ОПТИКА И СПЕКТРОСКОПИЯ, 2014, том 116, № 5, с. 801-804
XV МЕЖДУНАРОДНЫЙ ФЕОФИЛОВСКИЙ СИМПОЗИУМ
УДК 548.0:535
PHOTOCONDUCTIVITY AND PHOTODIELECTRIC EFFECT IN LiY1- *Lu,F4 CRYSTALS DOPED WITH Ce3+ AND Yb3+ IONS
© 2014 г. V. V. Pavlov, V. V. Semashko, R. M. Rakhmatullin, and S. L. Korableva
Kazan Federal University, 420008 Kazan, Russia E-mail: Vitaly.V.Pavlov@gmail.com Received November 18, 2013
The time and spectral dependences of the dielectric permittivity of the LiYi _ xLuxF4 (x = 0, 0.5, and 1) crystals doped with Ce3+ and co-doped with Yb3+ ions under UV laser excitation were studied by the 8-mm microwave resonant technique at room temperature. The obtained photoconductivity spectrum in 240—310 nm spectral range was interpreted as a stepwise photoionization spectrum of the Ce3+ ions due to sequential 4f—5d and 5d—6s transitions. Average lifetimes of free and defect trapped (color centers) charge carriers were estimated.
DOI: 10.7868/S0030403414050183
INTRODUCTION
The dielectric properties investigations are important for the UV laser media in which UV excitation leads to the photoionization of the impurity ions. Pho-todynamic processes (PDP) caused by the impurity photoionization are the main obstacle that prevent or impair laser action on 5d—4f interconfigurational transitions of the rare-earth ions [1]. These processes significantly change optical and dielectric properties of the doped crystals because of one- or multi photon impurity ionization, free charge carriers (electrons and holes) production in the appropriate energy bands of the crystal and subsequent their capture by the lattice defects (color center formation).
Therefore, exploring dielectric properties of the crystals under UV excitation, we can determine the basic characteristic of PDP and select suitable excitation conditions to achieve laser action avoiding negative effects of these processes. To describe the influence of PDP on the optical and laser properties of laser media, we need to study the numerical values of their basic parameters, their spectral and kinetics dependences. In this way the most important parameter, characterizing an impact of PDP on the UV laser performances, is the active ions stepwise photoionization cross-section at pumping wavelength.
In this work, the microwave resonant technique has been used to study the photoionization spectrum of activator ions and the timing performances of PDP. The microwave resonant technique is the powerful instrument for research of dielectric properties of the crystals. The potential of this technique is illustrated by the progress of investigations of the electro-physical parameters of semiconductors, such as: surface recombination rate, mobility and lifetime of the free charge carriers [2—4].
MATERIALS
In this work LiY1- xLuxF4 (x = 0, 0.5, and 1) fluoride crystals doped with Ce3+ and Yb3+ ions have been studied. The crystals were grown by means of the Bridgman—Stockbarger technique in graphite crucibles. The concentrations of impurity ions were 1 at, % in the melt. The samples were polished and shaped as a parallelepiped with the size of 1.5 x 1.5 x 0.5 mm.
EXPERIMENTAL
The microwave resonant technique permits the research of the variations of the complex dielectric permittivity of the matter s = s1 — js2 undergoing different external influences. Concept and experimental realization of this technique to study impurity photoion-ization in dielectric crystals were previously described in detail [5, 6].
In this technique, the sample is placed inside the microwave cavity in the antinode of the electric field. If the sample is irradiated by UV radiation, the variations of dielectric properties of the sample will lead to a shift of the resonance frequency and change of the cavity Q factor. It impacts on the amplitude and phase of the microwave signal reflected from the cavity. Changes of microwave signal are detected by means of a quadrature balanced mixer and registered by 8 bits 200 MHz-bandwidth digital oscilloscope AKTAKOM AOC-2282.
The resonance frequency shift is caused by the change of real part of the dielectric permittivity and the variation of the cavity Q factor is attributed to the change of its imaginary part. In activated dielectric crystals, the variations of imaginary part of the dielectric permittivity are mainly contributed by the free charge carriers generated into host bands as a result of
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13
te
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te et
Q
1 -
0 -
200
400
Time, ns
10
20
Time, ns
Fig. 1. Time decays of the real and imaginary parts of complex dielectric permittivity for LiLuF4:Ce (black) LiLuF4:Ce3+,Yb3+ (grey) crystals (a) and LiYF4:Ce3+ crystal (b) excited by radiation at 240 nm (T = 300 K).
0
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the impurity photoionization, in other words, it is photoconductivity effect. The variation of real part is caused by the change of dielectric polarization of the crystal, in other words, it is photodielectric effect. In this case, electrons and holes localized at the impurity ions or electron and holes traps play the key role.
