Temporal coupled-mode theory for PT-symmetric chiral metasurfaces.

We have developed a temporal coupled-mode theory based on quasi-normal modes to investigate the chiroptical effects in parity-time (PT) symmetric metasurfaces. The PT symmetry enforces a different constraint for the direct scattering matrix and the coupling constants, which is verified by calculating the transmission spectra originating from the chiral quasi-bound states in the continuum. What's more, the scattering matrix can be analytically continued to the complex frequency plane. We find that the zero and pole singularities of the transmission coefficients and scattering matrix play an important role in the optical chirality. The pole singularities carry a quantized topological charge of -1. Our work paves the way for studying the enhanced optical chirality in non-Hermitian metasurfaces.

2.7 µm dual-wavelength laser performance of LD end-pumped Er:YAP crystal.

We demonstrate a laser diode (LD) end-pumped Er:YAP laser with dual-wavelength outputs of 2710 and 2728 nm. The maximum average powers of 739 and 738 mW are achieved in the continuous wave (CW) and pulse modes, which corresponds to optical-to-optical efficiencies of 10.1% and 12.3%, and the slope efficiencies of 12.1% and 13.8%, respectively. In addition, a comparison of laser performance on differently sized crystals indicates that the 1 × 1 × 5 mm3 Er:YAP crystal has a best cooling efficiency. This helps to decrease the thermal lensing effect, which contributes to improving laser efficiency and beam quality.

Laser performance of a 966 nm LD side-pumped Er,Pr:GYSGG laser crystal operated at 2.79 µm.

We demonstrate a 966 nm laser diode (LD) side-pumped Er,Pr:GYSGG laser crystal operated at 2.79 µm under a high repetition rate. The lifetimes of the upper level I11/24 and lower level I13/24 are 0.66 and 0.85 ms, respectively. The laser performance under different repetition rates and pulse widths is experimentally studied with the optimal cavity structure. A maximum output power of 8.86 W is achieved at 125 Hz and 200 µs pulse widths, corresponding to the slope efficiency of 14.8% and electrical-to-optical efficiency of 7.7%. With increasing frequency from 50 to 200 Hz, the slope efficiency varies from 24.7% to 11.7% operated at a 125 µs pulse width. Moreover, the Mx2/My2 factors of 7.52/7.59 and Θx/Θy far-field divergences of 16.1/16.5 mrad are also measured. The results indicate that a high-performance 2.79 µm laser could be realized on the Er,Pr:GYSGG radiation resistant crystal by deactivation and LD side-pumping.

Thermal analysis and laser performance of a GYSGG/Cr,Er,Pr:GYSGG composite laser crystal operated at 2.79 µm.

We demonstrate the thermal analysis and laser performance of a GYSGG/Cr,Er,Pr:GYSGG composite crystal. The lifetime ratio of lower and upper levels of Er3+ in Cr,Er,Pr:GYSGG crystal is further reduced due to the optimized doping concentrations. The thermal effect of composite crystal is lower than that of Cr,Er,Pr:GYSGG crystal. A maximum pulse energy 342.8 mJ operated at 5 Hz and 2.79 µm is obtained on the composite crystal, corresponding to electrical-to-optical efficiency of 0.86% and slope efficiency of 1.08%. Under the same condition, these values on the Cr,Er,Pr:GYSGG crystal are only 315.8 mJ, 0.79% and 1.04%, respectively. Moreover, the composite crystal has also a relative high laser beam quality, exhibiting obvious advantage in reducing thermal effects and improving laser performances.

Growth, spectroscopy, and laser performance of a 2.79 µm Cr,Er,Pr:GYSGG radiation-resistant crystal.

We demonstrate the growth, spectroscopy, and laser performance of a 2.79 µm Cr,Er,Pr:GYSGG radiation-resistant crystal. The lifetimes for the upper laser level (4)I(11/2) and lower laser level (4)I(13/2) are 0.59 and 0.84 ms, respectively, which are due to the doping of the Pr(3+) ions. A maximum pulse energy of 278 mJ operated at 10 Hz and 2.79 µm is obtained when pumped with a flash lamp, which corresponds to the electrical-to-optical efficiency of 0.6% and a slope efficiency of 0.7%. A maximum average power of 2.9 W at 60 Hz is achieved, which corresponds to the electrical-to-optical efficiency of 0.4% and slope efficiency of 0.8%. Compared with a Cr,Er:YSGG crystal, the Cr,Er,Pr:GYSGG crystal can be operated at a higher pulse repetition rate. These results suggest that doping deactivator Pr(3+) ions can effectively decrease the lower laser level lifetime and improve the laser repetition rate. Therefore, the application fields and range of the Cr,Er,Pr:GYSGG laser can be extended greatly due to its properties of radiation resistance and high repetition frequency.