KINETICS OF GENERATION, RELAXATION, AND ACCUMULATION OF ELECTRONIC EXCITATIONS UNDER TWO-PHOTON INTERBAND PICOSECOND ABSORPTION IN TUNGSTATE AND MOLIBDATE CRYSTALS
V. I. Lukanin, A. Ya. Karasik*
Prokhorov General Physics Institute, Russian Academy of Sciences 119991, Moscow, Russia
Received August 6, 2012
Under two-photon 523.5 nm interband picosecond laser excitation, we measured the kinetics of induced absorption in PbWOi, ZnWOi, and PbMoOi crystals with 532 to 633 nm continuous probe radiation. We obtained real-time information about the dynamics of the generation, relaxation, and accumulations of electronic excitations over a wide time range (from picoseconds to hundreds of seconds) and the 77-300 K temperature range. For the studied crystals, exponential temperature-independent growth of the induced absorption (IA) with 60 ns rise time reflects the dynamics of the generation of electronic excitation. The kinetics of the IA exponential growth with temperature-dependent 3.5—lips time constants reflect the dynamics of energy migration between neighboring tungstate (molibdate) ions to traps for the studied crystals. The multiexponential relaxation absorption kinetics strongly depend on temperature, and the relaxation decay time of induced absorption increased from tens to hundreds of ms to seconds under crystal cooling from 300 K to 77 K. We found that the increase in the laser pump repetition rate (0-10 Hz) leads to the accumulation of electronic excitations. Control of the repetition rate and the number of excitations allowed us to change the relaxation time of the induced absorption by more than two orders of magnitude. Due to accumulation of excitations at 77 K, the absorption relaxation time can exceed 100 s for PbWO.i and PbMoO.i crystals. In the initially transparent crystals, two-photon interband absorption (2PA) leads to crystals opacity at the 523 and 633 nm wavelengths. (An inverse optical transmission of the crystals exceeds 50-55 at a 50-100 GW/crrr pump intensity.) Measured at ~ 1 mW probe radiation of 532 and 633 nm wavelengths, the induced absorption values are comparable with those obtained under two-photon absorption at ~ 5 kW pump power. An optical 2PA shutter for the visible spectral range is proposed with a variable shutting time from hundreds of microseconds to tens of seconds.
DOI: 10.7868/S0044451013080026
1. INTRODUCTION
Processes concerning the generation and relaxation of electronic excitations in inorganic fl] and organic [2] media are of particular interest. Methods of laser spectroscopy are widely used for investigations of basic properties of materials and interaction of light with matter. In this respect, the nonlinear two-photon absorption (2PA) technique attracts attention duo to a number of unique properties. The 2PA technique is applicable to controlling the energy, time, spectral, and spatial parameters of laser radiation [3]. In addition,
E-mail: karasik'ölst.gpi.ru
the 2PA technique allows increasing the spatial resolution in laser microstructuring of materials and microscopy. The list of 2PA applications can be significantly extended [2].
Two-photon nonlinear spectroscopy can provide new information that is inaccessible by traditional one-photon spectroscopy [4,5]. The use of the 2PA technique is advantageous, in particular, for excitation of electronic states in the conduction band of materials. For one-photon excitation in a nontransparent spectral range of the conduction band, large nonradiative losses do not allow obtaining bulk homogeneous excitation of a sample [G]. In the case of interband 2PA, the energy of one of the excitation photons can correspond to the transparency region of the material. In this case, selective laser excitation of levels under 2PA can allow di-
root examination of luminescence and absorption from the interior of a bulk sample. The difference between the selection rules for one- and two-photon excitation processes provides additional capabilities. The measurement of the 2PA coefficients and cross sections are of independent interest.
The optical properties of oxide tungstate and molib-date crystals are of current interest because of the use of these crystals in scintillation detector of ionizing radiation fl], in addition to their promising use as nonlinear optical materials, e.g., as shifters of a laser radiation frequency via stimulated Raman scattering (SRS) [7,8]. Applications of the crystals require knowledge of the scintillation response rate. As a rule, this rate is measured using methods of the one-photon florescence spectroscopy. However, a search of new methods for investigations of the dynamics of generation and relaxation of electronic excitations is important. In [9], we demonstrated a method to analyze the dynamics of interband 2PA in tungstate crystals excited by a sequence of picosecond laser pulses of variable intensity while under continuous probe radiation. We measured the 2PA coefficients for several tungstates and molibdates and analyzed the competition between 2PA and SRS [10,11].
