Anomalous Temperature Effect in Weakly Coupled Superlattices: Carrier Transport in a THz Quantum Cascade Laser.

We present an investigation into the vertical transport through 13 different superlattice structures, where the well and barrier widths, doping concentration, dopant position, and contact layers were varied. Although superlattices have been extensively studied since 1970, there is a lack of publications on transport through superlattices similarly low doped as THz quantum cascade lasers (QCLs), for which the doping is in the 3-5×10^{10} cm^{-2} range. The superlattices presented are doped in the same range as THz QCLs, with contact layers and fabrication comparable to high-temperature THz QCLs. The temperature-dependent current-voltage characteristics were measured starting from 5 K and an anomalous temperature effect was observed at the first plateau. The measured current through the superlattice first decreases before increasing again with increasing temperature, resulting in the lowest current occurring at 75-110 K. This behavior is also observed in some THz QCLs. The effect disappears for thinner barriers, higher quantum well doping, or modified contact layers, indicating a strong dependency on band bending, due to the large difference in the doping of the contact layers and the superlattice, which is confirmed with multiscattering Büttiker simulations.

Flexible terahertz gas sensing platform based on substrate-integrated hollow waveguides and an opto-electronic light source.

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.

Ultrabroadband Heterogeneous THz Quantum Cascade Laser.

Broadband emission in the terahertz spectral region is a prerequisite for applications such as spectroscopy or white light sources. Appropriate signal powers and a compact design are advantageous for this use. A technology which meets these requirements are terahertz quantum cascade lasers. These electrically pumped, on-chip semiconductor lasers provide high output powers and the freedom of tailoring their emission wavelength by bandstructure engineering. By combining multiple active region designs emitting at different wavelengths in a single structure, one can obtain broadband emission from a single device. Here, we present a heterogeneous terahertz quantum cascade laser consisting of five individual active regions based on a three-well, LO-phonon depopulation design. The devices lase in pulsed and continuous-wave operation and emit in a spectral range from 1.9 to 4.5 THz, covering a bandwidth of 1.37 octaves. The use of the three-well design, which was optimized for high temperature operation, leads to a maximum operating temperature in the pulsed operation of 143 K.

Flexible terahertz opto-electronic frequency comb light source tunable over 3.5 THz.

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 <10MHz by resolving the pressure-dependent collisional broadening of an ammonia molecule rotational mode.

Deep learning control of THz QCLs.

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.

Slot-Waveguide Silicon Nitride Organic Hybrid Distributed Feedback Laser.

One of the major barriers for a widespread commercial uptake of silicon nitride photonic integrated circuits for cost-sensitive applications is the lack of low-cost monolithically integrated laser light sources directly emitting into single-mode waveguides. In this work, we demonstrate an optically pumped organic solid-state slot-waveguide distributed feedback laser designed for a silicon nitride organic hybrid photonic platform. Pulsed optical excitation of the gain medium is achieved by a 450 nm laser diode. The optical feedback for lasing is based on a second-order laterally coupled Bragg grating with a slot-waveguide core. Optimized material gain properties of the organic dye together with the increased modal gain of the laser mode arising from the improved overlap of the slot-waveguide geometry with the gain material enable single-mode lasing at a wavelength of 600 nm. The straightforward integration and operation with a blue laser diode leads to a cost-effective coherent light source for photonic integrated devices.

Analysis of silicon nitride partial Euler waveguide bends.

In this work, we present a detailed analysis of individual loss mechanisms in silicon nitride partial Euler bends at a wavelength of 850 nm. This structure optimizes the transmission through small radii optical waveguide bends. The partial Euler bend geometry balances losses arising from the transition from the straight to the bend waveguide mode and radiative losses of the bend waveguide mode. Numerical analyses are presented for 45-degree bends commonly employed in S-bend configurations to create lateral offsets, as well as 90- and 180-degree bends. Additionally, 90-degree partial Euler bends were fabricated on a silicon nitride photonic platform to experimentally complement the theoretical findings. The optimized waveguide bends allow for a reduced effective radius without increasing the total bend loss and, thus, enable a higher component density in photonic integrated circuits.

