Single-Particle Photothermal Circular Dichroism and Photothermal Magnetic Circular Dichroism Microscopy.

Recent advances in single-particle photothermal circular dichroism (PT CD) and photothermal magnetic circular dichroism (PT MCD) microscopy have shown strong promise for diverse applications in chirality and magnetism. Photothermal circular dichroism microscopy measures direct differential absorption of left- and right-circularly polarized light by a chiral nanoobject and thus can measure a pure circular dichroism signal, which is free from the contribution of circular birefringence and linear dichroism. Photothermal magnetic circular dichroism, which is based on the polar magneto-optical Kerr effect, can probe the magnetic properties of a single nanoparticle (of sizes down to 20 nm) optically. Single-particle measurements enable studies of the spatiotemporal heterogeneity of magnetism at the nanoscale. Both PT CD and PT MCD have already found applications in chiral plasmonics and magnetic nanomaterials. Most importantly, the advent of these microscopic techniques opens possibilities for many novel applications in biology and nanomaterial science.

Future Paths in Cryogenic Single-Molecule Fluorescence Spectroscopy.

In the last three decades, cryogenic single-molecule fluorescence spectroscopy has provided average-free understanding of the photophysics and of fundamental interactions at molecular scales. Furthermore, they propose original pathways and applications in the treatment and storage of quantum information. The ultranarrow lifetime-limited zero-phonon line acts as an excellent sensor to local perturbations caused either by intrinsic dynamical degrees of freedom, or by external perturbations, such as those caused by electric fields, elastic and acoustic deformations, or light-induced dynamics. Single aromatic hydrocarbon molecules, being sensitive to nanoscale probing at nanometer scales, are potential miniaturized platforms for integrated quantum photonics. In this Perspective, we look back at some of the past advances in cryogenic optical microscopy and propose some perspectives for future development.

Thousand-Fold Enhancement of Photothermal Signals in Near-Critical CO2.

Photothermal (PT) microscopy has shown strong promise in imaging single absorbing nano-objects in soft matter and biological systems. PT imaging at ambient conditions usually requires a high laser power for a sensitive detection, which prevents application to light-sensitive nanoparticles. In a previous study of single gold nanoparticles, we showed that the photothermal signal can be enhanced more than 1000-fold in near-critical xenon compared to that in glycerol, a typical medium for PT detection. In this report, we show that carbon dioxide (CO2), a much cheaper gas than xenon, can enhance PT signals in a similar way. We confine near-critical CO2 in a thin capillary which easily withstands the high near-critical pressure (around 74 bar) and facilitates sample preparation. We also demonstrate enhancement of the magnetic circular dichroism signal of single magnetite nanoparticle clusters in supercritical CO2. We have performed COMSOL simulations to support and explain our experimental findings.

Photothermal Circular Dichroism Measurements of Single Chiral Gold Nanoparticles Correlated with Electron Tomography.

Chemically synthesized metal nanoparticles with morphological chiral features are known to exhibit strong circular dichroism. However, we still lack understanding of the correlation between morphological and chiroptical features of plasmonic nanoparticles. To shed light on that question, single nanoparticle experiments are required. We performed photothermal circular dichroism measurements of single chiral and achiral gold nanoparticles and correlated the chiroptical response to the 3D morphology of the same nanoparticles retrieved by electron tomography. In contrast to an ensemble measurement, we show that individual particles within the ensemble display a broad distribution of strength and handedness of circular dichroism signals. Whereas obvious structural chiral features, such as helical wrinkles, translate into chiroptical ones, nanoparticles with less obvious chiral morphological features can also display strong circular dichroism signals. Interestingly, we find that even seemingly achiral nanoparticles can display large g-factors. The origin of this circular dichroism signal is discussed in terms of plasmonics and other potentially relevant factors.

Optically Probing the Chirality of Single Plasmonic Nanostructures and of Single Molecules: Potential and Obstacles.

