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Abstract
This work reports on the nonlinear optical properties of Er doped ZnO (EZO) films, measured by the Z-scan technique with different laser parameters (pulse width, wavelengths and energy). The nonlinear absorption mechanism of EZO films is analyzed by a singlet state three and four-level model, respectively. The effect of the direct current sputtering power of EZO films on the nonlinear optical properties was studied. From the results, the samples show the self-focusing effect and two photon absorption (TPA) induced excited state reverse saturable absorption (RSA) behavior. Under the different laser parameter excitation, it has been determined that the TPA induced saturable absorption (SA) or RSA properties for EZO films with 8 W dc sputtering power. Moreover, these findings indicate that free carrier density increases with Er doping and this behavior leads to excited state absorption. In addition, the ultrafast dynamics of EZO films have been investigated using the two-color pump-probe (TCPP) technique with 325, 380 and 400 nm wavelength excitation and probing over a broad range in the visible region. The decay of the positive signal is found to be biexponential, which we have assigned to the pump-induced deep-level (DL) or excited state absorption. We also present experimental results that the few decade picosecond component has been assigned to vibrational relaxation in the excited electronic states, and the slow components represent the decay from singlet excited state to the ground state (< 1.2 ns) and DL to the ground state (> 1.2 ns). Moreover, these findings indicate that the excitation wavelength increases and the detection wave band widens. Our results show that EZO films are a promising candidate in further optoelectronic device applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
It is well known that ZnO with various metal dopants can effectively improve the optical and electrical properties of ZnO [1–3]. The properties of metal doped ZnO change when it transfers from bulk to nanostructures form that leads to recent advances in nanostructures fabrications [4,5]. Recently, rare earth (RE) ions doped ZnO has attracted the attention of many research works, and they have reported the optical and electrical properties of RE ions doped ZnO [6–14]. RE ions-doped ZnO has been studied widely for industrial applications in the fields of photoluminescence (PL) [15], varistors [16], laser [17], flat panel displays [18], piezoelectric transducers [19], fiber amplifier [20], ferroelectric [21], solar cells [22,23], light-emitting diodes [24], sensors [25], and so on. Among the optoelectronic materials, the Erbium (Er)-doped semiconductors designed devices have attracted much interest from the viewpoint of applications. Er is an essential element in optical telecommunication technology due to its 1.54 μm emission at which silica-based fiber has minimum attenuation and near zero dispersion [26–29]. Er-doped semiconductors have been reported to be promising optical materials for optoelectronic devices [30,31]. And the Er-doped ZnO (EZO) films have been fabricated using various techniques [32–36], such as wet chemical precipitation method, sol–gel method, pulsed-laser deposition, and direct current reactive magnetron sputtering technique. Despite many works on the optical properties of EZO films were studied [36,37]. R. P. Casero et al. [36] proved that Er-ions are probably located not only inside of the crystalline grains but also at the grain boundaries leading to strong luminescent quenching. V. Kumari et al. [37] studied the optical limiting (OL) of semiconductor EZO films by sol–gel method using spin coating technique with different doping concentration. The OL experimental results reveal that the EZO films exhibited strong OL performance for nanosecond laser pulses. M. A. Lamrani et al [38] have demonstrated the third order nonlinear optical susceptibility values of EZO films were in the remarkable range of 10−12 esu. However, there are no detail reports on the study of deposition parameter, which affect the film properties. The analyses about the influence factor of direct current (dc) sputtering power under nonlinear optical behavior of the EZO films were seldom reported. Furthermore, under different laser parameters, the nonlinear optical properties and carrier dynamics of the EZO films have not been reported.
In Secs. 3.1 of this paper, we characterized the films using X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV-visible absorption. The EZO films in the effect of dc power on the structure and optical properties were investigated. In Sec. 3.2, the nonlinear optical characteristic of the films was studied using Z-scan with different laser parameters (pulse width, wavelengths and energy). The nonlinear absorption mechanism responsible for the difference among the results is analyzed by using singlet state three and four-level model, respectively. The effect of direct current sputtering power of EZO films on the nonlinear optical properties was studied. In Sec. 3.3, we present two-color pump-probe (TCPP) data on the EZO film, taken at nine probe wavelengths around the exciton resonance when excited with three pump wavelengths.
