CARS-Background suppression by phase-controlled nonlinear interferometry

Contrast-enhanced phase-resolved second harmonic generation microscopy

The characterization of inverted structures (crystallographic, ferroelectric, or magnetic domains) is crucial in the development and application of novel multi-state devices. However, determining these inverted structures needs a sensitive probe capable of revealing their phase correlation. Here a contrast-enhanced phase-resolved second harmonic generation (SHG) microscopy is presented, which utilizes a phase-tunable Soleil-Babinet compensator and the interference between the SHG fields from the inverted structures and a homogeneous reference. By this means, such inverted structures are correlated through the π-phase difference of SHG, and the phase difference is ultimately converted into the intensity contrast. As a demonstration, we have applied this microscopy in two scenarios to determine the inverted crystallographic domains in two-dimensional van der Waals material MoS2. Our method is particularly suitable for applying in vacuum and cryogenic environments while providing optical diffraction-limited resolution and arbitrarily adjustable contrast. Without loss of generality, this contrast-enhanced phase-resolved SHG microscopy can also be used to resolve other non-centrosymmetric inverted structures, e.g. ferroelectric, magnetic, or multiferroic phases.

Observation of Stimulated Brillouin Scattering in Silicon Nitride Integrated Waveguides

Silicon nitride (Si_{3}N_{4}) has emerged as a promising material for integrated nonlinear photonics and has been used for broadband soliton microcombs and low-pulse-energy supercontinuum generation. Therefore, understanding all nonlinear optical properties of Si_{3}N_{4} is important. So far, only stimulated Brillouin scattering (SBS) has not yet been reported. Here we observe, for the first time, backward SBS in fully cladded Si_{3}N_{4} waveguides. The Brillouin gain spectrum exhibits an unusual multipeak structure resulting from hybridization with high-overtone bulk acoustic resonances of the silica cladding. The reported intrinsic Si_{3}N_{4} Brillouin gain at 25 GHz is estimated as 4×10^{-13}  m/W. Moreover, the magnitude of the Si_{3}N_{4} photoelastic constant is estimated as |p_{12}|=0.047±0.004, which is nearly 6 times smaller than for silica. Since SBS imposes an optical power limitation for waveguides, our results explain the capability of Si_{3}N_{4} to handle high optical power, central for integrated nonlinear photonics.

Background-free 3D chemical imaging based on polarization coherent Raman holography

We report on a holographic coherent anti-Stokes Raman scattering imaging by using polarization discrimination for nonresonant background suppression. With reduced polarization scrambling effect under weakly focused excitation, nonresonant background-free coherent Raman imaging is demonstrated. A fast chemically selective imaging is achieved in a three-dimensional volume of 70  μm ×70  μm ×100  μm in 2 s.

Implementation of vibrational phase contrast coherent anti-Stokes Raman scattering microscopy

Detection of molecules using vibrational resonances in the fingerprint region for narrowband coherent anti-Stokes Raman scattering (CARS) is challenging. The spectrum is highly congested resulting in a large background and a reduced specificity. Recently we introduced vibrational phase contrast CARS (VPC-CARS) microscopy as a technique capable of detecting both the amplitude and phase of the CARS signal, providing background-free images and high specificity. In this paper we present a new implementation of VPC-CARS based on a third-order cascaded phase-preserving chain, where the CARS signal is generated at a single (constant) wavelength independent of the vibrational frequency that is addressed. This implementation will simplify the detection side considerably.

Interferometric switching of coherent anti-Stokes Raman scattering signals in microscopy

Coherent anti-Stokes Raman scattering (CARS) interferometry is used to deplete the anti-Stokes radiation emerging from a tightly focused spot. Near-to-complete depletion of the anti-Stokes radiation is obtained when a phase-controlled local oscillator field at the anti-Stokes frequency is out of phase with the induced CARS field in the focal volume. Unlike in traditional interferometry, this depleted energy is not spatially redistributed. A theoretical analysis shows that the energy loss in the anti-Stokes channel is accompanied by an energy gain in the pump and Stokes channels. Interferometric switching of anti-Stokes radiation may offer a route toward developing high-resolution CARS microscopy.

Nonlinear optical interference of two successive coherent anti-Stokes Raman scattering signals for biological imaging applications

The nonlinear optical interference of two successively generated coherent anti-Stokes Raman scattering (CARS) signals from two different samples placed in series is demonstrated for the imaging performance, in which a collinear phase matching geometry is used. The relative phase of two CARS signals is controlled by a phase-shifting unit made of dispersive glass materials of which the thickness can be precisely varied. The clear interference fringes are observed as the thickness of the phase-shifting unit changes. The interference effect is then utilized to achieve a better quality CARS image of a biological tissue taken from a mouse skin. Placing the tissue in the second sample position and performing raster scans of the laser beams on it, we can acquire a CARS image of higher contrast compared to the normal image obtained without interferometric implementation.

Relative phase control between two successive coherent anti-Stokes Raman-scattering signals for the recovery of spectral lines

We have controlled the relative phase between two successively generated coherent anti-Stokes Raman-scattering signals with a homemade phase-shifting unit, the thickness of which could be automatically adjusted as the wavelength of Stokes beam scanned in a wide spectral range. With this technique we have recovered the Q-branch spectrum of carbon monoxide distorted by a large nonresonant contribution from propane. Also, we have selectively recorded the spectrum of hydrogen chloride buried in the spectrum of propane.