Received Month X, XXXX; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc.ID XXXXX); published Month X, XXXX Mid-infrared light generation through four-wave mixing-based frequency down-conversion in a normal group velocity dispersion silicon waveguide is demonstrated. A telecom-wavelength signal is down-converted across more than 1.2 octaves using a pump at 2190 nm in a 1cm long waveguide. At the same time 13dB parametric gain of the telecom signal is obtained. OCIS Codes: (130.3120) Integrated optics devices; (190.0190) Nonlinear optics;The broadband transparency of the silicon-on-insulator waveguide platform from 1.1µm (limited by the absorption of silicon) to ~4µm (limited by the absorption of SiO2) [1] enables the realization of photonic integrated circuits outside the telecommunication band. Such circuits can be valuable for spectroscopic sensing applications, which leverage the strong rovibrational absorption lines of molecules in the mid-infrared wavelength spectrum -the so-called molecular fingerprint region [2]. While silicon provides an excellent platform for passive waveguiding in this 1.1 -4 µm wavelength range [1,3], the generation and detection of mid-infrared radiation is not straightforward. Recent research has been geared towards the integration of III-V semiconductor devices onto the silicon photonics platform to implement this functionality [4,5]. However, mid-infrared semiconductor photodetectors suffer from poor sensitivity at room temperature due to their narrow band gap, while semiconductor light sources only offer a limited gain bandwidth and hence a limited emission wavelength range. Efficient nonlinear optical effects on the silicon photonic platform, making use of the instantaneous thirdorder χ(3) nonlinear effect, can provide a solution to many of these challenges. Recent work has shown that fourwave mixing-based nonlinear optical functions including supercontinuum generation [6], optical parametric amplification [7,8] and wavelength conversion [7][8][9][10] can be integrated for mid-infrared light generation within compact silicon photonic integrated circuits. Moreover, silicon waveguides accomplishing bi-directional broadband spectral translation of optical signals between 1620 nm and 2440 nm in the mid-infrared [11], and between 1312 nm and 1884 nm in the short-wave infrared [12] have been demonstrated using four-wave mixing phase matched by anomalous dispersion. Such spectral translation technology can be applied to mid-infrared spectroscopic functions on a silicon photonic integrated circuit, by using advanced telecom-wavelength laser sources and high-sensitivity photodetectors. In this paper, we demonstrate that by using silicon waveguides with normal dispersion, the range of mid-infrared light generation and spectral translation can be extended to 3.6 µm wavelength, by frequency down-conversion across 1.2 octaves from the telecom band.The spectral translator waveguide used in this work is dispersion engineered specifically to allow for phase matching far ...

Background: In a recent high-profile case study, we used functional magnetic resonance imaging (fMRI) to monitor improvements in motor function related to neuroplasticity following rehabilitation for severe traumatic brain injury (TBI). The findings demonstrated that motor function improvements can occur years beyond current established limits. The current study extends the functional imaging investigation to characterize neuromodulation effects on neuroplasticity to further push the limits. Methods: Canadian Soldier Captain (retired) Trevor Greene (TG) survived a severe open-TBI when attacked with an axe during a 2006 combat tour in Afghanistan. TG has since continued intensive daily rehabilitation to recover motor function, experiencing an extended plateau using conventional physical therapy. To overcome this plateau, we paired translingual neurostimulation (TLNS) with the continuing rehabilitation program. Results: Combining TLNS with rehabilitation resulted in demonstrable clinical improvements along with corresponding changes in movement evoked electro-encephalography (EEG) activity. High-density magneto-encephalography (MEG) characterized cortical activation changes in corresponding beta frequency range (27Hz). MEG activation changes corresponded with reduced interhemispheric inhibition in the post-central gyri regions together with increased right superior/middle frontal activation suggesting large scale network level changes. Conclusions: The findings provide valuable insight into the potential importance of non-invasive neuromodulation to enhance neuroplasticity mechanisms for recovery beyond the perceived limits of rehabilitation.

We numerically investigate the trade-offs between the dispersion properties, coupling efficiency, and geometrical constraints in dual-wire (twin-lead) terahertz (THz) waveguides. In particular, we show that their inherent linearly polarized quasi-transverse electromagnetic (TEM) modes exist for waveguide transverse dimensions comparable with the wavelength, enabling significant end-fire coupling (>10%) for numericalaperture limited Gaussian beams while supporting a relatively low-dispersion propagation of below 0.5 ps 2 /m, as desired for short-pulse time-domain spectroscopy applications. Starting from the dual-wire structure, we also demonstrate that low-dispersion tapers can be designed to improve coupling efficiency.OCIS Terahertz (THz) bandwidth has been attracting much interest in material characterization, owing to its demonstrated advantages in probing and recognizing different materials, for various potential applications in fields spanning from biology to security. These applications rely on the free space propagation of broadband singlecycle THz pulses, which enables time-domain spectroscopy (TDS), capable of retrieving the amplitude and phase information of the spectral signature of various compounds [1] . One of the recent challenges on the topic is the development of practical waveguides that are capable of transporting this signal to arbitrary locations.Many studies highlight that the most confining structures do not exhibit the wide bandwidth required by typical pulses used in THz-TDS. The strong dispersion scrambles the phase-time information and renders the extraction of the complex spectrum rather difficult. Most studies have been based either on conventional metallic guiding structures such as hollow circular waveguides [2] and hollow rectangular waveguides [3] ; or dielectric waveguides such as sapphire fibers [4] , plastic ribbon waveguides [5] , plastic photonic crystal fibers [6] , and parallel-plate photonic waveguides [7] ; or dielectric waveguides coated with metal sheets [8] . Some structures, such as metallic parallel-plate waveguides [9−11] , are known to support the propagation of almost dispersiveless transverse electromagnetic (TEM) or quasi-TEM modes at the expense of providing confinement in only one dimension, which largely limits their practical impact.Other structures, such as coaxial [12] and metal wire waveguides [13] , better known as Sommerfeld rods [14] , exhibit ring-shaped coaxial modes with a radial polarization distribution; hence, they have poor end-fire coupling efficiency when used with conventional THz sources, that generally output linearly polarized quasi-Gaussian beams.Recent studies report low loss propagation in THz twowire waveguides [15] with a wire separation of 4.7 mm [16] . However, mode analysis ( Fig. 1) confirms that when the conductors' separation exceeds three to four times the wavelength, the propagation mode no longer exists between the two wires; instead, there are individual Sommerfeld modes around each wire, resulting in negligible coupling w...

We experimentally demonstrate the first integrated temporal Fourier transformer based on a linearly chirped Bragg grating waveguide written in silica glass with a femtosecond laser. The operation is based on mapping the energy spectrum of the input optical signal to the output temporal waveform by making use of first-order chromatic dispersion. The device operates in reflection, has a bandwidth of 10 nm, and can be used for incident temporal waveforms as long as 20 ps. Experimental results, obtained through both temporal oscilloscope traces and Fourier transform spectral interferometry, display a successful Fourier transformation of in-phase and out-of-phase pairs of input optical pulses, and demonstrate the correct functionality of the device for both amplitude and phase of the temporal output.

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