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.
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.
The extreme temperature sensitivity of whispering-gallery-mode (WGM) microresonators holds great promise as a detection strategy for single-particle photothermal microscopy and spectroscopy. The detection limit is currently partially constrained by frequency noise from the laser used to probe the cavity resonance wavelength. We present a measurement technique capable of simultaneously detecting backscattered and transmitted light from a wavelengthlocked optical microresonator, with laser intensity noise and frequency noise partitioned into the two independent detection channels. Photothermal mapping of single absorbing nano-objects demonstrates that both methods are capable of high signal/noise, exceeding 30,000:1 in the backscattering channel for a photothermally-induced microresonator resonance shift of 93 fm.
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.