Patent GB-986589-A (GB)
Abstract: 986,589. Solid state devices. STANDARD TELEPHONES & CABLES Ltd. (International Telephone & Telegraph Corporation). May 1, 1963, No. 17175/63. Heading H1K. [Also in Division H3] Infra-red or microwave signals are amplified by a body comprising two dissimilar monocrystalline materials which are clamped together in intimate contact and have normal (+) and abnormal (-) g factors respectively, direct current being passed from one material to the other while the body is maintained at a low temperature in a magnetic field. Depending on the direction of current through the body, the arrangement is capable of emitting continuous and amplified radiation of either infra red or microwave frequency in response to stimulation by the same frequency, the precise frequency of operation being determined by the strength of the magnetic field. Theory.-The device utilizes the principle that in a condition of thermal equilibrium the magnetic field establishes Zeeman sub-levels each characterized by the spin direction of conduction electrons. For a material having a normal + g factor, the more densely populated lower sublevel has a spin quantum number of m = - ¢ corresponding to a spin direction anti parallel with the applied field, while for a material having an abnormal - g factor, the lower sublevel has a spin quantum number m = + ¢ corresponding to a spin direction parallel with the applied field. Similarly for the upper sublevels m = + ¢ for + g materials and m = - ¢ for - g materials. The energy diagram for a body formed of non-ferrous metal (portion 2) in contact with indium antimonide (portion 3) is shown in Fig. 2, the g factors being + 2 for the metal and - 58 for the indium antimonide giving respective energy separations of #E 2  and #E 3 , where #E 3  = 29#E 2 , between sublevels 25 and 26 and between sublevels 27 and 28. Pumping is effected continuously by the D.C. potential which causes conduction electrons to flow from portion 2 to portion 3. Thus electrons from level 26 are forced into level 27 since their spin directions correspond, and similarly the electrons from level 25 are forced into level 28. As both portions initially have an excess of population in the lower energy levels 26, 28, it follows that pumping produces a greater population in level 27 than in level 28, and the material of portion 3 will exhibit a negative temperature characteristic due to population inversion. The inversion is maintained by the D.C. potential, and continuous amplification of an incident infra-red signal of frequency corresponding to the energy level separation # 3  takes place by the process of stimulated emission. In a modification, Fig. 4, silicon is used as the + g factor material and microwave amplification is obtained in this case by reversing the polarity of the D.C. source. As shown the pump action transfers conduction electrons from energy levels 42 and 43 to levels 41 and 40 respectively and so inverts the population, the separation #E 2  of the levels in portion 2 corresponding to the microwave frequency to be amplified. An alternative microwave amplifier uses metal (g factor + 2) for portion 2 and gallium arsenide (g factor - 1.6) for portion 3, Fig. 5 (not shown the positive D.C. terminal being connected to portion 2. Suitable + g factor materials are stated to be copper, gold, silver, caesium, and rubidium, and suitable - g factor materials are semi-conductors such as indium arsenide, indium antimonide, gallium arsenide, mercury telluride, mercury selenide, bismuth and antimony. To prevent a build-up of space charge in the material connected to the positive source terminal, the injected electrons must be rapidly removed either by an electronic sweeping field or by the use of P-type semi-conductor material. Infra-red amplifier.-As shown in Fig. 1 the body 1 comprises a metal block 2 having a normal g factor and a semi-conductor block 3 have an abnormal g factor, the blocks being held together by a plastic clamp 4 with optically true surfaces in contact to form an interface 5. A D.C. source 14 has its positive and negative terminals connected to electrodes 15 and 16 fused on to blocks 3 and 2 respectively. A resonant cavity is formed by the metallic wall of electrode 15 and the highly polished surface of interface 5. Half silvered mirrors 21, 22 optionally provide a further resonant cavity. The arrangement is housed within a dewar vessel 6 comprising concentric shells 8, 9, 11 providing an evacuated region 7 and a region 10 containing liquid nitrogen or helium, apertures 6a, 6b being provided for the incident and emitted radiations. The arrangement is positioned between magnet poles 12, 13, and parabolic mirrors 17, 19 and a collimating lens 18 establish the necessary electromagnetic wave paths between an infra-red source, the body 1 and a detector 20. Microwave amplifier.-A body 1 formed as described for the infra-red amplifier is. disposed between magnetic polepieces 12, 13, Fig. 3, and is contained within a dewar vessel 6. The positive and negative terminals of a D.C. source 14 are connected to blocks 2 and 3 respectively through electrodes 15, 16, one connection utilizing as a conductor the metallic wall of a waveguide resonant cavity 33 which is tuned to the microwave frequency to be amplified. The input signal is applied from an antenna 36 to the resonant cavity 33 through a circulator 37 and a transmission line 38, and the extracted electromagnetic waves are coupled through the circulator to a detector 39.