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Abstract
The nonlinear response of wide bandgap oxide thin films gradually emerges and attracts attention with the development of an ultra-short and ultra-intense laser. In Z-scan technique, due to the extremely lower nonlinear response of thin film compared with the common substrate, it isn’t easy to measure the multiphoton absorption coefficient of wide bandgap oxide thin films. In this study, a method is proposed to suppress the substrate impact and improve the thin film measurement sensitivity. To make the thin film nonlinear intensity dominate the total intensity, including unwanted substrate impact, material and thickness of the substrate are analyzed. Considering the nonlinear effects of different substrates and the adhesion between the substrate and the thin film, 50 μm MgF2 and quartz glass are selected as the substrate for deposition. The nonlinear intensity of substrate is suppressed to at least 80% of the whole element or can even be ignored so that the normalized transmittance of the thin film can be obtained effectively. The two-photon and three-photon absorption coefficients of HfO2, Al2O3, and SiO2 thin film are measured at different wavelengths. The nonlinear absorption response measurements of wide bandgap oxide thin films can advance the design and fabrication of low-loss photonic devices in ultra-fast lasers.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Thin-film elements play an extremely important role in all kinds of laser systems. For example, high-reflective mirrors, anti-reflective mirrors, and polarizers are essential in the high-power laser systems. These thin-films are usually consisted of wide bandgap oxide materials with high laser damage threshold and mature deposition technologies [1–3], such as HfO2 and SiO2, as the high and low refractive index material [4,5]. With the development of ultra-short and ultra-intense laser, the optical nonlinear effects in films cannot be ignored, such as nonlinear absorption [6,7], Kerr effect [8,9], and frequency conversion [10,11]. From the perspective of output laser, these effects are harmful to the pulse quality and need to be restrained. In Ref. [7], due to the two-photon absorption of the dielectric multilayer, the high reflection performance of the mirror was seriously affected. On the other hand, the exploitation of the nonlinear effect in films is desirable. It was predicated that multilayer structure composed of wide bandgap oxide material could achieve a triple frequency conversion efficiencies larger than ten percent [12]. It is important to determine the optical nonlinear coefficient of wideband oxide layers for manipulating the optical nonlinear performance.
The Z-scan technique [13–20] is widely used to measure optical nonlinearity of the material because of its efficiency and convenience. The nonlinear properties of the thin film are quite different from the bulk, especially as for the wide bandgap oxide materials. The thin film is fabricated on the substrate with a similar optical bandgap and a thickness that is thousands of times larger. There are challenges to obtain the film’s nonlinear response, due to extracting it from the total Z-scan intensity including unwanted contribution of the substrate. In previous work, the nonlinear absorption coefficient of the thin metal films [15], semiconductor films [17,18], etc., which were prepared on the quartz or BK7 with millimeter thickness, were measured by the Z-scan method. These films with narrow or zero optical bandgap showed a much larger nonlinear response than the substrate material, so there is no need to consider the substrate. In 2019, T. R. Ensley, S. Benis et al. proposed dual-arm Z-scan method when the substrate background signal is large during the measurement [19]. By scanning the thin film sample and the bare in two laser beams simultaneously, the substrate effect can be correctly eliminated if the Z-Position between two arms are completely identical. As for the wide bandgap oxide thin film, its nonlinear response is very weak. Suppressing the nonlinear response of the substrate is one of the key to improve the measurement ability. The former method of dual-arm Z-scan is efficient, but increases the difficulty of operating in matching two arms. Highly suppressing the nonlinear response of the substrate by regulating the substrate material and thickness is another way to obtain the thin film nonlinear response efficiently, and has the advantages of simplicity and high-sensitivity.
In this paper, 50 μm MgF2 and quartz glass are selected as the substrate to prepared wide bandgap oxide thin films. Due to the efficient suppression of the unwanted substrate impacts during Z-scan measurement, the normalized transmittance of thin films can be obtained directly with ignorance of substrate or extracted from the whole sample effectively. To verify the better performance of the wider bandgap and thinner thickness substrate, the two-photon absorption (2PA) and three-photon absorption (3PA) coefficient of HfO2, Al2O3, and SiO2 thin film are measured at laser wavelengths of 343 nm and 515 nm with 1 kHz repetition rate. The measurement results are important for their application in high-power laser.
