GENERATION AND DETECTION OF ULTRASONIC
WAVES BEYOND 100 GHz
E. Jacobsen
To cite this version:
E. Jacobsen.
GENERATION AND DETECTION OF ULTRASONIC WAVES BEYOND 100 GHz.
Journal de Physique Colloques, 1972, 33 (C6), pp.C6-25-C6-31.
<10.1051/jphyscol:1972605>. <jpa-00215123>
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Submitted on 1 Jan 1972
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JOURNAL DE PHYSIQUE
Colloque C6, suppliment au no 11-12, Tome 33, Novembre-Dicembre 1972, page 25
GENERATION AND DETECTION OF ULTRASONIC
WAVES BEYOND 100 GHz
E. H. JACOBSEN
Department of Physics and Astronomy, University of Rochester, Rochester, New York, U. S. A.
R6sum6. - Le phenomene de pido6lectricit6 devrait thkoriquement permettre la production
et la detection des ondes sonores dans un domaine de frequences couvrant la totalite du spectre
de vibrations d'un solide; soit des frequences atteignant quelque 1013 Hz. Un avantage important
de la methode est l'independance pratiquement totale de l'effet piBzoClectrique vis-a-vis des
conditions locales de temperature, de pression et des chamsp Blectriques continus.
Un inconvenient apparait dans la detection des ondes elastiques de frequence superieure A
100 GHz ; en effet, le front d'onde doit %re parallkle a la surface piCzo6lectrique ou faire avec
celle-ci un angle infkrieur a 1/D environ (1 reprisente la longueur d'onde sonore et D la dimension linkaire de la surface utiliske du transducteur). Cette condition devient de plus en plus difficile
a respecter a haute frequence lorsque 1atteint quelque 10 a 100 A, D mesurant habituellement
10-1 cm environ.
M6me si cette condition geomktrique Btait parfaitement remplie, de petits dkfauts de l'khantillon
crkraient une distorsion suffisante du front d'onde.
Ces conditions n'empechent pas, cependant, la production, par effet pi6zo6lectrique, d'ondes
klastiques de haute frequence.
Diffkrents procedes permettent la detection d'ondes sonores de haute frkquence :
1) utilisation de dktecteurs incoherents (par exemple spins) sensibles a des champs sur des
distances de l'ordre de grandeur des dimensions atomiques, ou
2) utilisation de syst6mes de detection macroscopiques (par exernple jonction Josephson) qui
bien que de dimensions superieures A celles de l'atome sont beaucoup plus petits que les dimensions transversales de l'ichantillon.
Ces mkthodes sont couramment CtudiCes dans notre laboratoire et seront discutkes plus longuement.
Abstract. - The phenomenon of piezoelectricity should permit the generation of sound waves
over virtually the entire elastic vibrational spectrum of a solid which covers frequencies up to
some 1013 Hz. A great advantage of the piezoelectric method is that its dynamical behavior is
almost totally independent of the local environment as regards temperature, pressure, and ambient
D. C. fields.
A disadvantage presents itself in the detection of elastic waves above roughly 100 GHz owing
to the fact that the wave front must be parallel to the piezoelectric surface to within an angle
of approximately (AID), where I is the sonic wavelength and D is the cross-sectional dimension
of the transducer surface. This condition becomes increasingly difficult to meet at high frequencies
where I may range from ten to hundreds of Angstroms, and D will be typically about 10-1 cm.
Even if perfect geometric alignment were possible, small imperfections in the sample would distort
the wave front to a significant degree. These conditions do not, however, prevent efficient generation of high frequency elastic waves by means of the piezoelectric effect.
Possible ways to detect high frequency sound are to : 1) employ incoherent detectors (e. g.
spins) which respond to fields over atomic dimensions or 2) use macroscopic detection systems
(e. g. Josephson junctions) which, though larger than an atom, are much smaller than typical
transverse dimensions of the sample. These methods are currently under study in our laboratory
and will be discussed at greater length.
