Coupling of erbium dopants to yttrium orthosilicate photonic crystal cavities for on-
chip optical quantum memories
Evan Miyazono, Tian Zhong, Ioana Craiciu, Jonathan M. Kindem, and Andrei Faraon
Citation: Applied Physics Letters
108
, 011111 (2016); doi: 10.1063/1.4939651
View online: http://dx.doi.org/10.1063/1.4939651
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/108/1?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
High-contrast all optical bistable switching in coupled nonlinear photonic crystal microcavities
Appl. Phys. Lett.
96
, 131114 (2010); 10.1063/1.3378812
Highly luminescent garnets for magneto-optical photonic crystals
Appl. Phys. Lett.
95
, 102503 (2009); 10.1063/1.3224204
Fabrication of high quality factor photonic crystal microcavities in In As P ∕ In P membranes combining reactive
ion beam etching and reactive ion etching
J. Vac. Sci. Technol. B
27
, 1801 (2009); 10.1116/1.3151832
Strong resonant luminescence from Ge quantum dots in photonic crystal microcavity at room temperature
Appl. Phys. Lett.
89
, 201102 (2006); 10.1063/1.2386915
APL Photonics
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
131.215.70.231 On: Tue, 19 Jan 2016 15:53:01
Coupling of erbium dopants to yttrium orthosilicate photonic crystal cavities
for on-chip optical quantum memories
Evan
Miyazono,
Tian
Zhong,
Ioana
Craiciu,
Jonathan M.
Kindem,
and Andrei
Faraon
a)
T. J. Watson Laboratory of Applied Physics, California Institute of Technology, 1200 E California Blvd,
Pasadena, California 91125, USA
(Received 26 September 2015; accepted 24 December 2015; published online 7 January 2016)
Erbium dopants in crystals exhibit highly coherent optical transitions well suited for solid-state op-
tical quantum memories operating in the telecom band. Here, we demonstrate coupling of erbium
dopant ions in yttrium orthosilicate to a photonic crystal cavity fabricated directly in the host crys-
tal using focused ion beam milling. The coupling leads to reduction of the photoluminescence life-
time and enhancement of the optical depth in microns-long devices, which will enable on-chip
quantum memories.
V
C
2016 AIP Publishing LLC
.[
http://dx.doi.org/10.1063/1.4939651
]
Optical quantum networks
1
are currently investigated
for applications that require distribution of quantum entan-
glement over long distances. Optical quantum memories are
essential network components that enable high fidelity stor-
age and retrieval of photonic states.
2
However, for practical
applications, solid-state components are desirable. Rare-
earth doped crystals are state-of-the-art materials that have
been used for high performance optical memories.
3
,
4
Of the
rare-earth elements investigated for this application, erbium
distinguishes itself as a natural choice for coupling directly
to the low-loss optical fibers wiring the internet
5
due to its
1.53
l
m optical transition in the telecommunications C band
with an optical coherence time of over 4 ms in yttrium ortho-
silicate crystals (YSO).
6
For quantum memories based on
atomic frequency combs or controlled reversible inhomoge-
neous broadening, initialization of the memory into a
Zeeman state is inefficient in erbium doped YSO
(Er
3
þ
:YSO) compared to other rare earth ions because the
optical lifetime (11 ms) is comparable to the Zeeman level
lifetime (
100 ms).
7
Detailed balance and parameters from
Lauritzen
et al
.
7
show spin initialization efficiency is limited
to 68% for a single pump beam. Thus, while a few demon-
strations of optical quantum memories based on Er
3
þ
:YSO
have already been reported,
8
the efficiency of those memo-
ries were low and can be increased significantly by improv-
ing the memory initialization. Spin mixing using an RF
source, or driving the spin-flip relaxation path with a second
laser can be used to achieve an efficiency of over 90%.
7
Alternatively, this lifetime reduction can also be accom-
plished by coupling the rare-earth atoms to on-chip microre-
sonators with high quality factors and small optical mode
volumes.
9
A spin initialization of 90% should also result
from a reduction in the effective excited state lifetime by a
factor of 6. In this letter, we demonstrate photonic crystal
resonators fabricated in Er
3
þ
:YSO. The coupling of Er ions
to the cavity results in enhanced interaction between the ions
and photons coupled to the cavity mode. This allows for
enhanced optical depth in a microns-long device and is thus
an enabling technology for on-chip integrated telecom quan-
tum memories. Excited state decay rate enhancement is
demonstrated, which can be used to achieve higher efficiency
optical pumping to a Zeeman level required to initialize the
memory in protocols like atomic frequency combs.
