We present a study on the design, growth and optical characterization of a GaN/AlGaN microcavity for the enhancement
of second order non linear effects. The proposed system exploits the high second order nonlinear optical response of
GaN due to the non centrosymmetric crystalline structure of this material. It consists of a GaN cavity embedded between
two GaN/AlGaN Distributed Bragg Reflectors designed for a reference mode coincident with a second harmonic field
generated in the near UV region (~ 400 nm). Critical issues for this target are the crystalline quality of the material,
together with sharp and abrupt interfaces among the multi-stacked layers. A detailed investigation on the growth
evolution of GaN and AlGaN epilayers in such a configuration is reported, with the aim to obtain high quality factor in
the desiderated spectral range. Non linear second harmonic generation experiments have been performed and the results
were compared with bulk GaN sample, highlighting the effect of the microcavity on the non linear optical response of
In this paper we present a reliable process to fabricate GaN/AlGaN one dimensional photonic crystal (1D-PhC)
microcavities with nonlinear optical properties. We used a heterostructure with a GaN layer embedded between two
AlGaN/GaN Distributed Bragg Reflectors on sapphire substrate, designed to generate a λ= 800 nm frequency downconverted
signal (χ(2) effect) from an incident pump signal at λ= 400 nm. The heterostructure was epitaxially grown by
metal organic chemical vapour deposition (MOCVD) and integrates a properly designed 1D-PhC grating, which
amplifies the signal by exploiting the double effect of cavity resonance and non linear GaN enhancement. The integrated
1D-PhC microcavity was fabricate combing a high resolution e-beam writing with a deep etching technique. For the
pattern transfer we used ~ 170 nm layer Cr metal etch mask obtained by means of high quality lift-off technique based
on the use of bi-layer resist (PMMA/MMA). At the same time, plasma conditions have been optimized in order to
achieve deeply etched structures (depth over 1 micron) with a good verticality of the sidewalls (very close to 90°).
Gratings with well controlled sizes (periods of 150 nm, 230 nm and 400 nm respectively) were achieved after the pattern
is transferred to the GaN/AlGaN heterostructure.
Theoretical analysis on second harmonic (SH) generation with phase matched grating in waveguide is presented from
the viewpoint of device design. Usually high intensity sources are necessary in order to observe a SH in a χ(2) nonlinear
structure. For this purpose, the novel proposed design takes into account a double grating effect which enhances the
guided SH signal along the waveguide. In the presented structure two grating are considered: the first grating,
considered at the interface between air and core, is designed in order to obtain an efficient SH conversion process by
considering the quasi phase matching (QPM) condition; the second grating, placed at the interface between the core and
the substrate region, increases the SH power along the propagation direction through the coupling with the substrate
modes generated by the diffraction effect. The novelty of this work is in the combined effect of the two gratings. The
grating lengths and periods are designed by considering the nonlinear coupled mode theory with the effective dielectric
constant (EDC) assumption. The analysis includes three dimensional (3D) cases where phase matching is involved, in
particular the model is applied to a GaAs/AlGAs waveguides with fundamental wavelength at λFU=1.55 μm and SH
signal at λSH =0.775 μm.
