Black phosphorus is a layered semiconducting material, demonstrating strong layer-dependent optical and electronic properties. Probing the photophysical properties on ultrafast time scales is of central importance in understanding many-body interactions and nonequilibrium quasiparticle dynamics. Here, we applied temporally, spectrally, and spatially resolved pump-probe microscopy to study the transient optical responses of mechanically exfoliated few-layer black phosphorus, with layer numbers ranging from 2 to 9. We have observed layer-dependent resonant transient absorption spectra with both photobleaching and red-shifted photoinduced absorption features, which could be attributed to band gap renormalization of higher subband transitions. Surprisingly, coherent phonon oscillations with unprecedented intensities were observed when the probe photons were in resonance with the optical transitions, which correspond to the low-frequency layer-breathing mode. Our results reveal strong Coulomb interactions and electron-phonon couplings in photoexcited black phosphorus, providing important insights into the ultrafast optical, nanomechanical, and optoelectronic properties of this novel two-dimensional material.

Photochromic probes with reversible fluorescence have revolutionized the fields of single molecule spectroscopy and super-resolution microscopy, but lack sufficient chemical specificity. In contrast, Raman probes with stimulated Raman scattering (SRS) microscopy provides superb chemical resolution for super-multiplexed imaging, but are relatively inert. Here we report vibrational photochromism by engineering alkyne tagged diarylethene to realize photo-switchable SRS imaging. The narrow Raman peak of the alkyne group shifts reversibly upon photoisomerization of the conjugated diarylethene when irradiated by ultraviolet (UV) or visible light, yielding “on” or “off” SRS images taken at the photoactive Raman frequency. We demonstrated photo-rewritable patterning and encryption on thin films, painting/erasing of cells with labelled alkyne-diarylethene, as well as pulse-chase experiments of mitochondria diffusion in living cells. The design principle provides potentials for super-resolution microscopy, optical memories and switches with vibrational specificity.

Hexagonal boron nitride (h-BN) is an important member of two-dimensional (2D) materials with a large direct bandgap, and has attracted growing interest in ultraviolet optoelectronics and nanoelectronics. Compared with graphene and graphite, h-BN has weak Raman effect because of the far off-resonance excitation; hence, it is difficult to exploit Raman spectroscopy to characterize important properties of 2D h-BN, such as thickness, doping, and strain effects. Here, we applied stimulated Raman scattering (SRS) to enhance the sensitivity of the E2g Raman mode of h-BN. We showed that SRS microscopy achieves rapid high resolution imaging of h-BN with a pixel dwell time 4 orders of magnitude smaller than conventional spontaneous Raman microscopy. Moreover, the near-perfect linear dependence of signal intensity on h-BN thickness and isotropic polarization dependence allow convenient determination of the flake thickness with SRS imaging. Our results indicated that SRS microscopy provides a promising tool for high-speed quantification of h-BN and holds the potential for vibrational imaging of 2D materials.

Two-dimensional (2D) materials have attracted broad research interests across various nonlinear optical (NLO) studies, including nonlinear photoluminescence (NPL), second harmonic generation (SHG), transient absorption (TA), and so forth. These studies have unveiled important features and information of 2D materials, such as in grain boundaries, defects, and crystal orientations. However, as most research studies focused on the intrinsic NLO processes, little attention has been paid to the substrates underneath. Here, we discovered that the NLO signal depends significantly on the thickness of SiO in SiO/Si substrates. A 40-fold enhancement of the NPL signal of graphene was observed when the SiO thickness was varied from 270 to 125 nm under 800 nm excitation. We systematically studied the NPL intensity of graphene on three different SiO thicknesses within a pump wavelength range of 800-1100 nm. The results agreed with a numerical model based on back reflection and interference. Furthermore, we have extended our measurements to include TA and SHG of graphene and MoS, confirming that SiO thickness has similar effects on all of the three major types of NLO signals. Our results will serve as an important guidance for choosing the optimum substrates to conduct NLO research studies on 2D materials.