This letter describes the design and operation of a polymer-based third order
distributed feed-back (DFB) microfluidic dye laser. The device relies on
light-confinement in a nano-structured 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 straight-forward to
integrate on Lab-on-a-Chip micro-systems where coherent, tunable light in the
visible range is desired.Comment: 4 pages, 3 figure

Gersborg-Hansen, Morten

Kristensen, Anders

English

arXiv

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. (c) 2006 American Institute of Physics

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Optofluidic third  order distributed  feedback dye  laserGersborg-Hansen, Morten; Kristensen, AndersPublished in:Applied Physics LettersLink to article, DOI:10.1063/1.2345602Publication date:2006Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Gersborg-Hansen, M., & Kristensen, A. (2006). Optofluidic third  order distributed  feedback dye  laser. AppliedPhysics Letters, 89(10), Article number: 103518. DOI: 10.1063/1.2345602Optofluidic third order distributed feedback dye laserMorten Gersborg-Hansen and Anders KristensenaMIC, Department of Micro and Nanotechnology, NanoDTU, Technical University of Denmark,Building 345 East, Ørsteds Plads, DK-2800 Kongens Lyngby, DenmarkReceived 5 May 2006; accepted 13 July 2006; published online 7 September 2006This letter describes the design and operation of a polymer-based third order distributed feedbackDFB microfluidic dye laser. The device relies on light confinement in a nanostructured polymerfilm where an array of nanofluidic channels is filled by capillary action with a liquid dye solutionwhich has a refractive index lower than that of the polymer. In combination with a third order DFBgrating, formed by the array of nanofluidic channels, this yields a low threshold for lasing. The laseris straightforward to integrate on lab-on-a-chip microsystems where coherent, tunable light in thevisible range is desired. © 2006 American Institute of Physics. DOI: 10.1063/1.2345602Integration of optical and fluidic functionalities on a chiphas recently been investigated by several research groups torealize novel optofluidic laser devices, microfluidic dye la-sers, suitable for lab-on-a-chip microsystems.1–4 These opti-cally pumped devices consist of microfluidic channels withan embedded optical resonator and a liquid laser dye is usedas active gain medium. In general, these devices can beadded to a microfluidic chip without adding additional fabri-cation steps,5 thus offering a simple way of integrating opti-cal transducers to lab-on-a-chip microsystems,6 and pave theway for advanced integrated sensor concepts.7The threshold for lasing is a key parameter for the fea-sibility of these devices in a future technology. Distributedfeedback DFB laser resonators have proven particularlysuited in this respect. A DFB microfluidic dye laser was firstdemonstrated by Balslev and Kristensen,2 who used a highorder Bragg grating in an 8 m thick polymer film to obtainfeedback. Single mode lasing with a threshold fluence ofapproximately 20 J /mm2 was obtained due to mode selec-tive losses in the multimode structure where light was notguided in the fluidic segments. Vezenov et al.3 obtainedliquid-core waveguiding using a low refractive index poly-mer and a high refractive index liquid. Li et al.4 exploitedthis concept to realize a 15th order DFB laser with a record-low threshold fluence for lasing of approximately 8 J /mm2.However, the 15th order Bragg grating still gives rise toscattering of light out of the chip plane.8 These losses can bereduced by employing a lower order Bragg grating. Ideally, afirst order Bragg grating combined with planar waveguidingin the polymer film could be used. The requirement of a highrefractive index liquid and low refractive index polymer3,4may be relaxed by reducing the dimensions of the resonatorsegments as the subwavelength regime is entered.9In this letter, we present the design and operation of apolymer-based third order DFB microfluidic dye laser. Thedevice is fabricated by a flexible lithography technique, com-bining electron beam lithography EBL and UV lithographyUVL in a single polymer film, enabling fast prototyping ofdevice designs. In the device, see Fig. 1, a third order DFBresonator is formed by infiltrating a periodic array of nano-fluidic channels with a liquid laser dye. The filling is medi-ated by capillary action, thus simplifying the operation of thedevice as opposed to conventional setups with external sy-ringe pumps. Replenishing of the dye in the cavity region isachieved for hours through capillary filling of a 16 cm long,100 m wide meandering channel following the cavity re-gion. The speed of the advancing fluid front in the meander-ing channel decreases with time.10 The dye solution in thecavity region is replaced in 30 s–5 min during the first30 min of experiments ethylene glycol solution. The flowrate in the 500500 m2 cavity region can be further in-creased by increasing the width of the meandering channel.By employing a third order Bragg grating, we demonstratelasing with a threshold fluence comparable to the results ofRef. 