Applications of photoinduced electron spin polarization at room temperature to microwave technology

Aharon Blank, Haim Levanon
2001 Applied Physics Letters  
We present a method for controlling the bulk permeability via spin polarization generated by light excitation. This process involves the magnetic interaction of photoexcited triplets with stable radicals in liquid solution. The resulting non-Boltzmann spin population of the stable radical is reflected by a significant change in the permeability of the chemical system. We demonstrate how these light-driven changes result in conspicuous changes in the reflection coefficient ͑amplitude and phase͒
more » ... plitude and phase͒ of a microwave cavity in which the active chemical system is placed. This effect can lead the way to ultralow noise microwave amplifiers and low-loss microwave phase shifters, operating at room temperature with very low spin temperature ͑Ͻ16 K͒. Moreover, the nonlinear character of the phenomenon can be utilized for devices, which protect sensitive instrumentation from a strong destructive microwave pulse. Controlling the magnetic permeability in paramagnetic materials has been recognized as an important feature in microwave technology. 1-3 The general concept is to exploit changes in the electron spin population of the magnetic Zeeman levels, thus allowing us to amplify the electromagnetic radiation in the bulk material. These early studies resulted in the three-level solid-state maser ͑microwave amplification by stimulated emission of radiation͒ amplifier based upon the paramagnetic properties of the electron. 4,5 The main advantage of using the maser is typified by its extremely low-noise figure as compared to conventional microwave amplifiers, e.g., vacuum tubes and GaAs field effect transistors. For example, maser technology was used in the discovery of the 3.5 K background blackbody radiation of the universe. 6 Even today, the most advanced state-of-the-art semiconductor microwave amplifiers, e.g., high electron mobility transistors, cooled to cryogenic temperatures can achieve noise temperature of ϳ6 K, while the maser achieves noise temperature of ϳ3 K at ϳ9 GHz. 7 In general, the gap between the maser noise performance relative to cryogenic semiconductor amplifiers increases with microwave frequency. To achieve amplification in a paramagnetic-based maser, one must achieve inverted spin population, which corresponds to a negative spin temperature. This is done in conventional masers by microwave pumping, but with the restriction of operating at very low temperatures ͑TϽ2 K͒. This restriction, which precluded the wide use of masers, can be accounted for by two reasons. First, by microwave pumping high population inversion can be achieved, only if k B T Ӷh. The second reason is that the active materials in the microwave pumped masers 8 exhibit a very steep dependence of the spin-lattice relaxation ͑SLR͒ time upon temperature. Thus, at high temperatures, the fast relaxation cannot allow for efficient pumping of the magnetic levels. Pumping the levels by optical excitation can overcome the first difficulty of very low temperature. [8] [9] [10] [11] Nevertheless, the second constraint of short SLR time still limits the maximum temperature of operation to ϳ10 K. It is noteworthy that a radio-frequency amplifier, based upon nuclear polarization at room temperature was suggested in the past. 12 However, this approach is restricted to low radio frequency ͑Ͻ50 MHz and magnetic fields of ϳ10-20 kG͒, where existing solid-state electronics provides better noise performances. The population difference ⌬N between the spin levels is directly related to the macroscopic magnetic permeability of the active material, ϭ1ϩ4, where the volume magnetic susceptibility, ϭЈϪiЉ, is expressed by 13 where ␥ is the electron gyromagnetic ratio (1.76 ϫ10 7 G Ϫ1 s Ϫ1 ) and f Љ(Ϫ 0 ) is the normalized absorption/emission line shape function ͑usually a Lorentzian͒ 14 with a maximum at 0 . The real part of the volume magnetic susceptibility Ј, which is related to the signal phase, can be obtained by replacing the line shape function f Љ(Ϫ 0 ) with the function f Ј(Ϫ 0 ). 13 We present here an approach to change via photoexcitation, by controlling ⌬N ͓Eq. ͑1͔͒. This effect is also known as electron spin polarization ͑ESP͒, generated in photoexcited chemical systems. 15 By optimizing the parameters affecting the ESP value, we have achieved changes in , which are large enough to produce significant changes in the microwave power reflected from the cavity loaded with the active material. Thus, by controlling , two necessary conditions for possible microwave devices can be materialized, namely: ͑a͒ achieving maser action, i.e., amplification of a microwave signal with very low noise; and ͑b͒ controlling the phase of a microwave signal ͑low-loss phase shifter͒. The latter condition was accomplished and can be implemented on a prototype device for protecting sensitive receivers from strong pulses ͑see below͒. In order to generate high ESP, we have utilized a unique process based on the interaction of photoexcited triplets with stable radicals in solution. 16, 17 This interaction is powered by the spin exchange and the triplet's zero-field splitting ͑ZFS͒
doi:10.1063/1.1401790 fatcat:rrhjvnptjfhe3giee4uk62gae4