The Luminescent Glasses

7.4.4. The Luminescent Glasses

The colour of glasses just studied show the phenomenon of absorption in the visible spectra. The colours due to the “fluoroscence” is shown by the electronic transition with an emission of a photon in the visible. One atom brought to an excited state by the absorption of a photon returns to its fundamental

NANO MATERIALS

level with the emission of light → either immediately, i.e. fluoroscence, or after a significant delay, i.e. phosphoroscence.

The centres of fluoroscence in the glasses can be either the metallic atoms in the “nano range” (e.g. Ag atom), or the crystalline phases (CdS), or certain ions → the most important of which are rare earth ions, which intervene in the amplification of coherent (stimulated) light, i.e. “The Lasers”.

The synthesis of novel ‘metal nano-particles’ and their application to nano-optical materials were explored by Shiraishi et al [11-13]. The metal nano particles stabilized by organic molecules are now creating a new class of materials that are different from both conventional bulk materials and the atoms, giving one of the smallest building blocks of matter. The stabilizers play important roles in not only protecting the metal nano-particles, but also controlling the properties for optical functions.

The ‘stimuli-responsive’ colour change of colloidal dispersions of Au nano particles as an appli- cation as sensors : The gold nano particles protected by 3-mercaptopropionic acid were prepared by reducing tetrachloroauric acid in the presence of 3-mercaptopropionic acid. The colour of the disper- sions changed from red to purple by adding hydrochloric acid, and changed back from purple to red with the addition of an aqueous sodium hydroxide solution. The change responsive to pH is reversible even after 5 repetitions. On the other hand, the colour of colloidal dispersion of Au nano particles stabilized by poly(beta-cyclodextrin) (PCyD) changed from red to purple with the addition of mercaptocarboxylic acid, suggesting the inclusion complex formation of the PCyD protecting Au nano particles with mercaptocarboxylic acid.

The electro-optic properties of ‘liquid crystalline display’ (LCD) system with the nano particles of palladium (Pd), which are protected by 'liquid crystalline molecules', was also explored by Shiraishi et al [13] :

This study is aimed at synthesizing liquid-crystalline (LC) molecule-protected metal nano parti- cles and developing novel liquid crystal display (LCD) materials. 4-Cyano-4′-pentylbiphenyl-covered palladium (5CB-Pd) nano-particles were prepared by UV irradiation of a tetrahydrofuran solution of palladium(II) acetate in the presence of 5CB. The nano particles of 5CB-Pd have higher solubility than fullerene in the liquid-crystalline medium. The twisted nematic LCD cell with Pd nano particles showed

a frequency modulation (FM) response even under low voltages. The response of this FM-LCD is 10 times faster than that of the conventional device. This approach can be extended to other modes of LCDs.

7.4.4.1. The Laser Glasses

A solid Laser is a ‘luminescent material’ wherein the light emitted by the fluoroscence from one of the “centres” stimulates the other ‘centres’ on its own in order to provoke the emission of light in phase with that of the first “centre” and in the same direction.

In order to obtain such a stimulated emission, it is necessary to provoke an “inversion of popula- tion”, i.e. to create a situation wherein the species in the excited state are more in numbers than that in the fundamental or lowest energy state, so that the ‘population inversion’ can take place. By limiting to the case of only excitation by photon, which is called optical pumping, it can be shown that it is neces- sary that the “excitable” ions dispose at least “three” energy levels, as shown in Figure 7.2.

OPTICAL PROPERTIES

3′ Q′

Q ′′

(A)

(B)

Figure 7.2 : Simplified Diagram of the Energy Levels for a Laser System (3 and 4 Levels).

This excitation, i.e. the optical pumping, elevates the atoms to the level 3 (or 3′′′′′), wherein they have a chance to return either to the fundamental level with emission of a photon, or to move to an intermediate level 2, by a non-radiative transition. This level 2 of fluoroscence is of fundamental impor- tance to understand the mechanism of Laser.

As the atoms return from the level 2 to the base level 1, it emits a light of the same wavelength as the ‘atom’ which has originally stimulated this transition, and this ‘atom’ in turn stimulates another transition and so forth. Hence, the process continues. In the absence of the level 2 one could only have an equalisation of population density between the levels 3 and 1.

Some systems have four levels. The effect of Laser is produced between the levels 2 and a level

1 above the fundamental level 0. The excitation is generally produced by an ‘external lamp’, which emits a light, which is absorbed by the ‘excitable’ ions.

In actual situation, the active solid is placed between two ‘mirrors’ with the reflectivities as : R 1 = 100% and R 2 < 100%. The light emitted between the mirrors provokes the ‘stimulated emission’ by the 'avalenche' effect. If N 2 and N 1 are the ‘populations’ in the higher and lower states respectively for an unit volume, for an inversion of population with N = N 2 –N 1 > 0 and a coefficient of gain/ion (β), it can be shown that the ‘amplification’ of light will be produced if the following condition is met as :

R 1 R 2 . exp ((βN – α)/2L) > 1

where, α is the normal absorption coefficient and L the length of the sample bar. The value of β depends on the indices of refraction n, which depends on the following :

NANO MATERIALS

(a) The wavelength (λ), (b) The variation of wavelength (Δλ) of the fluoroscent rays, and also on (c) The Einstein Coefficient (A). The dependence of the Einstein coefficient (A) is expressed as follows :

β = (1/8πc) . (λ 4 /n 2 ) . (A/Δλ)

Originally, the solid Lasers were essentially the ‘rubies’ (i.e. alumina doped with Cr 3+ ions), or the YAG (Yttrium Alumina Garnet) containing Nd 3+ ions. The ‘ruby’ Laser emits around 690 nm and the YAG around 1060 nm.

7.4.4.2. Some Examples of Nano Particles