Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Multiphoton-Excited Fluorescence of Silicon-Vacancy Color Centers in
  Diamond   Silicon-vacancy color centers in nanodiamonds are promising as fluorescent
labels for biological applications, with a narrow, non-bleaching emission line
at 738\,nm. Two-photon excitation of this fluorescence offers the possibility
of low-background detection at significant tissue depth with high
three-dimensional spatial resolution. We have measured the two-photon
fluorescence cross section of a negatively-charged silicon vacancy (SiV$^-$) in
ion-implanted bulk diamond to be $0.74(19) \times 10^{-50}{\rm cm^4\;s/photon}$
at an excitation wavelength of 1040\,nm. In comparison to the diamond nitrogen
vacancy (NV) center, the expected detection threshold of a two-photon excited
SiV center is more than an order of magnitude lower, largely due to its much
narrower linewidth. We also present measurements of two- and three-photon
excitation spectra, finding an increase in the two-photon cross section with
decreasing wavelength, and discuss the physical interpretation of the spectra
in the context of existing models of the SiV energy-level structure.

21
The categories of this paper are: adap-org
33
The categories of this paper are: plasm-ph
16
The categories of this paper are: stat
28
The categories of this paper are: alg-geom
34
The categories of this paper are: chem-ph
8
The categories of this paper are: hep-ex
10
The categories of this paper are: q-bio
2
The categories of this paper are: math
1
The categories of this paper are: cs
0
The categories of this paper are: hep-ph
24
The categories of this paper are: dg-ga
7
The categories of this paper are: hep-th
11
The categories of this paper are: hep-lat
17
The categories of this paper are: q-fin
19
The categories of this paper are: eess
9
The categories of this paper are: nlin