Photon recycling

Photon momentum., 14 834 Photon recycling, 14 846 Photon scanning tunneling microscopy (PSTM), 16 503... 

All expressions on radiative recombination presented in this text until this point denoted microscopic radiative lifetime, i.e., single acts of radiative recombination (a measure of the efficiency of photons escaping from the detector bulk). However, a photon emitted within a semiconductor sample during recombination may be absorbed again, which effectively increases total radiative lifetime [27]— the phenomenon of reabsorption or photon recycling [28, 29]. 

When considering photon recycling processes one can always neglect the component of radiative recombination caused by the photons, which do not leave the sample. This is because reabsorption processes are very fast, so that they do not contribute to the lifetime within the photodetector bandwidth. If the real part of the refractive index of semiconductor is a its absorption coefficient and c velocity of light, the photon lifetime will be avg) = n ac, i.e., in real situations (10 -10 ) s, several orders of magnitude smaller than carrier recombination lifetime. This means that reabsorption acts do not influence the noise level within the detector and only total reabsorption radiative lifetime is important for its performance. 

The photon management methods also represent a pathway toward a decrease of generation-recombination processes and the related noise phenomena and pose a viable way to overcome obstacles that until recently appeared insurmountable. The mechanism of photon recycling by cavity enhancement ensures direct suppression of radiative noise, while the possibility to localize fields at a subwavelength level ensures vastly smaller photodetector volumes, thus helping overcome problems with Auger and Shockley-Read phenomena as well. 

Since the stmctures and processes used for detector enhancement are interconnected, it often occurs that an approach introduced because of one effect also brings to an improvement of another one. A typical example can be found with methods for optical trapping that were introduced to improve the optical path through the detector (absorption coeflhcient-thickness product), to turn out that these methods not only improve radiative lifetime through photon recycling but even Auger and Shockley-Read lifetimes due to a decrease of overall dimensions of the detector. 

The second equihbrium group encompasses strucmres for increase in the optical path of the beam which already entered the active area of the detector, the so-called light trapping structures. These structures simultaneously increase radiative lifetime through the mechanism of reabsorption—photon recycling. They include different surface rehef stmctures for the increase in total internal reflection, from diflractive to macroscopic ones. Reflective detector surfaces also belong to this group, both the back-side ones and fuU resonant cavities (RCE—resonant cavity enhancement) with reflective surfaces both of the front and the back side of the detector. The most advanced stmctures for optical path and radiative time increase are radiative shields and photonic crystal enhancement stmcmres, which represent a fuU cavity enhancement and may support the existence of multiple modes. 

In steady-state PL, the shape of the spectrum is determined by the level of excitation intensity as the defect-related PL often saturates at power densities on the order of to 10 Wcm, and the overall PL spectrum may be skewed in favor of the excitonic emission at higher excitation densities. Similarly, focusing the laser beam and using small monochromator slit widths would also skew the PL in favor of excitonic transitions. In such a case, the chromatic dispersion of the lenses used to collect the PL, as well as the different effective sizes of the emission spots for the ultraviolet (UV) and visible emission attributed in particular to photon recycling process [24], may lead to a noticeable artificial enhancement of the UV (near band edge) over the visible part in the PL spectrum (mainly defect related). Qualitative terms such as "very intense PL attesting to the high quality of the material are omnipresent in the literature on ZnO. In contrast to the wide use of PL measurements, relatively little effort has been made to estimate the absolute value of the PL intensity or its quantum efficiency (QE) for a quantitative analysis. 

A laser consists in a medium where stimulated emission dominates over spontaneous emission placed inside an optical cavity which recycles the optical field. Above threshold, the photon number probability density distribution is poissonian, that means that the photon arrival time are a random variable. The probability of obtaining m photons during a given time interval is thus... 

Since the sensitizer Rose Bengal is recyclable, relatively high concentrations (5 10 M) can be used without raising cost issues and optical detection problems [21]. Under these conditions, it was shovm that molecules at any position in the micro channel have a similar photon flux. 

New natural polymers based on synthesis from renewable resources, improved recyclability based on retrosynthesis to reusable precursors, and molecular suicide switches to initiate biodegradation on demand are the exciting areas in polymer science. In the area of biomolecular materials, new materials for implants with improved durability and biocompatibility, light-harvesting materials based on biomimicry of photosynthetic systems, and biosensors for analysis and artificial enzymes for bioremediation will present the breakthrough opportunities. Finally, in the field of electronics and photonics, the new challenges are molecular switches, transistors, and other electronic components molecular photoad-dressable memory devices and ferroelectrics and ferromagnets based on nonmetals. 

The conjugated diene 173, irradiated at 350 nm, isomerized via a two-photon process, to give the spiro heterocycle 174 (Scheme 37). The reaction, carried out in pure acetone, produced the spiro 174 in 52% yield but polymer formation on the wall of the reaction vessel was also evident. This tendency was significantly reduced upon dilution with MeCN (acetone/MeCN 2 1) the photoisomerization was slower with lower yield (42%) but the starting material was easily recycled <20040L1313>. 

Some new luminescent and fluorescent reporters (some of them even non-substrate proteins ) are very attractive because of their easy and fast detection, explaining their current frequent use. The bacterial luciferase isolated from the Vibrio fischeri lux operon contains luxAB encoding the functional subunits and luxCDE for the synthesis and recycling of the aldehyde substrate (Prosser, 1996). Firefly (Photinus pyralis) luciferase, encoded by the luc gene catalyses the oxidative carboxylation of beetle luciferin, in which photons are emitted (LaRossa, 1998). Its short half-life and lack of any post-translational modification makes it ideal to look after effects in gene expression (Naylor, 1999). Detection of... 

A quantum-beat laser with external coherent driving [67,78]. Shown in Fig. 13 is a pumping and coupling scheme. The atoms are pumped from the ground state 10 ) to the excited states (/=1,2) to provide necessary population for the laser gain. An external coherent field of circular frequency coq is applied to the 2 )- 3 ) transitions to create atomic coherence, by which the system can operate without population inversion. Atoms emit photons into the laser modes o of circular frequencies a 12. In the dynamics, the atoms recycle throngh the snccessive channels... 

Chelnokov A, Wang K, Rowson S, Garoche P, Lourtioz J-M (2000) Near-inlrared Yablonovite-like photonic crystals by focused-ion-beam etching of macroporous silicon. Appl Phys Lett 77 2943 Chen X, Steinhart M, Hess C, Gosele U (2006) Ordered arrays of mesoporous microrods Ifom recyclable macroporous silicon templates. Adv Mater 18 2153-2156... 

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