Single-particle absorption spectroscopy by photothermal contrast.

Area of Science:

• Nanophotonics and Spectroscopy
• Materials Science
• Plasmonics

Background:

• Single-particle spectroscopy advances fields by removing sample heterogeneity effects.
• While background-free luminescence and scattering spectroscopy are common, pure absorption spectroscopy of individual nanostructures remains challenging.
• Extinction measurements typically combine absorption and scattering, obscuring pure absorption features.

Purpose of the Study:

• To develop and demonstrate a method for recording pure absorption spectra of individual nanostructures.
• To investigate the spectral differences between absorption and scattering in plasmonic nanoparticles.
• To analyze the relationship between nanorod aspect ratio and the spectral shift between absorption and scattering.

Main Methods:

• Implementation of single-particle absorption spectroscopy using photothermal microscopy.
• Combination with a supercontinuum laser and an advanced calibration procedure.
• Accounting for chromatic aberrations and wavelength-dependent excitation power.

Main Results:

• Successful recording of pure absorption spectra from individual gold nanoparticles.
• Observed blueshift of absorption spectra compared to scattering spectra, consistent with Mie theory.
• Demonstrated the ability to record absorption spectra across a 300 nm wavelength range for gold nanorods of varying aspect ratios.
• Found that the spectral shift between absorption and scattering for longitudinal plasmon resonance decreases with increasing nanorod aspect ratio.

Conclusions:

• The developed method enables pure absorption spectroscopy on individual nanostructures, overcoming limitations of extinction measurements.
• The study confirms theoretical predictions regarding spectral shifts and provides new insights into plasmonic behavior.
• Results offer a valuable tool for characterizing nanoscale optical properties and understanding structure-property relationships in nanomaterials.