J.-F. Lampin

Phase-locking of a 2.5 THz quantum cascade laser to a frequency comb using a GaAs photomixer

We report the heterodyne detection and phase locking of a 2.5 THz quantum cascade laser (QCL) using a terahertz frequency comb generated in a GaAs photomixer using a femtosecond fiber laser. With 10 mW emitted by the QCL, the phase-locked signal at the intermediate frequency yields 80 dB of signal-to-noise ratio in a bandwidth of 1 Hz.

Microwave modulation of terahertz quantum cascade lasers: a transmission-line approach

We report on microwave impedance measurements of metal-metal ridge-waveguide terahertz quantum cascade lasers. Experimental data, recorded at 4 K in the 100 MHz–55 GHz range, are well reproduced by distributed-parameter transmission-line simulations, showing that the modulation cutoff is limited by the propagation losses that increase for higher microwave frequencies, yielding a 3 dB modulation bandwidth of ∼70 GHz for a 1 mm-long ridge. By using a shunt-stub matching we demonstrate amplitude modulation of a 2.3 THz QCL up to 24 GHz.

Wide-band continuous-wave terahertz source with a vertically integrated photomixer

A transverse electromagnetic horn antenna is monolithically integrated with a low temperature grown GaAs vertical photodetector on a silicon substrate forming a vertically integrated photomixer. Continuous-wave terahertz radiation is generated at frequencies up to 3.5 THz with a power level reaching 20 nW around 3 THz. Microwave and material concepts allow both qualitative and quantitative explanations of the experimental results. The thin film microstrip line topology has been adapted for active devices by an Au–Au thermocompression layer transfer technique and seems to be a promising generic tool for a new generation of efficient terahertz devices.

THz long range plasmonic waveguide in membrane topology

In this communication, we present THz planar strip plasmonic (THz-PSP) structures based on planar Goubau lines (PGL). In order to improve different propagation characteristics, a membrane approach is studied because it is more convenient for low dispersion propagation and for a long range plasmonic behaviour. Resonating structures such as split ring resonators (SRR) along the plasmonic waveguide can be added to this structure in order to maximize the coupling between them.

QCL with terahertz TEM-Horn Antennas

Quantum Cascade Lasers are one the most promising approach for the generation of THz waves and particularly in a double plasmon waveguide topology. Stripline resonators with a quasi-TEM mode are indeed interesting for THz-QCLs thanks to a high confinement factor combined with low losses. However they present two major drawbacks: the first is a poor outcoupling of the radiation, and the second is a highly divergent beam. In this communication we propose an original approach to solve simultaneously these two identified problems. It consists on the use of a metallic triangle connected at one end of the top metallic strip in order to realize with the ground plane a TEM- Horn Antenna (TEM-HA). We have first validated this approach with electromagnetic simulations based on a finite integration technique (FIT). The use of the TEM-HA improves the far-field radiation pattern, which presents a higher directivity in the direction of the antenna. We have also validated experimentally this concept on a QCL designed to emit at f=2 THz (lambda = 150 mum). The use of the antenna increases the collected power by a factor 2-to-3 compared to the case without antenna. We expect an even higher outcoupling by optimising the connection of the antenna to the QCL (electrical connection and position). Finally, as we believe that the realisation of a full monolithic device is the best approach to integrate the TEM-HA on the QCL, we will discuss on the technological issues raised by such a device.

Highly efficient terahertz detection by optical mixing in a GaAs photoconductor

It is shown from accurate on-wafer measurement that a low-temperature-grown GaAs photoconductor using a metallic mirror Fabry-Perot cavity can serve as highly efficient optoelectronic heterodyne mixer in the terahertz frequency range. Conversion losses of 22 dB at 100 GHz and ∼27 dB at 300 GHz were measured, which is an improvement by a factor of about 40 dB as compared with the previous values obtained with photoconductors. Experimental results are interpreted satisfactorily by means of a simple electrical model of the optoelectronic mixing process.

Injection-locking of terahertz quantum cascade lasers up to 35GHz via RF amplitude modulation

We demonstrate that the cavity resonance frequency — the round-trip frequency — of terahertz quantum cascade lasers can be injection-locked by direct modulation of the bias current using an RF source. Metal-metal and single-plasmon waveguide devices with roundtrip frequencies up to 35GHz have been studied, and show locking ranges above 200MHz. Inside this locking range the laser round-trip frequency is phase-locked, with a phase noise determined by the RF-synthesizer. We find a square-root dependence of the locking range with RF-power in agreement with classical injection-locking theory. These results are discussed in the context of mode-locking operation.

Integrated Horn Antenna for THz Photomixing in LTG-GaAs

The investigation of THz generation using low-temperature-grown GaAs (LTG-GaAs) photoconductors integrated with horn antennas is reported. Results obtained using a photomixing experiment are presented. A maximum generated power of 1 muW at 780 GHz was achieved for an optical power of 2times80 mW and a photocurrent of 1 mA.

THz photomixers based on nitrogen-ion-implanted GaAs

Ultrafast photoconductors using GaAs implanted by low energy N+ ions (<55 keV) are fabricated and characterized up to 320 GHz by means of a photomixing experiment. Around 90 μW of output power was obtained at 290 GHz with a 2-μm-diameter photoconductor based on a 160-nm-thick GaAs layer implanted with a main dose of 1.1 × 1012 cm−2 and a subsequent annealing at 600 °C. These performances are similar to those obtained with LT-GaAs photoconductors having same electrodes geometry. Besides, it is shown that the frequency dependence of the output power is not correlated to the carrier lifetime as measured by time-resolved photoreflectance.

Sub-terahertz resistive mixing in an AlGaN/GaN FET

An AlGaN/GaN based field effect transistor (FET) has been designed, fabricated, and used as a resistive mixer for heterodyne detection in the 140–220 GHz frequency range. A double VNA heterodyne measurement setup has been used in an on-wafer configuration to accurately quantify the incident radiation absorbed by the device. The appropriate selection of optimum biasing conditions for minimum conversion losses is investigated. 47.3 dB conversion losses are obtained at 150 GHz and the device linearity is confirmed.