2. Experiment
EZO films were deposited on quartz substrates by simultaneous direct current (dc) and radio frequency (rf) magnetron sputtering method. A disc of ZnO with 60 mm in diameter and a purity of 99.99% were used as a rf target. Er foils (purity 99.999%) were used as a dc target. The base pressure in the deposition chamber was 4.0 × 10−4 Pa. During deposition, the reactant pressure was maintained at 3 Pa, the rf sputtering power was kept at 100 W and the substrate temperature was 100 °C. The gas flow rate during the deposition was 20 sccm for Ar and 10 sccm for O2. Under different dc power, the thickness of the films is about 120 nm after deposition for 30 min. XRD measurements were performed using a Siemens D5000 advanced diffraction system with a Cu Kα radiation (λ = 0.154 nm). The SEM images were measured using a Hitachi SEM, model Quanta 200F. UV-visible absorption and the transmission spectrum of the films on fused silica were measured from 300 to 800 nm using a UV-vis-NIR Perkin-Elmer spectrophotometer. The nonlinear optical properties were measured with Z-scan technique [39–41]. For measurements of ns and ps Z-scan, a second (532 nm) of a Q-switched YAG (1064 nm) laser were used as the excitation source. The measurements were performed with 5 ns pulses at a 10 Hz repetition rate and 21 ps pulses at a 1 kHz. The fs Z-scan experiments are performed using a Q-switched, frequency doubled Ti: sapphire laser producing 100 fs laser pulses with a pulse repetition rate of 1 KHz at different wavelengths. Basically, in this technique the nonlinear sample is scanned through the focal plane of a tightly focused Gaussian beam. Unstability of the Gaussian beam intensity is ± 0.2%. The time-resolved transient absorption spectra were recorded using a femtosecond TCPP spectrometer [41,42], a Ti:sapphire laser system, supplied by Thales Optronique SA, Elancourt, France, provides the pulses of about 100 fs. Which has been described in detail elsewhere [43–45].
3. Results and discussion
3.1 The morphologies and structure for EZO films
Figure 1 shows XRD patterns of EZO films with different dc powers deposited on quartz substrates. From the inset of Fig. 1 it can be observed that the (002) peaks of EZO films are 34.151° (for 2W), 34.283° (for 4W-12W), respectively. Compared with the strain-free ZnO films (34.431°) [46], the (002) diffraction peaks at 2θ of all the samples were smaller. The Er ions are incorporated by two pathways, one population is found inside the crystallites and another one at the grain boundaries, as a consequence of the differences in valence and ionic radius of Zn and Er. The slightly smaller angle found in the doped films may be due to that the radius of Er3+ ions (0.103 nm) are larger than Zn2+ ions (0.065 nm), which may lead to the increase of lattice constant after Zn2+ ions were substituted by Er3+ ions. As dc powers increased, the intensity of (002) peak decreased at 2-12 W. To be a very important sputtering parameter, the power, supplies the kinetic energy to the sputtered atoms and increases the growing rate. However, more highly c-axis oriented EZO films were grown at 2 W rather than at 12 W, even though higher kinetic energy could be provided to the sputtered particles at 12 W.
SEM image was collected to examine the surface morphology of EZO films as shown in Fig. 2, which show regular surface with small particle size and uniform distribution. From the inset (i) of Fig. 2, it can be observed that is size distribution diagram of EZO films for each sample. The average diameter of low-dimensional structure nanocrystal for all the samples were approximately 38 nm (for 2 W), 42 nm (for 4 W), 44 nm (for 8 W), and 48 nm (for 12 W), respectively, referring to the error value of ± 0.16 nm. With the increasing of dc sputtering power, the amounts of the Er dopant increased, leading to the crystallization of the films increasing. From the inset (ii) of Fig. 2, the cross section thickness of the EZO films was about 120 nm. Under different dc power, the thickness of the sample has a little change.
Fig. 2 SEM (a-d) image of AZO films at dc power of 2 W, 4 W, 8 W and 12W, respectively. The inset shows the distribution of the particle size for EZO films.
Figure 3 shows the UV-visible absorption (a) and the transmittance (b) spectra of all the films. The spectra of all films show a near-UV absorption peak followed by a sharp absorption edge, corresponding to the excitonic energies and band-edge of the ZnO nano-crystals. In Fig. 3(a), it is evident that all the EZO films show a sharp absorption in the UV region, and the absorption edge shifts with the change of the Er doping concentration. No absorption peaks due to Er3+ ions could be observed because of the weak intra-4f transitions in nature. UV- vis spectra of EZO material showed a slightly red shifted peak at 355 nm. It indicates impurity band that was introduced by Er doping [47]. In Fig. 3(b), on an average, nearly 80% transmissions were observed in the films. Tauc relationship [48] was used to determine the band gap of EZO films. The values of band gap for all the samples were 3.14 eV (for 2 W), 3.17 eV (for 4 W), 3.18 eV (for 8 W), and 3.22 eV (for 12 W), respectively. The optical band gap increases with the amounts of the Er dopant increased. Since oxygen vacancies can provide many electrons to fill the electronic states of the conduction band [49], which lead the Fermi level shifted to higher energies. The optical absorption edge corresponds to the transition from valence band to conduction band, thus optical energy band gap of the film widen with much oxygen deficiency.
