No abstract

Anisotropic nanoparticle (NP) arrays with useful optical properties, such as localized plasmon resonances (LPRs), can be grown by self-assembly on substrates. However, these systems often have significant dispersion in NP dimensions and distribution, which makes a numerical approach to modeling the LPRs very difficult. An improved analytic approach to this problem is discussed in detail and applied successfully to NP arrays from three systems that differ in NP metal, shape and distribution, and in substrate and capping layer. The materials and anisotropic NP structures that will produce LPRs in desired spectral regions can be determined using this approach.

The molecules of life are chiral. Advanced materials for heterogeneous enantioselective catalysis and biomolecular sensing and recognition will need nanoscale chiral structures, and methods of characterizing them. Optical circular dichroism (CD) is the main technique for probing chiral molecules in solution. Amino acids are levorotatory, but CD cannot detect chirality in ultrathin films of these small molecules at surfaces, as the response is too weak. Chiral second-harmonic generation (SHG) from ultrathin layers of larger, more polarizable organic molecules has been detected at silica surfaces, where the presence of electronic resonances enhanced the signal. Here chiral SHG is reported from ultrathin films of 1,1 0 -binaphthalene-2,2 0 -diol (binol) and cysteine. The SH response from the different chiral enantiomers was measured away from resonance, using unamplified femtosecond excitation at 800 nm, by rotating a quarter-wave plate polarizer, and detecting the ppolarized SH output. The SH response from the molecules is measurable and differs significantly between the enantiomers. Simultaneous fitting of the polarizer rotation plots, using a simple phenomenological model, produces reasonable agreement with the experimental results. This exploratory work shows that SHG has sufficient sensitivity to detect the chirality of small molecules adsorbed on surfaces.

A holographic device characterised by a large angular range of operation is under development. The aim of this study is to increase the angular working range of the diffractive lens by stacking three layers of high efficiency optical elements on top of each other so that light is collected (and focussed) from a broader range of angles. The angular range of each individual lens element is important, and work has already been done in an acrylamide-based photosensitive polymer to broaden the angular range of individual elements using holographic recording at a low spatial frequency. This paper reports new results on the angular selectivity of stacked diffractive lenses. A working range of 12° is achieved. The diffractive focussing elements were recorded holographically with a central spatial frequency of 300 l/mm using exposure energy of 60 mJ/cm2at a range of recording angles. At this spatial frequency with layers of thickness 50 ± 5 µm, a diffraction efficiency of 80% and 50% was achieved in the single lens element and combined device, respectively. The optical recording process and the properties of the multilayer structure are described and discussed. Holographic recording of a single lens element is also successfully demonstrated on a flexible glass substrate (Corning(R) Willow(R) Glass) for the first time.

Fabrication of dense arrays of nanowires by growth on nanostructured substrates appears to be a promising approach for producing functional nanoscale devices. Ion beam irradiation under carefully controlled conditions produces selforganized ripple patterns on silicon substrates, on which Ag nanowire arrays with nanoscale periodicity have been grown successfully. Here, the linear and nonlinear optical response from native-oxide-covered Si(001) templates, with ripple periodicity between $20 and $50 nm, is reported. Reflection anisotropy spectroscopy (RAS) shows a small, broad peak at $2.5 eV of $1 Â 10 À3 amplitude. The RAS response decreases as the periodicity of the ripple structure increases. The nonlinear second-harmonic (SH) response from the samples was also measured, using unamplified femtosecond excitation at 800 nm, by rotating the linear polarized input and detecting either the sor p-polarized SH output. A decrease in response with increased ripple structure periodicity was observed for the p-in/p-out configuration, when the plane of incidence was orthogonal to the average ripple orientation. Possible origins of the response and future experiments are discussed. For the substrates themselves, self-organized fabrication of nanoscale templates has been explored, for example by the controlled annealing of vicinal alumina [6,7]. Low-energy ion beam irradiation is another established method for fabricating self-organized, nanoscale periodic patterns on flat solid substrates [8], which attracted interest as a method to create templates for functional nanostructures [9]. Nanodot arrays and nanoripples with short range order and adjustable periodicity have been fabricated on various