Nonlinear optical imaging by detection with optical parametric amplification(invited paper)

Nonlinear optical imaging is a versatile tool that has been proven to be exceptionally useful in various research fields.However,due to the use of photomultiplier tubes(PMTs),the wide application of nonlinear optical imaging is limited by the incapability of imaging under am-bient light.In this paper,we propose and demonstrate a new optical imaging detection method based on optical parametric amplification(OPA).As a nonlinear optical process,OPA in-trinsically rejects ambient light photons by coherence gating.Periodical poled lithium niobate(PPLN)crystals are used in this study as the media for OPA.Compared to bulk nonlinear optical crystals,PPLN crystals support the generation of OPA signal with lower pump power.Therefore,this characteristic of PPLN crystals is particularly beneficial when using high-repetition-rate lasers,which facilitate high-speed optical signal detection,such as in spec-troscopy and imaging.A PPLN-based OPA system was built to amplify the emitted imaging signal from second harmonic generation(SHG)and coherent anti-Stokes Raman scattering(CARS)microscopy imaging,and the amplified optical signal was strong enough to be detected by a biased photodiode under ordinary room light conditions.With OPA detection,ambient-light-on SHG and CARS imaging becomes possible,and achieves a similar result as PMT detection under strictly dark environments.These results demonstrate that OPA can be used as a substitute for PMTs in nonlinear optical imaging to adapt it to various applications with complex.light ing conditions.

Confocal nonlinear optical imaging on hexagonal boron nitride nanosheets

Optical microscopy with optimal axial resolution is critical for precise visualization of two-dimensional flat-top structures.Here,we present sub-diffraction-limited ultrafast imaging of hexagonal boron nitride(hBN)nanosheets using a confocal focus-engineered coherent anti-Stokes Raman scattering(cFE-CARS)microscopic system.By incorporating a pinhole with a diameter of approximately 30μm,we effectively minimized the intensity of side lobes induced by circular partial pi-phase shift in the wavefront(diameter,d0)of the probe beam,as well as nonresonant background CARS intensities.Using axial-resolution-improved cFE-CARS(acFE-CARS),the achieved axial resolution is 350 nm,exhibiting a 4.3-folded increase in the signal-to-noise ratio compared to the previous case with 0.58 d0 phase mask.This improvement can be accomplished by using a phase mask of 0.24 d0.Additionally,we employed nonde-generate phase matching with three temporally separable incident beams,which facilitated cross-sectional visualization of highly-sample-specific and vibration-sensitive signals in a pump-probe fashion with subpicosecond time resolution.Our observations reveal time-dependent CARS dephasing in hBN nanosheets,induced by Raman-free induction decay(0.66 ps)in the 1373 cm^(−1) mode.

Stimulated Raman scattering microscopy on biological cellular machinery

Beneting from the developments of advanced optical microscopy techniques,the mysteries of biological functions at the cellular and subcellular levels have been continuously revealed.Stimulated Raman scattering(SRS)microscopy is a rapidly growing technique that has attracted broad attentions and become a powerful tool for biology and biomedicine,largely thanks to its chemical specicity,high sensitivity and fast image speed.This review paper introduces the principles of SRS,discusses the technical developments and implementations of SRS microscopy,then highlights and summarizes its applications on biological cellular machinery andnally shares our visions of potential breakthroughs in the future.

High spatial resolution collinear chiral sum-frequency generation microscopy

Chiral sum-frequency generation(SFG)has proven to be a versatile spectroscopic and imaging tool for probing chirality.However,due to polarization restriction,the conventional chiral SFG microscopes have mostly adopted noncollinear beam configurations,which only partially cover the aperture of microscope and strongly spoil the spatial resolution.In this study,we report the first experimental demonstration of collinear chiral SFG microscopy,which fundamentally supports diffraction-limited resolution.This advancement is attributed to the collinear focus of a radially polarized vectorial beam and a linearly polarized(LP)beam.The tightly focused vectorial beam has a very strong longitudinal component,which interacts with the LP beam and produces the chiral SFG.The collinear configuration can utilize the full aperture and thus push the spatial resolution close to the diffraction limit.This technique can potentially boost the understanding of chiral systems.