Optofluidics

Alvaro Casas-Bedoya, Ross McPhedran, Benjamin Eggleton

What is Optofluidics?

Optofluidics is the marriage of two relatively new fields of science:

Micro-Photonics
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Micro-Fluidics

Similar to electronics, photonics involves the controlled transport of photons (instead of electrons) usually generated by a laser (rather than a voltage). Whilst optics, the science of large scale control of light, has been around for centuries, the micron-scale control of light, or micro-photonics, was first explored around 30 years ago. The most widely known application of micro-photonics has been the optical fibre, which now is the foundation of the global information networks, e.g. the internet.
The electronics revolution occurred after the invention of the integrated solid-state transistor. It allowed for highly compact and integrated circuits to be made with an increasing number of functions for a given chip size, eventually leading to PC’s, laptop computers, MP3 players etc. Similarly, photonics is now undergoing its own revolution in chip-based miniaturization and integration. Performing the science to enable this breakthrough is one of the goals of CUDOS.
To continue the comparison to electronics, fluidics (fluid dynamics as it is usually known) involves the controlled transportation of fluid mass driven by pressure; plumbing is one of the most familiar and oldest examples. Again, just like photonics and electronics, recent developments in miniaturization have given birth to the field of micro-fluidics: the science of fluid constrained on the micron scale. The major application of this has been the lab-on-a-chip, where large-scale laboratory reactions and diagnostic processes have been shrunk to occupy a millimetre-sized plastic chip that only uses minute fractions of samples and reagents.
So why use microfluidics in conjunction with microphotonics? The combination of these fields potentially allows one to impart adjustable photonic control in new ways that are highly compact and tuneable. We may also turn the technology around and use photonics to sense fluid properties, which is of increasing importance to medical diagnostics.
In this context, some of the recent research in the Optofluidics group are highlighted below:

Slow-light dispersion engineering of photonic crystal waveguides using selective microfluidic infiltration

infiltration

Slow light dispersion engineering in planar Photonic Crystal (PhC) waveguides is of fundamental and practical interest for enhancing light-matter interactions over substantial bandwidths and is increasingly implemented in demonstrations of all-optical signal processing. The slow light flat band in the PhC dispersion is obtained by locally modifying the effective index in the surrounding of the PhC waveguide, and altering the coupling between ‘index guided’ and ‘gap guided’ modes. This linearizes a region of the PhC waveguide dispersion curve, which corresponds to a regime of flat band, slow light.
The local effective index modification is typically obtained by varying the radius or position of the PhC air holes close to the waveguide center, on the nanometer scale, using lithographic techniques. Recently, as a flexible alternative, our group demonstrates experimentally, for the first time, a slow light dispersion engineered PhC waveguide, obtained by selectively infiltrating ionic liquids into the first two rows of air holes adjacent to the PhC waveguide. We also investigate how the contact angle between the liquid and the silicon surface changes over time due to water absorption and how this phenomenon modifies the effective refractive index and, in turn, the slow light dispersion, opening up prospects for realizing tunable slow light structures.

group index

(Top) Measured group index evolution of the slow light dispersion engineered PhC waveguide obtained by selectively infiltration of ionic liquids into the first two rows of air holes adjacent to the PhC waveguide. (Bottom) Theoretical calculation of the same structure showing the change in effective index due to the liquid water absorption.

Photonic Crystal Microfluidic Waveguides

micro pipette

Our group recently demonstrates experimentally how PhC waveguides can be directly created by selectively infiltrating high refractive index liquids into the air holes of a two dimensional silicon PhC periodic lattice. The resulting effective index contrast achieved by infiltration is large enough that a single row of infiltrated air holes allows light propagation, i.e. effectively creating a single line waveguide in an otherwise "blank" and reflective PhC. Due to the mobile nature of the infused liquid, the optofluidic PhC waveguides thus created are intrinsically reconfigurable. This approach also offers the possibility of creating more complex reconfigurable photonic circuits (e.g. including optofluidic waveguides and cavities). Our technique not only allows for the infiltration of selected air holes (position and number) with a relatively high accuracy, almost down to the single hole level, but it also enables us to control the volume of liquid introduced into the holes, through varying the "writing" speed during the infiltration process. Combined with the use of high index liquids, this controlled infiltration technique gives us an additional degree of freedom for achieving the desired optical functionality.

