Nanowire photonic crystal waveguides for single-atom trapping and strong light-matter
interactions
S.-P. Yu, J. D. Hood, J. A. Muniz, M. J. Martin, Richard Norte, C.-L. Hung, Seán M. Meenehan, Justin D. Cohen,
Oskar Painter, and H. J. Kimble
Citation: Applied Physics Letters
104
, 111103 (2014); doi: 10.1063/1.4868975
View online: http://dx.doi.org/10.1063/1.4868975
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/11?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Design for ultrahigh-Q position-controlled nanocavities of single semiconductor nanowires in two-dimensional
photonic crystals
J. Appl. Phys.
112
, 113106 (2012); 10.1063/1.4768437
High-Q (>5000) AlN nanobeam photonic crystal cavity embedding GaN quantum dots
Appl. Phys. Lett.
100
, 121103 (2012); 10.1063/1.3695331
Compact coupling of light from conventional photonic wire to slow light waveguides
J. Appl. Phys.
110
, 113109 (2011); 10.1063/1.3665878
Photonic crystal slot nanobeam slow light waveguides for refractive index sensing
Appl. Phys. Lett.
97
, 151105 (2010); 10.1063/1.3497296
Efficient photonic crystal cavity-waveguide couplers
Appl. Phys. Lett.
90
, 073102 (2007); 10.1063/1.2472534
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: Thu, 01 May 2014 14:49:07
Nanowire photonic crystal waveguides for single-atom trapping and strong
light-matter interactions
S.-P. Yu,
1,2,
a)
J. D. Hood,
1,2,
a)
J. A. Muniz,
1,2
M. J. Martin,
1,2
Richard Norte,
2,3
C.-L. Hung,
1,2
Se
an M. Meenehan,
2,3
Justin D. Cohen,
2,3
Oskar Painter,
2,3,
b)
and H. J. Kimble
1,2,
c)
1
Norman Bridge Laboratory of Physics 12-33, California Institute of Technology, Pasadena,
California 91125, USA
2
Institute for Quantum Information and Matter, California Institute of Technology, Pasadena,
California 91125, USA
3
Thomas J. Watson, Sr., Laboratory of Applied Physics 128-95, California Institute of Technology, Pasadena,
California 91125, USA
(Received 2 February 2014; accepted 26 February 2014; published online 18 March 2014)
We present a comprehensive study of dispersion-engineered nanowire photonic crystal waveguides
suitable for experiments in quantum optics and atomic physics with optically trapped atoms.
Detailed design methodology and specifications are provided, as are the processing steps used to
create silicon nitride waveguides of low optical loss in the near-IR. Measurements of the
waveguide optical properties and power-handling capability are also presented.
V
C
2014 Author(s).
All article content, except where otherwise noted, is licensed under a Creative Commons
Attribution 3.0 Unported License.
[
http://dx.doi.org/10.1063/1.4868975
]
A promising frontier for optical physics would become
accessible with the integration of atomic systems and nano-
photonics, which have made remarkable advances in the last
decade.
1
–
5
Significant progress toward integration of atomic
systems with photonic devices has progressed on several
fronts, including cavity quantum electrodynamics (cavity
QED), where atom-photon interactions can be enhanced in
micro- and nanoscopic optical cavities,
6
–
11
and nanoscopic
dielectric waveguides, where the effective area of a guided
mode can be comparable to atomic radiative cross sections
leading to complex photon transport in 1D,
12
–
19
as recently
demonstrated in Refs.
20
–
23
.
Beyond traditional settings of cavity QED and waveguides,
compelling paradigms emerge by combining atomic physics
with photonic crystal waveguides. One- and two-dimensional
photonic crystal structures fo
rmed from planar dielectrics
24
offer
a configurable platform for engine
ering strong light-matter cou-
pling for single atoms and photons with circuit-level complex-
ity. For instance, dispersion-engineered photonic crystal
waveguides permit the trapping an
d probing of ultracold neutral
atoms with commensurate spati
al periodicity for both trap and
probe optical fields that have d
isparate free-space wave-
lengths.