Integrated silicon nitride organic hybrid DFB laser with inkjet printed gain medium.

The provision of a coherent light source is a prerequisite for a variety of photonic integrated circuits. The integration of semiconductor laser diodes in disposable photonic devices in fields such as biosensing is, however, impeded by the competitive pricing in this application area. In this work, we demonstrate lasing of an alternative laser light source, namely an integrated hybrid organic solid-state distributed feedback laser for a silicon nitride photonic platform. The laser is optically pumped with a high power 450 nm laser diode and emits in the visible at 630 nm into a waveguide taper to reduce the cross-section to a single mode geometry. Inkjet printing of the organic gain medium enables a local, cost-effective, and flexible processing technology. The fabrication of the presented coherent light source is CMOS compatible and therefore highly interesting for co-integrated sensing platforms.

Thermoelectric-cooled terahertz quantum cascade lasers.

We demonstrate the first lasing emission of a thermo-electrically cooled terahertz quantum cascade laser (THz QCL). A high temperature three-well THz QCL emitting at 3.8 THz is mounted to a novel five-stage thermoelectric cooler reaching a temperature difference of ΔT = 124 K. The temperature and time-dependent laser performance is investigated and shows a peak pulse power of 4.4 mW and a peak average output power of 100 µW for steady-state operation.

High-energy diode side-pumped Er:LiYF4 laser.

This work presents a novel diode side-pumping scheme for Er:YLF with a characteristic laser emission at 2.81 µm. Average output powers greater than 10 W and corresponding pulse energies exceeding 100 mJ with a high slope efficiency of 18.7% are achieved. The Er:YLF laser efficiently operates at room-temperature cooling water and maintains a good beam quality of M2<12 in the major axis and M2<6 in the minor axis of the beam profile. The laser cavity is studied by means of Findlay-Clay analysis, and the operation at characteristic pump parameters is investigated. In addition, a dynamic laser rate equation simulation is introduced for quantitative and qualitative comparison. This compact and scalable free-running Er:YLF laser provides an effective building block for a subsequent Q-switching concept.

Barrier Height Tuning of Terahertz Quantum Cascade Lasers for High-Temperature Operation.

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 Ga1-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.

Disk patch resonators for cavity quantum electrodynamics at the terahertz frequency.

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/ω0 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.

High-Power Growth-Robust InGaAs/InAlAs Terahertz Quantum Cascade Lasers.

We report on high-power terahertz quantum cascade lasers based on low effective electron mass InGaAs/InAlAs semiconductor heterostructures with excellent reproducibility. Growth-related asymmetries in the form of interface roughness and dopant migration play a crucial role in this material system. These bias polarity dependent phenomena are studied using a nominally symmetric active region resulting in a preferential electron transport in the growth direction. A structure based on a three-well optical phonon depletion scheme was optimized for this bias direction. Depending on the sheet doping density, the performance of this structure shows a trade-off between high maximum operating temperature and high output power. While the highest operating temperature of 155 K is observed for a moderate sheet doping density of 2 × 1010 cm-2, the highest peak output power of 151 mW is found for 7.3 × 1010 cm-2. Furthermore, by abutting a hyperhemispherical GaAs lens to a device with the highest doping level a record output power of 587 mW is achieved for double-metal waveguide structures.

Spectrally resolved far-fields of terahertz quantum cascade lasers.

We demonstrate a convenient and fast method to measure the spectrally resolved far-fields of multimode terahertz quantum cascade lasers by combining a microbolometer focal plane array with an FTIR spectrometer. Far-fields of fundamental TM0 and higher lateral order TM1 modes of multimode Fabry-Pérot type lasers have been distinguished, which very well fit to the results obtained by a 3D finite-element simulation. Furthermore, multimode random laser cavities have been investigated, analyzing the contribution of each single laser mode to the total far-field. The presented method is thus an important tool to gain in-depth knowledge of the emission properties of multimode laser cavities at terahertz frequencies, which become increasingly important for future sensing applications.

Broadband terahertz amplification in a heterogeneous quantum cascade laser.

