US3274406A

US3274406A - Acoustic-electromagnetic device

Info

Publication number: US3274406A
Application number: US255237A
Authority: US
Inventors: Jr Henry S Sommers
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1963-01-31
Filing date: 1963-01-31
Publication date: 1966-09-20
Anticipated expiration: 1983-09-20

Description

@a @www j SEARCH ROQM Sept. 20, 1966 H. s. SOMMERS, JR 3,274,406

ACOUSTIC-ELECTROMAGNETIC DEVICE Filed Jan. 5l 1965 5 Sheets-Sheet 1 y l Fig a y j' fs/1 L? 5452; 455 y 427 fsf@ Sept. 20, 1966 H. s. SOMMERS, JR 3,274,406

ACOUSTIC-ELEGTROMAGNETIC DEVICE Filed Jan. 3l 1965 .'5 Sheets-Sheet 2 1N VENTOR. HENRY 5. @MME/65k 46E/vr Sept. 20, 1966 H. s. SOMMERS, JR 3,274,406

ACOUSTIC-ELECTROMAGNETIC DEVICE Filed Jan` 3l 1963 5 Sheets-Sheet '7 WQ' O fa/ /aa ,g5

INVENTOR. Hf/VKY 5. SOMA/f'gJ United States Patent O 3,274,406 ACOUSTIC-ELECTRMAGNETIC DEVICE Henry S. Sommers, Jr., Princeton, NJ., assignor to Radio Corporation of America, a corporation oft' Delaware Filed Ilan. 31, 1963, Ser. No. 255,237 17 Claims. (Cl. S10-8.1)

This invention relates to a device for coupling acoustic energy and electromagnetic energy; and includes various embodiments wherein energy of one type is converted to energy of the other type for some useful purpose.

Acoustic energy, which is vibrational energy of the molecules, can be produced in the form of waves in a substance in various ways. For example, acoustic waves may be produced in a piezoelectric body by applying thereto an appropriate electric field. The velocity of propagation of the acoustic waves is generally about 105 to 106 cm./sec. (centimeters per second) in most solids and liquids Iat room temperature. Light waves or other electromagnetic energy can also be introduced into or produced in a body in various ways and in most modes propagate in a velocity range laround 1010 cm./sec. in many materials at room temperature. Because of the difference in velocities, there is only a very weak coupling between electromagnetic waves and acoustic waves even if they pass close to each other and in the same direction in a medium permitting interaction.

An object of this invention is to provide a novel device for coupling acoustic and electromagnetic waves.

Another object is to provide various embodiments in which acoustic and electromagnetic waves interact for some useful purpose.

According to the invention, a device for coupling an acoustic wave and an electromagnetic wave comprises a piezoelectric medium for supporting the propagation of an acoustic wave along `a path therein `and means for supporting the propagation of an electromagnetic wave with at least a component thereof propagating in the path in the same direction as the acoustic wave. The velocity of the component of the electromagnetic wave is related to the velocity of the acoustic wave for selectively transferring energy from one of the waves to the other. Preferably the velocity of the component of the electromagnetic wave is substantially the same as the velocity of the acoustic Wave. When the two waves have substanitally the same velocity, they interact or couple with one another and the slower of the waves may be made to grow in amplitude at the expense of the faster wave. As pointed out above, electromagnetic waves propagate in most modes at a velocity of the order of lo cm./sec. However, there is a mode of propagation of circularly polarized electromagnetic waves, which can have a phase velocity of about 105 to 106 cm./sec., and still be transported in a solid or other medium with small attenuation. This mode is sometimes called the helicon mode of propagation, or simply helicon waves, and may be used advantageously in the devices of the invention.

In a preferred embodiment, an electromagnetic wave in the helicon mode is propagated in the same direction as and close to an acoustic wave in a medium giving an interaction between the two waves. When the velocities ofthe two waves are substantially the same, the two waves interact cooperatively over a relatively long path length and for a relatively long time. This results in a traveling-wave-type interaction between the two waves. Energy may be transferred from the wave traveling at the faster velocity to the wave traveling at the slower velocity. Thus, either the acoustic or the electromagnetic wave can be amplified at the expense of the other by adjusting the velocity of the electromagnetic wave.

The invention may be applied to various electro- ICC acoustic devices, such as phonograph pickups, speakers, microphones, strain gauges, ultrasonic cutters, ultrasonic cleaners, sonars, millimeter and sub-millimeter wave generators, etc. The invention may be applied to amplify an input electric signal or an acoustic signal, or to increase an output electric signal or an acoustic signal.

A more detailed description of the invention and illustrative embodiments thereof appear below in conjunction with the drawings in which:

FIGURE 1 is a perspective view of a device illustrating some of the principles of the invention,

FIGURE 2 is a perspective view of a device similar to that of FIGURE 1 but adapted as an acoustic-to-electromagnetic transducer, such as a phonograph pickup,

FIGURE 3 is a perspective view of a device similar to that of FIGURE 1 but adapted as an electromagnetic-toacoustic transducer, such yas for a loud speaker,

FIGURE 4 is a perspective View of a device similar to that of FIGURE 1 but adapted as an amplifier or attenuator of acoustic waves,

FIGURE 5 is a perspective view of `a device similar to that of FIGURE 1 but adapted as an amplifier or attenuator of electromagnetic Waves,

FIGURE 6 is a perspective view of a device similar to that of FIGURE 1 but adapted as an acoustic-to-electromagnetic transducer with amplification or attenuation,

FIGURE 7 is a perspective view of a device similar to that of FIGURE 1 but adapted as an electromagnetic- 'to-acoustic transducer with amplification or attenuation,

FIGURE 8 is a perspective view of another embodiment of the invention designed to illustrate additional principles of the invention,

FIGURES 8A, 8B, and 8C are embodiments simi-lar to FIGURE 8 except that they include, respectively, means for injecting, means for photo-exciting, and means for thermally-exciting free charge carriers in a semiconductor portion of the device,

FIGURE 9 is a perspective view of a device similar to that of FIGURE 8 but adapted for use in an ultrasonic cutter, and,

FIGURE l0 is a perspective view of a device similar to that of FIGURE 8 but adapted to generate ultrasonic waves in a liquid.

Similar reference numerals are used for similar elements throughout the drawings.

