Donald G. McDonald

Lithographic Spiral Antennas at Short Wavelengths

We have extended the high efficiency of lithographic antennas to mid‐infrared wavelengths. Pattern measurements made at 9.5 μm wavelength on a 65°, self‐complementary, spiral antenna exhibit a ratio of response to orthogonal linear polarizations of 1.35 dB, a beamwidth of 85° (3 dB full width), a directivity of 8.2 dB, and surprisingly, a close resemblance to the theoretical pattern for a 65° spiral in free space. Direct detection measurements made with an ambient temperature blackbody source yield an antenna efficiency of 52±7%, when corrected for incomplete filling of the antenna beam by the source, at a mean effective wavelength of 19 μm.

Four‐Hundredth‐Order Harmonic Mixing of Microwave and Infrared Laser Radiation Using a Josephson Junction and a Maser

For mixing in a Josephson junction at infrared frequencies, we have shown that the available power from the junction increases as the intermediate frequency is increased. Following this result an infrared receiver has been developed incorporating a 9‐GHz maser preamplifier at the i.f. Using this system, the beat between the 401st harmonic of a high‐quality microwave source and a 3.8‐THz infrared laser has been observed. Also, for low‐order mixing at 3.8 THz, a comparison of beat signals from a Josephson junction and a room‐temperature mixer has been made.

Novel superconducting thermometer for bolometric applications

The temperature dependence of the magnetic penetration depth in a superconductor, in the kinetic inductance limit, is proposed as a basis for a sensitive thermometer. Considered as a bolometer, the noise equivalent power from the sum of the Johnson noise and the preamplifier noise can be reduced to about 7×10−20 W/(Hz)1/2, which is approximately four orders of magnitude below currently realized values.

Construction and performance of a high-temperature-superconductor composite bolometer

A high‐Tc superconducting bolometer has been constructed using a YBa2Cu3Ox thin‐film meander line 20 μm wide and 76 000 μm long, deposited on a SrTiO3 substrate. Radiation is absorbed by a thin film of Bi with well‐characterized absorption properties deposited on a Si substrate in contact with the SrTiO3. At 1.8 Hz the measured bolometer response to a 500‐K blackbody is 5.2 V/W (820 V/W extrapolated to dc), and the NEP is 5.7×10−8 W/(Hz)1/2 . The impact of apparent nonohmic behavior at the transition is discussed, as are ways of reducing the observed 1/f noise. The response time is 32 s and is dominated by the heat capacity of the SrTiO3 substrate.

Picosecond applications of Josephson junctions

The behavior of simple superconducting circuits in the picosecond regime is described in a comprehensive way, with primary emphasis being given to the step function and pulse responses of these circuits. Topics receiving detailed discussion include Josephson-junction modeling with both the microscopic and shunted-junction models. Limitations of the shunted-junction model are explored by comparing it with experimental results and with the microscopic model. An approximate evaluation is given of the important dynamical properties of junctions made with the dominant fabrication technology (Pb-alloy systems), as a function of tunneling barrier thickness. Rounding out the device aspects of the discussion, we describe in detail the properties of superconducting microstrip transmission lines, with an emphasis on their high-speed behavior. Turning to simple circuits we review experimental results on the measurement of picosecond regime transient signals. The concept of turn-on delay is analyzed anew, providing simplified and extended results. Details of concepts for pulse height and pulsewidth measurements are explored, leading to the conclusion that the time resolution of superconducting circuits is limited to approximately the period of one plasma oscillation. With present Pb-alloy fabrication technology this limit is 2 ps.

HARMONIC GENERATION AND SUBMILLIMETER WAVE MIXING WITH THE JOSEPHSON EFFECT

By observing constant‐voltage steps from Josephson junctions at voltages as high as 17 mV we deduce that junctions can generate harmonics up to frequencies as high as 8200 GHz. In consonance with this, submillimeter wave laser detection, harmonic generation, and mixing are demonstrated. These results suggest a model for the upper frequency limit of the Josephson effect.

Harmonic Mixing of Microwave and Far-Infrared Laser Radiation Using a Josephson Junction

Simultaneous irradiation of a Josephson junction with ≈ 10 GHz microwave power and 891‐GHz laser radiation produces a 60‐MHz beat between the fundamental laser frequency and harmonics of the klystron, ranging from the 84th to the 100th as the klystron is tuned. Observation of such high‐order harmonic mixing of klystron and laser signals is unprecedented.

Far-infrared kinetic-inductance detectors

Extremely sensitive far-infrared detectors suitable for both direct detection and heterodyne applications, based on mu m-sized thin films with thickness less than a superconducting penetration depth are possible. The penetration depth of such a film, and therefore its inductance, varies with temperature and with quasiparticle population (described by an effective temperature T*), resulting in both bolometric and nonequilibrium photoinductive responses. Incident radiation is coupled into the small-area kinetic inductor by a lithographic antenna, and the resulting inductance changes are amplified and converted to a voltage signal by an integrated microstrip DC SQUID. The device is sensitive because, unlike junction-based devices with large capacitive reactances, the kinetic inductor is naturally well matched to the antenna impedance at the far-IR frequency ( nu >2 Delta /h) and to the preamplifier (SQUID) impedance at microwave or video frequencies ( nu <<2 Delta /h). The best kinetic inductor materials are those with low electronic mean free path, large penetration depth, and high critical current density. Thus, common magnet alloys such as NbTi are the natural choice for liquid-He temperature operation. A detailed analysis predicts a (phonon-limited) NEP of 4*10/sup -17/ W/ square root Hz for a bolometer with an iridium kinetic inductor operated at 0.1 K. A heterodyne noise temperature of 2250 K (single-sideband) at 3 THz, with a 200-MHz bandwidth, is predicted for a Nb-Ti mixer operated at 4 K.

Partially coherent transmittance of dielectric lamellae

We derive an analytic formula for the transmittance of a dielectric amelia when the interference between successive internal reflections is only partially spatially coherent. This allows effects such as surface roughness and nonparallelism, which produce cumulative distortions in the phase front with each reflection, and which result in a loss of fringe contrast at high frequencies, to be accounted for quantitatively. The transmittance of a Si lamella, measured with a Fourier-transform interferometer over the range 20 to 1000 cm -1 agrees with our formula to within the accuracy of the data, which is dominated by systematic instrumental effects.

Josephson junctions at 45 times the energy‐gap frequency

Superconductive Nb–Nb point contacts have been studied with 9.5‐μm radiation from CO2 lasers. Two models are considered to explain the experiments: one is Werthamers Josephson junction model and the other is a thermally modulated Josephson junction. The evidence favors Werthamers model but is not conclusive.