Patent GB-1050160-A (GB)
IPC: H01L33/00
Abstract: 1,050,160. Light amplifiers. INTERNATIONAL BUSINESS MACHINES CORPORATION. Aug. 21, 1963 [Aug. 29, 1962], No. 33014/63. Heading H3B. [Also in Division H1] The drift velocity of a flow of charge carriers in a crystalline body is made greater than the phase velocity of a lattice optical mode of vibration in the crystalline body so that energy is exchanged between the optical mode and the flow of carriers to cause amplification of the vibration. As shown in Fig. 2 an N-type gallium arsenide crystal 2 is provided with N-type germanium contacts 3, 4 across which is connected potential source 9 providing a field of 2000 volts/cm. in the crystal body so as to produce a drift velocity V 0  for the electron charge carriers in the direction shown. It is shown theoretically that strong interaction is possible between polar waves of the crystal lattice and the free carriers where the phase velocity of the polar waves is greater than the carriers' drift velocity, the polar waves being those optical modes of vibration of a polar lattice for which the particle has a longitudinal component and which consist of a motion which can be described as a collective displacement of the positive ions with respect to the negative ones. If the drift velocity V o  is made larger than the phase velocity, i.e. greater than #/K where # is the frequency and K is the wave vector of the polar wave, the polar wave generated in the crystal body grows with time and is amplified in the -X direction. If the drift velocity is less than the phase velocity attenuation of the polar wave takes place. By coupling external electromagnetic waves to the polar waves inside the crystal amplification of the electromagnetic waves can be achieved. In order to couple light waves having the same frequency as the polar waves to the latter a surface of discontinuity such as 5, 6 is provided across which current can flow. The contacting surfaces 5, 6 are preferably sloping so as to prevent the possibility of a perfectly symmetrical condition existing within the crystal lattice such that the individual wavelets which are produced will tend to cancel each other. If light waves of about 10<SP>3</SP> times that of the polar waves are coupled thereto, then the light waves can be diffracted by polar waves since the latter behave like a stationary diffraction grating, the incident and diffracted waves differing in frequency by the frequency of the polar waves. A beam of X-rays may be modulated and shifted in frequency under the control of an electromagnetic wave applied to a crystal so as to generate polar waves. Alternatively if the electromagnetic wave which generates the polar wave is itself an X-ray there is provided the mixer and I.F. amplifier of an X-ray superheterodyne receiver. If two discontinuents are situated in the solid such that they reflect waves back and forth along the direction of carrier flow they will give rise to oscillations provided that the product of amplification along the path between them times the attenuation in the opposite direction times their reflection coefficients exceeds unity. The polar waves give rise to electromagnetic waves at the interface 6 and are taken from the crystal body 2.