Hsiang Chen Chui

Tapered optical fiber sensor based on localized surface plasmon resonance.

A tapered fiber localized surface plasmon resonance (LSPR) sensor is demonstrated for refractive index sensing and label-free biochemical detection. The sensing strategy relies on the interrogation of the transmission intensity change due to the evanescent field absorption of immobilized gold nanoparticles on the tapered fiber surface. The refractive index resolution based on the interrogation of transmission intensity change is calculated to be 3.2×10⁻⁵ RIU. The feasibility of DNP-functionalized tapered fiber LSPR sensor in monitoring anti-DNP antibody with different concentrations spiked in buffer is examined. Results suggest that the compact sensor can perform qualitative and quantitative biochemical detection in real-time and thus has potential to be used in biomolecular sensing applications.

Direct near-field optical imaging of plasmonic resonances in metal nanoparticle pairs.

In this paper we investigate the near-field optical behavior of plasmon coupling in gold nanoparticle pairs. In particular, by performing series measurements through a fiber-collection mode near-field scanning optical microscope (NSOM), we directly observed the localized electromagnetic (EM) field distribution between two nanospheres is sensitively depended on the incident polarization and interparticle distance. The qualitative near-field observation and quantitative analysis facilitate more understanding of localized hot spots in surface-enhanced Raman scattering (SERS), and nano-applications in selectively controlling the spatial distribution of localized surface plasmon (SP) modes on a fabricated nanostructure by adjusting the polarization direction.

Second harmonic generation imaging - a new method for unraveling molecular information of starch.

We present a new method, second harmonic generation (SHG) imaging for the study of starch structure. SHG imaging can provide the structural organization and molecular orientation information of bio-tissues without centrosymmetry. In recent years, SHG has proven its capability in the study of crystallized bio-molecules such as collagen and myosin. Starch, the most important food source and a promising future energy candidate, has, for a decade, been shown to exhibit strong SHG response. By comparing SHG intensity from different starch species, we first identified that the SHG-active molecule is amylopectin, which accounts for the crystallinity in starch granules. With the aid of SHG polarization anisotropy, we extracted the complete χ((2)) tensor of amylopectin, which reflects the underlying molecular details. Through χ((2)) tensor analysis, three-dimensional orientation and packing symmetry of amylopectin are determined. The helical angle of the double-helix in amylopectin is also deduced from the tensor, and the value corresponds well to previous X-ray studies, further verifying amylopectin as SHG source. It is noteworthy that the nm-sized structure of amylopectin inside a starch granule can be determined by this far-field optical method with 1-μm excitation wavelength. Since SHG is a relatively new tool for plant research, a detailed understanding of SHG in starch structure will be useful for future high-resolution imaging and quantitative analyses for food/energy applications.

Electrochemically fabricated self-aligned 2-D silver/alumina arrays as reliable SERS sensors.

A novel SERS sensor for adenine molecules is fabricated electrochemically using an ordered two-dimensional array of self-aligned silver nanoparticles encapsulated by alumina. Silver is electro-deposited on the interior surfaces at the bottom of nano-channels in a porous anodic aluminum oxide (AAO) film. After etching aluminum, the back-end alumina serves as a SERS substrate. SERS enhancement factor greater than 10(6) is measured by 532 nm illumination. It exhibits robust chemical stability and emits reproducible Raman signals from repetitive uses for eight weeks. The inexpensive mass production process makes this reliable, durable and sensitive plasmon based optical device promising for many applications.

Plasmon-induced optical switching of electrical conductivity in porous anodic aluminum oxide films encapsulated with silver nanoparticle arrays

We report on plasmon induced optical switching of electrical conductivity in two-dimensional (2D) arrays of silver (Ag) nanoparticles encapsulated inside nanochannels of porous anodic aluminum oxide (AAO) films. The reversible switching of photoconductivity greatly enhanced by an array of closely spaced Ag nanoparticles which are isolated from each other and from the ambient by thin aluminum oxide barrier layers are attributed to the improved electron transport due to the localized surface plasmon resonance and coupling among Ag nanoparticles. The photoconductivity is proportional to the power, and strongly dependent on the wavelength of light illumination. With Ag nanoparticles being isolated from the ambient environments by a thin layer of aluminum oxide barrier layer of controlled thickness in nanometers to tens of nanometers, deterioration of silver nanoparticles caused by environments is minimized. The electrochemically fabricated nanostructured Ag/AAO is inexpensive and promising for applications to integrated plasmonic circuits and sensors.

