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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 16 — Jul. 30, 2012
  • pp: 17467–17473
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Direct laser writing for nanoporous liquid core laser sensors

Tobias Grossmann, Mads Brøkner Christiansen, Jeffrey Peterson, Heinz Kalt, Timo Mappes, and Anders Kristensen  »View Author Affiliations


Optics Express, Vol. 20, Issue 16, pp. 17467-17473 (2012)
http://dx.doi.org/10.1364/OE.20.017467


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Abstract

We report the fabrication of nanoporous liquid core lasers via direct laser writing based on two-photon absorption in combination with thiolene-chemistry. As gain medium Rhodamine 6G was embedded in the nanoporous polybutadiene matrix. The lasing devices with thresholds of 19 µJ/mm2 were measured to have bulk refractive index sensitivities of 169 nm/RIU at a laser wavelength of 600 nm, demonstrating strongly increased overlap of the modes with the analyte in comparison to solid state evanescent wave sensors.

© 2012 OSA

1. Introduction

In this work, we demonstrate how direct laser writing (DLW) based on two-photon absorption (TPA) in combination with thiolene-chemistry can be used to define hydrophilic regions within nanoporous polybutadiene, which allows for three-dimensional control of the SLCW geometry and in principle lateral feature sizes of several hundred nanometers [14

14. G. von Freymann, A. Ledermann, M. Thiel, I. Staude, S. Essig, K. Busch, and M. Wegener, “Three-dimensional nanostructures for photonics,” Adv. Funct. Mater. 20(7), 1038–1052 (2010). [CrossRef]

]. We realize nanoporous liquid core ring resonator lasers by doping the NP with Rhodamine 6G, resulting in lasing thresholds around 19 µJ/mm2 and determine spectrometer-limited laser linewidths of 70 pm. The lasing devices were measured to have bulk refractive index sensitivities of 169 nm/RIU at a laser wavelength of 600 nm, demonstrating these ring resonators based on nanoporous liquid core waveguides to have an increased light matter interaction compared to solid state WGM sensors where only the evanescent field probes the liquid analyte in the surrounding.

2. Fabrication of nanoporous dye lasers by direct laser writing

In the following, the fabrication of the nanoporous ring resonator lasers is described before we turn to the lasing properties.

A commercial DLW system (Photonic Professional, Nanoscribe, Karlsruhe, Germany), exposing with a frequency-doubled, pulsed fiberlaser (pulse length below 150 fs, repetition rate 100 MHz at 780 nm wavelength) was used. The laser beam was focused into the sample by an immersion oil objective (numerical aperture NA = 1.4, 100x). A schematic of the layer system used for the exposure is depicted in Fig. 1(a)
Fig. 1 (a) Schematic of the layer system used for fabrication of nanoporous ring resonator lasers via DLW. (b) Optical micrograph during the exposure showing the laserspot and the exposed ring. (c) Schematic of the sample after the exposure. The exposed areas contain hydrophilic nanopores and are embedded within the hydrophobic nanoporous polymer. (d) Optical micrograph of the sample after the direct laser writing process. The ring has a diameter of 150 µm and a width of 5 µm.
. The laser beam passes the immersion oil and the cover slip before it is focused into the transparent polymer sample. The size of the ellipsoidal volume pixel (voxel) in which the TPA occurs was 1 µm in the propagation direction of the beam and several hundred micrometers in the lateral direction. During the exposure, the modification of the nanoporous polymer is easily visible by a refractive index change of the material due to the formation of hydrophilic end-groups in the nanopores. Figure 1(b) shows an image during the exposure of a ring, where the laserspot and the exposed ring can be seen. After the exposure, the sample was washed in ethanol to completely remove the thiol solution and dried afterwards. The exposed rings with hydrophilic nanopores were embedded within the hydrophobic nanoporous polymer as depicted schematically in Fig. 1(c). Figure 1(d) shows a microscope image of an exposed ring with a diameter of 150 µm and a width of 5 µm after the exposure. The thickness of the exposed ring is around 10 µm and was written by stacking 10 layers each with a thickness of 1 µm, which was defined by the voxel size.

