Optical confinement

Boriskina, S.V., 2005, Symmetry, degeneracy and optical confinement of modes in coupled microdisk resonators and photonic crystal cavities, submitted to J. Quantum Electron. 

There is a complete set of unique modes for each polarization, parallel (TE) and orthogonal (TM) to the film surface. For channel waveguides with optical confinement in two dimensions (like a fiber), shown in Fig. 1, the modes are de-... 

Fig. 7.30. Photoluminescence spectra (2K) of PLD MgZnO-ZnO-MgZnO quantum well heterostructures on sapphire with nominal thickness of the ZnO quantum well of 25, 12, 6, and 3nm [53]. The blueshift of the excitonic peak combined with the intensity enhancement is a clear indication of optical confinement in the ZnO layer...

First successful ZnO device demonstrations as for example stable homo-and heteroepitaxial pn-junctions and LED structures, thin film scintillators, and quantum well structures with optical confinement, and oxide-based Bragg reflectors, and high-quality Schottky contacts are based on PLD grown thin films. Several techniques as for example the PLD in UHV conditions (laser MBE), and gradient and combinatorial PLD, and high-pressure PLD for nano-heterostructures show the innovative potential of the advanced growth technique PLD. 

In modem heterostructure lasers, the actual active layer is usually very thin, much thinner than the optical mode to be supported by its optical gain. Therefore, it is convenient to introduce the so-called optical confinement factor T, which relates the material optical gain gmlt to the net optical gain geff seen by the lasing mode [6] ... 

FIGURE 4(a), the relation between device diameter and threshold current is shown. In this estimation, the structure shown in FIGURE 4(b) has been assumed. The transverse optical confinement factor for a cylindrical waveguide with GaN core and AIN cladding was taken into account in this estimation [8,17]. If we can fabricate the device with a diameter less than 10 pm, a GaN-based VCSEL with sub-milliampere threshold currents can be expected. 

Optical confinement can be achieved in channel configurations. These onedimensional structures are obtained by evaporation of PZG glass, as described above, onto an etched ZBLAN substrate showing strips with 60-120 /im width [249]. Typically, PZG films of 3- m thickness are deposited within 20-min evaporation time. Depending on evaporation time and temperature, thickness from 0.5 to 100 fim can be achieved. Near-field measurements show that multimode light propagation is observed in the channel waveguide formed at the top of the strips, not in the valleys between the strips. 

The RT modal gain (Gmod) is calculated by using the relation where G ax is the peak gain of Fig. 3, Np is the number of "W" or "M" periods taken equal to 5 and Fp is the optical confinement factor per period. Results as a function of carrier concentration Nsd (cm ) are shown in Fig. 4. The 3D carrier concentration is deduced from N3D = N2D / Leff, where Leff is the thickness of one "W" or "M" cell. For a N3D equal to 1.5 10 cm , the modal gain exceeds 50 cm, showing that it would be possible to achieve RT lasing operation. 

Thickness The solar spectra absorption has to be maximised. About 50 pm thick film of crystalline silicon is necessary to absorb 80% of the solar spectra. If there is an efficient optical confinement, it can be reduced to 20 pm or even less... 

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