Julien Madéo

Phase seeding of a terahertz quantum cascade laser

The amplification of spontaneous emission is used to initiate laser action. As the phase of spontaneous emission is random, the phase of the coherent laser emission (the carrier phase) will also be random each time laser action begins. This prevents phase-resolved detection of the laser field. Here, we demonstrate how the carrier phase can be fixed in a semiconductor laser: a quantum cascade laser (QCL). This is performed by injection seeding a QCL with coherent terahertz pulses, which forces laser action to start on a fixed phase. This permits the emitted laser field to be synchronously sampled with a femtosecond laser beam, and measured in the time domain. We observe the phase-resolved buildup of the laser field, which can give insights into the laser dynamics. In addition, as the electric field oscillations are directly measured in the time domain, QCLs can now be used as sources for time-domain spectroscopy.

Terahertz time domain spectroscopy of phonon-depopulation based quantum cascade lasers

A 3.1THz phonon depopulation-based quantum-cascade-laser is investigated using terahertz time domain spectroscopy. A gain of 25cm-1 and absorption features due to the lower laser level being populated from a parasitic electronic channel are highlighted.

Mode-locking of a terahertz laser by direct phase synchronization.

A novel scheme to achieve mode-locking of a multimode laser is demonstrated. Traditional methods to produce ultrashort laser pulses are based on modulating the cavity gain or losses at the cavity roundtrip frequency, favoring the pulsed emission. Here, we rather directly act on the phases of the modes, resulting in constructive interference for the appropriated phase relationship. This was performed on a terahertz quantum cascade laser by multimode injection seeding with an external terahertz pulse, resulting in phase mode-locked terahertz laser pulses of 9 ps duration, characterized unambiguously in the time domain.

Integrated terahertz pulse generation and amplification in quantum cascade lasers

Terahertz pulse generation is demonstrated by a resonant femtosecond interband excitation of the miniband of a quantum-cascade-laser. The laser gain is subsequently used to amplify the terahertz pulse generated as it propagates through the cavity.

Patch antenna terahertz photodetectors

We report on the implementation of 5 THz quantum well photodetector exploiting a patch antenna cavity array. The benefit of our plasmonic architecture on the detector performance is assessed by comparing it with detectors made using the same quantum well absorbing region, but processed into a standard 45° polished facet mesa. Our results demonstrate a clear improvement in responsivity, polarization insensitivity, and background limited performance. Peak detectivities in excess of 5 × 1012 cmHz1/2/W have been obtained, a value comparable with that of the best cryogenic cooled bolometers.

Dual wavelength emission from a terahertz quantum cascade laser

We describe a heterogeneous terahertz (THz) quantum cascade laser that is composed of two different active region designs. This device emits simultaneously at around 2.5 and 2.9 THz with certain frequency tunability by applied current. We also investigate the spectral gain in the structure by THz time-domain spectroscopy and correlate the gain spectral bandwidth with the alignment and wavelength emission behavior of the two stack device.

Gain enhancement in a terahertz quantum cascade laser with parylene antireflection coatings

We study the effect of parylene antireflection coatings on the gain of a 2.8 THz quantum cascade laser using terahertz time-domain spectroscopy. With antireflection coatings the threshold current increases as the mirror losses are increased, and the gain clamps at 16 cm−1, compared to 10 cm−1 for an uncoated device. These values are consistent with a drop in reflectivity from 0.320 to 0.053 as a consequence of the coating deposition. Further improvements could reveal the bare cavity gain and permit the quantum cascade laser to be used as an efficient terahertz amplifier.

Measuring the sampling coherence of a terahertz quantum cascade laser

The emission of a quantum cascade laser can be synchronized to the repetition rate of a femtosecond laser through the use of coherent injection seeding. This synchronization defines a sampling coherence between the terahertz laser emission and the femtosecond laser which enables coherent field detection. In this letter the sampling coherence is measured in the time-domain through the use of coherent and incoherent detection. For large seed amplitudes the emission is synchronized, while for small seed amplitudes the emission is non-synchronized. For intermediate seed amplitudes the emission exhibits a partial sampling coherence that is time-dependent.

Broad gain in a bound-to-continuum quantum cascade laser with heterogeneous active region

We demonstrate the operation of heterogeneous terahertz quantum cascade lasers with broadened gain by optimising the sub-stacks to align at the same field. In single plasmon waveguides, we find two-colour operation for nearly the entire dynamic range of the lasers with similar performance to homogeneous lasers. Time domain spectroscopy measurements confirm that a flat gain spectrum is present and the sub-stacks align at the same time. When incorporated into metal-metal waveguides, we find that performance is consistent with the constituent sub-stacks and there is broadband operation over 380 GHz.

Antenna-coupled microcavities for terahertz emission

We have investigated the capacitive coupling between dipolar antennas and metal-dielectric-metal wire microcavities with strong sub-wavelength confinement in the terahertz region. The coupling appears in reflectivity measurements performed on arrays of antenna-coupled elements, which display asymmetric Fano lineshapes. The experimental data are compared to a temporal coupled-mode theory and finite elements electromagnetic simulations. We show that the Fano interferences correspond to coupling between a subradiant mode (microcavity) and a superradiant mode (antennas). This phenomenon allows one to enhance and control the radiative coupling of the strongly confined mode with the vacuum. These concepts are very useful for terahertz optoelectronic devices based on deep-sub-wavelength active regions.