Optical cavities are of central importance in numerous areas of physics, including precision measurement, cavity optomechanics and cavity quantum electrodynamics. The miniaturisation and scaling to large numbers of sites is of interest for many of these applications, in particular for quantum computation and simulation. Here we present the first scaled microcavity system which enables the creation of large numbers of highly uniform, tunable light-matter interfaces using ions, neutral atoms or solid-state qubits. The microcavities are created by means of silicon micro-fabrication, are coupled directly to optical fibres and can be independently tuned to the chosen frequency, paving the way for arbitrarily large networks of optical microcavities.

The gain recovery time of a heterogeneous active region terahertz quantum cascade laser is studied by terahertz-pump terahertz-probe spectroscopy. The investigated active region, which is based on a boundto-continuum optical transition with an optical phonon assisted extraction, exhibits a gain recovery time in the range of 34 -50 ps dependent on the operation condition of the laser. The recovery time gets shorter for stronger pumping of the laser while the recovery dynamics slows down with increasing operation temperature. These results indicate the important role of the intracavity light intensity for the fast gain recovery.Semiconductor terahertz lasers based on the quantum cascade design, so called THz QCLs, have become an established THz technology. 1 Besides the generation of spectrally bright THz light, todays THz QCL can be used as frequency comb source, 2 as transceiver at THz frequencies for sensing and communication, 3 or as a source of tailored coherent pulses generated on demand. 4,5 Although the physics of QCLs is well understood 6 and can be modeled to a satisfactory level, 7 their performance is still not under full control. In experimental studies, discrepancies between the theoretical and actual performance of THz QCLs were observed that were attributed to deviations of the fabricated devices from the design model. To address these issues and to identify their origin THz time-domain spectroscopy (THz-TDS) has been applied to access the internal processes in QCLs, 8,9 which allowed to study the spectral gain curve at all operation points of a QCL (hence even above the lasing threshold), 8,10-12 the gain clamping dynamics, 4 and the gain induced dispersion. [13][14][15] This flexible spectroscopic tool helped to identify individual gain degradation mechanisms, 16 and provided direct access to the gain recovery dynamics. 17,18 Fast gain dynamics, which is described by a short characteristic time -the gain recovery time (GRT) -is important for the high speed modulation of THz QCLs and for the formation and the sustainability of THz pulses in QCLs. Thereby the gain modulation and the pulse dispersion are key issues for the active/passive mode-locking 19,20 of lasers. All present studies of the gain dynamics have been performed on QCLs with the bound-to-continuum (BTC) design that features a relatively narrow gain spectrum of ∼ 200 GHz and have reported a GRT in the range of 15 -26 ps without the phonon extraction scheme. 17,18,21,22 Following the recent successful demonstration of a THz QCL with octave spanning gain, 23 heterogeneous QCL active regions composed from several different quantum cascades structures become increasingly important. 24 While for standard active regions of THz QCLs, a single cascade design (typically 3 -8 quantum wells) is repeated hundreds of times to form a quantum cascade heterostructure (QCH), in a heterogeneous active region several heterostructures with different design details are stacked to form a broadband QCL. As a consequence, the different heterostructures can fea...

We designed disk patch resonators to meet the requirements for enhanced coupling of optical cavities to intersubband transitions in heterostructures in the terahertz frequency regime. We applied modifications to the standard patch resonator in the form of a chain of holes and slits to control the resonator eigenmodes featuring quality factors ωFWHM/ω as high as 40. Due to the broken rotational symmetry of the resonators the individual eigenmodes can be accessed selectively depending on the incidence and the polarization of the THz wave. The demonstrated post-process blue-shifting of the resonance frequency up to 50% is a key tuning knob for an optimization of light-matter interaction in a quantum system.

We demonstrate resonant tunneling diodes, embedded in double metal cavities, strongly coupled to the cavity field, while maintaining their electronic properties. We measure the polariton dispersion and find a relative vacuum Rabi splitting of 11%, which explicitly qualifies for the strong-coupling regime. Additionally, we show that electronic transport has a significant influence on the polaritons by modulating the coupling strength. The merge between electronic transport and polaritonic physics in our devices opens up different perspectives of cavity quantum electro-dynamics and integrated photonics.

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