Bikash Das-Mohapatra

We report a technique for generating narrowband, high-frequency electrical signals using spintronic terahertz emitters (STEs) integrated onto coplanar waveguides (CPWs). Conventional STE operation typically yields ultrashort, broadband pulses spanning tens of terahertz. Our method employs multiple STEs positioned at precisely defined intervals along the CPW to produce a burst like signal, where the inter-emitter spacing determines the time delay between pulses, thus the fundamental burst frequency, which theoretically can be tuned across the STE bandwidth. Meander-shaped CPW geometries are used to enable compact integration and uniform optical excitation. First test devices demonstrate electrical signals up to 30 GHz with a bandwidth of 4.7 GHz corresponding to a quality factor of 6.38.

We demonstrate a proof-of-principle for a device in which we can realize binary encoding of the amplitude of an ultrafast signal similar to a multiplying digital-to-analog converter. The device was developed using multiple on-chip spintronic THz emitters with different respective emission amplitudes and coercive fields, representing the different bits of the digital-to-analog converter. A resolution of up to four bits with an integral non-linearity of less than 2 least significant bits could be achieved. Although programming these demonstrators is a slow process, spin-transfer torque switching can easily move the switching frequency to the GHz regime.

Transmission and processing of information at the nanoscale using spin waves and their quanta—magnons—offer numerous advantages and opportunities that make it a promising next-generation technology for integrated electronics. The main challenges that still need to be addressed to ensure high competitiveness of magnonic devices include finding ways to efficiently amplify spin waves in nanostructures and developing nanocircuits that can operate without the need for an external bias magnetic field. Here, we demonstrate how these two challenges can be solved using nanowaveguides fabricated from a low-loss magnetic insulator. We show that using local parametric pumping with a power of only a few milliwatts, one can achieve coherent amplification of spin-wave pulses by more than two orders of magnitude at zero bias magnetic field. Our results provide a simple solution to problems that have long prevented the implementation of efficient integrated magnonic circuits.

We experimentally study the nonlinear generation of the second harmonic by backward volume spin waves propagating in microscopic magnonic waveguides fabricated from a low-loss magnetic insulator with a thickness of several tens of nanometers. We show that highly efficient resonant second harmonic generation is possible in the inter-mode regime at microwave powers of the order of 10−4 W. In contrast to previously observed second harmonic generation processes, the generation by backward waves is characterized by the nonlinearly generated waves propagating opposite to the initial waves and can be realized at zero bias magnetic field.

We show that to correctly describe the ultrafast currents in spintronic terahertz emitters it is necessary to take charge equilibration into account. It has been extensively described that in a spintronic terahertz emitter a femtosecond laser pulse can induce a perpendicular spin current that is then converted by the inverse spin Hall effect into an ultrafast lateral charge-current pulse, and the underlying physics have been thoroughly analyzed. We show that subsequent to spin-to-charge conversion, this lateral charge current leads to ultrafast charging phenomena at the edge of the illuminated area. The following charge relaxation leads to a current backflow with a delay and a time constant that depend mainly on the conductivity and the dielectric properties of the emitter. On the one hand, only this delayed charge equilibration allows the detection of the primary current pulse via terahertz emission because an instantaneous backflow would cancel any far-field emission. On the other hand, especially for longer light pulses, the backflow can significantly change the emitted spectrum compared with the initial spin-current pulse by suppressing low-frequency components. For the analysis of spin physics based on the charge-current profile, it is important to understand that the timing of the spin current cannot be inferred from the charge current unless the contribution from the initially induced charge current and the backflow can be deconvoluted.