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Optical Design & Engineering

Using fluids to manipulate nanoscale optics

Silicon nanophotonics combined with nanofluidics can create large refractive-index modulation and optofluidically address parts of photonic structures at sub-wavelength distances.
19 September 2006, SPIE Newsroom. DOI: 10.1117/2.1200608.0117

Optical devices that incorporate liquids as a fundamental part of their structures can be traced as far back as the 18th century, when rotating pools of mercury were proposed as a simple technique to create smooth spherical mirrors for use in reflecting telescopes. Modern microfluidics1,2 enabled the development of a present-day equivalent centered on the marriage of fluidics and optics: optofluidics. Such devices have substantial advantages for creating adaptive optical elements including high refractive-index modulation, inherently smooth optical interfaces, and thermal stabilization.

Photonic crystals3 are attractive for controlling optical propagation by introducing pre-engineered defects into an otherwise regular lattice: for example, to create waveguides, resonant cavities, or filters. At present, techniques for locally modulating the refractive index in silicon photonic structures exploit relatively weak nonlinearities, and thus require either long interaction lengths, high power, or resonant elements to enhance the effect. Although tuning techniques such as mechanical deformation, thermo-optics, liquid-crystal infusion, and others have been developed, they rely on globally modifying the optical properties of the element. Nanofluidics provides a solution that enables both localized control and high refractive-index modulation.

As a first step toward creating 2D reconfigurable photonic circuits, we have developed a technique for integrating nanofluidics made using soft lithography with silicon nanophotonics and demonstrate that we could use the device to address a single row of holes within a photonic crystal.4Figure 1 illustrates our fabrication and integration approach, which follows a three-level architecture: the nanophotonic level; the nanofluidic-delivery level, which delivers liquids directly into the photonic structure; and the microfluidics control engine, which manipulates the fluids. This multiscale approach allows the device to perform fluidic operations—specifically switching and mixing—at lengths on the scale of tens of microns. The short distances allow operations to be accomplished rapidly, while still enabling precise fluidic addressing of individual elements at scales limited only by the size of the nanofluidic element. In this case, our channel cross-section measured 350nm wide by 200nm tall.

Figure 1. One microscale device combines nanophotonics and fluidics. (a) The nanophotonics structure is a photonic crystal with numerous wells. Both (b) the nanofluidics structure and (c) the valve-control layer are made using soft lithography. (d) The layers are assembled into (e) a device about the length of a dime. SOI: Silicon on insulator. PDMS: Polydimethylsiloxane. (Click to enlarge.)

To directly examine the channel alignment and seal integrity at the nanoscale, we infused a surfactant solution into the fluidics and allowed the solvent to evaporate overnight. Figure 2(a) shows the system after removal of the soft-lithography fluidics. The conformable nature of the silicone elastomer polydimethylsiloxane (PDMS) ensures that fluid remains confined to the targeted row of holes within the photonic structure.

Figure 2. Transmission through a photonic crystal is tuned by changing the liquid filling one row of the structure. (a) First, subwavelength alignment, sealing, and complete infusion of the wells with deionized water are performed. (b) Switching is then achieved by replacing the water with a salt (5M CaCl2) solution. (c) Transmission spectra through photonic crystal for water and CaCl2 show a wavelength offset. (d) Switching between water and CaCl2 at a/λ = 0.291 results in variable transmission, where a is the lattice constant of the photonic crystal. (Click to enlarge.)

To illustrate that we can dynamically tune the optical transmission properties of the structure, we infused the system with alternating solutions of deionized water (which has a refractive index of about 1.33) and 5M CaCl2 (with refractive index of about 1.44) as shown in Figure 2(b).

We used a tunable infrared laser source, the output of which was coupled—using a tapered fiber lens—into the ridge waveguide that corresponded to the fluidically aligned photonic crystal. Figure 2(c) shows the normalized quasi-transverse-electric-mode transmission through the structure for both liquids.

The peak transmission wavelength of the guided mode shifts from 1491nm to 1502nm—which in Figure 2(c) corresponds to a/λ = 0.291 and a/λ = 0.289—when the higher-index salt solution displaces the lower-index water. We modulated the transmitted power by switching between the solution and water, as shown in Figure 2(d).

Extensions of this technique could be used create fully reconfigurable photonic devices. Because fluidically-defined defects can activate passive structures, we could arbitrarily redefine the functionality of the optics. Other possibilities include using the microfluidics to deliver optical gain media, nonlinear liquids, or colloidal particles into arbitrary regions of these structures. Such integration could also enable a new class of resonant-cavity sensors that incorporate targeted delivery of a single or a few molecules.

Demetri Psaltis
Electrical Engineering, California Institute of Technology
Pasadena, CA

Dr. Psaltis is the Thomas G. Myers Professor of Electrical Engineering at the California Institute of Technology. His research interests are in the areas of holography, optical memories, optical information processing, optofluidics, nonlinear optics and holography. He has authored or co-authored over 455 publications in these areas.

David Erickson
Mechanical and Aerospace Engineering, Cornell University
Ithaca, NY

David Erickson is a faculty member of Cornell University whose research interests involve the fundamental and applied study of micro- and nano-fluidics as well as their application to optofluidics, integrated microfluidic devices, and electrokinetics.

1. G. Whitesides, A. Stroock, Flexible methods for microfluidics,
Physics Today,
Vol: 54, pp. 42-48, 2001.
3. J. Joannopoulos, R. Meade, J. Winn,
Photonic Crystals: Molding the Flow of Light,
Princeton University Press, Princeton, New Jersey, 1995.