Highly efficient and ultra small Bragg mirrors

Silicon photonics technology offers promising low-cost optoelectronic solutions for many applications, ranging from optical communications to emerging areas such as optical sensing and analysis. Silicon is an ideal material platform for integrated photonics because of the maturity it has reached in the electronics industry, offering the possibility to combine photonic and electronic devices on one chip.

In recent years, rapid progress has been made in developing various silicon-based photonics building blocks, in particular optical microcavities, which confine light in wavelength-sized modal volumes, V. Microcavities are essential components for many optical devices and effects, from spontaneous-emission enhancement, threshold laser reduction, wavelength conversion, and a wide range of processing functions in integrated systems.¹ For all of these applications, high reflectivity combined with low absorption and scattering losses are critical to achieving a high quality factor (Q) and a correspondingly sharp resonance.

We have made microcavities on a silicon-on-insulator substrates using standard silicon fabrication techniques, as shown in the scanning-electron micrograph in Figure 1. As in ring resonators, light is confined in the transverse directions by using a classical, translation-invariant, ridge waveguide. Longitudinal confinement is achieved within the waveguide by mirrors that consist of subwavelength holes, as in photonic-crystal cavities. The mirrors are not fully periodic. Instead, through a fine geometry tuning designed with Bloch-mode-engineering concepts,^2,3 they also incorporate a taper section composed of four holes. The taper section reduces the transverse-mode-profile mismatch at the interface between the periodic section of the mirrors and the ridge waveguide, and thus reduces out-of-plane radiation losses.

Figure 1. Scanning electron micrograph of an ultra small cavity fabricated in a silicon ridge waveguide. Measured Q values are in the range of 100,000 at telecommunication wavelengths, and the mode volume is V=0.05μm³. The taper hole sections are composed of four holes with different diameters and separation-distances. (Fabrication courtesy David Peyrade, Laboratoire des Technologies de la Microélectronique.)

The design of the taper sections has been a key feature for achieving high Q. In 2006, transmission measurements⁴ on cavities with four holes in the mirror periodic sections had a Q of 10,000 at telecommunication wavelengths (λ =1.55 μm). At that time, the measured Q represented a 20-fold improvement over previous results obtained with similar in-line geometries.⁵ Over the past two years, the Q has been boosted by one further order of magnitude with essentially the same design^6,7 to reach a value of 110,000.⁸ Note that an experimental Q factor of 10⁵ implies a modal reflectance as large as 0.9998 for the mirror illuminated by the fundamental mode of the ridge waveguide.⁴

In comparison to photonic-crystal microcavities fabricated on semiconductor membranes (air-clad on both sides), the present cavities are mechanically robust, making them attractive for integration in large systems. They can be considered as the photonic-crystal counterparts of the classical ring resonators, but with much smaller mode volumes and footprints.

Ultra small microcavities that durably trap photons in small volumes have many applications in modern optics and various related fields. So far we have shown that these cavities can be manufactured on fully standard silicon-on-insulator wafers. Our next step will be to show that these new cavities can be coupled together to slow down light and to implement optical delay lines and switches.