Mid-infrared frequency comb spanning an octave based on an Er fiber laser and difference-frequency generation

We describe a coherent mid-infrared continuum source with 700 cm-1 usable bandwidth, readily tuned within 600 - 2500 cm-1 (4 - 17 \mum) and thus covering much of the infrared "fingerprint" molecular vibration region. It is based on nonlinear frequency conversion in GaSe using a compact commercial 100-fs-pulsed Er fiber laser system providing two amplified near-infrared beams, one of them broadened by a nonlinear optical fiber. The resulting collimated mid-infrared continuum beam of 1 mW quasi-cw power represents a coherent infrared frequency comb with zero carrier-envelope phase, containing about 500,000 modes that are exact multiples of the pulse repetition rate of 40 MHz. The beam's diffraction-limited performance enables long-distance spectroscopic probing as well as maximal focusability for classical and ultraresolving near-field microscopies. Applications are foreseen also in studies of transient chemical phenomena even at ultrafast pump-probe scale, and in high-resolution gas spectroscopy for e.g. breath analysis.Comment: 8 pages, 2 figures revised version, added reference

Infrared-spectroscopic, dynamic near-field microscopy of living cells and nanoparticles in water

Infrared fingerprint spectra can reveal the chemical nature of materials down to 20-nm detail, far below the diffraction limit, when probed by scattering-type scanning near-field optical microscopy (s-SNOM). But this was impossible with living cells or aqueous processes as in corrosion, due to water-related absorption and tip contamination. Here, we demonstrate infrared s-SNOM of water-suspended objects by probing them through a 10-nm thick SiN membrane. This separator stretches freely over up to 250~µm, providing an upper, stable surface to the scanning tip, while its lower surface is in contact with the liquid and localises adhering objects. We present its proof-of-principle applicability in biology by observing simply drop-casted, living E. coli in nutrient medium, as well as living A549 cancer cells, as they divide, move and develop rich sub-cellular morphology and adhesion patterns, at 150~nm resolution. Their infrared spectra reveal the local abundances of water, proteins, and lipids within a depth of ca. 100~nm below the SiN membrane, as we verify by analysing well-defined, suspended polymer spheres and through model calculations. SiN-membrane based s-SNOM thus establishes a novel tool of live cell nano-imaging that returns structure, dynamics and chemical composition. This method should benefit the nanoscale analysis of any aqueous system, from physics to medicine

Nanoscale Mechanical Manipulation of Ultrathin SiN Membranes Enabling Infrared Near‐Field Microscopy of Liquid‐Immersed samples

Scattering scanning near-field optical microscopy (s-SNOM) is a powerful technique for mid-infrared spectroscopy at nanometer length scales. By investigating objects in aqueous environments through ultrathin membranes, s-SNOM has recently been extended toward label-free nanoscopy of the dynamics of living cells and nanoparticles, assessing both the optical and the mechanical interactions between the tip, the membrane and the liquid suspension underneath. Here, the study reports that the tapping AFM tip induces a reversible nanometric deformation of the membrane manifested as either an indentation or protrusion. This mechanism depends on the driving force of the tapping cantilever, which is exploited to minimize topographical deformations of the membrane to improve optical measurements. Furthermore, it is shown that the tapping phase delay between driving signal and tip oscillation is a highly sensitive observable to study the mechanics of adhering objects, exhibiting highest contrast at low tapping amplitudes where the membrane remains nearly flat. Mechanical responses are correlated with simultaneously recorded spectroscopy data to reveal the thickness of nanometric water layers between membrane and adhering objects. Besides a general applicability of depth profiling, the technique holds great promise for studying mechano-active biopolymers and living cells, biomaterials that exhibit complex behaviors when under a mechanical load

Nanoscale mechanical manipulation of ultrathin SiN membranes enabling infrared near-field microscopy of liquid-immersed samples