3.2 The nonlinear absorption for EZO films
Assuming Gaussian profile for laser pulses and using the open aperture Z-scan curves, the multiphoton absorption coefficient (αn, n = 2 for two-photon absorption; n = 3 for three-photon absorption, and so on) can be obtained from the standard open aperture z-scan data that were fitted by using the equation in [50,51]. Figure 4 shows Z-scan transmittance curves of EZO films, which are obtained at different dc powers of EZO films and energy of 14 μJ for an irradiation wavelength of 532 nm. For the closed-aperture case, the signal profile shows a peak followed by a valley. This indicates a negative (self-defocusing) optical nonlinearity. For the open-aperture case, the transmittance curve of open-aperture Z-scans exhibits the valley characteristic shape. The nonlinear absorption mechanism of EZO films is exhibited by 532 nm laser irradiation due to the population of conduction band through two photon absorption (TPA). At different dc powers, the order of magnitude of the nonlinear absorption and nonlinear refractive index for all the samples were 10−6 cm/W and 10−5 esu, respectively. For 2, 4, 8 and 12 W dc powers, the nonlinear absorption and nonlinear refractive index coefficient of the sample were 1.6 × 10−6, 4.5 × 10−6, 8.4 × 10−6 and 3.8 × 10−6 cm/W and −2.37 × 10−5, −4.64 × 10−5, −5.18 × 10−5 and −4.37 × 10−5 esu, respectively.
Fig. 4 Normalized open (a) and closed (b) aperture Z-scan transmittance curves of EZO films measured at 532 nm.
According to the above analysis, the nonlinear absorption and nonlinear refractive index coefficient of the sample at 8 W dc powers is larger than other samples. In addition, from inset of Fig. 2 it can be seen that the distribution of the particle size for EZO films at 8 W dc powers is uniform. Next, it has not been analyzed that the effects of laser pulse width, wavelength and energy impact on the nonlinear absorption characteristics of the sample at 8 W dc powers.
Figure 5(a) (scattered points) shows the open- aperture Z-scans obtained the sample at 8 W dc powers under 5 ns, 21 ps and 100 fs laser pulse. The experiment was performed at 60 nj (for 100fs pulse), 42 μj (for 21 ps pulse) and 14 mj (for 5 ns pulse) energy and an irradiation wavelength of 532 nm. The nonlinear absorption behavior of EZO films is exhibited by 532 nm laser irradiation due to the population of conduction band (CB) through TPA. Because the laser excitation wavelength falls in the wing region of the exciton absorption, satisfying the condition for the occurrence of a TPA process (Eg/2 < ℏω < Eg, in Fig. 3) the nonlinear absorption was contributed by the TPA process also, in addition to the strong absorption of the defect states. To discuss the various parameters in the ns, ps and fs regime that influence the reverse saturable absorption (RSA) and saturable absorption (SA), a general model of multi-level energy diagram [52,53] is used. For the ns and ps laser, three level model analyses are adopted. Therefore, the nonlinear absorption mechanism of the EZO films should be repopulated from the VB to the CB caused by TPA, which induced ground state SA and TPA induced low excited states (or DL) RSA. For the fs laser, four level model analyses are adopted. The nonlinear absorption mechanism of the EZO films should be repopulated from the VB to the CB caused by TPA, which induced low excited states (or DL) SA and TPA induced high excited states (CB) RSA.
Fig. 5 Open aperture Z-scan curves of sample irradiated by 5ns, 21 ps and 100 fs laser pulses at 532 nm (a), and three distinct wavelengths: 400, 532 and 800 nm, all measured with energies of 60 nj at 100 fs laser pulses (b).
