High-Speed Spectral Characterization of Single-Molecule SERS Fluctuations.

The concept of plasmonic "hotspots" is central to the broad field of nanophotonics. In surface-enhanced Raman scattering (SERS), hotspots can increase Raman scattering efficiency by orders of magnitude. Hotspot dimensions may range from a few nanometers down to the atomic scale and are able to generate SERS signals from single molecules. However, these single-molecule SERS signals often show significant fluctuations, and the concept of intense, localized, yet static hotspots has come into question. Recent experiments have shown these SERS intensity fluctuations (SIFs) to occur over an extremely wide range of timescales, from seconds to microseconds, due to the various physical mechanisms causing SERS and the dynamic nature of light-matter interaction at the nanoscale. The underlying source of single-molecule SERS fluctuations is therefore likely to be a complex interplay of several different effects at different timescales. A high-speed acquisition system that captures a full SERS spectrum with microsecond time resolution can therefore provide information about these dynamic processes. Here, we show an acquisition system that collects at a rate of 100,000 SERS spectra per second, allowing high-speed characterization. We find that while each individual SIF event will enhance a different portion of the SERS spectrum, including a single peak, over 10s to 100s of microseconds, the SIF events overall do not favor one region of the spectrum over another. These high-speed SIF events can therefore occur with relatively equal probability over a broad spectral range, covering both the anti-Stokes and the Stokes sides of the spectrum, sometimes leading to anomalously large anti-Stokes peaks. This indicates that both temporally and spectrally transient hotspots drive the SERS fluctuations at high speeds.

Measurement of Optical Rubidium Clock Frequency Spanning 65 Days.

Optical clocks are emerging as next-generation timekeeping devices with technological and scientific use cases. Simplified atomic sources such as vapor cells may offer a straightforward path to field use, but suffer from long-term frequency drifts and environmental sensitivities. Here, we measure a laboratory optical clock based on warm rubidium atoms and find low levels of drift on the month-long timescale. We observe and quantify helium contamination inside the glass vapor cell by gradually removing the helium via a vacuum apparatus. We quantify a drift rate of 4×10-15/day, a 10 day Allan deviation less than 5×10-15, and an absolute frequency of the Rb-87 two-photon clock transition of 385,284,566,371,190(1970) Hz. These results support the premise that optical vapor cell clocks will be able to meet future technology needs in navigation and communications as sensors of time and frequency.

Ultralow phase noise microwave generation with an Er:fiber-based optical frequency divider.

We present an optical frequency divider based on a 200 MHz repetition rate Er:fiber mode-locked laser that, when locked to a stable optical frequency reference, generates microwave signals with absolute phase noise that is equal to or better than cryogenic microwave oscillators. At 1 Hz offset from a 10 GHz carrier, the phase noise is below -100 dBc/Hz, limited by the optical reference. For offset frequencies >10 kHz, the phase noise is shot noise limited at -145 dBc/Hz. An analysis of the contribution of the residual noise from the Er:fiber optical frequency divider is also presented.