Q-switched fiber lasers emitting at around 3 μm have been widely applied in various scientific and industrial fields, such as molecular spectroscopy, laser surgery, material processing, remote sensing, and mid-infrared (mid-IR) supercontinuum source generation. Au nanocages (Au-NCs) have attracted much attention recently due to their outstanding saturable absorption properties including broadband absorption, ultrafast optical response (a few picoseconds), and large third-order optical nonlinearity coefficient that caused by local surface plasmon resonance (LSPR). We propose and demonstrate a tunable Er3+ doped ZBLAN fiber laser using Au-NCs as a Q-switcher for the first time. Nonlinear absorption of the Au-NCs was measured by a home-made Ho3+/Pr3+ co-doped mode-locked fiber laser at 2850 nm. The measured modulation depth, saturation intensity and non-saturation loss are 10.73%, 0.11 MW/cm2 and 3.26%, respectively. The central wavelength of the Q-switched pulses could be tuned across 54.1 nm (from 2753.0 to 2807.1 nm). The Q-switched fiber laser delivers a maximum average power of 253.7 mW with corresponding pulse energy of 4.06 μJ and pulse width of 1.30 μs at repetition rate of 62.5 kHz. Our work shows the Au-NCs are promising saturable absorbers (SAs) for 3 μm mid-infrared (mid-IR) pulse generation.
High-resolution, real-time and three-dimensional imaging in thick scattering specimens is of great significance in biology, yet meeting these requirements at the same time is fraught with challenges. In this work, we describe a method that combines structured illumination microscopy (SIM) with dual nonlinear effects, two-photon excitation (2PE) technique and stimulated emission depletion (STED), to further improve the imaging resolution in optical-thick samples relative to SIM. Utilizing a line-scanning geometry shaped by cylindrical lens to form structured illumination pattern, the imaging speed is greatly improved. Theoretical study and simulations are both performed to demonstrate the capability of this method to enhance resolution laterally and the potential for applications in real-time imaging for living tissue.
Structured illumination microscopy (SIM) breaks the resolution limit caused by optical diffraction, and nonlinear SIM can further improve the resolution with nonlinear effect. However, current nonlinear SIM methods such as Saturated SIM and Photo-switching SIM are unsatisfactory in biomedical imaging. The stimulated emission depletion (STED) effect is considered as a great nonlinear effect with fast switching response, negligible stochastic noise during switching, low shot noise and theoretical unlimited resolution. We propose an original nonlinear structured illumination microscopy based on both patterned excitation illumination and structured STED field (SSTED-SIM). Theoretical study and simulation results demonstrated that SSTED-SIM is capable of providing the ability of fast imaging speed, and low imaging noise at the same time compared with other nonlinear SIM techniques.
Nonlinear structured illumination microscopy (SIM) allows full-field imaging at resolutions <100 nm. Two
nonlinear effects, excitation saturation (SSIM) and the photo-switching of protein had been applied to nonlinear
SIM. We report a new SIM technique which utilizes the nonlinearity of STED effect. Resolution and signal noise
ratio simulation shows that STED-SIM may serve as a better alternative to SSIM and SIM with photo-switchable
SIM requires a strong nonlinear effect in a large area. We use Surface Plasmon Resonant to enhance of evanescence
field near a dielectric-metal-dielectric interface. An 8 times STED effect enhancement is achieved on an optimized
glass-silver-glass-water planar structure. We further use the interference of two SPR-enhanced STED fields
propagating at opposite direction to generate a 1D structured STED field. Combined with a uniform excitation field,
the structure STED field allows full field total internal reflection imaging with an enhanced resolution along the
structured dimension. Less than 50 nm resolution is demonstrated.
A STED-SIM microscope with 2D structured STED field is under development. Future research will apply the
microscope to superresolution imaging of membrane resident or near membrane structure at super-resolution in live