Circular dichroism (CD) is a standard method for the analysis of biomolecular conformation. It is very reliable when applied to molecules, but requires relatively large amounts of solution. Plasmonics offer the perspective of enhancement of CD signals, which would extend CD spectrometry to smaller amounts of molecules and to weaker chiral signals. However, plasmonic enhancement comes at the cost of additional complications: averaging over all orientations is no longer possible or reliable, linear dichroism leaks into CD signals because of experimental imperfections, scattering and its interference with the incident beam must be taken into account, and the interaction between chiral molecules and possibly chiral plasmonic structures considerably complicates the interpretation of measured signals. This Perspective aims to explore the motivations and problems of plasmonic chirality and to re-evaluate present and future solutions.

Progress and perspectives in single-molecule optical spectroscopy.

We review some of the progress of single-molecule optical experiments in the past 20 years and propose some perspectives for the coming years. We particularly focus on methodological advances in fluorescence, super-resolution, photothermal contrast, and interferometric scattering and briefly discuss a few of the applications. These advances have enabled the exploration of new emitters and quantum optics; the chemistry and biology of complex heterogeneous systems, nanoparticles, and plasmonics; and the detection and study of non-fluorescing and non-absorbing nano-objects. We conclude by proposing some ideas for future experiments. The field will move toward more and better signals of a broader variety of objects and toward a sharper view of the surprising complexity of the nanoscale world of single (bio-)molecules, nanoparticles, and their nano-environments.

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

Magnetic imaging is a versatile tool in biological and condensed-matter physics. Existing magnetic imaging techniques either require demanding experimental conditions which restrict the range of their applications or lack the spatial resolution required for single-particle measurements. Here, we combine photothermal (PT) microscopy with magnetic circular dichroism (MCD) to develop a versatile magnetic imaging technique using visible light. Unlike most magnetic imaging techniques, photothermal magnetic circular dichroism (PT MCD) microscopy works particularly well for single nanoparticles immersed in liquids. As a proof of principle, we demonstrate magnetic CD imaging of superparamagnetic magnetite nanoparticulate clusters immersed in microscope immersion oil. The sensitivity of our method allowed us to probe the magnetization curve of single ∼400-nm-diameter magnetite nanoparticulate clusters.

Photothermal Spectro-Microscopy as Benchmark for Optoplasmonic Bio-Detection Assays.

Optoplasmonic bio-detection assays commonly probe the response of plasmonic nanostructures to changes in their dielectric environment. The accurate detection of nanoscale entities such as virus particles, micelles and proteins requires optimization of multiple experimental parameters. Performing such optimization directly via analyte recognition is often not desirable or feasible, especially if the nanostructures exhibit limited numbers of analyte binding sites and if binding is irreversible. Here we introduce photothermal spectro-microscopy as a benchmarking tool for the characterization and optimization of optoplasmonic detection assays.

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

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.

Photothermal Microscopy: Imaging the Optical Absorption of Single Nanoparticles and Single Molecules.

The photothermal (PT) signal arises from slight changes of the index of refraction in a sample due to absorption of a heating light beam. Refractive index changes are measured with a second probing beam, usually of a different color. In the past two decades, this all-optical detection method has reached the sensitivity of single particles and single molecules, which gave birth to original applications in material science and biology. PT microscopy enables shot-noise-limited detection of individual nanoabsorbers among strong scatterers and circumvents many of the limitations of fluorescence-based detection. This review describes the theoretical basis of PT microscopy, the methodological developments that improved its sensitivity toward single-nanoparticle and single-molecule imaging, and a vast number of applications to single-nanoparticle imaging and tracking in material science and in cellular biology.

Laser-Induced Frequency Tuning of Fourier-Limited Single-Molecule Emitters.

The local interaction of charges and light in organic solids is the basis of distinct and fundamental effects. We here observe, at the single-molecule scale, how a focused laser beam can locally shift by hundreds of times their natural line width and, in a persistent way, the transition frequency of organic chromophores cooled at liquid helium temperature in different host matrices. Supported by quantum chemistry calculations, the results can be interpreted as effects of a photoionization cascade, leading to a stable electric field, which Stark-shifts the molecular electronic levels. The experimental observation is then applied to a common challenge in quantum photonics, i.e., the independent tuning and synchronization of close-by quantum emitters, which is desirable for multiphoton experiments. Five molecules that are spatially separated by about 50 µm and originally 20 GHz apart are brought into resonance within twice their line width. This tuning method, which does not require additional fabrication steps, is here independently applied to multiple emitters, with an emission line width that is only limited by the spontaneous decay and an inhomogeneous broadening limited to 1 nm. The system hence shows promise for photonic quantum technologies.

