PHOTONIC CRYSTALS: Nanocavity lasers exceed 100 GHz modulation speeds

Stanford University (Palo Alto, CA) researchers have demonstrated ultrafast modulation at greater than 100 GHz in photonic-crystal (PC) nanocavity lasers. In previously reported work, they had overcome a primary disadvantage of single-nanocavity PC lasers-low output power-by packing multiple lasers into 2-D arrays. Doing so enabled them to obtain output powers comparable with vertical-cavity surface-emitting lasers (VCSELs) and almost two orders of magnitude faster than state-of-the-art edge-emitting semiconductor lasers, but at lower lasing thresholds.

Now they are reporting experimental demonstration of nanocavity lasers that achieve turn-on and turn-off times as short as 1 to 2 ps. The high signal-modulation speeds were enabled by a 75-fold spontaneous-emission-rate enhancement in the cavity due to the high ratio of cavity quality factor to mode volume (this ratio is known as the Purcell factor).

Large mode volumes tend to limit Purcell-factor effects in conventional lasers. But advances in semiconductor fabrication and crystal growth have made it possible to produce high-quality PC structures with alternating refractive index and unprecedented control over the electromagnetic environment. The resulting high ratios of quality factor to mode volume have enabled the large Purcell factors needed to take advantage of spontaneous-emission effects.

Currently observed 100 GHz modulation speeds are limited by detector response times, and signal-modulation speeds potentially in the terahertz regime are indicated by results to date, according to Hatice Altug, a member of the Stanford research team led by Jelena Vučković. Such performance further indicates a potentially strong future for this technology in applications such as high-speed communications, information processing and on-chip optical interconnects.¹

Curves of output intensity versus time (top), optically modulated at repetition periods of 9 ps (I) and 15 ps (II) using a Fabry-Perot etalon to generate pump laser pulses (bottom), demonstrate on and off switching of a single-defect PC nanocavity laser. The spacing of the pulse train is controlled by the Fabry-Perot mirror separation. In this demonstration, only the first three pump pulses had sufficient power to turn on the nanocavity laser.

Ultrafast electrical modulation will be another important step toward practical devices, and the researchers expect to achieve this because electrical pumping of PC nanocavities (although not ultrafast) has been demonstrated; time constants below 10 ps have already been seen in electrical devices using micron-scale contacts with subfemtofarad capacitance, and PC nanocavity lasers do not require the highly resistive Bragg mirror layers that limit electrical modulation speeds in fast VCSELs.

REFERENCE

1. H. Altug et al., Nature Physics 2, 484 (July 2006).