Patent Application
20140072008
A point defect in diamond, known as a nitrogen-vacancy (NV) center, exhibits magnetic resonances and can be used as a magnetometer. However, sensitivity for this type of magnetometer is limited, due to the limited amount of light that can be collected from a luminescing NV center, such as an NV center in bulk diamond or diamond nano-particles. The amount of photoluminescence in such systems is limited by the spontaneous emission rate of nitrogen-vacancy centers.
Magnetometry may be performed with an emitter (e.g., a color center) that has a ground state composed of multiple states whose splitting is magnetic field dependent, and where the multiple states exhibit different output by the color center. The output of color centers may change with application of a magnetic field under certain conditions, such as during application of a microwave field. Examples may provide Optically Detected Magnetic Resonance (ODMR), including continuous-wave ODMR. The luminescence signal from color centers may be enhanced through the use of lasing, where a gain medium includes color centers and has been placed in a resonant cavity. The laser action may be taken advantage of to provide a much more sensitive detector. For example, the sensitivity of a magnetometer may depend on the amount of collected light. By achieving lasing, it is possible to rely on stimulated emission, rather than spontaneous emission, and achieve significant amplification of the signal (e.g., output from a color center). Thus, examples may use stimulated emission, enhanced with a cavity, to create a laser to detect output from color centers. Lasing may be enabled, for example, by coupling the color centers to optical cavities, to increase the sensitivity of magnetometers. Further, examples provide output with enhanced spatial coherence, and enable produced light to be channeled into an optical subsystem, such as a waveguide, in contrast to non-lasing systems based on merely collecting spontaneous emission.
The color center 120 is associated with a plurality of spin states 122, which may be referred to as spin (sub) levels, and a lower laser level 124. The lower laser level 124 is identified as the lower level used to define the population inversion associated with lasing for system 100. The lower laser level 124 may be a spin state 122. The lower laser level 124 may be a true “ground state” or ensemble of ground state(s) that are the spin levels that color center 120 would be pumped from (e.g., based on pump energy 102). The plurality of spin states 122 associated with the lower laser level 124 are affected by magnetic field 104.
The plurality of spin states 122 may be associated with energy spacing between the spin states 122. Applying magnetic field 104 may change the energy spacing between the spin states 122. Depending on which state, different laser output may be provided when applying a microwave field (e.g., microwaves 106). Thus, by detecting output of the color center 120, it is possible to infer information regarding the magnetic field 104. If the plurality of spin states 122 exhibit a plurality of different outputs, the spin states 122 also may provide gain at different wavelengths.
Detection of output from the color center 120 may be enhanced based on lasing. The lasing is based on materials (e.g., color center 120) that exhibit ODMR, producing a gain that depends on the applied magnetic field 104 and/or microwaves 106 (i.e., providing an ODMR signal).
The resonant cavity 110 enables system 100 to lase, such that produced light 108 has increased availability due to stimulated emission. Lasing may provide other benefits, such as emitting produced light 108 in a single mode to facilitate optical collection (e.g., using a single-mode waveguide or other optical subsystem). Other examples may emit produced light 108 in multiple modes (i.e., examples are not limited to single mode). To lase, a gain medium 112, such as diamond or silicon carbide (SiC), may be associated with a resonant cavity 110, such as an optical cavity. For example, the gain medium 112 may be formed directly into a resonant cavity 110. The resonant cavity 110 may include other types of cavities, such as Bragg gratings coupled with a single mode diamond waveguide, or otherwise sandwiching the gain medium 112 between two mirrors.
For system 100 to lase, the associated gain of the system 100 is to be higher than the losses associated with the system 100. For a resonant cavity 110 having a quality factor of 104 (e.g., for a resonant cavity 110 formed in single crystal diamond), the associated gain may be achieved based on a minimum number of color centers (e.g., NV centers for a diamond gain medium 112) of approximately ˜105/micron3 (for lasing through vibronic states).
