Erik H. Horak

Chip-Scale Fabrication of High-Q All-Glass Toroidal Microresonators for Single-Particle Label-Free Imaging.

Whispering-gallery-mode microresonators enable materials for single-molecule label-free detection and imaging because of their high sensitivity to their micro-environment. However, fabrication and materials challenges prevent scalability and limit functionality. All-glass on-chip microresonators significantly reduce these difficulties. Construction of all-glass toroidal microresonators with high quality factor and low mode volume is reported and these are used as platforms for label-free single-particle imaging.

Optical Microresonators for Sensing and Transduction: A Materials Perspective

Optical microresonators confine light to a particular microscale trajectory, are exquisitely sensitive to their microenvironment, and offer convenient readout of their optical properties. Taken together, this is an immensely attractive combination that makes optical microresonators highly effective as sensors and transducers. Meanwhile, advances in material science, fabrication techniques, and photonic sensing strategies endow optical microresonators with new functionalities, unique transduction mechanisms, and in some cases, unparalleled sensitivities. In this progress report, the operating principles of these sensors are reviewed, and different methods of signal transduction are evaluated. Examples are shown of how choice of materials must be suited to the analyte, and how innovations in fabrication and sensing are coupled together in a mutually reinforcing cycle. A tremendously broad range of capabilities of microresonator sensors is described, from electric and magnetic field sensing to mechanical sensing, from single-molecule detection to imaging and spectroscopy, from operation at high vacuum to in live cells. Emerging sensing capabilities are highlighted and put into context in the field. Future directions are imagined, where the diverse capabilities laid out are combined and advances in scalability and integration are implemented, leading to the creation of a sensor unparalleled in sensitivity and information content.

Exploring Electronic Structure and Order in Polymers via Single-Particle Microresonator Spectroscopy

PEDOT PSS, a transparent electrically conductive polymer, finds widespread use in electronic devices. While empirical efforts have increased conductivity, a detailed understanding of the coupled electronic and morphological landscapes in PEDOT:PSS has lagged due to substantial structural heterogeneity on multiple length-scales. We use an optical microresonator-based absorption spectrometer to perform single-particle measurements, providing a bottom-up examination of electronic structure and morphology ranging from single PEDOT:PSS polymers to nascent films. Using single-particle spectroscopy with complementary theoretical calculations and ultrafast spectroscopy, we demonstrate that PEDOT:PSS displays bulk-like optical response even in single polymers. We find highly ordered PEDOT assemblies with long-range ordering mediated by the insulating PSS matrix and reveal a preferential surface orientation of PEDOT nanocrystallites absent in bulk films with implications for interfacial electronic communication. Our single-particle perspective provides a unique window into the microscopic structure and electronic properties of PEDOT:PSS.

Sculpting Fano Resonances To Control Photonic–Plasmonic Hybridization

Hybrid photonic-plasmonic systems have tremendous potential as versatile platforms for the study and control of nanoscale light-matter interactions since their respective components have either high-quality factors or low mode volumes. Individual metallic nanoparticles deposited on optical microresonators provide an excellent example where ultrahigh-quality optical whispering-gallery modes can be combined with nanoscopic plasmonic mode volumes to maximize the systems photonic performance. Such optimization, however, is difficult in practice because of the inability to easily measure and tune critical system parameters. In this Letter, we present a general and practical method to determine the coupling strength and tailor the degree of hybridization in composite optical microresonator-plasmonic nanoparticle systems based on experimentally measured absorption spectra. Specifically, we use thermal annealing to control the detuning between a metal nanoparticles localized surface plasmon resonance and the whispering-gallery modes of an optical microresonator cavity. We demonstrate the ability to sculpt Fano resonance lineshapes in the absorption spectrum and infer system parameters critical to elucidating the underlying photonic-plasmonic hybridization. We show that including decoherence processes is necessary to capture the evolution of the lineshapes. As a result, thermal annealing allows us to directly tune the degree of hybridization and various hybrid mode quantities such as the quality factor and mode volume and ultimately maximize the Purcell factor to be 104.

