C. ONSRT. Exploring the optical properties of nitrogen- vacancy color centers in diamond Andrew Cook and Hailin Wang Department of Physics, University of Oregon, Eugene, Oregon 97403 . Andrew Cook. Dr. Hailin Wang. Introduction:
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Exploring the optical properties of nitrogen- vacancy color centers in diamond
Andrew Cook and Hailin Wang
Department of Physics, University of Oregon, Eugene, Oregon 97403
Dr. Hailin Wang
Nitrogen-Vacancy (NV) color centers are defects in diamond with unique optical properties. Diamond containing NV centers is one of only a handful of crystals that exhibit Electromagnetcally Induced Transparency (EIT). Electron spin resonance in NV centers has decay times on the order of tens of microseconds at room temperature. It is our goal to find new ways to optically manipulate the electronic spin states of NV centers, as well as to use diamond nanocrystals containing NV centers in whispering gallery cavity QED experiments.
Energy levels of NV centers:
The NV center has a spin triplet ground state that with the spin 0 level split from the spin +/-1 levels by 2.9 GHz. The excited state is a triplet and no breaking of degeneracy has bee observed.. The spin +1 and spin –1 levels are nearly degenerate, but split by 17 MHz due to a lack of crystal symmetry. A decay emits light of 637 nm if the electron is does not decay into a metastable state (not shown).
A nitrogen-vacancy center consists of a missing carbon atom adjacent to a nitrogen impurity. NV centers occur only very rarely in natural diamond, but are more common in synthetic diamond which tends to have a more vacancies and nitrogen impurities. Proximity to other lone nitrogen atoms reduces spin coherence times.
PL spectra of irradiated diamond nanocrystals:
Producing NV Centers in diamond nanocrystals:
Currently we are studying the optical properties of synthetic diamond nanocrystals (50 nm diameter). We start with a few grams of diamond nanocrystals produced for polishing hard drive surfaces. We then irradiate the diamond nanocrystals with electrons or neutrons to knock carbon atoms out of the lattice, producing vacancies. We irradiate with 6 MeV electrons at a food irradiation facility in China with an exposure time of 20 hours (~ 10^18 e-/cm^2 total). We also irradiate samples with an equivalent fluence of 1-6 MeV neutrons in the core of the Oregon State University. This process, however, leaves the samples radioactive due to trace heavy metal impurities. Since the size of the diamond nanoparticles is much smaller than the wavelength of light corresponding to energy transitions (637 nm), the diamond nanocrystals can readily emit and absorb light regardless of diamond’s high index of refraction.
Photoluminescent spectrum of electron irradiated and annealed 50 nm diamond nanocrystals bought from element 6 (deBeers). The 637 nm peak to the left is the inhomogeneously broadened transition of interest.
NV centers via CVD:
We are also studying NV centers produced by Chemical Vapor Deposition (CVD) in collaboration with Steve Rand at the University of Michigan. In CVD, a diamond crystal is formed by when a high pressure and high temperature jet of gas in the presence of Methane, composed of carbon and hydrogen. Room temperature Electromagnetically Induced Tranparency experiments are underway on these samples.
The diagram to the right displays EIT results from an experiment by Hemmer. A magnetic field was applied along the  diamond crystal direction to reduce the splitting between spin -1 and spin 0 levels to 120 MHz, allowing mixing between transitions. This experiment was performed at 15 K. However, Rabi oscillations have been observed at room temerature by Jelezko.
NV centers via and Cavity QED:
Work has begun to couple individual NV centers in nanocrystals to high Q silicon microspheres such as the one pictured to the left. A laser evanescnetly couples into the cavity and shows cavity mode structure as it scans wavelength. When the cavity couples to a transition of an atom on the waist of the cavity, changes in mode structure are observed. It is our goal to couple NV centers to the microsphere whispering gallery without reducing its Q factor.
* Jelezko, “Spectroscopy of Single N-V centers in DiamondSingle Molecules 2 (2001) 4, 255-260
** Hemmer, “Raman-excited spin coherences in nitrogen-vacancy diamond” Optics Letters Vol 26 No. 6 March 15 2001
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