10
Triangular nanobeam optical cavities, like those demon-
strated in our other work,
11
were scaled to have a resonant
wavelength matching the 1536 nm transition in Er
3
þ
:YSO.
The cavity consists of an equilateral triangular beam with
rectangular grooves milled into the top. Each side of the tri-
angle is 1.38
l
m wide, and the grooves are 200 nm wide and
800 nm deep with a 570 nm period. The TE cavity mode pos-
sesses a simulated quality factor of Q
sim
¼
70 000 and a
mode volume of
V
mode
¼
1.65(
k
/
n
YSO
)
3
¼
1.05
l
m
3
. Here,
we have used the Purcell definition of mode volume.
12
The
cavity cross-section and optical mode profiles from finite dif-
ference time domain simulations using MEEP
13
are shown
in Fig.
1(a)
.
The YSO crystal was grown by Scientific Materials,
Inc., with 0.02% Er dopants. The crystal was cut such that
the
b
axis was normal to the polished top surface. The reso-
nator was milled using a focused ion beam system (FEI
Nova 600). The completed device is shown in Fig.
1(b)
. The
nanobeam was oriented along the
D
2
direction of the YSO
crystal, so that the electric field of the TE mode was oriented
along the
D
1
direction of the crystal, where
D
1
and
D
2
are
the optical axes of the biaxial birefringent YSO crystal.
The device was optically characterized using a custom-
made confocal microscope. The transmission spectrum was
measured over a broad range of frequencies using a super-
continuum laser for input, with the output measured directly
on a spectrometer with an InGaAs photodiode array (PyLoN
IR 1024-1.7). The transmission through the beam is shown
in Fig.
1(c)
showing the cavity resonance peak near
1536 nm. The quality factor of this resonance was deter-
mined to be Q
¼
11 400 by least-squares fitting of a
Lorentzian to the transmission curve. To attain higher resolu-
tion spectra, a tunable external cavity diode laser was
stepped in frequency across the cavity, with the spectrometer
integrating the transmission for 0.2 s per data point on the
photodiode array. Upon cooling to 4.7 K using a continuous
flow liquid helium cryostat, the cavity resonance shifted to a
higher frequency than the atomic resonance. The atomic
transition targeted was the 1536 nm transition between the
a)
Electronic mail: faraon@caltech.edu
0003-6951/2016/108(1)/011111/4/$30.00
V
C
2016 AIP Publishing LLC
108
, 011111-1
APPLIED PHYSICS LETTERS
108
, 011111 (2016)
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
131.215.70.231 On: Tue, 19 Jan 2016 15:53:01
lowest states of the
4
I
15
=
2
and
4
I
13
=
2
multiplets in the 4f or-
bital, labeled as Z
1
and Y
1
in Fig.
2(a)
. This transition has an
inhomogeneous broadening of
500 MHz at liquid helium
temperatures. Following the method presented in Mosor
et al
.,
14
the optical resonance of the structure is precisely
tuned to match the erbium absorption line, illustrated in Fig.
2(b)
, by slowly letting nitrogen gas into the cryostat, which
deposits onto the nanobeam. The bottom scan on Fig.
2(b)
shows a dip that is 25% the height of the full Lorentzian
transmission peak. As the power is lowered to reduce satura-
tion, the size of the atomic absorption dip increases to
40%. The expected absorption coefficient in bulk for a field
polarized along the
D
1
direction of the YSO crystal is
24.5 cm
1
.
15
A waveguide of the same length (26
l
m) would
have an attenuation of 3.8%. The resulting substantial
increase in the optical depth is due to the interaction between
the cavity mode and the Er ensemble.
FIG. 1. (a) Cross sectional views of the triangular nanobeam through the center of the beam showing the structure of the beam (top) and the simulated cav
ity
mode profiles (bottom). (b) Scanning electron microscope image of the triangular nanobeam YSO cavity. Angled trenches at the ends of the beam allow cou
-
pling from free space for transmission measurements. The x and y axes correspond to the optical axes
D
2
and
D
1
, respectively, while z corresponds to the
b
axis of the orthorhombic YSO crystal. (c) Measured transmission through the nanobeam. Broad-spectrum data taken with a supercontinuum laser; inset
shows
high-resolution frequency scan of a narrow linewidth laser in transmission through the cavity resonance at room temperature; fitting of a Lorentzian
to the
transmission spectrum shows the quality factor to be 11 400.
FIG. 2. (a) Erbium level diagram showing the crystal field splitting of the
lowest and second lowest energy states. (b) Resonator transmission spectra
as the cavity resonance was tuned using nitrogen deposition onto the 4.7 K
device. The three steps show high resolution frequency scans as the cavity is
tuned to the 1536 nm Er transition indicated in red.