In this paper, we investigate the real-time kinetics of the generation and relaxation of electronic excitations under interband picosecond 2PA and induced absorption (IA) from excited levels in PIAVO4 (PWO), I'li.MoO 1 (I'.MO). and Z11WO4 (/WO) crystals. The use of 532 to 633 mil continuous probe radiation allows obtaining real-time information about the dynamics of the generation, relaxation, and accumulations of electronic excitations over a wide time range (from picoseconds to hundreds of seconds) and the 77 300 K temperature range. We demonstrate an effect of accumulation of excitations by varying the laser pumping rate and the sample temperature. We show a possibility of an optical shutter creation with a variable shutting time from hundreds of microseconds to tens of seconds.
2. EXPERIMENTAL METHODS AND RESULTS
The experimental setup shown in [10] was modified for this work. The crystals were excited by 523.5 11111 trains of transform-limited 20 ps pulses of the second harmonic of a passively mode-locked and Q-switched Nd:YLiF4 laser [12]. The edge of the absorption band varied from 330 11111 in (PWO) to 400 11111 in (PMO) [10]. The pump photon energy, hv, corresponds to the transparency region of the crystals and the condition hv < Eg < 2hv = 4.74 eV is satisfied
for the two-photon interband absorption. The linearly polarized single-mode laser 523.5 11111 radiation was focused 011 the 0.5 3 cm long crystal under study to a spot with a beam waist radius of about 26/mi by a lens with a focal length of 112 111111. (The Gaussian beam profile of the pump laser radiation was measured with a silicon CCD camera.) Radiation before and after the crystal was directed onto fast Ge or Si photodiodes. The signals were analyzed with a Tektronix DPO 4104 digital oscilloscope with an amplification band of 1 GHz, which determined the time resolution of the recording system. For direct measurement of IA kinetics, ~ 1 111W of a continuous wave (cw) probe, at either 632.8 11111 from a He No laser or 532 11111 from a frequency doubled Nd laser, was introduced to the crystal collinearly to the picosecond pumping radiation. The output probe radiation after a crystal and diffraction grating was directed to the Si (Ge) photodiode or to a photomultiplicr tube (PMT-136) with the resolution time ~ 7 11s. As a rule, an oscilloscope trace was measured in one laser burst. Because of this, we were able to substantially decrease the effect of a geometrical factor on the measurement accuracy as compared to the use of single pulse for the excitation followed by signal accumulation and averaging with many laser bursts.
Figure la shows the dependence of the output pulse intensity I on the input pulse intensity J0 for one of the two orthogonal linear directions of the 523.5 11111 excitation polarization with respect to the crystallographic axes of an 18 111111 long ZWO crystal. This dependence was obtained by measuring the amplitudes of the corresponding pulses of the train (Fig. 2a) in oscilloscope traces recorded at the input and output of the crystal. Here, the arrow directions indicate a sequential increase or decrease in the pulse intensity when passing from the first to the second half of a train. The essential increase in the input value J0 was limited by optical breakdown in the crystal. (For the ZWO and PWO crystals, damage was observed at J0 > 100 GW/'cm2, and for PMO, at J0 < 10 GW/'cm2.) Figure 16 shows the ratio of the radiation intensities at the input and output of the crystal, 1/T = Jo//, as a function of J). Using the linear part of the inverse transmission (1/T) dependence in the initial stage, we determined the 2PA coefficients [10,11]. The 2PA coefficient measured for ZWO is equal to 3 =1.1 cni/'GW. (For orthogonal linear directions of the excitation polarization, 3 = 0.7 cni/'GW.) For PWO and PMO, 3 = 2.0 0.8 cni/'GW and 3 = 2.4 0.9 cni/'GW, respectively. The crystals are promising for their use as limit ors of laser intensity. Due to 2PA, the output intensity for ZWO is limited at the level ~ 1.85 GW/'cm2
J, GW/cm2 2.5
2.0
1.5
1.0
0.5
1-1-1-r
• • • •
• •
W ▼
0
1/T 50
20 40 60 80 100
Jo, GW/cm2
40 30 20 10
~l-T"
-r"
Y
▼ •
20
40 60 80 100
2
Jo, GW/cm2
Fig. 1. a) Radiation intensity at the output (J) vs. the 532.5 nm excitation intensity at the input (Jo) of an 18 mm long ZnWo04 crystal, b) the inverse transmission 1/T = Jo/1 vs. Jo for linear directions of the excitation polarization with respect to the crystallographic axis, E || C2. The arrows associated with circles indicate the direction of a sequential increase in the intensity Jo of the excitation pulses in the first half of the train, and the arrows associated with triangles indicate a sequential decrease in the intensity of the excitation pulses in the second half of the train
(Fig. la). (From a calculation, the Jo-independent limiting intensity is I ma
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