Photonic waveguides evanescently coupled with single NV-centers

The interaction of photons with individual quantum systems is a
very fundamental process in physics. Thereby, the emission rate as
well as the angular emission pattern of a quantum emitter are not
only a function of intrinsic properties of the emitter itself, but
are also strongly modified by its surrounding. For instance, by
restricting the optical modes which are allowed at the position of
the dipole, the emission rate can be strongly modified and the
emitted photons can be directed into specific optical modes. This
effect can be demonstrated by the interaction of a single optically
active quantum emitter with the strongly confined optical mode of a
single-mode dielectric waveguide. Efficient coupling of the emitter
to the dielectric structure can be achieved by placing the quantum
emitter inside the evanescent field of the guided mode. This
evanescent field coupling mechanism is discussed and demonstrated
experimentally. A single nitrogen-vacancy center (NV-center),
hosted in a nanodiamond is deterministically coupled to a tapered
optical fiber (TOF) via the evanescent field of its guided mode
(coupling efficiencies exceeding 30% are predicted). By employing
an AFM-based nanomanipulation technique, the diamond nanocrystal is
placed on the nanofiber waist of the TOF. Beforehand, the diamond
nanocrystal has been characterized to guarantee that it hosts only
one fluorescing NV-center. While the diamond nanocrystal is
optically exited, single photon fluorescence of the NV-center is
detected at both outputs of the tapered optical fiber. This
verifies the evanescent coupling of the emitter to the guided mode.
In order to quantify the coupling, the comparison of the emission
rate into free space with the rate into the fiber yields that
10.0(5) of the emitted photons are coupled into the tapered optical
fiber. In the determination of this value, the orientation of the
emitting dipoles and the emission pattern, which are modified by
the TOF, have been considered. The NV-center features a broad
emission spectrum which can be used to investigate the
wavelength-dependence of the coupling. Comparing the spectra of the
emission into the fiber mode with the emission into free space
modes roughly resembles the expected wavelength dependency of the
coupling efficiency. The evanescent coupling and the deterministic
positioning of preselected fluorescing diamond nanocrystals, which
has been demonstrated with the TOF, can be applied to other
waveguide structures as well. Dielectric single-mode waveguides
made of Ta2O5 on a SiO2 substrate promise similar coupling
efficiencies to tapered optical fibers (above 30%). With the design
of the on-chip wave-guiding structure being flexible, the
combination with other optical on-chip elements is feasible,
rendering it a promising platform for on-chip photonic experiments.
Test structures of this waveguide design are realized using
lithographic processes and are characterized. These waveguides are
equipped with inverted taper structures to allow efficient off-chip
coupling with butt-coupling to standard single-mode fibers. The
evanescent coupling of a single quantum emitter to a singe optical
mode can be used to efficiently collect emission of the quantum
emitter. This can help building a compact single photon source and
is beneficial for the optical read-out of the quantum emitter's
internal degree of freedom, which can be either used as probe
(sensing) or as information-storage. Utilizing the high coupling
efficiency, for instance, the non-linearities of the quantum system
can be exploited to build a single photon transistor. The
evanescent coupling is very broadband (about hundred nanometers),
allowing to efficiently collect emission from broadband emitters
like the NV-center, but it can also be used for multi-wavelength
manipulation schemes.