Nitrogen-vacancy centers in diamond are widely studied both as a testbed for solid state quantum optics and for their applications in quantum information processing and magnetometry. Here we demonstrate coupling of the nitrogen-vacancy centers to gap plasmons in metal nano-slits. We use diamond samples where nitrogen-vacancy centers are implanted tens of nanometers under the surface. Silver nano-slits are patterned on the sample such that diamond ridges tens of nanometers wide fill the slit gap. We measure enhancement of the spontaneous emission rate of the zero photon line by a factor of 3 at a temperature of 8K

Barclay, Paul E.

Beausoleil, Raymond G.

Brongersma, Mark L.

Faraon, Andrei

Fu, Kai-Mei C.

Jun, Young Chul

Santori, Charles M.

English

Caltech Authors - Main

Modiﬁcation of the spontaneous emission rate of
nitrogen-vacancy centers in diamond by coupling to plasmons
Andrei Faraon1*, Young Chul Jun2, Paul E. Barclay1, Kai-Mei C. Fu1, Charles M. Santori1,
Mark L. Brongersma2, Raymond G. Beausoleil1
1 Hewlett Packard Laboratories, 1501 Page Mill Rd., Palo Alto, CA, 94304, USA
2 Department of Materials Science and Engineering, Stanford University, Stanford, CA,
94305,USA
ABSTRACT
Nitrogen-vacancy centers in diamond are widely studied both as a testbed for solid state quantum optics and for
their applications in quantum information processing and magnetometry. Here we demonstrate coupling of the
nitrogen-vacancy centers to gap plasmons in metal nano-slits. We use diamond samples where nitrogen-vacancy
centers are implanted tens of nanometers under the surface. Silver nano-slits are patterned on the sample such
that diamond ridges tens of nanometers wide ﬁll the slit gap. We measure enhancement of the spontaneous
emission rate of the zero photon line by a factor of 3 at a temperature of 8K.
Keywords: quantum information, plasmon, diamond, nitrogen-vacancy center
1. INTRODUCTION
Large-scale quantum photonic circuits could enable applications that are impossible to realize on any classi-
cal computer, such as fundamentally secure communication,1 eﬃcient factoring algorithms,2 and simulation of
large quantum systems.3 Because of their spin properties, nitrogen vacancy (NV) centers in diamond are one
of the most promising quantum optical emitters for integration in quantum networks. Key experiments on op-
tical control of the spin states in nitrogen-vacancy centers,4,5 and entanglement between the state of a single
nitrogen-vacancy and a single photon have already been demonstrated.6 One of the remaining challenges is to
integrate the nitrogen-vacancy centers in an on-chip photonic network. This requires coupling to nanophotonic
devices with an optical mode that can be further coupled into the network. Coupling has been observed using
diamond nanoparticles, but the nitrogen-vacancies in these structures generally suﬀer from bad spectral prop-
erties.7–12 The eﬀorts for nanophotonic integration of nitrogen-vacancies located in single crystal diamond have
been hindered so far by the diﬃculties with single crystal diamond fabrication13,14 or integration of diamond
with other material systems that are easier to fabricate.15,16 Here we demonstrate coupling of nitrogen-vacancy
centers to gap plasmons metal-insulator-metal (MIM) waveguides fabricated in single crystal diamond. Similar
results have been observed using colloidal quantum dots coupled to metal nanoslits.17
2. DEVICE FABRICATION AND CHARACTERIZATION
A schematic of the device is shown in Fig. 1(a). It consists of diamond ridge (width w and height H) in
between two diamond slabs. This structure behaves like a metal-insulator-metal plasmonic waveguide. Two
dimensional ﬁnite diﬀerence time domain (FDTD) simulations indicate that for a ﬁxed ridge width (w=50nm),
the spontaneous emission rate enhancement (γ/γ0) for an optical dipole located in the middle of the ridge and
oriented perpendicular to the metal facets has a strong dependence on H. Here γ is the spontaneous emission
rate inside the metal-insulator-metal waveguide and γ0 is the spontaneous emission rate in bulk diamond. For
an operating wavelength of 637nm, corresponding to the zero photon line of the negatively charged nitrogen-
vacancy centers, the maximum enhancement is obtained for H ≈120nm. Our simulations indicate that the
enhancement becomes higher for smaller waveguide widths. However, as the slit width reaches the nano-meter
range, non-radiative recombination may become the dominant process.
* To whom correspondence should be addressed. Email: andrei.faraon@hp.com
Advances in Photonics of Quantum Computing, Memory, and Communication IV,
edited by Zameer Ul Hasan, Philip R. Hemmer, Hwang Lee, Charles M. Santori, Proc. of SPIE
Vol. 7948, 794814 · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.874221
Proc. of SPIE Vol. 7948  794814-1
Diamond (n=2.42)
HwSilver (Ag)
Air (n=1)
(a) (b)
Figure 1. (a) Schematic of the simulated structure consisting of a diamond ridge of width w and height H in between two
silver (Ag) slabs. (b) Simulated (FDTD) spontaneous emission rate enhancement as a function of ridge height for a ﬁxed
width w=50nm
Diamond (n=2.42) Diamond (n=2.42) Diamond (n=2.42)
1. Diamond Sample 2. Spin Fox 12 3. Pattern ridges using e-beam
Diamond (n=2.42)
4. Etch ridges in diamond
Diamond (n=2.42)
5. Deposit Silver
Diamond (n=2.42)
6. Liftoff
Figure 2. Schematic representation of the sequence of fabrication steps.
Proc. of SPIE Vol. 7948  794814-2
- AccV Spot Magn Det WD Exp
300 kV 30 200000x SE 77 7
(a) (b)
Figure 3. (a) Fabricated diamond ridges shown after the etching (120nm deep) step. (b) Top view of the ridge after silver
deposition and liftoﬀ.
