Microfluidic Waveguide Can Be Single- or Multimode

Breck Hitz

Optofluidics — the technology created by integrating photonics and microfluidics — offers the promise of unique functionalities in ultrasmall photonic integrated circuits. Fluid-filled optical waveguides, for example, might be doped with gain media or with biological molecules to create tiny lasers or biosensors, respectively.

Figure 1. The scientists ablated a trench in the glass sample and filled it with a high-index fluid to create an optical waveguide. Images reprinted with permission of Optics Letters.

Recently, researchers in China and Japan reported what they believe is the first microfluidic optical waveguide fabricated with femtosecond laser pulses. The scientists, associated with Shanghai Institute of Optics and Fine Mechanics in China and with RIKEN in Wako, Japan, used a Ti:sapphire laser from Spectra-Physics of Mountain View, Calif., to ablate a tiny trench in a small microscope slide (Figure 1). The trench became an optical waveguide when they filled it with a liquid whose refractive index was greater than that of the surrounding glass.

One of the waveguides they fabricated was a 90° arc with a 5-mm radius of curvature (Figure 2). The Ti:sapphire laser, operating at 1 kHz and 800 nm, produced 7.1-μJ, 40-fs pulses that they focused to an ~2-μm spot on the surface of the glass sample with a microscope objective. They translated the glass sample with a scanning speed of 160 μm/s and repeated the scan 25 times to fabricate the curved trench shown in the figure.

Figure 2. One of the waveguides was a 90° arc with a 5-mm radius of curvature. The trench was about 7 μm wide at the top and 16 μm deep (inset).

By filling the trench with varying mixtures of two liquids with different refractive indices (paraffin, n = 1.474, and α-bromnaphtalene, n = 1.658), they varied the waveguide core’s refractive index from 1.52 to 1.658.

To evaluate the optical properties of the microfluidic waveguide, they illuminated the input port with a HeNe laser (Figure 3). As can be seen in the figure, most of the scattering occurs along the edges of the waveguide. The scientists believe that most of the total propagation loss, which they estimate to be 3 to 5 dB/cm, is caused by imperfections in the waveguide’s fabrication. They found it difficult to keep the focus of the Ti:sapphire laser precisely on the glass surface during fabrication, so the waveguide probably was not of uniform cross section over its entire length. Such nonuniformity could introduce significant loss.

Figure 3. HeNe laser light entered the microfluidic waveguide at top right and emerged at bottom left. By changing the refractive index of the fluid filling the channel, the scientists could make it function as either a single- or multimode waveguide.

By observing the spatial profile of the light emerging from the far end of the waveguide, they concluded that fluids with a refractive-index contrast that was high in comparison with that of glass allowed multimode propagation in the waveguide, whereas fluids with a lower contrast forced single-mode propagation.

They noted that it was somewhat surprising that an asymmetrical waveguide such as the trench pictured in Figure 2 could support single-mode propagation, but they explained that the bottom portion of the trench is too small to guide light, so guiding occurs only in the more-or-less symmetric upper portion of the trench.

Optics Letters, June 1, 2007, pp. 1536-1538.