Photothermal Microscopy of Nonluminescent Single Particles Enabled by Optical Microresonators

Abstract

A powerful new paradigm for single-particle microscopy on nonluminescent targets is reported using ultrahigh-quality factor optical microresonators as the critical detecting element. The approach is photothermal in nature as the microresonators are used to detect heat dissipated from individual photoexcited nano-objects. The method potentially satisfies an outstanding need for single-particle microscopy on nonluminescent objects of increasingly smaller absorption cross section. Simultaneously, our approach couples the sensitivity of label-free detection using optical microresonators with a means of deriving chemical information on the target species, a significant benefit. As a demonstration, individual nonphotoluminescent multiwalled carbon nanotubes are spatially mapped, and the per-atom absorption cross section is determined. Finite-element simulations are employed to model the relevant thermal processes and elucidate the sensing mechanism. Finally, a direct pathway to the extension of this new technique to molecules is laid out, leading to a potent new method of performing measurements on individual molecules.

Manipulating optical nonlinearities of molecular polaritons by delocalization

“…In the collective basis, the quadratic part of the Hamiltonian is written as: (5) where the operators αd are the annihilation operators of dark modes. The bright normal modes of the above Hamiltonian are given by the lower (LP) and upper polaritons (UP) with annihilation operators written as (8): αUP(k) = cos(θk/2)a(k) + sin (θk/2)b(k) (6) αUP(k) = −sin(θk/2)a(k) + cos(θk/2)b(k) (7) where the specific form of the mixing angles θk and polariton frequencies ωLP(k) and ωUP(k) are inessential for our discussion.…”

“…This interplay between microscopic and macroscopic characteristics is at the heart of the versatility of polaritons. Much of the recent research on molecular polaritons has focused on the modification of molecular properties through the interaction with an optical cavity (5)(6)(7)(8)(9)(10)(11)(12)(13)(14). Conversely, control of polariton optical response via manipulation of the macroscopic parameters which define the electromagnetic modes (e.g., cavity length) has received far less attention (15) (Fig.1a) and, in fact, we are not aware of any investigations of these effects in the nonlinear photonic regime.…”

Applications of Optical Microcavity Resonators in Analytical Chemistry

“…While not necessarily a hybrid sensor geometry, the Goldsmith group has uniquely utilized microtoroidal resonators in concert with an orthogonal pump bead to perform single particle absorption measurements (151). Silica microtoroids were coated with multi-walled carbon nanotubes.…”

Single-Particle Absorption Spectroscopy by Photothermal Contrast

“…9,10 Although absorption-based imaging has recently been demonstrated. [11][12][13][14][15][16][17][18] no absorption spectroscopy is available for nanostructures of arbitrary size, geometry, and material. Absorption spectroscopy of single molecules, complex nanostructures, and macromolecular systems would be a significant advancement with great impact as, for example, the identification of degenerate intermediates in enzymatic and catalytic processes have so far only been examined on the ensemble level.…”