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A Squirt of Water Writes Waveguides

Photonics Spectra
Jan 2007
Customizing photonic crystals may lead to sophisticated integrated photonic devices.

Breck Hitz

Photonic integrated circuits will surely replace electronic chips in the coming decades, and although simple versions have already entered the commercial mainstream and have earned their own acronyms, more complex and elegant devices exist only on the drawing board of engineers and scientists. Many of the most appealing of these hypothetical designs are based on photonic crystals, but the real-world difficulties of fabricating the intricate devices have loomed as an insurmountable barrier.

Now a team of European scientists has developed a technique for manufacturing photonic crystal circuits that may lead to advanced but inexpensive photonic integrated circuits.

In a photonic crystal, a periodic variation of the refractive index creates a photonic bandgap, where photons of a given energy cannot propagate, analogous to the electronic bandgap in a semiconductor. Scientists around the world have demonstrated a variety of devices in photonic crystals, including lasers, spectral filters and optical switches. The chief advantage of these devices over their conventional counterparts is their tiny size, which can enable an enormous packing density of devices in an integrated circuit.

But the periodic variation of a photonic crystal’s refractive index must be on a subwavelength scale, making the devices difficult and expensive to fabricate. The European collaborators, including scientists from the European Laboratory for Non-Linear Spectroscopy in Florence and from Università di Trento, both in Italy, and from the University of Paderborn in Germany, have demonstrated a technique that is capable not only of implementing complex photonic integrated circuits relatively easily, but also of rewriting the circuits after they have been fabricated.

The method is based on squirting water into ~1-μm-diameter airholes etched into bulk silicon (Figure 1). The water creates a defect in the photonic crystal that defines a waveguide. But injecting water into selected holes — and not into adjacent ones — is not as easy as it sounds. The scientists used a commercial micropipette from Eppendorf International of Hamburg, Germany, with an outer diameter of less than a micron, which they could maneuver with 0.1-μm precision across the surface of the etched silicon.

Figure 1. Scientists created a defect in the silicon photonic crystal by injecting water into selected holes (a). They monitored the process by observing both the reflection of the silicon surface (b) and the luminescence of a dye dissolved in the water (c). In the latter case, they added a filter to eliminate any residual excitation light reflected by the dichroic mirror. CLSM = confocal laser scanning microscope. Images reprinted with permission of Applied Physics Letters.

Because capillary forces rule in this minuscule scale, the scientists could deposit water into the holes only when the liquid meniscus was in contact with the sample. Perhaps surprisingly, they found no problem with air trapped at the bottoms of the holes. They speculate that the water must fill a hole by running down the sides, filling it from the bottom up.

Rather than pure water, the researchers used a solution of rhodamine 6G dye in water, which enabled them to make two images of the solution-filled holes. Using a confocal laser scanning microscope, they made straightforward reflection images simply by monitoring the light backreflected with a beamsplitter (Figure 1b). Alternatively, they imaged the dye’s luminescence by replacing the beamsplitter with a dichroic mirror (Figure 1c).

These images convinced the scientists that they were filling the target holes with water and leaving adjacent holes dry (Figure 2). Moreover, by observing a wavelength shift that occurred when the water evaporated, leaving behind dried dye, they concluded that any liquid accidentally splattered on the surface of the silicon would evaporate in a day, while the water was retained in the holes for a much longer period.

Figure 2. An image of light reflected from the surface of the silicon matrix in Figure 1 shows a periodic array of holes with triangular symmetry and a 1.5-μm lattice constant (a). An image of the same surface area is filtered to show only the luminescence of the dye dissolved in water (b). Clearly, the water has filled only a single hole, and the adjacent holes are dry.

As an indication of the possibilities opened by the technique, the investigators fabricated an S-shaped waveguide by repeating a pattern of four water-filled holes in the block of silicon (Figure 3). They calculated the fields of modes that would be guided and found that light could propagate around the sharp curves without loss.

Figure 3. The scientists fabricated an S-shaped waveguide by filling selected holes in the bulk silicon with water. The image shows luminescence from the water-dissolved dye in the holes (a). They calculated the spatial distribution of the magnetic field that would occur for one of the modes guided in this waveguide (b).

The scale of the structure in Figure 3 makes it suitable for light in the 9-μm spectral range, but there is greater interest, for both communication and information-technology applications, in shorter wavelengths. However, the researchers believe that straightforward modifications of their approach — invoking readily available micropipettes 0.5 μm in diameter or smaller, and perhaps imaging with ultraviolet wavelengths — would make it possible to inject water into pores of 200 to 600 μm, typical of photonic crystals that operate in the 1.5-μm range.

Applied Physics Letters, Nov. 20, 2006, 211117.

The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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