Morten Gersborg-Hansen

Tunability of optofluidic distributed feedback dye lasers

We investigate the tunability of optofluidic distributed feedback (DFB) dye lasers. The lasers rely on light-confinement in a nano-structured polymer film where an array of nanofluidic channels constitutes a third order Bragg grating DFB laser resonator with a central phase-shift. The lasers are operated by filling the DFB laser resonator with a dye solution by capillary action and optical pumping with a frequency doubled Nd:YAG laser. The low reflection order of the DFB laser resonator yields low out-of-plane scattering losses as well as a large free spectral range (FSR), and low threshold fluences down to ~7 microJ/mm2 are observed. The large FSR facilitates wavelength tuning over the full gain spectrum of the chosen laser dye and we demonstrate 45 nm tunability using a single laser dye by changing the grating period and dye solution refractive index. The lasers are straight-forward to integrate on lab-on-a-chip microsystems, e.g. for novel sensor concepts, where coherent light in the visible range is desired.

Optofluidic third order distributed feedback dye laser

This letter describes the design and operation of a polymer-based third order distributed feedback (DFB) microfluidic dye laser. The device relies on light confinement in a nanostructured polymer film where an array of nanofluidic channels is filled by capillary action with a liquid dye solution which has a refractive index lower than that of the polymer. In combination with a third order DFB grating, formed by the array of nanofluidic channels, this yields a low threshold for lasing. The laser is straightforward to integrate on lab-on-a-chip microsystems where coherent, tunable light in the visible range is desired.

Optofluidic tuning of photonic crystal band edge lasers

We demonstrate optofluidic tuning of polymer photonic crystal band edge lasers with an imposed rectangular symmetry. The emission wavelength depends on both lattice constant and cladding refractive index. The emission wavelength is shown to change 1nm with a cladding refractive index change of 10−2. The rectangular symmetry modification alters the emission characteristics of the devices and the relative emission intensities along the symmetry axes depend on cladding refractive index, suggesting a sensor concept based on detection of intensity rather than wavelength.

Finite-element simulation of cavity modes in a microfluidic dye ring laser

We consider a recently reported microfluidic dye ring laser and study the full wave nature of TE modes in the cavity by means of finite-element simulations. The resonance wave-patterns of the cavity modes support a ray-tracing view and we are also able to explain the spectrum in terms of standing waves with a mode spacing δk = 2π/Leff, where Leff is the effective optical path length in the cavity.

Bleaching and diffusion dynamics in optofluidic dye lasers

The authors have investigated the bleaching dynamics that occur in optofluidic dye lasers where the liquid laser dye in a microfluidic channel is locally bleached due to optical pumping. They find that for microfluidic 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, their observation potentially allows for a significant simplification of optofluidic dye laser device layouts, omitting the need for cumbersome and costly external fluidic handling or on-chip microfluidic pumping devices.

Single-mode and tunable microfluidic dye lasers

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.

Polymer-based lab-on-a-chip lasers

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.

Optofluidic evanescent sensing by polymer photonic crystal band edge lasers

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.

Diffusion dynamics in microfluidic dye lasers

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.

Capillary driven tunable optofluidic DFB dye lasers

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.