Hung-Chih Kan

Effect of gold nanopillar arrays on the absorption spectrum of a bulk heterojunction organic solar cell

We report on the effect of arrays of Au nanopillars of controlled size and spacing on the spectral response of a P3HT: PCBM bulk heterojunction solar cell. Prototype nanopillar-patterned devices have nearly the same overall power conversion efficiency as those without nanopillars. The patterned devices do show higher external quantum efficiency and calculated absorption in the wavelength range from approximately 640 nm to 720 nm, where the active layer is not very absorbing. The peak enhancement was approximately 60% at 675 nm. We find evidence that the corresponding resonance involves both localized particle plasmon excitation and multiple reflections/diffraction within the cavity formed by the electrodes. We explore the role of the attenuation coefficient of the active layer on the optical absorption of such an organic photovoltaic device.

Spacer Layer Effect in Fluorescence Enhancement from Silver Nanowires over a Silver Film; Switching of Optimum Polarization

We explore the role of coupling between silver nanowires and an underlying silver film in fluorescence enhancement from proximal molecules. Variation of the thickness of an oxide layer separating nanowire arrays from the Ag film causes an alternation in the incident light polarization that produces the highest enhancement. Finite difference time domain calculations show that it results from an alternation of regions of high field above and between nanowires as the spacer thickness is increased.

Doubly resonant surface-enhanced Raman scattering on gold nanorod decorated inverse opal photonic crystals

We present a novel type of surface-enhanced Raman scattering (SERS) substrate constituted of a 3-dimensinal polymeric inverse opal (IO) photonic crystal frame with gold nanorods (Au-NRs) decorating on the top layer. This substrate employs resonant excitation as well as constructive backward scattering of Raman signals to produce large enhancement of SERS output. For the incoming excitation, Au-NRs with appropriate aspect ratio were adopted to align their longitudinal localized surface plasmon band with the excitation laser wavelength. For the outgoing SERS signal, the spectral position of the photonic band gap was tuned to reflect Raman-scattered light constructively. This SERS substrate produces not only strong but also uniform SERS output due to the well control of Au-NRs distribution by the periodic IO structure, readily suitable for sensing applications.

Enhanced fluorescence and near-field intensity for Ag nanowire/nanocolumn arrays: evidence for the role of surface plasmon standing waves

We use scanning fluorescence microscopy and electron beam lithography to probe the mechanism of fluorescence enhancement by periodic arrays of silver nanostructures, determining the optimum size and spacing of both Ag nanowires and Ag nanocolumns for incident light of different wavelengths and polarizations. Finite difference time domain (FDTD) calculations show a systematic variation with spatial period and incident polarization of the local electric field above the surface of the arrays which correlate well with that of the measured fluorescence enhancement, but a lack of a simple proportionality indicates that the dependence of the radiative and nonradiative decay rates on array geometry must be included in models for this effect. The dependence of the enhancement on spatial period and polarization indicates the importance of surface plasmon standing waves in this effect.

Hybrid surface-enhanced Raman scattering substrate from gold nanoparticle and photonic crystal: maneuverability and uniformity of Raman spectra.

A novel hybrid surface-enhanced Raman scattering (SERS) substrate based on Au nanoparticles decorated inverse opal (IO) photonic crystal (PhC) is presented. In addition to the enhancement contributed from Au nanoparticles, a desired Raman signal can be selectively further enhanced by appropriately overlapping the center of photonic bandgap of the IO PhC with the wavelength of the Raman signal. Furthermore, the lattice structure of the IO PhC provides excellent control of the distribution of Au nanoparticles to produce SERS spectra with high uniformity. The new design of SERS substrate provides extra maneuverability for ultra-high sensitivity sensor applications.

Enhancing light extraction efficiency of polymer light-emitting diodes with a 12-fold photonic quasi crystal.