19
Such systems can lead to atom-atom interactions effi-
ciently mediated by photons within the waveguide.
25
–
28
In
photonic crystal waveguides,
atom-photon coupling can be
enhanced near the band-edge via slow-light effects
19
,
29
and can
be tailored to explore quantum many-body physics with atom-
atom interactions that can be readily engineered.
28
Significant technical challenges exist for developing
hybrid atom-photonic systems arising from the following
requirements: (1) The fabrication is sufficiently precise to
match waveguide photonic properties to atomic spectral lines;
(2) atoms are stably trapped in the presence of substantial
Casimir–Polder (CP) forces
19
yet achieve strong atom-field
interaction; (3) coupling to and from guided modes of nano-
photonic elements is efficient; (4) sufficient optical access
exits for external laser cooling and trapping; and (5) optical
absorption is low, and the net device thermal conductivity is
high, permitting optical power handling to support
1mK
trap depths. In this Letter, we describe nanowire photonic
crystal waveguides that meet these stringent requirements for
integration of nanophotonics with ultracold atom experiments.
The central component of our device is the ‘alligator’
photonic crystal waveguide (APCW) region shown in Fig.
1(a)
. It consists of two parallel Si
3
N
4
waveguides (refractive
index
n
¼
2). This configuration is similar to that proposed in
Ref.
19
for the trapping of atoms in the gap between the
dielectrics, where the atomic spontaneous emission rate into
a single guided mode,
C
1
D
, can be greatly enhanced with
respect to spontaneous emission into all other free-space and
guided modes,
C
0
, which here is approximately equal to the
free-space spontaneous emission rate,
C
0
. Figure
1(b)
shows
the theoretical optical bandstructure of the TE-like modes
(electric field polarized in the plane of the waveguide) for
the APCW studied in this work, computed using the MIT
Photonic Bands (MPB)
30
software package. The waveguides
are designed such that the Cs D1 (
1
¼
335.1 THz) and D2
(
1
¼
351.7 THz) transitions are aligned near the lower/‘di-
electric’ (
D
) and upper/‘air’ (
A
) band-edges, respectively.
As discussed in detail in Ref.
19
, the enhanced density
of states near the
X
-point band-edge, along with the strong
field confinement of the even-parity supermodes in the gap,
can be used to create large atom-photon interactions.
Intensity images of the dielectric and air band modes are
plotted in Figs.
2(a)
and
2(c)
, respectively.
30
The corre-
sponding enhancement of
C
1
D
is shown in Fig.
2(f)
.
One strategy for trapping Cs atoms within the gap of the
APCW is to use the dielectric-band mode blue-detuned from
the Cs D1 line as a trapping beam and the air-band mode as
a probe on the D2 line of the trapped atoms. In this sce-
nario,
19
Cs atoms are trapped between the parallel dielectrics
where the dielectric-band mode has an intensity null in the
a)
S.-P. Yu and J. D. Hood contributed equally to this work.
b)
opainter@caltech.edu
c)
hjkimble@caltech.edu
0003-6951/2014/104(11)/111103/5
V
C
Author(s) 2014
104
, 111103-1
APPLIED PHYSICS LETTERS
104
, 111103 (2014)
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: Thu, 01 May 2014 14:49:07
x-y
plane [Fig.
2(c)
] and the Casimir–Polder force provides
additional confinement in the vertical
z
-direction [Figs.
2(d)
and
2(e)
]. Further vertical confinement can be provided by
an additional guided mode red-detuned from the Cs D2 tran-
sition. For the band structure shown in Fig.
1(b)
and with
counter-propagating 30
l
W TE-mode fields blue-detuned
30-GHz from the Cs D1
F
¼
4
!
F
0
¼
4 transition com-
bined with 15
l
W of counterpropagating TE-mode fields
red-detuned 300 GHz from the D2
F
¼
4
!
