High spectral resolution second harmonic generation microspectroscopy at thin layer interfaces with broadband continuum pulses

We demonstrate an effective microspectroscopy technique by tracing the dispersion of second order nonlinear optical susceptibility χ(2) in single atomic layer materials. The experimental method relies on the detection of single-shot second harmonic (SH) spectra from the materials and the subsequent data normalization. The key point in our study is that we used a broadband (˜350 nm) near-infrared femtosecond continuum pulses generated at high repetition rates in a photonic crystal fiber with superior spatial quality and stable spectral power density. This is opposite to the point-by-point laser tuning method in SH generation spectroscopy that was applied extensively in the past and has shown limited precision in obtaining χ(2) dispersion. The continuum pulse technique produces spectral resolution better than 2 meV (\u3c0.3 nm at 450 nm) and shows low (\u3c5–6% rms) signal detection noise allowing the detection of subtle features in the χ(2) spectrum at room temperatures. Fine sub-structure features within the main peak of χ(2) spectra indicate the impact of broadened resonances due to exciton transitions in the single layer materials. Tailored continuum pulses are used to generate second harmonic signal in non-centrosymmetric semiconductors. SHG spectrum carries fingerprints of the bandstructure around the direct gap states. The technique produces fine spectral resolution and much better signal-to-noise ratio compared to point-by-point wavelength tuning methods

Tracing molecular dephasing in biological tissue

We demonstrate the quantitative spectroscopic characterization and imaging of biological tissue using coherent time-domain microscopy with a femtosecond resolution. We identify tissue constituents and perform dephasing time (T2) measurements of characteristic Raman active vibrations. This was shown in subcutaneous mouse fat embedded within collagen rich areas of the dermis and the muscle connective tissue. The demonstrated equivalent spectral resolution (-1) is an order of magnitude better compared to commonly used frequency-domain methods for characterization of biological media. This provides with the important dimensions and parameters in biological media characterization and can become an effective tool in detecting minute changes in the bio-molecular composition and environment that is critical for molecular level diagnosis

Resolving Fine Spectral Features in Lattice Vibrational Modes Using Femtosecond Coherent Spectroscopy

We show resolution of fine spectral features within several Raman active vibrational modes in potassium titanyl phosphate (KTP) crystal. Measurements are performed using a femtosecond time-domain coherent anti-Stokes Raman scattering spectroscopy technique that is capable of delivering equivalent spectral resolution of 0.1 cm−1. The Raman spectra retrieved from our measurements show several spectral components corresponding to vibrations of different symmetry with distinctly different damping rates. In particular, linewidths for unassigned optical phonon mode triplet centered at around 820 cm−1 are found to be 7.5 ± 0.2 cm−1, 9.1 ± 0.3 cm−1, and 11.2 ± 0.3 cm−1. Results of our experiments will ultimately help to design an all-solid-state source for sub-optical-wavelength waveform generation that is based on stimulated Raman scattering

Imaging skeletal muscle using second harmonic generation and coherent anti-Stokes Raman scattering microscopy

We describe experimental results on label free imaging of striated skeletal muscle using second harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS) microscopy. The complementarity of the SHG and CARS data makes it possible to clearly identify the main sarcomere sub-structures such as actin, myosin, acto-myosin, and the intact T-tubular system as it emanates from the sarcolemma. Owing to sub-micron spatial resolution and the high sensitivity of the CARS microscopy technique we were able to resolve individual myofibrils. In addition, key organelles such as mitochondria, cell nuclei and their structural constituents were observed revealing the entire structure of the muscle functional units. There is a noticeable difference in the CARS response of the muscle structure within actin, myosin and t-tubule areas with respect to laser polarization. We attribute this to a preferential alignment of the probed molecular bonds along certain directions. The combined CARS and SHG microscopy approach yields more extensive and complementary information and has a potential to become an indispensable method for live skeletal muscle characterization

Nonlinear optical susceptibility of atomically thin WX\u3csub\u3e2\u3c/sub\u3e crystals

We have studied tungsten diselenide (WSe2) and tungsten disulfide (WS2) monolayer materials in second harmonic generation spectroscopy and microscopy experiments. Ultra-broadband continuum pulses served as the fundamental beam while its second harmonic spectrum in the visible and ultraviolet (UV) range was detected and analyzed with a better than 0.3 nm spectral resolution (\u3c2 \u3emeV). We provide dispersion data and absolute values for χ(2) for the materials within a photon energy range of 2.3–3.2 eV. Fine spectral features that were detected within the dispersion data for the optical nonlinearities indicate the impact of near bandgap exciton transitions. The fundamental bandgap of 2.35 eV and exciton binding energy of 0.38 eV were determined from the measurements for WS2 monolayers while the corresponding values in WSe2 monolayers were 2.22 eV and 0.71 eV. Ranges for the absolute values of the sheet nonlinearity for WS2 and WSe2 are shown to be 0.58–1.65 × 10−18 m2/V and 0.21–0.92 × 10−18 m2/V, correspondingly

Seeded fiber laser with nonlinear mirror feedback

Optical waveform seeding provided an efficient control of key nonlinear effects in fiber laser and resulted in nearly ultimate stabilization of output of Yb-doped fiber laser operating in Q-switching mode due to distributed nonlinear mirror

Dispersion of the resonant nonlinear optical susceptibility obtained with femtosecond time-domain coherent anti-Stokes Raman scattering

We propose and experimentally demonstrate a method that is capable of resolving both real and imaginary parts of third-order nonlinearity (X (3)) in the vicinity of Raman resonances. Dispersion of X (3) can be obtained from a medium probed within microscopic volumes with a spectral resolution of better than 0.10 cm-1. © 2013 Optical Society of America

Seeded fiber laser with nonlinear mirror feedback

Optical waveform seeding provided an efficient control of key nonlinear effects in fiber laser and resulted in nearly ultimate stabilization of output of Yb-doped fiber laser operating in Qswitching mode due to distributed nonlinear mirror

Quantitative Ultrafast Spectroscopy and Microscopy of Traditional and Soft Condensed Matter

We demonstrate and analyze a series of experiments in traditional and soft condensed matter using coherent optical spectroscopy and microscopy with ultrafast time resolution. We show the capabilities of resolving both real and imaginary parts of the third-order nonlinearity in the vicinity of Raman resonances from a medium probed within microscopic volumes with an equivalent spectral resolution of better than 0.1 cm−1. We can differentiate between vibrations of various types within unit cells of crystals, as well as perform targeted probes of areas within biological tissue. Vibrations within the TiO6 octahedron and the ones for the Ti-O-P intergroup were studied in potassium titanyl phosphate crystal to reveal a multiline structure within targeted phonon modes with closely spaced vibrations having distinctly different damping rates (~0.5 ps−1 versus ~1.1 ps−1). We also detected a 1.7–2.6 ps−1 decay of C-C stretching vibrations in fat tissue and compared that with the corresponding vibration in oil

Dispersion of the resonant second order nonlinearity in 2D semiconductors probed by femtosecond continuum pulses

We demonstrate an effective microspectroscopy technique by tracing the dispersion of second order nonlinear susceptibility (χ(2)) in a monolayer tungsten diselenide (WSe2). The χ(2) dispersion obtained with better than 3 meV photon energy resolution showed peak value being within 6.3-8.4×10-19 m2/V range. We estimate the fundamental bandgap to be at 2.2 eV. Sub-structure in the χ(2) dispersion reveals a contribution to the nonlinearity due to exciton transitions with exciton binding energy estimated to be at 0.7 eV