Nonreciprocity, the defining characteristic of isolators, circulators, and a wealth of other applications in radio/microwave communications technologies, is generally difficult to achieve as most physical systems incorporate symmetries that prevent the effect. In particular, acoustic waves are an important medium for information transport, but they are inherently symmetric in time. In this work, we report giant nonreciprocity in the transmission of surface acoustic waves (SAWs) on lithium niobate substrate coated with ferromagnet/insulator/ferromagnet (FeGaB/Al2O3/FeGaB) multilayer structure. We exploit this structure with a unique asymmetric band diagram and expand on magnetoelastic coupling theory to show how the magnetic bands couple with acoustic waves only in a single direction. We measure 48.4-dB (power ratio of 1:69,200) isolation that outperforms current state-of-the-art microwave isolator devices in a previously unidentified acoustic wave system that facilitates unprecedented size, weight, and power reduction. In addition, these results offer a promising platform to study nonreciprocal SAW devices.

Optical injection and detection of charge currents can complement conventional transport and photoemission measurements without the necessity of invasive contact that may disturb the system being examined. This is a particular concern for the surface states of a topological insulator. In this work one-and two-color sources of photocurrents are examined in epitaxial, thin films of Bi2Se3. We demonstrate that optical excitation and terahertz detection simultaneously captures one-and twocolor photocurrent contributions, as previously not required in other material systems. A method is devised to isolate the two components, and in doing so each can be related to surface or bulk excitations through symmetry. This strategy allows surface states to be examined in a model system, where they have independently been verified with angle-resolved photoemission spectroscopy.

Films of the topological insulator Bi 2 Se 3 are grown by molecular beam epitaxy with in-situ reflection high-energy electron diffraction. The films are shown to be high-quality by X-ray reflectivity and diffraction and atomic-force microscopy. Quantum interference control of photocurrents is observed by excitation with harmonically related pulses and detected by terahertz radiation. The injection current obeys the expected excitation irradiance dependence, showing linear dependence on the fundamental pulse irradiance and square-root irradiance dependence of the frequency-doubled optical pulses. The injection current also follows a sinusoidal relative-phase dependence between the two excitation pulses.These results confirm the third-order nonlinear optical origins of the coherently controlled injection current. Experiments are compared to a tight-binding band structure to illustrate the possible optical transitions that occur in creating the injection current.2 Bi 2 Se 3 is a well-known thermoelectric material 1 and currently of great interest in condensed matter physics because the surface states exhibit massless Dirac dispersions in the gaps between the bulk valence and conduction bands. 2 The surface states have an electron spin structure defined by strong spin-orbit coupling which locks the spin vector perpendicular to the momentum vector. 3,4 Fundamental properties of Bi 2 Se 3 have been widely investigated because of the potential opportunities for developing spintronic and optically controlled devices.A great deal of attention has been paid to the light-matter interactions because the surface states of Bi 2 Se 3 are obscured in electrical measurements, due to inherent n-doping in the as-grown material. This means that the Fermi level is in the bulk conduction band above the 0.3-eV band gap. Despite this fact, optical measurements have been widely explored in the near infrared, revealing anomalous absorption coefficients with film thickness, 5 strong coupling between electrons and phonons 6,7 and a second set of massless surface states at about 1.7 eV above the commonly studied Dirac point below the Fermi level. 8 These developments, among others, 9 indicate that near-infrared optical interactions provide mechanisms that can be exploited for photonic devices and also access surface-surface state excitations. In which case, coupling of optical radiation to inject and control photocurrents is then relevant for the performance of this material system and coupling to the all-important surface states.Photocurrents in Bi 2 Se 3 have been predicted 10 and measured 11 using circular polarization, through the circular photogalvanic effect. Experimental studies have been performed in exfoliated Bi 2 Se 3 patterned with contacts that are sensitive to both photocurrents and photo-thermal currents. Nonetheless, this study has demonstrated that photocurrents can indeed be controlled. A more discrete scheme has been proposed for coherent control of the surface states using two-color quantum interference. 12 Quant...

Doping a continuum of states is shown to preserve excited carrier lifetimes and mobility, increasing photocatalysis across the UV-visible spectrum.