1. Introduction. - The piezoelectric effect should
permit the generation and detection of elastic waves
in matter over virtually the entire vibrational spectrum, i. e., from zero to about loi3 Hz. An attractive
feature of the piezoelectric transducer lies in its
insensitivity to temperature, and ambient electric
and magnetic fields. Moreover, it can be deposited
as a thin film directly on a sample [I], and electrically
coupled in a simple manner to the driving and receiv-
ing circuit. Aside from signal loss due to attenuation,
which can be reduced by subjecting the sample to
liquid helium temperatures [2], a major problem
arises, however, in detecting sound at high frequencies.
The difficulty centers on the short sonic wave length ;
e. g., the wave length will be thousands of Angstroms
at 10'' Hz, and hundreds of Angstroms or less at
1012 Hz. These numbers refer to a solid ; in a liquid
the wave length will be some five to ten times less
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972605
C6-26
E. H. JACOBSEN
for the same frequency because of the reduced interatomic forces.
For maximum detection sensitivity, the piezoelectric surface should be flat and parallel to the
wave front (assuming a plane wave) to within an
angle of approximately 1/D, where I is the sound
wave length and D the average cross sectional dimension (typically millimeters) of the surface. This
condition insures that the entire surface is driven
at the same phase so that the piezoelectric polarization
field is coupled maximally to the external circuit,
e. g., resonant cavity. (Transformation from electric
to sonic energy, and vice versa, takes place over a
surface depth of one half the sonic wave length [3]).
At the higher frequencies this condition becomes
increasingly difficult to meet because imperfections
in the medium will scatter the wave and so produce
an irregular wave front, and because even small
misalignments between the wave front and piezoelectric surface will easily exceed AID. It is to be noted
that these geometrical conditions need not be satisfied
in the generation of high frequency sound : complex
and misaligned wave fronts will be generated readily
by irregular piezoelectric surfaces and will propagate
accordingly. Conventional piezoelectric detection of
such waves, however, will prove difficult if not impossible at high frequencies.
In recent years, spin systems [4] and superconducting tunnel junctions [5] have been used to detect
(and also generate) elastic waves above 100 GHz.
These systems respond to elastic waves over atomic
volumes and therefore can detect wave fronts which
lack phase coherence over macroscopic dimensions.
Detection sensitivity is further enhanced by operating
these systems in conjunction with synchronous
amplifiers.
2. Conventional piezoelectric generation and detection of sound up to 100 GHz. - The piezoelectric
generation and detection of sound up to perhaps
25 GHz is quite straight forward : a re-entrant cavity,
see figure 1, provides a convenient method of applying
a microwave electric field to the surface of a piezoelectric crystal during sound generation. The same
FIG. 1. -Typical re-entrant cavity used in the range of 2 to
25 GHz to match piezoelectric transducer to exciting and
receiving microwave circuits.
cavity also serves to couple the piezoelectric surface
polarization, produced by an incident sound wave,
to the receiver. For simplicity we assume that the
sound field is a pulse which propagates as a plane
wave normal to thg crystal surface. (This assumption
is usually valid if the cross-sectional dimensions of the
sample are much larger than the sonic wave length.)
Maximum detection sensitivity will obtain when the
sound wave front is parallel to the piezoelectric
surface. As departure from parallelism occurs, part
of the surface will produce a polarization field of
opposite phase so that the average polarization field
seen by the cavity is accordingly reduced. As the angle
between wave front and surface increases further, the
average polarization oscillates through a series of
nulls and decreasing maxima. The limits in angle
are set, of course, by the sound intensity and receiver
sensitivity. Typical pulse, superheterodyne microwave
receivers in this frequency range routinely provide
sensitivities of some 90 dbm (decibels below milliwatt).
Using such a receiver we have been able to detect a
I milliwatt sound pulse in quartz at 24 GHz with
a misorientation angle exceeding 100 AID.
At higher frequencies, e. g., beyond 100 GHz,
the ratio 1/D becomes smaller and harder to satisfy.
