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We investigate the atomic rearrangement in graphene under femtosecond pulse illumination with reactive molecular dynamics simulations and compare with ultra‐fast laser ablation experiments. To model the impact of the laser pulse irradiation, heat is locally applied to a selected area of the graphene layer and the resulting structural deformation is simulated as a function of time, providing a detailed understanding of the bond breaking process under laser illumination and subsequent re‐equilibration after the pulse is turned off. Analysis of the atomic dynamics indicates that the types of defects formed depend on the pulse energy and exposure duration. By varying the exposed area, we determine that the shape of the ablated area is not only a function of the pulse energy, but also of the beam spot size and pulse repetition. Furthermore, we apply a machine learning approach to extrapolate our simulated data to experimental length scales and reproduce the trends in ablated area as a function of temperature. Our study provides a first step towards understanding the design parameters for graphene nano‐patterning.

Mid-infrared photothermal imaging is a novel chemical imaging modality that combines high sensitivity with enhanced spatial resolution. Subcellular features in fibroblast cells and tissues are imaged and analyzed with regards to their molecular structures without the need of exogenous fluorophores at a resolution that overcomes the diffraction limited spot size of the mid-infrared excitation beam. With a phase-sensitive lock-in detection scheme, changes in the thermal diffusion properties can be detected and can provide a complementary sample characterization.

Polarization rotation dynamics for group velocity locked vector solitons in higher order harmonically mode-locked soliton fiber lasers are studied for the first time, in a thulium/holmium fiber laser. In a linear ultrafast fiber laser with net anomalous dispersion, harmonically mode-locked states are generated continuously by increasing the pump power for specific net cavity birefringence settings. The polarization evolution frequency remains the same for all harmonically mode-locked pulse trains, converging toward a singular solution close to the maximum pulse intensity modulation depth for each respective harmonically mode-locked state. With a polarization resolved output, an intensity modulation of the ultrafast pulse train with varying pulse binning is recorded, corresponding to the polarization rotation of the vector solitons. In addition, a polarization rotation vector soliton state with two different sets of RF sidebands symmetric to the repetition rate is observed, which leads to a more complex modulated pulse train. The experimental results of the polarization evolution also matche well with the presented theoretical modeling. Similar relative intensity noise

This publisher’s notes amends the funding of [Biomed. Opt. Express 12, 303 (2021) 10.1364/BOE.411888].

Imaging of the phase output of a lock-in amplifier in mid-infrared photothermal vibrational microscopy is demonstrated for the first time in combination with nonlinear demodulation. In general, thermal blurring and heat transport phenomena contribute to the resolution and sensitivity of mid-infrared photothermal imaging. For heterogeneous samples with multiple absorbing features, if imaged in a spectral regime of comparable absorption with their embedding medium, it is demonstrated that differentiation with high contrast is achieved in complementary imaging of the phase signal obtained from a lock-in amplifier compared to standard imaging of the photothermal amplitude signal. Specifically, by investigating the relative contribution of the out-of-phase lock-in signal, information based on changes in the rate of heat transport can be extracted, and inhomogeneities in the thermal diffusion properties across the sample plane can be mapped with high sensitivity and sub-diffraction limited resolution. Under these imaging conditions, wavenumber regimes can be identified in which the thermal diffusion contributions are minimized and an enhancement of the spatial resolution beyond the diffraction limited spot size of the probe beam in the corresponding phase images is achieved. By combining relative diffusive phase imaging with nonlinear demodulation at the second harmonic, it is demonstrated that 1-μm-size melamine beads embedded in a thin layer of 4-octyl-4'-cyanobiphenyl (8CB) liquid crystal can be detected with a 1.3-μm spatial full-width at half-maximum (FWHM) resolution. Thus, imaging with a resolving power that exceeds the probe diffraction limited spot size by a factor of 2.5 is presented, which paves the route towards super-resolution, label-free imaging in the mid-infrared.

Label-free vibrational imaging of biological samples has attracted significant interest due to its integration of structural and chemical information. Vibrational infrared photothermal amplitude and phase signal (VIPPS) imaging provide label-free chemical identification by targeting the characteristic resonances of biological compounds that are present in the mid-infrared fingerprint region (3 µm - 12 µm). High contrast imaging of subcellular features and chemical identification of protein secondary structures in unlabeled and labeled fibroblast cells embedded in a collagen-rich extracellular matrix is demonstrated by combining contrast from absorption signatures (amplitude signals) with sensitive detection of different heat properties (lock-in phase signals). We present that the detectability of nano-sized cell membranes is enhanced to well below the optical diffraction limit since the membranes are found to act as thermal barriers. VIPPS offers a novel combination of chemical imaging and thermal diffusion characterization that paves the way towards label-free imaging of cell models and tissues as well as the study of intracellular heat dynamics.

Femtosecond micromachining offers a contact-free and mask-less technique for material patterning. With ultrafast laser irradiation, permanent modifications to the properties of single layer graphene through material ablation or defect introduction can be induced. Multiple femtosecond pulse interactions with a single layer graphene are studied and a low laser ablation threshold ~9.2 mJ/cm2 is reported for a 15 second illumination time. Clean ablated structures are generated in such a multi pulse irradiation configuration at low pulse energies as an attractive alternative to ablation with single femtosecond, high energy pulses. For a fully ablated graphene hole, a radially symmetric region extending around 2 um from the ablated edge is characterized by strong defect generation. Average distances between point-defects down to ~58 nm are derived and Raman spectroscopy implies that overall there is a strong resemblance to amorphous structures. For fluence values around 75% of the ablation threshold, modification with defect generation down to ~48 nm average defects lengths is reported, while the underlying graphene structure is maintained. Thus, depending on the laser parameter choice, the same laser configuration can be used to ablate graphene or to primarily introduce defect states. The presented findings offer interesting insights into femtosecond induced structural modifications of graphene that can lead to improved precision ablation and patterning of single-layer materials at the micro- and nano-scale. Further, this can be attractive for graphene or carbon-based device fabrication as well as sensor and transistor applications, where regions of varying carrier concentrations and different electrical, optical or physical properties are desired.

Mid-infrared spectroscopy and imaging has attracted significant interest due to the presence of characteristic vibrational resonances of numerous molecular compounds in the fingerprint region. However, the imaging resolution in the midinfrared can be significantly weakened by the relatively large spot sizes of the beam used to target the absorption resonances of the sample. Photothermal spectroscopy and imaging as a label-free and nondestructive technique has shown great potential for pushing the imaging resolution to values below the mid-infrared diffraction limit while offering enhanced chemical sensitivity and specificity. Mid-infrared photothermal spectroscopy relies on a pump-probe setup, in which a shorter wavelength probe (removed from the targeted resonances) is co-aligned with the resonant pump beam. The modulated photothermal signal is then detected by a lock-in amplifier, whose output can provide information on the signal amplitude as well as phase. However, most commonly, amplitude images have been evaluated, without considering the recorded photothermal phase. We will provide a detailed analysis of the mid-infrared photothermal amplitude and phase information from a melamine bead sample embedded in a 2.5 μm-thick 4-Octyl-4’-Cyanobiphenyl (8CB) liquid crystal environment. Studies related to thermal transport phenomena as well as increases in imaging sensitivity will be presented for sub-diffraction limited resolution, label-free imaging. The concept of wavelength optimization to minimize thermal blurring effects at the zerocrossing for the out-of-phase contribution is introduced as a novel approach to enhance spatial resolution and sensitivity in mid-infrared label-free photothermal imaging.

Vector solitons and the evolution of their polarization evolution frequency are studied in harmonically mode-locked Tm/Ho co-doped fiber laser states.