F
0
¼
5 transi-
tion, we expect a trap depth of
’
5 mK and trap frequencies
of {
x
¼
3.5,
y
¼
1.4,
z
¼
0.7} MHz. Here, the total power
within the device is 90
l
W.
As shown in Fig.
3
, we incorporate several elements
into the waveguide structure to in- and out-couple light, to
provide mechanical support, and to improve heat dissipation.
SEM images taken along the length of the SiN waveguide
show the various sections of the device, including a wave-
guide-to-fiber coupling region [Fig.
3(b)
], mechanical sup-
port and thermal tethers [Fig.
3(c)
], a tapered region of the
APCW [Fig.
3(d)
], and finally the central APCW region
[Fig.
3(e)
].
The waveguide-to-fiber coupling in Fig.
3(b)
consists of
a slow tapering of the nanowire-waveguide from a nominal
width of 300 nm down to an endpoint near the fiber facet of
FIG. 1. (a) Schematic of the APCW with dimensional parameters thickness
t
¼
200 nm, inner waveguide width
w
¼
187 nm, gap
g
¼
260 nm, discrete periodic-
ity
a
¼
371 nm, and sinusoidally-modulated outer waveguide edge with ‘tooth’ amplitude
A
¼
129 nm. (b) Photonic bandstructure of the fundamental TE-like
modes of a nominal alligator waveguide device calculated with the dimensions derived from a typical fabricated device.
30
Small adjustments are made to the
waveguide parameters within the absolute uncertainty of the SEM (
<
5%) to obtain better agreement between measured band structures and those computed
from SEM images.
FIG. 2. Finite-element-method (FEM) simulation of the near-
X
-point guided mode electric field magnitudes
j
~
E
ð
~
r
Þj
in the
x-y
plane for the (a) air band and (c)
dielectric band of the even parity TE-like supermodes for the periodic structure shown in (b). The optical frequencies correspond to the Cs D1 and D2 li
nes,
and the corresponding band structure is shown in Fig.
1(b)
. ((d) and (e)) Numerically computed Casimir–Polder potential along directions (
x
m
,
y
m
,
z
) (d) and
(
x
m
,
y
,
z
m
) (e) for the dielectric-band trapping mode around minima of the optical trapping potential at (
x
m
,
y
m
,
z
m
) [i.e., the positions of the green spheres in
(c)]. (f) Calculated rate of radiative decay
C
1
D
into the guided mode in (a) for the cases of an initially excited atom trapped at (
x
m
,
y
m
,
z
m
) in an infinite photonic
crystal for transitions between atomic levels as depicted in the figure. The shaded area indicates the photonic bandgap region and the dashed lines the
Cs D1
and D2 transition frequencies. Here,
C
0
is the free-space decay rate.
111103-2 Yu
etal.
Appl. Phys. Lett.
104
, 111103 (2014)
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: Thu, 01 May 2014 14:49:07
width 130 nm and provides efficient optical mode-matching
to an optical fiber
31
(Nufern 780HP fiber; mode field diame-
ter 5
l
m). To support the nanowire-waveguide, nanoscale
tethers are run from the side of the waveguide either directly
to the substrate or to side ‘rails’ of 7.5
l
m wide SiN that
extend the entire waveguide length and connect to the sub-
strate at either end of the waveguide (labeled sections B, C,
and D of Fig.
3
show the tethers). The tethers are each 90 nm
wide and consist of a single tether for fiber alignment at the
ends of the waveguide and multi-tether arrays of 15 tethers,
spaced at a 220 nm pitch. Finite-difference time-domain
(FDTD) simulations
32
show that the input coupling effi-
ciency of the taper and single alignment tether is
’
75% for
light near the D2 line of Cs. The multi-tether supports pro-
vide anchoring against the high stresses within the device
and increase device-substrate thermal contact. Optical scat-
tering is minimized at the multi-tether attachment points by
tapering the waveguide width up to 1
l
m [see Fig.
3(c)
].
FDTD simulation shows that the scattering loss at the multi-
tether points is
0
:
5%.
The nanowire waveguides as shown in Fig.
