In-plane Losses Research Articles

Resonator embedded photonic crystal surface emitting lasers

The finite size of 2D photonic crystals results in them being a lossy resonator, with the normally emitting modes of conventional photonic crystal surface emitting lasers (PCSELs) differing in photon lifetime via their different radiative rates, and the different in-plane losses of higher order spatial modes. As a consequence, the fundamental spatial mode (lowest in-plane loss) with lowest out-of-plane scattering is the primary lasing mode. For electrically driven PCSELs, as current is increased, incomplete gain clamping results in additional spatial (and spectral) modes leading to a reduction in beam quality. A number of approaches have been discussed to enhance the area (power) scalability of epitaxy regrown PCSELs through careful design of the photonic crystal atom1–3. None of these approaches tackle the inflexibility in being unable to independently modify the photon lifetime of the different modes at the Γ2 point. As a method to introduce design flexibility, resonator embedded photonic crystal surface emitting lasers (REPCSELs) are introduced. This device, combining comparatively low coupling strength photonic crystal structures along with perimeter mirrors, allow a Fabry–Pérot resonance effect to be realised that provides wavelength selective modification of the photon lifetime. We show that surface emission of different surface emitting modes may be selectively enhanced, effectively changing the character of the modes at the Γ2 point. This is a consequence of the selective modification of in-plane loss for particular modes, and is dependent upon the alignment of the photonic crystal (PhC) band-structure and distributed Bragg reflectors’ (DBRs) reflectance spectrum. These findings offer new avenues in surface emitting laser diode engineering. The use of DBRs to reduce the lateral size of a PCSEL opens the route to small, low threshold current (Ith), high output efficiency epitaxy regrown PCSELs for high-speed communication and power sensitive sensing applications.

Coherent power scaling in photonic crystal surface emitting laser arrays

A key benefit of photonic crystal surface emitting lasers (PCSELs) is the ability to increase output power through scaling the emission area while maintaining high quality single mode emission, allowing them to close the brightness gap which exists between semiconductor lasers and gas and fiber lasers. However, there are practical limits to the size, and hence power, of an individual PCSEL device, and there are trade-offs between single-mode stability and parasitic in-plane losses with increasing device size. In this paper, we discuss 2D coherent arrays as an approach to area and coherent power scaling of PCSELs. We demonstrate in two and three element PCSEL arrays an increase in the differential efficiency of the system due to a reduction in in-plane loss.

Compact photonic crystal disk cavity optimized using the gentle confinement method and boundary design

A compact, novel photonic crystal cavity aimed at applications with strict area limitations is presented. Optimization shows that the gentle confinement method previously used for line-defect cavities can be applied to more limited geometries. It also shows that it is paramount to consider the boundary region to minimize in-plane losses. The investigation show that a near optimum boundary thickness can be found by considering the boundary region as a Fabry–Pérot resonator. This optimization strategy is shown to be deterministic in terms of resonance wavelength. For an optimized air-clad, silicon cavity, finite-difference time-domain simulations give Q-values as high as 75,000 which is comparable to other photonic crystal cavities of similar size.

Three-dimensional coupled-wave analysis for triangular-lattice photonic-crystal surface-emitting lasers with transverse-electric polarization

Three-dimensional coupled-wave theory is extended to model triangular-lattice photonic-crystal surface-emitting lasers with transverse-electric polarization. A generalized coupled-wave equation is derived to describe the sixfold symmetry of the eigenmodes in a triangular lattice. The extended theory includes the effects of both surface radiation and in-plane losses in a finite-size laser structure. Modal properties of interest including the band structure, radiation constant, threshold gain, field intensity profile, and far-field pattern (FFP) are calculated. The calculated band structure and FFP, as well as the predicted lasing mode, agree well with experimental observations. The effect of air-hole size on mode selection is also studied and confirmed by experiment.

An electrode-free method of characterizing the microwave dielectric properties of high-permittivity thin films

A thin dielectric resonator consisting of a dielectric substrate and the thin film deposited upon it is shown to suffice for microwave characterization and dielectric parameter measurement of high-permittivity thin films without electrodes. The TE01δ resonance mode was excited and measured in thin (down to 0.1 mm) rectangular- or disk-shaped low-loss dielectric substrates (D∼10 mm) with permittivity ε′≥10 inserted into a cylindrical shielding cavity or rectangular waveguide. The in-plane dielectric permittivity and losses of alumina, DyScO3, SmScO3, and (LaAlO3)0.29(SrAl1/2Ta1/2O3)0.71 (LSAT) substrates were measured from 10 to 18 GHz. The substrate thickness optimal for characterization of the overlying thin film was determined as a function of the substrate permittivity. The high sensitivity and efficiency of the method, i.e., of a thin dielectric resonator to the dielectric parameters of an overlying film, was demonstrated by characterizing ultrathin strained EuTiO3 films. A 22 nm thick EuTiO3 film grown on a (100) LSAT substrate and strained in biaxial compression by 0.9% exhibited an increase in microwave permittivity at low temperatures consistent with it being an incipient ferroelectric; no strain-induced ferroelectric phase transition was seen. In contrast, a 100 nm thick EuTiO3 film grown on a (110) DyScO3 substrate and strained in biaxial tension by 1% showed two peaks as a function of temperature in microwave permittivity and loss. These peaks correspond to a strain-induced ferroelectric phase transition near 250 K and to domain wall motion.