2. Theories and experiment
2.1 Two-photon absorption (2PA) and three-photon absorption (3PA)
Two-photon absorption (2PA) and three-photon absorption (3PA) are nonlinear absorption processes that can be regarded as electronic transitions from the valence band to the conduction band via one and two intermediate virtual states respectively. In an optical absorption process, the irradiance-dependent optical absorption coefficient α (I) can be written as:
where I is the irradiation intensity; α0 is the linear absorption coefficient; β2 and β3 represent 2PA and 3PA coefficient, respectively. The 2PA and 3PA process occurs when the material bandgap Eg and excitation photon energy Ep satisfy Ep<Eg<2Ep and 2Ep<Eg<3Ep, respectively. In our experiment, the photon energy is 3.6 eV (343 nm) and 2.4 eV (515 nm). The optical bandgap Eg of the materials is shown in Table 1. Based on the comparison of material bandgap and the energy of two (and three) photons, it is believed that HfO2 and Al2O3 both mainly perform 2PA at 343 nm and 3PA at 515 nm, and SiO2 mainly shows 3PA response in 343 nm.
Table 1. Optical bandgap Eg of HfO2, Al2O3, and SiO2 thin film
2.2 Z-Scan measurement
The experimental setup diagram is shown in Fig. 1. The setup consists of a femtosecond laser (Light Conversion, CB5-05) with wavelength of 1030 nm, which pumps a harmonic module (Light Conversion, 2H-3H) and produces pulses in the wavelengths of 515 nm and 343 nm with ∼300 fs and ∼200 fs pulse width (FWHM) respectively, providing excitation sources for our experiment. In our experiment, the laser is operated at 1 KHz repetition rate. According to the related research in Ref. [29–31], this rate is not enough to produce the thermal lens effect. An attenuator composed of a half-wave plate and a polarizer is used to change the irradiation energy. An expander makes the laser diameter ∼4 mm (measured by a beam quality analyzer, Coherent). Two beams of laser are obtained by a beam splitter and focused by the lens with the focal length of 150 mm. The waist radius of the focused beams are about 8 μm and 12 μm in the wavelengths of 343 nm and 515 nm, respectively. The sample is placed on a mobile platform controlled by a computer and moves along the optical axis (Z axis). The transmittance data are collected by two power meters (Ophir).
The spectrum (detected by a spectrometer, Ocean Optics) of the laser in front or behind the sample shows no different or shift during scanning. It is regarded that the self-phase modulation (SPM) and the white-light continua aren’t induced in our experiment. According to the Ref. [32,33], influence of group velocity dispersion (GVD) is very weak and can be negligible.
The traditional models are applied to describe the relations between the normalized transmittance and the 2PA coefficient β2 and 3PA coefficient β3, which is described as [14,34] :
and
2.3 Thin-film material preparation
The HfO2, Al2O3, and SiO2 thin film are fabricated by electron beam evaporation (EBE) technique. The borosilicate glass (18 mm×18 mm×0.17 mm), quartz glass (Φ10 mm×50 μm), and MgF2 (Φ10 mm×50 μm) are used as the substrate to prepare the thin film. During the deposition process, the substrate temperature is 25 ℃. The base pressure is ∼2×10−3 Pa. The deposition rate is 0.1 nm/s for HfO2 film, 0.8 nm/s for Al2O3 film and 0.3 nm/s for SiO2 film respectively.
2.4 Selection of substrate
Initially, the borosilicate glass with thickness of 0.17 mm is used as the substrate for preparing wide bandgap oxide thin-films. As shown in Fig. 2(a), during the open aperture (OA) Z-scan measurement, the normalized transmittance signal of the film sample is almost overlapped with the bare substrate. The nonlinear response of the thin film cannot be measured because of the huge contribution from the substrate. To suppress the substrate impact, the material and thickness of the substrate are analyzed. In Ref. [35], it is reported that the MgF2 bulk shows no nonlinear response at 355 nm or 266 nm. According to Eq. (3) ∼ Eq. (6), the thicker sample will produce a stronger nonlinear response, so the thickness of the substrate is important for the thin film measurement. Fig. 2(b) shows the normalized transmittance measured by OA Z-scan when different substrates move along the optical axis. The intensity fluctuations of 50 μm MgF2 (blue), 0.1 mm MgF2 (purple), 50 μm CaF2 (yellow) and 50 μm quartz glass (green) are 0.38%, 0.49%, 0.68% and 0.89% respectively. It can be concluded that even under high-intensity laser irradiation, 50 μm MgF2 appears minimal nonlinear response and can be selected for preparing wide bandgap oxide thin films. Particularly, as for SiO2 thin film, considering the poor adhesion between SiO2 film and fluoride substrate, it is prepared on the 50 μm quartz glass substrate.
