Distributed Bragg reflectors

Eig. 3. Depiction of the light extraction, ie, escape cones of light emission, for various LED chip stmctures consisting of absorbing substrate devices having (a) thin window layers (top cone) (b) thick window layers (top cone and four one-half side cones) (c) thin window plus the implementation of a distributed Bragg reflector between the active layer and the substrate (top and bottom cone). Also shown is (d), the optimal stmcture for light extraction, a... 

We consider a radially symmetric structure as illustrated in Fig. 12.2. The guiding defect, consisting of a material of refractive index ndefect, is surrounded by distributed Bragg reflectors on both sides, where the reflectors layers are of refractive indices ni and n2. All the electromagnetic field components can be expressed in terms of the z-component of the electric and magnetic fields14. These components satisfy the scalar Helmholtz equation, which in cylindrical coordinates is given by ... 

To demonstrate the method an example of a slow-wave optical structure is modelled. Such structures consist of a cascade of directly coupled optical resonators in order to enhance the nonlinear effects. The structure used here was recently defined within Working Group 2 of the European Action COST Pll (http //w3.uniromal.it/energetica/slow waves.doc). One period of the structure consists of one-dimensional Fabry-Perot cavity placed between two distributed Bragg reflectors (DBR) and can be described by the sequence... 

Figure 10-12. Left hand side Structure of a PPV microcavity. A thin film of the conjugated polymer is deposited on top of a highly reflective distributed Bragg reflector (DBR). The second mirror is then fabricated by evaporation of a silver layer. Right hand side Emission spectra of the microcavity at excitation energies of 0.05 pJ (dashed hne) and 1.1 pJ (solid line), respectively. Laser pulses of duration 200-300 ps and a wavelength of 355 nm were used for optical excitation (according to Ref. [39]).

In planar cavities, the optical modes are discrete and the frequencies of these modes are integer multiples of the fundamental mode frequency, as shown schematically in Fig. 1.7. The fundamental and first excited mode occur at frequencies of vQ and 2v0, respectively. For a cavity with two metallic reflectors (no distributed Bragg reflectors) and a jt phase shift of the optical wave upon reflection, the fundamental frequency is given by v0 = c / 2nlcaw where c is the velocity of light in vacuum and Icav is the length of the cavity. In a resonant cavity, the emission frequency of an optically active medium located inside the cavity equals the frequency of one of the cavity modes. 

Chiou S. W., Lee C. P., Huang C. K., Chen C. W. Wide-angle distributed Bragg reflectors for 590 nm amber AlGalnP light-emitting diodes J. Appl. Phys. 87, 2052 (2000). 

Modem design of efficient surface emitting semiconductor lasers implies monolithic solid state stmctures with an active layer and periodic multilayer stacks comprising Distributed Bragg Reflectors (DBR). The latter provides reflection band at the emission wavelength due to multiple reflection/interference in a complex medium with periodically graded refraction index of the layers [1], The larger is the refraction index difference A n = U/ - between a couple of materials chosen to... 

Microcavity OLEDs fabricated on distributed Bragg reflectors (quarter wave stacks) [94]. Such OLEDs are fabricated on dielectric layers with significant dielectric contrast, so they narrow the emission spectrum by constructive interference. The narrow emission spectra also result in more efficient and more stable devices than regular OLEDs. The emission spectrum can be tailored to the specific sensor requirements (i.e., the absorption peak of the sensing element). 

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