Thermal bistability

Using (5.14) and the determined value of aabs, we can estimate 8 if ab is known, and vice versa. Two examples of thermal bistability data, fit to a calculated tuning curve based on (5.12), are shown below. Figure 5.7 is for the bare sphere, and Fig. 5.8 is for the PDDA-coated sphere. In the figures, the laser scans slowly across a TM-polarized WGM dip (taking several thermal relaxation times to scan Av), first down in frequency, then reversing at the vertical dashed line, and scanning back up in frequency across the same mode. The continuous smooth lines are the theoretical fits. 

Fig. 5.7 WGM resonance dip showing thermal bistability. For this undercoupled mode in the bare sphere with Q 3.14 x 107, the fit gives aabs 0.00419 m Reprinted from Ref. 5 with permission. 2008 International Society for Optical Engineering...

The sensing methods summarized thus far are intended for absorption detection of molecules in the ambient, but molecules (or indeed thin films) on the microresonator surface can also be detected. In particular, if the surface is covered to such an extent that the optical energy absorbed heats the microresonator, the resulting thermal bistability in the frequency-scan response can be used to determine the absorption and/or thickness of the thin-film coating. This and surface characterization by measurement of the thermal accommodation coefficient were described in Sect. 5.5. These methods offer quite precise measurement, provided that certain reasonable and easily implemented assumptions are satisfied. 

Rokhsari, H. Spillane, S. M. Vahala, K. J., Loss characterization in microcavities using the thermal bistability effect, Appl. Phys. Lett. 2004, 85, 3029 3031... 

Recently R.Burzynski et a1[10] reported ultra-high speed electronic bistabilities in polyamic acid waveguide with grating excitation. They classified electronic or thermal bistabilities usi-... 

Several other light-induced phenomena associated with spin transition systems have recently been reported. These include light induced thermal hysteresis (LITH), which is another example of light induced bistability, discovered for the SCO compound [Fe(PMBiA)2(NCS)2] which undergoes a very abrupt thermal ST around 170 K with hysteresis [174]. Irradiation of the sample at 10 Kwith green light resulted in the population of the LIESST state. When the temperature was raised to 100 K and lowered back to 10 K under continuous irradiation a wide thermal hysteresis loop resulted. The... 

Thermally or photochemically induced proton transfers represent bistable switching processes and are of interest for information storage. A lateral transfer of information on the surface of biological membranes is thought to occur by fast proton conduction through protonic networks [8.234]. 

Switching also implies molecular and supramolecular bistability since it resides in the reversible interconversion of a molecular species or supramolecular system between two thermally stable states by sweeping a given external stimulus or field. Bistability in isolated molecules or supermolecules is, for instance, found in optical systems such as photochromic [8.229] or thermochromic substances or devices, in electron transfer or magnetic processes [8.239], in the internal transfer of a bound substrate between the two binding sites of a ditopic receptor (see Section 4.1 see also Fig. 33) [6.77]. Bistability of polymolecular systems is of a supramolecular nature as in a phase transition or a spin transition, both of which involve an assembly of interacting species. 

Positional changes of atoms in a molecule or supermolecule correspond on the molecular scale to mechanical processes at the macroscopic level. One may therefore imagine the engineering of molecular machines that would be thermally, photochem-ically or electrochemically activated [1.7,1.9,8.3,8.109,8.278]. Mechanical switching processes consist of the reversible conversion of a bistable (or multistable) entity between two (or more) structurally or conformationally different states. Hindered internal rotation, configurational changes (for instance, cis-trans isomerization in azobenzene derivatives), intercomponent reorientations in supramolecular species (see Section 4.5) embody mechanical aspects of molecular behaviour. 

However, in many cases the fluctuating systems of interest are far from thermal equilibrium. Examples include optical bistable devices [45], lasers [23,46], pattern forming systems [47], trapped electrons that display bistability and switching in a strong periodic field [48-50], and spatially periodic systems (ratchets) that display a unidirectional current when driven away from thermal equilibrium [51-56]. 

In this report, vacuum evaporated PDA(12-8) film is used as an optically nonlinear layer with a grating coupler for nonlinear coupling for all optical bistability. Grating coupler on a substrate was prepared at the same periodicity and depth as the SHG devices. Vacuum evaporation of PDA on a substrate with previously rf-sputtered Corning 7059 buffer layer film were carried out at 5 x 10 5 torr with tungsten boat heater. Rapid evaporation can avoid thermal polymerization of the undesirable red phase PDA during the process. UV polymerization of the film for the useful blue phase PDA was carried out by Xe lamp 500 w for 20 min. at a... 

Also thermal process governs the phenomenon for longer pulse operation. We used two pulse widths, 200 ns and 20 ns. It is said the former corresponds to thermal process and the latter is mixture of electronic- and thermal-process[13]. In any way vacuum evaporated PDA is randomly oriented and more oriented polymer is desired together with shorter pulse excitation to analize basic process of this type of bistability. 

All-optical bistability angular dependence, 324 experimental setup, 321,323/ output-input curves, 321,324,325/ thermal effect, 324 All-optical guided wave devices examples, 124,125/126... 

We show that application of a constant force (bias held) results in shifting the position of the ordinary SR peak together with the anticipated reduction of its height and sharpness. For the quadratic SR the situation is more complicated. There, the joint action of the thermal noise and constant bias leads to formation of a mountain-like surface over the plane of those parameters. In other words, for each given value of the bias held there exists a unique value of the noise strength that maximizes SNR and vice versa. The discovered effect can be useful, for example, for evaluation of the parameters of bistable systems through susceptibility measurements. In addition, it has to be taken into account when designing any devices where the nonlinear SR is employed. 

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