Arrow ncpt

Our setup is based on a commercial s-SNOM (Neaspec GmbH), which allows single laser-line mid-IR nano-imaging simultaneously with the sample topography mapping. The s-SNOM incorporates tapping-mode atomic-force microscope (AFM) performing measurements with Arrow-NCPt (Nanoworld) metal-coated Si tips, oscillating at frequency of ~270–285 kHz with 70–90 nm amplitude. Temperature-tunable CO₂ laser (Access Laser) radiation is focused on the sample in oblique configuration. Pseudo-heterodyne interferometric detection at nonlinear harmonics of the tapping frequency^{38 (link)} (third in this work) allows us to measure amplitude (s₃) and phase of the tip-scattered signal, which represent near-field distribution at the sample. Cross-sections of the optical signal (taken always along the fringe evolution direction) were obtained by averaging several neighbouring lines in order to increase signal-to-noise ratio.

For time-domain interferometric nanoimaging experiments, we used a commercial nano-FTIR system from Neaspec GmbH based on an atomic force microscope (AFM). The Pt-coated AFM tip oscillates vertically with an amplitude of about 70 nm at a frequency Ω ≈ 250 kHz (Arrow NCPt, NanoWorld). It is illuminated by a broadband laser pulse of 100-fs duration (range from 1000 to 2000 cm⁻¹; see fig. S3). A piezo-controlled linearly moving mirror is used to precisely control the time delay τ between the tip-scattered field and the reference field. The 2D mappings were carried out at different time delays with a temporal resolution of Δτ = 16.7 fs (corresponding to the reference-mirror moving interval of 2.5 μm) satisfying the band-pass sampling (see note S1 for details). The interferometric detector signal was demodulated at a higher harmonic nΩ (n ≥ 2), yielding near-field amplitude s_n and phase ϕ_n images as a function of both the 2D (x, y) coordinates and time delay τ. In this work, we recorded the signal of the second order for producing a raw space-time imaging dataset (see Fig. 1B and fig. S1), e.g., the real part of near fields, E(x,y,τ) = s₂ cos(ϕ₂). We display the mappings of s₁ in Fig. 3E since the higher-order signal is weak for an incidence angle of 45°.

The ultrafast nanoscopy contains three Erbium-doped fiber amplifiers connected to the same oscillator with a 76-fs pulse duration and 80 MHz repetition frequency (TOPTICA Photonics AG). Amplifiers 1 and 2 emitted near-infrared (NIR) pulses with a wavelength of 1500–1600 nm and a power of 400 mW, and amplifier 3 emitted super-continuum pulses (980–2200 nm) using a nonlinear fiber. The pump branch was emitted from amplifier 1, and the probe branch was produced by difference frequency generation processes between amplifiers 2 and 3. By tuning the pitch angle of the nonlinear crystal (GaSe-1000H1, EKSMA OPTICS) to meet the phase-matching condition, The mid-infrared (MIR) broadband probe pulses (v/c = 850–1200 cm⁻¹) were obtained. The pump and probe pulses were spatially overlapped on the metalized Pt/Ir tip (ARROW-NCPt, Nanoworld) through a parabolic mirror of a commercial scattering-type scanning near-field optical microscopy (attocube systems AG). Samples were settled on a customized rotation platform to change the probe incident angles.

The near-field scans were obtained by commercial s-SNOM (Neaspec GmbH) coupled with a tunable quantum cascade laser (Daylight Solutions, MIRcat), which illuminates the Pt-coated AFM tip (Nano World, ARROW-NCPt). The background-free interferometric signal^{44 (link)}, demodulated at third harmonic 3Ω (where Ω is the tapping frequency of the AFM tip), was used for near-field imaging. s-SNOM in AFM tapping mode was used to perform surface scans with 5 nm step and tip oscillation amplitude of ≈70 nm.

A commercial near-field microscope system (neaSNOM, NeaSpec, Germany) equipped with a QCL system (MIRcat-QT, Daylight Solutions, USA) with four lasing chips covering a spectral range of $~$ (900–1900) cm⁻¹ was used for s-SNOM imaging. A pseudoheterodyne detection scheme^{25 (link)} was employed to obtain background-free phase measurements. Commercially available probes (Arrow NCPt, NanoWorld, Switzerland) with resonant frequency $~$ 285 kHz were driven in tapping mode with $~$ 50 nm amplitude. Gwyddion was used for basic image processing, such as line noise correction and extracting line profiles from image data.

Infrared nano-imaging of the amelogenin proteins was performed using a commercial scattering scanning near-field optical microscope (s-SNOM), the Inspire with PeakForce Tapping (Bruker Nano Surfaces, Santa Barbara, CA, USA). In the AFM based instrument the tip is illuminated via a monochromatic, tunable, low-noise quantum cascade laser (Daylight Solutions, San Diego, CA, USA). The tip-scattered light is then analyzed in an asymmetric Michelson interferometer with a two-phase homodyne detection scheme to directly obtain the nanoscale infrared absorption with a tip-limited spatial resolution of 10–20 nm52 (link). Metal-coated AFM tips (Nanoworld Arrow NCPT) were used in tapping-mode at a frequency and amplitude around 250 kHz and 40 nm, respectively. Background-free imaging was achieved via signal demodulation at higher harmonics of the tapping frequency; in this case the second (main text) and third (SI) harmonics were chosen.