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We report on a flexible platform for molecular sensing in the terahertz range. Merging the well-established technologies of near-infrared electro-optic modulation and photomixing realizes a spectrally adaptable terahertz source, which is combined with a new generation of compact gas cells, so-called substrate-integrated hollow waveguides (iHWGs). iHWGs have been developed in the mid-infrared and provide flexibility in the design of the optical absorption path. Here, we demonstrate its suitability for the terahertz domain by presenting its low propagation losses and by measuring rotational transitions of nitrious oxide (N2O). A fast frequency sideband modulation technique results in substantially reduced measurement times and increased accuracy compared to a standard wavelength tuning method.

Artificial neural networks are capable of fitting highly non-linear and complex systems. Such complicated systems can be found everywhere in nature, including the non-linear interaction between optical modes in laser resonators. In this work, we demonstrate artificial neural networks trained to model these complex interactions in the cavity of a Quantum Cascade Random Laser. The neural networks are able to predict modulation schemes for desired laser spectra in real-time. This radically novel approach makes it possible to adapt spectra to individual requirements without the need for lengthy and costly simulation and fabrication iterations.

We demonstrate a terahertz (THz) frequency comb that is flexible in terms of its frequency range and the number and spacing of comb lines. We use a combination of near-infrared laser diodes, phase modulation and opto-electronic frequency conversion. The THz comb lines are characterized to be < 10 MHz by resolving the pressure dependent collisional broadening of an ammonia molecule rotational mode., manuscript version 2

Machine learning and its applications are rapidly advancing. Even-though the majority of machine learning is performed on computers, the fundamental methods of machine learning are based on linear algebra, which can be readily transferred to other systems.

In recent years, optical frequency comb sources have been realized in the mid-infrared and terahertz region by using quantum cascade lasers (QCLs) [1] , [2] . These electrically driven, on-chip sources provide high output power and a compact design. Commonly used waveguide designs include ridge type resonators supporting Fabry-Perot modes. The appearing effect of spatial hole burning in this resonator type prevents the exploitation of the whole gain bandwidth of the active region [3] . This limitation can be overcome using ring-shaped resonators, which exhibit either a standing-wave pattern formed by two counterpropagating waves, or support a traveling wave, which does not cause spatial hole burning.

Amplifiers of Terahertz (THz) radiation are quite useful, because in this spectral range radiation is normally weak and sensitive detectors are hard to get. Monolithically integrated master-oscillator power-amplifier (MOPA) [1] , [2] as well as free-space amplifiers [3] have been demonstrated by using Quantum Cascade laser (QCL) structures with amplifications in the range of 3 – 12 dB. Although these devices are very promising, they all provide a linear amplification. However, for most telecommunication applications an amplification only above a certain threshold is advantageous, since this avoids the amplification of noise.

We present comb formation in ring-shaped THz quantum cascade lasers. Devices are spontaneously operating in a harmonic state transiting to a dense comb regime exhibiting over 30 equidistant modes covering a bandwidth of 622 GHz.

A Terahertz optical amplifier based on a Quantum Cascade laser structure with a lossy double-metal cavity is demonstrated. Amplification appears only above a certain threshold and an amplification of ~17 dB is achieved.

The control of the output beam shape and direction of THz Quantum Cascade Lasers is presented. For the improvement of the operation temperature different barrier heights are investigated. As result a very efficient device is realized which requires only thermo-electric cooling and thus can be integrated into compact gas spectroscopy systems.