FIGURE 1 illustrates a device 21 including a piezoelectric body 23 of generally tabular shape. Among suitable piezoelectric materials are barium titanate, quartz, Rochelle salt, gallium phosphide, indium arsenide, cadmium sulfide, and zinc oxide. The body 23 need not be a crystalline solid, and may be any piezoelectric medium. A pair of electrodes 25 is attached to opposite faces of the piezoelectric body 23 at one end thereof. A pair of leads 27 connects to the electrodes 25 providing means for applying an input voltage Ea, for exciting an acoustic wave in the piezoelectric body 23. A pair of electrodes 47 is attached to opposite faces of the piezoelectric body 23 at the other end thereof. A pair of leads 49 connect to the electrodes 47 providing rneans for deriving an output voltage EE0 from the acoustic wave.

As illustrated, when the input voltage Ea, is applied to the electrodes 2S, an ac-oustic wave propagates down the piezoelectric body 23 in the direction shown by the arrow 29 with a velocity va. Any of the known piezoelectric materials -rnay be used as the body 23 in any of the known geometric shapes and electrode arrangements for producing an acoustic wave in the piezoelectric body 23. Alternatively, the acoustic wave may be produced mechanically, as with a phonograph needle attached to the piezoelectric body 23 as described below with respect to FIGURE 2. Similarly, the acoustic wave may be used directly with a mechanical linkage as with the loudspeaker described below with respect to FIGURE 3.

The device of FIGURE 1 includes also a second body 31 or other medium which is capable of transporting a helicon wave with l-ow attenuation. The second body 31 may be solid or liquid and may be a metal, a semi-metal, or semiconductor. Some suitable materials are semiconduct-ors such as germanium, silicon, cadmium sulfide, gallium phosphide, gallium arsenide and indium arsenide; metals such as mercury, copper, gold, and sodium; and semi-metals such as bismuth and antmony. The body 31 has a generally tabular shape and is positioned adjacent to the piezoelectric body 23. A pair of electrodes 33 are attached to opposite faces -of the body 31 at one end thereof. A pair of leads 35 which may be conductors of a waveguide connect to the electrodes 33 providing a means for applying a voltage Ehi to the electrodes 33. A magnetic field Hna is applied to the body 31 in the direction shown by the arrow 39 from the poles 36 and coils 38 -of an electromagnet actuated from a constant current source 40. When a voltage Ehi is applied to the electrodes 33, a helicon wave is generated in the second body 31 and propagates down the body 31 in the direction shown by the arrow 37 with a velocity vh. It is noteworthy that the acoustic Wave, the helicon wave, and the magnetic field are substantially parallel to one another and the two waves move in the same direction, as illustrated by the

arrows

29, 37 and 39 respectively.

A helicon wave, or technically the helicon mode for propagating an electromagnetic wave, is a mode of propagation -of circularly polarized electromagnetic waves which can have an arbitrarily low phase velocity. The helicon wave is transmitted by a conducting material along the direction of a magnetic field HDC. The conditions for propagation with low attenuation are that the dielectric constant be essentially real and positive. For instance in the case of a solid body, if the dielectric constant is designated eh then:

Eh= =`11i12 :tare two senses of circular polarization e=dielectric constant of free space [=1 in CGS units used here] eh=relative dielectric constant of the lattice w=frequency of signal wp=plasma frequency of conducting solid n=carrier density, m*=effective mass, e=funda mental charge in E.S.U. wc=cyclotron frequency=He/m*, H=malgnetic field intensity in E.M.U. t=carrier scattering time=m/e;t; n=mobility in E.S.U. =loss tangent For illustration, assume the material has high enough mobility so the loss tangent is small, that the plasma frequency is very high, and the signal frequency considerably less than the cyclotron frequency; these are all practical conditions which have been observed. w wc; l for helicons to propagate: wp2 wzzrc to simplify the equation. Then the helicon phase velocity in terms of the velocity of light in free space, c, is

l This velocity can be very slow indeed; Rose, Taylor and Bonars, Phys. Rev. 127, 1122 (1962) observed a dimensional resonance in a small sphere of metallic sodium at 60 cps.

The body 31 may be a conducting solid with an appropriately high plasma frequency (this frequency ranges from 10m/sec. for highly conducting metals to zero for insulators), and the magnetic field can be adjusted so a desired signal frequency w is conducted in a helicon mode at a velocity about that of sound. This electromagnetic signal can interact with the sound waves through the piezoelectric interaction. Since the propagation velocities of both waves are about the same, the interaction cau continue over a considerable length of crystal and for a considerable time, giving a travelling wave interaction between the electromagnetic wave w and an acoustic signal of about the same frequency, with the wave of higher velocity transferring energy to the slower wave. Thus, either the acoustic or the electromagnetic wave can be amplified at the expense of the other by adjusting the magnetic field appropriately.

Because of the dispersive nature of the helicon propagation, its velocity increases with the square root of the frequency. In a solid, the acoustical branch of the phonon spectrum has a constant velocity in much of its frequency range; hence, the interaction will be confined to a relatively narrow spectrum of frequencies. This seems also to be true for the optical branch of the phonon spectrum, for although it is dispersive, the velocity seems to generally decrease with increasing frequency. However, the interaction can still be effective between the electromagnetic and acoustical waves for a large fractional band width, as is found in travelling wave amplifier tubes using electron beams in a slow wave structrue which is also dispersive.

The helicon velocity is controlled by the free carrier concentration, the magnetic field, and the signal velocity. The velocity increases with increasing magnetic field, with increasing signal frequency, or with reduction of the free carrier concentration. Since the helicon waves can be transmitted through polycrystalline materials as well as single crystals, one can build up layers of different materials as well as adjust the doping gradient and the magnetic field gradient. The range of control is such as to permit changing the helicon velocity over a range of orders of magnitude.

In FIGURE l, an electric potential Ehi applied to the electrodes 2S through the leads 27 generates an acoustic wave in the piezoelectric body 23 with velocity va in the direction indicated by the arrow 29. This acoustic wave has the same frequency as the driving voltage Ehi. The acoustic wave propagating through the piezoelectric body 23 is accompanied by an associated electric field moving with the same velocity and frequency. As the associated electric field changes in time, it radiates an electromagnetic wave of the same frequency, a part of which will propagate in the nearby body 31 `with velocity vh in the direction indicated by the arrow 37. In this way, the piezoelectric nature of the body 23 serves as a means for transforming some of the energy of an acoustic wave in the body 23 into a helicon wave in body 31.

