F.K. Kneubühl

Nanometer thin-film Ni-NiO-Ni diodes for detection and mixing of 30 THz radiation

Abstract We report on the realization and the experimental study of thin-film Ni–NiO–Ni diodes with integrated infrared antennas. These diodes are applied as detectors and mixers of 28-THz CO 2 -laser radiation with difference frequencies up to 176 GHz. They constitute a mechanically stable alternative to the point-contact MOM diodes used today in heterodyne detection of such high frequencies. Thus, they represent the extension of present millimeter-wave and microwave thin-film and antenna techniques to the infrared. Our thin-film Ni–NiO–Ni diodes are fabricated on SiO 2 /Si substrates with the help of electron-beam lithography at the IBM Research Laboratory (Ruschlikon, Switzerland). We have reduced the contact area to 110 nm×110 nm in order to achieve a fast response of the device. This contact area is in the order of those of point-contact diodes and represents the smallest ever reported for thin-film MOM diodes. The thin NiO layer with a thickness of about 35 A is deposited by sputtering. Our thin-film diodes are integrated with planar dipole, bow-tie and spiral antennas that couples the incident field to the contact. The second derivative I″ ( V ) of the nonlinear I ( V ) characteristics at the bias voltage applied to the diode is measured at a frequency of 10 kHz. It determines the detection and second-order mixing performed with the diode for frequencies from dc to at least 30 THz. The I″ ( V ) characteristics exhibit for low bias voltage V bias a linear dependence, which is followed by a saturation and a maximum for high V bias . The zero-bias resistance of the diode is in the order of 100 Ω. It is not strictly inversely proportional to the contact area of the diode. The first application of our thin-film diodes was the detection of cw CO 2 -laser radiation. The measured dc signal generated by the diode when illuminated with 10.6- μ m radiation includes a polarization-independent contribution, caused by thermal effects. This contribution is independent of the contact area and of the type of integrated antenna. The polarization-dependent contribution of the signal originates in the rectification of the antenna currents in the diode by nonlinear tunneling through the thin NiO layer. It follows a cosine-squared dependence on the angle of orientation of the linear polarization, as expected from antenna theory. For the linearly polarized dipole and bow-tie antennas, the maximum detection signals are therefore measured for the polarization parallel to the antenna axis. Bow-tie antennas with a half length of 2.3 μ m generate the highest detection signals. The full length of these antennas corresponds to 3/2 of the wavelength of the incident 10.6- μ m radiation in the supporting Si substrate. The relevance of the substrate wavelength confirms that our antennas are more sensitive to the radiation incident from the substrate side. The time of response of our thin-film diode is not limited by the speed of the electron-tunneling effect, but by the RC time constant of the diode circuitry. Thus, the overall best performances are attained by the diodes with the smallest contact areas and corresponding capacitances. The study of the polarization response of our integrated asymmetric spiral antennas revealed the contribution of an unbalanced mode propagating on the antenna arms beside the fundamental balanced mode. The imbalance is caused by the reactive impedance of the diode and by the asymmetry of the antenna arms in the feed region. In addition, the response of the diode is influenced by reflection of the antenna currents near the end of the spiral arms. The resulting polarization of our spiral antenna is therefore not the expected circular polarization, yet an elliptical polarization with an axial ratio in the order of 0.12. Furthermore, we have demonstrated the presence of two distinct additive thermal effects besides the fast antenna-induced contribution by the measurement of the response of our thin-film diodes to 35 ps optical-free-induction decay (OFID) CO 2 -laser pulses. The measured characteristic times of these two relatively slow relaxations are τ 1 ≈100 ns and τ 2 ≈15 ns. These exponential relaxations observed are explained by thermal diffusion in the SiO 2 and in the Ni layers of our structures. These time constants show that thermal effects influence mixing processes at low difference frequencies. For the first time, the operation of thin-film diodes as mixers of 28-THz radiation was demonstrated. Difference frequencies up to 176 GHz have been measured when the diode was irradiated by two CO 2 -laser beams and microwaves generated by a Gunn oscillator working at 58.8 GHz. These difference frequencies were generated in mixing processes from the second to the fifth order. These experiments were performed with thin-film Ni–NiO–Ni diodes with the minimum contact area of 0.012 μ m 2 and integrated resonant bow-tie antennas. The transmission of the high-frequency signals to the spectrum analyzer was accomplished using integrated rhodium waveguides and flip-chip connections. The diode and the antenna were irradiated through the substrate, taking advantage of the better sensitivity of the antenna to radiation incident from the substrate side. The dependence on the linear polarization of the mixing signal matches almost perfectly the ideal cosine-squared dependence predicted by antenna theory for bow-tie antennas. A ratio of the mixing signals for the polarization parallel to the axis vs. the cross-polarization of over 50 was attained. The signal-to-noise ratios of our mixing signals demonstrate the potential of our type of diodes to respond to even higher carrier and difference frequencies. Also higher-order mixing can be achieved with our thin-film diodes.

