Computational modeling and simulations are increasingly being used to complement experimental testing for analysis of safety and efficacy of medical devices. Multiple voxel- and surface-based whole- and partial-body models have been proposed in the literature, typically with spatial resolution in the range of 1–2 mm and with 10–50 different tissue types resolved. We have developed a multimodal imaging-based detailed anatomical model of the human head and neck, named “MIDA”. The model was obtained by integrating three different magnetic resonance imaging (MRI) modalities, the parameters of which were tailored to enhance the signals of specific tissues: i) structural T1- and T2-weighted MRIs; a specific heavily T2-weighted MRI slab with high nerve contrast optimized to enhance the structures of the ear and eye; ii) magnetic resonance angiography (MRA) data to image the vasculature, and iii) diffusion tensor imaging (DTI) to obtain information on anisotropy and fiber orientation. The unique multimodal high-resolution approach allowed resolving 153 structures, including several distinct muscles, bones and skull layers, arteries and veins, nerves, as well as salivary glands. The model offers also a detailed characterization of eyes, ears, and deep brain structures. A special automatic atlas-based segmentation procedure was adopted to include a detailed map of the nuclei of the thalamus and midbrain into the head model. The suitability of the model to simulations involving different numerical methods, discretization approaches, as well as DTI-based tensorial electrical conductivity, was examined in a case-study, in which the electric field was generated by transcranial alternating current stimulation. The voxel- and the surface-based versions of the models are freely available to the scientific community.

Optical frequency combs have revolutionized the fields of high-resolution and precision atomic spectroscopy due to their high coherence, wide spectral bandwidth and absolute traceability [1,2]. Initially developed in the nearinfrared (NIR) spectral region, frequency combs are now being extended to other parts of the spectrum. In particular, extending the spectral range of frequency combs into the mid-infrared (MIR) and terahertz (THz) regions will open new possibilities in the fields of frequency metrology, molecular spectroscopy, chemical analysis and medical diagnosis [3], as the fundamental roto-vibrational absorption lines of a variety of molecules lie in this spectral region.Different schemes have been investigated for generating MIR frequency combs. A well-established approach consists of transferring frequency combs from the near-infrared region into the MIR region through nonlinear processes using, for example, optical parametric oscillators [4,5] or difference frequency generation in fiber-based NIR combs [6][7][8].Other examples include MIR combs generated by transition metals incorporated into chalcogenide hosts [9,10] or Thulium-doped silica fiber lasers [11]. These sources are now well-established and applications such as MIR highresolution spectroscopy are possible. These methods guarantee good spectral coverage and coherence, but usually require delicate experimental set-ups with large footprints.Significant effort has been recently made for achieving chip-based MIR frequency combs. Microresonator frequency combs (Kerr-combs) have been significantly improved [12][13][14][15] and have been extended to the MIR region [16][17][18]. Although Kerr-combs can be produced in different material platforms, they still require a high-power continuous wave (CW) laser as well as an evanescent coupling system, especially difficult to achieve in the MIR and THz regions.Quantum cascade lasers (QCL) have proven to be semiconductor lasers capable of generating comb radiation in the MIR and THz regions [19][20][21]. As the comb formation takes place directly in the QCL active region, QCL frequency combs (QCL-combs) offer the unique possibility of a completely integrated chip-based system capable of performing broadband high-resolution spectroscopy. Such a compact system is ideal for applications requiring the detection of several different molecules masked by a complex background matrix.Meanwhile, dual-comb spectroscopy using QCL-combs has been demonstrated [22] and a theoretical description of the comb formation has recently been developed [23,24]. However, key characteristics of QCL-combs -such as optical bandwidth and power-per-mode distribution -still need to be improved in order to better address spectroscopy applications.Group delay dispersion (GDD) plays an important role in the formation of 20,24]. In this work, we investigate a scheme for controlling the dispersion in MIR QCL-combs. We demonstrate that a dispersion compensation scheme based on a Gires-Tournois Interferometer [25] (GTI) directly integrated int...

In this work, we report the characterization of a quantum cascade laser frequency comb with an optical power of 1.05 W at λ∼8.2 μm. A 4.5 mm long device has a high reflectivity coating on the back facet as well as a top cladding designed to lower the group velocity dispersion and is operated at 258 K. Very strong (more than 60 dB) narrow beatnotes are shown, and frequency comb operation is obtained on a bandwidth of 85 cm−1 in a very large range of light-versus current characteristics. A bandwidth of 82 cm−1 has a power per mode of more than 1 mW and an average power per mode of 4.1 mW. Finally, a multi-heterodyne spectrum with 215 lines covering an optical bandwidth of more than 70 cm−1 measured with lasers showing similar performances is presented with very good line separation.

We study the operation of an 8.5 µm quantum cascade laser based on GaInAs/AlInAs lattice matched to InP using three different simulation models based on density matrix (DM) and nonequilibrium Green's function (NEGF) formulations. The latter advanced scheme serves as a validation for the simpler DM schemes and, at the same time, provides additional insight, such as the temperatures of the sub-band carrier distributions. We find that for the particular quantum cascade laser studied here, the behavior is well described by simple quantum mechanical estimates based on Fermi's golden rule. As a consequence, the DM model, which includes second order currents, agrees well with the NEGF results. Both these simulations are in accordance with previously reported data and a second regrown device