Imaging and Absolute Extinction Cross-Section Measurements of Nanorods and Nanowires through Polarization Modulation Microscopy

Abstract

This paper describes an experimental technique for measuring the extinction cross-section of anisotropic nanomaterials. The experiments are performed by focusing a laser beam to a diffraction-limited spot under an optical microscope, and using a photoelastic modulator (PEM) to rotate the polarization of the light beam. Monitoring the transmitted beam with a lock-in amplifier referenced to twice the resonant frequency of the PEM yields the difference in extinction for light polarized parallel and perpendicular to the optical axis of the nanostructure. These experiments are the single particle analog of polarization modulation microscopy (PMM). Experimental results for gold nanorods and CdSe nanowires are presented that demonstrate the sensitivity and properties of this technique. In particular, we show that by collecting images at two laser polarizations separated by 45°it is possible to construct an extinction cross-section image. The advantages and disadvantages of the PMM technique compared to existing ways of measuring the extinction of nanoparticles (photothermal heterodyne imaging and spatial modulation spectroscopy) are discussed. Results from experiments where we collected simultaneous absorption and emission images are also presented for different shaped CdSe nanowires. These measurements provide insight into how the structure of the nanowire affects its photophysics. † Part of the special issue "Protected Metallic Clusters, Quantum Wells, and Metallic Nanocrystal Molecules".

Single Cell Optical Imaging and Spectroscopy

“…434-437 Nanoparticles absorb energy from a time-modulated excitation laser and convert the energy into heat. Subsequently, as the heat is dissipated from the nanoparticle, the surrounding medium undergoes a time-modulated variation in its refractive index.…”

“…437 Additionally, PHI can be used to detect noble metal or semiconductor nanoparticles, and it can detect the orientation of a nanoparticle when polarized light is utilized. 434,438 …”

Single-Particle Absorption Spectroscopy by Photothermal Contrast

“…The first single-molecule optical detection experiment was achieved by absorption using a doublemodulation technique, but had to be carried out at cryogenic conditions, under which the absorption cross-section of the zero-phonon line is 10 6 times larger. 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.…”

Metrological Investigation of the (6,5) Carbon Nanotube Absorption Cross Section

“…One limiting factor of this technique was that only relatively low frequencies (< kHz) could be used for modulating the sample position due to mass loading 17 . Thus, these measurements suffer from 1/f noise.…”