The waveguides were fabricated in single crystal CVD diamond (Element 6). We use diamond samples
where nitrogen-vacancy centers are implanted at an accelerating voltage of 50KeV and implantation density of
2×1013/cm2. The nitrogen-vacancy centers can reach implantation depths as deep as 100nm, with the maximum
concentration at a depth of 60nm, as determined from simulations.
Electron-beam lithography and and reactive ion etching were using in the fabrication process (Fig. 2). First,
negative e-beam resist (FOX-12) was spun on the sample and the ridges were lithographically patterned. An
oxygen plasma etching step (Oxford reactive ion-etching machine) was then used to etch 120nm into the diamond
sample thus obtaining thin (tens of nm) diamond ridges as shown in Fig. 3(a). Then 120nm silver was deposited
on the sample and a liftoﬀ procedure was used to remove the silver from the top of the ridges. A top image of
the device is shown in Fig. 3(b). The diamond can be observed in between the silver slabs. The measurements
done using the scanning electron microscope indicate that the top of the ridge has a thickness slightly smaller
than 30nm. However, the etching proﬁle of the ridge is not perfectly vertical. We used an FIB to determine that
the thickness of the width of the ridge increases up to ≈60nm at its bottom.
The sample characterization was done in a continuous ﬂow He cryostat operating at 8K. A confocal microscope
setup was used both for excitation and for collection of the photoluminescence signal. The spectrum of the NV
centers under continuous wave green (532nm) excitation is shown in Fig. 4(a). This sample exhibits a pronounced
zero phonon line at 637nm that corresponds to the negatively charged nitrogen vacancy centers, and another
zero phonon line at 575nm that corresponds to the neutral nitrogen-vacancy centers. From the relative intensity
of the zero phonon lines, it can be concluded that the nitrogen-vacancies in this sample are mainly negatively
charged.
The lifetime of the NVs coupled to the metallic slit was determined from the photoluminescence signal under
pulse excitation. For this measurement we used pulsed green light obtained by ﬁltering a super continuum source.
The super continuum generation was done in a photonic crystal ﬁber using laser pulses from a Ti:Sapphire laser.
The average intensity of the pulsed light source was 200μW. A grating ﬁlter, was used to ﬁlter the signal from
the negatively charged zero phonon line and the output was sent to a avalanche silicon photodiode detector.
The experimental data both from nitrogen-vacancies in the slit and nitrogen-vacancies in bulk diamond is
shown Fig. 4(b). The bulk lifetime data was acquired in ridges with large width (tens of microns), such that
Proc. of SPIE Vol. 7948  794814-3
(a) (b)
NV- ZPL
NV0 ZPL
Figure 4. (a) Photoluminescence spectrum of an ensemble of NV centers under green excitation.(b) Lifetime measurement
indicating that the NV lifetime (measured on the zero phonon line) decreases from ≈ 9ns to ≈ 3ns
there is no coupling to gap plasmons. A single exponential ﬁt is used to determine that the lifetime decreases
by a factor of ≈ 3 from 8.8ns (in bulk) to 2.7ns (in the slit). The bulk lifetime is signiﬁcantly smaller than
the value reported in the literature for NV centers (≈12ns). We observe this in samples that have been heavily
implanted. The measured enhancement factor is smaller than the factor of 15 estimated in Fig. 1(b) because the
NV centers span a variety of orientations and locations with respect to the slit, and because of imperfections in
the device fabrications. These imperfections can be observed in Fig. 3(b), where the size of the metal roughness
is a signiﬁcant fraction of the slit width. More measurements (currently in progress), as polarization dependence
and lifetime dependence on slit width, are needed for an exact quantiﬁcation of the coupling between the NVs
and the gap plasmon mode. Most importantly, it is necessary to determine how much of the lifetime reduction
is caused by radiative versus non-radiative processes.
One of the next steps for this type of experiment is to use samples with lower concentration of nitrogen-
vacancy centers, so the lifetime modiﬁcation of single nitrogen-vacancy centers can be measured. This can be
done in electronic-grade diamond sample. The fabrication process needs to be improved in order to obtain
smooth metal surfaces that reduce the scattering loss. Signiﬁcant scattering is in fact useful for experiments
where the samples are probed from the top because more signal can be collected. However, the idea is to use the
well deﬁned mode of the slit waveguides for further coupling into optical networks. Also, once good NV coupling
is obtained, one of the next steps is to do coherent probing measurements by coupling laser beams into the slit
waveguide.
3. CONCLUSION
In conclusion, we demonstrate modiﬁcation of the spontaneous emission rate for nitrogen-vacancy centers coupled
to gap plasmons in silver nanoslits. The gap plasmon modes can be further coupled into a nano-photonic optical
network, which is relevant for future quantum photonic technologies based on nitrogen-vacancy centers.
Other possibilities for coupling nitrogen-vacancy centers into photonic networks is to use dielectric resonators
and waveguides. The dielectric resonators have high quality factors, which allows for resonant enhancement of
Proc. of SPIE Vol. 7948  794814-4
the zero-phono line with respect to the phonon sidebands as recently demonstrated18 using ring resonators in
single crystal diamond.
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Proc. of SPIE Vol. 7948  794814-5