Figure 5(b) (scattered points) presents typical Z-scan traces at different wavelengths all measured at 60 nJ. This laser with the different wavelength excitation causes to TPA induced SA or RSA and TPA induced excited state absorptions (ESA) for EZO films. In the case of 400 nm, the EZO films show good SA in the studied intensity range. We hardly see RSA in the plots of Fig. 5(bIII). However, in the case of 532 nm, The EZO films show RSA behaviour, as seen in Fig. 5(bII). At further higher excitation wavelengths (800 nm), the switching of the nonlinear absorption is from SA to RSA (Fig. 5(bI)). Furthermore, Clearly, EZO exhibits a strong linear absorption band with a peak in the near-UV range, therefore, one may choose the excitation wavelength (λexc) to be 400 nm and 532 nm for the sample in order to fulfill the requirements (2λabs <λexc<3λabs) of 2PA, and 800 nm for the sample in order to fulfill the requirements (2λabs <λexc<3λabs) of 3PA studies. For the femtosecond laser, four level model analyses are adopted. The nonlinear absorption mechanism of the EZO films should be repopulated from the VB to the CB caused by TPA, which induced low excited states (or DL) SA and TPA induced high excited states (CB) RSA. So the above analysis process is shown in Fig. 7. The similar analysis process has been reported by many other papers [54–59].
Figure 6 (scattered points) shows the open- aperture Z-scans obtained the sample at 8 W dc powers under different laser energy with 400, 532 and 800 nm laser wavelengths at femtosecond pulse. Under different laser energy, the EZO films exhibit different nonlinear absorption process. The low energy EZO films display a normalized transmittance peak at the beam focus, indicating a negative nonlinear absorption due to typical two ‒ photon excitation ground state SA. However, the high energy EZO films demonstrate a strong positive nonlinear absorption (RSA) in addition to SA, and the nonlinear absorption is completely switched over to RSA. The change from SA to RSA near the beam focal region can be interpreted by considering the intensity-dependent contributions from SA to RSA. With the increasing of the excitation power, the intensity of SA in the ground state was being reduced. When excitation power is relatively large, “valley” curve (near the focal point of the transmittance decreases), is typical of the two ‒ photon induced RSA effect (σ2>σ1). The theoretical explanation of the process of mechanism transformation is as follows: because of the maximum power density of the probe beam occurred at focal point of the lens, the nonlinear absorption of the sample should be the strongest here. For two photon excitation ground state SA, the electrons in the ground state are excited to the excited state, and there are no more electrons to absorb the extra photons, called “Ground bleaching effect”. So the closer the focal point get to, the higher the transmittance is. Reacting on the test curve, it will appear the “peak” curve. For TPA induced the RSA, the sample absorbed two photons at a time firstly, which will lead to lower transmittance, and the more closer the focal point get to, the more obvious the effect is. This effect reacted on the measurement curve, is the “valley” curve.
Fig. 6 Open-aperture Z-scan curves of the EZO films in different input light intensities at 400 nm (a), 532 nm (b) and 800 nm (c).
Figure 6(a) represents the normalized transmittance curves of the EZO with different laser energy at 400 nm. At the lower input laser energy of 30-60 nj, the transmittance curve exhibits a symmetrical peak with respect to the focus (z = 0), indicating that SA occurs in the EZO by now. With the input light intensity increased to 80 nj, the transmittance curve exhibits a symmetrical valley with respect to the focus (z = 0), indicating that RSA that occurs in the EZO. The results indicate that the transformation from SA to RSA occurs in the EZO with the increase of the input laser energy. The nonlinear absorption mechanism, the low energy EZO films display a normalized transmittance peak at the beam focus, indicating a negative nonlinear absorption due to typical two ‒ photon excitation ground state SA (see Fig. 7(I)). However, the high energy EZO films demonstrate a strong positive nonlinear absorption (RSA) TPA induced the RSA (see Fig. 7(II)). The Er doping could be the main cause of the transformation from SA to RSA in the EZO films. For 532 nm wavelength excitations, the open- aperture Z-scans is showed in Fig. 6(b). In the ns case photon energy at 532 nm corresponding to 2.33 eV and since the maximum band gap of films is ~3.18 eV, 2PA could be the plausible mechanism for nonlinear absorption. It can be seen from Fig. 6(b), SA decreases when increasing irradiance, and for irradiances larger than 60 nj, RSA starts to show. Similarly, the change from SA to RSA of different gold nanorod has been reported by E. V. G. Ramírez et al [56].The transformation from SA to RSA is an interesting effect that can be used for optical pulse compressor, optical switching and laser pulse narrowing.
