In this paper we report on optical pump-probe measurements on BDN-doped polycarbonate irradiated with laser pulses ranging from 25fs to 1.2ps in duration and up to fluences of 0.1 J/cm2 at 800nm wavelength. Transient change of the transmission has been measured as a function of pump-probe delay and pump fluence. Pulse duration dependent channels of saturation have been identified and the ground state repopulation (GSR) time has been determined.
© 2006 Optical Society of America
Polycarbonate-host materials doped with absorbing chemical complexes have found widespread applications in different areas of research as well as industry. Special attention has to be paid to the dopant Bis-Dithiobenzil-Nickel (BDN), which is highly absorbing in the near infrared and transparent in the visible spectral range. Since its invention in 1974, this dye has been used mainly as a saturable absorber in Q-switched or mode-locked Nd:Glass lasers [1–2], but it has also found a wide range of applications as a dopant of plastic-host optical filters in the near infrared [3–4].
Recently, we have studied the temporally and spatially integrated nonlinear transmission properties of polycarbonate based filters doped with BDN. The measurements revealed a pronounced difference in the transmission behavior for picosecond and femtosecond pulses . Previous measurements have been performed on the nano- and picosecond time scale [5–13], describing the optical nonlinear [5–9], spectroscopic  and dynamic [11–13] properties of BDN in various solutions and solid hosts. One of the most important general conclusions of these studies is that optical properties of the dye substantially differ depending on the host material. Nevertheless, the femtosecond saturation dynamics of this chemical compound remained undiscovered until now. In order to gain insight into the dynamics of the underlying physical mechanisms, a series of time-resolved experiments was conducted. In conclusion, we have identified pulse duration dependent channels of saturation; furthermore, characterization of the relaxation of BDN in the femtosecond to nanosecond delay range was achieved.
The optical setup used for the time resolved measurements is depicted in Fig. 1. For the experiments, we used a 25fs Ti: Sapphire laser system (λ0=800nm, Δλ=50nm FWHM) with 1 kHz repetition rate described elsewhere . The incoming laser pulse was split up by a fused silica wedge. The transmitted (pump) beam passed the optional pulse stretcher, which consisted of heavy flint glass blocks of different thickness. The beam could be attenuated, if necessary, with pellicle beam splitters or a reduced-reflectivity silver mirror. Finally, the pump beam was focused onto the target with an f#=50 silver coated spherical mirror. A DC-motor driven XY translation stage was used to accomplish the target positioning.
The reflected beam from the back surface of the wedged beam splitter was injected into the pyroelectric head of the energy meter and served for single-shot pulse energy monitoring. The front reflection (referred as probe beam) was sent to the delay stage, which enabled ≈0.66 fs temporal resolution over the entire range. After passing the delay line and the polarization rotator (that rotates the polarization by 90°), the probe pulse was focused onto the target. In order to realize a uniform illumination on the imaged sample area the diameter of the probe beam was set to about a factor of 3 larger as compared to the pump. In all our experiments special care was taken to keep the temporal resolution as high as possible, by using only reflective optics and minimizing the angle between pump and probe beam. Note however, that some applications may take advantage of large pump-probe angles . In experiments with longer pulse durations (τp=175fs and 1.2ps) only pump pulses were stretched; the duration of the probe pulses was kept as short as 25fs for all measurements.
The transmitted probe beam was recorded with a cooled slow-scan CCD camera with 16-bit resolution. Lenses of f=80 and 200 mm were used for imaging, allowing magnifications of a factor of about 14 and 3, respectively. Due to the slight angular offset, the pump and probe beams were spatially well separated behind the target. For image processing, we used LabView-based software, which allowed us to exploit the spatial resolution of the camera. The intensity profile of the pump beam has been measured and finally projected onto the recorded images. Thus, it was possible to assign a fluence value to each pixel. This method permits the extraction of the transmission information from each individual pixel, and so the generation of statistics for each single shot (or image), making several scans with a tightly focused probe over a weakly focused pump beam unnecessary.
3. Results and discussion
We have performed time-resolved single-pulse experiments with the above-described system. The change of the transmission (T pump+probe/T probe) has been measured as a function of pump-probe delay, pump pulse duration and pump fluence. The sample has been irradiated with pump pulses of 25, 175fs and 1.2ps duration. Peak fluences of up to 0.1J/cm2 have been applied, which is orders of magnitude above the measured saturation fluence FSAT ~ 5×10-3 J/cm2, but well below single-shot damage threshold in the order of ~ 1 J/cm2 . Images, that were recorded 5 seconds after pumping (referred to as “infinite delay” images), furthermore, surface microscopy analysis showed no degradation of the surface, providing evidence to the absence of damage. The duration of the probe pulses was kept as short as 25fs for all experiments, and its fluence was set to be close to, but below FSAT. In other words, it is reasonable to assume that the sample exposed only to probe pulses behaved as a reversible linear absorber.
The results focusing on the transmission change for short delay times are depicted in Fig. 2 for 25fs and 1.2ps excitation pulses, F≈0.1J/cm2. The points represent the mean value of the transmission change. The average deviation (scattering) of the measurement data is about 12% or less, thus no error bars are displayed. The temporal evolution of the curves reveals quite different behaviour for different pulse durations. For 25fs-excitation one can observe instantaneous, fast bleaching, which peaks at about 50fs delay time; the transmission increases rapidly by a factor of 40 (see also range I in Fig. 3). The dynamics of the transient bleaching follows qualitatively the temporal evolution of the cross-correlation signal obtained by using a thin BBO crystal. After a short decay, the transmission increases relatively slowly, and reaches a quasi-steady saturated state (range II in Fig. 3) up to the nanosecond time scale. In this saturation regime, the steady-state value of the transmission is proportional to the incident pump fluence. Note, that the amplitude of the transient bleaching decreases for decreasing fluences, and no transient bleaching could be observed for fluences below 0.01J/cm2.
