Magnetic imaging is a versatile tool in biological andcondensed-matter physics. Existing magnetic imaging techniqueseither require demanding experimental conditions which restrict therange of their... Show moreMagnetic imaging is a versatile tool in biological andcondensed-matter physics. Existing magnetic imaging techniqueseither require demanding experimental conditions which restrict therange of their applications or lack the spatial resolution required forsingle-particle measurements. Here, we combine photothermal (PT)microscopy with magnetic circular dichroism (MCD) to develop aversatile magnetic imaging technique using visible light. Unlike mostmagnetic imaging techniques, photothermal magnetic circulardichroism (PT MCD) microscopy works particularly well for singlenanoparticles immersed in liquids. As a proof of principle, wedemonstrate magnetic CD imaging of superparamagnetic magnetitenanoparticulate clusters immersed in microscope immersion oil. Thesensitivity of our method allowed us to probe the magnetizationcurve of single similar to 400-nm-diameter magnetite nanoparticulate clusters Show less
Circular dichroism (CD) is the property of chiral nanoobjects to absorb circularly polarized light of either handedness to different extents. Photothermal microscopy enables the detection of CD... Show moreCircular dichroism (CD) is the property of chiral nanoobjects to absorb circularly polarized light of either handedness to different extents. Photothermal microscopy enables the detection of CD signals with high sensitivity and provides a direct absorptive response of the samples under study. To achieve CD measurements at the single-particle level, one must reduce such artifacts as leakage of linear dichroism (LD) and residual intensity modulation. We have simulated our setup with a simple model, which allows us to tune modulation parameters to obtain a CD signal virtually free from artifacts. We demonstrate the sensitivity of our setup by measuring the very weak inherent CD signals of single gold nanospheres. We furthermore demonstrate that our method can be extended to obtain spectra of the full absorptive properties of single nanoparticles, including isotropic absorption, linear dichroism, and circular dichroism. We then investigate nominally achiral gold nanoparticles immersed in a chiral liquid. Carefully taking into account the intrinsic chirality of the particles and its change due to heat-induced reshaping, we find that the chiral liquid carvone surrounding the particle has no measurable effect on the particles' chirality, down to g-factors of 3 x 10(-4). Show less
Notwithstanding its unique power for imaging and investigation of transparent condensed and biological matter, fluorescence presents severe limitations: it requires special fluorescent labels,... Show moreNotwithstanding its unique power for imaging and investigation of transparent condensed and biological matter, fluorescence presents severe limitations: it requires special fluorescent labels, which are prone to photobleaching, and the photon streams it provides are relatively weak. In the past 10 to 20 years nonfluorescent optical methods have appeared, which can also provide information on matter at the nanoscale, while presenting different limitations. In the present paper, we review some of these methods, with special emphasis on work from our group. We consider mostly the optical detection and study of single immobilized or transiently bound molecules and nanoparticles through their scattering, the heat they dissipate in the environment upon light absorption, or their coupling to auxiliary optical resonators such as whispering-gallery modes. Show less
