Derek A. Bas

Coherent control of injection currents in high-quality films of Bi2Se3

Films of the topological insulator Bi2Se3 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.

Identification of photocurrents in topological insulators

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 oneand two-color sources of photocurrents are examined in epitaxial, thin films of Bi2Se3. We demonstrate that optical excitation and terahertz detection simultaneously captures oneand 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.Optical injection and detection of charge currents is an alternative to conventional transport and photoemission measurements, avoiding the necessity of invasive contact that may disturb the system being examined. This is a particular concern for analyzing the surface states of topological insulators. In this work one- and two-color sources of photocurrents are isolated and examined in epitaxial thin films of Bi2Se3. We demonstrate that optical excitation and terahertz detection simultaneously captures one- and two-color photocurrent contributions, which has not been required for other material systems. A method is devised to extract the two components, and in doing so each can be related to surface or bulk excitations through symmetry. The separation of such photocurrents in topological insulators opens a new avenue for studying these materials by all-optical methods.

Terahertz time-domain spectroscopy of magnons in antiferromagnetic MnF2(Conference Presentation)

Antiferromagnets are an important class of ordered spin systems, common in spintronic applications and providing a testbed for studying magnetism. Recently, the injection of magnons – coherent spin waves – has been explored by broadband terahertz pulses in antoferromagnets, such as MnO. Here, terahertz time-domain spectroscopy is used to detect magnon resonances in MnF2, which is a model antiferromagnet with uniaxial anisotropy and a Néel temperature of 67 K. Temperature dependence of a one-magnon resonances is examined from 5 K to 70 K. The center frequency of the one-magnon is recorded below the Néel temperature and fit to a Brillouin function. It is found that the degree of correlation between neighboring spins is j = 1.1. Namely, a weak correlation and appropriately modeled by mean-field theory befitting this simple system. From low temperature to room temperature, a two-magnon resonance is observed to broaden and strengthen as the temperature increases. Two-magnon modes arise due to zone-edge magnons being stimulated with -k and +k momenta and do not require magnetic ordering. Over this same temperature range, THz transients are used to monitor the time-of flight through the crystal, the refractive index, the internal energy and the heat capacity. Overall these quantities decrease with decreasing temperature, with behavior that falls into three regimes: a thermal dominated region above the Néel temperature, a magnetic regime below the Néel temperature; and a hyperfine interaction region at temperatures below 6 K. The latter is the first direct observation of the hyperfine interaction using terahertz time-domain spectroscopy.

Quantum interference control of photocurrents in topological insulator films

Charge currents in thin films of Bi2Se3 are controlled using a two-color scheme that exploits quantum interference of single- and two-photon absorption pathways. Injection and shift currents are observed and linked to surface state excitations.