We present in this work the scalar potential formulation of second harmonic generation process in χ(2) nonlinear
analysis. This approach is intrinsically well suited to the application of the concept of circuit analysis and synthesis to
nonlinear optical problems, and represents a novel alternative method in the analysis of nonlinear optical waveguide, by
providing a good convergent numerical solution. The time domain modeling is applied to nonlinear waveguide with
dielectric discontinuities in the hypothesis of quasi phase matching condition in order to evaluate the conversion
efficiency of the second harmonic signal. With the introduction of the presented rigorous time domain method it is
possible to represent the physical phenomena such as light propagation and second harmonic generation process inside a
nonlinear optical device with a good convergent solution and low computational cost. Moreover, this powerful approach
minimizes the numerical error of the second derivatives of the Helmholtz wave equation through the generator
modeling. The novel simulation algorithm is based on nonlinear wave equations associated to the circuital approach
which considers the time-domain wave propagating in nonlinear transmission lines. The transmission lines represent the
propagating modes of the nonlinear optical waveguide. The application of quasi phase matching in high efficiency
second harmonic generation process is analyzed in this work. In particular we model the χ(2) non linear process in an
asymmetrical GaAs slab waveguide with nonlinear core and dielectric discontinuities: in the nonlinear planar
waveguides a fundamental mode at λ=1.55 μm is coupled to a second-harmonic mode (λ=0.775 μm) through an
appropriate nonlinear susceptibility coefficient. The novel method is also applied to three dimensional structures such as
We analyze in this work the second harmonic amplification of χ(2) nonlinear process in membrane type GaAs circular
photonic crystal. This unconventional kind of photonic crystal is well suited for the generation of whispering gallery
modes due to the circular symmetric periodic pattern. The Gaussian beam of a fundamental pump signal at 1.55 μm
defines a whispering gallery mode resonance and generates a second harmonic mode at 0.775 μm in the central missing
hole micro-cavity. The periodic pattern and the micro-cavity are tailored and optimized in order to generate a second
harmonic conversion efficiency of 50 %. We predict the resonances by an accurate 2D time domain model including χ(2)
nonlinearity and also by a 3D Finite Element Method FEM. Moreover, by using a 3D membrane configuration, we
predict a quality factor of the second harmonic mode of the order of 35000.
This work presents a detailed numerical Finite Element Method FEM modeling for passive optical components such as
photonic crystals (PhCs). The accurate modeling characterizes the PhCs structures by considering the field resonance
and the radiation behavior of the periodic pattern. The frequency responses at each side of the photonic crystal are
evaluated by considering the 3D periodic structure enclosed in a black box with six input/output ports. This scattering
matrix approach (SMA) is useful in order to evaluate in plane and vertical PhCs the resonance of the photonic crystal.
Through the analysis of all the frequency responses we characterize the passband regions and the stopband regions of
the PhC slab.
In this contribution, we measure and compare the polarization properties of 3-D photonic crystal fibers and of a classic single-mode polarization maintaining fiber. The polarization-maintaining property depends on the cross-section geometry of the fiber and is therefore related to the strong intrinsic birefringence and to the well-defined principal axis of the fiber.
This work analyzes a new efficient numerical algorithm for evaluating the time-domain electromagnetic (EM) field and the filtering behaviour of periodic waveguides, by reducing the number of equations to solve. The scalar Helmholtz-equation is utilized in order to determine the electric and magnetic Hertzian-potentials that represent the EM field. The principle of the method is demonstrated by a simple 2D application to a multilayer dielectric stack at optical frequencies.
In this contribution we present the accurate analysis and modeling of periodic optical structures that are finding wide application in photonics. The EM analysis is performed by two different 3D full-wave methods, the Transmission Line Matrix-Integral Equation (TLMIE) and the Generalized Transverse Resonance Diffraction (GTRD). TLMIE is a 3D full-wave hybrid technique in the time-domain which combines the advantages of the numerical TLM method and those of the analytical Green's functions representation for the free-space region, thus providing exact boundary conditions at optical frequencies. In GTRD the dyadic Green's function of a loaded box is used for the modeling of the layered structure, combined with Ohm's law formulation of the volume currents. By using the pre- and post-processing tools of TLMIE and GTRD methods, we investigate the dynamic of the EM field in and outside the structure and evaluate the frequency response of the laminated polarizer behaving as a negative uniaxial crystals. The calculated S-parameters are compared with measured data showing good agreement.
This paper considers the application of the Generalized Telegrapher’s Equations (GTE) to the electromagnetic modelling and designing of integrated electro-optical devices. This approach allows to eliminate the restrictions introduced by others models, as: weakly guiding condition and isotropic unperturbed medium and to value the modulator time response for a generic modulating signal. The presence of small dielectric perturbations and the large difference between the optical and modulating signal frequencies are the hypotheses considered in deriving the model. The analysis has been applied to a GaAs phase modulator in order to validate the equations and to evidence the effects of the induced anisotropy on the time-domain response. The model can be extended to the analysis of multimode dielectric waveguides, such as independent polarization and directional coupler modulator.