it requires that the phase velocities of the frequency‐doubled and the incident waves are the same within the crystal.
Then, the third harmonic of frequency ω p traverses the OPO, a birefringent nonlinear BBO (β‐BaB2O4) crystal, and is converted into two beams, signal and idler, with frequencies ω s and ω i , respectively. Because of the energy conservation, the following condition applies:
(2.73)
To achieve the phase matching condition within the OPO, the conservation of momentum has to be fulfilled as well:
(2.74)
The previous equation can be written in the form:
(2.75)
where n p , n s , and n i are the refractive indices of the nonlinear crystal at the frequencies ω p , ω s , and ω i , respectively. Since the refractive index depends on the polarization of the light and the angle of incidence with respect to the optical axis of the BBO crystal, the OPO output can be thus continuously tuned over a wide spectral range by varying the crystal orientation. The idler polarization is perpendicular to the optical bench while the signal polarization is parallel; then a polarizer is placed in front of the OPO to select one of them. Usually, the signal wavelength can be varied from 410 to 710 nm and the idler wavelength from 710 to 2400 nm. Moreover, the output wavelength range can be extended down to 210 nm by suitable UV modules, SHG nonlinear crystals, that halve the wavelength of the OPO beam. In such tunable laser systems, the beam intensity can reach tens of mJ pulse−1, the linewidth is ~1 meV.
2.2.3.2 Time‐Resolved Detection System: Spectrograph and Intensified CCD Camera
The time‐resolved detection of luminescence spectra usually combines a Spectrograph and an Intensified CCD Camera. The light emitted by the sample enters a slit, whose width can be changed, then it is spectrally resolved by a spectrograph equipped with more gratings differing in the blaze wavelength, λ blaze, and in the spectral resolution depending on number of grooves 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
