Controllable Patterning of Metallic Photonic Crystals for Waveguide-Plasmon Interaction.

Waveguide-plasmon polaritons sustained in metallic photonic crystal slabs show fascinating properties, such as narrow bandwidth and ultrafast dynamics crucial for biosensing, light emitting, and ultrafast switching. However, the patterning of metallic photonic crystals using electron beam lithography is challenging in terms of high efficiency, large area coverage, and cost control. This paper describes a controllable patterning technique for the fabrication of an Ag grating structure on an indium-tin oxide (ITO) slab that enables strong photon-plasmon interaction to obtain waveguide-plasmon polaritons. The Ag grating consisting of self-assembled silver nanoparticles (NPs) exhibits polarization-independent properties for the excitation of the hybrid waveguide-plasmon mode. The Ag NP grating can also be annealed at high temperature to form a continuous nanoline grating that supports the hybrid waveguide-plasmon mode only under transverse magnetic (TM) polarization. We tuned the morphology and the periodicity of the Ag grating through the concentration of silver salt and the photoresist template, respectively, to manipulate the strong coupling between the plasmon and the waveguide modes of different orders.

Switchable Plasmonic Chirality for Light Modulation: From Near-Field to Far-Field Coupling.

This paper describes a quasi-planar chiral metamaterial of metal-insulator-metal (MIM) tetramer arrays that support multiplasmon modes from a hybridization scheme to achieve significant chiroptical responses with the largest circular dichroism (CD) value of 42%. The chiroptical responses can be actively switched on and off by tuning the field coupling regime from near field to far field through the insulator (or spacer) thickness. Numerical calculations demonstrate that near-field coupling of the hybridized plasmons on the stacked metallic tetramers governs the chiroptical responses at small insulator thickness (tSiO2 < 160 nm). In contrast, far-field coupling of the plasmon radiations dominates at large spacing (tSiO2 > 160 nm) as phase retardation plays a crucial role. The quasi-planar chiral metamaterial with tunable plasmonic chirality enables efficient light modulation for polarization conversion: from circular to elliptical/linear polarization.

Ultrafast particle-plasmon enhancement by energy-band modification in nanostructured tungsten carbide.

Ultrafast optical excitation induced transient modification on the energy-band structures in tungsten, which resulted in the expansion and shift toward the Fermi-level of d-band. This process led to enhanced interband transitions at reduced photon energies. Meanwhile, enhanced interband excitation led to increased electron density above the Fermi level, resulting in enhanced optical scattering by localized surface plasmon resonance (LSPR). These mechanisms are responsible for balancing the direct heating of bulk electrons by optical pulses. The corresponding studies not only revealed the physics for the electronic dynamics in tungsten carbide, but also proposed that the modified electronic and electron-phononic interactions are one of the important responsible mechanisms for the enhanced laser-damage threshold of the hard-metal coating. Furthermore, the nanostructured hard-metal coating integrates functions of enhancement of the damage-threshold and anti-reflection coating, which is important for exploring new tools or materials in laser engineering.

Plasmonic microcavity using photo-reduced silver nanoparticles and light-emitting polymer.

We report a plasmonic Fabry-Perot (F-P) microcavity with silver nanoparticles and a continuous silver film function as the end mirrors, where the silver nanoparticles were produced through photo-reduction. Filled with a layer of light-emitting polymer, the F-P microcavity becomes an active device with its output spectrum dependent on localized surface plasmon resonance (LSPR) of silver nanoparticles. A phase shift as large as π is resolved in the resonance modes of the F-P microcavity due to the modulation by LSPR. This leads to complementary spectroscopic performance in the output of the plasmonic microcavity with respect to its non-plasmonic counterparts.

A cross-stacked plasmonic nanowire network for high-contrast femtosecond optical switching.

We report an ultrafast optical switching device constructed by stacking two layers of gold nanowires into a perpendicularly crossed network, which works at a speed faster than 280 fs with an on/off modulation depth of about 22.4%. The two stacks play different roles in enhancing consistently the optical switching performance due to their different dependence on the polarization of optical electric fields. The cross-plasmon resonance based on the interaction between the perpendicularly stacked gold nanowires and its Fano-coupling with Rayleigh anomaly is the dominant mechanism for such a high-contrast optical switching device.

