per mm. Then, the luminescence light hits a photocathode, placed at the entrance of the Intensified CCD Camera, that releases electrons that are accelerated into the Micro‐Channel Plate (MCP). The MPC is an image intensifier consisting of a thin semiconductive glass plate, which is perforated by more than 106 small holes (channels) with a diameter in the range 10 ÷ 25 μm. Since the inner surface has a high secondary emission coefficient, the electrons that hit the channel walls generate additional electrons with a gain depending on the voltage at the MCP output. The electrons are further accelerated by a high voltage (5 ÷ 8 kV) and strike the phosphor coating on the fluorescent screen causing it to release photons. These photons are transferred to the surface of CCD, by optical fibers and produce charges at the pixels they strike. These charges, which are proportional to the number of incident photons, are then converted to an analog voltage, that is input to a A/D converter where it is digitally encoded and transmitted to the interface of the computer. Therefore, due to the MCP gain, for each photon that strikes the photocathode surface, many photons (>103) are produced. Moreover, the possibility of varying the photocathode voltage allows to enable or to disable the CCD: in the Gate ON mode, the photocathode voltage is usually set at −200 V and the CCD sees the light; in the Gate OFF mode, the photocathode voltage is zero and the CCD does not see the light.
As shown in the diagram of Figure 2.5, the Gate Width Δt determines the amplitude of the time window during which the CCD is enabled to reveal the luminescence light (Gate ON mode); while the Gate Delay T D regulates the temporal shift of the acquisition window with respect to the trigger signal (that is the arrival of the laser pulse). This setup allows the detection of time‐resolved luminescence spectra synchronized with the laser excitation pulses. The measured PL intensity is the light emitted from the sample, given by Eq. (2.65), integrated from T D to T D + Δt, in agreement with:
Figure 2.5 Diagram of the CCD timing: Gate ON and Gate OFF modes.
To acquire the decay curve, from which the lifetime τ can be estimated, it is necessary to acquire a set of PL spectra with a fixed Δt and T D ranging from zero to several τ, when the luminescence signal is extinguished. In fact, if (Δt ≪ τ), the integral in (2.76) reduces to:
(2.77)
that well describes the luminescence decay on increasing T D.
2.3 Case Studies: Luminescent Point Defects in Amorphous SiO2
Interest toward the optical properties of point defects in amorphous SiO 2 (silica) is a timely debated issue for its fundamental aspects in the science of amorphous solids and is constantly motivated by the key role of this material in high‐tech devices, see, for instance, review papers [13, 14] and references therein. Silica is a model material both for its simple structure and for the possibility to compare its properties with those of its crystalline counterpart (α‐quartz). Moreover, due to its excellent transparency, from the mid‐IR to vacuum‐UV, is indispensable for long‐range low‐loss optical communication fibers and is the best glassy material for high‐power pulsed laser optics. Point defects are relevant because they determine a wide number of optical phenomena. They can be not only detrimental for the use of silica, as it is the case of the transmission losses, but they are also successfully exploited to build modern devices, such as fiber Bragg gratings based on the change of refractive index induced by radiation (photosensitivity). One of the most relevant optically active defects in the silica network is the oxygen dangling bond or nonbridging oxygen hole center (NBOHC), (
This section deals with the luminescence of the NBOHC at the silica surface: a model system to evidence the effectiveness of the time‐resolved technique in the study of electronic transitions and their coupling with phonons. The reported results are set in an important and timely issue concerning a wide class of optical phenomena influenced by the size reduction of materials to nanoscale. Nanometer‐sized silica particles (nanosilica) are, indeed, characterized by a large specific surface area that favors a large concentration of defects that have a crucial role in determining the high emissivity, which is surprising if compared to the optical properties of their bulk counterpart, and are therefore the subject of huge attention in the specialized literature [15–20]. It also worth noting that in these nanosystems, the generation of defects is strongly conditioned by the accessibility of surface sites for molecules of the environment [21]. This is particularly advantageous from a fundamental viewpoint because the possibility of stabilizing specific defects, by controlled thermochemical processes, is crucial to better understand their structural and electronic properties.
At silica surfaces, the structure of NBOHC is in fact determined by its generation occurring by the chemical reaction of an axially symmetric surface‐E' center,
Figure 2.6 Structure of the NBOHC and p orbitals of the dangling oxygen in the (panel a) C 3v and (panel b) C S symmetries.
2.3.1 Emission Spectra and Lifetime Measurements
Several studies have evidenced that NBOHC at the silica surface emits