Michael James Ventura

Nanoplasmonics: a frontier of photovoltaic solar cells

Abstract Nanoplasmonics recently has emerged as a new frontier of photovoltaic research. Noble metal nanostructures that can concentrate and guide light have demonstrated great capability for dramatically improving the energy conversion efficiency of both laboratory and industrial solar cells, providing an innovative pathway potentially transforming the solar industry. However, to make the nanoplasmonic technology fully appreciated by the solar industry, key challenges need to be addressed; including the detrimental absorption of metals, broadband light trapping mechanisms, cost of plasmonic nanomaterials, simple and inexpensive fabrication and integration methods of the plasmonic nanostructures, which are scalable for full size manufacture. This article reviews the recent progress of plasmonic solar cells including the fundamental mechanisms, material fabrication, theoretical modelling and emerging directions with a distinct emphasis on solutions tackling the above-mentioned challenges for industrial relevant applications.

Void channel microstructures in resin solids as an efficient way to infrared photonic crystals

Microvoid channels were generated by local melting in a solidified polymer resin sample moving perpendicular to the focus of a high numerical-aperture objective under visible femtosecond-pulsed illumination. Channel size, surface quality, and high density channel vicinity depended on laser intensity and scanning speed. Electron microscope images revealed elliptical channel cross sections of 0.7–1.3 μm in lateral diameter and an elongation in the focusing direction of approximately 50%. A 20 layer woodpile-type photonic crystal structure with a 1.7 μm layer spacing and a 1.8 μm in-plane channel spacing provided a sharp peak in reflection and a suppression of infrared transmission in the stacking direction by 85% at wavelength 4.8 μm with a gap/midgap ratio of 0.11.

Hybrid high-resolution three-dimensional nanofabrication for metamaterials and nanoplasmonics.

We propose a novel hybrid fabrication approach that combines direct laser writing with a subsequent electron-beam lithography step. This approach allows realizing out-of-plane plasmonic nanostructures with truly nanoscopic feature sizes. The excellent quality of the obtained structures is evidenced by optical characterization of upright-standing split-ring resonator arrays fabricated along these lines.

Use of ultrafast-laser-driven microexplosion for fabricating three-dimensional void-based diamond-lattice photonic crystals in a solid polymer material

Micro-sized void spheres are successfully generated in a solid polymer by use of a tightly focused femtosecond laser beam from a high-repetition-rate laser oscillator. Confocal reflection images show that the void spheres are longitudinal rotational symmetric ellipsoids with a ratio of long to short axes of approximately 1.5. Layers of void spheres are then stacked to create three-dimensional diamond-lattice photonic crystals. Three gaps are observed in the [100] direction with a suppression rate of the second gap of up to approximately 75% for a 32-layer structure. The observed first- and second-order gaps shift to longer and shorter wavelengths, respectively, as the angle of incidence increases.

Observation of multiple higher-order stopgaps from three-dimensional chalcogenide glass photonic crystals

For the first time to our knowledge the observation of near-IR multiple higher-order stopgaps in three-dimensional photonic crystals (PhCs) fabricated using the direct-laser-writing method in thick chalcogenide glass films is reported. The fabrication and etching conditions necessary to realize well-defined structures are presented. The fabricated PhCs exhibit higher-order stopgaps, which are only evident in high-quality structures. The higher-order stopgaps observed permit these high-refractive-index and high-nonlinear PhCs to be used directly as functional photonic devices operating at telecommunication wavelengths without further miniaturizing structural dimensions.

Fabrication and characterization of face-centered-cubic void dots photonic crystals in a solid polymer material

Spherical void dots with a diameter of 1.2–1.8 μm have been generated in a solid polymer material by use of the ultrafast laser driven micro-explosion method. Micron-sized structures with a face-centered cubic lattice stacked in the [100] and [111] directions have been fabricated. Confocal microscopic images show the high uniformity of the fabricated structures. Photonic stopgaps with a suppression rate of approximately 70% as well as the second-order stopgaps have been observed in both directions. It is shown that the dependence of the stopgaps on the illumination angle in the [100] direction is significantly different from that in the [111] direction.

In-plane and out-of-plane band-gap properties of a two-dimensional triangular polymer-based void channel photonic crystal

The in-plane and out-of-plane band-gap properties of two-dimensional triangular void channel photonic crystals fabricated by femtosecond laser drilling in a solid polymer material were characterized for transverse electric (TE) and transverse magnetic (TM) polarization illumination. For a 24 layer structure stacked in the Γ–M direction, the fundamental stop gap resulted in the suppression of infrared transmission of as much as 96% for TE- and 85% for TM-polarized incident light. The midgap wavelength for the TM polarization was longer by 2.5% than that for the TE polarization. Increasing the angle of incidence for both the in-plane and out-of-plane cases shifted the stop gap to short wavelengths for both TE and TM polarizations. The experimental results allowed for the estimation of the cross section of void channels and the effective refractive index of the polymer after the fabrication.

Direct laser writing of three-dimensional photonic crystal lattices within a PbS quantum-dot-doped polymer material

We report on the synthesis of an optically homogenous PbS quantum-dot-doped polymer material of thickness up to 100 micrometers. It is shown that high quality micro-void channels of submicrometer diameters can be directly fabricated into this nanocomposite by using an ultrafast femtosecond laser beam. Periodically stacked channels in the form of a three-dimensional photonic crystal woodpile lattices reveals a main stop gaps as well as higher-order gaps that overlaps the near-infrared emission wavelength range of PbS quantum dots. These partial stop gaps are well defined in an angular range from zero to 15 degrees in the stacking direction.

Planar cavity modes in void channel polymer photonic crystals

Planar dielectric microcavities embedded in woodpile void channel photonic crystals with stop bands in the stacking direction ranging from 4.3 to 4.8 microm in wavelength were generated by femtosecond-laser direct writing in a solid polymer. Infrared transmission spectra revealed fundamental and second-order modes crossing the stop gap region with a free spectral range of 430 nm on varying the microcavity size from 0.3 to 2.25 microm. Supercell calculations confirmed the cavity size dependence of highly localized cavity modes, whereas the angle of incidence was accounted for using a simple Fabry-Perot model.

Photonic bandgap properties of void-based body-centered-cubic photonic crystals in polymer

We report on the fabrication and characterization of void-based body-centered-cubic (bcc) photonic crystals in a solidified transparent polymer by the use of a femtosecond laser-driven microexplosion method. The change in the refractive index in the region surrounding the void dots that form the bcc structures is verified by presenting confocal microscope images, and the bandgap properties are characterized by using a Fourier transform infrared spectrometer. The effect of the angle of incidence on the photonic bandgaps is also studied. We observe multiple stop gaps with a suppression rate of the main gap of 47% for a bcc structure with a lattice constant of 2.77 microm, where the first and second stop gaps are located at 3.7 microm and 2.2 microm, respectively. We also present a theoretical approach to characterize the refractive index of the material for calculating the bandgap spectra, and confirm that the wavelengths of the observed bandgaps are in good correlation with the analytical predictions.