Polarization control in flexible interference lithography for nano-patterning of different photonic structures with optimized contrast.

Half-wave plates were introduced into an interference-lithography scheme consisting of three fibers that were arranged into a rectangular triangle. Such a flexible and compact geometry allows convenient tuning of the polarizations of both the UV laser source and each branch arm. This not only enables optimization of the contrast of the produced photonic structures with expected square lattices, but also multiplies the nano-patterning functions of a fixed design of fiber-based interference lithography. The patterns of the photonic structures can be thus tuned simply by rotating a half-wave plate.

Fiber-based flexible interference lithography for photonic nanopatterning.

Flexible interference lithography using fibers as laser beam splitting and delivering system is demonstrated. A laser beam at 325 nm is used as the ultraviolet light source. Fiber bundles consisting of two, three, or four optical fibers have been utilized for the fabrication of two-dimensional photonic structures with different lattice configurations and different periods. The effective area of the fabricated waveguide grating structures is in the scale of centimeters in diameter with excellent homogeneity, which has much space for further improvement. This flexible interference lithography technique enables simple, compact, and efficient fabrication of photonic structures.

Nanoscale heat transfer in direct nanopatterning into gold films by a nanosecond laser pulse.

We investigate nanoscale heat transfer and heat-flux overlapping effects in nanopatterning through interactions between interferogram produced by 5-ns laser pulses at 355 nm and gold films. These mechanisms played different roles in direct writing of gold nanolines with different periods. Continuous gold nanolines were produced for large periods, where heat-flux overlapping is too small to effect the laser-metal interactions. Thus, the heat-transfer distance and direct laser-ablation determined the width of resultant gold nanolines. However, gold nanolines consisting of isolated gold nanoparticles were produced for small periods, where the overlapped heat-flux exceeds the threshold for removing or melting gold films.

Nanoscale tensile stress approach for the direct writing of plasmonic nanostructures.

One- and two-dimensional plasmonic nanostructures can be fabricated using nanoscale tensile stress. A polymer layer, coated with a thin metal film, is exposed to an interference pattern produced by ultraviolet laser beams. Crosslinking is induced between the polymeric molecules located within the bright fringes. This process not only increases the refractive index but also reduces the polymer layer thickness. Corrugations occur on the continuous thin metal film due to the nanoscale stress in the polymer layer. Thus, a periodic nanostructure of area 3 × 3 mm and depth 50 nm is created both in the polymer and metal films with excellent homogeneity and reproducibility. This method enables direct writing of a large-area plasmonic nanostructure at low cost which can be used in the design of optoelectronic devices and sensors.

Compact bandwidth-tunable polarization filter based on a plasmonic heterograting.

A plasmonic heterograting device, consisting of two juxtaposed parallel gratings with different periods, is demonstrated to function as a compact bandwidth-tunable polarization filter. The essential aspect of the structure is that the grating couples into a photonic mode of the substrate. Using this device, a linearly polarized spectrum can be conveniently and selectively picked out from nonpolarized white light. The bandwidth depends on the incident angle and the overlap of the first-order diffraction spectra of the two different grating, and can be freely narrowed. The tuning characteristics of the heterograting are investigated both theoretically and experimentally. The unique physical features potentially enable the development of new polarization elements and optical devices.

Solution-processable complex plasmonic quasicrystals.

Large-area plasmonic photonic structures containing a proportion of quasicrystals can be fabricated by a solution-processable method. A photoresist film is exposed to a multi-beam interference pattern to form a quasicrystal template. A gold nanoparticle colloid is then spin-coated onto the template. An inverse pattern can be obtained after annealing to afford greater control over the sample morphologies and spectroscopic characteristics. Coupling between the waveguide modes and particle plasmons strengthens with increasing annealing temperature. After mode degeneration is removed, a multi-mode coupling process is observed. These results are helpful in understanding the mechanisms and design strategies of complex plasmonic nanostructures.