The gain material was integrated after the lithographic definition of the resonator. This was achieved by doping the entire polymer matrix with the laser dye Rhodamine 6G (rh6G) as depicted schematically in Fig. 2(a)
Fig. 2 (a) Schematic and (b) microscope image of a dye-doped nanoporous sample without water. (c) Schematic and (d) microscope image of a sample with a waterfilm on top resulting in an infiltrated ring with increased refractive index compared to the surrounding polymer matrix.
. The substrate was submerged for 24 h in a saturated solution of rh6G dissolved in THF. The solvent causes the polymer matrix to swell, enabling diffusion of dye into the matrix. The density of dye molecules is strongly increased in the nanopores with hydrophilic end-groups as can be seen in the microscope image depicted in Fig. 2(b), due to binding of the relatively polar rh6G molecules onto the surface of the nanopores. In order to guide light in the liquid core resonator, the exposed ring has to be infiltrated with water. A waterfilm on top of the substrate is already sufficient to cause condensation of evaporated water within the ring and to infiltrate the hydrophilic nanopores (see Fig. 2(c)). The infiltrated region has a refractive index of 1.42, which is about 0.16 RIU higher than the hydrophobic regions without water and thus sufficient to guide light [13

13. N. Gopalakrishnan, M. B. Christiansen, and A. Kristensen, “Nanofiltering via integrated liquid core waveguides,” Opt. Lett. 36(17), 3350–3352 (2011). [CrossRef] [PubMed]

]. Figure 2(d) shows a microscope image of an exposed ring with a water film on top of the sample. In this case the sample was undoped and the contrast between exposed ring and unexposed surrounding was increased due to uptake of water within the ring. This can be clearly seen by comparison with a dry sample depicted in Fig. 1(d). The interference effect in Fig. 2(d) is due to a thin air film between the transparent sample and the stage it was layed upon.

3. Lasing properties of nanoporous liquid core dye lasers

To characterize the lasing properties, the dye-doped samples were infiltrated with water and optically pumped from above with 8 ns pulses of a frequency doubled Nd:YAG laser at a pump wavelength of 532 nm and a repetition rate of 10 Hz. Output emission was collected either at the edge of the chip with a multimode optical fiber, or perpendicularly to the sample with a microscope objective (NA = 0.4, 20x) and analyzed in a spectrometer. Spectra of the laser output for increasing pump fluence are depicted in Fig. 3(a)
Fig. 3 (a) Output spectrum of an optically pumped dye-doped liquid core ring resonator laser for different pump fluencies. (b) Input-output curve of the grey marked mode in (a) at 587 nm with a threshold pump fluence of 19 µJ/mm2. (c) High-resolution spectrum above lasing threshold, showing multiple laser modes between 603 nm and 606 nm with linewidths of 70 pm and a free spectral range of 0.5 nm.
. Above a pump fluence of 12 µJ/mm2 several sharp lasing modes appear in the spectrum due to amplification of WGMs by the dye. Output intensity as function of increasing excitation pump fluence is exemplarily shown in Fig. 3(b) for the grey marked mode in Fig. 3(a). The input-output curve has a kink at a threshold pump fluence of 19 µJ/mm2, determining the onset of lasing for this particular mode. The value of the threshold inferred from the intercept of the two linear regimes is the best possible upper estimation in this case. This takes into account both fluorescence backgrounds, the one from the ring resonator itself as well as the other one from the surrounding doped polymer. The inset of Fig. 3(b) shows a microscope image of the ring laser with a diameter of 150 µm under optical excitation, where the excitation wavelength is filtered out. Above threshold equally spaced laser modes were observed as shown with higher resolution in Fig. 3(c). The free spectral range ΔλFSR measured in the spectrum was 0.5 nm, which is in agreement with the value expected from the equation ΔλFSR = λ2/(2π n R)=0.54 nm, with a refractive index of n=1.42, a radius of R=75µm, and a wavelength of λ = 600 nm. The linewidth of the laser modes was measured to be 70 pm and limited by the spectrometer resolution using a grating with 1200 lines/mm.

Another interesting property of the present platform is the huge inner surface area of the NP of 283 ± 14 m2/g [12

12. K. Sagar, N. Gopalakrishnan, M. B. Christiansen, A. Kristensen, and S. Ndoni, “Photolithographic fabrication of solid–liquid core waveguides by thiol-ene chemistry,” J. Micromech. Microeng. 21(9), 095001 (2011). [CrossRef]

]. This would provide an inherently large sensitivity to surface refractive index changes, if the inner pore surfaces were modified to bind specific analytes.

4. Conclusion

In summary, we have applied direct laser writing (DLW) in combination with thiolene-chemistry to define hydrophilic regions within a nanoporous polymer which can then be infiltrated with water and thus forms a liquid core waveguide. Nanoporous liquid core ring resonator lasers were fabricated by doping the polymer matrix with dye. Thus the liquid applied to these devices did not require any treatment or admixing to serve as liquid core of the cavity. The liquid core lasing devices were measured to have bulk refractive index sensitivities of 169 nm/RIU at a wavelength of 600 nm, which is an order of magnitude larger sensitivity than in comparable solid state WGM sensors, demonstrating the large light matter interaction in these active intra-cavity sensing devices.

