Stand-Off Magnetometry with Directional Emission from Sodium Vapors

Stand-off magnetometry allows measuring magnetic field at a distance, and can be employed in geophysical research, hazardous environment monitoring, and security applications. Stand-off magnetometry based on resonant scattering from atoms or molecules is often limited by the scarce amounts of detected light. The situation would be dramatically improved if the light emitted by excited atoms were to propagate towards the excitation light source in a directional manner. Here, we demonstrate that this is possible by means of mirrorless lasing. In a tabletop experiment, we detect free-precession signals of ground-state sodium spins under the influence of an external magnetic field by measuring backward-directed light. This method enables scalar magnetometry in the Earth field range, that can be extended to long-range remote sensing.

Figure 1

Schematic of stand-off magnetometer. (a) Energy levels of sodium atoms and excitation scheme. Pump beam is a left circularly polarized 589 nm laser exciting the $3S_{1/2}$ to $3P_{3/2}$ transition, which generates atomic spin polarization in the $3S_{1/2}$ state; the probe beam is a combination of right circularly polarized 589 nm and 569 nm light, which excites atoms to the $4D_{5/2}$ state and therefore produces population inversion between the $4P_{3/2}$ and $4S_{1/2}$ states, thus enabling gain for $2.21\mu \mathrm{m}$ spontaneous emission and mirrorless lasing. (b) Experimental setup. EOM, electro-optic modulator driven by 1.713 GHz signal to generate a sideband for hyperfine repumping; AOM1, acousto-optic modulator used to pulse the pump beam; AOM2, acousto-optic modulator used to pulse the probe beam; BE, beam expander; $\lambda /2$, half-wave plate; $\lambda /4$, quater-wave plate; PBS, polarizing beam splitter; BS, beam splitter; BD, beam dump; L, a convex lens; GP, glass plate for reflecting the mirrorless-lasing emission; PD, photodiode placed about 1.5 m away from the center of the vapor cell; DSO, digital storage oscilloscope; PC, personal computer. (c) The timing diagram. The waveforms at the top and the middle are the control pulses of AOM1 and AOM2, respectively, and the waveform on the bottom represents the light pulses sent into the vapor cell, in which $\tau$ is the free evolution of the atomic spins “in the dark.”

Figure 2

(a) Orange dots (purple triangles) represent the free-precession signal with $28.4\mu \mathrm{T}$ ($56.7\mu \mathrm{T}$) bias magnetic field applied along the $x$ direction. For each evolution time, we average readouts from 3000 modulation periods ($20\mu \mathrm{s}$) and the total integration time for each free-precession signal is about 6 s. (b) The amplitude spectrum of the free precession signal with $28.4\mu \mathrm{T}$ ($56.7\mu \mathrm{T}$) bias magnetic field shown in (a) and its Lorentzian fitting are represented by orange dots (purple triangles) and the light orange solid line (light purple dashed line), respectively. (c) Blue points: measured magnetic fields at different $B_x$ applied, which are derived from the central frequencies of the Lorentzian fitting, as shown in (b); red line: a fitting of the measured magnetic fields, which suggests a $1.0(\pm 0.4)\mu \mathrm{T}$ residual magnetic field along $-x$ direction and a $8.7(\pm 1.2)\mu \mathrm{T}$ residual magnetic field in the $y\text{−}z$ plane in the magnetic shield; gray dashed line: a line that assumes no residual magnetic field in the magnetic shield, so that the total magnetic strength $|B|$ equals to the applied $|B_x|$.