Photothermal single particle microscopy using a single laser beam

Abstract

We introduce a single-laser-beam photothermal microscopy scheme for the detection of single absorbing nano-objects. Here, a modulated incident laser beam with a constant intensity offset serves as pump and probe beam at the same time. Using the out-of-phase scattering response of the retarded thermorefractive wave field, the method provides a selective contrast for absorbers over a possible background of scatterers. The use of a single wavelength and a single beam, considerably simplifies the setup and integration of photothermal detection in existing microscopy schemes.

Single-Particle Absorption Spectroscopy by Photothermal Contrast

“…21 Indirect detection schemes combined with modulation techniques have been developed for absorption-based detection, [22][23][24][25] 4 including ground-state depletion microscopy 15,26 and photothermal microscopy. 11,13,14,16,[27][28][29][30] However, most studies using these methods have reported single-wavelength imaging without yielding spectroscopic information. A tunable dye laser, incorporated as the excitation source into a photothermal microscope, has allowed the acquisition of absorption, albeit in a limited spectral range of 515 nm to 585 nm, for which chromatic aberrations were mostly negligible.…”

Mie and potential scattering by a refractive 1/r inhomogeneity: Electromagnetic scattering by an infinite Coulomb-like scatterer

“…(4) for radii larger than the particle radius R 5 λ. This is the situation commonly encountered in photothermal single particle microscopy [4][5][6][7][8][9]. Using Eq.…”

“…In particle scattering, the latter form may be taken to represent the electronic shielding of the nuclear charge which limits the range of its Coulomb potential. Similarly, the optical inhomogeneity in photothermal microscopy does not extend indefinitely under realistic conditions [9]. In heterodyne photothermal microscopy, an exponential damping is determined by the thermal diffusivity of the embedding medium and the modulation frequency of heating of the absorbing nanoparticle [4,9].…”

“…This analog was explicated in classical and wave-mechanics. Specifically, the situation of focused beam scattering [3] was of technical importance, as it closely approximates the situation encountered in photothermal single particle microscopy [4][5][6][7][8][9]. In contrast, here the plane-wave scattering behavior for the same idealized refractive index inhomogeneity is studied, as depicted in Fig.…”

Photothermal Microscopy for High Sensitivity and High Resolution Absorption Contrast Imaging of Biological Tissues

“…In stimulated emission and stimulated Raman microscopy, the intensity of the pump beam is modulated at high frequency (>1 MHz) because the laser intensity noise of a solid state laser used for probing occurs primarily at low frequencies (from kilohertz to DC) in the form of 1/f noise [14,23,24]. However, in PT microscopy, the high-frequency modulation scheme reduces signal intensity because the time response of the PT signal is, in principle, determined by heat conductivity and is decreased by half at~1 MHz for a tightly focused laser beam [25][26][27][28]. Moreover, the high-frequency modulation scheme cannot circumvent the high-frequency laser noise that usually presents in the laser diodes and amplified fiber lasers.…”