For 800 nm wavelength excitation, Fig. 6(c) shows the normalized transmittance curve, which exhibits a symmetrical valley with respect to the focus (z = 0), indicating that RSA occurred in the EZO films by now. The nonlinear absorption mechanism, the low energy EZO films display a normalized transmittance peak at the beam focus, indicating a weak positive nonlinear absorption due to typical three ‒ photon excitation defect state or high excited state RSA. However, the high energy EZO films demonstrate a strong positive nonlinear absorption (RSA) defect state or excited state RSA induced the RSA (see Fig. 7). The nonlinear absorption mechanism of the EZO films should be repopulated from the VB to the CB caused by 3PA, which induced low excited states (or DL) RSA and high excited states (CB) RSA, respectively. The above analysis process is shown in Fig. 7. The nonlinear absorption mechanism, studied near 800 nm using 100 fs pulses, switched from 3PA induced low excited states (or DL) RSA type at low laser energy to 3PA induced high excited states (CB) RSA at high laser energy. The photon energy corresponding to 800 nm wavelength is 1.55 eV, while the band gap of the films is 3.18 eV. It clearly suggests that the nonlinear absorption mechanism has to engage at least three photons.
The electronic transitions in these systems consisting of four energy levels S0, S1, S2 and Sn are shown in Fig. 7. It is the differet energy between the top of the VB and the bottom of the CB. In our experiment, the transformation will occur between discrete states that arise from the top of the VB and the lowest energy discrete states which appear from the bottom of the CB. The density of states is increased tremendously, and the energy of the band gap absorption is also increased. The more transformations from ground state to excited state, SA should be obvious, which is observed in Fig. 7(I). RSA could be mainly derived from the interband transitions among excited states, which is observed in Fig. 7(II and III). In order to explain the results of transient transmittance, a photonic excitation absorption model was established. We propose a following several mechanism involving singlet-singlet exciton annihilation for the trap formation and PIA, which is operatived under femtosecond pulse excitation. PIA of excited state, which results in a highly reactive excited state, has been proposed in explaining single electron experiments with two-photon excitation [60–62].
The exciting at 400, 532 and 800 nm excites the transition singlet S0 → S2 (two or three - photon excitation) state. The theoretical explanation of the process of mechanism nonlinear absorption is as follows: when excited with 400 nm pumps, which is derived from two-photon induced ground state or excited-state S1 SA (Eq. (1a)). When excited with 532 nm excitation, which is derived from two-photon induced excited-state DL or S2 RSA (Eq. (1b)). When excited with 800 nm pump, which is derived from three - photon induced excited-state RSA (Eq. (1c)).
At different pules, wavelengths and energy, the order of magnitude of the nonlinear absorption for the samples were 10−6 cm/W. The value of the NLA coefficient was evaluated from the fits to the experimental data that was obtained using the equation in Ref [50,51]. The best fit was obtained with the transmission equation for n = 2 (TPA) or 3 (3PA). Obviously, the nonlinear absorption process involved is TPA (α2) certainly. The α2 and α3 values estimated for the sample show in Table 1 and Table 2. Black represents the TPA coefficient, while dark blue represents the 3PA coefficient.
Table 1. Nonlinear absorption coefficients of EZO film at different pules and wavelength, respectively.
Table 2. Nonlinear absorption coefficients of EZO film under different laser energy with 400, 532 and 800 nm laser wavelengths at femtosecond pulse, respectively.