Moreover, anharmonic attenuation becomes more
serious : the attenuation is typically proportional
to fT4 at microwave frequencies and low temperatures, where f is the frequency and T is the sample
temperature [6]. Finally geometrical scattering of
the wave by small imperfections will increase and
tend to produce a nonplanar front. Consequently
the net piezoelectric polarization presented to the
microwave cavity for coupling to the receiver is
substantially reduced. Nonetheless, it is possible to
piezoelectrically generate and detect sound waves
in the 100 GHz range by paying careful attention to
transducer alignment and receiver design. Such
experiments have been carried out at 114 GHz 121,
and are described below.
At frequencies beyond about 25 GHz the re-entrant
cavity becomes impractical as a means of coupling
the external microwave circuit to a piezoelectric crystal
because the cavity dimensions and Q become very
small [7]. A way around this limitation is to replace
the re-entrant cavity by an enclosed cylindrical or
rectangular cavity operating in a higher-order mode.
Such an arrangement is shown in figure 2. If the
quartz crystal is made to fill the cavity such that the
x-axis of the crystal coincides with the long dimension,
a corrugated wave front will be generated at each
end as indicated. The corrugation results from the
fact that the phase of the electric driving field Ex
oscillates with distance along the transverse direction
of the cavity. The resulting sound wave is mainly
compressional but contains a small shear component.
Such a wave will propagate in fact over several
centimeters before the phase of the shear wave
changes enough, relative to that of the compressional
GENERATION AND DETECTION OF IJLTRASONIC WAVES BEYOND 100 GHz
wave, to alter significantly the shape of the wave
front [S]. It is likely, however, that other sources of
wave front distortion, e. g., misalignment and scattering, are much more effective in diminishing the
average surface polarization available to the cavity
for (( reading out )) the signal. A rectangular TM3,4,8
cavity filled with quartz and bathed in liquid helium
will have dimensions of 0.328 x 0.425 x 0.85 cm3,
a Q
1000 (assuming the use of copper), and be
resonant at 114 GHz [2].
-
Waveguide
Quartz
\
avity
x-axis
/
Sound Wave
fronts
,
\
FIG. 2. - Cylindrical high mode cavity for use above 25 GHz.
A receiver sensitivity exceeding 100 dbm will be
required for detection at these frequencies unless
microwave driving powers of at least a hundred
milliwatts are available. We found it expeditious to
operate a superheterodyne receiver of standard design
as a phase sensitive detector so as to achieve the
required sensitivity. A block diagram of such a
scheme appears in figure 3. The basic operation
entails comparing the average of a series of receiver
output pulses containing signal plus noise with the
average of an equal number of receiver pulses
containing noise only. (Prior to taking the noise only
samples, the phonon generator klystron is quiescent.).
05-27
The pulses are selected and averaged by a delayable
gate and box car integrator, the output of which is
fed to a synchronous amplifier, and hence to a recorder.
In this way the effective receiver band pass is greatly
reduced and the noise averaged out. The delay of the
pulse sampling gate is slowly and continuously increased
so that the output of synchronous amplifier gradually
sweeps through the various phonon echo pulses.
This procedure will yield a receiver sensitivity of
about 100 dbm, and will permit detecting phonon
echoes generated by a microwave drive power of
some tens of milliwatts. Non-piezoelectric samples
can be studied in the same system by evaporating a
piezoelectric film on one surface [I]. Since geometric
alignment is critical at these frequencies, it will often
prove rewarding to test the propagation of phonon
signals in the sample first at 10 GHz. The samples
which yield the best signals at 10 GHz usually also
yield the best signals at 100 GHz.
It is clear that successful experiments in elastic
wave propagation in the 100 GHz range depend on
the combination of sample perfection, attenuation,
available microwave source power, and receiver
sensitivity. Of these items, increases in microwave
source power and detection sensitivity appear to be
the only practical route for improved research at
100 GHz frequencies and beyond by means of the
piezoelectric effect. At this writing carcinotrons are
available which produce a watt of power between
100 and 300 GHz. At 900 GHz, the HCN laser can
produce about the same power. At frequencies
above 100 GHz the attenuation of electromagnetic
waves in metal wave guides increases rapidly so that
dielectric guides, mirrors, or hybrid systems will be
required for energy transmission to (and possibly
from) the sample. For the same reason it will be
necessary to replace the enclosed resonant cavity
by structures such as the Fabry-Perot and zone plate
systems shown in figures 4 and 5. While it seems
Sample
Cavity
'2 = f l / "
I
Pulse
11111
Ulli
-
closed
closed
I
FIG. 4. - Fabry-Perot cavity for coupling to piezoelectric
transducer above 200 GHz.
