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arXiv:1410.6239 · New Journal of Physics

Laser threshold magnetometry

Jan Jeske, Jared H. Cole, Andrew D. Greentree

Published Jan 8, 2016• 58 citations
arXivHTMLPDF

Abstract

We propose a new type of sensor, which uses diamond containing the optically active nitrogen-vacancy (NV − ) centres as a laser medium. The magnetometer can be operated at room-temperature and generates light that can be readily fibre coupled, thereby permitting use in industrial applications and remote sensing. By combining laser pumping with a radio-frequency Rabi-drive field, an external magnetic field changes the fluorescence of the NV − centres. We use this change in fluorescence level to push the laser above threshold, turning it on with an intensity controlled by the external magnetic field, which provides a coherent amplification of the readout signal with very high contrast. This mechanism is qualitatively different from conventional NV − –based magnetometers which use fluorescence measurements, based on incoherent photon emission. We term our approach laser threshold magnetometer (LTM). We predict that an NV − –based LTM with a volume of 1 mm 3 can achieve shot-noise limited dc sensitivity of 1.86 fT and ac sensitivity of 3.97 fT .

DOI
10.1088/1367-2630/18/1/013015

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We propose a new type of sensor, which uses diamond containing the optically active nitrogen-vacancy (NV − ) centres as a laser medium. The magnetometer can be operated at room-temperature and generates light that can be readily fibre coupled, thereby permitting use in industrial applications and remote sensing. By combining laser pumping with a radio-frequency Rabi-drive field, an external magnetic field changes the fluorescence of the NV − centres. We use this change in fluorescence level to push the laser above threshold, turning it on with an intensity controlled by the external magnetic field, which provides a coherent amplification of the readout signal with very high contrast. This mechanism is qualitatively different from conventional NV − –based magnetometers which use fluorescence measurements, based on incoherent photon emission. We term our approach laser threshold magnetometer (LTM). We predict that an NV − –based LTM with a volume of 1 mm 3 can achieve shot-noise limited dc sensitivity of 1.86 fT and ac sensitivity of 3.97 fT .
High precision magnetometry is important for a range of applications from the monitoring of biologically generated magnetic fields (e.g. magnetoencephalography and magnetocardiography), to navigation in GPS denied environments, to the detection of gravitational waves. Diamond containing the negatively-charged nitrogen vacancy colour centre (NV-) has emerged as a powerful room-temperature sensing solution. Here we explore NV- centres as a laser medium for a new form of magnetometry: laser threshold magnetometry (LTM). LTM works by placing NV- inside an optical cavity and uses the coherent laser output as a potentially more sensitive readout channel than is possible using conventional (incoherent) optically detected magnetic resonance. Here we show progress towards LTM with diamond. We show twolaser excitation and stimulated emission in free space, and report progress towards diamond-cavity experiments. Our studies highlight the need for different NV- optimisation for laser applications, rather than those conventionally used for quantum information applications
Laser threshold magnetometry (LTM) promises a path towards significant reductions in optical power and volume for high sensitivity diamond nitrogen-vacancy (NV) magnetometers. LTM utilizes the difference in laser threshold due to intra-cavity diamond absorption while driving the RF spin resonance. Power consumption is directly related to the sizes of the laser spot on the gain chip and the diamond. By confining the spot size, we can drastically reduce the laser threshold pump power needed (up to ~34x for proposed linear cavity design). By physically connecting all the pieces of our sensor, we also reduce laser noise allowing us to approach closer to threshold, therefore increasing our contrast and decreasing our photon shot noise limit.