Lasing may enhance the sensitivity of the laser magnetometer system 100, and may achieve approximately ˜1 nTVsqrt(Hz) sensitivity. Enhanced sensitivity enables various applications, at levels not previously achievable, such as detection of electrical neuron signals or other sensitive magnetic field sensing applications.
The gain medium 112 can be used to support at least one color center 120. The gain medium 112 may include a variety of color centers 120 that may be distributed throughout the gain medium 112 according to various arrangements, such as a uniform distribution. The gain medium 112 may be diamond, SiC, and/or various wide-bandgap, group IV semiconductor materials. The gain medium 112 may exhibit dielectric and even magnetic properties, and may be used to form a laser cavity (resonant cavity 110).
The color center 120 is to provide the actual gain for the gain medium 112, and the color center 120 is embedded in the gain medium 112. The color center 120 may be a nitrogen vacancy (NV) center in examples where the gain medium 112 is diamond, and the color center 120 may be a divacancy in examples where the gain medium 112 is silicon carbide (SiC). A divacancy may be formed by a missing silicon atom whose nearest carbon neighbor is also missing. Other forms of NV centers and/or divacancies may be used.
In an example, the gain medium 112 is a single-crystal diamond. The single crystal diamond may be engineered to contain a high concentration of color centers 120, which may be NV centers in the single-crystal diamond gain medium 112. The color centers 120 may emit red light output when excited with pump energy 102 (e.g., green light). Stimulated emission, and thus lasing, may be achieved with sufficient concentration of color centers 120, high quality resonant cavity 110, and sufficient pump energy 102. When lasing is achieved, the amount of output emitted from the color centers 120 increases dramatically.
A nitrogen vacancy center (color center 120) may be formed in diamond by removing two neighboring carbon atoms in the diamond lattice, and replacing only one of them with a nitrogen atom. A divacancy center (color center 120) may exist in SiC, and also may exist in diamond (e.g., two missing carbon atoms at nearest neighbor lattice sites). Divacancies may correspond to spin-1 ground states that can be spin-polarized with incident light.
Pump energy 102 may be optical, electrical, or other forms of energy, that may be absorbed by and/or transferred to the color center 120. The resonant cavity 110 then may provide feed-back, and the gain medium 112 may serve as an amplifier, to provide lasing. The feedback caused by the resonant cavity 110 allows the gain medium 112 to amplify color center 120 output in a coherent manner to lase.
The magnetic field 104 is to affect the energy splitting, i.e., the energy difference, between the spin states 122. Magnetometers based on color centers 120 rely on changes in the intensity of a color center's output as a function of the magnetic field 104. The magnetic field 104 is to affect system 100 in the presence of the microwaves 106.
The microwaves 106 may be applied to the system 100 as, e.g., a microwave field having a frequency, to observe corresponding changes in output of the color centers 120. The frequency of microwaves 106 may correspond to the energy level structure of the color centers 120. The color center 120 may be associated with multiple energy levels, and a frequency of the microwaves 106 may be directly related to the difference of the energy levels. Microwave excitation may be used to depolarize the spins that are precessing at the same frequency as determined by the magnetic field 104. The depolarization affects the system gain and how much produced light 108 is emitted from the system 100. Exposure to microwaves 106 may affect the populations of the plurality of spin states 122 associated with the lower laser level 124. However, when the system 100 is then excited by the pump energy 102, distribution of spin states 122 in the excited laser levels may change.
Microwaves 106 may be applied continuously. Other techniques may include sequences of pulses of microwave excitation, and changing a delay between pulses of microwave excitation. Thus, a relationship between the magnetic field 104 and the microwaves 106 may be developed based on sweeping, pulsing, delaying, or otherwise varying the microwaves 106.
The system 100 therefore enables produced light 108 to be based on lasing. The gain produced may be dependent on the plurality of spin states 122. The produced light 108 may exhibit different gain for different spin states 122, which may be affected by magnetic field 104, thus providing a laser magnetometer.
