Optical confinement factor

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. 

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. 

Shimizu J, Yamada H, Murata S, Tomita A, Kitamura M, Suzuki A (1991) Optical-confinement-factor dependencies of the K-factor, differential gain, and nonlinear gain coefficient for 1.55 im InGaAs/InGaAsP MQW and strained-MQW lasca. TEF.F. Photonics Technol Lett 3 773-776... 

Depending on the details of the structure, particularly on the confinement factor, the pump power density needed to achieve an effective optical gain of, say, 50 cm 1 is of the order of a few hundred kilowatts per square centimetre at room temperature [14-16],... 

Because the excitation intensity varies as the square of the distance from the focal plane, the probability of two-photon absorption outside the focal region falls off with the fourth power of the distance along the z optical axis. Excitation of fluorophores can occur only at the point of focus. Using an objective with a numerical aperture of 1.25 and an excitation beam at 780 nm, over 80% of total fluorescence intensity is confined to within 1 pm of the focal plane. The excitation volume is of the order of 0.1-1 femtoliter. Compared to conventional fluorometers, this represents a reduction by a factor of 1010 of the excitation volume. 

The DNA-carbon nanotube interaction is a complicated and dynamic process. Many studies on this subject have been pursued through a series of techniques, including molecular dynamic simulation, microscopy, circular dichroism, and optical spectroscopy.57,58 Although the detailed mechanism is not fully understood at present, several physical factors have been proposed to be driving DNA-carbon nanotube interactions,46,59-61 such as entropy loss due to confinement of the DNA backbone, van der Waals and hydrophobic (rr-stacking) interactions, electronic interactions between DNA and carbon nanotubes, and nanotube deformation. A recent UV optical spectroscopy study of the ssDNA-SWNT system demonstrated experimentally that... 

Itoh and Eujishima reported a sensitivity factor for the optical signal of 20-40 for a multimode structure and approximately 150 for a single-mode waveguide [27, 28]. These sensitivity factors are consistent with the weak confinement provided by a gradient-refractive index waveguide. Another relevant work was... 

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