Integrated nanostrip dipole antennas for coherent 30 THz infrared radiation

We report on the experimental study of infrared nanostrip dipole antennas which are connected to thin-film nanometer Ni-NiO-Ni diodes. The integrated Ni-NiO-Ni diodes are used to detect 30 THz (≈10 µm) CO2-laser radiation.The diodes are deposited on 385 µm silicon substrates which are covered with a layer of 1.6 µm SiO2 on both sides. We have found that in low-power applications 1.6 µm of SiO2 yields excellent quarter-wave matching layers for wavelengths centered at ⋋0 = 10.8 µm. By this method 79% of the incident CO2-laser radiation is transmitted into the Si substrate compared to 48% without SiO2 layer. The use of SiO2 quarter-wave matching layers considerably improves the efficiency of infrared nanostrip dipole antennas. This has been confirmed by the study of the laser-induced response of the Ni-NiO-Ni diode detectors as a function of the lengthL of the dipole antenna. Thus, we have observed that the laser-induced response strongly increases for shorter antennas and exhibits a distinct maximum atL=2.8 ± 0.3 µm. For the first time, we have investigated the 30 THz radiation patterns of nanostrip dipole antennas of different lengths. On this occasion, we have observed that the radiation pattern changes when the lengthL of the dipole antenna is varied. This observation indicates that antenna currents propagate on the nanostrip dipole antenna.

Polarization response of asymmetric-spiral infrared antennas

We present measurements on the polarization response of Ni-NiO-Ni diodes coupled to asymmetric spiral antennas. Our data are for the wavelength dependence of the orientation of the major axis of the polarization ellipse over the wavelength range 10.2-10.7 mum. The data are well fit by a two-wire antenna model. We find that the modes excited on the antenna are a combination of the balanced and unbalanced modes of a two-wire lossy transmission line.

Mixing of 30 THz laser radiation with nanometer thin-film Ni-NiO-Ni diodes and integrated bow-tie antennas

Mixing experiments with 30 THz CO2-laser radiation as well as the detection of 35 ps 30 THz pulses of an optical-free-induction-decay CO2-laser system have been performed with the first nanometer thin-film Ni-NiO-Ni diodes with a minimum contact area of 0.012 µm2. Difference frequencies up to 85 MHz were detected by mixing two different CO2-laser beams coupled to the diode with an integrated bow-tie antenna. The dependence of the beat signal on bias voltage, laser power and polarization of the infrared laser radiation was determined.

Spatial impulse response of lithographic infrared antennas

We present measurements of the spatial response of infrared dipole and bow-tie lithographic antennas. Focused 10.6-microm radiation was scanned in two dimensions across the receiving area of each antenna. Deconvolution of the beam profile allowed the spatial response to be measured. The in-plane width of the antennas spatial response extends approximately one dielectric wavelength beyond the metallic structure. Determination of an antennas spatial response is important for several reasons. The power collected by the antenna can be calculated, if the collection area and the input irradiance (watts per square centimeter) are known. The actual power collected by the antenna is required for computation of responsivity and noise-equivalent power. In addition, the spatial response provides insight into the current-wave modes that propagate on an antenna and the nature of the fringe fields that exist in the adjacent dielectric.

Tunable polarization response of a planar asymmetric-spiral infrared antenna

We present measurements at 10.6 microm that demonstrate electronic tuning of the polarization response of asymmetric-spiral infrared antennas connected to Ni-NiO-Ni diodes. Continuous variation of the bias voltage applied to the diode results in a rotation of the principal axis of the polarization ellipse of the spiral antenna. A 90 degrees tuning range is measured for a bias voltage that varies from -160 to +160 mV .This effect is caused by a small asymmetry of the deposited diode contact or by a variation of the detector capacitance with the applied bias voltage.