The inverse of this transfer can `also occur. An electric potential EhiL applied to electrodes 33 through leads 35 generates a helicon wave in the body 31 with a velocity vh in [the direction indicated by the arrow 37. This Ihelicon wave has the same frequency as the driving voltage Ehi. The helicon wave propagating through the body 31 is accompanied by an electric and a magnetic field with the same velocity and frequency. Part of these alternatin-g fields will penetrate into :the nearby piezoelectric body 23 and generate in it an acoustic wave of the same frequency. Part of this acoustic wave will propagate in the piezoelectric body 23 with velocity vh in the direction indicated by the arrow 29. In this way the piezoelectric nature of the piezoelectric body 23 serves as a means for transforming some of the energy of a helicon wave in the body 31 into an acoustic w-ave in the body 23.

If the -velocity of the acoustic wave va exceeds the velocity of the helicon wave vh, then transfer of energy from the acoustic wave to the helicon wave favored, and the helicon excitation tends to grow at the expense of the acoustic oscillation. Under this condition, the energy of the electrical signal Ehi applied to leads 27 is transferred to the helicon wave of the same frequency in the body 31. Conversely, when the helicon velocity vh exceeds the acoustic velocity va, transfer of energy from t-he helicon wave to the acoustic wave is favored, and the acoustic excitation tends to grow at the expense of the electromagnetic oscillation. In this case, the energy of an electric signal Ehi applied to leads 35 is transferred to the acoustic wave of the same frequency in body 23. The velocity of the helicon wave may be adjusted by 4adjusting the magnitude of the applied magnetic field HDC; its direction may be changed by changing the direction of the magnetic field. Thus, the one structure of FIGURE l can be either a unilateral coupler from the piezoelectric body 23 to the body 31 or a unilateral coupler from the body 31 to the piezoelectric body 23, the direction and strength of the coupling being determined by the direction and strength of the external magnetic field HDC.

The :acoustic wave may be amplified .or attenuated by an applied electric field through interaction with the electric current fiowing in the piezoelectric body 23 (FIGURE l), as in the manner described by A. R. Hutson et al., Phys. Rev. Letters 7, 237 (1961). The helicon Wave in the body 31 may be amplified or attenuated by an applied electric field through interaction with the electron current fiowinzg in the body 31, as in the manner described in A. Libchaber and R. Veilex, Phys. Rev. 127, 774 (1962). By such :an `arrangement an electric field Eac is applied (as with the electrodes 56 and associated circuit illustrated in FIGURE 4), in `addition to the applied magnetic field HDC in the direction of propagation of the helicon wave. Such an arrangement permits gain of either or both the acoustic Iand the electromagnetic waves as they traverse their respective media giving a very high ratio of output to input signal. Under these conditions, the device may exhibit a gain greater than unity.

The output from the electromagnetic wave energy may be derived from the body 31 as a voltage Eho through the electrodes 41 and leads 43 (which may be the conductors of a waveguide). The output from the acoustic wave ener-gy is derived from the body 23 through a mechanical linkage (see below) or as a voltage Eo through the electrodes 47 and the leads 49. In most applications, only one means for deriving an output is required, although several such means may be provided `as in FIGURE l.

In an acoustic-to-electromagnetic transducer, the piezoelectric body 23 may be excited by a mechanical source, such Ias a phonograph needle, as input, `a microphone, or a strain gauge, to produce acoustic waves in the body. The acoustic energy is then converted to helicon waves, and the helicon waves coupled out as an electrical signal across an output load. In an electromafgnetic-to-acoustic transducer, an electric signal is used to generate helicon waves which travel parallel to the piezoelectric path. The electric and/ or magnetic components of the helicon waves produce acoustic waves in the piezoelectric path, which actuate a linkage to a load, such las a loud speaker or other mechanical output.

Other combinations are possible, such as those in which one or more of the inputs or outputs is an acoustic or electromagnetic wave transmitted from or to lanother medium. Also, the effects can be modified by introducing reflecting members into the structures so as to enhance the standing wave patterns when resonant structures are desired, or by proper matching and proper introduction of absorbing materials 'to reduce refiections in the various ways known in the art.

FIGURE 2 illustartes an acoustic-to-electromagnetic transducer 21 adapted for use as a phonograph pickup, which is similar in structure to the device of FIGURE 1. The transducer 21 includes a piezoelectric body 23, a body 31 for supporting the propagation of helicon waves, and electrodes 41 and leads 43 for receiving lan output voltage Eho from the body 31. A phonograph needle 45 is attached to the piezoelectric body 23, providing a means for producing an acoustic wave in the body 23. The needle 45 is physically attached or mechanically linked to the piezoelectric body 23 in Iany of the ways ordinarily used in the art.

In operation, a constant magnetic field HDC is applied to the body 31 in the direction parallel or antiparallel to the arrow 39. When the needle 45 contacts the phonograph record grooves, it is vibrated, producing an acoustic wave in the piezoelectric body 23 which propagates down the body in the direction shown by the arrow 29 with a velocity va. Accompany-ing the acoustic Wave is an alternating electric field which penetrates the body 31, generating a helicon wave of the same frequency in it. The magnetic field HDC is adjusted so that the velocity of the helicon wave traveling down the body 31 in the direction shown by the arrow 37 has a velocity vh slightly less than the velocity of the acoustic wave. In this condition, the acoustic wave interacts with the helicon wave in the manner of a travelling wave device, amplifying the helicon wave and imparting much of the energy of the pick-up signal to the helicon wave. When the helicon wave reaches the other end `of the body 31, it is coupled to the output through the electrodes 41 and sensed as a voltage Eho on leads 43.

FIGURE 3 illustrates an electromagnetic-to-acoustic transducer 21 adapted for use as a loudspeaker and which is similar in structure to the dev-ice of IFIGURE 1. The device 21 of FIGURE 3 comprises a piezoelectric body 23 and a body 3=1 for supporting the propagation of electromagnetic (helicon) waves. Electrodes 33 and leads 35 connect to one end of the body 311. A linkage 53 mechanically couples -a speaker cone 455 with the other end of the piezoelectric body 23 by any means conventionally used in the art. In operation, a magnete field HDC indicated by the arrow 39 is adjusted t-o provide the desired helicon velocity vh in the body 311, which velocity is slightly faster than the acoustic velocity va in the piezoelectric body 23. A voltage Ehi is applied to the leads 3S generating helicon waves in the body 31 which propagate continuously down the body 31C in lthe direction shown by the arrow 37. The alternating field components of the helicon wave generate an acoustic Wave in the body 23 which is a replica of the helicon wave. The acoustic wave propagates down the body 2'3 in the direction indicated by the arrow 29 with a velocity va. The acoustic wave and the helicon Wave continue to interact during their travel. Since the acoustic wave velocity is slower than the helicon wave velocity, the acoustic wave grows in amplitude at the expense of the helic-cn wave. When the acoustic wave reaches the other end of the piezoelectric body 23, the linkage S3 is moved physically by the acoustic wave, providing the output of the device in the form of a movement of the linkage fwhich, in turn, actuates the speaker cone 55. The motion of the linkage is thus controlled by the electrical input Ehi.