Broadband tunable optical parametric amplification from a single 50 MHz ultrafast fiber laser

We have demonstrated a 0.7 microm - 1.9 microm wavelength-tunable light source based on a single-pass optical parametric amplification (OPA) in a multiperiod magnesium oxide-doped periodically poled lithium niobate crystal. The OPA pump was a frequency-doubled ultrafast ytterbium-doped fiber oscillator, and the residual 1040 nm laser power after frequency doubling was recycled to generate a supercontinuum seeding source. Compared with conventional OPAs, this system is free from timing jitter between the pump laser and the seeding source. Over 50% conversion efficiency was obtained with 10 nJ pump energy. Combined with a 50 MHz repetition rate, this versatile source is ideal for biomedical and spectroscopic applications.

The phase-response effect of size-dependent optical enhancement in a single nanoparticle

We demonstrate detailed simulations and experiments of near-field phase-response in a single silver nanoparticle. The plasmon-photon interaction is directly observed in the vicinity of silver nanoparticles through a near-field scanning optical microscope (NSOM). Our results manifest the correlation of phase-response and size-dependent optical enhancement. Detailed interference behaviors between optical excitation and plasmon mediated re-radiation are revealed on a single particle basis. This observation facilitates nano-applications in controlling the spatial distribution of surface plasmon (SP) modes by means of nanostructures.

Revealing anisotropic strain in exfoliated graphene by polarized Raman spectroscopy

We report on a polarized Raman study on mechanically cleaved single-layer graphene films. Under a specific orientation of scattering measurement, the width and position of the G peak change with the incident polarization direction, while the integrated intensity of that is unaltered. This phenomenon is explained by a proposed mode in which the peak is contributed by a mixture of un-, compressive-, and tensile-strained G sub-modes. The compression and tension are both uniaxial and approximately perpendicular to each other. They are undesigned and located in either separated or overlapped sub-areas within the probed local region. Compared to the unstrained wavenumber of 1580 cm(-1), compression induces a blue shift while tension causes a red one. The sub-modes correlated with the light polarization through different relationships split the G peak into three sub-ones. We develop a method to quantitatively analyze the positions, widths, intensities, and polarization dependences of sub-peaks. This analysis quantitatively reveals local strain, which changes with the detected area of a graphene film. The method presented here can be extended to detect the strain distribution in the film and thus is a promising technology for graphene characterization.

Layer-dependent morphologies of silver on n-layer graphene.

The distributions of sizes of silver nanoparticles that were deposited on monolayer, bilayer, and trilayer graphene films were observed. Deposition was carried out by thermal evaporation and the graphene films, placed on SiO2/Si substrates, were obtained by the mechanical splitting of graphite. Before the deposition, optical microscopy and Raman spectroscopy were utilized to identify the number of the graphene layers. After the deposition, scanning electron microscopy was used to observe the morphologies of the particles. Systematic analysis revealed that the average sizes of the nanoparticles increased with the number of graphene layers. The density of nanoparticles decreased as the number of graphene layers increased, revealing a large variation in the surface diffusion strength of nanoparticles on the different substrates. The mechanisms of formation of these layer-dependent morphologies of silver on n-layer graphene are related to the surface free energy and surface diffusion of the n-layer graphene. The effect of the substrate such as SiO2/Si was investigated by fabricating suspended graphene, and the size and density were similar to those of supported graphene. Based on a comparison of the results, the different morphologies of the silver nanoparticles on different graphene layers were theorized to be caused only by the variation of the diffusion barriers with the number of layers of graphene.

Interferometric Interrogation of the Inclination and Displacement of Tapered Fiber Mach-Zehnder Interferometers

All-fiber inclinometers and displacement sensors using tunable Mach-Zehnder interferometers with an extinction ratio of are produced by introducing two abrupt tapers in a short length of highly Er/Yb co-doped fiber. The abrupt tapers partially convert the core mode into cladding modes and interference occurs when the phase difference between the core and cladding modes accumulate to π after a certain propagation length. The maximal wavelength shift for the resonant transmission dips can reach 40.8 nm (from 1648 to 1607.2 nm) when the inclination angle, θ, varies from 0 ° to 32 °. The wavelength shift can reach 26 nm (from 1404.4 to 1430.4 nm) when the compressive strain ε is 0.004.