Modification of the spontaneous emission rate of

nitrogen-vacancy centers in diamond by coupling to plasmons

Andrei Faraon

Young Chul Jun

Paul E. Barclay

Kai-Mei C. Fu

Charles M. Santori

Mark L. Brongersma

Raymond G. Beausoleil

Crossref

Modification of the spontaneous emission rate of nitrogen-vacancy centers in diamond by coupling to plasmons

Cavity qed with diamond nanocrystals and silica microspheres.

Chip-based microcavities coupled to nitrogen-vacancy centers in single crystal diamond.

Coherent interference effects in a nano-assembled diamond nv center cavity-qed system.

Coherent population trapping of single spins in diamond under optical excitation.

Coupling of nitrogen-vacancy centers in diamond to a gap waveguide.

Design and focused ion beam fabrication of single crystal diamond nanobeam cavities. arXiv:1008.1431v2 [quant-ph],

Deterministic coupling of a single nitrogen vacancy center to a photonic crystal cavity.

Deterministic nano-assembly of a coupled quantum emitter - photonic crystal cavity system. arXiv:1008.4097v1 [quant-ph],

Diamond waveguides fabricated by reactive ion etching.

Enhancement of the zero phonon line emission from a single nitrogen vacancy center in a nanodiamond via coupling to a photonic crystal cavity.

Fault-tolerant quantum repeaters with minimal physical resources and implementations based on single-photon emitters.

Plasmon-enhanced single photon emission from a nanoassembled metal-diamond hybrid structure at room temperature.

Quantum Computation and Quantum Information.

Quantum entanglement between an optical photon and a solid-state spin qubit.

Resonant enhancement of the zero-phonon emission from a color center in a diamond cavity. arXiv:1012.3815v1 [quant-ph],

Spin-Light Coherence for Single-Spin Measurement and Control in Diamond.

Strong modification of quantum dot spontaneous emission via gap plasmon coupling in metal nanoslits.

Towards quantum chemistry on a quantum computer.

http://authors.library.caltech.edu/34132/1/Faraon_794814.pdf

Modification of the spontaneous emission rate of nitrogen-vacancy centers in diamond by coupling to plasmons

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