Simplified analytical models with predictive capability enable simpler and faster optimization of the performance in applications of complex photonic devices. We recently demonstrated the most simplified analytical model still showing predictive capability for parallel-aligned liquid crystal on silicon (PA-LCoS) devices, which provides the voltage-dependent retardance for a very wide range of incidence angles and any wavelength in the visible. We further show that the proposed model is not only phenomenological but also physically meaningful, since two of its parameters provide the correct values for important internal properties of these devices related to the birefringence, cell gap, and director profile. Therefore, the proposed model can be used as a means to inspect internal physical properties of the cell. As an innovation, we also show the applicability of the split-field finite-difference time-domain (SF-FDTD) technique for phase-shift and retardance evaluation of PA-LCoS devices under oblique incidence. As a simplified model for PA-LCoS devices, we also consider the exact description of homogeneous birefringent slabs. However, we show that, despite its higher degree of simplification, the proposed model is more robust, providing unambiguous and physically meaningful solutions when fitting its parameters.
Recently we demonstrated a novel and simplified model enabling to calculate the voltage dependent retardance provided
by parallel aligned liquid crystal devices (PA-LCoS) for a very wide range of incidence angles and any wavelength in the
visible. To our knowledge it represents the most simplified approach still showing predictive capability. Deeper insight
into the physics behind the simplified model is necessary to understand if the parameters in the model are physically
meaningful. Since the PA-LCoS is a black-box where we do not have information about the physical parameters of the
device, we cannot perform this kind of analysis using the experimental retardance measurements. In this work we
develop realistic simulations for the non-linear tilt of the liquid crystal director across the thickness of the liquid crystal
layer in the PA devices. We consider these profiles to have a sine-like shape, which is a good approximation for typical
ranges of applied voltage in commercial PA-LCoS microdisplays. For these simulations we develop a rigorous method
based on the split-field finite difference time domain (SF-FDTD) technique which provides realistic retardance values.
These values are used as the experimental measurements to which the simplified model is fitted. From this analysis we
learn that the simplified model is very robust, providing unambiguous solutions when fitting its parameters. We also
learn that two of the parameters in the model are physically meaningful, proving a useful reverse-engineering approach,
with predictive capability, to probe into internal characteristics of the PA-LCoS device.
The concentration dependence of amplified spontaneous emission (ASE) in optically pumped polystyrene (PS) films containing a variable concentration (between 2.5 and 100 % by weight (wt)) of the luminescent and hole-transporting organic molecule N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) is studied. It is observed that the photoluminiscence (PL) efficiency, the ASE threshold and the linewidth above threshold, decrease with concentration up to 20 wt % doped films and then keeps a constant value up to concentrations of 100 wt % (neat films). The position of ASE can be tuned between 413nm and 421 nm by changing the concentration of TPD. It was also possible to tune the ASE position (between 404 and 417 nm) trough control of the film thickness (between 100 and 200 nm). The observed shifts of the ASE position due to changes in concentration are determined by the PL efficiency (not by the waveguiding characteristics of the films). On the other hand, the observed shifts in the ASE position due to changes in film thickness depend on the shape of the PL spectrum and on cut-off thickness limitations. In this case, the ASE thresholds depend on the different confinement of the propagation modes due to thickness variations.