  Thermo-optic microheater is indispensable in silicon photonic devices for
smart and reconfigurable photonic networks. Much efforts have been made to
improve the metallic microheater performance in the past decades. However,
because of the metallic nature of light absorption, placing the metallic
microheater very close to the waveguide for fast response is impractical and
has not been done experimentally. Here, we experimentally demonstrate a
metallic microheater placed very close to the waveguide based on parity-time
(PT) symmetry breaking. The intrinsic high loss of metallic heater ensures the
system will operate in the PT-symmetry-broken region, which guarantee the low
loss of light in the silicon waveguide. Moreover, heating at a close range
significantly reduces the response time. A fast response time of ~1 us is
achieved without introducing extra loss. The insertion loss is only 0.1 dB for
the long heater. The modulation bandwidth is 280 kHz, which is an order of
magnitude improvement when compared with that of the mainstream thermo-optic
phase shifters. To verify the capability of large-scale integration, a 1*8
phased array for beam steering is also demonstrated experimentally with the
PT-symmetry-broken metallic heaters. Our work provides a novel design concept
for low-loss fast-response optical switches with dissipative materials and
offers a new approach to enhance the performance of thermo-optic phase
shifters.