3
are formed
from a thin film of stoichiometric SiN 200 nm in thickness,
grown via low-pressure chemical vapor deposition on a
(100) Si substrate of 200
l
m thickness. This sort of SiN has
exhibited low optical loss in the near-infrared
33
–
35
and large
tensile stress (
1 GPa).
36
A1
5 mm window opened
through the Si substrate provides optical access for laser
trapping and cooling, with the nanowire waveguides extend-
ing across the length of window. Even with the extreme as-
pect ratio of the nanowire waveguides, the high tensile stress
of SiN preserves mechanical stability and alignment.
In order to obtain smooth waveguide side walls of verti-
cal profile and to avoid damage during the SiN etch, we
employ an inductively-coupled reactive-ion etch (ICP-RIE)
of low DC-bias and optimized C
4
F
8
and SF
6
gas ratios. A
similar etch has been used to create record-high Q SiN
micro-ring optical cavities near 800 nm.
1
,
33
Fabrication of
the waveguide chip begins with a UV lithography step to
define the back window region. We then use a single
e
-beam
lithography step to define the fine features of the waveguide,
and to set the fiber v-groove position and width (which ulti-
mately determine the fiber-waveguide alignment). A piranha
clean removes any resist residue prior to a potassium hydrox-
ide (KOH) wet etch, which opens a through-hole in the Si
substrate defined by the two SiN windows on back and front.
After additional Nanostrip cleaning, the chip is transferred to
an isopropyl alcohol solution where it is dried using a critical
point drying step to prevent stiction of the double-wire
APCW section. Last, an O
2
plasma clean removes any resid-
ual particles on the waveguide surface.
Once fabricated, anti-reflection coated optical fibers are
mounted into the input and output v-grooves in the Si sub-
strate. The fiber-waveguide separation is set for optimal cou-
pling (typically
10
l
m) before the fibers are affixed in
place with UV curing epoxy. The Si chip and fibers are then
attached to a vacuum-compatible mount [see Fig.
3(a)
] and
loaded into a vacuum enclosure (reaching
10
–9
Torr) with
optical fiber feedthroughs.
37
In order to measure the broadband reflectivity and
transmission of the APCW, we utilize a broadband super-
luminescent diode optical source and optical spectrum ana-
lyzer. Figure
4(a)
shows the measured normalized reflection
R
and transmission
T
spectra over a frequency range of
320–360 THz for a typical APCW waveguide. The measured
spectra demonstrate that the fabricated APCW has the
desired photonic bandgap, with the dielectric and air band-
edges closely aligned with the D1 and D2 lines of Cs,
respectively, and in reasonable agreement with the theoreti-
cal spectra in Figs.
4(c)
and
4(d)
. From the average reflection
level within the photonic bandgap, we estimate the total
single-pass coupling from optical fiber to APCW to be
’
(60
6
5)%. The high-frequency oscillatory behavior of the
reflected and transmitted intensities is due to parasitic reflec-
tions from the AR-coated input fiber facet (
0.1% reflection)
and the input tether (
0.2%). Based upon previous measure-
ments for similar waveguides, we estimate that the power
loss coefficient of the unpatterned nanobeam sections is
4 dB/cm.
FIG. 3. Center: Schematic of the waveguide chip, illustrating the various regions of the waveguide. Bottom: (a) Optical image of the fiber-coupled wav
eguide
chip showing the through-hole for optical access. Zoom-in SEM image of (b) the adiabatic fiber-coupling region (A), (c) the alignment, mechanical sup
port,
and thermal heat-sink tethers (B, C, D), (d) the tapered region of the APCW (E), and (e) the central APCW region (F). The sinusoidal modulation facilita
tes
high-precision fabrication. Other elements (not shown) are side thermal contacts which consist of a pair of 7.5
l
m wide SiN rails extending across the entire
length of the waveguide and connecting to the substrate.
111103-3 Yu
etal.
Appl. Phys. Lett.
104
, 111103 (2014)
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: Thu, 01 May 2014 14:49:07