Stimulated Smith-Purcell semiconductor THz sources

Impedance matching and heat management are important factors influencing performance of THz sources. In this work we analyze thermal and radiative properties of such devices based on mesa structures of a layered high-temperature superconductor Bi2Sr2CaCu2O8+δ. Two types of devices are considered, containing either a conventional large single crystal, or a whisker. We perform numerical simulations for various geometrical configurations and parameters and make a comparison with experimental data for the two types of devices. It is demonstrated that the structure and the geometry of both the superconductor and the electrodes are playing important roles. In crystal-based devices an overlap between the crystal and the electrode leads to appearance of a large parasitic capacitance, which shunts THz emission and prevents impedance matching with open space. The overlap is avoided in whisker-based devices. Furthermore, the whisker and the electrodes form a turnstile (crossed-dipole) antenna facilitating good impedance matching. This leads to more than an order of magnitude enhancement of the radiation power efficiency in whisker-based, compared to crystal-based devices. These results are in good agreement with presented experimental data.

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Plasmonics in semiconductors and in Ag submicron wires and graphene

Lithuanian Journal of Physics ◽

10.3952/physics.v55i4.3225 ◽

2016 ◽

Vol 55 (4) ◽

Author(s):

Hans Hartnagel ◽

Shihab Al-Daffaie ◽

Oktay Yilmazoglu

Keyword(s):

Continuous Wave ◽

Silver Nanowires ◽

Field Enhancement ◽

New Device ◽

Domain Structures ◽

New Concepts ◽

Metal Nanostructure ◽

Thz Sources ◽

Nitrogen Ion ◽

Optical Focusing

Some 50 years ago discussions of plasmonics in semiconductors lead to many new concepts such as travelling domain structures with applications leading even to logic systems. Now plasmonics of submicron wires of Ag and graphene bring new device concepts for the fabrication of compact THz sources and optical focusing of the beat signal into the active area. Here as an experimental example such new opportunities are involved with compact THz sources based on optical laser mixing. They include resonant plasmonic structures at THz and optical frequencies to locally enhance the electromagnetic fields at THz as well as optical frequencies by the combination of semiconductor-graphene plasmons respectively by the semiconductor-metal-nanostructure plasmons. Of particular interest is the usage of graphene, which is optically transmitting and which is either a semimetal or can be transformed into a semiconductor by reducing the width of its strips to about 30 nm, opening a band gap in the meV to tens of the meV range. A successful experimental structure for continuous-wave THz photomixing is fabricated using 1D and 2D nanocontacts either on low-temperature-grown (LTG) GaAs or on nitrogen ion-implanted (N+i) GaAs and graphene sheets. The overlaying 1D and 2D nanocontacts were formed by silver nanowires with a diameter of 60 or 120 nm. They can handle currents of >10 and >30 mA, respectively, without electromigration enabling reliably high photocurrents and field enhancement at THz frequencies by plasmonic effects. The nanomaterial structurization in connection with present-day plasmonic applications is now to be discussed in a similar manner as past opportunities with semiconductor plasmonics.

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