Acknowledgments

This work has been supported by the DFG Research Center for Functional Nanostructures (CFN) Karlsruhe, by a grant from the Ministry of Science, Research, and the Arts of Baden-Württemberg (Grant No. Az:7713.14-300) and by the German Federal Ministry for Education and Research BMBF (Grant No. FKZ 13N8168A). T. M.’s Young Investigator Group (YIG 08) received financial support from the Concept for the Future of the Karlsruhe Institute of Technology (KIT) within the framework of the German Excellence Initiative. T. G. gratefully acknowledges financial support of the Deutsche Telekom Stiftung, the Karlsruhe House of Young Scientists (KHYS) and the Karlsruhe School of Optics and Photonics (KSOP). M. B. C. is financially supported by the Danish Research Council for Technology and Production Sciences (Grant No. 274-09-0105). The partial support of the EC funded project NaPANIL (Contract No. NMP2-LA-2008-214249) is gratefully acknowledged. We acknowledge support by Deutsche Forschungsgemeinschaft and Open Access Publishing Fund of Karlsruhe Institute of Technology.

References and links

1.

D. Erickson, D. Sinton, and D. Psaltis, “Optofluidics for energy applications,” Nat. Photonics 5(10), 583–590 (2011). [CrossRef]

2.

H. Schmidt and A. R. Hawkins, “The photonic integration of non-solid media using optofluidics,” Nat. Photonics 5(10), 598–604 (2011). [CrossRef]

3.

X. Fan and I. M. White, “Optofluidic microsystems for chemical and biological analysis,” Nat. Photonics 5(10), 591–597 (2011). [CrossRef] [PubMed]

4.

M. Mancuso, J. M. Goddard, and D. Erickson, “Nanoporous polymer ring resonators for biosensing,” Opt. Express 20(1), 245–255 (2012). [CrossRef] [PubMed]

5.

H. Schmidt and A. R. Hawkins, “Optofluidic waveguides: I. Concepts and implementations,” Microfluid. Nanofluid. 4(1-2), 3–16 (2008). [CrossRef] [PubMed]

6.

T. Dallas and P. K. Dasgupta, “Light at the end of the tunnel: recent analytical applications of liquid-core waveguides,” TRAC-Trend. Anal. Chem. 23, 385–392 (2004).

7.

H. Li and X. Fan, “Characterization of sensing capability of optofluidic ring resonator biosensors,” Appl. Phys. Lett. 97(1), 011105 (2010). [CrossRef]

8.

L. He, S. K. Ozdemir, J. Zhu, W. Kim, and L. Yang, “Detecting single viruses and nanoparticles using whispering gallery microlasers,” Nat. Nanotechnol. 6(7), 428–432 (2011). [CrossRef] [PubMed]

9.

Y. Sun and X. Fan, “Distinguishing DNA by analog-to-digital-like conversion by using optofluidic lasers,” Angew. Chem. 124(5), 1262–1265 (2012). [CrossRef]

10.

A. M. Armani, D. K. Armani, B. Min, K. J. Vahala, and S. M. Spillane, “Ultra-high-Q microcavity operation in H2O and D2O,” Appl. Phys. Lett. 87(15), 151118 (2005). [CrossRef]

11.

N. Gopalakrishnan, K. S. Sagar, M. B. Christiansen, M. E. Vigild, S. Ndoni, and A. Kristensen, “UV patterned nanoporous solid-liquid core waveguides,” Opt. Express 18(12), 12903–12908 (2010). [CrossRef] [PubMed]

12.

K. Sagar, N. Gopalakrishnan, M. B. Christiansen, A. Kristensen, and S. Ndoni, “Photolithographic fabrication of solid–liquid core waveguides by thiol-ene chemistry,” J. Micromech. Microeng. 21(9), 095001 (2011). [CrossRef]

13.

N. Gopalakrishnan, M. B. Christiansen, and A. Kristensen, “Nanofiltering via integrated liquid core waveguides,” Opt. Lett. 36(17), 3350–3352 (2011). [CrossRef] [PubMed]

14.

G. von Freymann, A. Ledermann, M. Thiel, I. Staude, S. Essig, K. Busch, and M. Wegener, “Three-dimensional nanostructures for photonics,” Adv. Funct. Mater. 20(7), 1038–1052 (2010). [CrossRef]

15.

L. Schulte, A. Grydgaard, M. R. Jakobsen, P. P. Szewczykowski, F. Guo, M. E. Vigild, R. H. Berg, and S. Ndoni, “Nanoporous materials from stable and metastable structures of 1,2-PB-b-PDMS block copolymers,” Polymer (Guildf.) 52(2), 422–429 (2011). [CrossRef]

16.

I. M. White and X. Fan, “On the performance quantification of resonant refractive index sensors,” Opt. Express 16(2), 1020–1028 (2008). [CrossRef] [PubMed]

17.