The interplay of high operating temperatures and good heat dissipation is crucial for high-performance terahertz quantum cascade lasers. We therefore study the influence on the cross-plane thermal conductivity of different aluminum concentrations in the barrier of ${\mathrm{Ga}\mathrm{As}/\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ active regions. The thermal conductivity is decreasing from $30\phantom{\rule{0.1em}{0ex}}\mathrm{W}\phantom{\rule{0.1em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}\phantom{\rule{0.1em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ to $12\phantom{\rule{0.1em}{0ex}}\mathrm{W}\phantom{\rule{0.1em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}\phantom{\rule{0.1em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}$ if the aluminum concentration is increased from 15% to 24%. The temperature during pulsed-laser operation is obtained by measuring the variation of the emission frequency for different laser pulse lengths. This shows, that besides the thermal conductivity, the amount of electric input power has a strong influence on the temperature reached internally during laser operation and is critical for creating high-power devices operating at high temperatures. We show that active regions with thin but high ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ barriers fulfill this need and are well suited for high-temperature operation. A thermal model of the devices allows prediction of the active-region temperature increase for very short pulse durations. For the structure with 24% Al barriers and a starting temperature of 10 K, the model shows an increase by 24 K for a pulse length of only 300 ns.

Spectral fingerprints of molecules are mostly accessible in the terahertz and mid-infrared ranges, such that efficient molecular-detection technologies rely on broadband coherent light sources at such frequencies. THz Quantum Cascade Lasers can achieve octave-spanning bandwidths. However, their tunability and wavelength selectivity is often constrained by the geometry of their cavity. The recently introduced Quantum Cascade Random Lasers represent alternative sources of THz light, in which random scattering provides the required field confinement. The random resonator geometry greatly relaxes wavelength selectivity, thus producing radiation that is both spectrally broadband and collimated in the far-field. Yet, the intrinsic randomness of these devices' spectral emission strongly restricts the scope of their potential applications. In this work, we demonstrate the all-optical adaptive control and tuning of Quantum Cascade Random Lasers. The specificity of our random-laser sources is exploited to locally modify the system's permittivity with a near-infrared (NIR) laser beam and thereby substantially reconfigure the distribution of disorder. Using a spatial light modulator combined with an optimization procedure, the NIR illumination is spatially modulated to reshape the spectral emission and transform the initially multimode laser into a single mode source, which could be harnessed to perform self-referenced spectroscopic measurements. Moreover, we show that local NIR perturbations can be used to sense linear and nonlinear interactions amongst modes in the near field. Our work points the way towards the design of broadly tunable THz sources with greatly relaxed fabrication constraints., 11 pages, 3 figures

[Image: see text] Terahertz quantum cascade lasers (QCLs) are excellent coherent light sources, but are still limited to an operating temperature below 200 K. To tackle this, we analyze the influence of the barrier height for the identical three-well terahertz QCL layer sequence by comparing different aluminum concentrations (x = 0.12–0.24) in the GaAs/Al(x)Ga(1-x)As material system, and then we present an optimized structure based on these findings. Electron injection and extraction mechanisms as well as LO-phonon depopulation processes play crucial roles in the efficient operation of these lasers and are investigated in this study. Experimental results of the barrier height study show the highest operating temperature of 186.5 K for the structure with 21% aluminum barriers, with a record k(B)T(max)/ℏω value of 1.36 for a three-well active region design. An optimized heterostructure with 21% aluminum concentration and reduced cavity waveguide losses is designed and enables a record operating temperature of 196 K for a 3.8 THz QCL.

We demonstrate the presence of Intersubband Polaritons [1] in Triple Barrier Resonant Tunneling Diodes (TBRTDs) [2]. Additionally, we investigated the influence of Intersubband Polaritons on the electrical transport and show that the coupling-strength can be modulated electrically.

In this contribution, we study the effects arising when using a metamaterial surface featuring a dense array of superradiant emitters as cavities for experiments in the ultrastrong coupling regime. We show that an increasing cavity loss does not always lead to the expected transition to the weakly coupled Purcell regime but more that it is still possible to observe polaritons as a feature of the strong coupling between the metaatoms and the intersubband transition.

The active region of a terahertz quantum cascade laser with two optical transitions is studied. The population of upper laser states, which correspond to 3.4 and 3.8 THz, are investigated at different operating temperatures and in a strong magnetic field.