FIG. 3. - Superheterodyne receiver operated as a synchronous
detector to achieve higher rensitivity at ultramicrowave frequencies. For any given series of echoes the synchronous amplifier compares the output of the I. F. amplifier containing the
noise-plus-signal with the output containing noise only.
likely that more powerful high frequency microwave
sources will be developed, a more immediate prospect
of improvement appears to lie in the direction of
greater detection sensitivity.
Fundamentally, detection of sound can proceed
by incoherent or coherent processes. Quantum detection by means of paramagnetic ions in crystals [4],
E. H. JACOBSEN
Zone Plate
J
Sample
/
/ Piezoelectric
Surface
FIG. 5, - Zone plate as a means of coupling to piezoelectric
transducer above several hundred GHz.
and Cooper pairs in superconductors [S] are examples
of the former. In this process only the intensity and
not the phase of the wave is detected, the obvious
advantage being that detection is unaffected by
wave shape and sample geometry. Piezoelectric
detection, by contrast, is a coherent process ; the
phase is preserved and the instantaneous amplitude
is amplified at the signal frequency. So long as wave
shape and sample geometry are not limiting factors,
coherent detection offers the advantage of relatively
high gain through the property of the superheterodyne
principle wherein the intensity at the difference
frequency (I. F.) is proportional to the square root
of the product of signal and local oscillator intensities.
In the light of these remarks it is of interest to
consider methods for improving both coherent and
incoherent detection of sound at ultra-microwave
frequencies and beyond. We propose in the following
paragraphs some possible schemes for doing so.
Two weak-link geometries suitable for coupling
to the piezoelectric polarization field and outlined
in figures 6 and 7. Figure 6 shows a point-contact
junction afixed to the end of a sample. The junction
comprises a sputtered film and point of niobium,
separated by a thin (1 000 A) piezoelectric layer of
cadmium sulfide. The point is pressed through the
piezoelectric layer by means of a differential spring
drive mechanism so as to establish a weak link.
Coupling to the local polarization field occurs over
a small volume surrounding the point. The piezoelectric properties of the thin transducer determine whether
transverse or longitudinal waves will be coupled to
the junction. In the arrangement outlined here both
a longitudinal and transverse wave can produce a
parallel field which will couple to the junction.
FIG.6. - Niobium point contact Josephson junction excited by
polarization field in thin piezoelectric film of cadmium sulfide.
This geometry is sensitive to vibration and ambient fields.
x-axis
3. New methods of detecting sound above 100 GHz.
- 3.1 JOSEPHSON
JUNCTION PIEZOELECTRICALLY COUPLED TO SOUND.- A novel method of phonon
detection arises in the use of a weak-link superconducting Josephson junction. This device is highly sensitive
to oscillating electric and magnetic fields [9], and
may be deposited on a piezoelectric surface in microgeometry. Such an arrangement is expected to couple
to the local piezoelectric polarization field on the
scale of the junction dimensions, which means that
the sound wave need be phase coherent over a much
smaller spacial extent.
The coupling of a Josephson junction to an oscillating field produces a series of steps in the d. c.
current-voltage characteristic of the junction. If
only one frequency is present, each step occurs at a
voltage corresponding to a multiple of the excitation
frequency, according to the relation V = nhf/2 e.
V is the d. c. voltage applied to the junction, n is
an integer, h is Planck's constant, f is the excitation
frequency, and e is the electron charge. If two frequencies are present, the steps in the I-V curve occur
at voltages corresponding to sum and difference
frequencies, and multiples and combinations thereof.
Junction
m
w
Wave Normal
Quartz
FIG.7. - Constricted film bridge Josephson junction excited
by transverse piezoelectric fieid produced by sound wave propagating along film normal. This geometry is relatively insensitive to ambient fields and is immune to vibration.