4. The subwavelength dimensions of the third orderDFB grating yield a low coupling loss for the light whentraversing the dye-filled nanofluidic channels, and combinedaElectronic mail: ak@mic.dtu.dk; URL: www.mic.dtu.dk/nilFIG. 1. Color online Overview of the fabricated microfluidic dye laser. aTop-view picture of the overall layout of the fabricated chip. The shallow300 nm high meandering channel facilitates capillary filling of the embed-ded laser resonator. b Optical micrograph of the DFB laser resonator em-bedded in the shallow meandering channel. c Side-view sketch showingthe layers of the DFB laser resonator. d Scanning electron micrographshowing the third order Bragg grating of nanofluidic channels which con-stitutes the DFB laser resonator prior to bonding.APPLIED PHYSICS LETTERS 89, 103518 20060003-6951/2006/8910/103518/3/$23.00 © 2006 American Institute of Physics89, 103518-1Downloaded 17 Jun 2010 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspwith the feedback of the DFB grating, this yields an efficientlaser device.The laser is based on a planar waveguide structure sup-porting a single propagating TE-TM mode. The basic wave-guide structure consists of a SiO2 refractive index n=1.46buffer substrate, a 300 nm thick core layer of the negative-tone resist SU-8 n=1.59, and a top cladding of polymethyl-methacrylate PMMA n=1.49. The device structure isformed by nanofluidic channels defined lithographically inthe SU-8 film. The laser resonator consists of an array of300 nm high nanochannels of period =601 nm which com-prises a third order Bragg grating with a central  /2 phaseshift, embedded in a 500 m wide shallow nanochannel.To fabricate the devices, a 430 nm thick film of SU-82000 resist11 is spin coated onto a Si substrate with a 2.5 mthick thermally grown oxide layer. The wafer is baked at90 °C for 1 min. The device structure is defined by com-bined EBL and UVL in the SU-8 film. The nanostructures ofthe third order DFB grating are defined by 100 kV electronbeam exposure JEOL-JBX9300FS, dose of 3 C/cm2. Thetotal writing time for all 32 devices on a 4 in. wafer is15 min. After electron beam exposure, the micron-sizedstructures meandering channel and reservoirs are defined inthe same polymer film by UV exposure 20 s at8.9 mW/cm2, the wafer is postexposure baked at 90 °C for1 min, and the micro and nanostructures are developed si-multaneously in propylene glycol monomethyl ether acetatefor 30 s, followed by an isopropyl alcohol rinse. The wafer issubsequently subjected to a 20 s soft oxygen plasma12 toremove any residues of SU-8 at the SiO2 surface in thenanochannels of the DFB resonator. After the plasma treat-ment, the structures are 300 nm high, and the heights of theEBL and UVL defined structures are matched within 30 nm.Fluidic access holes are formed by micropowder blasting13and the channels are sealed by a glass lid, using adhesivebonding by means of a 5 m thick PMMA film 2200 N,140 °C, 10 min.14The laser is characterized by adding a droplet of dyesolution to the fluid inlet, thus filling the DFB resonatorstructure by capillary action. In this letter, the laser dyerhodamine 6G R6G is dissolved in ethylene glycoln=1.43, ethanol n=1.33, and in a 1:1 mixture of ethyleneglycol and benzyl alcohol n=1.485 by experimentally con-firmed linear extrapolation of the refractive indices, all witha concentration of 210−2 mol/L and a refractive index be-low that of the polymer surrounding the channel. The laser isoptically pumped through the glass lid by a frequencydoubled neodymium-doped yttrium aluminum garnet laser at532 nm 5 ns pulse duration, 10 Hz repetition rate.Figure 2 shows laser spectra from four different chipsfilled with R6G in ethylene glycol. The results demonstratewafer-scale spectral reproducibility of the laser, exhibitingnarrow-linewidth emission, polarized perpendicularly to thechip plane TM. The average laser wavelength is 582.72 nmwith a standard deviation of 0.14 nm, at the resolution limitof our spectrometer 0.15 nm. The deviation and the minorpeaks in Figs. 2c and 2d may arise from grating imper-fections due to fabrication defects. Due to limited spectrom-eter resolution, our equipment does not allow us to determinewhether the laser is truly operating in a single mode, andthe spectral shape and linewidth of the laser remainundetermined.Figure 3 shows laser spectra from chips with R6G dis-solved in different solvents, thus demonstrating tunability ofthe laser from 581.41 to 587.86 nm. A quantitative theoreti-cal prediction of the shift in wavelength requires a full solu-tion of Maxwell’s equations in the fabricated structure, sincethe structures are of subwavelength dimensions.9 In Fig. 3c,the minor side mode shifted 1.8 nm from the main peak maybe attributed to an additional phase shift introduced in theDFB grating by a stitching error during electron beam expo-sure. The dye laser output power is found by integrating themeasured laser spectrum for each value of the average pumppulse