fluid cavity diagram

(Left) Simulated dispersion diagram for a W1 liquid infiltrated PhC (Right) Experimental transmission spectrum for the different lengths W1 liquid PhC waveguides 20µm (Dark blue) 60 µm long PhC membranes (light-blue) infiltrated at 10µm/s and empty PhC membrane (Red)

Photonic Crystal Microfluidic Cavities

nano fluidic cavity

Microcavities are very useful in applications such as telecommunications, low-threshold lasers and optical sensing, because they can potentially give rise to a dramatic enhancement of light-matter interaction over a compact space. Research into optical microcavities based on photonic crystals has attracted a lot of attention in the last years. The realization of photonic crystal microcavities has so far widely exploited structural modifications of the photonic crystal structure being introduced during the fabrication step. However, the extreme nanometre-scale precision required to realize these geometries is a limiting factor in achieving practical microcavities.

before after

To avoid this, our group at CUDOS has demonstrated a novel way of creating microcavities: post-processed and reconfigurable photonic crystal double-heterostructure cavities using selective fluid infiltration. These microcavities are formed within photonic crystals after their fabrication. Instead of exploiting a change of the periodicity of the artificial crystal, the cavities are created by selectively filling a controlled region of the photonic crystal with a liquid using a micropipette (Image from experiment to left, experiment before and after cavity creation, right). Our fluid-writing technique does not require nanometre-scale alterations in the geometry and may be undertaken at any time after photonic crystal fabrication. The reversible nature of this process offers a “rewriting” potential, paving the way for reconfigurable microphotonic devices and sensing architectures.

Integrated Optofluidic Refractometer

Basic optical components such as optical filters can be achieved in an interferometer structure, typically millimetres in length, which incorporates a phase delay in one arm. We use a novel single-beam compact microfluidic Mach-Zehnder interferometer design, where half of the beam is phase delayed (travels through fluid) before recombination with the other half (travels through the silica glass). The large refractive index contrast between fluid and silica reduces the device footprint

fluidRefraOpt

(a) 3D Schematic representation of the single beam fluid Mach–Zender interferometer. (b) 2D Schematic cross-section. (c) Experimental response measured from the ionic liquid [C2mim][I7]. Values with high noise are caused due to the low power coupling.

As lab-on-a-chip technology becomes more widely utilized, monitoring of reaction conditions becomes vital. Refractive index is a useful process parameter to monitor as it can indicate reactant concentration or the relative health of a patient. We demonstrate a chip based integrated optofluidic refractometer able to measure the dispersive properties of liquids with interesting properties
For example Ionic liquids possess a unique combination of attractive material properties for applications in thermal transfer technologies. These are essentially salts that exist as liquids at room temperature due to their bulky, asymmetric organic cations and/or anions, which make efficient stacking of ions difficult and therefore lower the lattice energy compared to typical solid salts. They are attractive due to their negligible vapour pressure and high refractive indices, up to approximately 2.1 reported for visible wavelengths for 1-Ethyl-3-methylimidazolium heptaiodide, [C2mim][I7].
In this work, our group reported the refractive index dispersion measurement of these liquids in the near-IR. We perform these measurements using a highly compact, single-beam, fluid-based Mach–Zender interferometer. This device provides an easy way to handle the liquids, and a robust and stable approach for measuring their refractive index dispersion without prior calibration step; it only requires small volumes of liquids (~25nL) and naturally isolates the liquids under test from the environment. It is therefore particularly well-suited for measuring the dispersion of liquids intended for near-IR optofluidic tunable devices.

table

Table 1. Ionic liquid refractive index measured at different time frames for the range between 1300nm and 1700nm using the interferometer.

