Towards a Photonic Crystal Mode-Locked Laser

For a given average power, the energy per pulse of a mode-locked laser increases with increasing cavity length, lowering the repetition rate. Photonic crystal slow light optical waveguides can be used to address the high repetition rates and resulting low pulse energies of conventional semiconductor lasers by substantially increasing the effective optical cavity length while keeping the device compact. Such a device could enable a semiconductor laser to power two-photon microscopy, an advanced non-linear technique for time-resolved deep-tissue imaging. We present a design for realizing a monolithic two-segment quantum dot passively mode-locked photonic crystal laser. The cavity consists of a novel photonic crystal waveguide designed for low dispersion and wide bandwidth by engineering the photonic crystal lattice structure. Group velocity dispersion of 2x10(4) ps(2)/km, more than an order of magnitude lower than similar dispersion engineered photonic crystal waveguides, is achieved over 2% bandwidth, more than sufficient for mode-locking. Gain is achieved by optically pumping epitaxially grown InAs/GaAs quantum dots in part of the photonic crystal waveguide, and the saturable absorber section is reversed biased to enable pulse shaping. A cladding scheme is used to apply reverse bias to the saturable absorber and shorten its recovery time. Devices are fabricated using a combination of electron beam lithography, anisotropic etching, and selective under-etching processes, similar to standard photonic crystal waveguides. The low-dispersion, wide bandwidth waveguide, combined with the fast dynamics of InAs quantum dots could enable a compact, low repetition rate mode-locked laser to be realized.