Operating frequency of the experimental setup is 35.4 GHz. The transient time of electromagnetic field in the microwave cavity is about 2 ns and the bandwidth of the mixer is 250 MHz. Taking into account 200 MHz bandwidth of digital oscilloscope, the time constant of the measuring system enables to investigate the transient responses of the dielectric permittivity of the crystals about 5 ns time resolution.
Studied crystals were excited by radiation of the third harmonic of tunable Al2O3: Ti laser in spectral range from 240 to 310 nm. This spectral range corresponds to the 4f—5d transitions of the Ce3+ ions. Pulse duration and pulse-repetition rate of the exciting radiation were 10 ns and 10 Hz, respectively. a-polariza-tion of laser beam was carried out. All the experiments were performed at the room temperature.
EXPERIMENTAL RESULTS AND ANALYSIS
Typical time decays of the real and imaginary parts of complex dielectric permittivity for LiLuF4:Ce3+, LiLuF4:Ce3+,Yb3+ and LiYF4:Ce3+ crystals excited by radiation at 240 nm are shown in Fig. 1. We presented the detected signals only for two samples in Fig. 1a in order to avoid cluttering figure.
Using the microwave resonant technique, we determined that the photoconductivity signal Ss2(t) for all samples seems as narrow spike. The decay time of photoconductivity signal in studied crystals appeared to be about 10 ns. Taking into account that the laser pulse duration was less than the duration of the photo-
is conductivity signal (Fig. 1b) and the time constant of
is the measuring system is equal to 4 ns, we can conclude
ie that the decay time of photoconductivity signal char-
n acterizes the average life-time of the free charge canity ers in host bands.
The photodielectric signal 8s1(i) is characterized ^d by the rise time of about 20 ns. The decay curve of pho-todielectric signal, which can be connected with the nt average life-time of color centers (host defects bound-ie ed charge carriers), has two components. The decay i- time of short-lived component is of the order of hundred nanoseconds, and the decay time of long-lived component appeared to be longer than the pulse-repetition excitation interval (>0.1 s). It was ascertained al from the fact that the prolonged irradiation of the - sample by the pulsed laser radiation led to saturation se of the photodielectric signal.
i- Saturation of the photodielectric signal under irra-i- diation of the samples during 50 s is shown on Fig. 2. ts Saturation cannot be caused by the temperature effect, when the increase of temperature of the crystal under irradiation leads to the microwave cavity detuning, because in this case the distinction between the thermal conductivities of crystals doped only with ts Ce3+ ions and crystals co-doped with Yb3+ ions would % lead to the change not only of decay time of photodi->y electric signal but of the rise time of photodielectric ;d signal as well. Besides, all samples studied demon-[n strate high 5d—4f fluorescence quantum yield and hence low light-induced heating. As it can be seen from Fig. 2, the co-doping of the crystal by Yb ions r does not change the rise time of the photodielectric of signal.
:d Therefore we propose that saturation of the photoer dielectric signal is attributed to several types of long)- lived color centers. The life-time of long-lived color
PHOTOCONDUCTIVITY AND PHOTODIELECTRIC EFFECT
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Time, s Time, s
Fig. 2. Saturation of the photodielectric signal of the fluoride crystals under irradiation during 50 s at room temperature (À,exc = = 240 nm).
Fig. 3. Spectral dependences of photoconductivity (a) and photodielectric (b) signals for the LiY1 _xLuxF4 (x = 0, 0.5, and 1) crystals doped with Ce3+, Yb3+ ions (T = 300 K): LiY1 _ xLuxF4:Ce,Yb (V - x = 0.5, O - x = 1), LiY1 _ xLuxF4:Ce (O - x = 0, A - x = 0.5, □ - x = 1).
centers for studied crystals single-doped with Ce3+ ions seems to be about 14 s and the co-doping of the crystals with Yb3+ ions leads to a decrease of this lifetime to about 9 s. It proves that the additional activation of Ce-doped crystals by Yb3+ ions creates a supplementary recombination channel for free charge carriers and leads to lowering of color centers formation probability. Such an essential role of Yb3+ ions in PDP was previously discussed in [7], where 4f-5d fluorescence decays of the Ce3+ ions in these crystals had been studied.
Spectra of photoconductivity and photodielectric signals for investigated crystals in 240-310 nm range are shown in Fig. 3. The photoconductivity spectra have a maximum at about 270 nm. This 270 nm band cannot be caused by 4f-5d transition of the Ce3+ ions because the ground-state absorption of the Ce3+ ions at 270 nm is extremely low (Fig. 4a) and, accordingly
[8], mainly associated with transitions from excited state of 5d-configuration to 2S1/2 state of 6^-configura-tion of Ce3+ ions located in conduct
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