FIG. 3. (a) Photoluminescence decay from erbium ions as a function of
time. The cavity coupling decreases the lifetime via the Purcell effect. The
cavity-coupled luminescence converges to the bulk curve because all of the
excited ions do not experience equal coupling. Inset shows the pulse used to
excite the luminescence. (b) Simplified schematic of the confocal setup used
to characterize the devices. With the flip mirror up, lifetime measurements
were performed by modulating the input with an electro-optic modulator
(EOM) synchronized with a single photon detector (SPD).
011111-2 Miyazono
etal.
Appl. Phys. Lett.
108
, 011111 (2016)
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
131.215.70.231 On: Tue, 19 Jan 2016 15:53:01
To measure the Purcell enhancement, the laser and reso-
nator were tuned to the erbium transition line, and an
electro-optic modulator was used to excite the ions with rec-
tangular pulses 20 ms long with a 75 ms repetition period.
The pulse and photoluminescence decay are shown in Fig.
3(a)
, and the additions to the confocal microscope used for
the lifetime measurement are shown in Fig.
3(b)
. Time
resolved photoluminescence measurements were taken with
an IDQuantique ID220 InGaAs/InP avalanche photodiode
detector. Given the measured quality factor and simulated
mode volume, the expected Purcell enhancement for an ion
positioned at the antinode of the cavity field is
F
P
¼
3
4
p
2
k
n
Q
V
E
ion
E
max
2
¼
517
:
(1)
Here,
F
P
is the Purcell factor,
k
is the cavity wavelength,
n
is
the cavity refractive index, and
Q
and
V
are the cavity qual-
ity factor and mode volume, respectively. The final term
accounts for positional misalignment between the field and
the dipole moment;
~
E
ion
is the electric field at the ion and
~
E
max
is the maximum of the electric field. Since the ensemble
of ions is distributed uniformly inside the photonic crystal
and the Purcell enhancement takes into account the emitter’s
dipole overlap with the field, most ions will not exhibit the
full Purcell enhancement. The non-zero width of the inhomo-
geneous linewidth (
500 MHz) was neglected, as it was
much smaller than the cavity linewidth (
17 GHz). Taking
these considerations into account, Ref.
9
gives an effective
enhancement of 116.
Furthermore, the excited electrons can follow many
decay paths from Y
1
, of which only the path directly to Z
1
couples to the cavity mode and is thus enhanced. We esti-
mate the branching ratio by comparing the expected emis-
sion rate, computed from the 1536 nm transition dipole
moment, and the measurable 1/11.4 ms spontaneous decay
rate.
16
For this calculation, we use the maximum absorption
coefficient 24.5 cm
1
with FWHM of 510 MHz for a 0.02%
erbium ion dopant density given an electric field polarized
along the
D
1
direction from B
€
ottger
et al
.
15
to compute an
oscillator strength
f
12
¼
1.095
10
7
. This is half the size of
the value in McAuslan
et al
.
16
for
D
2
polarization due to the
factor of two difference between absorption coefficient for
light polarized along the
D
1
and
D
2
directions. Following the
results from McAuslan
et al
.,
16
we find the spontaneous
emission rate that we would expect from only this decay
path to be 10.03 Hz. Comparing this value to the measured
excited state decay rate of 87.7 Hz (11.4 ms lifetime), we
determine that the branching ratio for Er:YSO in our cavity
is
0.11. When taking this into account, the aforementioned
factor of 116 increase in the spontaneous emission rate aver-
aged over the cavity leads to a reduction in the excited state
lifetime by a factor of 13, down to
900
l
s.
Fitting a single exponential, the lifetime in the bulk was
found to be 10.8 ms, which is in agreement with values in
the literature.
16
This was compared to the decay rate for ions
in the cavity when the cavity is resonant with the ions. In
this case, two exponential decays were fit, analogous to the
fitting procedure by Gong
et al
.,
17
and one of the decay
curves had a time constant fixed at the bulk lifetime. The
bulk lifetime in this fit corresponds to ions that are not
coupled to the optical cavity, because they are located in the
mirror sections. The shorter lifetime was 1.8 ms. The lumi-
nescence data after the cavity had been tuned to be resonant
with the ions are shown in comparison to bulk lifetime data
in Fig.
3(a)
. The data were normalized by scaling the coeffi-
cient of the bulk decay rate.
Accounting for the branching ratio, the observed reduc-
tion in lifetime would correspond to an effective Purcell
enhancement of
53. Due to the difficulty quantifying the
number of excited ions per homogeneous linewidth, this
analysis does not take into account the collective coupling
effect, which could have contributed to the observed
enhancement. Future studies in this system will involve mak-
ing cavities with higher quality factors, different dopant den-
sities, and with the mode aligned to the
D
1
direction, which
will allow a better assessment of the discrepancy between
the expected reduction by a factor of 13 and measured reduc-
tion by a factor of 6.