Either an acoustic wave or a helicon wave may be amplified or attenuated. The acoustic wave amplifier illustrated in FIGURE 4 is similar in structure to the device of FIGURE l. In operation, an input Ea, produces an acoustic wave which propagates in the piezoelectric 4body 23 with a velocity v., las described above. The electric field associated with the acoustic waves produces a helicon Wave which propagates in the body 31 with a velocity vh. The velocity vh may be adjusted by the applied magnetic field HDC. When HDC is adjusted so that vh is greater than va, the helicon wave does not absorb energy from the acoustic Wave and the acoustic 7 wave is transmitted through body 23 with an attenuation determined principally by the material of the piezoelectric body 23. As shown in FIGURE 4, the acoustic wave -generates an output Voltage Eao which is a replica of Ea, but decreased in power or voltage or both.

If HDC is adjusted so that vh is equal to or smaller than va, then the acoustic wave is further attenuated by transferring energy to the helicons, that is the output acoustic wave is a replica of the input acoustic wave but -is smaller in amplitude or power or both than when vh exceeds va.

An applied electric field EDC may be included to amplify or further attenuate the acoustic wave. The electric field EDC is shown schematically by the arrow 57 (FIG- URES 1, 4, 5, 6 and 7) and may be produced, for example by a pair of electrodes 56 attached to the ends of the second body 31 and energized by a constant voltage source 58 (FIGURE 4). Amplification or attenuation is achieved by the interaction of the helicon wave with the fiow of electric current produced by the field EDC in the second body 431 in which the helicon wave is propagated. Changes in both the applied magnetic field HDC and the applied electric field EDC may be made at the same time to obtain a combined effect. A flow of current produced in body 23 by the field EDC will interact with the acoustic wave in body 23 in a similar wave, giving amplification or attenuation of the acoustic wave.

The helicon wave amplifier illustrated in FIGURE is similar in structure to the device of FIGURE 1 except that there are only input electrodes 33 for the helicon signal Ehi and the output electrodes 41 the helicon signal Ehh. In operation, a signal voltage Em causes a helicon wave to propagate in the second body 31 in the direction shown by the arrow 37 with a velocity vh. The alternating field of the helicon Wave causes an acoustic wave to propagate in the piezoelectric body 23 in the direction shown by the arrow 29 with a velocity va. As described above, the helicon wave may be attenuated by a controllable amount by adjusting the velocity va thereof with an applied magnetic field HDC; or it may be further amplified or attenuated 'by interaction with charge carriers in the body 31 with an applied electric field EDC; or both, while the acoustic wave may be further amplified or attenuated through current flow in body 23 produced by an applied electric field EDC.

FIGURE 6 illustrates an acoustic-to-electromagnetic transducer 21 with gain or attenuation. The structure of FIGURE 6 is similar to that of FIGURE 2 except that an adjustable magnetic -field HDC or an applied electric field EDC or both are included for the purpose of amplifying or of further attenuating the output in the manner described with respect to the device of FIGURE 4.

FIGURE 7 illustrates an electromagnetic-to-acoustic transducer 21 with `gain or attenuation. The structure of FIGURE 7 is similar to that `of FIGURE 3 except that an adjustable magnetic field HDC or an applied electric field EDC or both are included for the purpose of amplifying or further attenuating the output in the manner described with respect t-o the device of FIGURE 5.

It should be noted that the structure of FIGURE 6 and FIGURE 7 may be the same; that is, the structure of FIGURE 6 with the signals propagated from left to right may be the same as the structure of FIGURE 7 with the signals propagated from right to left. Because of the difference between the velocities vh and va, these structures are non-reciprocal, and the signal is propagated more strongly from the input in the higher velocity material to the output in the lower velocity material, as already described. Hence, if va exceeds vh, the propagation from the acoustic input on the electrodes 25 to electromagnetic out-put on the electrodes 41 is stronger than the reverse of electromagnetic input on the electrodes 41 and acoustic output on the electrodes 25. Alternatively, Viewed as a coupler, between electric signals on the electrodes 25 and electrodes 41., the coupling is stronger from the electrodes 25 to the electrodes 41 than in the reverse direction from the electrodes 41 to the electrodes 2S. When the external or internal conditions iare chosen so vh exceeds va, then the coupling is stronger from the electrodes 33 to the electrodes 47 of FIGURE 7, or from the electrodes 41 to the electrodes 25 in FIGURE 6 than in the reverse mode from the electrodes 47 to the electrodes 33 in FIGURE 7 or from the electrodes 25 to the electrodes 41 in FIGURE 6. With the one material structure of FIGURE 6 (or the equivalent structure of FIGURE 7) the various combinations of unidirectional couplers with variable gain or attenuation between electrical input and electrical output, between electric input and acoustic output, and between acoustic input and electric output can be achieved by appropriate adjustment of the external parameters, as already described. By electric, we include also electromagnetic waves, and by acoustic, we include also mechanical motions.

Any of the devices of FIGURES 2 through 7 may be used as a mixer of two or more signals with amplification or attenuation. Mixing may be achieved by adding or substituting an alternating electric field EAC of a desired carrier frequency, for the applied constantv electric field EDC indicated by the arrow 57. In the embodiment of FIGURE 4, for example, an applied alternating electric field EAC may be produced by positioning the switch 60 so as to disconnect the constant voltage source 58 and to connect a signal voltage source 62 across the electrodes 56. Mixing may also be achieved by adding or substituting an alternating magnetic field HAC of a desired carrier frequency, to the applied constant magnetic field HDC indicated by the arrow 39. In the embodiment of FIGURE 1, for example, an applied alternating magnetic field HAC may be produced by positioning the switch 42 so as to disconnect the constant current source 4t) and to connect the signal cur-rent source 44 across to the coils 38 of the electromagnets. Combinations of electric and magnetic field, both alternating and direct, may be used. Alternatively, any of the embodiments of FIGURES 2 through 7 may be used as a mixer as described above except that the signal input is applied through the applied field EAC, HAC or both, and the carrier frequency is applied as an input voltage Ehi, or Ea, or both.