Microfluidic Tuneable Photonic Crystal Fibre

We have shown previously that a photonic crystal fibre probed transversely acts essentially as a planar photonic crystal. If we introduce fluid into the fibre it can be moved by an external pressure (here a thin film gold heater). The fluid modifies the transmission of the photonic crystal, and, if index matched to silica, completely hides the microstructure.

fluidPcSwitch fluidPcSwitchResp


Figure 6: (left) A schematic of the fluid switchable photonic crystal fiber. We drive the device using a square wave voltage on a thin film capillary heater. (right) The spectral response of the device, showing a periodic temporal response due to the periodic driving voltage.

Optical Trapping and Optofluidic Control

Optical trapping (or optical tweezers) has seen growing adoption in biological fields for the ability to remotely manipulate cells. A strongly focused laser beam exerts a force on a dielectric particle (e.g. a cell) that traps it at the focus of the beam. We have used a trapped silica micro-sphere acting as a ball lens to steer an optical beam in the optofluidic environment. This demonstrates the possibility of manipulating other optical components ‘all-optically’.

fluidTrap


Figure 7: Schematic of the bulk optics used to trap the microsphere.

fluidTrap2


Figure 8: Schematic of the optically trapped microsphere modulating an optical beam in the microfluidic environment.


Publications:

  1. Neetesh Singh, Alvaro Casas-Bedoya, Darren D. Hudson, Andrew Read, Eric Mägi, and Benjamin J. Eggleton,
    "Mid-IR absorption sensing of heavy water using a silicon-on-sapphire waveguide,"
    Opt. Lett. 41, 5776-5779 (2016)
  2. Casas-Bedoya, A. and Shahnia, S. and Di Battista, D. and Mägi, E. and Eggleton, B. J.
    "Chip scale humidity sensing based on a microfluidic infiltrated photonic crystal,"
    Applied Physics Letters, 103, 181109 (2013)
  3. Alvaro Casas Bedoya, Christelle Monat, Peter Domachuk, Christian Grillet, and Benjamin J. Eggleton
    "Measuring the dispersive properties of liquids using a microinterferometer,"
    Applied Optics, Vol. 50, Issue 16, pp. 2408-2412 (2011)
  4. A. Casas-Bedoya, C. Husko, C. Monat, C. Grillet, N. Gutman, P. Domachuk, and B. J. Eggleton,
    "Slow-light dispersion engineering of photonic crystal waveguides using selective microfluidic infiltration,"
    Opt. Lett. 37, 4215-4217 (2012)
  5. A. Casas Bedoya, P. Domachuk, C. Grillet, C. Monat, E.C. Mägi, E. Li, and B. J. Eggleton,
    "Reconfigurable photonic crystal waveguides created by selective liquid infiltration,"
    Opt. Express 20, 11046-11056 (2012)
  6. A. Casas Bedoya, S. Mahmoodian, C. Monat, S. Tomljenovic-Hanic, C. Grillet, P. Domachuk, E.C. Mägi, B. J. Eggleton, and R. W. van der Heijden,
    "Liquid crystal dynamics in a photonic crystal cavity created by selective microfluidic infiltration"
    Optics Express 18, 27280-27290 (2010).
  7. M. Vieweg, T. Gissibl, S. Pricking, B. T. Kuhlmey, D. C. Wu, B. J. Eggleton, and H. Giessen,
    "Ultrafast nonlinear optofluidics in selectively liquid-filled photonic crystal fibers
    Optics Express Vol. 18, pp. 25232-25240 (2010).
  8. Snjezana Tomljenovic-Hanic, Adel Rahmani, M. J. Steel, and C. Martijn de Sterke
    "Comparison of the sensitivity of air and dielectric modes in photonic crystal slab sensors"
    Optics Express Vol. 17, No. 17, pp. 14552–14557 (2009).
  9. Christian Karnutsch, Cameron L. C. Smith, Alexandra Graham, Snjezana Tomljenovic-Hanic, Ross McPhedran, Benjamin J. Eggleton, Liam O'Faolain, Thomas F. Krauss, Sanshui Xiao, and N. Asger Mortensen
    "Temperature stabilization of optofluidic photonic crystal cavities"
    Appl. Phys. Lett. 94, 231114 (2009), DOI:10.1063/1.3152998
  10. Darran K. C. Wu, Boris T. Kuhlmey, and Benjamin J. Eggleton,
    "Ultrasensitive photonic crystal fiber refractive index sensor,"
    Opt. Lett. 34, 322-324 (2009)
  11. M. Ebnali-Heidari, C. Grillet, C. Monat, and B. J. Eggleton,
    "Dispersion engineering of slow light photonic crystal waveguides using microfluidic infiltration,"
    Opt. Express 17, 1628-1635 (2009)
  12. Fok S, Domachuk P, Rosengarten G, Krause N, Braet F, Eggleton BJ , Soon LL,
    "Planar microfluidic chamber for generation of stable and steep chemoattractant gradients,"
    BIOPHYSICAL JOURNAL, Volume: 95, Issue: 3 Pages: 1523-1530 AUG 1 2008
  13. U. Bog, C. L. Smith, M. W. Lee, S. Tomljenovic-Hanic, C. Grillet, C. Monat, L. O'Faolain, C. Karnutsch, T. F. Krauss, R. C. McPhedran, and B. J. Eggleton
    "High-Q microfluidic cavities in silicon-based two-dimensional photonic crystal structures,"
    Opt. Lett. 33, 2206-2208 (2008)
  14. C. L. Smith, U. Bog, S. Tomljenovic-Hanic, M. W. Lee, D. K. Wu, L. O'Faolain, C. Monat, C. Grillet, T. F. Krauss, C. Karnutsch, R. C. McPhedran, and B. J. Eggleton
    "Reconfigurable microfluidic photonic crystal slab cavities"
    Opt. Express 16, 15887-15896 (2008)
  15. C. Monat, P. Domachuk, C. Grillet, M. Collins, B. J. Eggleton, M. Cronin-Golomb, S. Mutzenich, T. Mahmud, G. Rosengarten, A. Mitchell
    "Optofluidics: a novel generation of reconfigurable and adaptive compact architectures,"
    Microfluidics and Nanofluidics, Publisher Springer Berlin / Heidelberg (2007).
