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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 22 — Oct. 24, 2011
  • pp: 22242–22247
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Coupled optofluidic ring laser for ultrahigh- sensitive sensing

Xingwang Zhang, Liqiang Ren, Xiang Wu, Hao Li, Liying Liu, and Lei Xu  »View Author Affiliations


Optics Express, Vol. 19, Issue 22, pp. 22242-22247 (2011)
http://dx.doi.org/10.1364/OE.19.022242


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Abstract

Ultrahigh sensitivity is achieved in a new active sensor structure: coupled optofluidic ring laser. The sensor consists of one ring laser and one optofluidic tube. The emission intensity of the multimode whispering gallery resonance from the coupled ring laser is strongly modulated. By using the optofluidic tube as the sensing element, and monitoring the envelope shift of the modulated lasing spectrum, we achieved a sensitivity of 5930 nm/RIU, which is two orders of magnitude higher than a conventional ring resonator sensor.

© 2011 OSA

1. Introduction

In this paper, we report on ultrahigh sensitivity of a coupled optofluidic ring laser (CORL). The new sensor consists of one ring laser (master resonator) and one optofluidic tube (slave resonator). The optofluidic tube also serves as sensing element. Majority portion of the WG mode distributes in the tube, therefore S~1. Combining large M and S, very high sensitivity is expected. Experimentally, a sensitivity of 5930 nm/RIU was achieved, which is two orders of magnitude higher than conventional ring resonator sensors, and one order of magnitude higher than evanescent field engineered ring resonator sensors.

2. Experiment method and results

The fabrication process of CORL is basically the same as in our earlier works [13

13. X. W. Zhang, H. Li, X. Tu, X. Wu, L. Y. Liu, and L. Xu, “Suppression and hopping of whispering gallery modes in multiple-ring-coupled microcavity lasers,” J. Opt. Soc. Am. B 28(3), 483–488 (2011). [CrossRef]

]. Two commercial glass fibers (125 μm diameter, 10 cm long) were used. One fiber was coated with a thin film (refractive index n = 1.52, thickness 2 μm). The coating materials were rhodamine B (RhB) dye doped organic/inorganic hybrid glass materials. The other fiber was treated in Trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane to form a hydrophobic surface. The two fibers were then stacked in parallel with each other and sealed in polydimethylsiloxane (PDMS, refractive index n = 1.42), and annealed at 120 °C for 1 hour. After annealing, the hydrophobic treated fiber was pulled out from the PDMS, leaving in the matrix a hollow tube which serves as the optofluidic channel. Figure 1(a)
Fig. 1 (a) The fabrication process of an optofluidic coupled cavity; (b) The cross-section of the coupled cavity.
is the schematic fabrication process of the coupled cavity. Figure 1(b) shows the schematic cross-section of the coupled cavity. When the fluidic channel was connected with a syringe pump and dimethyl sulfoxide (DMSO, refractive index n = 1.477) flows through the channel, the RhB coated fiber and the fluidic channel form a CORL. The two cavities in the device are 129 μm and 125 μm in diameter. Fluid flow rate was kept at 2 μl/minute in experiment.

The CORL was pumped by a 532 nm mode-locked laser (30 ps pulse width, 10 Hz repetition rate). The emitted laser light was collected by a multimode fiber (1 mm in diameter) and was transmitted to a spectrometer which was equipped with a scientific CCD detector.

The emission spectra from a CORL are shown in Fig. 2
Fig. 2 Emission spectra from a 129 μm/125 μm coupled cavity. Upper plot: optofluidic channel is empty; Lower plot: DMSO flows through the optofluidic channel, red dash line is a Lorentz-shape fitted envelope.
. When the optofluidic channel is empty (air filled), it does not support WG mode resonance, the RhB ring works as a circular oscillator, and its multimode lasing spectrum forms a single broad band envelope. When DMSO flows through the optofluidic channel, WG modes exist in both the RhB ring and the optofluidic tube, the device works as a coupled ring laser. As a result, we saw strong spectrum modulation. The emission envelope can be fitted well with a Lorentz shape, envelope center can thus be derived.

We examined the lasing spectrum change when different fluid passed through the fluidic channel. We used mixture of DMSO/water to adjust the refractive index of the fluid in a step of ~0.0004. When the fluid refractive index increases, the modulated spectrum envelope substantially shifts to shorter wavelength (see Fig. 3
Fig. 3 Emission spectra of a coupled optofluidic ring laser when DMSO/water flows through the optofluidic channel. From top to bottom, the emission envelope moves to shorter wavelength when fluid refractive index increases. Red arrow indicates the envelope shift.
). Figure 4
Fig. 4 Fitted envelope center versus fluid refractive index. A linear fitting gives a sensitivity of 5930 ± 360 nm/RIU.
plots the changes of envelope center wavelength versus refractive index of the fluid. A linear fitting gives a giant refractive index sensitivity of 5930 ± 360 nm/RIU.

