18 research outputs found
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
MHz-repetition-rate, sub-mW, multi-octave THz wave generation in HMQ-TMS
We demonstrate the first megahertz (MHz) repetition-rate, broadband terahertz (THz) source based on optical rectification in the organic crystal HMQ-TMS driven by a femtosecond Yb:fibre laser. Pumping at 1035 nm with 30 fs pulses, we achieve few-cycle THz emission with a smooth multi-octave spectrum that extends up to 6 THz at -30 dB, with conversion efficiencies reaching 10-4 and an average output power of up to 0.38 mW. We assess the thermal damage limit of the crystal and conclude a maximum fluence of ∼1.8 mJ·cm-2 at 10 MHz with a 1/e2 pump beam diameter of 0.10 mm. We compare the performance of HMQ-TMS with the prototypical inorganic crystal gallium phosphide (GaP), yielding a tenfold electric field increase with a peak on-axis field strength of 7 kV·cm-1 and almost double the THz bandwidth. Our results further demonstrate the suitability of organic crystals in combination with fibre lasers for repetition-rate scaling of broadband, high-power THz sources for time-domain spectroscopic applications
Quantitative near-field characterization of surface plasmon polaritons on monocrystalline gold platelets
The subwavelength confinement of surface plasmon polaritons (SPPs) makes them
attractive for various applications such as sensing, light generation and solar
energy conversion. Near-field microscopy associated with interferometric
detection allows to visualize both the amplitude and phase of SPPs. However,
their full quantitative characterization in a reflection configuration is
challenging due to complex wave patterns arising from the interference between
several excitation channels. Here, we present near-field measurements of SPPs
on large monocrystalline gold platelets in the visible spectral range. We study
systematically the influence of the incident angle of the exciting light on the
SPPs launched by an atomic force microscope tip. We find that the amplitude and
phase signals of these SPPs are best disentangled from other signals at grazing
incident angle relative to the edge of the gold platelet. Furthermore, we
introduce a simple model to explain the phase shift observed between the SPP
amplitude and phase profiles. Using this model, the wavelength and propagation
length of the tip-launched plasmons are retrieved by isolating and fitting
their signals far from the platelets edges. Our experimental results are in
excellent agreement with theoretical models using gold refractive index values.
The presented method to fully characterize the SPP complex wavevector could
enable the quantitative analysis of polaritons occurring in different materials
at visible wavelengths.Comment: 36 pages, 8 figure