  16. Cameron L. C. Smith, Darran K. C. Wu, Michael W. Lee, Christelle Monat, Snjezana Tomljenovic-Hanic, Christian Grillet, Benjamin J. Eggleton, Darren Freeman, Yinlan Ruan, Steve Madden, Barry Luther-Davies, Harald Giessen and Yong-Hee Lee
    "Microfluidic photonic crystal double heterostructures,"
    Applied Physics Letters, Volume 91, Issue 12, 121103, 17 September 2007
  17. P Domachuk, F G Omenetto, B J Eggleton and M Cronin-Golomb
    "Optofluidic sensing and actuation with optical tweezers"
    J. Opt. A: Pure Appl. Opt. vol. 9 S129-S133 (2007).
  18. C. Monat, P. Domachuk, and B. J. Eggleton
    Integrated optofluidics: A new river of light
    Nature Photonics 1, 106 - 114 (2007).
  19. Christelle Monat, Peter Domachuk, Vincent Jaouen, Christian Grillet, Ian Littler, Mark Croning-Golomb, Benjamin J. Eggleton, Simon Mutzenich, Tanveer Mahmud, Gary Rosengarten, Arnan Mitchell, " Micron-scale tunability in photonic devices using microfluidics," Invited paper, SPIE Meeting, San Diego 2006.
  20. C. Monat, C. Grillet, P. Domachuk, C. Smith, E. Magi, D. J. Moss, H. C. Nguyen, S. Tomljenovic-Hanic, M. Cronin-Golomb, B. J. Eggleton, D. Freeman, S. Madden, B. Luther-Davies, S. Mutzenich, G. Rosengarten, and A. Mitchell
    Frontiers in microphotonics: tunability and all-optical control
    Laser Physics Letters, Published Online: 7 Dec 2006
  21. S. Tomljenovic-Hanic, C. M. de Sterke, and M. J. Steel
    Design of high-Q cavities in photonic crystal slab heterostructures by air-holes infiltration
    Opt. Express 14, 12451-12456 (2006)
  22. Peter Domachuk, Eric Magi, Benjamin J. Eggleton, and Mark Cronin-Golomb,
    Actuation of cantilevers by optical trapping
    Appl. Phys. Lett. 89, 071106 (2006).
  23. P. Domachuk, I. C. M. Littler, M. Cronin-Golomb, and B. J. Eggleton
    Compact resonant integrated microfluidic refractometer
    Appl. Phys. Lett. 88, 093513 (2006)
  24. P. Domachuk, M. Cronin-Golomb, B. J. Eggleton, S. Mutzenich, G. Rosengarten, and A. Mitchell
    Application of optical trapping to beam manipulation in optofluidics
    Opt. Express 13, 7265-7275 (2005)
  25. Grillet C, Domachuck P, Eggleton B, Cooper-White J
    Optofluidics enables compact tunable interferometer
    Laser Focus World 41 (2), 100+ (2005)
  26. P. Domachuk, C. Grillet, V. Ta'eed, E. Mägi, J. Bolger, B. J. Eggleton, L. E. Rodd, and J. Cooper-White
    Microfluidic interferometer
    Applied Physics Letters, 86, 024103 (2005)
  27. Domachuk, P. Nguyen, H.C. Eggleton, B.J.
    Transverse Probed Microfluidic Switchable Photonic Crystal Fiber Devices
    Photonics Technology Letters, IEEE, 16 (8), 1900-1902 (2004)
  28. Kerbage C, Eggleton BJ
    Manipulating light by microfluidic motion in microstructured optical fibers
    Optical Fiber Technology 10 (2): 133-149 APR 2004
  29. Domachuk P, Nguyen HC, Eggleton BJ, et al.
    Microfluidic tunable photonic band-gap device
    Applied Physics Letters 84 (11): 1838-1840 MAR 15 2004
  30. H.C. Nguyen, P. Domachuk, B.J. Eggleton, M.J. Steel, M. Straub, M. Gu, M. Sumetsky
    New slant on photonic crystal fibers
    Opt. Exp. 12 (8): 1528-1539 APR 19 2004
  31. H.C. Nguyen, P.Domachuk, M.J. Steel, B.J. Eggleton
    Experimental and finite difference time domain technique characterization of transverse in-line photonic crystal fiber
    IEEE Phot. Tech. Lett. 16 (8): 1852-1854 AUG 2004
  32. C. Grillet, P. Domachuk, V. Ta'eed, E. Magi. J.A. Bolger, B.J. Eggleton, L.E. Rodd, J. Cooper-White
    Compact tunable microfluidic interferometer
    Opt. Exp. 12 (22): 5440-5447 NOV 1 2004
  33. P. Domachuk, A. Chapman, E. Magi, M.J. Steel, H.C. Nguyen, B.J. Eggleton
    Transverse characterization of high air-fill fraction tapered photonic crystal fiber
    App. Opt. 44 (19): 3885-3892 JUL 1 2005
  34. H.C. Nguyen, B.T. Kuhlmey, E.C. Magi, M.J. Steel, P. Domachuk, C.L. Smith, B.J. Eggleton
    Tapered photonic crystal fibers: properties, characterisation and applications
    Appl. Phys. B 81: 377-387 JUL 15 2005