NEMS and to propagate this field to a detector in the far field. The noise analysis based upon this model allows us to elucidate the displacement sensitivity limits of optical interferometry as a function of device dimensions as well as important optical parameters. Our results may provide benefits for the design of next generation, improved optical NEMS.

Magnetic Properties and Crystalline Transition for the NiCr 1.7 Fe 0.3 O

We have studied the temperature dependent magnetic properties and crystalline phase transitionn in small amount Fe doped nickel chromite. The Crystalline structure of NiCr 1.7 Fe 0.3 O₄ is spinel cubic (Fd-3m) structure with a lattice constant ^a0 = 8.317 A at room temperature. The magnetic Neel temperature (T N ) of the Fe doped nickel chromite sample is determined to be 250 K. The Mossbauer spectra exhibit that there are two magnetic phases with the two different sites for the Cr 3+ ions. The spectrum at 4.2 K is fitted to two magnetic components of the magnetic hyperfine fields Hhf = 496 and 485 kOe. From the spectrum at 295 K, the electric quadrupole splittings are observed with large values of 0.49 and 0.50 ㎜/s, respectively. The values of the isomer shifts at all temperature ranges show that the Fe ions are ferric states. We are suggested that the dynamic Jahn-Teller distortion and anisotropic magnetic relaxation effects due to the crystalline phase transition.

Room-temperature operation of a nanoelectromechanical resonator embedded in a phase-locked loop

We describe the operation of a phase-locked loop (PLL) that tracks the high-frequency electromechanical resonance of a nanoscale beam resonator. The fundamental in-plane flexural resonance of the beam resonator embedded in the PLL is actuated electrostatically and detected optically. PLL operation is demonstrated by locking stably to the resonance frequency of the beam, and by tracking this resonance with high fidelity as the beam is mass loaded. Our analysis reproduces the observed locking behavior. Feedback control schemes for nanoelectromechanical resonators may offer prospects for miniature timekeeping devices and ultrasensitive sensors.

Highly efficient full-color display based on blue LED backlight and electrochromic light-valve coupled with front-emitting phosphors.

We report a novel full-color display based on the generation of full-color by blue light approach, so called color-by-blue display. This newly proposed color-by-blue light-valve display combines a blue backlight excitation source, a blue light-valve shutter, and front-emitting phosphor pixels. Careful evaluation shows that the detailed display characteristics as well as excellent cycling durability under a low operation voltage of 3 V easily satisfy the requirements for the current display application. Also, we would like to emphasize that the proposed method shows a conversion efficiency of 20%, surpassing the value (≈5%) seen in the typical liquid crystal displays. Although the switching response reported here is slower than in a commercial display module due to the solution-phase electrochromic nature of the shutter used, a response time close to that of a liquid crystal display is highly feasible, as we suggest.

Subgap Density of States in Superconductor-Normal Metal Bilayers in the Cooper Limit

We present transport and tunneling measurements of Pb-Ag bilayers with thicknesses, d(Pb) and d(Ag), that are much less than the superconducting coherence length. The transition temperature, T(c), and energy gap, Delta, in the tunneling density of states (DOS) decrease exponentially with d(Ag) at fixed d(Pb). Simultaneously, a DOS that increases linearly from the Fermi energy grows and introduces states within the gap. The integrated subgap DOS approaches 40% of the normal state value in the lowest T(c) film investigated (T(c) approximately 0.1 T(Pb)(c,bulk)). This behavior suggests that a growing fraction of quasiparticles decouple from the superconductor as T(c)-->0. The linear dependence is consistent with the quasiparticles becoming trapped on integrable trajectories in the metal layer.

Photothermal Effect and Heat Dissipation in a Micromechanical Resonator

We describe the photothermal effect in an aluminium–silicon nitride doubly-clamped beam with an optical deflection scheme. Incident optical power results in the temperature rise in the composite beam and the shift in the resonance frequency due to thermal stress. The observed dynamic response is consistent with the detailed beam equation as well as the thermal conduction model. The pressure-dependent dynamics of the beam allows the investigation of convective heat dissipation due to the surrounding gas molecules as well as determination of heat transfer coefficient. The photothermally coupled operation presented here opens up the prospects for miniaturized pressure-sensing elements.

Mössbauer analysis of silicate Li2FeSiO4 and delithiated Li2−xFeSiO4 (x = 0.66) compounds

Lithium iron silicate compounds of Li2FeSiO4 and partially delithiated Li2−xFeSiO4 (x = 0.66) were synthesized by vacuum-sealed solid-state and chemical delithiation reactions, and their magnetic properties were characterized based on Mossbauer analysis. Crystal structures of both Li2FeSiO4 and Li2−xFeSiO4 (x = 0.66) compounds are found to be γs-type (P21/n) monoclinic structures with difference in the lattice parameters due to lithium delithiation. Mossbauer spectrum of Li2FeSiO4 below TN1 = 20 K exhibits eight Lorentzians of Fe2+ with antiferromagnetic ordering. However, the spectrum of intermediate Li2−xFeSiO4 (x = 0.66) compound shows the appearance of magnetically ordered Fe3+ sextet below TN2 = 28 K. The temperature-dependent isomer shift of Li2−xFeSiO4 indicates the coexistence of nonequivalent Fe2+/Fe3+ valence states with the partial oxidation of FeO4, enhanced by the lithium ion deficiency. Also, we have observed a considerable change in electric quadrupole interaction between Fe2+/Fe3+ ions in ...

Pressure-sensing based on photothermally coupled operation of micromechanical beam resonator

Here, we demonstrate the pressure-sensing scheme based on the photothermal effect in the miniaturized beam resonator in the moderate pressure range. Since the resonance frequency of the small beam resonator under thermal stress can be easily modulated by the convective cooling of the gas molecules, the pressure change has been monitored by tracking the frequency shift under constant optical power. Our experimental measurements as well as the analytical model show that the described technique ensures the fast response to the external pressure variation with high responsivity as well as much sought-after scalability, desirable for many technological applications.