Energy resolution of terahertz single-photon-sensitive bolometric detectors
D. F. Santavicca,
1
B. Reulet,
2
B. S. Karasik,
3
S. V. Pereverzev,
3
D. Olaya,
4
M. E. Gershenson,
4
L. Frunzio,
1
and D. E. Prober
1,a͒
1
Department of Applied Physics, Yale University, New Haven, Connecticut 06520, USA
2
Laboratoire de Physique des Solides, Universite Paris-Sud, 91405 Orsay, France
3
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA
4
Department of Physics, Rutgers University, Piscataway, New Jersey 08854, USA
͑Received 18 May 2009; accepted 5 February 2010; published online 25 February 2010͒
We report measurements of the energy resolution of ultrasensitive superconducting bolometric
detectors. The device is a superconducting titanium nanobridge with niobium contacts. A fast
microwave pulse is used to simulate a single higher-frequency photon, where the absorbed energy
of the pulse is equal to the photon energy. This technique allows precise calibration of the input
coupling and avoids problems with unwanted background photons. Present devices have an intrinsic
full-width at half-maximum energy resolution of approximately 23 THz, near the predicted value
due to intrinsic thermal fluctuation noise. © 2010 American Institute of Physics.
͓doi:10.1063/1.3336008͔
Terahertz ͑THz͒ detectors have seen rapid development
during the past decade. However, an energy-resolving THz
single-photon detector—i.e., a THz calorimeter—has re-
mained elusive. Previous work on semiconductor quantum
dot detectors has demonstrated THz single-photon detection,
but with a complex device geometry, low quantum efficiency
͑ϳ1%͒, and without photon energy resolution.
1,2
The super-
conducting bolometric detector has the potential to achieve
energy-resolved THz single-photon detection with high
quantum efficiency in a device with a relatively simple
geometry.
3,4
For a hot electron bolometric calorimeter, with a mea-
surement bandwidth equal to the intrinsic device response
bandwidth, the energy resolution is limited by thermody-
namic fluctuations, and scales as
␦
E
intrinsic
ϳ
ͱ
k
B
T
2
C
e
, ͑1͒
where C
e
is the electronic heat capacity, proportional to the
active device volume and the operating temperature T.
5,6
Thus, for sensitive detection, operation is at low temperature
and all dimensions of the device are much smaller than a
wavelength. Efficient photon coupling can be achieved by
integrating the device in a planar THz antenna.
7
An array of
such detectors is essential for proposed next-generation
space-based far-infrared telescopes.
8,9
This detector would
also create possibilities for THz spectroscopic studies at the
single-photon level, such as measurements of the THz emis-
sion from individual nanostructures.
10
The detector we have studied consists of a superconduct-
ing titanium ͑Ti͒ nanobridge approximately 4
m long, 350
nm wide, and 70 nm thick, with T
c
Ϸ0.30 K ͑Fig. 1͒. The Ti
nanobridge spans contacts consisting of thick niobium ͑Nb͒
with T
c
Ϸ8 K. The fabrication process has been described
previously.
4
The dimensions of the Ti nanobridge were cho-
sen to have an impedance close to 50 ⍀ in the normal ͑non-
superconducting͒ state to facilitate efficient high-frequency
coupling.
For photons with a frequency greater than the upper
frequency scale for superconductivity in the Ti, f
Ti
Ϸ3.5k
B
T
c
/h=22 GHz at T Ӷ T
c
, the nanobridge impedance
is approximately equal to the normal state resistance R
n
Ϸ40 ⍀. In practice, the superconducting energy gap in the
Ti is strongly suppressed by the bias current and temperature,
so the relevant frequency scale is well below 22 GHz. The
temperature rise due to an absorbed photon is ⌬T=hf/C
e
,
where f is the photon frequency, assuming that no energy is
lost while the electron system reaches a thermal distribution.
The larger superconducting energy gap in the Nb contacts,
⌬
Nb
Ϸ1.2 meV in our films, creates Andreev mirrors that
inhibit the outdiffusion of heat from the Ti nanobridge.
3
The time for the initially excited photoelectron to share its
energy with other electrons in the Ti and relax below ⌬
Nb
is
e-e
ϳ͑2 ϫ 10
8
R
sq
⌬
Nb
/k
B
͒
−1
ϳ0.1 ns, where R
sq
is the sheet
resistance.
11
The initial excitations will spread a distance
ϳ͑D
e-e
͒
1/2
ϳ0.1
m, where D is the diffusion constant,
while the excitations cool to below ⌬
Nb
. The subsequent en-
ergy removal is by electron-phonon coupling within the Ti,
with an intrinsic thermal time constant
0
=C
e
/G
th
ϳ
s,
where G
th
is the electron-phonon thermal conductance.
3,4
A test system to study the detector response to single
THz photons is under development but has presented signifi-
cant technical challenges. A THz source coupled from out-
side the cryostat must be highly attenuated due to room tem-
perature blackbody photons. Even with a source internal to
the cryostat, the radiation power absorbed in the device must
be fW to avoid exceeding the detector count rate. This
a͒
Electronic mail: daniel.prober@yale.edu.
FIG. 1. ͑Color online͒ dc resistance as a function of temperature measured
with 1 nA bias current. Inset: scanning electron micrograph of Ti nanobo-
lometer device on silicon substrate. The strips of Ti below the Nb contacts
are an artifact of the fabrication process.
APPLIED PHYSICS LETTERS 96, 083505 ͑2010͒
0003-6951/2010/96͑8͒/083505/3/$30.00 © 2010 American Institute of Physics96, 083505-1