Drumming up single-molecule beats

Single-particle and single-molecule techniques have become invaluable tools to unravel complex chemical phenomena and biological processes. While fluorescence-based techniques have flourished due to advances in wavelength-dependent optical filtering and highly sensitive detectors (1), other methodologies (Fig. 1) have only recently achieved single-molecule sensitivity limits. A technique based on an oscillating nanodrum offers a promising new direction (2). Fig. 1. Various photothermal transducers for single-particle and single-molecule detection. ( A ) Nanomechanical resonator. A driven high-Q mechanical resonator oscillates at its stress-dependent frequency (red arrow). The photothermal plume produces thermal expansion, relieving stress and altering the resonance frequency measured by a Doppler vibrometer (green). ( B ) Surrounding medium. The heat plume produces a thermal plume (depicted by black circles) that a probe beam (green) scatters off of (dashed green arrows). The use of a high thermooptic coefficient medium (purple) enables sensitive detection. ( C ) Optical microresonator. A high-Q optical microresonator is photothermally heated, shifting the resonance condition monitored by a frequency-locked probe laser (green). ( D ) Atomic force microscope. A photothermal plume introduces thermal expansion in the sample, measured by deflection of the cantilever of the atomic force microscope (green arrow). ( E ) Surface plasmon resonance. Surface plasmons (green) are excited in a wide-field configuration. The thermal plume alters the local refractive index, shifting the plasmon. Room-temperature absorption-based techniques, while commonplace at the bulk level, offer significant challenges at the single-molecule level. A major obstacle arises when applying a typical transmission-type experimental geometry to single molecules from the extremely small single-molecule absorption cross-section, with a molecule yielding an almost imperceptible change in a large transmitted signal. At low temperature, … [↵][1]1To whom correspondence should be addressed. Email: rhg{at}chem.wisc.edu. [1]: #xref-corresp-1-1

Optical power diodes based on phase-transition materials (Conference Presentation)

We present several designs and experimental implementations of optical power diodes – devices that are designed to be transparent from one direction, but opaque from the other, when illuminated by a beam with sufficient intensity. Optical power diodes can be used to protect optical devices that both detect and transmit light. Our designs are based on phase-change material vanadium dioxide (VO2), which undergoes an insulator-to-metal transition (IMT) that can be triggered thermally or optically. Here, VO2 films serve as nonlinear elements that can be transformed from transparent to opaque by intense illumination. We build thin-film metallic structures on top of the VO2 films such that the optical absorption becomes asymmetric – light impinging from one direction is absorbed at a higher rate than from the other direction, triggering the transition, and turning the device opaque. This results in asymmetric transmission. The designs are optimized with finite-difference time-domain (FDTD) simulations, using optical constants of VO2 extracted using ellipsometry, and are shown to be scalable across the near- and mid-infrared. Our initial experimental results using a simple design comprised of metal and VO2 films on sapphire, designed for an operating wavelength of 1.35µm, show a transmission asymmetry ratio of ~2, and experiments with superior designs are ongoing. Future work will include the use of defect-engineered VO2 to engineer the intensity threshold of optical power diodes.

Cleaning procedure for improved photothermal background of toroidal optical microresonators

High Q-factors and small mode volumes have made toroidal optical microresonators exquisite sensors to small shifts in the effective refractive index of the WGM modes. Eliminating contaminants and improving quality factors is key for many different sensing techniques, and is particularly important for photothermal imaging as contaminants add photothermal background obscuring objects of interest. Several different cleaning procedures including wet- and dry-chemical procedures are tested for their effect on Q-factors and photothermal background. RCA cleaning was shown to be successful in contrast to previously described acid cleaning procedures, most likely due to the different surface reactivity of the acid reagents used. UV-ozone cleaning was shown to be vastly superior to O2 plasma cleaning procedures, significantly reducing the photothermal background of the resonator.

Optical Microresonators: Chip‐Scale Fabrication of High‐Q All‐Glass Toroidal Microresonators for Single‐Particle Label‐Free Imaging (Adv. Mater. 15/2016)

On page 2945, R. H. Goldsmith and co-workers describe an all-glass, high-Q, low-mode-volume optical microresonator for label-free sensing and imaging, which overcomes scalability barriers and also brings other advantages, including optical transparency. By using the microresonators as transducers to detect heat dissipated by individual nano-objects upon photoexcitation, label-free single-particle imaging is demonstrated.

Optical microresonators as single-molecule spectrometers

A new approach is described for sensing and spectroscopy of single non-luminescent nano-objects which combines the spatial and spectral selectivity of laser microscopy with the tremendous sensitivity of ultrahigh-quality factor optical microresonators.

Photothermal imaging of individual carbon nanofibers with optical microresonators

A new method is described for measuring the absorption of light by single non-emissive nanoparticles. Individual carbon nanofibers are imaged using a photonic transducer to quantify the heat dissipated after the electronic energy is thermalized. Leveraging the high sensitivity of ultrahigh-quality-factor optical microresonators as photothermal transducers provides high sensitivity. Polarization-resolved measurements indicate that the orientation of the absorption dipole of a nanofiber matches the long axis of the fiber. The per-atom absorption cross-section is determined to be (2.9 x 10-18 cm2 /carbon atom), in close agreement with the value for bulk graphite.