Fig. 1. Concept of MEIP microscopy. (a) Energy diagram of mid-IR photothermal spectroscopy. Molecules absorb IR photons to vibrational excited states and the energy dissipates through nonradiative decay, thus leading to temperature rise. (b) Pump–probe detection method of MEIP. A visible wavelength is selected for the probe beam to match the size of the modulated temperature field. Mid-IR field enhancement around nanoantenna results in a rise in localized temperature and thus refractive index decrease of the solutions. (c) SEM image of the metasurface resonant at protein amide-I band. L, length of 1.8μm; W, width of 200 nm; and height of 70 nm. (d) MEIP tomography of the hot spots of the metasurface at different depths.

Fig. 2. MEIP tomography setup and reflection MEIP microscopy setup. (a) Schematic of photothermal diffraction tomography setup. TL, tube lens. Obj, objective. The pump beam is illuminated by a mid-IR laser loosely focused with a parabolic mirror. The probe beam consists of a 16-angle fiber diode laser, and the transmitted photons are collected by water immersion objective. IR-on multi-angle images and IR-off multi-angle references are captured by the camera. (b) Synchronization of pump pulses, probe pulses, and camera in the MEIP system. (c) Schematic of intensity diffraction tomography reconstruction. 3D k-space information is covered by scanning the illumination angles. 3D refractive index distribution is retrieved by inverse model through Tikhonov regularization. (d) Schematic of reflection MEIP setup. BS, beam splitter. PM, parabolic mirror. Pump mid-IR light was loosely focused by a parabolic mirror onto the sample plane. The probe beam was counter-propagated with a pump and detected with a home-built widefield microscope.

Fig. 3. Simulation of MEIP signal generation. (a) IR reflection spectra of metasurface in H2O (blue) and D2O (red) from experiments (solid lines) and simulations (dashed lines). (b) IR intensity enhancement of metasurface excited at 1650cm−1. (c) Heat dissipation around nanoantenna excited at 1650cm−1. (d) Line plot of temperature increase (blue, red) and heat dissipation (green) through the center of nanoantenna under a heating pulse of 250 ns at 1650cm−1. (e) y–z cross-sectional map of the temperature distribution in the small volume surrounding nanoantenna at 250 ns. (f) Time-dependent temperature plot at various heights from the gold surface and 40 nm away from the center of nanoantenna tip.

Fig. 4. Photothermal imaging and spectroscopy of metasurface in H2O and D2O. (a) Depth-resolved photothermal images and spectra of nanoantenna in water. (b) Depth-resolved photothermal images and spectra of nanoantenna in D2O. Solid line, Lorentzian fitted curve; error bar, standard deviation of MEIP signals from single nanoantennas (n=10). (c) Time-dependent photothermal images and spectra of nanoantenna in water. Solid line, fitted as a convolution of a Gaussian function and exponential function; error bar, standard deviation of MEIP signals from single nanoantennas (n=10).

Fig. 5. MEIP sensing of proteins of different secondary structures. (a) Schematic of MEIP sensing with MEIP tomography. (b) MEIP spectra of BSA in water. (c) MEIP spectra of avidin in water. (d) MEIP images of BSA solution at a concentration of 500 nM. (e) MEIP spectra of BSA at various concentrations. Solid line, Lorentzian fitted curve. (f) Concentration-dependence of MEIP signals. Error bar, the standard deviation of MEIP signals from single nanoantennas (n=10).

Fig. 6. Sensing of nitrile molecules with reflection MEIP system. (a) Bright-field image of metasurface in nitrile solution. (b) MEIP spectra of nitrile molecules in water with a concentration of 0 to 10μM. (c) MEIP images of metasurface in nitrile solutions.