As discussed in Chapter 1, rare-earth doped crystals often contain more than one type of optically active centre. In consequence, the associated excitation and emission spectra can be quite complex, and may actually overlap each other. With the development of a narrow
bandwidth tunable dye laser, the so-called selective excitation and emission (or site-selective laser spectroscopic) technique has been established to separate out the excitation and emission spectra of individual centres (Tallant and Wright 1975). Due to the small linewidth and the tunability of dye lasers, the laser frequency can be chosen to selectively excite a particular type of centre without exciting other centres. Therefore, the resulting emission spectrum will be due to one centre only (selective emission). Correspondingly, by monitoring a certain emission line while continuously scanning the wavelength of the laser, the excitation spectrum for the associated centre is obtained (selective excitation).
Preliminary work on the Ho3+ centres in CaF2 crystal was carried out with a pulsed tunable dye
laser system. The basic experiment set-up is schematically illustrated in Fig.3.2. The laser system is a Molectron dye laser model DL14P (linewidth ~ 0.3 cm"1) pumped by a Molectron nitrogen laser model UV12 (lasing transition at 337 nm (UV) and pulse repetition rate 0-50 Hz). The dye laser pulses are 10 nsec long and the laser is normally run at a repetition rate of 20 Hz. The crystal is cooled to approximately 10 K in a 'flow tube' by a stream of cold helium gas boiled off at variable rates by means of a heater from a dewar containing liquid helium.
Fluorescence is analysed by a 3/4m Spex monochromator, and detected by an EMI 9816 photomultiplier tube. To avoid laser scatter from being detected, the photomultiplier tube is gated in such a way that it is turned on ~ 20 psec after the laser pulse has finished. The signal
from the photomultiplier tube is processed by a Princeton Applied Research Model 162 Boxcar
Integrator which is usually set at a small delay and with a width depending on the lifetime of the emission. The signal is then plotted on a National VP-643 IB x-y recorder.
sample in flow tube
xy-recorder
pulse generator boxcar integrator
monochromator pulse
nitrogen laser dye laser
Figure 3.2 Site-selective laser spectroscopy with pulse dye laser
Three types of experiments using the pulsed dye laser have been undertaken. Firstly, an overall
excitation spectrum was obtained by detecting all emission (i.e., setting the monochromator to
zero-order) from the crystal while scanning the laser wavelength. An excitation peak appears
whenever the laser is in resonant with a transition which induces fluorescence, and hence the obtained excitation spectrum contains information about all optically active centres presented in
the crystal.
Secondly, the emission spectrum, which resulted from exciting each excitation peak, was obtained by keeping the laser frequency fixed at the frequency of that particular peak and
spectrum is due to that one centre only (selective emission)^.
Finally, the excitation spectrum associated with a particular centre was obtained by setting the monochromator at a wavelength where only that centre emits while the wavelength of the laser is continuously changed. Since the emission is now selectively monitored, only excitation
transitions belonging to the chosen centre will show up as signals when the laser is scanned (selective excitation).
Once the wavelength positions of the 5Ig <-> 5F5 optical transitions are known, the selective excitation technique can be performed with the replacement of the pulse dye laser by a continuous wave (cw) dye laser. A schematic of the experimental set-up in this case is shown in Fig.3.3. A Coherent CR 699-21 ring dye laser (with DCM red dye) pumped by a Spectra- Physics 5030 argon ion laser (6 W at 514.5 nm) is used as an excitation source. There are two reasons for this replacement:
(a) in comparison with the pulsed dye laser previously used, the CR 699-21 cw ring dye laser operating in broadband mode (without any intracavity elements) can deliver higher power and, hence, is more suitable for studying very low-doped crystals (the associate spectra have a better signal-to-noise ratio); and
(b) when operated in high-resolution (single-frequency) mode, the CR 699-21 cw ring dye laser has a linewidth of ~ 1 MHz§ and, hence, can be used to study hyperfine and
superhyperfine interactions - in particular, the resolved hyperfine structure in several 5Ig —> 5F5 optical excitation transitions was observed (Secs.5.1 and 6.1).
The energy transfer among different centres in the crystal may occur. However, the emission from a centre other than that being excited by the laser will have a different time dependence. For the CaF2: H o'+ (0.0005 mole %) crystals used, energy transfer among different centres has not been observed.
In general, the laser linewidth depends on the number of lasing cavity modes and on the stability of the cavity, and hence can be minimized by limiting the number of lasing cavity modes to one and improving the stability of the cavity. Mode selection in the laser used can be achieved by adjusting the birefringent filter and two intracavity elements, the thin etalon and the thick etalon. Further details on frequency tuning and single frequency operation can be found in the Coherent CR 699-21 model ring dye laser manual.
The crystal sample was cooled to temperatures of 1.6 - 4.2 K in an ANU constructed hybrid glass-metal cryostat, which is an immersion type where the sample is directly in contact with liquid helium. Temperatures lower than 4.2 K can be achieved by reducing the pressure above
the liquid helium surface. The emission from the crystal is dispersed by a Spex 1400 double pass monochromator, and detected by an EMI 9658R thermoelectrically-cooled photomultiplier.
When the cw dye laser is used, there is no need to gate the photomultiplier tube or use the boxcar integrator. In the cases where the CR 699-21 cw ring dye laser is operated in broadband mode, the laser wavelength was mechanically scanned at the rate of approximately 2 Ä per second, and the signal from the photomultiplier tube is plotted directly on a National VP-643 IB x-y recorder. As the basic dye laser system without any intracavity elements has a linewidth of about 30 GHz (1 cm '1), the resulting excitation spectra, in comparison with those obtained by using pulsed dye laser, will have lower resolution but higher signal-to-noise ratio.
sample in cryostat dye laser
cw argon ion laser
xy-recorder monochromator
Figure 3.3 Site-selective laser spectroscopy with broad band cw dye laser
For the high-resolution experiments, the CR 699-21 cw ring laser was operated in scanning
mode normally over a 20 GHz frequency range (the laser can be scanned by a preset amount
filter and, then, using the thin etalon horizontal tilt control together with the thin etalon offset to fine tune to the excitation line. In this case, the signal-to-noise of the observed optical spectrum was improved by averaging many scans in a Princeton Applied Research 4202 signal averager before it was plotted on a National VP-643 IB x-y recorder (Fig.3.4).
sample in cryostat PMT dye laser dye laser controller
cw argon ion laser
xy-recorder signal averager
monochromator
Figure 3.4 Site-selective laser spectroscopy with high resolution cw dye laser
The technique of selective excitation and emission has been used to characterize the two dominant centres in low-doped CaF2: Ho3+ crystal. The results will be presented in Chapter 4 where it will be shown that setting the monochromator to 6440.6 Ä and scanning the wavelength of the laser between 6350 and 6450 Ä selectively produces the excitation spectrum in the 5Ig —» 5F5 optical transition region of the CaF2: Ho3+ C4v centre. The excitation spectrum of the Ho3+ ions in trigonal (C3v) sites can be separated from that of the tetragonal (C4v) centres by monitoring the intensity of the 6432.8 Ä emission line and scanning the wavelength of the laser. The excitation in the tetragonal centres does not give rise to emission at 6432.8 Ä and thus does not affect the monitored emission intensity, resulting in the intensity variation to be proportional to the absorption of only the trigonal centre.