Any of the embodiments of FIGURES 2 to 7 may be used as a demodulator of a modulated signal. This may be achieved by applying the modulated signal as an input voltage Ear, Ehi, 0r both; and simultaneously applying a beat frequency through an applied field EAC, HAC, or both. Alternatively, the modulated signal may be applied through an applied field EAC, HAC, or both, and the beat frequency may be applied as an input voltage Em, Ehi, or both. A mechanical input such as a mounted phonograph needle may be substituted for an input voltage Ea, in any of the embodiments.

The

bodies

23 and 31 of FIGURE 1 for supporting the acoustic wave and electromagnetic wave, may in some embodiments be combined in the same piece of material; that is, the helicon waves and acoustic waves may exist simultaneously in the same volume. Normally, the electric plasma may be expected to give unwanted action to the acoustic wave. But if gain or attenuation is provided by passing a current parallel to the direction of propagation, then the presence of a plasma can enhance the effect.

FIGURE 8 illustrates one embodiment of an integrated unit. The unit comprises three sections: a germanium body comprising a first section 81 and a second section 83, and a gallium arsenide body comprising a third section 85 epitaxially grown on the second section 83. A pair of electrodes 87 attached to opposite faces at one end of the first section 31 provides means for generating a helicon wave in the first section 81. When a magnetic field HDC is applied to the first section 8l in the direction shown by the arrow 89, the helicon wave propagates in the first germanium portion in the direction shown by the arrow 91 with a velocity vh. The helicon wave passes into the second germanium portion and then into the third section 85. The free carrier concentrations in 81 and 83 are so adjusted that the helicon is slowed down in successive steps from a high velocity in 81 to essentially the acoustical Velocity in 85. The helicon waves generate acoustic waves as they pass through 85 by a continuous interaction, transferring an appreciable amount of their energy to the acoustic Waves as they propagate together through section 85.

The structure of FIGURE 8 may be fabricated by the following procedure: An N-type crystal body of germanium with antimony, arsenic, or phosphorous doping is pulled along the 111 direction with initial doping of 2 l015/cm.3 changing abruptly to 2 101'7/crn.3 at a plane inside the single crystal, by known pulling techniques. From this is cut a body with cross section about 0.6 x 0.6 cm. (or 0.6 cm. in diameter) and length about 0.7 cm. along the 111 direction, with the doping step 0.06 cm. from the high doped end of the piece. The lower doped portion comprises the first section 81 and the higher doped germanium portion comprises the second section 83. The axis of the germanium body is along the 111 crystallographic direction with a nominal accuracy of a few degrees. The ends of the germanium body should be parallel to within about a mil.

On the end of the more heavily doped second section 83, a layer comprising the third section 85 of gallium arsenide (GaAs) is grown. This layer should be doped N-type with standard dope such as tin or silicon to 2 l019 electrons/cm2. The third section 85 can be grown epitaxially from vapor phase deposition, by solution-regrowth, or other tsandard techniques, so as to have its 111 axis essentially parallel to the 111 axis of the germanium. After growth, the GaAs layer is ground to a thickness of 0.12 cm. with its free end parallel to the ends of the germanium body to within about a mil.

When a constant magnetic field HDC of approximately 10,000 gauss is applied along the 111 direction of the germanium body, this unit satisfies the basic conditions for operation according to the invention. A signal Ehi applied to the electrodes 87 causes a helican to propagate along the 111 direction of the body. In the gallium arsenide portion 85, the velocity of propagation of a helicon wave in the 111 direction is essentially the velocity of sound in this direction, for a helicon of frequency of about 20 mc./ sec. When the magnetic field is somewhat larger than this, the helicon Velocity will be larger than the velocity of sound; for lower magnetic fields it will be smaller. Hence, for a magnetic field much in excess of 10,000 gauss, a coherent sound wave in GaAs will grow at the expense of the helicon wave. Since the wavelength of the acoustic wave of this frequency is about 0.025 cm., the two waves will interact in the GaAs for about ve wavelengths, giving a long region for a travelling wave interaction. This interaction length can be changed for particular applications to increase or decrease the interaction length.

The first section 81 serves the function of a transformer section from the electrical circuit composed of leads connected to electrodes 87 with moderately high characteristic impedance. The second section 83 serves the function of a waveguide of low characteristic impedance. The second section 83 is designed to be a quarter wave section, of intermediate impedance between the first and third sections 81 and 8S. Specifically, in the first section 81, the wavelength of a helicon Wave of frequency 20 mc./s. in a magnetic field of 10,000 gauss is about 2.5 cm. The first section 81 is about 1A Wavelengths long. In the second section 83, because of its increased doping, the helicon Wave has a wavelength of 0.25 cm., and section 83 has a thickness of one-quarter wavelength for the same magnetic field and frequency. In the third section 85, the wavelength of the helicon 10 wave is about 0.025 cm. at this same magnetic field and frequency.

The structure of FIGURE 8 can be modified in accordance with the known practices of match-ing waveguides from one impedance level to another. Thus, instead of an abrupt doping step, the doping can be smoothly graded from 2.3 l01'1 to 2.3)(1019. This will look like an exponential horn type of matching if the doping n follows a pseudo-exponential law This can be approximately achieved by diffusing arsenic into a crystal doped to the lower density. Here n is the doping at the position x measured from the ow doped end of the crystal; no is the doping at x=0; and x0 is the distance over which the doping changes by the factor 2.3. To duplicate the structure of the example, no is and x0 is 0.075 cm. This will give a match from the input end where the germanium is doped to 2X1015/cm.3 to the GaAs section 85, which has the uniform doping of 2X1019/cm.3. The cross sectional area can also be reduced from larger at the left end of the first germanium portion 81 to smaller at the right end of the second germanium portion 83, as the Wavelength decreases.

In some embodiments, as in the embodiment of FIG- URE 8, the piezoelectric medium and/or a transitional medium and/or the helicon transmitting medium is of a semiconductor. It has been noted above that some of the operating characteristics of the device are a function of the free charge carrier density in the semiconductor. Where this is the case, light or heat incident upon a portion of the semiconductor may change the free charge carrier density in that portion, thereby changing the operating characteristics of the device. Thus, incident light and/ or ambient heat may be used to modify the operating characteristics of the device, or `to provide another input to the device. Some examples of embodiments of the invention including means for changing the free charge carrier density in a semiconductor portion are shown in FIGURES 8A, 8B and 8C.

