The force exerted by electromagnetic fields is of fundamental importance in physics. Intense debates on the conventionally accepted Lorentz formulation and the recently suggested Einstein–Laub formulation still continue due to lack of experimental evidences. To distinguish these two formulations, we experimentally investigated the topological charge of optical force in a solid dielectric, and found that the force exerted by a Gaussian beam has components with topological charge of both 2 and 0, which agrees with neither the Lorentz nor Einstein–Laub formulation. Instead, we found a modified Helmholtz theory could explain our experimental results. This work not only contributes to the ultimate determination of the correct force formulation in classical electrodynamics, but also has broad and far-reaching impact on many subjects involving electromagnetic forces.
A remarkable self-sustained thermo-optomechanical oscillator has been observed in various optical micro-cavities, which caused by competition among the thermal expansion, the thermo-optic effect, and Kerr effects as we scan the probing laser across a cavity resonance at various tuning rates. Oscillation periods in the thermo-optomechanical oscillator are considered to be related to the heat dissipate rate from the micro-cavity resonator bulk to the environment, and it is possible that the thermo dissipate rate can be measured by detecting oscillation periods. Although a nonlinear relationship between the heat dissipate rate and oscillation periods is exhibited, an artificial neural network is applied to identify the heat dissipate rate. Numerical results demonstrate that the method can be used to measure the heat dissipate rate effectively in the thermo-optomechanical oscillator based on a CaF2 whispering-gallery-mode resonator.
Mode coupling is a important issue in a resonant system. A concept of dissipative coupling is proposed in the frame of coupled mode theory. This is a indirect coupling based on coupling of both modes to a highly lossy mode. It is shown that the lossy mode provide a equivalent coupling between two coherent modes. As an example, the theory is applied to a micro-ring to break its chiral symmetry. By carefully designing dissipation and scattering coupling we break chial symmetry of light in the modes-coupling system. The resonant frequencies and modes both in theoretical and numerical results show good agreements. The reflection spectrum show also asymmetry feature. Moreover, the dissipation is usually considered to be harmful for applications and should be avoid in the designation of photonic systems. We believe our finding of symmetry breaking by dissipation coupling will provoke people to utilizing dissipations as a tool for manipulating photons.
Optical whispering gallery mode (WGM) microcavities are promising candidates for basic research and optoelectronic applications. Due to the isotropic emission property resulting from the rotational symmetry, traditional WGM microcavities have to rely on external couplers to excite the modes and collect their emission. One of the most possible solutions is to deform microcavities from rotational symmetry, which could provide directional emission instead of isotropic characteristic. Here we report the first experimental realization of on-chip microcavities which support both highly unidirectional emission and ultra-high Q factors. The demonstrated Q factor exceeds 100 million in near infrared. By doping erbium into the deformed microcavity, lasing action in 1550 nm band was observed under convenient freespace optical pumping, with the threshold as low as 2 μW. Remarkably, the lasing emission is along a single direction with a narrow divergence angle about 10 degrees.