Unfortunately the scheme in figure 6 is rather
sensitive to mechanical vibration and external a. c.
fields. An alternative arrangement which is much
less sensitive to external influences is outlined in
figure 7. Here the junction comprises a thin film in
the shape of an (( H )) with the central region functioning as the weak-link. The assembly is deposited directly
C6-29
GENERATION AND DETECTION OF ULTRASONIC WAVES BEYOND 100 GHz
on a piezoelectric surface, so that the lateral polarization field between the legs of the (( H )) excite the
junction and produce steps in the I-V curve as in
figure 6 . The central (weak-link) region of the junction
must be small, typically 0.5 micron long by 0.25 micron
wide. An ingenious method of producing such microgeometry has been reported [lo]. Electron beam
lithography [ l l ] appears especially attractive as a
means of fabrication, since all dimensions of the
junction can then be made small, e. g., a cross-sectional width of order ten microns. The orientation
of the junction relative to the piezoelectric substrate
will determine the polarization of the sound wave
to be detected. Figure 7 shows a junction oriented
on quartz in such a way that a transverse wave polarized along the c-axis and propagating along the
x-axis will generate a polarization field along the
c-axis and thereby excite the junction. A different
choice of orientation, crystal, and piezoelectric
tensor would permit detection of a longitudinal
wave by means of the same configuration. An alternative procedure would be to misorient the plane
of the junction by a small angle relative to the wave
front so that the two halves of the junction would
be driven out of phase. The angle would be of order
AID, where D is the cross sectional dimension of the
junction. Our experience with waves at 10 GHz
indicates that it is possible to create controlled small
angle misorientations by applying a small force at
right angles to the sample. Hence, it may be possible
to mechanically (( tune )) such a system so as to
achieve maximum phase difference between the two
halves of the junction.
In addition to detecting high frequency elastic
waves by means of a Josephson junction, it appears
possible in principle to generate them according to
the relation f = 2 eV/h. This relation states that an
a. c. voltage at frequency f will appear across the
junction when the applied d. c. voltage is V. The
a. c. voltages and resulting fields will be small under
typical conditions. However, it may be possible to
develop a resonant geometry which will substantially
increase the effective a. c. field. Generation of sound
by such a device is attractive in that the frequency
can be continuously tuned over a vast range. Further
efforts in this direction may prove rewarding.
As noted earlier, superheterodyne detection is
attractive because of the possibility of power gain
and the convenience of processing the signal at the
difference frequency. It is well known that the Josephson junction is a non-linear device which permits the
mixing of two microwave signals [9]. This feature
offers the possibility of operating the junction configurations of Figures 6 and 7 in the superheterodyne
mode, instead of searching for steps in the I-V curve,
as a means of detecting the presence of high frequency
phonons. While this method appears interesting,
there exists an alternative method of perhaps greater
potential by which to employ the superheterodyne
principle with its consequent enhancement of detection
sensitivity. This method, non-piezoelectric in nature,
utilizes the relatively strong coupling between Cooper
pairs and phonons with energies which exceed the
gap energy of a superconductor. Hence the system
functions as a square law detector in the same way
as a photoelectric cell exposed to light. These ideas
are described further below.
3.2 SUPERHETERODYNE
DETECTOR (FOR [hw > E
- When a superconductor-insulator-superconductor sandwich is biased as in figure 8, very
little quasiparticle tunneling current will flow from
side [2] to [I] since there are relatively few particles
above the gap at low temperatures. If, however,
GAP).
