Tianrui Zhai

Random Laser Based on Waveguided Plasmonic Gain Channels

A waveguide-plasmonic scheme is constructed by coating the matrix of randomly distributed gold nanoisland structures with a layer of dye-doped polymer, which provides strong feedback or gain channels for the emission from the dye molecules and enables successful running of a random laser. Excellent overlap of the plasmonic resonance of the gold nanoislands with the photoluminescence spectrum of the dye molecules and the strong confinement mechanism provided by the active waveguide layer are the key essentials for the narrow-band and low-threshold operation of this random laser. This kind of feedback configuration potentially enables directional output from such random lasers. The flexible solution-processable fabrication of the plasmonic gold nanostructures not only enables easy realization of such a random laser but also provides mechanisms for the tuning and multicolor operation of the laser emission.

Sensors Based on Plasmonic-Photonic Coupling in Metallic Photonic Crystals

An optical sensor based on the coupling between the plasmonic and photonic resonance modes in metallic photonic crystals is investigated. Large-area metallic photonic crystals consisting of periodically arranged gold nanostructures with dimensions down to sub-100 nm are fabricated using solution-processible gold nanoparticles in combination with interference lithography or interference ablation, which introduces a variety of fabrication techniques for the construction of this kind of sensor device. Sensitivity of the plasmonic response of the gold nanostructures to the changes in the environmental refractive index is enhanced through the coupling between the narrow-band photonic resonance mode and the relatively broad-band plasmon resonance, which is recognized as a Fano-like effect and is utilized to explore sensors. Theoretical modeling shows the characterization and the optimization of the sensitivity of this kind of sensor device. Theoretical and experimental results are demonstrated for the approaches to improve the sensitivity of the sensor device.

Polarization-dependent SERS effects of laser-generated sub-100 nm antenna structures

Sub-100 nm antenna arrays consisting of a star-like ridge or dome-like structures with needles in their centers are prepared in thin gold films on glass substrates using femtosecond laser pulses. The needles can be bent mechanically to be horizontally aligned to the substrate surface. Controlled variation of the pulse energy allows one to obtain nanostructures of different defined morphologies. These arrays of nanostructures are covered with a thin homogeneous layer of rhodamine molecules. Raman spectra using linearly polarized laser light of 632.8 nm are taken with the laser spot centered on individual nanostructures and at positions on the unstructured film. The average Raman enhancement within the laser spot focused onto a nanostructure is two orders of magnitude higher than on the unstructured film. The nanostructures with bent needles exhibit a polarization dependence of the SERS effect, i.e., typically the enhancement is larger by about a factor of two for excitation light polarized parallel to the needle direction than for the perpendicular case. The enhancement factor of the star-like ridge structures with needles is analyzed by the finite-element method, which agrees with the experiment. We show that the variation of the SERS activity of almost similar structures arises from the inherent randomness of the hot spots created in the fabrication process. Nevertheless, these antenna structures may be useful as elements in novel SERS devices as they can be accurately positioned on a device using a cheap fabrication process compatible with microfabrication technology.

Plasmonic nano-ring arrays through patterning gold nanoparticles into interferograms

Large-area gold nanoring arrays were fabricated using interference lithography and metallic transformation through annealing of colloidal gold nanoparticles. The strong surface tension of the suspension solution and the molten gold, as well as the effective distance of these interaction mechanisms, is responsible for the creation of gold nanorings. The size and shape of the gold nanorings can be controlled by adjusting the size of the holes in the template photoresist grating, which is accomplished in the stage of interference lithography. Furthermore, the concentration of the colloidal gold nanoparticles and the annealing temperature can be utilized to achieve further optimization of the gold nanoring structures. Optical spectroscopic measurements show unique plasmonic response of the nanoring arrays in the visible and in the infrared spectral ranges, which agrees well with the theoretical simulation. This fabrication method provides a simple and low-cost route for achieving metallic nanoring arrays in a large scale for practical applications.

Ultra-thin plasmonic random lasers

An ultrathin plasmonic random laser is fabricated by a simple lift off process, which consists of a free-standing polymer membrane embedded with silver nanoparticles. Low threshold random lasing is observed when the 200-nm-thick membrane device is optically pumped, due to the strong plasmonic feedback and high-quality waveguide confinement provided by the silver nanoparticles and the polymer membrane, respectively. The free-standing polymer membrane is very flexible and transplantable, which can be attached to an optical fiber end face to achieve random lasing. This fabrication technique provides a promising way to realize plasmonic random lasing on surfaces with arbitrary shapes.

Plasmonic random laser on the fiber facet

A random laser on the optical fiber facet is constructed by dipping an optical fiber end face into the solution of polydimethylsiloxane doped with rhodamine 6G organic dye and silver nanowires. The PDMS film doped with rhodamine 6G acts as the active waveguide layer, and the silver nanowires provide a three-dimensional plasmonic feedback. The plasmon resonance of silver nanowires significantly improves the pump efficiency of the random laser. The most output energy of random laser concentrates in a small angle range along the axis of the optical fiber. This fabrication technique provides a simple and efficient way for the fabrication of random lasers on the optical fiber facet with low cost.

Plasmonic random lasing in polymer fiber.

A random fiber laser is achieved based on the plasmonic feedback mechanism, which is constructed by first siphoning the polymer solution doped with silver nanoparticles into a 300-μm capillary tube and then evaporating the solvent. Strong amplification of the radiation can be obtained by employing the variable gain region, the fiber waveguide scheme and three-dimensional plasmonic feedback provided by the silver nanoparticles. Low-threshold directional random lasing is observed in the polymer fiber. This simple and straightforward approach facilitates the investigation of plasmonic random fiber lasers.

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.

Red-green-blue plasmonic random laser

A red-green-blue plasmonic random laser is achieved in a multilayer structure, which is fabricated by spin-coating three polymer solutions successively on a silica substrate. Under optical pumping, strong amplification of the polymer radiation can be observed due to the localized surface plasmon resonance of silver nanoparticles embedded in the multilayer structure. Red-green-blue random lasing is simultaneously obtained from the sample based on the enhanced scattering strength of silver nanoparticles. These results are useful for designing compact integrated random laser sources.

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.