  Photonic integrated circuits (PICs) with rapid prototyping and reprogramming
capabilities promise revolutionary impacts on a plethora of photonic
technologies. Here, we report direct-write and rewritable photonic circuits on
a low-loss phase change material (PCM) thin film. Complete end-to-end PICs are
directly laser written in one step without additional fabrication processes,
and any part of the circuit can be erased and rewritten, facilitating rapid
design modification. We demonstrate the versatility of this technique for
diverse applications, including an optical interconnect fabric for
reconfigurable networking, a photonic crossbar array for optical computing, and
a tunable optical filter for optical signal processing. By combining the
programmability of the direct laser writing technique with PCM, our technique
unlocks opportunities for programmable photonic networking, computing, and
signal processing. Moreover, the rewritable photonic circuits enable rapid
prototyping and testing in a convenient and cost-efficient manner, eliminate
the need for nanofabrication facilities, and thus promote the proliferation of
photonics research and education to a broader community.