The embodiment of FIGURE 8A is the same as that of FIGURE 8 except that it includes means for injecting free charge carriers into the first `germanium portion 81. The injecting means includes a rectifying contact 82 and an ohmic contact 84 to the germanium first portion 81. The

contacts

82 and 84 are connected through a voltage source 86 and a signal source 88 which are connected in series. When the rectifying contact is forward biased, carriers are injected into the first portion 81. The density of injected carriers is varied by the -signal voltage from the signal source 88. Different types of contacts permit changing injection or extraction of majority or minority carriers by known semiconductor techniques.

The embodiment of FIGURE 8B is the same as that of FIGURE 8 except that it includes means for exciting free charge carriers in the first germanium portion 81 with light. The exciting means comprises a source of light of a suitable band of wavelengths, and means, such as a lens 92, Ifor directing light from the light source 90 upon the first germanium portion 81. When light is incident on the first portion 81, free charge carriers are photoexcited therein. The density of photoexcited carriers may be varied by changing the opening of an iris 94 between the light source 90 and the first portion 81.

The embodiment of FIGURE 8C is the same as that of FIGURE 8 except that it includes means for varying the temperature of the first germanium portion 81. The temperature Varying means includes for exam-ple, an insulated chamber 96, a heating coil 98 in the chamber 96, and means 100 for variably energizing the heating coil 98. When the ambient temperature is raised, the temperature of the first section 81 is also raised, thereby increasing the density of thermally-excited free charge carriers in the first section 81.

In employing the devices described above, special pro- Vision is sometimes desirable for coupling the helicon wave in the body 31 to an external electric circuit, because the helicon wave inside the body 31 (FIGURE 1) has a far shorter wavelength than a normal electromagnetic wave of the same frequency in a conventional waveguide or conductor. One way of improving the electrical coupling between the body 31 and an external circuit, is to connect the body 31 to the external circuit through a graded structure in which the helicon velocity increases progressively from the value of v,L to one approaching the velocity of light in free space (which is 3 1010 cm./sec.). In such a structure, the helicon wave length increases progressively from that of the acoustic signal toward the free space wave length. When the wave length of the electromagnetic wave becomes somewhat larger than it is in the body 31, it becomes possible to insert probes or loops so as to couple out the wave to a coaxial structure or other circuit. In order to couple in this mode, the wave length of the electromagnetic wave needs to be only large enough to be comparable with a feasible mechanical structure such as a small wire probe or a comb structure like a grating.

FIGURE 8 illustrates a graded connection between electrical inputs vh and a helicon structure 85 with vhiva. If 85 is piezoelectric, it may have the transducer action in it, as described with FIGURE 7. In any case, it is a graded structure.

FIGURE 9 shows a structure in which the transducer of FIGURE 8 is incorporated for use as an ultrasonic cutter. Four

ohmic electrodes

93, 93, 95, and 95 contact the first germanium portion 81, on opposite side faces of the body or on 90 arcs for a circular cross section. These electrodes are driven in quadrature from a two phase electrical signal of frequency in the range of 20 mc./s. for which the transducer is designed through transformers 97 and 99. The sense of rotation of the driving iield is such as to generate helicon waves in the first germanium portion 81 which are propagated with sma-ll attenuation toward the gallium arsenide portion 85 in the 111 direction. (If the sense of rotation of the electric field is wrong, it can be corrected by reversing either one pair of connections or the direction of the magnetic field HDC.) 'Ihe output is an ultrasonic cutting tool 101 bonded to the GaAs portion 85, in conventional manner.

Two phase electrical power at a nominal frequency of about 20 mc./s. is supplied to the quadrature transformer, producing a rotating electrical eld in the first germanium portion 81 and generating helicon waves of the same frequency therein. Since the helicon wavelength in the irst germanium portion 81 is fairly long, about 2.5 cm., the

contacts

93 and 95 can be easily placed on the crystal so as to give optimum coupling to the helicon waves. The helicon waves pass to the right through the variable transmission line and are transmitted with good efficiency to the GaAs portion 85. In the GaAs portion 85, the velocity of the helicon Wave is slightly higher than the velocity of sound, and so it transfers its energy via the piezoelectric interaction in the GaAs to a coherent acoustical wave of the same frequency travelling in the same direction. This acoustical wave gives a mechanical distortion to the GaAs portion 85 which drives the cutting tool 101 in the conventional way. The efliciency of the unit can be optimized by slight changes in the driving frequency yand by adjustment of the strength and direction of the magnetic iield.

FIGURE 10` is a device for generating an ultrasonic wave in a liquid. A rectangular piece of germanium about 1.5 x 3.3 cm, with 111 crystallographic axis in the long dimension is comprised of three

sections

103, 105 and 107. The

sections

103 and 105 are the same as already de- 12 scribed for portions 81 and 83 of FIGURE 7 except for the cross sectional dimensions. The section 107 is a more heavily doped section with a doping concentration to give 2 1019 electrons/ cm.3 and length 0.30 cm. The section 107 can be added to the end of the section 105 by expitaxial or solution-regrowth in known ways. Alternatively, the entire piece of germanium can be made to `have a graded doping profile going from 2 1015 at the extreme end of the section 103 to 2 1019 at the opposite extreme end of section 105, as previously described. The section 107 can then be given a constant doping concentration of about 2 1019 by outdiifusion.

Section 109 is a rectangular piece of GaAs, which can be high resistivity, and which is grown onto the germanium, with 111 axis in the same direction as the gremanium body. The section 109 joins to the section 107 so that wave enengy may couple to the germanium over a number of wavelengths. The body 109 can protrude slightly beyond the germanium section 107 for convenience of insertion into the liquid in the beaker below it. A magnetic field of about 25,000 gauss is supplied in the direction shown by the arrow 111. Two pairs of ohmic contacts 113, 115, 117 and 119 connect to opposite faces of the germanium body at one end of the section 103. The electrode pads are driven in quadrature from the two phase supply through

transformers

121 and 123. When the germanium is driven at about 8 mc./s. a helicon mode electromagnetic wave is generated in the section 103 which passes down through the germanium from the section 103 through section to section 107. In the section 107, the velocity of the helicon mode wave is slightly higher than the velocity of sound in the adjacent GaAs body 109, and so the helicon energy excites and amplies coherent acoustic waves of the same frequency in the GaAs body 109. These acoustic waves travel through the GaAs body 109 and couple out into a fluid 125 contacting the free end of the GaAs body 109.