System 200 illustrates an example of a vertical-cavity surface-emitting laser (VCSEL) based on color centers 220 affected by magnetic field 204. Produced light 208 may be provided based on lasing and coupling-to-free-space. The system 200 may be provided as a diamond pillar supported by substrate 218. A Bragg reflector of some kind (e.g., to serve as a dielectric) may be provided as the lower reflector 216. A grating and/or Bragg reflector may be provided as the upper reflector 214. The reflectors 214, 216, and the gain medium 212, thus provide a resonant cavity including color centers 220. The resulting system 200 may operate like a VCSEL to generate produced light 208 based on lasing.
Optical subsystem 230 is to couple the produced light 208 for output. As illustrated, the optical subsystem 230 is an output coupler to free space. In alternate examples, the optical subsystem 230 may be provided as a planar waveguide for a planar cavity and/or coupling. The optical subsystem 230 may be a non-waveguide structure to couple light, such as a lens, grating structure, or other structure for collecting produced light 208 and/or free-space coupling.
The resonant cavity 310 may be a microring to enable lasing, based on confining light (emitted by the color centers 320) to circulate around the microring until the light is coupled out via the optical subsystem 330 (e.g., based on waveguide coupling). The light from the resonant cavity 310 can be immediately coupled to optical subsystem 330 (waveguide) and then to an optical fiber, providing efficient and compact coupling for emitting produced light 308 based on lasing. The optical subsystem 330 may include a single-mode waveguide, for simplicity, though examples may use waveguides that are not single mode, and may be associated with mode competition. Thus, system 300 may be provided in a compact form-factor for magnetic sensing.
A lower laser level 424, as well as an upper laser level, may be associated a plurality of spin states (e.g., spin states 122 shown in
In an example, excitation may be based on a green laser having a wavelength of 532 nm at 2 mW. Excitation may be based on other pump energies, as appropriate for a given system 400. The ground states 424 and excited states 440 may include a plurality of spin states, including ms=±1 and ms=0. When in the ms=0 spin state, optical transitions are mainly cycling, which means that while exciting the system with pump energy, the system will get excited from ms=0 ground state 424 to ms=0 excited state 440, and then decay back down to ms=0 ground state 424 by emitting a photon. The system may continue getting reexcited according to this cycle. However, if the system is in the ms=±1 spin state, there is a possibility, once excited to the excited states 440, that the system will decay through singlet levels, which are shown on the right of
Various aspects of the system may be referred to as an upper laser manifold, lower laser manifold, ground state, i.e., lower pump manifold, and excited state, i.e., the upper pump manifold. System 400 may be a quasi three-level system, where three spin levels are shown, and the lower laser level is depopulated by pumping the system out of a lowest quantum state. System 400 may be a four-level system, lasing with sidebands, depending on the level of the lower and/or upper laser levels.
At room temperature, some transitions may not be spectrally resolved within a zero-phonon line. Rapid phonon assisted relaxation between two orbital branches (e.g., between 3Ex and 3Ey excited state manifolds), i.e., orbital relaxation, may be so fast that there is a motional averaging and the excited states may appear as if there were a single orbital state.
In order for lasing to occur, population inversion is to be created, resulting in gain to be stored in an amplifier (e.g., resonant cavity). A population inversion that is created is to be large enough such that the gain in the laser medium (e.g., for a round trip of generated light through the amplifier) exceeds losses that the generated light might see during the round trip, enabling a feedback system. This is relatively straight forward to achieve when lasing occurs using the vibronic states 444 and/or the vibronic states above excited states 442, because these states are not occupied. By manipulating the coupling between upper levels and lower levels, the conditions to create the population inversion are correspondingly changed. Placing population in the excited state 440 may create population inversion. Inversion also may be achieved with respect to the ground states 424, but in such a situation more than half of the population is to be excited from the ground states 424.