The ultrahigh sensitivity of the coupled optofluidic ring laser comes from two parts: Vernier effect induced significant amplification of spectral shift (M) and large ratio of effective refractive index/fluid refractive index change (S). In Ref. [9

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

], S was optimized by controlling the capillary wall thickness, consequently, sensitivity rose to 570 nm/RIU. In the present work, by using coupled cavity structure, sensitivity was further improved by one order of magnitude when envelope center of the modulated spectrum was monitored.

The detection limit (DL) of the CORL is determined by accuracy in the modulated spectrum envelope center measurement. In experiment, for each refractive index of fluid, we take one spectrum every minute. As it is shown in Fig. 7
Fig. 7 Fluctuation of the envelope center versus time for different refractive indices of fluid.
, the fitted envelope center fluctuates slightly, because the pump laser power fluctuation is over 10% therefore the resonant mode intensities vary. The mean standard deviation, δ, is 0.16 nm, leading to a bulk refractive index noise equivalent detection limit (NEDL) of 2.7 × 10−5 RIU. Lower DL is expectable, if the modulation spectrum becomes more stable by using more stable pump light, or by generating passive modulation spectrum instead of active modulation spectrum which depends nonlinearly on pump power. If intensity fluctuation can be lowered down efficiently, the accuracy in determining modulated spectrum envelope center will be only limited by spectral resolution. For a spectrometer resolution of 0.01 nm (as in our experiment), envelope center uncertainty is 0.004nm after multi-peak fitting. In that case, a DL of 7 × 10−7 RIU can be reached. Better spectrometer resolution will help in reducing DL furthermore, nevertheless, we would like to emphasize that rapid and convenient detection with high sensitivity are the key advantages of a CORL sensor.

In the present work, the fluidic channel forms in PDMS (n = 1.42), thus the coupled cavity works only for fluids that have higher refractive indices. However, an alternative coupled cavity structure is a ring laser coupling with a fluidic glass capillary tube. In that case, the device can be exposed in air, and water can be used as the fluid. Hence CORL can be a high sensitivity bio-sensor.

3. Conclusions

In summary, very large sensitivity was achieved in a coupled optofluidic ring laser. Sensitivity as high as 5930 nm/RIU is obtained experimentally and agrees well with theoretical calculation. The new sensing scheme and senor structure open a new way in achieving ultrahigh sensitive bio and chemical sensing.

Acknowledgments

This work is supported in part by National Natural Science Foundation of China (grant # 10874033, 60977047, 60907011, 61078052, 11074051), National Basic Research Program of China (973 Program) (grant # 2011CB921802) and Natural Science Foundation of Shanghai (grant # 09ZR1402800).

References and links

1.

F. Vollmer and S. Arnold, “Whispering-gallery-mode biosensing: label-free detection down to single molecules,” Nat. Methods 5(7), 591–596 (2008). [CrossRef] [PubMed]

2.

J. G. Zhu, S. K. Ozdemir, Y.-F. Xiao, L. Li, L. He, D.-R. Chen, and L. Yang, “On-chip single nanoparticle detection and sizing by mode splitting in an ultrahigh-Q microresonator,” Nat. Photonics 4(1), 46–49 (2010). [CrossRef]

3.

F. Vollmer, S. Arnold, and D. Keng, “Single virus detection from the reactive shift of a whispering-gallery mode,” Proc. Natl. Acad. Sci. U.S.A. 105(52), 20701–20704 (2008). [CrossRef] [PubMed]

4.

A. M. Armani, R. P. Kulkarni, S. E. Fraser, R. C. Flagan, and K. J. Vahala, “Label-free, single-molecule detection with optical microcavities,” Science 317(5839), 783–787 (2007). [CrossRef] [PubMed]

5.

N. M. Hanumegowda, C. J. Stica, B. C. Patel, I. White, and X. D. Fan, “Refractometric sensors based on microsphere resonators,” Appl. Phys. Lett. 87(20), 201107 (2005). [CrossRef]

6.

I. M. White, H. Zhu, J. Suter, N. M. Hanumegowda, H. Oveys, M. Zourob, and X. D. Fan, “Refractometric sensors for lab-on-a-chip based on optical ring resonators,” IEEE Sens. J. 7(1), 28–35 (2007). [CrossRef]

7.

I. Teraoka and S. Arnold, “Enhancing the sensitivity of a whispering-gallery mode microsphere sensor by a high-refractive-index surface layer,” J. Opt. Soc. Am. B 23(7), 1434–1441 (2006). [CrossRef]

8.

F. Xu, P. Horak, and G. Brambilla, “Optical microfiber coil resonator refractometric sensor,” Opt. Express 15(12), 7888–7893 (2007). [CrossRef] [PubMed]

9.