FIG. 8. - Density of states and gap for two identical superconductors biased across an insulating barrier. For excitations
exceeding the gap energy, quasiparticles are lifted above the gap
resulting in net current flow from film (2) to film (1).
phonons with hw > E gap are incident on the sandwich
the tunnel current from [2] to [I] will increase as
a result of breaking Cooper pairs and sending quasiparticles above the gap [5], [Ill. The same process
should also occur for incident photons provided that
hw > E gap [12]. Moreover, the increase in tunnel
current will be proportional to the number of phonons
or photons present, i. e., proportional to the intensity
of the incident sound or light wave. This situation
raises the possibility of detecting phonons in a superconducting sandwich by means of the superheterodyne principle (i. e., by using the superconductor
as a square law detector). In this arrangement the
sandwich would be simultaneously excited by phonons
(signal at o,) and photons at a slightly different
frequency from a klystron (local oscillator at o,)
through a suitable wave guide assembly. Assuming
the sandwich to be excited coherently by the two
waves, the tunnel current will be of the form
I
=
I [ ~ Asin o, t C bE sin (o, t
+
q)I2
where A and E are, respectively, the amplitudes of
the sound and electromagnetic waves, and a and b
are the corresponding coupling constants between
wave and Cooper pairs. The current will contain
frequency components at 2 w,, 2 o,, o, + o, and
w, - o,. We are interested in the latter which represents the cross e m s in the above expression. Since
C6-30
E. H. JACOBSEN
the current component which vaires at (o, - 0.1,)
is proportional to the product (AE)', power gain is
possible by making the local oscillator amplitude E
large. (Should the cross mixing of phonon and photon
amplitudes appear questionable, it should be recalled
that the sound wave is really an oscillating electric
field on the local scale.) The difference frequency
(a,- o,) could easily be adjusted to fall between 1
and 100 MHz, and it should be quite practical to
extract currents at these low frequencies from the
junction and amplify them with conventional I. F.
amplifiers. Figure 9 outlines such an arrangement.
-
exceeds AID so that significant detection is still possible
for 6
10 AID. Also, the effective lateral dimension
of the sandwich can be made very small if necessary,
although doing so will reduce the maximum amplitude
of tunnel current. Further encouragement as to the
feasibility of achieving phase coherence over the
detector area comes from recent experiments on
phonon interference in liquid helium I1 films via
tunnel junctions at 100-150 GHz [12]. Hence it
appears that sufficient leeway exists for achieving
superior detection sensitivity up to several hundred
GHz, and possibly into the 10'' Hz region, a range
offering considerable exploratory interest.
4. Piezoelectric generation of sound above 100 GHz.
Whereas the piezoelectric detection of high frequency sound presents difficulties because of the
short wave length, the generation of sound does
not. Generation merely requires an electromagnetic
source of sufficient intensity and a means of coupling
to the piezoelectric transducer. With present technology two approaches to this problem suggest themselves as likely candidates for development in the
near future.
One approach is to drive the piezoelectric transducer
at the frequency of the source by a coupling scheme
similar to that of figures 4 and 5. At the present time
molecular lasers produce powers of about a watt
at discrete frequencies near 900 GHz. The upper
frequency limit of electron tube devices is about
300 GHz so that a gap exists between 300 and 900 GHz
for which no monochromatic source exists presently.
It seems likely, however, that this part of the spectrum
will be filled in due course by some form of laser
oscillator. Above 900 GHz molecular gas lasers
furnishing milliwatts at discrete frequencies are
becoming practical 1131. Future laser development
will undoubtedly provide more powerful monochromatic sources in the range between 10'' and 1013 Hz.
A second approach to sound generation by piezoelectric conversion of electromagnetic energy is to
mix two laser beams in a non-linear device and
drive the transducer at the difference frequency.
Recent experiments in mixing the outputs of two
CO, lasers (f
3 x 1013 HZ) in a thin slab of GaAs
yielded microwatts of power at a difference frequency
of some 900 Hz [14]. The lasers were pulsed and
yielded a peak output power of about 2 kW. Calculations indicate that such a system could generate
output at a difference frequency over the range of
300 GHz to about 6 000 GHz. The low conversion
efficiency resulted from the lack of phase matching
of the primary waves in the crystal with the result
that generation occurred over a coherence length
of 0.7 mm. I1 the conversion efficiency could be
improved to the level of yielding watts of power
at the difference frequency, very useful sources for
generating monochromatic sound at thermal frequencies would become available.
-
FIG. 9. - Biased, tunnel junction (two superconductors separated by an insulating layer) deposited on sample and used as
square law detector. Output is taken at the difference frequency
between phonon signal at f~ and local oscillator at f2.
