Ultrafast Carrier Dynamics and Bandgap Renormalization in Layered PtSe2.

Carrier interactions in 2D nanostructures are of central importance not only in condensed-matter physics but also for a wide range of optoelectronic and photonic applications. Here, new insights into the behavior of photoinduced carriers in layered platinum diselenide (PtSe2 ) through ultrafast time-resolved pump-probe and nonlinear optical measurements are presented. The measurements reveal the temporal evolution of carrier relaxation, chemical potential and bandgap renormalization in PtSe2 . These results imply that few-layer PtSe2 has a semiconductor-like carrier relaxation instead of a metal-like one. The relaxation follows a triple-exponential decay process and exhibits thickness-dependent relaxation times. This occurs along with a band-filling effect, which can be controlled based on the number of layers and may be applied in saturable absorption for generating ultrafast laser pulses. The findings may provide means to study many-body physics in 2D materials as well as potentially leading to applications in the field of optoelectronics and ultrafast photonics.

Mid-infrared laser emission from Cr:ZnS channel waveguide fabricated by femtosecond laser helical writing.

The operation of a mid-infrared laser at 2244 nm in a Cr:ZnS polycrystalline channel waveguide fabricated using direct femtosecond laser writing with a helical movement technique is demonstrated. A maximum power output of 78 mW and an optical-to-optical slope efficiency of 8.6% are achieved. The compact waveguide structure with 2 mm length was obtained through direct femtosecond laser writing, which was moved on a helical trajectory along the laser medium axis and parallel to the writing direction.

Experimental demonstration of laser damage caused by interface coupling effects of substrate surface and coating layers.

The laser damage resistance of the coatings for high-power laser systems depends greatly on the surface quality of the substrate. In our work, experimental approaches were employed to understand the interface coupling effect of the substrate surface and coatings on the laser resistance of the coatings. A 1064 nm anti-reflection (AR) coating was deposited by an e-beam coater onto fused silica with and without micro-scale pits (structural defects). The micro-scale pits were precisely fabricated by femtosecond laser processing to prevent the emergence of subsurface cracks. Different deposition temperatures were characterized in order to verify the intensity of the interface coupling effect of the substrate and coating layers. Our experimental results indicate that impurities that are introduced in the finishing process, shifted to the substrate surface, and aggregated during the heating process, play a much more crucial role than structural defects (length: ∼7 µm; width: ∼3 µm; depth: ∼0.8 µm) in the laser-induced damage process. By effectively reducing the intensity of the interface coupling effect, the e-beam AR coatings, whose laser-induced damage resistance was closed to the bare substrate, was prepared.

Equivalent-nanocircuit-theory-based design to infrared broad band-stop filters.

We theoretically introduced a design paradigm and tool by extending the circuit functionalities from radio frequency to near infrared domain, and a broad band-stop filter, is successfully demonstrated by cascading triple layers of nano-square arrays. The feasibility is confirmed by its consistency with the rigorous FDTD calculation. Moreover, such a third-order Butterworth filter is not only insensitive to the incident angle and but also to input light's polarization. The new paradigm forms a theoretical foundation for designing optical devices and also enriches the classic circuit operations at the optical frequency region.

Theoretical analysis and design of a near-infrared broadband absorber based on EC model.

We theoretically introduced a design paradigm and tool by extending the circuit functionalities from radio frequency to near infrared domain, and its first usage to design a broadband near-infrared (1.5µm~3.5µm) absorber, is successfully demonstrated. After extracting the equivalent circuit (EC) model of the absorber structure, the formerly relatively complicated frequency response can be evaluated relatively easily based on classic circuit formulas. The feasibility is confirmed by its consistency with the rigorous FDTD calculation. The absorber is an array of truncated metal-dielectric multilayer composited pyramid unit structure, and the gradually modified square patch design makes the absorber be not sensitive to the incident angle and polarization of light.

Impact of substrate pits on laser-induced damage performance of 1064-nm high-reflective coatings.

The laser damage resistance of coatings in high-power laser systems depends significantly on the surface quality of the substrate. In our experiment, pits were precisely fabricated on the surface of fused silica substrate using a femtosecond laser processing bench. The HfO2/SiO2 high-reflective coatings at 1064 nm were deposited by conventional e-beam evaporation onto fused silica substrates with and without pits, respectively. The internal crack that was induced by the substrate geometrical structure was first observed in our experiment. The laser-induced damage threshold test showed negative effects of the substrate pits on the laser resistance of high-reflective coatings. Simulations by the finite element method were carried out, and results demonstrated that the modulation of a high reflector multilayer geometry could lead to electrical-field amplification and reduce laser damage resistance. Combined with its poor mechanical properties, the pits on substrate could contribute to the occurrence of damages.

Temperature-independent fiber inclinometer based on orthogonally polarized modes coupling using a polarization-maintaining fiber bragg grating.

A reflection fiber inclinometer is proposed and experimentally demonstrated based on two linearly polarized (LP) modes coupling. The configuration consists of a section of polarization-maintaining fiber (PMF) containing a fiber Bragg grating (FBG) splicing with single mode fiber (SMF). Bending the PMF in the upstream of FBG can induce an additional birefringence of PMF, which results in the intensity changes of two LP modes owing to orthogonal polarization coupling. The experimental results represent that the device shows different bending responses at the angle range from 0° to 40°and from 64° to 88°, respectively. Moreover, the temperature change just shifts the wavelengths of LP modes reflected and does not influence their intensities, which effectively avoid the temperature cross-sensitivity and make it a good candidate for measuring inclinometer and temperature simultaneously.