Terahertz (THz) quantum cascade lasers (QCLs) are powerful coherent light sources covering the frequency range from 1.2 to 5.4 THz, but still suffer from operation temperatures below 200 K [1]. Beside the standard material system GaAs/AlGaAs novel material systems with lower effective electron masses were investigated to improve the operation temperature of THz QCLs. These materials include InGaAs and InAs based systems [2–4] which benefit from a higher optical gain but suffer from molecular beam epitaxy growth issues due to their very thin barrier layers.

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.

With the tremendous development of ultrafast lasers we are provided with a tool for efficient wavelength conversion. Down conversion allows generation of mid-infrared and THz light and provides in addition also the ability to control the phase. This additional control knob is a new feature for optical experiments which we are just beginning to use. I will show a few experiments with semiconductor nanostructures and quantum cascade lasers where the phase information allows observing physical processes directly; this includes population transfer, amplification, and short pulse formation. In addition to the phase information, down conversion and quantum cascade lasers provide us with very large bandwidth- spanning more than one octave. Handling these bandwidths is an interesting challenge and also extremely attractive for new optical methods like frequency comb sensing.

The mode formation of a terahertz Quantum Cascade laser with two optical transitions is studied experimentally. The emission spectrum shows two well separated frequency regions at 3.4 THz and 3.8 THz corresponding to two different upper laser states | 3 ⟩ and | 4 ⟩. From the relative strength of the two colors, population and electron scattering effects for the two states are investigated at different operating temperatures and in the presence of a strong magnetic field. At elevated temperatures, the population of state | 3 ⟩ is continuously reduced, resulting in only the 3.8 THz transition lasing at 140 K. For an applied magnetic field, the elastic scattering channel from | 4 ⟩ → | 3 ⟩ is progressively suppressed, resulting in laser emission switching from 3.4 THz to 3.8 THz.The mode formation of a terahertz Quantum Cascade laser with two optical transitions is studied experimentally. The emission spectrum shows two well separated frequency regions at 3.4 THz and 3.8 THz corresponding to two different upper laser states | 3 ⟩ and | 4 ⟩. From the relative strength of the two colors, population and electron scattering effects for the two states are investigated at different operating temperatures and in the presence of a strong magnetic field. At elevated temperatures, the population of state | 3 ⟩ is continuously reduced, resulting in only the 3.8 THz transition lasing at 140 K. For an applied magnetic field, the elastic scattering channel from | 4 ⟩ → | 3 ⟩ is progressively suppressed, resulting in laser emission switching from 3.4 THz to 3.8 THz.

Random lasing operation requires an active region, a gain medium that supports multiple scattering, and, especially for integrated optoelectronic devices, a nonresonant outcoupling mechanism over a continuous spectrum. For broadband operation, the resonator geometry must provide frequency nonselective, strong feedback over a large bandwidth. The feedback mechanism by multiple scattering in terahertz semiconductor random lasers and the bandwidth of such cavities are presented and discussed. We demonstrate the influence of shape and scattering strength of the scatterers on the lasing process and determine the bandwidth of such resonator structures. We use passive resonator structures to prove that the feedback as well as the outcoupling is frequency independent over a large bandwidth.Random lasing operation requires an active region, a gain medium that supports multiple scattering, and, especially for integrated optoelectronic devices, a nonresonant outcoupling mechanism over a continuous spectrum. For broadband operation, the resonator geometry must provide frequency nonselective, strong feedback over a large bandwidth. The feedback mechanism by multiple scattering in terahertz semiconductor random lasers and the bandwidth of such cavities are presented and discussed. We demonstrate the influence of shape and scattering strength of the scatterers on the lasing process and determine the bandwidth of such resonator structures. We use passive resonator structures to prove that the feedback as well as the outcoupling is frequency independent over a large bandwidth.

We present a simple method to generate almost arbitrary Terahertz frequency combs, between 0.1 and 3.6 THz with single line linewidths of 10 MHz, based on a robust optical system built from standard fiber-optic components.

We present a method to gain control of Quantum Cascade Random Lasers by illuminating them with spatially modulated near-infrared light. We employ deep learning to predict the response of the system to the near-infrared illumination.