Abstract We propose a high-sensitivity magnetometry scheme based on a diamond Raman laser with visible pump absorption by an ensemble of coherently microwave driven negatively charged nitrogen-vacancy centres (NV−) in the same diamond crystal. The NV− centres’ absorption and emission are spin-dependent. We show how the varying absorption of the NV− centres changes the Raman laser output. A shift in the diamond Raman laser threshold and output occurs with the external magnetic field and microwave driving. We develop a theoretical framework with steady-state solutions to describe the effects of coherently driven NV− centres including the charge state switching between NV− and its neutral charge state NV0 in a diamond Raman laser. We discuss that such a laser working at the threshold can be employed for magnetic field sensing. In contrast to previous studies on NV− magnetometry with visible laser absorption, the laser threshold magnetometry method is expected to have low technical noise, due to low background light in the measurement signal. For magnetic-field sensing, we project a shot-noise limited DC sensitivity of a few pT / H z in a well-calibrated cavity with realistic parameters. This sensor employs the broad visible absorption of NV− centres and unlike previous laser threshold magnetometry proposals it does not rely on active NV− centre lasing or an infrared laser medium at the specific wavelength of the NV− centre’s infrared absorption line.
AbstractWe propose a high-sensitivity magnetometry scheme based on a diamond Raman laser with visible pump absorption by an ensemble of coherently microwave driven negatively charged nitrogen-vacancy centres (NV−) in the same diamond crystal. The NV− centres’ absorption and emission are spin-dependent. We show how the varying absorption of the NV− centres changes the Raman laser output. A shift in the diamond Raman laser threshold and output occurs with the external magnetic field and microwave driving. We develop a theoretical framework with steady-state solutions to describe the effects of coherently driven NV− centres including the charge state switching between NV− and its neutral charge state NV0 in a diamond Raman laser. We discuss that such a laser working at the threshold can be employed for magnetic field sensing. In contrast to previous studies on NV− magnetometry with visible laser absorption, the laser threshold magnetometry method is expected to have low technical noise, due to low background light in the measurement signal. For magnetic-field sensing, we project a shot-noise limited DC sensitivity of a few pT /Hz in a well-calibrated cavity with realistic parameters. This sensor employs the broad visible absorption of NV− centres and unlike previous laser threshold magnetometry proposals it does not rely on active NV− centre lasing or an infrared laser medium at the specific wavelength of the NV− centre’s infrared absorption line.
Nitrogen vacancy (NV) centers in diamond have attracted considerable recent interest for use in quantum sensing, promising increased sensitivity for applications ranging from geophysics to biomedicine. Conventional sensing schemes involve monitoring the change in red fluorescence from the NV center under green laser and microwave illumination. Due to the strong fluorescence background from emission in the NV triplet state and low relative contrast of any change in output, sensitivity is severely restricted by a high optical shot noise level. Here, we propose a means to avoid this issue, by using the change in green pump absorption through the diamond as part of a semiconductor external cavity laser run close to the lasing threshold. We show that theoretical sensitivity to the magnetic field on the pT/Hz\sqrt{\mathrm{Hz}}Hz​ level is possible using a diamond with an optimal density of NV centers. We discuss the physical requirements and limitations of the method, particularly the role of amplified spontaneous emission near threshold and explore realistic implementations using current technology.
Nitrogen-vacancy (NV) centers in diamond are a promising platform for high-precision magnetometry. In contrast to the use of spontaneous emission in a number of NV-magnetometers, laser threshold magnetometry exploits stimulated emission of NV centers by placing an NV-doped diamond inside an optical cavity. The NV laser system is predicted to reach a high magnetic-field-dependent contrast and coherent signal strength, leading to an improved magnetic field sensitivity combined with high linearity. Here, we consider a two-media setup where the cavity additionally includes a vertical external cavity surface emitting a laser. This optically active material compensates cavity losses at 750 nm while still allowing for magnetic-field-dependent effects from the NV-diamond. We demonstrate a magnetic-field-dependent laser threshold and investigate the effects of pump laser induced absorption of the diamond. The experimental data are supported by an analytical simulation based on a rate model. Furthermore, we derive a generalized formula to compute the shot-noise-limited magnetic field sensitivity in the regime of high contrast, yielding 49.07(0.33)pT/Hz for the present setup. Simulations with an optimized NV-diamond suggest that values down to 4.9fT/Hz are possible.