If the strength of the magnetic field is reduced so that va in body 109 is now greater than vh in body 107, an acoustic wave of about 8 mc./ sec. travelling upward in the iiuid 125 will produce an electrical signal in electrodes 113, 115, 117 and 119 because now the entire process and direction of energy transport is reversed. The electrical output can be taken either -as single phase or two phase. Adjustment of the size and shape of the cross section of theGaAs and of the Ge may enhance or diminish the different waves modes in the transducer so as to favor the desired one, in analogy with practice in electrical waveguides or musical instruments.

The efficiencies and power outputs of the devices described in FIGURES 8, 8A, 8B, 8C, 9 and 10 can be enhanced by application of suitable electric fields EDC or EAC as already discussed in connection with the previous embodiments.

What is claimed is:

1. An acoustic-electromagnetic device comprising means `for generating an acoustic wave, a piezoelectric medium for supporting the propagation of said acoustic wave along a path therein and means for supporting the propagation of an electromagnetic wave with at least a component thereof propagating in said path in the same direction as said acoustic wave, said electromagnetic wave supporting means having associated therewith magnetic ileld means for maintaining said electromagnetic wave at a velocity related to the velocity of said acoustic wave for selectively transferring energy from one of said waves to the other.

2. An acoustic-electromagnetic device comprising means for generating an acoustic wave, a piezoelectric medium for supporting the propagation of said acoustic wave along a path therein, and means for supporting the propagation of an electromagnetic wave in the helicon mode with at least a component thereof propagating in said path in the same direction as said acoustic wave, said electromagnetic wave supporting means having asso- 13 ciated therewith magnetic iield means for maintaining the velocity of said electromagnetic wave at substantially the velocity of said acoustic wave.

3. An acoustic-electromagnetic device comprising a means for generating an -acoustic wave, piezoelectric medium for supporting the propagation of said acoustic wave along a path therein, means including another medium adjacent said piezoelectric medium, for supporting the propagation of an electromagnetic wave in the helicon mode in said other medium with at least a component of said electromagnetic wave propagating in said path in substantially the same direction as said acoustic wave, said electromagnetic wave supporting means having associated therewith means for producing a magnetic field along .said path in the direction of propagation of said acoustic wave for maintaining the velocity of said electromagnetic Wave at substantially the velocity of said acoustic wave.

4. An acoustic-electromagnetic device comprising means for generating an acoustic wave, a piezoelectric medium for supporting the propagation of said acoustic wave along a path therein, means including a semiconductor medium adjacent said piezoelectric medium, for supporting the propagation of an electromagnetic Wave in the helicon mode in said semiconductor medium with at least a component of said electromagnetic Wave propagating in said path in substantially the same direction as said acoustic Wave, said electromagnetic wave supporting means having associated therewith means for producing a unidirectional magnetic field along said path in the direction of propagation of said acoustic wave for maintaining the velocity of said electromagnet wave at substantially the velocity of said acoustic Wave.

5. An acoustic-electromagnetic device comprising means for generating an acoustic wave, a piezoelectric medium for supporting the propagation of said acoustic wave along a path therein, means including a semiconductor medium adjacent said piezoelectric medium, for supporting the propagation of an electromagnetic wave in the helicon mode in said semiconductor medium with at least a component of said electromagnetic wave propagating in said path in substantially the same direction as said acoustic wave, said electromagnetic wave supporting means having associated therewith means for producing a magnetic eld for maintaining the velocity of said electromagnetic wave at substantially the velocity of said acoustic wave, and means for adjusting the velocity of propagation of said electromagnetic wave in said semiconductor medium for selectively transferring energy from one of said Waves to the other.

6. An acoustic-electromagnetic device comprising means for generating an acoustic wave, a piezoelectric medium for supporting the propagation of said acoustic wave along a path therein, means including a semiconductor medium adjacent said piezoelectric medium, for

supporting the propagation of an electromagnetic Wave in the helicon mode in said semiconductor medium with at leasta component of said electromagnetic wave propagating in said path in substantially the same direction as said acoustic wave at substantially the velocity of said acoustic Wave, said electromagnetic wave supporting means having associated therewith means for producing a magnetic field in said semiconductor body in the direction of propagation of said electromagnetic Wave, and means for adjusting the velocity of propagation of said electromagnetic wave in said semiconductor medium for selectively transferring energy from one of said waves to the other and including means for changing the strength of said magnetic lield.

7. An acoustic-electromagnetic device comprising means for generating an acoustic wave, a piezoelectric semiconductor medium for supporting the propagation of said acoustic wave along a path therein, means including said semiconductor medium for supporting the propagation of an electromagnetic wave in the helicon mode in said semiconductor medium with at least a component of said electromagnetic wave propagating in said path in substantially the same direction as said acoustic wave, said electromagnetic wave supporting means having associated therewith magnetic field means for maintaining the velocity of said electromagnetic wave at substantially the velocity of said acoustic wave, and means for adjusting the velocity of propagation of said electromagnetic wave in said semiconductor medium for selectively transferring energy from one of said waves to the other, said adjusting means including means for changing the density of charge carriers in said semiconductor medium.

8. An acoustic-electromagnetic device comprising means for generating an acoustic wave, a piezoelectric semiconductor medium for supporting the propagation of said acoustic wave along a path therein, means including said semiconductor medium for supporting the propagation of an electromagnetic wave in the helicon mode in said semiconductor medium with at least a component of said electromagnetic wave propagating in said path in substantially the same direction as said acoustic wave, said electromagnetic wave supporting means having associated therewith magnetic eld means for maintaining the velocity of said electromagnetic wave at substantially the velocity of said acousti-c Wave, and means for adjusting the velocity of propagation of said electromagnetic Wave in said semiconductor medium for selectively transferring energy from one of said waves to the other, said adjusting means including means for variably injecting free charge carriers into said semiconductor medium from an external source.

9. An acoustic-electromagnetic device comprising means for generating an acoustic wave, a piezoelectric semiconductor medium for supporting the propagation of said acoustic wave along a path therein, means including said semiconductor medium for supporting the propagation of an electromagnetic wave in the helicon mode in said semiconductor medium with at least a component of said electromagnetic wave propagating in said path in substantially the same direction as said acoustic Wave, said electromagnetic Wave supporting means having associated therewith means for maintaining the velocity of said electromagnetic wave at substantially the velocity of said acoustic wave, and means for adjusting the velocity of propagation of said electromagnetic Wave in said semiconductor medium for selectively transferring energy from one of said waves to the other, said adjusting means including means for directing light of controlled intensity upon said semiconductor medium.

