The radical suppression of the photodarkening effect and laser performance deterioration via H2 loading were demonstrated in high-power Yb-doped fiber (YDF) amplifiers. The photodarkening loss at equilibrium was 114.4 dB/m at 702 nm in the pristine fiber, while it vanished in the H2-loaded fiber. To obtain a deeper understanding of the impact of photodarkening on laser properties, the evolution of the mode instability threshold and output power in fiber amplifiers was investigated. After pumping for 300 min, the mode instability threshold of the pristine fiber dropped from 770 to 612 W, and the periodic fluctuation of the output power became intense, finally reaching 100 W. To address the detrimental effects originating from photodarkening, H2 loading was applied in contrast experiments. The output power remained stable, and no sign of mode instability was observed in the H2-loaded fiber. Moreover, the transmittance at 638 nm confirmed the absence of the photodarkening effect. The results pave the way for the further development of high-power fiber lasers.

Fast and sensitive air-coupled ultrasound detection is essential for many applications such as radar, ultrasound imaging, and defect detection. Here we present a novel approach based on a digital optical frequency comb (DOFC) technique combined with high-Q optical microbubble resonators (MBRs). DOFC enables precise spectroscopy on resonators that can trace the ultrasound pressure with its resonant frequency shift with femtometer resolution and sub-microsecond response time. The noise equivalent pressure of air-coupled ultrasound as low as 4.4mPa/√Hz is achieved by combining a high-Q (～3×107) MBR with the DOFC method. Moreover, it can observe multi-resonance peaks from multiple MBRs to directly monitor the precise spatial location of the ultrasonic source. This approach has a potential to be applied in 3D air-coupled photoacoustic and ultrasonic imaging.

An equivalence is made between the exceptional points proposed by the field of non-Hermitian quantum mechanics and the dead band observed in laser gyroscopes. The sensitivity enhancement near this exceptional point is plagued by increased uncertainty due to broadening of the beat-note bandwidth. Also, near the dead band the gyroscope response is caused by Rabi intensity oscillations and not solely by a phase modulation. Finally, a distinction is made between conservative and non-conservative coupling.

In this paper, a technique combining cascaded energy-transfer pumping (CEP) method and master-oscillator power-amplifier (MOPA) configuration is proposed for power scaling of 1.6-μm-band single-frequency fiber lasers (SFFLs), where the Er3+ ion has a limited gain. The CEP technique is fulfilled by coupling a primary signal light at 1.6 μm and a C-band auxiliary laser. The numerical model of the fiber amplifier with the CEP technique reveals that the energy transfer process involves the pump competition and the in-band particle transition between the signal and auxiliary lights. Moreover, for the signal emission, the population density in the upper level is enhanced, and the effective population inversion is achieved thanks to the CEP. A single-frequency MOPA laser at 1603 nm with an output power of 52.6 W and an improved slope efficiency of 30.4% is experimentally obtained through the CEP technique. Besides, a laser linewidth of 5.2 kHz and a signal-to-auxiliary laser ratio of 60.7 dB are obtained at the maximum output power. The proposed technique is anticipated to be promising for increasing the slope efficiency and power scaling for fiber lasers operating at L band.

Vacuum Rabi splitting, which stems from a single photon interaction with a quantum emitter (a single atom, molecule, or quantum dot), is a fundamental quantum phenomenon. Many reports have claimed that using J aggregate coupling to highly localized plasmon can produce giant Rabi splitting (in scattering spectra) that is proportional to N, where N is the number of excitons in J aggregates, and this splitting originates purely from quantum interaction between excitons and plasmons. In this work, we show that the scattering spectra are very sensitive to the surrounding matter, and the giant spectral splitting stems both from the quantum interaction of a single molecule with plasmons (Rabi splitting) and from the classical optical interaction of multiple molecules with plasmons. We develop a Lorentzian model to describe molecules and plasmon and find that the collective optical interaction is dominant in generating the giant splitting (in scattering spectra), which is also proportional to N, upon the quantum interaction of single-molecule Rabi splitting. Simply speaking, the observed giant spectral splitting is not a pure quantum Rabi splitting effect, but rather a mixture contribution from the large spectral modulation by the collective optical interaction of all molecules with plasmons and the modest quantum Rabi splitting of a single molecule strongly coupled with plasmons.

Chip-scale, tunable narrow-linewidth hybrid integrated diode lasers based on quantum-dot RSOAs at 1.3 μm are demonstrated through butt-coupling to a silicon nitride photonic integrated circuit. The hybrid laser linewidth is around 85 kHz, and the tuning range is around 47 nm. Then, a fully integrated beam steerer is demonstrated by combining the tunable diode laser with a waveguide surface grating. Our system can provide beam steering of 4.1° in one direction by tuning the wavelength of the hybrid laser. Besides, a wavelength-tunable triple-band hybrid laser system working at ～1, ～1.3, and ～1.55μm bands is demonstrated for wide-angle beam steering in a single chip.

Metamaterials play an important role in the modulation of amplitude and group delay in the terahertz (THz) regime on account of their optical properties, which are rare in natural materials. Here an ultrafast anisotropic switch of the plasmon-induced transparency (PIT) effect is experimentally and numerically demonstrated by metamaterial devices composed of two pairs of planar split-ring resonators and a pair of closed-ring resonators. By integration with a germanium (Ge) film, a recovery time of 3 ps and a decay constant of 785 fs are realized in the metadevice. Stimulated by the exterior optical pump, the PIT windows at different frequencies are switched off with an excellent property of slow light for vertical and horizontal THz polarizations, realizing an astonishing modulation depth as high as 99.06%. This work provides a new platform for ultrafast anisotropic metadevices tunable for amplitude and group delay.