Since their initial demonstration, random lasers have attracted a lot of interest due to their complementary characteristics compared to conventional lasers [1]. In a random laser, the light is trapped by multiple scattering in the gain medium, which has been demonstrated in different kinds of materials, ranging from grinded laser crystals to polymer films and ceramics. Recently we demonstrated [2] random lasers at terahertz (THz) frequencies by making use of quantum cascade laser (QCL) active regions, resulting in broadband surface emission with a highly collimated output beam.

Non-coherent thermal light emitters are employed in many spectroscopic systems due to their broadband emission spectrum. A novel laser class, which is broadband as well as coherent is the random laser [1] . A prominent subset of these lasers are the quantum cascade random lasers (QCRLs), which employ a semiconductor heterostructure based gain material.

Quantum cascade lasers (QCLs) are compact and powerful sources that cover a wide spectral range from infrared to terahertz (THz) radiation. The emission characteristics of QCLs depend on design parameters such as layer thickness, material composition and doping. Therefore, the material system has to be chosen accurately. Most commonly used material systems for THz QCLs are GaAs/AlGaAs and InGaAs/InAlAs. The latter requires very thin layers of InAlAs and is therefore difficult to manufacture epitaxially [1]. One solution to overcome this issue, while still making use of the benefits provided by InGaAs, namely lower effective electron mass (m∗= 0.043m 0 ) which leads to a higher optical gain, is the usage of different barrier materials such as the ternary GaAsSb [2] and the quaternary InAlGaAs [3]. Crucial for the barrier thickness is the conduction band offset (CBO) of the material system. The common notion is to employ barrier materials having lower CBOs and therefore thicker barriers.

Nowadays, frequency combs (FCs) are regarded as state-of-the-art measurement tools, when it comes to high precision experiments. Among the various possibilities to generate FCs, techniques based on electro-optic (EO) modulation provide the advantage of great freedom in the FC shaping process. Therefore, tunable FC sources utilizing EO modulators find application in numerous research fields including interferometry, metrology and spectroscopy [1] . So far, EO FCs are mainly restricted to the near-infrared (NIR) spectral region, since the realization of EO modulators for the mid-infrared (MIR) and Terahertz (THz) domain is a challenging task. These limitations can be overcome, when NIR EO FCs are converted into the desired wavelength range by a nonlinear frequency conversion process [2] . Especially spectroscopic applications in the MIR and THz benefit, when EO FCs can be used to target the many rotational and vibrational transitions lying in these parts of the electromagnetic spectrum.

The control of heat dissipation in semiconductor lasers is in general of importance. Especially in the case of quantum cascade lasers (QCLs) [1], this problem has to be treated with great attention. The superlattice structure of the active region strongly reduces the thermal conductivity in the direction of the growth direction [2]. Due to the low energy spacing of the upper and lower laser state in terahertz QCLs, population inversion in difficult to achieve at higher temperatures, what limits the maximum operating temperature to 200 K [3]. Additionally, the inherent emission of phonons in the active region converts a large amount of electrical input power into heat that has to be dissipated.

Intersubband Polaritons [1] are quasi-particles that form under strong light-matter coupling conditions in semiconductor heterostructures. The two main requirements are a high-Q tunable cavity and a suitable intersubband transition. Intersubband polaritons are predicted to enable novel applications such as the intersubband polariton laser [2]. The most characteristic feature of intersubband polaritons is their avoided-crossing property. When the detuning between the cavity resonance and the intersubband transition is very small (i.e. the cavity resonance frequency equals the intersubband transition energy) the two absorptions caused by the cavity and the intersubband transitions become indistinguishable. For the matter part we used triple-barrier resonant tunneling diodes (TBRTDs) [3]. TBRTDs are semiconductor heterostructures that consist of two quantum wells. When applying an electric field, the eigen-energies of the quantum wells can align, which leads to a resonant tunneling current. This causes sharp peaks in the I-V-characteristic of the devices. TBRTDs also feature intersubband transitions, which are crucial for the formation of intersubband polaritons. The intersubband transition of interest is located at 100 meV.