A batch of experimental diffusion-cooled hot-electron bolometers (HEBs), suitable for use as mixers having input frequencies in the terahertz range and output frequencies up to about a gigahertz, exploit the superconducting/normal-conducting transition in a thin strip of tantalum. The design and operation of these HEB mixers are based on mostly the same principles as those of a prior HEB mixer that exploited the superconducting/normal- conducting transition in a thin strip of niobium and that was described elsewhere

Bumble, Bruce

LeDuc, Henry

McGrath, William

Skalare, Anders

NASA Technical Reports Server

26 NASA Tech Briefs, October 2004
Diffusion-Cooled Tantalum Hot-Electron Bolometer Mixers
Lower TCs should translate to lower noise and lower required local-oscillator power.
NASA’s Jet Propulsion Laboratory, Pasadena, California
A batch of experimental diffusion-
cooled hot-electron bolometers (HEBs),
suitable for use as mixers having input fre-
quencies in the terahertz range and out-
put frequencies up to about a gigahertz,
exploit the superconducting/normal-con-
ducting transition in a thin strip of tanta-
lum. The design and operation of these
HEB mixers are based on mostly the same
principles as those of a prior HEB mixer
that exploited the superconducting/nor-
mal-conducting transition in a thin strip of
niobium and that was described in “Diffu-
sion-Cooled Hot-Electron Bolometer
Mixer” (NPO-19719), NASA Tech Briefs,
Vol. 21, No. 1 (January 1997), page 12a.
One reason for now choosing tanta-
lum instead of niobium arises from the
fact that the superconducting-transition
temperature (TC) of tantalum lies be-
tween 2 and 3 K, while that of niobium
lies between 6 and 7 K. Theoretically,
the input mixer noise of a supercon-
ducting HEB is proportional to TC and
the power demand on the local oscilla-
tor that supplies one of the input signals
to the mixer is proportional to TC2. The
lower noise and power demand associ-
ated with the lower TC of tantalum
could make tantalum HEBs more at-
tractive, relative to niobium HEBs, in
applications in which there are require-
ments to minimize noise and/or to pro-
vide mixers that can function well using
the weak signals generated by typical
solid-state local oscillators. Of course, to
reach the required lower TC, it is neces-
sary to use more complex cryogenic
equipment. Fortunately, such equip-
ment (e.g., helium-3 cryostats) is com-
mercially available.
In order to make a practical tantalum
HEB, it is necessary to overcome a chal-
lenge posed by the fact that thin films of
tantalum tend to contain grains of two
different crystalline phases. The pres-
ence of the two phases would be unac-
ceptable in a practical device because
(1) the additional electron scattering at
the grain boundaries would tend to sup-
press the diffusion-cooling mechanism,
and (2) the different TCs of the two
phases would lead to broadening of the
transition.
The present HEB mixers contain tan-
talum microbridges having lengths of
100 to 400 nm, widths of 100 to 200 nm,
and thicknesses of 10 nm. The bridges
were made from a 10-nm-thick film of
tantalum deposited by sputtering onto a
1.5-nm-thick seed layer of niobium on a
silicon wafer. The niobium seed layer
was used to promote the growth of one
of the two crystalline phases (the α
phase) to ensure the required crystalline
purity and thereby keep the supercon-
ducting transition (see figure) as sharp
as possible.
The results of microwave impedance
tests of one of the experimental tanta-
lum HEBs have been interpreted as sig-
nifying that the 3-dB roll-off frequency
for mixer conversion efficiency can be
expected to be about 1 GHz, neglect-
ing the effect of electrothermal feed-
back. End effects are small enough (as
illustrated by the smallness of the
“foot” of the resistance-versus-tempera-
ture curve in the figure) that it should
be possible to use devices as short as
100 nm and possibly even shorter. Inas-
80
60
40
20
0
1 2
Temperature, Kelvins
R
es
is
ta
nc
e,
 O
hm
s
3
DC Resistance of an HEB was measured as a function of temperature from below to above TC. Except
for the lower TC, this plot is similar to the resistance-vs.-temperature plots of niobium HEBs. The
nonzero-resistance “foot” of this plot is an effect of normally conductive gold contact pads at the
ends of the device.
thicknesses of layers, this device could
be made to operate in either of two
regimes (see figure):
• Under low bias, in a resonant-inter-
band-tunneling regime, in which elec-
trons would traverse valence subband
states in GaSb or
• Under moderate bias, in an intraband-
resonant-tunneling regime, in which
electrons would traverse conduction
subband states in InAs. Computational
simulations have led to an expectation
that the interband regime would yield
better performance.
This work was done by David Z.-Y. Ting,
Xavier Cartoixà, and Thomas C. McGill of
Caltech; Jeong S. Moon, David H. Chow,
and Joel N. Schulman of HRL Laboratories,
LLC; and Darryl L. Smith of Los Alamos
National Laboratory for NASA’s Jet
Propulsion Laboratory. Further informa-
tion is contained in a TSP (see page 1).
In accordance with Public Law 96-517,
the contractor has elected to retain title to this
invention. Inquiries concerning rights for its
commercial use should be addressed to:
Innovative Technology Assets Management
JPL
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
(818) 354-2240
E-mail: iaoffice@jpl.nasa.gov
Refer to NPO-30635, volume and number of
this NASA Tech Briefs issue, and the page
number.
https://ntrs.nasa.gov/search.jsp?R=20110020292 2019-08-30T17:49:08+00:00Z
NASA Tech Briefs, October 2004 27
much as the thermal-relaxation time
for diffusion cooling is proportional to
the square of the device length, the
100-nm-long devices should be capable
of “raw” speeds of about 16 GHz. Since
only a few gigahertz of bandwidth is
needed for most mixer applications, it