We demonstrate a broadband terahertz amplifier based on ultrafast gain switching in a quantum cascade laser. A heterogeneous active region is processed into a coupled cavity metal-metal waveguide device and provides broadband terahertz gain that allows achieving an amplification bandwidth of more than 500 GHz. The temporal and spectral evolution of a terahertz seed pulse, which is generated in an integrated emitter section, is presented and an amplification factor of 21 dB is reached. Furthermore, the quantum cascade amplifier emission spectrum of the emerging sub-nanosecond terahertz pulse train is measured by time-domain spectroscopy and reveals discrete modes between 2.14 and 2.68 THz.

Coupled cavity terahertz quantum cascade lasers with integrated emission monitoring.

We demonstrate the on-chip generation and detection of terahertz radiation in coupled cavity systems using a single semiconductor heterostructure. Multiple sections of a terahertz quantum cascade laser structure in a double-metal waveguide are optically coupled and operate either as a laser or an integrated emission monitor. A detailed analysis of the photon-assisted carrier transport in the active region below threshold reveals the detection mechanism for photons emitted by the very same structure above threshold. Configurations with a single laser cavity and two coupled laser cavities are studied. It is shown that the integrated detector can be used for spatial sensing of the light intensity within a coupled cavity.

Subwavelength micropillar array terahertz lasers.

We report on micropillar-based terahertz lasers with active pillars that are much smaller than the emission wavelength. These micropillar array lasers correspond to scaled-down band-edge photonic crystal lasers forming an active photonic metamaterial. In contrast to photonic crystal lasers which use significantly larger pillar structures, lasing emission is not observed close to high-symmetry points in the photonic band diagram, but in the effective medium regime. We measure stimulated emission at 4 THz for micropillar array lasers with pillar diameters of 5 µm. Our results not only demonstrate the integration of active subwavelength optics in a terahertz laser, but are also an important step towards the realization of nanowire-based terahertz lasers.

Spectrally coded optical nanosectioning (SpecON) with biocompatible metal-dielectric-coated substrates.

Fluorescence nanosectioning within a submicron region above an interface is desirable for many disciplines in the life sciences. A drawback, however, to most current approaches is the a priori need to physically scan a sculptured point spread function in the axial dimension, which can be undesirable for optically sensitive or highly dynamic samples. Here we demonstrate a fluorescence imaging approach that can overcome the need for scanning by exploiting the position-dependent emission spectrum of fluorophores above a simple biocompatible nanostructure. To achieve this we have designed a thin metal-dielectric-coated substrate, where the spectral modification to the total measured fluorescence can be used to estimate the axial fluorophore distribution within distances of 10-150 nm above the substrate with an accuracy of up to 5-10 nm. The modeling and feasibility of the approach are verified and successfully applied to elucidate nanoscale adhesion protein and filopodia dynamics in migrating cells. It is likely that the general principle can find broader applications in, for example, single-molecule studies, biosensing, and studying fast dynamic processes.

Probing scattering mechanisms with symmetric quantum cascade lasers.

A characteristic feature of quantum cascade lasers is their unipolar carrier transport. We exploit this feature and realize nominally symmetric active regions for terahertz quantum cascade lasers, which should yield equal performance with either bias polarity. However, symmetric devices exhibit a strongly bias polarity dependent performance due to growth direction asymmetries, making them an ideal tool to study the related scattering mechanisms. In the case of an InGaAs/GaAsSb heterostructure, the pronounced interface asymmetry leads to a significantly better performance with negative bias polarity and can even lead to unidirectionally working devices, although the nominal band structure is symmetric. The results are a direct experimental proof that interface roughness scattering has a major impact on transport/lasing performance.

Rotating polarization spectroscopy for single nano-antenna characterization.

We report on a novel micro-spectroscopic technique to quantitatively measure the extinction cross-section σ(ext) of few and single linearly polarizing nano-antennas. This technique relies on rotating the linear polarization of a monochromatic laser beam at a frequency ω(1) while optically chopping the incident beam at ω(2) and using lock-in detection with a switched reference frequency input to measure the amount of scattered and absorbed power. The amount of power removed from the beam corresponds to σ(ext) of the polarizing nano-structure. Furthermore, this technique is easy to integrate into existing microscopy or micro-photoluminescence setups and does not depend on the sample's temperature.