Fig. 2. At laser wavelength of 515 nm, (a) the normalized transmittances of 200 nm SiO2 plus 0.17 mm borosilicate glass (black) and the bare 0.17 mm borosilicate glass (red) measured in OA Z-scan at 1.6 μJ input; (b) the normalized transmittances of 50 μm MgF2 (blue), 0.1 mm MgF2 (purple), 50 μm CaF2 (yellow), and 50 μm quartz glass (green) measured by OA Z-Scan at 2 μJ input.
2.5 Acquisition of thin-film nonlinear absorption signal
To determine the nonlinear absorption, the thin film element and the bare substrate are measured by two sequential OA Z-scans. Although the nonlinear response of the thin film is weak, the substrate impact is greatly suppressed or even can be ignored. As can be seen from Fig. 3(a), compared to the nonlinear transmittance of the whole thin film sample (Tf+s), the bare substrate (Ts) has little nonlinear response, so there is no need to consider the substrate impact. The signal Tf+s can represent the nonlinear response of the thin film directly. In Fig. 3(b), the nonlinear response of the bare substrate (Ts) occupies a proportion in the total intensity (Tf+s). The normalized transmittance of thin film needs to be extracted. In our experiment, the influence of Fresnel reflections and low finesses cavities are ignored. We regard that the optical nonlinear response intensity of the thin film with the substrate is the sum of the thin film and the bare substrate. The normalized transmittance of the thin-film Tf can be extracted as:
Fig. 3. The normalized transmittances measured by OA Z-Scan at λ=343 nm of (a) 623 nm HfO2 with 50 μm MgF2 (Tf+s, black dotted line) and bare 50 μm MgF2 (Ts, red dotted line) at 0.6 μJ; (b) 700 nm SiO2 with 50 μm quartz glass (Tf+s, black dotted line), bare 50 μm quartz glass (Ts, red dotted line) and SiO2 thin film extracted by 1+(Tf+s-Ts) (blue dotted line) at 1.6 μJ.
3. Results and discussion
3.1 Measurement result of HfO2 thin film with 50 μm MgF2 substrate at 343 nm and 515 nm
As for 623 nm HfO2 thin film with 50 μm MgF2 substrate, it can be concluded from Fig. 4 that the 50 μm MgF2 shows no nonlinear response at λ=343 nm with laser intensities I1 = 1.45×103 GW/cm2 and λ=515 nm with laser intensities I2 = 3.90×103 GW/cm2. The substrate impact can be ignored when the laser peak intensity at focus point I00 ≤ I1 at λ=343 nm (or ≤ I2 at λ=515 nm) during Z-scan measurement. The measured data and best fitting with a laser intensity of 0.71×103 GW/cm2∼1.45×103 GW/cm2 at λ=343 nm are shown in Fig. 5(a), and 3.21×103 GW/cm2∼3.90×103 GW/cm2 at λ=515 nm are shown in Fig. 5(b). The corresponding 2PA coefficient (β2) and 3PA coefficient (β3) of HfO2 thin film are listed in Table 2.
Fig. 4. The normalized transmittance of 623 nm HfO2 with 50 μm MgF2 substrate (black dotted line) and the bare 50 μm MgF2 (red dotted line) at (a) λ=343 nm with laser intensities of 1.45×103 GW/cm2 and (b) λ=515 nm with laser intensities of 3.90×103 GW/cm2 respectively.
Fig. 5. Experimental data (dotted line) and fitting curve (solid line) of 623 nm HfO2 thin film at (a) λ=343 nm with laser intensities of 0.71×103 GW/cm2, 0.95×103 GW/cm2, 1.20×103 GW/cm2, 1.45×103 GW/cm2 and (b) λ=515 nm with laser intensities of 3.21×103 GW/cm2, 3.46×103 GW/cm2, 3.71×103 GW/cm2, 3.90×103 GW/cm2 respectively.
