Using planar waveguides as a platform for optical biosensors allows an efficient and selective fluorescence excitation in
close proximity to the waveguide surface. Usually, the fluorescence light that is emitted in the space above the sensor
chip is collected and analyzed by suitable free space optics and a detector. Due to the vicinity of the fluorescent
molecules to the interface of the waveguide layer, a substantial part of the fluorescence light is coupled back into and
collected by the waveguide. The coupling efficiency depends on position, environment and orientation of the molecules.
The utilization of this signal for fluorescence detection and analysis can allow a significant simplification of the optical
instrumentation. We present a fundamental investigation of the fluorescence collection efficiency into the waveguide by
theoretical and experimental means.
Resonant reflection filters -- also known as grating waveguide structures -- are characterised by a multilayer configuration including a substrate, waveguide layer and grating(s) at the top of and, in this investigation, also under the waveguide layer. For a specific wavelength at a specific angular and polarisation orientation an incident beam is partly diffracted, guided and rediffracted, leading to vanishing transmission due to destructive interference with the directly transmitted beam, while most of the light is reflected. Since this resonance is a guided mode phenomenon these devices can be used as tunable filters or dichroic elements (reflected wavelength as a function of incident angle) as long as the guided mode condition holds. In this experimental study the behaviour of ultrashort pulses of ~100 fs within structures with various grating depths and, therefore, different spectral resonance bandwidths was investigated under resonance conditions. Spectral and time-resolved measurements in transmission as well as reflection geometry revealed that the ultrashort pulses leaving the structures are time-bandwidth limited, i.e. the spectral bandwidth of the resonant filter determines the pulse length. Group velocity dispersion (GVD) has no important influence since the light is immediately rediffracted after having been coupled into the waveguide layer of the sample.
Submicron surface-relief gratings were fabricated in ultrathin dielectric films by F2-laser ablation. Projection mask imaging by a Schwarzschild objective applying nanosecond duration pulses from a high-resolution 157-nm optical processing system generated 780-nm-period gratings in various thin oxide layers. The grating modulation depths were controlled within tens of nanometers by applying suitable energy densities and number of pulses. Thus, high-resolution laser ablation proves to be a promising alternative approach to well-known lithographic methods for the fabrication of submicron-period gratings in thin films.
Such gratings are the most critical component of grating waveguide structures (GWS) that comprise of a substrate, a thin waveguide, and a grating layer in a planar multilayer structure. Interference effects in a GWS will provide high reflection efficiency under resonance conditions for an ideal grating with no absorption losses. The resonance spectral responses of the F2-laser ablated gratings have been investigated using an ultrashort-pulse titanium-sapphire laser. Their potential for optical applications will be shown and discussed. GWS are attractive for optical switches or modulators, narrow-band spectral filters, high reflectivity mirrors, bio-sensor chips and many other applications.