Nanophotonics Heads into the Third Dimension

Hank Hogan

Scientists at the University of California, Los Angeles, have demonstrated a way to exploit the third dimension to create tiny optical components. Going vertical should enable the fabrication of densely integrated optical structures and offer precise control over the interaction between optical elements. Moreover, the method promises total integration of optics and electronics.

The add/drop filter comprises microdisk resonators vertically coupled to waveguides. The resonant wavelength of each microdisk is determined by its size. Courtesy of Prakash Koonath.

“This processing is essentially compatible with the standard CMOS fabrication techniques,” said Prakash Koonath, a postdoctoral member of the research team.

The scientists begin by implanting oxygen ions into a silicon substrate with a photolithographically patterned SiO₂ mask atop it. They then anneal the substrate at a high temperature, repairing the implant damage and creating a buried SiO₂ layer that divides the silicon into two levels. The large difference in the refractive indices of silicon and silicon dioxide enables nanophotonics, and CMOS circuitry can be fabricated in the top silicon layer.

In a demonstration of the technique, the researchers created a 1 × 3 add/drop optical filter based on 40-, 41- and 42-μm-diameter microdisk resonators in the uppermost silicon layer coupled to waveguides in the bottom one through the buried SiO₂. The size of the microdisks determined their resonant wavelength.

They tested the filters using polarized light and found that the device worked in the 1534- to 1560-nm range, with crosstalk suppression between adjacent channels of 6.2 to 12.1 dB. Koonath noted that this is not as good as the 30 dB or so from commercially available filters, but he expects better performance after further process tweaking.

Besides add/drop filters, the technique could find application in optical delay elements. Future work will investigate combining nanophotonic structures with transistors.

Applied Physics Letters, Feb. 28, 2005, 091102.