Dual-media laser system: Nitrogen vacancy diamond and red semiconductor laser

Abstract

Diamond is a potential host material for laser applications due to its exceptional thermal properties, ultrawide bandgap, and color centers, which promise gain across the visible spectrum. More recently, coherent laser methods offer improved sensitivity for magnetometry. However, diamond fabrication is difficult in comparison to other crystalline matrices, and many optical loss channels are not yet understood. Here, we demonstrate a continuous-wave laser threshold as a function of the pump intensity on nitrogen-vacancy (NV) color centers. To achieve this, we constructed a laser cavity with both an NV diamond medium and an intracavity antireflection-coated diode laser. This dual-medium approach compensates intrinsic losses of the cavity by providing a fixed additional gain below threshold of the diode laser. We observe a continuous-wave laser threshold of the laser system and linewidth narrowing with increasing green pump power on the NV centers. Our results are a major development toward coherent approaches to magnetometry.

Fig. 1.. Experimental design.

(A) Energy levels of the NV center: The green pump laser addresses a transition from the ground state to a phonon-added excited state, and the laser transition is from the excited state to a phonon-added ground state. (B) Sketch of the linear cavity setup: The backside mirror of the diode laser (DL) and the external outcoupling mirror (M₁) form the cavity, the diode laser and the NV-diamond (NV-D) are the active media, and the lens (L₁) is necessary to focus the output of the antireflection-coated side of the diode laser.

Fig. 2.. Output power of the dual-media laser system.

(A) Output power (P) of the laser system as a function of the laser diode current (I_red). Gray curve represents without NV gain (green pump laser turned off), and green curve denotes with fixed NV gain. Dashed lines denote the results of a simulation based on the experimental parameters. (B) Relative change in output power (∆P) between the measured values shown in (A). (C) Demonstration of a cw NV laser threshold: Green denotes maximum amplitude of the main laser line of the dual-media laser system as a function of green pump power on the NV-diamond sample. The laser diode is operated with a fixed-diode current below the laser threshold, which is then overcome by the NV gain. Orange denotes the result of the simulation. a.u., arbitrary unit.

Fig. 3.. Spectral output and linewidth narrowing of the NV laser system.

(A) Spectra of the diode-assisted NV laser system. The color corresponds to the green pump power [color scale on the right of (B)]. (B) Detailed view of the main laser line (measured values shown as crosses) with a fitted Gaussian profile (dashed). (C) Laser linewidth [full width at half maximum (FWHM), obtained from the fitted Gaussian curves] as a function of pump power. A reduction of the linewidth above the threshold (at ∼2.5 W) is visible. (D) Laser linewidth as a function of reciprocal amplitude (blue) of the fitted spectra shown in (B). Orange curve denotes the adapted theoretical model.