Conventional 3D TIs are the van der Waals bound, binary compounds such as Bi 2 Te 3 , [10] Bi 2 Se 3 , [11,12] Sb 2 Te 3 , [13] as well as alloys of these elements. [14,15] For a quasi 1D nanowire, due to the inclusion of the Berryphase of a particle traversing the perimeter of the nanowire, the surface band structure is determined to be gapped. [16-18] By applying a magnetic flux parallel to the nanowire axis Φ = ±Φ 0 /2, where Φ 0 = h/e, this gap will be closed. The non-degenerate, topologically protected linear surface bands will re-emerge periodically with a period of a full flux quantum Φ = (n + 1/2) Φ 0 threading the wire. The observation of such periodic Aharonov-Bohm (AB) oscillations has previously been reported. [19-24] Making use of a selective area growth approach, nanowires of aforementioned materials can as well be deposited by molecular beam epitaxy (MBE). [25-28] These nanowires have a rather rectangular cross-section and are therefore referred to as nano ribbons. As a scalable bottom-up approach, these selectively grown nanoribbons are beneficial for desired Majorana surface architectures. [29] However, MBE grown 3D TI compounds usually Quasi-1D nanowires of topological insulators are candidate structures in superconductor hybrid architectures for Majorana fermion based quantum computation schemes. Here, selectively grown Bi 2 Te 3 topological insulator nanoribbons at cryogenic temperatures are investigated. The nanoribbons are defined in deep-etched Si 3 N 4 /SiO 2 nano-trenches on a silicon (111) substrate followed by a selective area growth process via molecular beam epitaxy. The selective area growth is beneficial to the device quality, as no subsequent fabrication needs to be performed to shape the nanoribbons. In the diffusive transport regime of these unintentionally n-doped Bi 2 Te 3 topological insulator nanoribbons, electron trajectories are identified by analyzing angle dependent universal conductance fluctuation spectra. When the sample is tilted from a perpendicular to a parallel magnetic field orientation, these high frequent conductance modulations merge with low frequent Aharonov-Bohm type oscillations originating from the topologically protected surface states along the nanoribbon perimeter. For 500 nm wide Hall bars low frequent Shubnikov-de Haas oscillations are identified in a perpendicular magnetic field orientation. These reveal a topological, high-mobility, 2D transport channel, partially decoupled from the bulk of the material.

We present a novel, free-standing low-temperature GaAs (LT-GaAs) photoconductive switch and demonstrate its femtosecond performance. A 1-microm-thick layer of a single-crystal LT-GaAs was patterned into 5-10-microm-wide and 15-30-microm-long bars, separated from their GaAs substrate and, subsequently, placed across gold coplanar transmission lines deposited on a Si substrate, forming a photoconductive switch. The switch was excited with 110-fs-wide optical pulses, and its photoresponse was measured with an electro-optic sampling system. Using 810-nm optical radiation, we recorded an electrical transient as short as 360 fs (1.25 THz, 3-dB bandwidth) and established that the photo-carrier lifetime in our LT-GaAs was 150 fs. Our free-standing devices exhibited quantum efficiency of the order of approximately 7%, and their photoresponse amplitude was a linear function of the applied voltage bias, as well as a linear function of the excitation power, below a well-defined saturation threshold.

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