Approach to high-frequency, cavity-enhanced Faraday rotation in fluids

Abstract

Recent work demonstrating detection of nuclear spin magnetization via Faraday rotation in transparent fluids promises novel opportunities for magnetic resonance imaging and spectroscopy. Unfortunately, low sensitivity is a serious concern. With this motivation in mind, we explore the use of an optical cavity to augment the Faraday rotation experienced by a linearly polarized beam traversing a sample fluid. Relying on a setup that affords reduced sample size and high-frequency modulation, we demonstrate amplification of regular (i.e., nonnuclear) Faraday rotation of order 20. Extensions of the present methodology that take into account the geometric constraints imposed by a high-field magnet may open the way to high-sensitivity, optically-detected magnetic resonance in the liquid state.

Time-resolved, optically detected NMR of fluids at high magnetic field

“…6 relied on a 50 cm long water sample prepolarized to the equivalent of 1.5 T. With the fluid continuously circulating from a high-to a low-field magnet ͑ϳ5 G͒ detection was carried out at 21 kHz using a cw "spin-locking" protocol. Here, we extend these results with observations of 19 F-and 1 H-induced optical Faraday rotation ͑OFR͒ in model liquid samples at high magnetic field. Our approach articulates OFR with detection schemes well known in modern nuclear magnetic resonance ͑NMR͒, a feature that allows us to implement time-resolved, multipulse sequences and measure light-encoded chemical-shift-resolved spectra.…”

“…In Fig. 3, we exploit the ab-sence of background signal to optically detect the timeresolved 19 F echo in a sample of hexafluorobenzene ͑C 6 F 6 ͒. We still use a CPMG train to optimize detection sensitivity, but in this case we extend the acquisition window to the interpulse interval.…”

“…2, detection is carried out during the duration of each interpulse interval e . ͑Main͒ ͑a͒ OFR-detected 19 F "T 2 -weighted" echo after coadding individual echoes in the train ͑see text͒. The "smooth" solid line corresponds to the inductively detected T 2 -weighted echo, superimposed for reference.…”

Magneto-optical contrast in liquid-state optically detected NMR spectroscopy

“…We then coadd these echoes to produce an average signal (8) whose amplitude will be a transverse coherence time-weighted measure of the system density; naturally, subsequent Fourier transform leads to the corresponding lowresolution (∼1 ppm) chemical shift-resolved spectrum. Although this strategy regains part of the sensitivity loss caused by field inhomogeneity and susceptibility broadening, we emphasize that the simpler scheme excitation-acquisition should suffice in a system designed to simultaneously suit the constraints of rf manipulation and optical sensing (8,10). Fig.…”

“…This trait makes OFR-detected NMR an approach well-suited to the emerging fields of microfluidics and chip-scale integrated A B C optical sensing. In this context, it is worth noting that the OFR angle doubles if the beam, after crossing the medium once, is reflected back through it a second time, a property that points to optical cavities as a signal enhancement route (10,19). Microfluidic distributed feedback grating (20) and liquid-liquid waveguide (21) architectures have already been used as the platform for lab-on-a-chip tunable fluidic lasers, and this approach could arguably be adapted to serve the purposes of OFR-detected NMR.…”