In conclusion, we have fabricated an optical microreso-
nator in an erbium-doped yttrium orthosilicate crystal and
used it to demonstrate enhanced optical depth and Purcell
enhancement of the optical decay rate of the coupled erbium
ions. This is the first step to efficient on-chip solid-state
quantum memories in the telecom C band. Next steps
include the measurement of optical coherence of the cavity-
coupled ions, as was demonstrated for the 883 nm transition
of neodymium in YSO,
9
and photon storage using the atomic
frequency comb and controlled reversible inhomogeneous
broadening techniques.
The authors sincerely thank Alexander E. Hartz for his
contributions. Financial support was provided by an AFOSR
Young Investigator Award (FA9550-15-1-0252), a Quantum
Transduction MURI (FA9550-15-1-002), a National Science
Foundation (NSF) CAREER award (1454607), and Caltech.
Equipment funding was also provided by the Institute of
Quantum Information and Matter (IQIM), an NSF Physics
Frontiers Center (PHY-1125565) with support of the Gordon
and Betty Moore Foundation (GBMF-12500028). The
device was fabricated in the Kavli Nanoscience Institute at
Caltech with support from Gordon and Betty Moore
Foundation.
1
H. J. Kimble,
Nature
453
, 1023 (2008).
2
T. E. Northup and R. Blatt,
Nat. Photonics
8
, 356 (2014).
3
A. I. Lvovsky, B. C. Sanders, and W. Tittel,
Nat. Photonics
3
, 706 (2009).
4
F. Bussie
`res, N. Sangouard, M. Afzelius, H. de Riedmatten, C. Simon, and
W. Tittel,
J. Mod. Opt.
60
, 1519 (2013).
5
W. Tittel, M. Afzelius, T. Chaneli
ere, R. Cone, S. Kr
€
oll, S. Moiseev, and
M. Sellars,
Laser Photonics Rev.
4
, 244 (2009).
6
T. Boettger, Y. Sun, C. W. Thiel, and R. L. Cone,
Proc. SPIE
4988
,51
(2003).
7
B. Lauritzen, S. R. Hastings-Simon, H. De Riedmatten, M. Afzelius, and
N. Gisin,
Phys. Rev. A
78
, 043402 (2008).
8
B. Lauritzen, J. Min
a
r, H. de Riedmatten, M. Afzelius, N. Sangouard, C.
Simon, and N. Gisin,
Phys. Rev. Lett.
104
, 080502 (2010).
9
T. Zhong, J. M. Kindem, E. Miyazono, and A. Faraon,
Nat. Commun.
6
,
8206 (2015).
10
M. Afzelius, C. Simon, H. De Riedmatten, and N. Gisin,
Phys. Rev. A
79
,
052329 (2009).
11
T. Zhong, J. Rochman, J. M. Kindem, E. Miyazono, and A. Faraon, e-print
arXiv:1512.03947
.
011111-3 Miyazono
etal.
Appl. Phys. Lett.
108
, 011111 (2016)
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
131.215.70.231 On: Tue, 19 Jan 2016 15:53:01
12
E. M. Purcell, in Proceedings of the Ame
rican Physical Society (1946), Vol. 69.
13
A. F. Oskooi, D. Roundy, M. Ibanescu, P. Bermel, J. D. Joannopoulos, and
S. G. Johnson,
Comput. Phys. Commun.
181
, 687 (2010).
14
S. Mosor, J. Hendrickson, B. C. Richards, J. Sweet, G. Khitrova, H. M.
Gibbs, T. Yoshie, a. Scherer, O. B. Shchekin, and D. G. Deppe,
Appl.
Phys. Lett.
87
, 141105 (2005).
15
T. B
€
ottger, Y. Sun, C. Thiel, and R. Cone,
Phys. Rev. B
74
, 075107 (2006).
16
D. L. McAuslan, J. J. Longdell, and M. J. Sellars,
Phys. Rev. A
80
,
062307 (2009).
17
Y. Gong, M. Makarova, S. Yerci, R. Li, M. J. Stevens, B. Baek, S. W.
Nam, R. H. Hadfield, S. N. Dorenbos, V. Zwiller, J. Vuckovic
́, and L. Dal
Negro,
Opt. Express
18
, 2601 (2010).
011111-4 Miyazono
etal.
Appl. Phys. Lett.
108
, 011111 (2016)
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
131.215.70.231 On: Tue, 19 Jan 2016 15:53:01