This work demonstrates the enhancement of light extraction of polymer light-emitting diodes (PLEDs) by incorporating a 12-fold photonic quasi crystal (PQC) in the device structure. Multi-exposure two-beam interference technique combined with inductively coupled plasma etching was employed to pattern a 12-fold PQC structure on the ITO film on a glass substrate of the diode. The air-hole coverage (AHC) and etching depth dependences of the light emitting performance of the 12-fold PQC patterned PLEDs were investigated. For AHC within the range between 6.4% and 32.3%, a nearly constant enhancement of the luminance efficiency of the PQC PLEDs was observed. On the other hand, the light emitting performance of the PQC PLEDs is very sensitive to the etching depth. The photoluminescence intensity of the PQC PLEDs increases monotonically with the etching depth. In contrast, the electro luminance efficiency shows a non-monotonic dependence on etching depth with a maximum occurring at 55 nm etching depth. The maximum improvement of luminance efficiency of the 12-fold PQC PLEDs reaches nearly 95% compared with an un-patterned PLED at an injection current of 110 mA.

Optical modulation of guided mode resonance in the waveguide grating structure incorporated with azo-doped-poly(methylmethacrylate) cladding layer

Optical modulation of guided mode resonance (GMR) is demonstrated in a waveguide grating structure (WGS) which contains a disperse-red1 (DR1)-doped poly(methylmethacrylate) (PMMA) cladding layer. The resonance wavelength of a GMR mode can be tuned by pumping the cladding layer with a 442 nm wavelength laser beam, because of photoinduced refractive index change in the layer. The resonance wavelength shifts to shorter wavelength side, and the shift increases with pumping power, up to a maximum shift of 5 nm. A detector was used to monitor the intensity of the light that was reflected from the WGS at the wavelengths of the GMR peak positions, and the WGS was found to exhibit optical modulation with a shortest switching time of less than 0.3s.

Guided-mode resonance enhanced excitation and extraction of two-photon photoluminescence in a resonant waveguide grating

Guided-mode resonances enhanced excitation and extraction of two-photon photoluminescence (TPP) is demonstrated with a one-dimensional resonant waveguide grating (RWG) with a layer of fluorescent polymer (polyfluorene, PFO) on top. In this work, we design and fabricate a PFO RWG, in which two dispersive resonant modes in TE-polarization were measured. By aligning the red-shifting resonant mode with excitation wavelength in the infrared range, and the blue-shifting resonant mode with TPP spectrum in the visible range, the intensity of TPP can be enhanced up to 300-fold compared with that from a flat film with the same thickness coated on a glass slide. Such high enhancement results from firstly the strong evanescent local field in the waveguide layer due to the resonance between the incident light and the waveguide structure according to the results of rigorous coupled-wave analysis calculation, and secondly the enhanced extraction of the emission light which also resonates with the waveguide structure.

Strong guided mode resonant local field enhanced visible harmonic generation in an azo-polymer resonant waveguide grating

Guided mode resonance (GMR) enhanced second- and third-harmonic generation (SHG and THG) is demonstrated in an azo-polymer resonant waveguide grating (RWG), comprised of a poled azo-polymer layer on top of a textured SU8 substrate with a thin intervening layer of TiO2. Strong SHG and THG outputs are observed by matching either in-coming fundamental- or out-going harmonic-wavelength to the GMR wavelengths of the azo-polymer RWG. Without the azo-polymer coating, pure TiO2 RWGs, do not generate any detectable SHG using a fundamental beam peak intensity of 2 MW/cm(2). Without the textured TiO2 layer, a planar poled azo-polymer layer results in 3650 times less SHG than the full nonlinear RWG structure under identical excitation conditions. Rigorous coupled-wave analysis calculations confirm that this enhancement of the nonlinear conversion is due to strong local electric fields that are generated at the interfaces of the TiO2 and azo-polymer layers when the RWG is excited at resonant wavelengths associated with both SHG and THG conversion processes.

Direct and quantitative evidence for buckling instability as a mechanism for roughening of polymer during plasma etching

We investigate elastic buckling as a driving force for roughening of polystyrene, a model resist, during plasma etching. Force curve measurements of the effective modulus of etched polystyrene films, along with modeling which accounts for adhesive forces and surface corrugation, show that an extremely stiff modified layer results from Ar-ion etching, with the modulus increasing with incident energy. The results are in good quantitative agreement with analysis based on the observed dominant corrugation wavelength, and buckling theory providing the corrugation is taken into account.