is expected that tantalum HEBs will be
fast enough even in the presence of
slowing effects of electrothermal feed-
back, which is present in most bolome-
ter mixer circuits.
This work was done by Anders Skalare,
William McGrath, Bruce Bumble, and
Henry LeDuc of Caltech for NASA’s Jet
Propulsion Laboratory. Further informa-
tion is contained in a TSP (see page 1).
NPO-30695
Tunable Optical True-Time Delay Devices Would Exploit EIT
Adjustable delays up to milliseconds would be generated within small volumes.
NASA’s Jet Propulsion Laboratory, Pasadena, California
Tunable optical true-time delay devices
that would exploit electromagnetically in-
duced transparency (EIT) have been pro-
posed. Relative to prior true-time delay de-
vices (for example, devices based on
ferroelectric and ferromagnetic materials)
and electronically controlled phase
shifters, the proposed devices would offer
much greater bandwidths. In a typical en-
visioned application, an optical pulse
would be modulated with an ultra-wide-
band radio-frequency (RF) signal that
would convey the information that one
seeks to communicate, and it would be re-
quired to couple differently delayed repli-
cas of the RF signal to the radiating ele-
ments of a phased-array antenna. One or
more of the proposed devices would be
used to impose the delays and/or generate
the delayed replicas of the RF-modulated
optical pulse. The beam radiated or re-
ceived by the antenna would be steered by
use of a microprocessor-based control sys-
tem that would adjust operational parame-
ters of the devices to tune the delays to the
required values.
EIT is a nonlinear quantum optical in-
terference effect that enables the propaga-
tion of light through an initially opaque
medium. A suitable medium must have,
among other properties, three quantum
states (see Figure 1): an excited state (state
3), an upper ground state (state 2), and a
lower ground state (state 1). These three
states must form a closed system that ex-
hibits no decays to other states in the pres-
ence of either or both of two laser beams:
(1) a probe beam having the wavelength
corresponding to the photon energy equal
to the energy difference between states 3
and 1; and (2) a coupling beam having the
wavelength corresponding to the photon
energy equal to the energy difference be-
tween states 3 and 2. The probe beam is
the one that is pulsed and modulated with
an RF signal.
If a properly adjusted probe pulse is sent
into the medium in the absence of the cou-
pling beam, the atoms completely absorb
the pulse and jump from state 1 to state 3.
This absorption is what makes the medium
opaque. In the presence of the coupling
beam, states 2 and 3 become coupled, pre-
venting the absorption of the probe beam
and thereby rendering the medium trans-
parent to the probe beam. At resonance
(that is, when the laser wavelengths corre-
spond exactly to the differences between
the energy levels of the quantum states of
the medium), the index of refraction of
the medium is exactly 1. At slightly differ-
ent frequencies, the degree of cancellation
of absorption of the probe beam is less and
the index of refraction differs. The nature
of the variation of the slope of the index of
refraction as a function of wavelength is
such as to substantially reduce the group
velocity of a probe light pulse.
The reduction in group velocity can be
exploited to slow and even stop the pulse. If
the coupling pulse is turned off when the
probe pulse reaches the middle of the
medium, the probe pulse becomes trapped
in the quantum states of the medium. The
ratio between the numbers of atoms in
states 1 and 2 is a measure of the ratio be-
tween the electric-field strengths of the
probe and the coupling beams immediately
before turn off. Also, the quantum state of
the RF modulation of a stopped light pulse
is stored in the spin quantum state of the
medium. After a desired delay, the quantum
state of the medium can be interrogated
optically: Turning the coupling beam back
on causes the regeneration of the optical
pulse, complete with the RF modulation.
Suitably prepared clouds of alkali atoms
(e.g., clouds of optically cooled sodium
vapor) have been observed to exhibit EIT
and would be used as the initially opaque
media in the proposed devices. Recent ex-
periments in compact cells (<1 cm3) of al-
kali-atom vapors have demonstrated that
light pulses can be effectively decelerated
to speeds of tens of meters per second and
even stopped for controlled amounts of
time that can range from microseconds to
milliseconds. In contrast, to obtain mil-
lisecond delays by use of optical fibers, it
would be necessary to make the fibers hun-
dreds of kilometers long, and the delays
would not be adjustable because the
lengths would not be adjustable.
A simple example of a device according
to the proposal is depicted schematically
in Figure 2. A probe laser beam would be
split into two parts: an undelayed (refer-
ence) beam and a beam to be delayed. A
single probe pulse would be generated,
yielding a single reference pulse. The
probe pulse would be trapped in the
medium, then released by use of suitably
timed coupling pulses. If the coupling
pulses were cycled on and off several times
with sufficient rapidity while the probe
pulse was still inside the medium, then sev-
eral differently delayed replicas of the
probe pulse would be generated.
This work was done by Igor Kulikov, Leo
DiDomenico, and Hwang Lee of Caltech for
NASA’s Jet Propulsion Laboratory. Fur-
ther information is contained in a TSP (see
page 1).
NPO-30776
Figure 1. A Closed System of Three Quantum
States that are accessible via laser beams is
among characteristics essential for electromag-
netically induced transparency.
State 3
State 1
Probe-Laser
Photon Energy
Coupling-Laser
Photon Energy
State 2


Diffusion-Cooled Tantalum Hot-Electron Bolometer Mixers

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server