CUDOS@USyd Research

Optofluidics

Christian Karnutsch (team leader), Christelle Monat, Snjezana Tomljenovic-Hanic, Christian Grillet, Cameron Smith, Uwe Bog, 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.

Photonic Crystal Microfluidic Cavities
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.
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 (diagram to left, image from experiment to 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.

Figure 1. (left) Schematic illustration of the fluid infiltration process. (right) Transmission spectrum for the nanowire in contact with the uninfiltrated W1 PCS structure (top), and for the nanowire being in close proximity to a microfluidic cavity of width d = 6 µm (bottom).

Integrated Microfluidic Interferometer
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 air). The large refractive index contrast between fluid and air reduces the device footprint while mobility of the fluid/air meniscus allows the device attenuation to be tuned.

Figure 2: (top) A schematic of a general Mach-Zender interferometer, incorporating a phase delay in one arm.
(bottom) A schematic of the operation of the microfluidic interferometer.

Figure 3: The optical response of the microfluidic interferometer, displaying a characteristic resonance.

Integrated Optofluidic Refractometer
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 utilizing a fibre Bragg grating Fabry-Perot interferometer. Fluid introduced into the interferometer cavity changes the device response, enabling monitoring of 0.2% changes in refractive index.

Figure 4: A schematic showing the design of the integrated optofluidic refractometer

Figure 5: The response of the integrated optofluidic interferometer, showing an 0.2% sensitivity to refractive index

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.

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’.

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

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

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