Changing the colour of light in a silicon resonator

As the demand for high bandwidths in microelectronic systems increases, optical interconnect architectures are now being considered that involve schemes commonly used in telecommunications, such as wavelength-division multiplexing (WDM) and wavelength conversion1. In such on-chip architectures, the ability to perform wavelength conversion is required. So far wavelength conversion on a silicon chip has only been demonstrated using schemes that are fundamentally alloptical, making their integration on a microelectronic chip challenging. In contrast, we show wavelength conversion obtained by inducing ultrafast electro–optic tuning of a microcavity. It is well known that tuning the parameters of an optical cavity induces filtering of different colours of light. Here we demonstrate that it can also change the colour of light. This is an effect often observed in other disciplines, for example, in acoustics, where the sound generated by a resonating guitar string can be modified by changing the length of the strings (that is, the resonators). Here we show this same tuning effect in optics, enabling compact on-chip electrical wavelength conversion. We demonstrate a change in wavelength of up to 2.5 nm with up to 34% on–off conversion efficiency. Previous approaches to wavelength conversion on a silicon chip rely on nonlinear effects, such as cross-gain modulation, crossphase modulation, cross-absorption modulation, four-wave mixing, difference-frequency generation2–5 or free-carrier effects6. However, all of these approaches have one thing in common— they are fundamentally all-optical, that is, they operate by imparting the optical signal carried by a high-intensity pump beam onto a probe beam. In contrast, in this work we show the feasibility of a new electro–optic approach to wavelength conversion that is based on dynamic cavity changes. The dynamic cavity change can be achieved by inducing small changes in refractive index in the cavity, either optically or electrically. Here we use the free-carrier plasma dispersion effect, an electro–optic effect, to induce the dynamic change in the cavity, where the free-carrier concentration is induced using linear absorption of an optical pump. The same dispersion effect can also be induced electrically using a PIN diode, as recently demonstrated10, enabling the possibility of electrical wavelength conversion. The opto–optic experiment performed here gives a result that is equivalent to the one that would have been obtained if electrical injection of electrons and holes had been used. We recently predicted that by dynamically tuning a resonator the wavelength of the light confined in the resonator would be changed. Notomi and Mitsugi showed that the physical effect behind this wavelength conversion process is the adiabatic tuning of an oscillator, which is a classical linear phenomenon commonly observed in oscillators, such as a guitar string. This effect was first investigated by Reed et al. in their theoretical studies of photonic crystals subjected to shock waves12. In addition, it is also possible to stop light by similarly dynamically changing a coupled resonator system as was recently predicted by Yanik and Fan. The only requirement for the dynamic wavelength conversion process is that the resonator is modified in a timescale much shorter than the photon lifetime. Until recently, most onchip resonators had a photon lifetime on the order of a picosecond, making it extremely difficult to meet this requirement for the wavelength-conversion process. However, we recently demonstrated the ultrafast tuning of compact silicon ring resonators with photon lifetimes of tens of picoseconds, making the work presented here possible. The cavity used here to change the frequency of incoming light is a silicon ring resonator of diameter 6 mm, with a waveguide cross-section of 0.45 0.25 mm, similar to the one in ref. 15. Unlike the resonator in ref. 15, we use an add/drop configuration here, where an additional waveguide is added adjacent to the ring, as seen in Fig. 1e. This additional waveguide is known as the drop port. The ring resonator is measured to have a free spectral range of 29.1 nm, corresponding to a group index of ng 1⁄4 4.45 (ref. 16). The quality factor, Q, is ffil0/DlFWHM 1⁄4 18,614, where l0 1⁄4 1,564.3 nm is the resonance wavelength and DlFWHM 1⁄4 0.084 nm is the resonance full width at half maximum (FWHM). This Q-factor corresponds to a photon lifetime of tph 1⁄4 l0 /(2pcDlFWHM) 1⁄4 15.5 ps, where c is the speed of light in vacuum. In order to induce a fast dynamic change in the resonator, we induce a refractive-index change using free-carrier injection. The carrier concentration is induced using short optical pump pulses; however, the injection of carriers can also be induced electrically. As seen in Fig. 2, the optical pump is incident on the top of the ring resonator and is linearly absorbed, which generates free carriers. The free carriers cause a reduction in the ring’s refractive index, which in turn causes the resonator’s resonance to blue shift. This resonance shift causes the wavelength of the probe light confined in the resonator also to blue-shift by the same amount, as depicted in Fig. 1e. In order to show the effect of the cavity on wavelength conversion, in Fig. 1a–d we show the normalized probe transmission for four different detunings (indicated as a–d on LETTERS