Circular Dichroism Measurement of Single Metal Nanoparticles Using Photothermal Imaging.

Circular dichroism (CD) spectroscopy is a powerful optical technique for the study of chiral materials and molecules. It gives access to an enantioselective signal based on the differential absorption of right and left circularly polarized light, usually obtained through polarization analysis of the light transmitted through a sample of interest. CD is routinely used to determine the secondary structure of proteins and their conformational state. However, CD signals are weak, limiting the use of this powerful technique to ensembles of many molecules. Here, we experimentally realize the concept of photothermal circular dichroism, a technique that combines the enantioselective signal from circular dichroism with the high sensitivity of photothermal microscopy, achieving a superior signal-to-noise ratio to detect chiral nano-objects. As a proof of principle, we studied the chiral response of single plasmonic nanostructures with CD in the visible range, demonstrating a signal-to-noise ratio better than 40 with only 30 ms integration time for these nanostructures. The high signal-to-noise ratio allows us to quantify the CD signal for individual nanoparticles. We show that we can distinguish relative absorption differences for right circularly and left circularly polarized light as small as gmin = 4 × 10-3 for a 30 ms integration time with our current experimental settings. The enhanced sensitivity of our technique extends CD studies to individual nano-objects and opens CD spectroscopy to numbers of molecules much lower than those in conventional experiments.

A Single-Crystalline Silver Plasmonic Circuit for Visible Quantum Emitters.

Plasmonic waveguides are key elements in nanophotonic devices, serving as optical interconnects between nanoscale light sources and detectors. Multimode operation in plasmonic two-wire transmission lines promises important degrees of freedom for near-field manipulation and information encoding. However, highly confined plasmon propagation along gold nanostructures is typically limited to the near-infrared region due to ohmic losses, excluding all visible quantum emitters from plasmonic circuitry. We report on the top-down fabrication of complex plasmonic nanostructures in single-crystalline silver plates. We demonstrate the controlled remote excitation of a small ensemble of fluorophores by a set of waveguide modes and the emission of the visible luminescence into the waveguide with high efficiency. This approach opens up the study of a nanoscale light-matter interaction between complex plasmonic waveguides and a large variety of quantum emitters available in the visible spectral range.

Absorption and Quantum Yield of Single Conjugated Polymer Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) Molecules.

We simultaneously measured the absorption and emission of single conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) molecules in a poly(methyl methacrylate) (PMMA) matrix using near-critical xenon to enhance the photothermal contrast for direct absorption measurements. We directly measured the number of monomers and the quantum yield of single conjugated polymer molecules. Simultaneous absorption and emission measurements provided new insight into the photophysics of single conjugated polymers under optical excitation: quenching in larger molecules is more efficient than in smaller ones. Photoinduced traps and defects formed under prolonged illumination lead to decrease of both polymer fluorescence and absorption signals with the latter declining slower.

Temperature dependent single molecule rotational dynamics in PMA.

Temperature dependent measurements of the rotational diffusion of single dye molecules in the polymer poly(methyl acrylate) (PMA) are presented and compared to shear viscosity data and numerical simulations of the rotational diffusion process. It is found that single molecule rotational diffusion very accurately follows the Debye-Stokes-Einstein predictions for the shear viscosity without any additional parameter. We employ a simple model of dynamic changes of the rotational speed of a single molecule. This dynamic heterogeneity model is based on a Gaussian distribution of activation energies in a VFTH (Vogel-Fulcher-Tammann-Hesse) type temperature dependence of the polymer viscosity. The simulations explain all experimental details concerning the stretched exponential single molecule relaxation dynamics and the related distributions. They also reveal that the observed distributions are related to the intrinsic physical properties of the polymer but do not in general reflect the instantaneous spread of local viscous properties.