3.3 The excited-state charge-transfer dynamics for EZO films
To understand the dynamics of the excited-state charge-transfer better, we performed ultrafast TCPP experiment to investigate the dynamics behavior of EZO films. The fs laser wavelength was tuned to 325, 380 and 400 nm, nearly in resonant with the band absorption of EZO films. The samples were probed at different wavelengths (480.18, 500.41, 550.39, 600.37, and 640.83 nm). Figure 8 shows the transient absorption signals ΔA(t) of EZO films by the TCPP experimental. This sample, when it was excited with 325 nm pump (Fig. 8(a)), displays a very fast component with a time which is constant of few ps and a small amplitude slow decay component with a lifetime of few hundred ps, in probing at nearly 559.91 nm wavelengths. The fast component is due to electron-phonon coupling; the slower decay is due to phonon relaxation. The excitation-transfer processes, including the energy transfer between CB state and internal conversion transition happened in few ps scale, leading to the fast decay component in pump-probe dynamics. The slow decay component corresponds to the evolution of the excitation without internal conversion transition state. Directly upon excitation with the laser pump pulse at 325 nm, a positive signal at different probed wavelengths (Fig. 8(a)) has been recorded. When excited with 325 nm pumps and probed with different wavelengths, the results show that EZO have the same mechanism derived from pump-induced excited-state absorption. This sample, when excited with 380 nm pump (Fig. 8(b)), displays as that the lifetime of fast component does, however, it varies in the range of 18-36 ps with increasing probe wavelength for probe wavelengths of 500.41 to 600.37 nm, and a small amplitude slow decay component with a lifetime of >1.2 ns. The fast component is due to excited-state to DL coupling; the slower decay is due to phonon relaxation. The excitation-transfer processes, including the nonradiative process from CB state directly to the DL transition happened in few ps scale, leading to the fast decay component in pump-probe dynamics. The slow decay component corresponds to the excitation without internal conversion transition state from DL state directly to CB state. When excited with 380 nm pumps and probed with different wavelengths, the results show that EZO have the same mechanism, which is derived from pump-induced excited-state absorption. When excited with 400 nm pump (Fig. 3(c)), a very fast component with a time which is constant of 12-26 ps and a small amplitude slow decay component with a lifetime of >1.2 ns that were obtained. The fast component is due to high excited-state to low excited-state coupling, while the slower decay is due to phonon relaxation. The excitation-transfer processes, including the nonradiative process from high excited-state directly to the low excited-state, lead to the fast decay component in pump-probe dynamics. The slow decay component corresponds to the evolution of the excitation wi internal conversion transition state. Directly upon excitation with the laser pump pulse at 400 nm, a positive signal at different probed wavelengths (Fig. 8(c)) has been recorded. When excited with 400 nm pump and probed with different wavelengths, the results show that A-ZnO has the same mechanism, which is derived from TPA induced ESA. The similar analysis process has been reported by many other papers [63–65].
Fig. 8 Measured transient absorption change of EZO for three different pump wavelength (a) 325 nm, (b) 380 nm and (c) 400 nm.
In order to explain the results of transient transmittance, a photonic excitation absorption model was established. We propose a following several mechanism involving singlet-singlet exciton annihilation for the trap formation and PIA, which is operated under femtosecond pulse excitation. PIA of excited state, which results in a highly reactive excited state, has been proposed in explaining single electron experiments with two-photon excitation [66].
The pumping at 325, 380 and 400 nm excites the transition singlet S0 → S1 (one-photon excitation) or S2 (two-photon excitation) state. If the trap formation or pump-induced excited-state absorption occurred via the singlet-singlet annihilation mechanism (Eq. (2), the photodamage of CW excitation would be much slower than we observed. The theoretical explanation of the process of mechanism transformation comes as follows: when excited with 325 nm pumps and probed with different wavelengths, which is derived from one-photon -induced excited-state S1 absorption. When excited with 380 nm pumps, which is derived from one-photon -induced excited-state DL absorption. When excited with 400 nm pump, which is derived from two-photon -induced excited-state ESA (Fig. 9).
Fig. 9 Principal pump-induced absorption processes for a) SPA induced DLA, b) SPA induced ESA and c) simultaneous two-photon absorption induced ESA.
4. Conclusions
In summary, we reported the nonlinear absorption properties and ultrafast carrier dynamics of EZO films. Moreover, the mechanisms of the nonlinear optical behavior for EZO films were analyzed. The transformation from SA to RSA is observed in the EZO films with the increase of the input laser energy of 30 nj in the open-aperture Z-scan model. The Er dopping could be the main cause of the transformation from SA to RSA in the EZO films. A further confirmation is still needed in the future work about the mechanism of the third-order optical nonlinear absorption in the EZO films. From the experimental results, the self-focusing effect and TPA induced excited state SA or RSA behavior of EZO films with different energy has been determined. This laser with the different pulse width and wavelengths excitation causes to SA or RSA and TPA induced excited state absorptions (ESA). These effects constitute nonlinear absorptions in samples. The results indicate that the carrier dynamics depends strongly on the pump and probe wavelengths and the bandgap of the material. We also presented experimental results that the few decades picosecond component has been assigned to vibrational relaxation in the excited electronic states, and the slow components represent the decay from singlet excited state to the ground state (< 1.2 ns) and DL to the ground state (> 1.2 ns). Our results indicate that TPA dominates at longer wavelengths and ESA dominates at shorter wavelengths. When the excitation is into the absorption band we see saturation type of behavior. At higher intensities we even observe RSA behavior at these wavelengths. This study of Er doped ZnO provides theoretical and methodological support for improving the multifunctional optical devices such as optical limiter, all-optical switch or planar optical amplifiers in the future.
Funding
National Natural Science Foundation of China (11504072); University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2016179).
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