Gap deformation and classical wave localization in disordered two-dimensional photonic band gap materials

By using two ab initio numerical methods we study the effects that disorder has on the spectral gaps and on wave localization in two-dimensional photonic band gap materials. We find that there are basically two different responses depending on the lattice realization (solid dielectric cylinders in air or vise versa), the wave polarization, and the particular form under which disorder is introduced. Two different pictures for the photonic states are employed, the ``nearly free'' photon and the ``strongly localized'' photon. These originate from the two different mechanisms responsible for the formation of the spectral gaps, ie. multiple scattering and single scatterer resonances, and they qualitatively explain our results.Comment: Accepted for publication in Phys. Rev.

Robustness of One-Dimensional Photonic Bandgaps Under Random Variations of Geometrical Parameters

The supercell method is used to study the variation of the photonic bandgaps in one-dimensional photonic crystals under random perturbations to thicknesses of the layers. The results of both plane wave and analytical band structure and density of states calculations are presented along with the transmission cofficient as the level of randomness and the supercell size is increased. It is found that higher bandgaps disappear first as the randomness is gradually increased. The lowest bandgap is found to persist up to a randomness level of 55 percent.Comment: Submitted to Physical Review B on April 8 200

Conductive nitrides: growth principles, optical and electronic properties, and their perspectives in photonics and plasmonics

The nitrides of most of the group IVb-Vb-VIb transition metals (TiN, ZrN, HfN, VN, NbN, TaN, MoN, WN) constitute the unique category of conductive ceramics. Having substantial electronic conductivity, exceptionally high melting points and covering a wide range of work function values, they were considered for a variety of electronic applications, which include diffusion barriers in metallizations of integrated circuits, Ohmic contacts on compound semiconductors, and thin film resistors, since early eighties. Among them, TiN and ZrN are recently emerging as significant candidates for plasmonic applications. So the possible plasmonic activity of the rest of transition metal nitrides (TMN) emerges as an important open question. In this work, we exhaustively review the experimental and computational (mostly ab initio) works in the literature dealing with the optical properties and electronic structure of TMN spanning over three decades of time and employing all the available growth techniques. We critically evaluate the optical properties of all TMN and we model their predicted plasmonic response. Hence, we provide a solid understanding of the intrinsic (e.g. the valence electron configuration of the constituent metal) and extrinsic (e.g. point defects and microstructure) factors that dictate the plasmonic performance. Based on the reported optical spectra, we evaluate the quality factors for surface plasmon polariton and localized surface plasmon for various TMN and critically compare them to each other. We demonstrate that, indeed TiN and ZrN along with HfN are the most well-performing plasmonic materials in the visible range, while VN and NbN may be viable alternatives for plasmonic devices in the blue, violet and near UV ranges, albeit in expense of increased electronic loss. Furthermore, we consider the alloyed ternary TMN and by critical evaluation and comparison of the reported experimental and computational works, we identify the emerging optimal tunable plasmonic conductors among the immense number of alloying combinations

Tight-binding parameterization for photonic band gap materials

The ideas of the linear combination of atomic orbitals (LCAO) method, well known from the study of electrons, is extended to the classical wave case. The Mie resonances of the isolated scatterer in the classical wave case, are analogous to the localized eigenstates in the electronic case. The matrix elements of the two-dimensional tight-binding (TB) Hamiltonian are obtained by fitting to ab initio results. The transferability of the TB model is tested by reproducing accurately the band structure of different 2D lattices, with and without defects, thus proving that the obtained TB parameters can be used to study other properties of the photonic band gap materials.Comment: 4 pages, 3 postscript figures, sumbitted to Phys. rev. Let

Simulating the opto-thermal processes involved in laser induced self-assembly of surface and sub-surface plasmonic nano-structuring

Nano-structuring of metals is one of the greatest challenges for the future of plasmonic and photonic devices. Such a technology calls for the development of ultra-fast, high-throughput and low cost fabrication techniques. Laser processing accounts for the aforementioned properties, representing an unrivalled tool towards the anticipated arrival of modules based in metallic nano-structures, with an extra advantage: the ease of scalability. Specifically, laser nano-structuring of an ultra-thin metal film or an alternating metal film on a substrate/metal film on a substrate results respectively on surface (metallic nanoparticles on the surface of the substrate) or subsurface (metallic nanoparticles embedded in a dielectric matrix) plasmonic patterns with many applications. In this work we investigate theoretically the photo-thermal processes involved in surface and sub-surface plasmonic nano-structuring and compare to experiments. To this end, we present a design process and develop functional plasmonic nano-structures with pre-determined morphology by tuning the annealing parameters like the laser fluence and wavelength and/or the structure parameters like the thickness of the metallic film and the volume ratio of the metal film on a substrate-metal composite. For the surface plasmonic nano-structuring we utilize the ability to tune the laser's wavelength to either match the absorption spectral profile of the metal or to be resonant with the plasma oscillation frequency, i.e. we utilize different optical absorption mechanisms that are size-selective. Thus, we overcome a great challenge of laser induced self assembly by combining simultaneously large-scale character with nanometer scale precision. For subsurface plasmonic nano-structuring, on the other hand, we utilize the temperature gradients that are developed spatially across the metal/dielectric nano-composite structure during the laser treatment. We find that the developed temperature gradients are strongly depended on the nanocrystalline character of the dielectric host which determines its thermal conductivity, the composition of the ceramic/metal and the total thickness of the nano-composite film. The aforementioned material parameters combined with the laser annealing parameters can be used to pre-design the final morphology of the sub-surface plasmonic structure. The proposed processes can serve as a platform that will stimulate further progress towards the engineering of plasmonic devices

Few-cycle pulses from a graphene mode-locked all-fiber laser

We combine a graphene mode-locked oscillator with an external compressor and achieve~29fs pulses with~52mW average power. This is a simple, low-cost, and robust setup, entirely fiber based, with no free-space optics, for applications requiring high temporal resolution

A stable, power scaling, graphene-mode-locked all-fiber oscillator

This is the final version. Available from AIP Publishing via the DOI in this record.We report power tunability in a fiber laser mode-locked with a solution-processed filtered graphene film on a fiber connector. 370 fs pulses are generated with output power continuously tunable from 4 up to 52 mW. This is a simple, low-cost, compact, portable, all-fiber ultrafast source for applications requiring environmentally stable, portable sources, such as imaging.European Research Council (ERC)Engineering and Physical Sciences Research Council (EPSRC)Emmanuel College, CambridgeIsaac Newton Trust, Trinity College Cambridg

A stable, power scaling, graphene-mode-locked all-fiber oscillator

We report power tunability in a fiber laser mode-locked with a solution-processed filtered graphene film on a fiber connector. ∼370 fs pulses are generated with output power continuously tunable from ∼4 up to ∼52 mW. This is a simple, low-cost, compact, portable, all-fiber ultrafast source for applications requiring environmentally stable, portable sources, such as imaging.</jats:p