The terahertz (THz) spectral region, covering frequencies from 1 to 10 THz, is highly interesting for chemical sensing. The energy of rotational and vibrational transitions of molecules lies within ...this frequency range. Therefore, chemical fingerprints can be derived, allowing for a simple detection scheme. Here, we present an optical sensor based on active photonic crystals (PhCs), i.e., the pillars are fabricated directly from an active THz quantum-cascade laser medium. The individual pillars are pumped electrically leading to laser emission at cryogenic temperatures. There is no need to couple light into the resonant structure because the PhC itself is used as the light source. An injected gas changes the resonance condition of the PhC and thereby the laser emission frequency. We achieve an experimental frequency shift of 10(-3) times the center lasing frequency. The minimum detectable refractive index change is 1.6 × 10(-5) RIU.
To establish thresholds for rheumatoid arthritis (RA) activity categories according to the RA Disease Activity Index-5 (RADAI-5).
Three hundred ninety-two patients with RA were categorized according ...to Disease Activity Score 28-joint count (DAS28), Clinical Disease Activity Index (CDAI), and their satisfaction (PATSAT) with disease status. These measures built the basis for the calculation of disease activity limits for the RADAI-5. Patient assessments simultaneously meeting the identical DAS28, CDAI, and PATSAT categories were taken as the references to establish the thresholds for the respective RADAI-5 categories by calculating the third quartile of the corresponding RADAI-5 values. Subsequently, these new thresholds were applied to all assessments.
Seven hundred fifty-eight assessments in 392 patients (2 assessments median/patient) could be obtained, most patients having mild to moderate disease according to DAS28 and CDAI. Calculating the third quartile, the RADAI-5 thresholds were as follows: 0.0-1.4 for a remission-like state, 1.6-3.0 for mild disease activity, 3.2-5.4 for moderate, and 5.6-10.0 for high disease activity. Categorization according to the RADAI-5 showed a normal distribution, while DAS28 and CDAI were somewhat shifted to the left. DAS28 and CDAI levels, as well as tender and swollen joint counts and physician's global assessment and erythrocyte sedimentation rate, proved to be highly significantly different within the different RADAI-5 categories (Kruskal-Wallis test p < 0.001).
RADAI-5 thresholds for RA activity could be elaborated. Patient self-report questionnaires may substitute composite disease activity scores and may contribute significantly to improving documentation in routine patient care.
We present the development of a transportable laser frequency stabilization system with application to both optical clocks and a next-generation gravity mission (NGGM) in space. This effort leverages ...a 5-cm long cubic cavity with crystalline coatings operating at room temperature and with a center wavelength of 1064 nm. The cavity is integrated in a custom vacuum chamber with dedicated low-noise locking electronics. Our vacuum-mounted cavity and control system are well suited for space applications, exhibiting state-of-the-art noise performance while being resilient to radiation exposure, vibration, shock, and temperature variations. Furthermore, we demonstrate a robust means of automatically (re)locking the laser to the cavity when resonance is lost. We show that the mounted cavity is capable of reaching technology readiness level (TRL) 6, paving the way for high-performance ultrastable laser systems and eventually optical atomic clocks amenable to future satellite platforms.
Given their excellent optical and mechanical properties, substrate-transferred crystalline coatings are an exciting alternative to amorphous multilayers for applications in precision interferometry. ...The high mechanical quality factor of these single-crystal interference coatings reduces the limiting thermal noise in precision optical instruments such as reference cavities for narrow-linewidth laser systems and interferometric gravitational wave detectors. In this manuscript, we explore the optical performance of GaAs/AlGaAs crystalline coatings transferred to 50.8-mm (2-inch) diameter fused silica and sapphire substrates. We present results for the transmission, scattering, absorption, and surface quality of these prototype samples including the defect density and micro-roughness. These novel coatings exhibit optical performance on par with state-of-the-art dielectric structures, encouraging further work focused on the fabrication of larger optics using this technique.
Coupling of broadband terahertz pulses into metal-metal terahertz quantum cascade lasers is presented. Mode matched terahertz transients are generated on the quantum cascade laser facet of ...subwavelength dimension. This method provides a full overlap of optical mode and active laser medium. A longitudinal optical-phonon depletion based active region design is investigated in a coupled cavity configuration. Modulation experiments reveal spectral gain and (broadband) losses. The observed gain shows high dynamic behavior when switching from loss to gain around threshold and is clamped at total laser losses.
Quantum cascade lasers (QCLs) have been realized in several different material systems. In the mid‐infrared, active regions are predominantly based on In0.53Ga0.47As and InAs as quantum well ...material. Market‐ready devices routinely provide continuous‐wave operation at room temperature. For their THz counterparts, the situation is less clear. The most common material system for THz QCLs is the inherently lattice‐matched combination of GaAs with Al0.15Ga0.85As barriers. Yet, these devices still only reach a maximum operating temperature of 200 K with a lack of progress within the past years. Based on the identification of key parameters, this work reviews material systems for quantum cascade lasers with an emphasis on material and growth‐related aspects and the goal to identify promising candidates for future device generations. Similar active regions realized in different material systems allow to estimate the gain per unit thickness, as well as total growth times and relative thickness errors.
This work evaluates different material systems for THz quantum cascade laser active regions. Different combinations of quantum well and barrier materials are discussed with respect to growth and design‐related issues including practical aspects like a thickness error estimation. Experimental data from established materials furthermore allow to gauge the suitability of novel candidate materials.
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