Hyper-numerical aperture (NA = 2.8) microscope using λ = 1.56 µm femtosecond source for multi-photon imaging

Area of Science:

• Optics and Photonics
• Materials Science
• Nanotechnology

Background:

• Advanced microscopy techniques are crucial for characterizing nanoscale materials and biological samples.
• Multi-photon microscopy offers enhanced depth penetration and reduced phototoxicity compared to single-photon methods.
• Investigating optical properties of nanomaterials like gold nanoparticles is essential for their applications.

Purpose of the Study:

• To introduce and characterize a new scanning microscope system.
• To investigate multi-photon emission from silicon interfaces and gold nanoparticles.
• To analyze the spectral and emission properties of gold nanoparticles of varying sizes.

Main Methods:

• Utilized a scanning microscope with a numerical aperture of 2.8 and a 1.56 µm femtosecond fiber laser.
• Imaged multi-photon emission in transmission using a cooled CCD detector.
• Employed a blazed transmission grating for spectral analysis of individual particles.
• Studied cells, reference spheres, and gold nanoparticles (20nm, 40nm, 60nm).

Main Results:

• Observed two-photon and three-photon effects at the silicon/water interface and from gold nanoparticles.
• Demonstrated the imaging capabilities of the microscope with various samples.
• Found that gold nanoparticle emission is a two-photon effect due to continuum generation.
• Observed similar spectral shapes and emission powers for 20nm, 40nm, and 60nm gold nanoparticles.

Conclusions:

• The developed microscope is effective for multi-photon imaging of nanoscale phenomena.
• Gold nanoparticle emission properties are consistent across different sizes studied, primarily driven by a two-photon continuum generation effect.
• This study provides valuable data on the optical behavior of gold nanoparticles relevant to sensing and imaging applications.