Søren Balslev

Lab-on-a-chip with integrated optical transducers

Taking the next step from individual functional components to higher integrated devices, we present a feasibility study of a lab-on-a-chip system with five different components monolithically integrated on one substrate. These five components represent three main domains of microchip technology: optics, fluidics and electronics. In particular, this device includes an on-chip optically pumped liquid dye laser, waveguides and fluidic channels with passive diffusive mixers, all defined in one layer of SU-8 polymer, as well as embedded photodiodes in the silicon substrate. The dye laser emits light at 576 nm, which is directly coupled into five waveguides that bring the light to five different locations along a fluidic channel for absorbance measurements. The transmitted portion of the light is collected at the other side of this cuvette, again by waveguides, and finally detected by the photodiodes. Electrical read-out is accomplished by integrated metal connectors. To our knowledge, this is the first time that integration of all these components has been demonstrated.

A microfluidic dye laser fabricated by nanoimprint lithography in a highly transparent and chemically resistant cyclo-olefin copolymer (COC)

We present a polymer-based, microfluidic dye laser suitable for integration into polymer- or silicon-based lab-on-a-chip systems. The laser is fabricated by nanoimprint lithography (NIL) in cyclo-olefin copolymer (COC). The polymer device consists of microfluidic channels, with sizes ranging from several mm down to a few µm, and integrated optical waveguides to couple the light out of the structure, all fabricated in one single NIL step and with approximately 10 nm roughness. COC is a highly transparent, chemically resistant thermoplastic polymer optimal for the integration of microfluidic systems with optical elements. Rhodamine 6G dissolved in ethanol is used as an active medium in the laser, and the resonator is based on multiple reflections from a periodic structure of 16 µm wide, parallel microfluidic channels. Lasing from the device is observed at 577 nm, when optically pumped with a frequency doubled Nd:YAG laser emitting at 532 nm, where Rhodamine 6G has its absorption maximum.

Levitated droplet dye laser

We present the first observation, to our knowledge, of lasing from a levitated, dye droplet. The levitated droplets are created by computer controlled pico-liter dispensing into one of the nodes of a standing ultrasonic wave (100 kHz), where the droplet is trapped. The free hanging droplet forms a high quality optical resonator. Our 750 nL lasing droplets consist of Rhodamine 6G dissolved in ethylene glycol, at a concentration of 0.02 M. The droplets are optically pumped at 532 nm light from a pulsed, frequency doubled Nd:YAG laser, and the dye laser emission is analyzed by a fixed grating spectrometer. With this setup we have achieved reproducible lasing spectra in the visible wavelength range from 610 nm to 650 nm. The levitated droplet technique has previously successfully been applied for a variety of bio-analytical applications at single cell level. In combination with the lasing droplets, the capability of this high precision setup has potential applications within highly sensitive intra-cavity absorbance detection.

Micro-fabricated single mode polymer dye laser.

We present a single mode, single polarization, distributed feedback polymer dye laser, based on a short high order Bragg grating defined in a dye doped polymer layer between two secondary polymer layers. The Bragg grating is defined solely with standard I-line UV lithography. In this device we obtain single mode operation in a multimode structure by means of mode loss differentiation without using sub-wavelength structures. The laser is fabricated using micro-fabrication technology, is pumped by a pulsed frequency doubled Nd:YAG laser, and emits light in the chip plane at 551.39 nm, with a FWHM linewidth below 150 pm.

Single mode solid state distributed feedback dye laser fabricated by gray scale electron beam lithography on a dye doped SU-8 resist

We demonstrate gray scale electron beam lithography on a functionalized SU-8 resist for fabrication of single mode solid state dye laser devices. The resist is doped with Rhodamine 6G perchlorate and the lasers are based on a first-order Bragg grating distributed feedback resonator. The lasers are optically pumped at 532 nm, and exhibit low lasing threshold from 530 nJ mm?2 and single mode output at selectable wavelengths from 580 to 630 nm, determined by the grating pitch. The lasers are well suited for integration into polymer based lab-on-chip circuits for interference based sensing.

Microfabricated solid-state dye lasers based on a photodefinable polymer

We present a solid polymer dye laser based on a single-mode planar waveguide. The all-polymer device is self-contained in the photodefinable polymer SU-8 and may therefore easily be placed on any substrate and be integrated with polymer-based systems. We use as the active medium for the laser the commercially available laser dye Rhodamine 6G, which is incorporated into the SU-8 polymer matrix. The single-mode slab waveguide is formed by three-step spin-coating deposition: a buffer layer of undoped SU-8, a core layer of SU-8 doped with Rhodamine, and a cladding layer of undoped SU-8.

Real-time tunability of chip-based light source enabled by microfluidic mixing

We demonstrate real-time tunability of a chip-based liquid light source enabled by microfluidic mixing. The mixer and light source are fabricated in SU-8 which is suitable for integration in SU-8-based laboratory-on-a-chip microsystems. The tunability of the light source is achieved by changing the concentration of rhodamine 6G dye inside two integrated vertical resonators, since both the refractive index and the gain profile are influenced by the dye concentration. The effect on the refractive index and the gain profile of rhodamine 6G in ethanol is investigated and the continuous tuning of the laser output wavelength is demonstrated using an ethanolic rhodamine 6G solution of 2×10−2mol∕l mixed with pure ethanol. This yields rhodamine 6G concentrations from 5×10−3 to 1.5×10−2mol∕l inside the laser resonators and a wavelength change of 10 nm with a response time of 110 s.

Fully integrated optical systems for lab-on-a-chip applications

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 the integration of miniaturized, polymer based lasers, with planar waveguides, microfluidic networks and substrates such as structured silicon. The flexibility of the polymer patterning process, enables fabrication of laser light sources and other optical components such as waveguides, lenses and prisms, in the same lithographic process step on a polymer. The optically functionalised polymer layer can be overlaid on any reasonably flat substrate, such as electrically functionalised Silicon containing photodiodes. This optical and microfluidic overlay, interfaces optically with the substrate through the polymer-substrate contact plane. Two types of integrable laser source devices are demonstrated: microfluidic- and solid polymer dye lasers. Both are based on laser resonators defined solely in the polymer layer. The polymer laser sources are optically pumped with an external laser, and emits light in the chip plane, suitable for coupling into chip waveguides. Integration of the light sources with polymer waveguides, micro-fluidic networks and photodiodes embedded in a Silicon substrate is shown in a device designed for measuring the time resolved absorption of two fluids mixed on-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).

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.