1,721,006 research outputs found
Gallium and titanium diffused optical waveguide devices in sapphire
Full text linkThis thesis describes new methods to realise an integrated Ti:Sapphire laser using thermal ionic diffusion. Passive and active waveguides were fabricated by thermal diffusion of gallium and titanium ions in sapphire.Theoretical simulations were established which describe the potential of planar Ti:Sapphire waveguide lasers and intracavity wavelength selection devices. A diffusion study of gallium, titanium and gallium/titanium co-doping is presented; the diffusion coefficient of gallium ions in sapphire was calculated to be 3.3x10-17 m2s-1 at 16000 °C and the diffusion coefficient of titanium ions in sapphire at 16000 °C was found to be 1.7x10-15 m2s-1. Planar Ga:Sapphire passive waveguides were realised by thermal diffusion of gallium ions. The refractive index of sapphire at 800nm is approximately 1.766 and the index change induced by gallium doping was found to be up to 6x10-3 and the mode sizes of the waveguides were as small as 1µm at 488nm. A fabrication procedure based on SU-8 photolithography and ion beam milling was developed for micromachining the surface of sapphire. Using this fabrication technique Ga:Sapphire ridge waveguides were fabricated by diffusion of gallium and exhibited mode sizes as small as 2µm at 633nm. Using the same fabrication technique the realisation of the first Ti-diffused sapphire ridge waveguide lasers is reported. Finally, the fabrication of sapphire optical circuits using titanium and gallium co-doping is described and fluorescence characterisation of these waveguides is given
Deep neural network ensembles for THz-TDS refractive index extraction exhibiting resilience to experimental and analytical errors
No full textTerahertz time-domain spectroscopy (THz-TDS) achieves excellent signal-to-noise ratios by measuring the amplitude of the electric field in the time-domain, resulting in the full, complex, frequency-domain information of materials’ optical parameters, such as the refractive index. However the data extraction process is non-trivial and standardization of practices are still yet to be cemented in the field leading to significant variation in sample measurements. One such contribution is low frequency noise offsetting the phase reconstruction of the Fourier transformed signal. Additionally, experimental errors such as fluctuations in the power of the laser driving the spectrometer (laser drift) can heavily contribute to erroneous measurements if not accounted for. We show that ensembles of deep neural networks trained with synthetic data extract the frequency-dependent complex refractive index, whereby required fitting steps are automated and show resilience to phase unwrapping variations and laser drift. We show that training with synthetic data allows for flexibility in the functionality of networks yet the produced ensemble supersedes current extraction techniques.</p
Simulation of terahertz generation from lateral diffusion currents in semiconductor devices
No full textArtificial neural networks for material parameter extraction in terahertz time-domain spectroscopy
No full textTerahertz time-domain spectroscopy (THz-TDS) is a proven technique whereby the complex refractive indices of materials can be obtained without requiring the use of the Kramers-Kronig relations, as phase and amplitude information can be extracted from the measurement. However, manual pre-processing of the data is still required and the material parameters require iterative fitting, resulting in complexity, loss of accuracy and inconsistencies between measurements. Alternatively approximations can be used to enable analytical extraction but with a considerable sacrifice of accuracy. We investigate the use of machine learning techniques for interpreting spectroscopic THz-TDS data by training with large data sets of simulated light-matter interactions, resulting in a computationally efficient artificial neural network for material parameter extraction. The trained model improves on the accuracy of analytical methods that need approximations while being easier to implement and faster to run than iterative root-finding methods. We envisage neural networks can alleviate many of the common hurdles involved in analyzing THz-TDS data such as phase unwrapping, time domain windowing, slow computation times, and extraction accuracy at the low frequency range.</p
Simulation of THz generation and propagation from photo-Dember emitters
No full textWe demonstrate a simulation of lateral photo-Dember terahertz (THz) emitters in two dimensions using the drift diffusion equation coupled to a finite-difference time-domain model with a far-field algorithm that can take into account the angular spectrum of emission and propagation. We show that the dynamics of the system are more dependent on the currents traveling perpendicular to the semiconductor surface, as opposed to just the lateral currents as previously thought. The 2D simulation is needed in order to correctly represent experiments that measure emission out of lateral photo-Dember emitters.</p
Optically Defined Reconfigurable THz Metasurfaces using Graphene on Iron‐Doped Lithium Niobate
No full textGraphene plasmonic devices have been demonstrated to show great potential for reconfigurable metasurfaces due to the tuneable electronic charge transport properties of graphene in response to electrostatic gating. Iron-doped lithium niobate is proposed as a platform for patterning-free optically reconfigurable graphene metasurfaces in the THz spectral region. Under structured illumination, the lithium niobate undergoes charge migration in the bulk, where carriers migrate away from illuminated regions, forming spatially patterned charge distributions capable of electrostatic tuning of graphene. These charge distributions are stable in the dark, however, can be redefined by subsequent illumination. Through the use of numerical simulations, it is demonstrated that optically defined charge distributions in lithium niobate can tune locally the graphene Fermi level allowing for plasmonic resonances at THz frequencies
Anomalous resonance frequency shift in liquid crystal-loaded metamaterials
No full textWe show that Babinet complementary patterns of metamaterials may not exhibit the same frequency tuning range when integrated with liquid crystals due to anisotropy of local fields and strong orientational optical nonlinearity of liquid crystals
THz spectroscopy of photogenerated carriers in Fe:LiNbO<sub>3</sub> for optical control of 2D materials
No full textWe report on significant photo-induced changes in the transmission of THz radiation through iron doped lithium niobate (Fe:LiNbO3). The effect is attributed to photo-excited charge carriers, originating from the Fe dopant energy levels in the band gap of the crystal. In previous work, we have demonstrated that Fe:LiNbO3 substrates allow control of graphene electronics and plasmonics through photo-induced electrostatic fields in the crystal [1]. THz time domain spectroscopy (TDS) measurements of Fe:LiNbO3 provide the means to calculate photoexcited charge carrier densities within the crystal, thereby providing further insight into Fe:LiNbO3 as a versatile platform for the transient photo-induced electrostatic doping of 2D materials
Diffused Ti:sapphire channel-waveguide lasers
No full textThe fabrication and operation of Ti:sapphire channel-waveguide lasers is presented, in which both the gain medium and the waveguide are formed by the thermal diffusion of titanium. Lasing was observed between wavelengths of 775 nm and 821 nm, with the lowest launched pump-power threshold being 210 ± 40 mW for a pump wavelength of 514.5 nm
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