fluence. The dye laser output power versus averagepump pulse fluence follows the standard pump/output rela-tion of two linear segments around a lasing threshold fluenceQth.The coupling loss  for a period of the DFB grating hasbeen modeled by a finite-difference beam propagationmethod.15,16 Light of the observed wavelength traveling inthe TM mode of the single mode polymer waveguide ispropagated through a 273 nm wide channel filled with a liq-FIG. 2. Normalized laser spectra from four nominally identical chips dem-onstrating wafer-scale spectral reproducibility of the laser. On each chip, theDFB laser resonator is filled by capillary action with rhodamine 6G R6Gdissolved in ethylene glycol. The average laser wavelength of the chips is582.72 nm with a standard deviation of 0.14 nm.FIG. 3. Normalized laser spectra and corresponding laser output powercurves as a function of pump pulse fluence for R6G in three different solu-tions: a R6G in ethanol, b R6G in ethylene glycol, and c R6G in a 1:1mixture of ethylene glycol and benzyl alcohol. The results are summarizedin Table I.103518-2 M. Gersborg-Hansen and A. Kristensen Appl. Phys. Lett. 89, 103518 2006Downloaded 17 Jun 2010 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspuid of refractive index nch, corresponding to the value of theused solvent. The calculations yield the energy propagationloss corresponding to the coupling loss for a period of theDFB grating. The loss decreases for increasing nch, see TableI. To obtain lasing in the resonator, the amplification of theR6G must be equal to the loss. As expected, the experimentsshow that the threshold fluence for lasing decreases as therefractive index of the R6G solution increases, see Table I.Further, when considering the absorbed pump energythroughout the device, the effective threshold fluence may beconsiderably lower.In summary, we have demonstrated a third order DFBmicrofluidic dye laser with a low threshold for lasing andoperating with liquids of refractive index below the refrac-tive indices of the surrounding polymer. Although experi-mental uncertainties make a direct comparison of lasingthresholds difficult, our results imply the importance of bothliquid-core waveguiding4 and low order Bragg reflection.The device relies on light confinement in a nanostructuredpolymer film where the individual resonator elements—nanofluidic channels and polymer walls—are of subwave-length dimensions. The resonator consists of an array ofnanofluidic channels forming a third order DFB Bragg grat-ing resonator. The laser is realized using a flexible fabrica-tion technique, combining electron beam lithography andUV lithography in a single polymer film, followed by adhe-sive polymer wafer bonding. The laser is straightforward tointegrate on lab-on-a-chip microsystems, e.g., for integratedsensors, where coherent, tunable light in the visible range isdesired.The authors thank S. Balslev and N. A. Mortensen forfruitful discussions on the work presented in this letter.1B. Helbo, A. Kristensen, and A. Menon, J. Micromech. Microeng. 13, 3072003.2S. Balslev and A. Kristensen, Opt. Express 13, 344 2005.3D. V. Vezenov, B. T. Mayers, R. S. Conroy, G. M. Whitesides, P. T. Snee,Y. Chan, D. G. Nocera, and M. G. Bawendi, J. Am. Chem. Soc. 127, 89522005.4Z. Li, Z. Zhang, T. Emery, A. Scherer, and D. Psaltis, Opt. Express 14,696 2006.5S. Balslev, A. M. Jorgensen, B. Bilenberg, K. B. Mogensen, D.Snakenborg, O. Geschke, J. P. Kutter, and A. Kristensen, Lab Chip 6, 2132006.6E. Verpoorte, Lab Chip 3, 42N 2003.7L. Lindvold and L. Lading, Proc. SPIE 3276, 220 2003.8R. G. Hunsperger, Integrated Optics: Theory and Technology, 5th ed.Springer, Berlin, 2002.9P. Lalanne and M. Hutley, Encyclopedia of Optical Engineering Dekker,New York, 2003, pp. 62–71.10N. R. Tas, J. Haneveld, H. V. Jansen, M. Elwenspoek, and A. van denBerg, Appl. Phys. Lett. 85, 3274 2004.11SU-8 formulation 2002 from MicroChem. Corp., www.microchem.com,diluted with pure cyclopentanone to a 13% mass percentage solid contentsolution.12B. Bilenberg, S. Jacobsen, M. S. Schmidt, L. H. D. Skjolding, P. Shi, P.Boggild, J. O. Tegenfeldt, and A. Kristensen, Microelectron. Eng. 83,1609 2006.13Microetcher II, Danville Engineering www.daneng.com.14B. Bilenberg, T. Nielsen, B. Clausen, and A. Kristensen, J. Micromech.Microeng. 14, 814 2004.15S. Balslev, A. Mironov, D. Nilsson, and A. Kristensen, Opt. Express 14,2170 2006.16H. J. W. M. Hoekstra, G. J. M. Krijnen, and P. V. Lambeck, J. LightwaveTechnol. 10, 1352 1992.TABLE I. Summary of the results. For R6G solutions of larger refractiveindices n, the laser wavelength  increases, and the threshold pump fluenceQth and coupling loss  decrease.Solvent n  nm Qth J /mm2 Ethanol 1.33 581.41 65 0.191Ethylene glycol 1.43 582.72 40 0.089Ethylene glycol/benzyl alcohol 1.485 587.86 10 0.043103518-3 M. Gersborg-Hansen and A. Kristensen Appl. Phys. Lett. 89, 103518 2006Downloaded 17 Jun 2010 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Opto-fluidic third order distributed feed-back dye laser

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