QTu1E.5.pdf CLEO Technical Digest © OSA 2012
!
"
# $ %
& ’
( #
)
* +
,
1
National Institute of Standards and Technology, 325 Broadway, MC 686.04, Boulder, CO 80305, US
2
Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, California 91109, USA
*
Currently with Photon Spot, Inc., 1212 S. Fifth Ave., Ste G, Monrovia, CA 91016
verma@nist.gov
#-. We report a system detection efficiency of 55 % at 1500 nm with an amorphous
tungstensilicide (WSi) superconducting nanowire singlephoton detector (SNSPD) using a self
aligned packaging scheme for alignment of the detector and optical fiber.
2011 Optical Society of America
/01 . (040.3780) Low light level detectors; (160.1890) Detector materials; (270.5565) Quantum communications;
(270.5570) Quantum detectors
1
Superconducting nanowire singlephoton detectors (SNSPDs) have played a crucial role in conducting experiments
in quantum optics, primarily due to their high sensitivity at nearinfrared wavelengths (10
20
W / Hz
0.5
noise
equivalent power at λ = 1550 nm), high timing resolution (35 ps FWHM jitter), low dark count rates
(DCR ~ 100 cps), and the simplicity of the readout circuit electronics [1, 2]. Standard SNSPDs, based on NbN and
NbTiN superconducting films, require the fabrication of narrow nanowires having widths less than 100 nm over a
large detection area (typically 10 × 10 @m
2
) with excellent uniformity (< 5 nm linewidth roughness), which limits
the yield of useful devices. Furthermore, NbN and NbTiNbased devices must be biased close to the critical current
(I
C
) in order to obtain the highest detection efficiencies, which results in often unacceptable dark count rates in
excess of 1 kcps. These stringent requirements limit the system detection efficiency (SDE) based on standard
SNSPDs to less than 10% [14].
Recently, we demonstrated SNSPDs based on amorphous tungstensilicide (WSi) superconducting films [5].
Due to the lower critical temperature (T
C
~ 3 K) and corresponding lower superconducting energy gap with respect
to NbN and NbTiN (T
C
~ 10 K), WSi allows the fabrication of SNSPDs with wider nanowires (up to 200 nm wide)
without any loss in SDE at nearinfrared wavelengths. Our WSi SNSPDs were based on 150nmwide nanowires,
which made them more robust with respect to fabrication defects and constrictions than standard SNSPDs [6], and
covered an active area of 16 × 16 @m
2
. These devices showed a saturated SDE of ~ 20 % over ~ 30 % of the bias
range for λ = 670 to 1850 nm.
Here we report the characterization of the detection efficiency, dark count rate, and jitter of a detector system
based on a WSi SNSPD integrated into an optical cavity using a selfaligned packaging scheme which greatly
simplifies the alignment of optical fibers to the detectors. We measured SDE = 55 % at λ = 1550 nm, DCR < 1 kcps,
and a jitter of 140 ps FWHM.
) *
WSi SNSPDs were fabricated on 200nmthick SiO
2
thermally grown on a 76.2 mm Si wafer. The SNSPDs were
embedded inside an optical cavity to enhance absorption. The bottom half of the cavity consists of an 80 nm gold
mirror and 276 nm SiO
2
. A 4.5nmthick layer of WSi with a Si composition of ~ 25 % was then deposited by
cosputtering W and Si at room temperature. We obtained an amorphous structure as verified by xray diffraction.
The nanowires were patterned by electron beam lithography using PMMA as the resist followed by etching in an
SF
6
plasma. A typical SNSPD is shown in Fig. 1 (a). The nanowires were 150 nm wide and had a pitch of 250 nm.
The active area of the device was ~ 15 × 15 @m
2
, allowing for efficient optical coupling of the device to a standard
telecommunications singlemode fiber with a core diameter of 10 @m. The top half of the cavity structure was then
deposited and consists of 189 nm SiO
2
followed by 222 nm TiO
2
. After device fabrication, a keyholeshaped via was
etched through the substrate around the device. This allowed the device to be freed from the substrate and mounted
inside a zirconia sleeve that holds the optical fiber as shown in Fig. 1 (b). Details of the packaging scheme can be
found in previous work [7].
The packaged devices were mounted in a cryogenfree adiabatic demagnetization refrigerator (ADR) and
operated at a temperature of ~ 150 mK (although the operating temperature can be as high as ~ 1 K with no
degradation in the peak detection efficiency [5]). Fig. 1 (c) shows the SDE as a function of bias current (I
B
) up to the