© 2005 Nature Publishing Group Active control of slow light on a chip with photonic crystal waveguides Yurii A. Vlasov 1 , Martin O’Boyle 1 , Hendrik F. Hamann 1 & Sharee J. McNab 1 It is known that light can be slowed down in dispersive materials near resonances 1 . Dramatic reduction of the light group velocity — and even bringing light pulses to a complete halt —has been demonstrated recently in various atomic 2–5 and solid state sys- tems 6–8 , where the material absorption is cancelled via quantum optical coherent effects 3–5,7 . Exploitation of slow light phenomena has potential for applications ranging from all-optical storage to all-optical switching 9,10 . Existing schemes, however, are restricted to the narrow frequency range of the material resonance, which limits the operation frequency, maximum data rate and storage capacity 10 . Moreover, the implementation of external lasers, low pressures and/or low temperatures prevents miniaturization and hinders practical applications. Here we experimentally demon- strate an over 300-fold reduction of the group velocity on a silicon chip via an ultra-compact photonic integrated circuit using low- loss silicon photonic crystal waveguides 11,12 that can support an optical mode with a submicrometre cross-section 13,14 . In addition, we show fast (,100 ns) and efficient (2 mW electric power) active control of the group velocity by localized heating of the photonic crystal waveguide with an integrated micro-heater. The propagation of light pulses in a dielectric medium with refractive index n(l) is described by the group velocity, defined as V g ¼ c=ðn 2 ldn=dlÞ¼ c=n g , where c is the speed of light in vacuum, n is the phase refractive index, l is the wavelength, and n g is the group index. When dispersion is negative and large ðdn=dl , 0Þ, the pulses can be significantly delayed with respect to free space propagation. Slow group velocity was measured recently in photonic crystal structures with ultrafast pulse propagation techniques 15,16 . Surface coupling to slow light modes was also inferred from the absence of transmission of light at wavelengths corresponding to strong dispersion 17,18 . However, the accuracy of the group velocity determination in both of these approaches is limited, as they are relying heavily on measurements of the amplitude of the transmitted light, whereas the dispersion is inherently connected with its phase. For example, significant amplitude reshaping of short optical pulses owing to strong group velocity dispersion makes the group delay assignment progressively inaccurate 15 in the slow group velocity regime. In contrast, phase-sensitive optical techniques based on observation of interference fringes in transmission spectra were successfully used to accurately measure group indices approaching 100 (refs 14, 19). To utilize this interferometric approach, we designed and fabricated an integrated Mach–Zehnder interferometer (MZI) employing photonic crystal waveguides. Silicon photonic crystal waveguides used in our experiments are shown in Fig. 1. These are resonant photonic structures formed by etching a periodic array (periodicity a ¼ 437 nm) of holes with radius 0.25a in a 223-nm-thick silicon suspended membrane. The light is coupled to the photonic crystal waveguide through a polymer- based fibre coupler and single-mode access strip waveguide butt- coupled to the photonic crystal 11 . Details of the structural parameters and device fabrication are described in Methods. The utilization of laterally tapered spot-sized converters and careful termination of the photonic crystal lattice 19 at a position half way through the holes nearest to the waveguide (see Fig. 1a) allows efficient coupling to the LETTERS Figure 1 | SEM images of a passive unbalanced Mach–Zehnder interferometer using photonic crystal waveguides. a, Input section of the photonic crystal waveguide showing the suspended silicon membrane etched with holes and butt-coupled to a strip waveguide. The termination of the photonic crystal lattice at the coupling interface is chosen to obtain highest coupling efficiency in the slow light regime. b, Broader view of the photonic crystal waveguide membrane and input strip waveguide. After passing through a sharp 908 bend with radius R ¼ 5 mm, the mode is widened in the tapered section to better match the photonic crystal slow light mode. c, View of the input of the Mach–Zehnder interferometer (MZI) with reference (left) and signal (right) arms and a compact 158 angle Y-junction that splits the light equally between the arms. The output side of the optical circuit has an analogous Y-junction and is terminated by a single output strip waveguide. 1 IBM T.J. Watson Research Center, Yorktown Heights, New York 10598, USA. Vol 438|3 November 2005|doi:10.1038/nature04210 65