Photothermal Circular Dichroism of Single Nanoparticles Rejecting Linear Dichroism by Dual Modulation

Abstract

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 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 × 10–4.

Correlating structure, morphology and properties of metal nanostructures by combining single-particle optical spectroscopy and electron microscopy

“…For example, Spaeth et al developed CD pump-probe photothermal microscopy with unprecedented sensitivity. 82,83 To test the validity of the technique, the measured CD signals of chiral nanogammadia were correlated to their SEM morphology, demonstrating that through photothermal CD the strength and handedness of chiral nanoobjects could be unambiguously determined (Fig. 1B).…”

Imaging the Magnetization of Single Magnetite Nanoparticle Clusters via Photothermal Circular Dichroism

“…Distinctions in magnetic behavior due to shape and size cannot be made by measuring an ensemble of nanoparticles, yet they are of vital importance to their applications in biomedical sciences and spintronic devices. Here, we overcome the limitation of visible-light MCD to ensemble measurements by implementing our newly developed photothermal circular dichroism (PT CD) microscopy , for the magnetic imaging of single nanoparticulate clusters. To image the magnetization of nanoparticles, PT MCD is considerably simpler than XMCD or scanning SQUID, making it far more accessible.…”

“…The setup is a photothermal microscope augmented with polarization modulation optics in the heating arm to facilitate photothermal circular dichroism measurements . We use a dual modulation of the polarization because it offers an excellent rejection of cross talk between linear and circular dichroism . By modulating either the intensity or the polarization of the heating beam, this setup can access absorption signals such as the total absorption (PT) and LD (linear dichroism) and CD.…”

“…This heating wavelength corresponds to intervalence charge transfer transitions of magnetite (Supporting Information). The technical details of the setup are explained in ref . In the circular dichroism mode, our technique measures the differential absorption between left and right circularly polarized light and therefore is most sensitive to magnetic moments that are parallel (or anti parallel) to the propagation direction of the light.…”

“…An additional set of static birefringent plates and a polarizer enable polarization and amplitude modulation of the heating beam. Details are described in previous work . A reference signal at the sum frequency ω 1 + ω 2 of the two modulators is sent to the lock-in amplifier.…”