Table 2. Laser intensity I00 and corresponding 2PA and 3PA coefficient for HfO2 thin film at 343 nm and 515 nm, respectively.
3.2 Measurement result of Al2O3 thin film with 50 μm MgF2 substrate at 343 nm and 515 nm
The 200 nm Al2O3 thin film is deposited on the 50 μm MgF2 substrate. Figure 6(a) shows that at λ=343 nm with laser intensity I1 = 962 GW/cm2, the substrate performs no nonlinear response (red dotted line). When the laser intensity I00 ≤ I1, the substrate impact can be ignored and the nonlinear response of the film are easily obtained. With I00 = 552 GW/cm2∼962 GW/cm2, the normalized transmittances of 200 nm Al2O3 (dotted line) and fitting curves based on 2PA theory (solid line) are shown in Fig. 7(a). As can be seen from Fig. 6(b), at irradiation wavelength of 515 nm with intensity I2= 4.67×103 GW/cm2, the nonlinear signal of 50 μm MgF2 occupies ∼20% of the total signal. Due to the existence of the substrate impact (red dotted line), the normalized transmittance of thin film (blue dotted line) needs to be extracted from the total signal (black dotted line). Under different intensities within 3.73×103 GW/cm2∼4.67×103 GW/cm2 (≤I2), the extraction results (dotted line) and best fitting based on 3PA theory (solid line) are shown in Fig. 7(b). Table 3 lists corresponding 2PA and 3PA coefficients.
Fig. 6. The normalized transmittance of 200 nm Al2O3 with 50 μm MgF2 substrate (black dotted line) and the bare 50 μm MgF2 (red dotted line) at (a) λ=343 nm with intensity of 962 GW/cm2 and (b) λ=515 nm with intensity of 4.67×103 GW/cm2 respectively. The extraction data in (b) represent the normalized transmittance of the Al2O3 thin film (blue dotted line).
Fig. 7. Experimental data (dotted line) and fitting curve (black solid line) of 200 nm Al2O3 thin film at (a) λ=343 nm with laser intensities of 552 GW/cm2, 697 GW/cm2, 825 GW/cm2, 962 GW/cm2 and (b) λ=515 nm with laser intensities of 3.73×103 GW/cm2, 4.30×103 GW/cm2, 4.49×103 GW/cm2, 4.67×103 GW/cm2 respectively.
Table 3. Laser intensity I00 and corresponding 2PA and 3PA coefficient for Al2O3 thin film at 343 nm and 515 nm, respectively.
3.3 Measurement result of SiO2 thin film with 50 μm quartz glass substrate at 343 nm
As for 700 nm SiO2 thin film fabricated on 50 μm quartz glass, at an irradiation wavelength of 343 nm with intensity of 5.55×103 GW/cm2, compared with the nonlinear response of the whole thin film element (Fig. 8(a), black dotted line), the substrate cannot be ignored (Fig. 8(a), red dotted line). The extracting process of the thin film normalized transmittance is made, and the result shows that it occupies about 20% of the total intensity (Fig. 8(a), blue dotted line). With different irradiation intensity, the normalized transmittance of SiO2 thin film (dotted line) and those best fitting with 3PA theory (solid line) are shown in Fig. 8(b). The corresponding 3PA coefficients during the fitting process with a laser intensity of 3.81×103 GW/cm2∼5.55×103 GW/cm2 are listed in Table 4.
Fig. 8. (a) The normalized transmittance of 700 nm SiO2 with 50 μm quartz glass (black dotted line), the bare 50 μm quartz glass (red dotted line) and extraction data (blue dotted line) at λ=343 nm with intensity of 5.55×103 GW/cm2. (b)The normalized transmittance of 700 nm SiO2 thin film extracted (dotted line) and best fitting (solid line) with laser intensities of 3.81×103 GW/cm2, 4.52×103 GW/cm2, 4.92×103 GW/cm2 and 5.55×103 GW/cm2 respectively.
Table 4. Laser intensity I00 and corresponding 3PA coefficient for SiO2 thin film at 343 nm.
