Solid-state quantum sensors are attracting wide interest because of their sensitivity at room temperature. In particular, the spin properties of individual nitrogen-vacancy (NV) colour centres in diamond 1-3 make them outstanding nanoscale sensors of magnetic fields 4-9 , electric fields 10 and temperature 11-13 under ambient conditions. Recent work on NV ensemble-based magnetometers 14-16 , inertial sensors 17 , and clocks 18 has employed unentangled colour centres to realize significant improvements in sensitivity 15,19 . However, to achieve this potential sensitivity enhancement in practice, new techniques are required to excite e ciently and to collect the optical signal from large NV ensembles. Here, we introduce a light-trapping diamond waveguide geometry with an excitation e ciency and signal collection that enables in excess of 5% conversion e ciency of pump photons into optically detected magnetic resonance 20 (ODMR) fluorescence-an improvement over previous single-pass geometries of more than three orders of magnitude. This marked enhancement of the ODMR signal enables precision broadband measurements of magnetic field and temperature in the low-frequency range, otherwise inaccessible by dynamical decoupling techniques.The NV's low absorption cross-section 21 has thus far prevented high conversion efficiency from excitation power to NV ODMR signal in bulk diamond samples using the typical single-pass excitation geometry illustrated in Fig. 1a. For NV densities around 10 15 cm −3 (refs 5,22), for which the NV spacing in the diamond lattice is sufficiently sparse to maintain long spin coherence times 6 , up to metre-long path length would be necessary for significant excitation absorption (see Supplementary Information). Efficient collection has been demonstrated from NVs in singlepass excitation geometries 14,15 , but absorption is less than 1% for a typical 300-µm-thick, electron-irradiated, type IIa diamond sample. Wide-field microscopy implemented with CCD (chargecoupled device) cameras can collect fluorescence from ensembles of up to 10 3 NV centres 23 in the focal volume of the objective, but suffers from low collection efficiency with most fluorescence emission trapped within the diamond owing to total internal reflection (TIR) at the diamond-air interface. External Fabry-Perot cavities could increase the optical depth 16 for green excitation, but would introduce bandwidth restrictions as well as the additional complexity of stabilized, narrow-linewidth excitation lasers.We have designed and realized a new ensemble measurement scheme that overcomes these limitations. The light-trapping diamond waveguide (LTDW) consists of a rectangular diamond slab with a small angled facet at one corner for input coupling of the pump beam, as illustrated in Fig. 1b. The input facet has a length a at an angle 45 • relative to the square sample sides, allowing pump light to couple into the structure while being confined by TIR (θ > θ c = 24.6 • ) on the other surfaces. Figure 1c shows an optical image of a pump beam...

Ultrashort pulses of extreme ultraviolet light from high-harmonic generation are a new tool for probing coupled charge, spin, and phonon dynamics with element specificity, attosecond pump-probe synchronization, and time resolution of a few femtoseconds in a tabletop apparatus. In this paper, we address an important question in magneto-optics that has implications for understanding magnetism on the fastest time scales: Is the signal from the transverse magneto-optical Kerr effect at the M 2;3 edges of a magnetic material purely magnetic or is it perturbed by nonmagnetic artifacts? Our measurements demonstrate conclusively that transverse magneto-optical Kerr measurements at the M 2;3 edges sensitively probe the magnetic state, with almost negligible contributions from the transient variation of the refractive index by the nonequilibrium hot-electron distribution. In addition, we compare pump-probe demagnetization dynamics measured by both high harmonics and conventional visible-wavelength magneto-optics and find that the measured demagnetization times are in agreement.

We demonstrate a robust, scale-factor-free vector magnetometer, which uses a closed-loop frequency-locking scheme to simultaneously track Zeeman-split resonance pairs of nitrogen-vacancy (NV) centers in diamond. Compared with open-loop methodologies, this technique is robust against fluctuations in temperature, resonance linewidth, and contrast; offers a three-order-of-magnitude increase in dynamic range; and allows for simultaneous interrogation of multiple transition frequencies.By directly detecting the resonance frequencies of NV centers aligned along each of the diamond's four tetrahedral crystallographic axes, we perform full vector reconstruction of an applied magnetic field. * These authors contributed equally to this work. †