Scattering scanning near-field optical microscopy (s-SNOM) is a powerful technique for mid-infrared spectroscopy at nanometer length scales. By investigating objects in aqueous environments through ultrathin membranes, s-SNOM has recently been extended towards label-free nanoscopy of the dynamics of living cells and nanoparticles, assessing both the optical and the mechanical interactions between the tip, the membrane and the liquid suspension underneath. Here, we report that the tapping AFM tip induces a reversible nanometric deformation of the membrane manifested as either an indentation or protrusion. This mechanism depends on the driving force of the tapping cantilever, which we exploit to minimize topographical deformations of the membrane to improve optical measurements. Furthermore, we show that the tapping phase, or phase delay between driving signal and tip oscillation, is a highly sensitive observable for quantifying the mechanics of adhering objects, exhibiting highest contrast for low tapping amplitudes where the membrane remains nearly flat. We correlate mechanical responses with simultaneously recorded spectroscopy data to reveal the thickness of nanometric water pockets between membrane and adhering objects. Besides a general applicability of depth profiling, our technique holds great promise for studying mechano-active biopolymers and living cells, biomaterials that exhibit complex behaviors when under a mechanical load.Comment: 31 pages, 7 figures, 7 supplementary figure

Revealing Mode Formation in Quasi‐Bound States in the Continuum Metasurfaces via Near‐Field Optical Microscopy

Photonic metasurfaces offer exceptional control over light at the nanoscale, facilitating applications spanning from biosensing, and nonlinear optics to photocatalysis. Many metasurfaces, especially resonant ones, rely on periodicity for the collective mode to form, which makes them subject to the influences of finite size effects, defects, and edge effects, which have considerable negative impact at the application level. These aspects are especially important for quasi-bound state in the continuum (BIC) metasurfaces, for which the collective mode is highly sensitive to perturbations due to high-quality factors and strong near-field enhancement. Here, the mode formation in quasi-BIC metasurfaces on the individual resonator level using scattering scanning near-field optical microscopy (s-SNOM) in combination with a new image processing technique, is quantitatively investigated. It is found that the quasi-BIC mode is formed at a minimum size of 10 × 10-unit cells much smaller than expected from far-field measurements. Furthermore, it is shown that the coupling direction of the resonators, defects and edge states have pronounced influence on the quasi-BIC mode. This study serves as a link between the far-field and near-field responses of metasurfaces, offering crucial insights for optimizing spatial footprint and active area, holding promise for augmenting applications such as catalysis and biospectroscopy

Infrared nanoscopy of Dirac plasmons at the graphene-SiO2 interface

We report on infrared (IR) nanoscopy of 2D plasmon excitations of Dirac fermions in graphene. This is achieved by confining mid-IR radiation at the apex of a nanoscale tip: an approach yielding two orders of magnitude increase in the value of in-plane component of incident wavevector q compared to free space propagation. At these high wavevectors, the Dirac plasmon is found to dramatically enhance the near-field interaction with mid-IR surface phonons of SiO2 substrate. Our data augmented by detailed modeling establish graphene as a new medium supporting plasmonic effects that can be controlled by gate voltage.Comment: 12 pages, 4 figure

Transient infrared nanoscopy resolves the millisecond photoswitching dynamics of single lipid vesicles in water

Understanding the biophysical and biochemical properties of molecular nanocarriers under physiological conditions and with minimal interference is crucial for advancing nanomedicine, photopharmacology, drug delivery, nanotheranostics and synthetic biology. Yet, analytical methods struggle to combine precise chemical imaging and measurements without perturbative labeling. This challenge is exemplified for azobenzene-based photoswitchable lipids, which are intriguing reagents for controlling nanocarrier properties on fast timescales, enabling, e.g., precise light-induced drug release processes. Here, we leverage the chemical recognition and high spatio-temporal resolution of scattering-type scanning near-field optical microscopy (s-SNOM) to demonstrate non-destructive, label-free mid-infrared (MIR) imaging and spectroscopy of photoswitchable liposomes below the diffraction limit and the tracking of their dynamics down to 50 ms resolution. The vesicles are adsorbed on an ultrathin 10-nm SiN membrane, which separates the sample space from the tip space for stable and hour-long observations. By implementing a transient nanoscopy approach, we accurately resolve, for the first time, photoinduced changes in both the shape and the MIR spectral signature of individual vesicles and reveal abrupt change dynamics of the underlying photoisomerization process. Our findings highlight the methods potential for future studies on the complex dynamics of unlabeled nanoscale soft matter, as well as, in a broader context, for host-guest systems, energy materials or drugs.Comment: 4 figures, 10 supplementary figure