Current state-of-the-art of pulsed laser deposition of optical waveguide structures: Existing capabilities and future trends

Pulsed laser deposition (PLD) has now reached a stage of maturity where the growth of thin films is routine. All that is required is a pulsed ultra-violet (UV) wavelength laser, a vacuum chamber, a target, and a substrate placed in near proximity to the plasma plume. Whether the film that you grow is the film that you need, and whether the thickness, uniformity, optical quality, stoichiometry, degree of crystallinity, orientation and much more is what is desired is another question entirely. PLD is both a science and an art and there are many tricks-of-the-trade that need to be considered to ensure that materials grown are the materials wanted. This paper discusses the practicalities of PLD systems, target geometries, heating regimes for successful epitaxial growth of crystalline films, the problem of particulates, laser sources to use, and in the context of our most recent PLD system, the number of independent lasers and targets used. We show that the use of multiple targets permits a combinatorial approach, whereby stoichiometry can be adjusted to grow designer materials, and in particular multilayer systems, ideally suited for active optical waveguides, a truly demanding end application where optical quality and in-plane losses must be reduced to an absolute minimum.

Photonic crystal heterostructure cavity lasers using kagome lattices

The lasing characteristics of a photonic crystal membrane heterostructure cavity that utilises a kagome type lattice is demonstrated. A heterostucture cavity is formed by interfacing two photonic crystals such that the dispersion maximum of the inner lattice falls within the photonic bandgap of the surrounding lattice. Feedback to slow Bloch modes allows for localisation of band edge modes and a reduction of in-plane losses. Singlemode lasing is observed in relatively large area lasers at 1550 nm.

Self-collimating photonic crystal polarization beam splitter

We present theoretical and experimental results of a polarization splitter device that consists of a photonic crystal (PhC) slab, which exhibits a large reflection coefficient for TE and a high transmission coefficient for TM polarization. The slab is embedded in a PhC tile operating in the self-collimation mode. Embedding the polarization-discriminating slab in a PhC with identical lattice symmetry suppresses the in-plane diffraction losses at the PhC-non-PhC interface. The optimization of the PhC-non-PhC interface is thereby decoupled from the optimization of the polarizing function. Transmissions as high as 35% for TM- and 30% for TE-polarized light are reported.

Silica glass bend waveguide assisted by two-dimensional photonic crystals

In this paper we report on the modeling of low index contrast silica glass 90° bend ridge waveguides assisted by a two-dimensional photonic crystal. A three-dimensional finite-difference time-domain (3D-FDTD) based computer code has been used in order to evaluate the transmission characteristics and the in-plane losses of the investigated waveguides having different values of the bend radius. The performance of the bend structure surrounded by two-dimensional photonic crystals is compared to that of a classical bend ridge waveguide and the phenomenon of light confinement is critically analyzed. The device design is optimized for quasi-TM polarization at the wavelength of 1.3 μm.

In-plane light propagation in Ta/sub 2/O/sub 5//SiO/sub 2/ autocloned photonic crystals

We investigate the characteristics of in-plane light propagation in 2-D photonic crystals fabricated by the autocloning method, by which multiple layers are stacked while preserving periodic surface corrugation. A numerical analysis shows that Ta/sub 2/O/sub 5//SiO/sub 2/ autocloned photonic crystals are applicable to waveguides and lenses since a sufficient amount of change of the effective refractive index is obtained by adjusting the aspect ratio of the unit structure. It is also shown that the crystals have a wide bandgap applicable to Bragg reflectors and resonators. Experimental results reveal that in-plane propagation losses for the two directions are less than 1.4 dB/mm at /spl lambda/=1.55 /spl mu/m, showing the crystals are useful for low loss in-plane devices.

Losses resulting from in-plane electricity conduction in tubular solid oxide fuel cells

A mathematical model has been developed that describes the potential distribution along the length of a tubular fuel cell. Insight into this distribution is essential because, as opposed to the planar fuel cell, the electricity has to be transported over a relatively long distance before connecting to the next cell. The model is based on an electrical network representation of the fuel cell and consists of resistors parallel and perpendicular to the plane cross section of the cell. The first result from the electrochemical process as would take place in any fuel cell. The second constitute losses resulting from electricity transport along the electrodes. The model leads to a differential equation, which is solved numerically. The modelling results show that in order to achieve high outputs there is a requirement to reduce electrolyte wall thickness and to limit the active length due to in-plane losses

In-plane magnetization and hysteresis losses in YBCO thick films

We present our observations of hysteretic magnetization of c-axis oriented thick (1-2 /spl mu/m) YBCO films in parallel fields. For other orientations of the field, the magnetization is reported to be mainly due to the c-axis component (M/sub c/sin/spl theta/) because of a large aspect ratio of the film. But our observations at low fields (/spl les/100 mT) did not follow such behavior. Moreover, the in-plane magnetization is affected by an artifact introduced during the deposition of such thick films. We discuss our results in the framework of Bean's model after removing the artifact.

A contribution to the in-plane stability of shallow arches

Abstract Practical design rules to cover in-plane instability losses of shallow steel arches are derived in this paper. Such guidelines do not yet exist in design codes. The recommendations in the form of limiting-slenderness ratios will allow such arches to be analysed and designed by simple rigid-plastic theory. Some numerical examples and experimental work are described. This paper is a sequel to a recent publication covering snap-through of pitched-roof steel frames.