Far-infrared, Waveguide and Distributed-feedback Gas Lasers

A survey is given of the history and the development of far-infrared and waveguide gas lasers. These lasers were the first to bridge the power gap between the thermal infrared and the microwave region, giving new impetus to spectrometry of gases, condensed matter and astronomical objects, as well as to plasma diagnostics. The basic types of far-infrared gas lasers are reviewed: electrically and transversely excited lasers, optically pumped lasers, waveguide lasers, distributed-feedback and helical-feedback lasers.

Photoresponse of the high-temperature superconductor YBa2Cu3O7−δ to ultrashort 10 μm CO2laser pulses

Abstract We have measured the photoresponse of the high-temperature superconductor YBa2Cu3O7−δ(YBCO) to pulsed infrared radiation of 10 μm wavelength and analysed its mechanisms. For this purpose, c-axis oriented YBCO films with a thickness between 130 and 280 nm were deposited on single crystal MgO (100) substrates and patterned in a meander structure. The aim of the meander structure was to increase the responsivity at temperatures below the transition temperature. These films were irradiated with 10 μm 35 ps pulses generated by an optical free induction decay (OFID) 10 μm CO2 laser system developed in our laboratory. We have observed that the form of the transients depends on the temperature. At temperatures below the transition temperature, the photoresponse consists of a 700 ps FWHM peak followed by a slow negative transient. This is the first time that such short transients were observed with 10 μm irradiation. At temperatures above the transition temperature, the transients exhibit a decay in the nanosecond range. We have developed a bolometric model which reproduces well the measured dependence of the peak amplitude of the slow transients on the temperature. We have also investigated the dependence of the fast transients on the bias current and temperature. The observed linear dependence of the peak amplitude of the fast transients on the bias current and the negative transient following the main peak exclude flux-flow and nonequilibrium effects as photoresponse mechanisms, whilst these characteristics agree well with a kinetic-inductance model. The measured dependence of the peak amplitude on the temperature permits one to distinguish between several models for the superfluid fraction ƒsc. Thus, we have established that the models with superfluid fractions ƒsc= 1 − (T/Tc)2 and ƒsc given by the BCS theory are compatible with our results, whereas the model with ƒsc = 1 − (T/Tc)4 corresponding to the Gorter-Casimir two-fluid model does not match our results. The analysis of the amplitude and temporal variation of the negative transient following the fast transient yields a London penetration depth λL(0 K) ≈ 220 nm in agreement with those determined by other techniques for superconducting films with similar resistivities and critical temperatures.

Mixing of 28 THz (10.7 μm) CO2-laser radiation by nanometer thin-film NiNiONi diodes with difference frequencies up to 176 GHz

Abstract Difference frequencies up to 176 GHz between CO 2 -laser transitions at 28 THz (10.7 μm) are generated by thin-film nanometer-scale NiNiONi diodes (MOM, MIM) with integrated bow-tie antennas and rhodium waveguides. A signal-to-noise (S/N) ratio of 47 dB was measured for a 58.7 GHz difference frequency and a 100 kHz bandwidth, while a S/N ratio of 14 dB was observed for a 176.2 GHz difference frequency and a 300 kHz bandwidth. The frequencies reported are considerably higher than those reported previously for thin-film diodes. The comparison of the mixing signals for the antenna parallel and perpendicular to the E-polarization of the infrared radiation yields a ratio of over 34 dB. These results imply the extension of millimeter-wave techniques to the infrared.

New phenomena in pulsed FIR gas lasers

Abstract New phenomena occur when pulsed far-infrared (FIR) gas lasers are pumped with the pulses of 10 μm-hybrid-CO2 lasers truncated within 10 ps as well as when truncating directly the FIR laser pulses with the first FIR plasma shutter developed in our laboratory. These new phenomena include, e.g. new interrelations between superradiance, swept-gain superradiance and Raman emission, anticorrelated fluctuations of pump radiation vs FIR emissions, first OFID of 10 μm-CO2-laser pulses with FIR emitting molecular gases.