In an example, the color centers may be provided as NV centers. The ground state of an NV center may have three states (a spin triplet), and the splitting between ms=0 and ms=±1 may be approximately 2.87 GHz. The output of the NV center is different when it is in the state ms=0 and ms=±1. When in state ms=0, the excited NV center emits a photon, while when in state ms=±1, the excited NV center can either emit a photon or (in about 30% of the cases) go into the state 1A1 which is mostly dark.
In an example, assume that the color center output, when in state ms=0, is I0. When in state ms=±1, the output is I1. When the NV center goes into the 1A1 state 446, the NV center will likely decay to the ms=0 state. The 1E state may have approximately equal chances to decay to the various triplet ground states, including ms=±1 and ms=0. Several cycles may be used to get very high probability into ms=0. Thus, if the system begins in the ms=±1 state, it will rapidly (on the order of microsecond(s)) transition into the ms=0 state. Thus, under continuous excitation, the system emits I0 regardless of the initial state. A microwave field, e.g., at approximately 2.87 GHz, may be applied such that the NV centers are constantly mixed in a 50/50 superposition of ms=0 and ms=±1, such that the output intensity is (I0+I1)/2. If a magnetic field changes the splitting between ms=0 and ms=±1, the NV centers quickly polarize to ms=0 and I0 is detected. This is one way the magnetic field may be sensed. Additional information associated with the magnitude of the magnetic field may be derived by sweeping a frequency of the microwaves, and/or monitoring changes in output.
In contrast to
In an example, the color centers may be NV centers. An NV center may have a trigonal symmetry (point group C3V). The ground states 524 (e.g., 3A2) may have an orbital singlet, spin triplet structure having a 2.87 GHz splitting between the ms=0 and ms=±1 spin sublevels. These states may be connected by optical transitions to a set of six excited states with an orbital doublet, spin triplet structure. Due to random strain fields present in the gain medium crystal, the orbital states denoted as Ex and Ey are typically nondegenerate. Here, x and y refer to principal axes in a plane perpendicular to the NV center, with an angle determined by the strain tensor. The magnitude of this splitting depends on the crystal quality, and may be on the order of 10 GHz (diamond). The optical transitions involving the Ex and Ey orbital states follow linear polarization selection rules. For the higher-energy orbital branch, the ms=0 transition may primarily be spin-conserving, useful as a cycling transition for spin readout. In the lower-energy orbital branch, non-spin-conserving transitions may be obtained, useful for optical spin manipulation or for schemes based on Raman scattering.
In contrast to
In contrast to showing lasing associated with a resonant cavity, diagram 750 shows luminescence of an ensemble of color centers (e.g., intensity 752 as a function of wavelength 754). More specifically, diagram 750 shows luminescence of an NV center in a diamond crystal. The luminescence can show how much gain may be expected for a gain medium/color center, including what wavelength will provide a given gain, and what wavelength might be desirable for initiating lasing (e.g., a wavelength with a high intensity).
The sharp peak below 650 nm is the transition from the E states, such as transitions from the Ex, Ey states to the A2 states directly, as shown in thick solid arrows shown in
The diagram 750 shows that lasing may be enabled anywhere from approximately 637 nm to 800 nm. Lasing through the vibronic states would involve a wavelength 754 in the broad hump, and lasing through the zero phonon line would involve a wavelength 754 within the sharp peak at approximately 637 nm.
Based on resonant cavity and other characteristics chosen to establish lasing, a sharp peak in intensity may be created (not shown), to be located typically between approximately 637 nm and 800 nm, corresponding to the chosen lasing wavelength 754. In an example, a desired lasing intensity may be chosen (which may be either in the zero phonon line or the vibronic states) corresponding to a resonant cavity, and a microwave frequency may be applied along with a pump energy and magnetic field, and the intensity 752 for lasing should be achieved.
Generally, as used in the specification and claims herein, the singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise.
