This system is used to investigate one of the most fascinating entities in modern nonlinear science; the soliton, or solitary wave. Solitons are formed in nonlinear dispersive media due to a balance between nonlinear compression and dispersive spreading. These nonlinear solitary waves have been observed in many physical systems, such as optical fibers, water, and electrical circuits to name a few. Here at Colorado State University we study microwave magnetic envelope solitons in thin magnetic films. The low loss characteristics of single crystal yttrium iron garnet (YIG) thin films make it possible to study a great number of nonlinear spin wave phenomena, including soliton formation and propagation. Thin YIG films are distinguished from other media, like optical fibers, in that their nonlinear and dispersive properties may be varied through experimentally variable parameters. In order to realize the formation and propagation of solitons it is necessary to introduce and receive microwave pulses in these films. To this end a simple microstrip delay line is used. A microstrip delay line consists of an input and output antenna with a YIG film placed face down on the antennas.
A simple explanation of the typical soliton experiment is as follows. First a microstrip delay line is placed in an external magnetic field. Next, its transmission vs frequency characteristic is measured for some fixed external field. From the transmission characteristic one may choose an operating point frequency. The narrow rectangular pulses are then applied to the input of the delay line at that carrier frequency. Through careful manipulation of the experimentally variable parameters, such as antenna separation, carrier frequency input pulse width and power, soliton formation and propagation is realizable. A soliton formed in a thin (5 - 10 mm) YIG film has the following typical characteristics: a carrier frequency of 4 - 8 GHz, a temporal width of 10 - 20 ns, a carrier wave number of 100 - 200 rad/cm and is formed from an input pulse of 25 - 30 ns at a power level below 1 W.
A subsystem of the soliton system in the magnetics lab at CSU is the inductive magnetodynamic probe (IMP) system. This system operates under the very simple idea of Faraday's law. When a spin wave propagates within a thin magnetic film, due to the continuity of B and H at the surface of the film, there is a microwave magnetic field near the surface of the film. Through the use of a carefully constructed inductive probe loop it is possible to probe these fields near the surface of the film. With this system it is possible to construct spatial and temporal contours of pulsed and cw excitations within the film. The IMP system here at CSU has a spatial and temporal resolution of better than 50 um and 1 ns, respectively. This makes it possible to investigate both linear and nonlinear pulse and cw propagation characteristics directly with a very good accuracy. The resolution of the probe is many orders of magnitude smaller than that of the excitations which it probes.
Besides the soliton system, the IMP system also works in tandem with the BLS system. While both the BLS and IMP systems have comparable resolutions, there are important differences between them. The most important difference is in the wave vector selectivity of the BLS system. However, while the IMP system has no wave vector selectivity, it does allow one to measure all characteristics of the waves in the film which are accessible to modern microwave measurement techniques. These include phase and frequency capabilities as well as many others which are inaccessible to the BLS system. Another important advantage of the IMP system, is its ability to probe planar microwave devices which are inaccessible to the BLS system. The IMP system combined with the soliton system and the BLS system make it possible to measure a wide variety of linear and nonlinear processes in thin ferrite films as well as device characterization capabilities.
Key instruments used for this system include:
- Agilent Infiniium Oscilloscope (1.5 GHz, 8 GSa/s)
- HP 8161A Pulse Generator
- HP 83650A Synthesizer Sweeper (10 MHz - 50 GHz)
- HP 83623B Synthesizer Sweeper (10 MHz - 20 GHz)
- HP 8510C Network Analyzer (45 MHz - 50 GHz)
- HP 8593E Spectrum Analyzer (9 kHz - 26.5 GHz)
- HP 70820a Microwave Transition Analyzer (DC - 40 GHz)