H. Li, L. Shang, X. Tu, L. Liu, and L. Xu, “Coupling variation induced ultrasensitive label-free biosensing by using single mode coupled microcavity laser,” J. Am. Chem. Soc. 131(46), 16612–16613 (2009). [CrossRef] [PubMed]

OCIS Codes
(140.2050) Lasers and laser optics : Dye lasers
(140.7300) Lasers and laser optics : Visible lasers
(160.5470) Materials : Polymers
(140.3948) Lasers and laser optics : Microcavity devices
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: May 29, 2012
Revised Manuscript: July 9, 2012
Manuscript Accepted: July 9, 2012
Published: July 17, 2012

Citation
Tobias Grossmann, Mads Brøkner Christiansen, Jeffrey Peterson, Heinz Kalt, Timo Mappes, and Anders Kristensen, "Direct laser writing for nanoporous liquid core laser sensors," Opt. Express 20, 17467-17473 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-16-17467


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References

  1. D. Erickson, D. Sinton, and D. Psaltis, “Optofluidics for energy applications,” Nat. Photonics5(10), 583–590 (2011). [CrossRef]
  2. H. Schmidt and A. R. Hawkins, “The photonic integration of non-solid media using optofluidics,” Nat. Photonics5(10), 598–604 (2011). [CrossRef]
  3. X. Fan and I. M. White, “Optofluidic microsystems for chemical and biological analysis,” Nat. Photonics5(10), 591–597 (2011). [CrossRef] [PubMed]
  4. M. Mancuso, J. M. Goddard, and D. Erickson, “Nanoporous polymer ring resonators for biosensing,” Opt. Express20(1), 245–255 (2012). [CrossRef] [PubMed]
  5. H. Schmidt and A. R. Hawkins, “Optofluidic waveguides: I. Concepts and implementations,” Microfluid. Nanofluid.4(1-2), 3–16 (2008). [CrossRef] [PubMed]
  6. T. Dallas and P. K. Dasgupta, “Light at the end of the tunnel: recent analytical applications of liquid-core waveguides,” TRAC-Trend. Anal. Chem.23, 385–392 (2004).
  7. H. Li and X. Fan, “Characterization of sensing capability of optofluidic ring resonator biosensors,” Appl. Phys. Lett.97(1), 011105 (2010). [CrossRef]
  8. L. He, S. K. Ozdemir, J. Zhu, W. Kim, and L. Yang, “Detecting single viruses and nanoparticles using whispering gallery microlasers,” Nat. Nanotechnol.6(7), 428–432 (2011). [CrossRef] [PubMed]
  9. Y. Sun and X. Fan, “Distinguishing DNA by analog-to-digital-like conversion by using optofluidic lasers,” Angew. Chem.124(5), 1262–1265 (2012). [CrossRef]
  10. A. M. Armani, D. K. Armani, B. Min, K. J. Vahala, and S. M. Spillane, “Ultra-high-Q microcavity operation in H2O and D2O,” Appl. Phys. Lett.87(15), 151118 (2005). [CrossRef]
  11. N. Gopalakrishnan, K. S. Sagar, M. B. Christiansen, M. E. Vigild, S. Ndoni, and A. Kristensen, “UV patterned nanoporous solid-liquid core waveguides,” Opt. Express18(12), 12903–12908 (2010). [CrossRef] [PubMed]
  12. K. Sagar, N. Gopalakrishnan, M. B. Christiansen, A. Kristensen, and S. Ndoni, “Photolithographic fabrication of solid–liquid core waveguides by thiol-ene chemistry,” J. Micromech. Microeng.21(9), 095001 (2011). [CrossRef]
  13. N. Gopalakrishnan, M. B. Christiansen, and A. Kristensen, “Nanofiltering via integrated liquid core waveguides,” Opt. Lett.36(17), 3350–3352 (2011). [CrossRef] [PubMed]
  14. G. von Freymann, A. Ledermann, M. Thiel, I. Staude, S. Essig, K. Busch, and M. Wegener, “Three-dimensional nanostructures for photonics,” Adv. Funct. Mater.20(7), 1038–1052 (2010). [CrossRef]
  15. L. Schulte, A. Grydgaard, M. R. Jakobsen, P. P. Szewczykowski, F. Guo, M. E. Vigild, R. H. Berg, and S. Ndoni, “Nanoporous materials from stable and metastable structures of 1,2-PB-b-PDMS block copolymers,” Polymer (Guildf.)52(2), 422–429 (2011). [CrossRef]
  16. I. M. White and X. Fan, “On the performance quantification of resonant refractive index sensors,” Opt. Express16(2), 1020–1028 (2008). [CrossRef] [PubMed]
  17. H. Li, L. Shang, X. Tu, L. Liu, and L. Xu, “Coupling variation induced ultrasensitive label-free biosensing by using single mode coupled microcavity laser,” J. Am. Chem. Soc.131(46), 16612–16613 (2009). [CrossRef] [PubMed]

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