We repeated the experiment with 1.2ps pump pulses, which saturate the filter more efficiently . While the curve shows qualitatively similar behavior in the quasi-steady-state regime, the fast component of the saturation is completely absent. During energy deposition, the transmission increases, and its slope is proportional to the deposited fluence. The rise of the transmission extends also after the energy deposition, however, with a rather moderate slope. From the subsequent increase of the transmission in absence of the laser pulse we have concluded that the extent of the saturation is also influenced by thermal processes. After approximately 3ps, (see Fig. 3) the saturation reaches its maximum value, which is proportional to the deposited energy. In accordance with earlier integrated transmission measurements , the increase in transmission amounts up to approximately two orders of magnitude.
Assuming a simple four-level energy scheme (applied e.g. in Refs. [6–8] and Ref. ) one could explain the instantaneous bleaching for short-pulse excitation as follows: the first excited state E1 is populated during energy deposition by the short pump pulse. The delayed probe pulse may partially deplete the first excited level E1 by stimulated emission, which can lead to the rapidly enhanced transmission (i.e. the fast component of saturation in range I). Stimulated emission has been generally observed in time-resolved spectroscopy measurements in organic dyes, e.g. Ref. . At the trailing edge of the pump pulse, the energy deposition and so the population of E1 decreases, which leads after all to the reduction of stimulated emission. On the other hand, we assume that the population of E1 decays at the same time into the triplet state T1 by a somewhat slower transition. This channel is mainly responsible for the remaining saturation in range II. Note, that no fast bleaching was observed for 175fs pump pulses, i.e. the time constant of the E1→T1 transition may be longer than 25fs but shorter than 175fs. In other words, if the pump pulse is much shorter than the decay time from E1 to T1, so that the trap-level T1 cannot be efficiently populated, a reduced remaining saturation in range II can be observed.
The same behavior is even more pronounced for ps pulses, where the pulse duration is substantially longer than this decay time. This results in a more efficient steady-state saturation for ps-pulses. Due to the relatively long lifetime of the triplet state, the trapped population is not available for ground-state absorption until relaxation sets in (range III in Fig. 3). Note, that excited-state absorption (transition T1→T2) can slightly reduce the transmission as reported in a previous publication (for details see ). However, the relatively low pulse energy used in this experiment was not enough to evoke this effect for any applied pulse duration.
After reaching the maximum value of the saturated transmission, the filter keeps its saturated quasi-steady-state up to ≈1 ns. We have noticed an exponentially decaying transmission from 2 ns, which characterizes range III in Fig. 3. In order to gain some information about the relaxation, a scan with an extended delay stage allowed us to perform measurements of up to 10 ns delay time. To be consistent with earlier studies on the ground-state repopulation time in BDN [11–12], we applied a transformation suggested by Greenhow et. al. on the measured data. Thus, in Fig. 4 we displayed ln[1-d(t)/d(∞)], whereas d(t) represents the temporal evolution of the optical density, and d(t)=log10[1/T(t)]. The slope of the linear regression (-1/τ) reveals the ground-state repopulation (GSR) time, which is estimated to be 25.6±0.5ns. Due to technical reasons we were unable to extend our measurements for longer delay times, however, the estimated GSR times were consistently independent for different deposited laser fluences as well as pulse durations. In conclusion, the GSR time of BDN in a polycarbonate matrix seems to be substantially longer compared to other liquid or plastic hosts. It is however, not surprising, given that the GSR time varies strongly (over two orders of magnitude ) depending on the particular host material, and it is generally longer for solid hosts. The independence of the GSR time on the pulse duration suggests that relaxation is mainly governed by thermal and/or vibronic processes.
Time-resolved optical transmission dynamics of BDN-doped polycarbonate has been studied in the femtosecond and picosecond regimes for laser fluences above saturation but below damage threshold. For pump pulses as short as 25fs we have observed ultrafast transient bleaching during excitation followed by a moderate residual saturation. For longer pulses (175fs and 1.2ps) we observed no instantaneous bleaching, however the extent of the residual saturation was higher for longer pulses and higher deposited laser fluences. We propose that this pulse duration dependent behavior of the transmission should be assigned to medium-fast transitions between the first excited state E1 and the triplet state T1 within a simple four-level energy scheme. The “trapped” population of the triplet state T1 could be the major source of the long-term residual saturation, which characterizes the second phase of the temporal evolution of the transmission for all pulse durations. For the shortest pulses (τp=25fs) stimulated emission prevails, causing ultrafast bleaching and a moderate population of T1. However, longer pulses (τp=175fs and 1.2 ps) can populate T1 more efficiently so that residual saturation becomes dominant.
Approximately 1ns after excitation slow relaxation sets in from T1 to the ground-state. The estimated ground state repopulation time is 25.6±0.5ns and almost independent on the pulse duration and the deposited laser fluence. Due to the relatively long time scale of the relaxation, we suggest that it can be mainly assigned to thermal processes. The long recovery time makes the applicability of the BDN-doped polycarbonate as a protective filter considerable in environments, where high-repetition rates and long pulses are present, such as long-cavity oscillators or regenerative amplifiers.
This work was supported by the Austrian Science Fund grant F016 (Advanced Light Sources) and by FemtoLasers GmbH. The authors thank A. Fiedler (LaserVision GmbH) for providing the samples and for helpful discussions.
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