Abstract
We evaluated the optimal detection angle for maximizing the signal to noise ratio (SNR) in sub-diffraction resolution photothermal microscopy. The angular dependent photothermal signal was calculated based on scattering theory using the temporally modulated Yukawa potential, and its detection angle and modulation frequency dependencies were analyzed. We verified the theoretical findings by imaging gold nanoparticles using laser diode based photothermal microscopy with balanced detection scheme. High-sensitivity (SNR ~40) photothermal biological imaging of a mouse brain was also demonstrated.
Highlights
Photothermal microscopy is an efficient method for observing single molecules or nanomaterials using only their optical absorption [1,2,3]
By virtue of this fact, it has been used in several biological applications in recent years, such as imaging of live cells using 5nm gold nanoparticles for arbitrarily long periods [4], evaluation of the hydrodynamic properties of cytosol by means of photothermal absorption correlation spectroscopy [5,6,7,8], label-free imaging of heme proteins with two-photon excitation [9], and the visualization or tracking of membrane proteins moving at the surface of live cells [10, 11]
We found that the full width at half maximum (FWHM) value of a single particle is ~240 nm, which is 19% smaller than the diffraction limited spot size of the pump beam and 32% smaller than that of the probe beam [25]
Summary
Photothermal microscopy is an efficient method for observing single molecules or nanomaterials using only their optical absorption [1,2,3]. Two laser beams with different wavelengths (pump and probe beams) are incident on the sample through a focusing lens. The pump beam increases the temperature around the focal point of the optical absorbing sample This results in local refractive index variations and induces deflection of the probe beam. The variation of the refractive index with temperature n / T is typically in the order of 10 4 K 1, which is readily detected by a lock-in detection scheme as a change in probe beam transmissivity. An auto-balanced detection scheme can be used for common mode noise reduction at a lower lock-in frequency (up to ~100 kHz) [14]. High-sensitivity photothermal imaging of a mouse brain was demonstrated with our subdiffraction resolution photothermal microscopy, using intensity modulated laser diodes with a balanced detection scheme
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