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

10.

A. Francois and M. Himmelhaus, “Whispering gallery mode biosensor operated in the stimulated emission regime,” Appl. Phys. Lett. 94(3), 031101 (2009). [CrossRef]

11.

L. Shang, L. Y. Liu, and L. Xu, “Single-frequency coupled asymmetric microcavity laser,” Opt. Lett. 33(10), 1150–1152 (2008). [CrossRef] [PubMed]

12.

H. Li, L. Shang, X. Tu, L. Y. 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]

13.

X. W. Zhang, H. Li, X. Tu, X. Wu, L. Y. Liu, and L. Xu, “Suppression and hopping of whispering gallery modes in multiple-ring-coupled microcavity lasers,” J. Opt. Soc. Am. B 28(3), 483–488 (2011). [CrossRef]

14.

C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (John Wiley & Sons, Inc., 1998).

OCIS Codes
(130.6010) Integrated optics : Sensors
(140.3945) Lasers and laser optics : Microcavities
(140.3948) Lasers and laser optics : Microcavity devices
(230.4555) Optical devices : Coupled resonators

ToC Category:
Coupled Resonators

History
Original Manuscript: June 30, 2011
Revised Manuscript: August 22, 2011
Manuscript Accepted: August 26, 2011
Published: October 24, 2011

Virtual Issues
Vol. 6, Iss. 11 Virtual Journal for Biomedical Optics
Collective Phenomena (2011) Optics Express

Citation
Xingwang Zhang, Liqiang Ren, Xiang Wu, Hao Li, Liying Liu, and Lei Xu, "Coupled optofluidic ring laser for ultrahigh- sensitive sensing," Opt. Express 19, 22242-22247 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-22-22242


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References

  1. F. Vollmer and S. Arnold, “Whispering-gallery-mode biosensing: label-free detection down to single molecules,” Nat. Methods5(7), 591–596 (2008). [CrossRef] [PubMed]
  2. J. G. Zhu, S. K. Ozdemir, Y.-F. Xiao, L. Li, L. He, D.-R. Chen, and L. Yang, “On-chip single nanoparticle detection and sizing by mode splitting in an ultrahigh-Q microresonator,” Nat. Photonics4(1), 46–49 (2010). [CrossRef]
  3. F. Vollmer, S. Arnold, and D. Keng, “Single virus detection from the reactive shift of a whispering-gallery mode,” Proc. Natl. Acad. Sci. U.S.A.105(52), 20701–20704 (2008). [CrossRef] [PubMed]
  4. A. M. Armani, R. P. Kulkarni, S. E. Fraser, R. C. Flagan, and K. J. Vahala, “Label-free, single-molecule detection with optical microcavities,” Science317(5839), 783–787 (2007). [CrossRef] [PubMed]
  5. N. M. Hanumegowda, C. J. Stica, B. C. Patel, I. White, and X. D. Fan, “Refractometric sensors based on microsphere resonators,” Appl. Phys. Lett.87(20), 201107 (2005). [CrossRef]
  6. I. M. White, H. Zhu, J. Suter, N. M. Hanumegowda, H. Oveys, M. Zourob, and X. D. Fan, “Refractometric sensors for lab-on-a-chip based on optical ring resonators,” IEEE Sens. J.7(1), 28–35 (2007). [CrossRef]
  7. I. Teraoka and S. Arnold, “Enhancing the sensitivity of a whispering-gallery mode microsphere sensor by a high-refractive-index surface layer,” J. Opt. Soc. Am. B23(7), 1434–1441 (2006). [CrossRef]
  8. F. Xu, P. Horak, and G. Brambilla, “Optical microfiber coil resonator refractometric sensor,” Opt. Express15(12), 7888–7893 (2007). [CrossRef] [PubMed]
  9. H. Li and X. D. Fan, “Characterization of sensing capability of optofluidic ring resonator biosensors,” Appl. Phys. Lett.97(1), 011105 (2010). [CrossRef]
  10. A. Francois and M. Himmelhaus, “Whispering gallery mode biosensor operated in the stimulated emission regime,” Appl. Phys. Lett.94(3), 031101 (2009). [CrossRef]
  11. L. Shang, L. Y. Liu, and L. Xu, “Single-frequency coupled asymmetric microcavity laser,” Opt. Lett.33(10), 1150–1152 (2008). [CrossRef] [PubMed]
  12. H. Li, L. Shang, X. Tu, L. Y. 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]
  13. X. W. Zhang, H. Li, X. Tu, X. Wu, L. Y. Liu, and L. Xu, “Suppression and hopping of whispering gallery modes in multiple-ring-coupled microcavity lasers,” J. Opt. Soc. Am. B28(3), 483–488 (2011). [CrossRef]
  14. C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (John Wiley & Sons, Inc., 1998).

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