The advantages of the above detection scheme
are : (i) Very high frequency phonons should be
detectable with good sensitivity because of the relatively strong coupling between Cooper pairs and
phonons for h o > E gap, and because of the superheterodyne principle. (ii) Accurate determination of
the signal phonon frequency spectrum is possible
provided a stable, monochromatic local oscillator
is available. (iii) The method is well suited to both
pulsed and steady state phonon signals. (iv) The
system is relatively simple, compact, and flexible over
a vast frequency range. For example, no magnetic
fields are needed to tune spin levels to a high frequency. More-over, as an aside, it should be possible
to explore in detail the frequency spectrum of phonons
generated by reconstituting Cooper pairs in a superconductor, which reflects the shape of the density
of states at the gap edges.
Maximum sensitivity requires that the detector
be driven coherently by the sound wave which implies
that the wave front be parallel to the plane of the
sandwich to within (ideally) an angle 6 AID, where A
is the sonic wavelength and D is the average lateral
dimension of the sandwich. Clearly, this condition
will be increasingly difficult to meet as the frequency
is raised. However, the situation is not as severe as
it may seem. As noted earlier the detection sensitivity
oscillates through a series of diminishing maxima as 8
-
-
GENERATION AND DETECTION OF ULTRASONIC WAVES BEYOND 100 GHz
C6-31
References
[I] DEKLERK
John, in Physical Acoustics, vol. IV, Part A,
edited by W. P. Mason (Academic Press, New
York, 1966), 195.
John and JACOBSEN
E. H., in Physical Acous[2] ILUKOR
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E. H., in Phonons and Phonon Interac[3] JACOBSEN
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[4] ANDERSON
C. H. and SABISKY
E. S., in Physical
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A. R., Appl. Phys. Lett.
20 (1972) 382.
DISCUSSION
B. R. TITTMANN.
- 1) In what materials do you
conduct your experiments ?
2) What is the phonon loss at 100 GHz ?
E. H. JACOBSEN.
- 1) Primarily natural quartz.
2) Approx. 0.2 db/cm at 4 OK. It would be nice
to conduct studies in other materials such as ZnO
but this for the future.
C. ELBAUM.
- YOU propose that the use of
tunnel junction detectors would permit detection of
waves with frequencies up to, or even in excess of
1012 CIS.Would the detection process not be limited,
essentially to the superconducting gap frequencies
(because of the falling efficiency in breaking Cooper
Alh) ?
pairs as the frequency increases above o
-
E. H. JACOBSEN.
- It's a question of how fast the
electron phonon interaction decreases as we go up
to frequency beyond 1012 c/s. It seem to recall that
the interaction is still sufficiently strong at these
frequencies, but should check this again.
M. PAPOULAR.
- Could you give a typical figure
for the sensitivity of the Josephson jonction device ?
E. H. JACOBSEN.
- We expect to be able to
detect sound power levels of lo-'' W/cm2 using
the weak link Josephson jonction.
J. LEWINER.
- Could YOU say a few words on
the technique you use to get a Josephson jonction
thiner than a micron ?
E. H. JACOBSEN.
- Method # 1 : Use a diamond
to scratch piezoelectric surface, then evaporate thin
(1 000 A) superconducting film, then make light
cut in direction at right angles. (See ref. to GregorHansen of Lenenson in my paper for details.)
Method # 2 : Use electron beam lithography
technique. (See ref's under IEEE in my paper for
details.)
H. LEVINE.- What is the nature of the interaction
between an acoustic wave and a Cooper pair ?
E. H. JACOBSEN.
- The acoustical wave produces
an oscillating electric field which modulates the
attractive potential between two electrons, comprising
the Cooper pair, at the phonon frequency. (The
attractive potential is formed by the electron-phonon
interaction, which is also electric in nature.) When
fiw of the phonon > E gap, there is a strong probability of breaking a pair, and threreby permitting
one of the resulting quasi-particules to be excited
above the gap and subsequently flowing across the
junction. The quasi-particle current is proportional
to the intensity of the exciting radiation, thus providing square law detection, hence non-linear mixing.
(The attractive potential is not simple represent.)