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arXiv:0711.4221 · Applied Physics Letters

Integrating superconductive and optical circuits

F. Stella, M. Casalboni, M. Cirillo, Vittorio Merlo, C. Palazzesi, G. P. Pepe, P. Prosposito, M. Salvato

Published May 19, 2008• 7 citations
arXivHTMLPDF

Abstract

We have integrated on oxidized silicon wafers superconductive films and Josephson junctions along with sol-gel optical channel waveguides. The fabrication process is carried out in two steps that result to be solid and noninvasive. It is demonstrated that 660nm light, coupled from an optical fiber into the channel sol-gel waveguide, can be directed toward superconducting tunnel junctions whose current-voltage characteristics are affected by the presence of the radiation. The response of the junction biased at various currents as a function of the optical pumping is presented and discussed according to a nonequilibrium superconductivity model.

DOI
10.1063/1.2931700

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We have integrated on oxidized silicon wafers superconductive films and Josephson junctions along with sol-gel optical channel waveguides. The fabrication process is carried out in two steps that result to be solid and noninvasive. It is demonstrated that 660nm light, coupled from an optical fiber into the channel sol-gel waveguide, can be directed toward superconducting tunnel junctions whose current-voltage characteristics are affected by the presence of the radiation. The response of the junction biased at various currents as a function of the optical pumping is presented and discussed according to a nonequilibrium superconductivity model.
The physics of the interaction between coherent light and superconductivity is a very intriguing subject but the alignment of a laser beam with integrated superconducting systems (thin films, striplines or tunnel junctions) has represented so far a real bottleneck for the experimentalists. The integration of both junctions and waveguides on the same chip surely represents a possible path around this difficulty. In this paper we report on the combined integration of optical structures based on hybrid channel waveguides with superconducting Josephson junctions circuits. The optical films are grown on silicon substrates by a spin-coating process using the hybrid organic/inorganic sol–gel route. The channel waveguides are deposited on a buffer layer and realized by photolithographic and ion milling techniques. We find that the optical buffer is a good substrate for SIS Josephson junctions and circuitry based on the Nb/Al–AlO x / Nb trilayer processing. The junctions produced present good current–voltage characteristics with critical current density of the order of 1 kA cm −2 , high subgap resistance and good uniformity. Our results show that the quality of the Josephson junctions is well preserved even when they are grown on the sol–gel buffer layer.
Single flux quantum (SFQ) circuits and optical circuits are expected to be used in next-generation integrated circuits. This study investigates basic technologies for integrating optical circuits with SFQ circuits to exploit the advantages of both technologies. We calculated, designed and evaluated metal-clad waveguides, which can be fabricated by the ISTEC standard process (STD2). Light propagates along an 830-μm-long waveguide with a propagation loss of 16 dB/mm. We also designed and evaluated an optical-electrical (O-E) converter that uses a Josephson junction, which can be fabricated by the same process used to fabricate the SFQ circuits. When an optical signal with a peak power of 1.15 mW at 1.548 μm is input, the Josephson O-E converter generates SFQ pulses.
We have measured the response of Nb-aSi-Nb junctions to 604 GHz radiation from an optically pumped far-infrared (FIR) laser source. These rugged and thermally cyclable junctions situated at the midpoint of 180 μm long dipole antennas, were fabricated on oxidized silicon wafers using the Selective Niobium Anodization Process (SNAP). Josephson current densities were\sim 9,000A/cm2, the McCumber parameters βcwere ∼3.5, and the Josephson plasma frequencies\omega_{J}\sim2.5\times10^{12}. On a junction with a normal state resistance of\sim7\Omega, we observed three Josephson steps and one photon-assisted tunneling step. The widths of the Josephson steps were studied as a function of the laser power. An RSJ model computer simulation with a nonlinear quasiparticle conductance and an rf current bias (assumed because of the low junction resistance) is able to account reasonably well for the laser-power dependence of the critical current (zeroth step) and the second step. However, the temperature dependence is more complex, and a discrepancy still exists between the RSJ model calculation and the data for the hysteretic first step.
Josephson voltage standards utilize microwave-induced constant voltage steps occurring due to the ac Josephson effect. Existing standards can be considerably simplified and their accuracy improved by using a large number of series-connected Josephson tunnel junctions which are operated in the zero current step mode. For this purpose superconducting millimeter wave integrated circuits have been designed, fabricated, and tested. The circuits consist of a broadband taper between the rectangular waveguide and the planar structure, the Josephson junction series, a well-matched load, and dc pads. Circuits with various numbers of junctions have been fabricated by photo-lithographic techniques and tested at 4.2 K in liquid helium. The version with 1474 junctions produced voltages up to 1.2 V when operated at 90 GHz.
Abstract Devices combining superconductors with semiconductors offer a wide range of applications, particularly in the growing field of quantum information processing. This is due to their ability to take advantage of both the extensive knowledge gathered in the field of semiconductors and the unique quantum properties of superconductors. This results in novel device concepts, such as structures generating and detecting entangled photon pairs as well as novel optical gain and laser realizations. In this review, we discuss the fundamental concepts and the underlying physical phenomena of superconductor–semiconductor optoelectronics as well as practical device implementations.
Ultrafast phenomena, optoelectronics, and superconductivity are acknowledged fields of technological importance and a very large amount of research has been performed in these areas in recent years. In modern, highly-advanced digital electronic circuits, the primary function of high-performance, high-speed signal processing is given to superconducting single-flux-quantum (SFQ) logical devices, based on resistively-shunted Josephson tunnel junctions. The development of this branch of digital electronics requires ultrafast, and ultra sensitive optical-to-electrical transducers for fiber optic interconnects, which must be able to transform to the electrical domain the input information coded in the form of a train of ultrafast optical pulses and, subsequently, feed it into the SFQ processor.