Compound-photonic structures with gain and loss 1 provide a powerful platform for testing various theoretical proposals on non-Hermitian parity-time-symmetric quantum mechanics 2-5 and initiate new possibilities for shaping optical beams and pulses beyond conservative structures. Such structures can be designed as optical analogues of complex parity-timesymmetric potentials with real spectra. However, the beam dynamics can exhibit unique features distinct from conservative systems due to non-trivial wave interference and phase-transition effects. Here, we experimentally realize paritytime-symmetric optics on a chip at the 1,550 nm wavelength in two directly coupled high-Q silica-microtoroid resonators with balanced effective gain and loss. With this composite system, we further implement switchable optical isolation with a non-reciprocal isolation ratio from 28 dB to 18 dB, by breaking time-reversal symmetry with gain-saturated nonlinearity in a large parameter-tunable space. Of importance, our scheme opens a door towards synthesizing novel microscale photonic structures for potential applications in optical isolators, on-chip light control and optical communications.One of the most fundamental postulates in canonical quantum mechanics, formulated by Dirac and von Neumann, mandates that the Hermiticity of each operator be directly associated with a physical observable. As such, the spectrum of a self-adjoint operator is ensured to be real and the total probability (or unitary evolution) is conserved. In 1998, however, Bender and colleagues 2 discovered a wide class of complex non-Hermitian Hamiltonians that can possess entirely real spectra below a certain phase-transition point, provided they satisfy combined parity-time (PT) symmetry. This counterintuitive discovery immediately aroused extensive theoretical interest in extending canonical quantum theory by including non-Hermitian but PT-symmetric operators 2-5 . For instance, a PT-symmetric Hamiltonian operator may contain a complex potential V(x) subject to a spatial-symmetry constraint V(x) ¼ V*(2x). One of the most striking properties of a PT-symmetric operator stems from the appearance of a sharp, symmetrybreaking transition once a non-Hermitian operator crosses a certain critical threshold 2-5 . On crossing that 'exceptional point', the spectrum ceases to be real and starts to become complex. This transition signifies the appearance of a spontaneous PT symmetry breaking from the exact-to the broken-PT phase. Despite much fundamental theoretical success in the development of PT-symmetric quantum mechanics, an experimental observation of pseudo-Hermiticity remains elusive and very challenging in real physical settings. Thanks to the formal equivalence between the quantum-mechanical Schrödinger equation and the paraxial optical diffraction equation, complex PT-symmetric potentials can be easily achieved in optics by spatially modulating the refractive index with properly placed gain and loss in a balanced manner 1 . This analogy immediately spurred theoretica...

In this work, we combine the large per-photon optical gradient force with the sensitive feedback of a high quality factor whispering-gallery microcavity. The cavity geometry, consisting of a pair of silica disks separated by a nanoscale gap, shows extremely strong dynamical backaction, powerful enough to excite coherent oscillations even under heavily damped conditions (mechanical Q approximately 4). In vacuum, the threshold for regenerative mechanical oscillation is lowered to an optical input power of only 270 nW, or roughly 1000 stored cavity photons, and efficient cooling of the mechanical motion is obtained with a temperature compression factor of nearly 14 dB with an input optical power of only 11 microW.

We demonstrate optical resonance from microfiber knots obtained by manipulating freestanding silica microfibers. Q factors as high as 57 000 with finesse of 22 are observed in knots with sizes less than 1mm. The free spectral range of the resonator can be easily tuned by tightening the knot structure in air. The knot resonators are highly stable in water with Q factors up to 31 000 and finesse of 13. The possibility of supporting the knot resonator with a solid MgF2 substrate is also demonstrated.

Oxidation from alcohols to carboxylic acids, a class of essential chemicals in daily life, academic laboratories, and industry, is a fundamental reaction, usually using at least a stoichiometric amount of an expensive and toxic oxidant. Here, an efficient and practical sustainable oxidation technology of alcohols to carboxylic acids using pure O2 or even O2 in air as the oxidant has been developed: utilizing a catalytic amount each of Fe(NO3)3·9H2O/TEMPO/MCl, a series of carboxylic acids were obtained from alcohols (also aldehydes) in high yields at room temperature. A 55 g-scale reaction was demonstrated using air. As a synthetic application, the first total synthesis of a naturally occurring allene, i.e., phlomic acid, was accomplished.

Despite being fundamentally challenging in integrated (nano)photonics, achieving chip-based light non-reciprocity becomes increasingly urgent in signal processing and optical communications. Because of material incompatibilities in conventional approaches based on the Faraday effect, alternative solutions have resorted to nonlinear processes to obtain one-way transmission. However, dynamic reciprocity in a recent theoretical analysis has pinned down the functionalities of these nonlinear isolators. To bypass such dynamic reciprocity, we here demonstrate an optical isolator on a silicon chip enforced by phase-matched parametric amplification in four-wave mixing. Using a high-Q microtoroid resonator, we realize highly non-reciprocal transport at the 1,550 nm wavelength when waves are injected from both directions in two different operating configurations. Our design, compatible with current complementary metal-oxide-semiconductor (CMOS) techniques, yields convincing isolation performance with sufficiently low insertion loss for a wide range of input power levels. Moreover, our work demonstrates the possibility of designing chip-based magnetic-free optical isolators for information processing and laser protection.

The authors demonstrate a composite microring laser formed by immersing a silica microfiber knot in a rhodamine 6G dye solution. When the dye molecules are evanescently pumped by 532nm wavelength laser pulses guided along a 350μm diameter knot, lasing oscillation occurs inside the evanescently coupled closed-ring microcavity with a linewidth of about 0.06nm. Laser emission around 570 and 580nm wavelengths, which is evanescently coupled back into the microfiber, is observed with a threshold of about 9.2μJ∕pulse. The use of the microfiber knot cavity suggests a convenient and efficient approach to both pumping and collection of the evanescent-wave-coupled dye laser.

The authors demonstrate a 1.5μm wavelength microfiber laser formed by tightening a doped microfiber into a knot in air. The 2-mm-diameter knot, assembled using a 3.8-μm-diameter microfiber that is directly drawn from Er:Yb-doped phosphate glass, serves as both active medium and resonating cavity for lasing. Single-longitudinal-mode laser with threshold of about 5mW and output power higher than 8μW is obtained. Their initial results suggest a simple approach to highly compact lasers based on doped microscale optical fibers.

Abstract:We design a double-disk microcavity consisting of a pair of silica microdisks separated by a nanoscale gap region on a silicon chip for cavity optomechanics. We show that this type of photonic structure can provide a per-photon gradient force with a magnitude much larger than for scattering-force-based structures. Moreover, this device provides for nearly independent optimization of optical and mechanical properties. We present the processing details of fabricated devices. ©2009 Optical Society of America