Organic dye doped polymer photonic crystal band-edge lasers, fabricated by combined nanoimprint and photolithography,
are applied as evanescent-wave refractometry sensors. The emission characteristics of the lasers
are altered in two ways, when the refractive index of the cladding is changed. Not only does the emission wavelength
change, with a sensitivity of 1 nm per 10-2 refractive index units, but also the relative emission intensity
along the two symmetry directions of the rectangular device. The latter phenomenon is caused by the interplay
between the symmetry of the triangular photonic crystal lattice and the rectangular device shape. This causes
two of the three emission axes expected from the photonic crystal geometry to collapse into one. The optical
losses of these two modes are influenced in different ways when the refractive index of the cladding is altered,
thus also causing the emitted intensities along the symmetry directions to change. This suggests an integrated
sensing scheme, where intensity is measured rather than emission wavelength. Since intensity measurements are
simpler to integrate than spectrometers, the concept can be implemented in compact lab-on-a-chip systems.
Optofluidic dye lasers have recently attracted much interest as potentially efficient light sources for integration
on lab-on-a-chip micro-systems. However, dye bleaching resulting in limited life-time could limit the applications
of such devices in lab-on-a-chip technology. Typically, the problem of dye bleaching is addressed by employing a
continuous convective flow of liquid-dissolved dye molecules, compensating the bleaching caused by the external
optical pump. In previously reported optofluidic light sources the required convective dye replenishing flow has
been achieved by external fluid handling apparatus (syringe pumps), on-chip microfluidic pumps, or by means
of capillary effect. We have investigated the bleaching dynamics that occur in optofluidic light sources where a
liquid laser dye in a micro-fluidic channel is locally bleached due to optical pumping. A simple one-dimensional
diffusion model is used to explore the characteristic evolution of the local un-bleached dye concentration in the
optically pumped or bleached volume of the device. In the absence of convective flow, the decay of the local
dye concentration in the optically pumped volume is governed by the diffusion rate and the resulting lifetime
of the device is mainly limited by the capacity of the fluidic reservoirs. Generic microfluidic platforms typically
allow for device layouts with a large volume ratio between the fluidic reservoir and the region being optically
pumped. These conclusions drawn from the simple model are supported by basic experiments. Our investigations
reveal the possibility that such optofluidic dye laser devices may potentially be operated for days by diffusion
without the need for a convective flow. Relying on diffusion rather than convection to generate the necessary
dye replenishment significantly simplifies optofluidic dye laser device layouts, omitting the need for cumbersome
and costly external fluidic handling or on-chip microfluidic pumping devices.
We present the design and operation of low-threshold and widely tunable polymer-based nanofluidic distributed
feedback (DFB) dye lasers. The devices rely on light-confinement in a nanostructured polymer film embedded
between two substrates. An array of nanofluidic channels forms a Bragg grating DFB laser resonator relying on
the third order Bragg reflection. The lasers are fabricated by Combined Electron beam and UV Lithography
(CEUL) in a thin film of SU-8 resist and polymer mediated wafer bonding. The devices are operated without the
need for external fluidic handling apparatus. Capillary action drives the liquid dye infiltration of the nanofluidic
DFB lasers and accounts for dye replenishment. The low Bragg reflection order yields: (i) low out-of-plane
scattering losses, (ii) low coupling losses for the light when traversing the dye-filled nanofluidic channels due to
the sub-wavelength dimensions of the resonator segments, and (iii) a large free spectral range (FSR). Points
(i)+(ii) enable a low threshold for lasing, point (iii) facilitates wavelength tuning over the full gain spectrum of
the chosen laser dye without mode-hopping. By combining different grating periods and dye solution refractive
indices, we demonstrate a tuning range of 45 nm using a single laser dye and obtain laser threshold fluences
down to ~ 7 μJ/mm2. The lasers are straightforward to integrate on lab-on-a-chip microsystems, e.g. for novel
sensor concepts, where coherent light in the visible range is desired.
We have investigated the bleaching dynamics that occur in opto-fluidic dye lasers, where the liquid laser dye
in a channel is locally bleached due to optical pumping. Our studies suggest that for micro-fluidic devices, the
dye bleaching may be compensated through diffusion of dye molecules alone. By relying on diffusion rather
than convection to generate the necessary dye replenishment, our observation potentially allows for a significant
simplification of opto-fluidic dye laser device layouts, omitting the need for cumbersome and costly external
fluidic handling or on-chip micro-fluidic pumping devices.
We present a technology for miniaturized, chip-based liquid dye lasers, which may be integrated with microfluidic networks and planar waveguides without addition of further process steps. The microfluidic dye lasers consist of a microfluidic channel with an embedded optical resonator. The lasers are operated with Rhodamine 6G laser dye dissolved in a suitable solvent, such as ethanol or ethylene glycol, and optically pumped at 532 nm with a pulsed, frequency doubled Nd:YAG laser. Both vertically and laterally emitting devices are realized. A vertically emitting Fabry-Perot microcavity laser is integrated with a microfluidic mixer, to demonstrate realtime wavelength tunability. Two major challenges of this technology are addressed: lasing threshold and fluidic handling. Low threshold, in-plane emission and integration with polymer waveguides and microfluidic networks is demonstrated with distributed feed-back lasers. The challenge of fluidic handling is addressed by hybridization with mini-dispensers, and by applying capillary filling of the laser devices.
This paper concerns automatic video surveillance of outdoor scenes using a single camera. The first step in automatic interpretation of the video stream is activity detection based on background subtraction. Usually, this process will generate a large number of false alarms in outdoor scenes due to e.g. movement of thicket and changes in illumination. To reduce the number of false alarms a Track Before Detect (TBD) approach is suggested. In this TBD implementation all objects detected in the background subtraction process are followed over a number of frames. An alarm is given only if a detected object shows a pattern of movement consistent with predefined rules. The method is tested on a number of video sequences and a substantial reduction in the number of false alarms is demonstrated.
The integration of optical transducers is generally considered a key issue in the further development of lab-on-a-chip Microsystems. We present a technology for miniaturized, polymer based lasers, suitable for integration with planar waveguides and microfluidic networks. The lasers rely on the commercial laser dye Rhodamine 6G as active medium, and the laser resonator is defined in a thin film of polymer on a low refractive index substrate. Two types of devices are demonstrated: solid and microfluidic polymer based dye lasers. In the microfluidic dye lasers, the laser dye is dissolved in a suitable solvent and flushed though a microfluidic channel, which has the laser resonator embedded. For solid state dye lasers, the laser dye is dissolved in the polymer forming the laser resonator. The miniaturized dye lasers are optically pumped by a frequency doubled, pulsed Nd:YAG laser (at 532 nm), and emit at wavelengths between 560 nm and 590 nm. The lasers emit in the plane of the chip, and the emitted light is coupled into planar polymer waveguides on the chip. The feasibility of three types of polymers is demonstrated: SU-8, PMMA and a cyclo-olefin co-polymer (COC) - Topas. SU-8 is a negative tone photoresist, allowing patterning with conventional UV lithography. PMMA and Topas are thermoplasts, which are patterned by nanoimprint lithography (NIL). The lasing wavelength of the microfluidic dye lasers can be coarse tuned over 30 nm by varying the concentration of laser dye, and fine tuned by varying the refractive index of the solvent. This is utilized to realize a tunable laser, by on-chip mixing of dye, and two solvents of different index of refraction. The lasers were also integrated with waveguides and microfluidic networks.