Particle accelerators are central to applications ranging from high-energy physics to medical treatments. However, the cost and size of conventional accelerators operating in radio-frequencies is prohibitive for widespread proliferation. Operating at optical and near-infrared frequencies, dielectric laser accelerators (DLAs) leverage the high damage threshold of dielectric materials, advances in nanofabrication techniques, and femtosecond pulsed lasers to produce miniaturized laser-driven accelerators. Previous demonstrations of dielectric laser acceleration have utilized free-space lasers directly incident on the accelerating structure. While this is acceptable for proof-of-principle, for DLAs to become a mature technology, it is necessary to integrate the accelerators on-chip to increase scalability and robustness of the system.
Here we demonstrate the first waveguide-integrated dielectric laser accelerator. In this scheme, a grating coupler is used to couple light from femtosecond pulsed laser to a 30 μm wide waveguide, fabricated on a silicon-on-insulator platform. The waveguide is then directly interfaced with an accelerating structure that is patterned with sub-wavelength features to produce near-fields phase-matched to electrons travelling through a vacuum-channel in the device. Both the input grating coupler and accelerator structure have been designed using the inverse design optimization approach.
We have experimentally demonstrated these waveguide-integrated accelerators by showing acceleration of subrelativistic electrons of initial energy 83.5 keV. We observe a maximum energy modulation of 1.19 keV over 30 μm. These results represent a significant step toward scalable and integrable on-chip DLAs for applications in ultrafast, medical, and high-energy technologies.
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 2x104 ps2/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.