10. An acoustic-electromagnetic device comprising means for generating an acoustic wave, a piezoelectric medium for supporting the propagation of said acoustic Wave along a path, means for supporting the propagation of an electromagnetic wave with at least a component thereof propagating in said path in the same direction as said acoustic wave, said electromagnetic wave supporting means having associated therewith magnetic iield means for maintaining said electromagnetic Wave at a velocity related to the velocity of said acoustic wave for selectively transferring energy from one of said waves to the other, whereby said waves interact with each other along said path, means for receiving said electromagnetic wave after said interaction, and means for substantially increasing the velocity of said received electromagnetic wave toward the free space velocity of an electromagnetic wave.

11. An acoustic-electromagnetic device comprising means for generating an acoustic wave, a piezoelectric medium for supporting the propagation of said acoustic wave along a path therein, a source of electromagnetic waves having a velocity substantially greater than the velocity of said acoustic wave, means for reducing the velocity of said electromagnetic waves, means for supporting the propagation of said electromagnetic Waves of reduced velocity with at least a component thereof propagating in said path in the same direction as said acoustic wave, said electromagnetic wave supporting means having associated therewith magnetic field means for maintaining the velocity of said component at substantially the velocity of said acoustic wave.

12. An acoustic-electromagnetic device comprising means for generating an acoustic wave, a piezoelectric medium for supporting the propagation of said acoustic wave along a path therein, means for supporting the propagation of an electromagnetic wave with at least a component thereof propagating in said path in the same direction as said acoustic wave, said electromagnetic Wave supporting means having associated therewith magnetic field means for maintaining said electromagnetic wave at a velocity related to the velocity of said acoustic wave for selectively transferring energy from one of said waves to the other, whereby said waves interact with each other along said path, and means connected to said device for deriving an output from at least one of said interacted Waves.

13. An acoustic-electromagnetic device comprising means for generating an input acoustic wave, a piezoelectric medium for supporting the propagation of said acoustic wave along a path therein, means for supporting the propagation of an electromagnetic wave in the helicon mode with at least a component thereof propagating in said path in the same direction as said acoustic wave, said helicon wave supporting means having associated therewith magnetic eld producing means for maintaining the velocity of said component at substantially the velocity of said acoustic wave in said medium, whereby said waves interact with each other along said path, means for applying at least one of a signal electric field and a signal magnetic lield having a substantial component along said path, and means for deriving an output from said helicon wave after said interaction.

14. An acoustic electromagnetic device ycomprising means for generating an acoustic wave, a piezoelectric medium for supporting the propagation of said acoustic wave along a path therein, means for generating an electromagnetic wave in the helicon mode, means for supporting the propagation of said helicon Wave with at least a component thereof propagating in said path in the same direction as said acoustic wave in said medium, said helicon wave supporting means having associated therewith magnetic iield producing meanslx for maintaining the velocity of said component at substantially the velocity of said acoustic wave in said medium, whereby said waves interact with each other along said path, means for applying at least one of a signal electric eld and a signal magnetic ield having a substantial component along said path, and means for deriving an output acoustic wave after said interaction.

1S. An acoustic-electromagnetic device comprising means for producing an input acoustic wave, a piezoelectric medium for supporting the propagation of said acoustic wave along a path, means including a semiconductor medium adjacent said piezoelectric medium, for

supporting the propagation of an electromagnetic wave with at least a component thereof propagating in said path in the same direction as said acoustic wave, said component having substantially the velocity of said acoustic wave for selectively transferring energy from one of said waves to the other, whereby said waves interact with each other along said path, means for applying at least one of a signal electric eld and a signal magnetic field having a substantial component along said path, means for applying a direct magnetic iield having a substantial component along said path, and means connected to said device yfor deriving an output acoustic wave after said interaction.

16. An acoustic-electromagnetic device comprising means for producing an input acoustic wave, a piezoelectric medium for supporting the propagation of said acoustic wave along a path therein, means including a semiconductor medium adjacent said piezoelectric medium, for supporting the propagation of an electromagnetic wave in the helicon mode with at least a component thereof propagating in said path in the same direction as said acoustic wave, said component having substantially the velocity of said acoustic wave in said medium, whereby said waves interact with each other along said path, means for applying a direct magnetic field having a substantial component along said path, means for applying at least one of a signal electric field and a signal magnetic field having a substantial component along "said path, and means for deriving an output helicon wave after said interaction.

17. An acoustic-electromagnetic device comprising means for generating an acoustic wave, a piezoelectric medium for supporting the propagation of said acoustic wave along a path therein, means for producing an input electromagnetic wave in the helicon mode, means including a semiconductor medium adjacent said piezoelectric medium for supporting the propagation of said helicon wave with at least a component thereof propagating in said path in the same direction as said acoustic wave in said medium, said component having substantially the velocity of said acoustic wave in said medium, whereby said waves interact with each other along said path, means for applying a direct magnetic eld having substantial component along said path, means for applying at least one of a signal electric iield and a signal magnetic eld having a substantial component along said path, and means for deriving an output acoustic wave after said interaction.

References Cited by the Examiner UNITED STATES PATENTS 2,898,477 8/1959 Hoesterey 3l08.l 3,158,819 11/1964 Tien S30-4.6

MILTON O. HIRSHFIELD, Primary Examiner.

ORIS L. RADER, Examiner.

A. I. ROSSI, Assistant Examiner'.

Claims

1. AN ACOUSTIC-ELECTROMAGNETIC DEVICE COMPRISING MEANS FOR GENERATING AN ACOUSTIC WAVE, A PIEZOELECTRIC MEDIUM FOR SUPPORTING THE PROPAGATION OF SAID ACOUSTIC WAVE ALONG A PATH THEREIN AND MEANS FOR SUPPORTING THE PROPAGATION OF AN ELECTROMAGNETIC WAVE WITH AT LEAST A COMPONENT THEREOF PROPAGATING IN SAID PATH IN THE SAME DIRECTION AS SAID ACOUSTIC WAVE, SAID ELECTROMAGNETIC WAVE SUPPORTING MEANS HAVING ASSOCIATED THEREWITH MAGNETIC FIELD MEANS FOR MAINTAINING SAID ELECTROMAGNETIC WAVE AT A VELOCITY RELATED TO THE VELOCITY OF SAID ACOUSTIC WAVE FOR SELECTIVELY TRANSFERRING ENERGY FROM ONE OF SAID WAVES TO THE OTHER.