3.4 Measurement result of Al2O3 and SiO2 bulk material
In addition to the thin film materials above, the nonlinear absorption response of Al2O3 bulk (sapphire window) with 0.5 mm thickness and SiO2 material (quartz glass) with 0.5 mm and 50 μm thickness are measured. The 2PA and 3PA coefficient of sapphire is measured to be 2.4×10−3∼7.1×10−3 cm/GW and 2.03×10−6∼5.01×10−6 cm3/GW2at laser wavelength of 343 nm and 515 nm, respectively. Their normalized transmittance and best fitting under different irradiation energy are shown in Fig. 9 (a) and (b). As for the quartz glass, the excitation energy required for 50μm bulk is greater than 0.5 mm bulk. 3PA theory is used to fitting the measured data of quartz glass at 343 nm, as shown in Fig. 9(c) and (d). 3PA coefficient of 4.76×10−6∼9.29×10−6 cm3/GW2 and 2.59×10−6∼2.77×10−6 cm3/GW2 are extracted with different laser energy for 0.5 mm and 50 μm quartz glass, respectively.
Fig. 9. The measured data (dots) and best fitting (solid line) under different irradiation energy of (a) 0.5 mm sapphire at 343 nm with 2PA theory, (b) 0.5 mm sapphire at 515 nm with 3PA theory, (c) 0.5 mm and (d) 50 μm quartz glass at 343 nm with 3PA theory.
3.5 Discussion
In our work, through regulating the substrate material and thickness, the nonlinear signal of the thin film is dominant in the total nonlinear signal of the whole sample. The proportions of the thin film are both ∼100% for HfO2 at 343 nm and 515 nm, ∼100% for Al2O3 at 515 nm, ∼80% for Al2O3 at 343 nm, and ∼20% for SiO2 at 343 nm. Based on the above, the nonlinear absorption coefficients of the thin film are measured with high sensitivity. All measurement results of the wide bandgap thin film and corresponding bulk materials are listed in Table 5. The uncertainties in β2 and β3 mainly originate from the determination of the irradiance distribution (for example, beam waist and pulse width) during the measurement. The 2PA coefficients are measured to be 2.19 ± 11% cm/GW for HfO2 thin film and 8.12 ± 13% cm/GW for Al2O3 thin film at 343 nm, respectively. The 3PA coefficients are measured to be 5.00×10−5±3% cm3/GW2 for HfO2 thin film at 515 nm, 1.80×10−4±59% cm3/GW2 Al2O3 thin film at 515 nm, and 9.00×10−5±6% cm3/GW2 for SiO2 thin film at 343 nm. The 2PA coefficient of crystalline sapphire was reported in Ref. [35] and Ref. [36]at laser wavelength of 264 nm and 266 nm, respectively. We note that their result ∼9×10−2 cm/GW is larger than our result at 343 nm for sapphire window glass with 4.50×10−3 cm/GW, which is possibly caused by laser wavelength change or the sample difference. From Table 5, it is can be seen that the β2 value of Al2O3 thin film is 1000 times larger than that of sapphire window. The β3 value of Al2O3 and SiO2 thin film are dozens of times that of their bulk state. This phenomenon that shows the big nonlinear coefficient difference between the film and the bulk also appears in other materials, such as ZnSe (film in Ref. [37] and bulk in Ref. [38]) and ZnO (film in Ref. [39] and bulk in Ref. [40]).
Table 5. 2PA and 3PA experimental result for HfO2, SiO2, and Al2O3 thin film or bulk materials
4. Conclusion
The method is proposed that the ultra-low nonlinear response and ultra-thin substrate is applied to prepare the wide bandgap oxide thin film. 50 μm MgF2 and quartz glass are selected as substrates for preparing thin films. The unwanted nonlinear intensity contributed from the substrate is considerably suppressed or even can be ignored during the Z-Scan technique, which makes high sensitivity for identifying the multiphoton absorption coefficients of the thin film material. Using this method, the nonlinear absorption response of HfO2, Al2O3 and SiO2 thin film are measured at laser wavelengths of 343 nm and 515 nm. By comparison with the corresponding bulk materials, it is found that the nonlinear coefficient of the thin film is much larger than that of the bulk. We believe the results in this paper to be the first measurement of the 2PA and 3PA coefficient of wide bandgap oxide monolayer film. Our work provides a highly sensitive method for the nonlinear optical parameter measurement of low-nonlinear-response oxide thin film, which is significant for manipulating the nonlinear performance of optical mirrors in ultra-fast lasers. In the future, other oxide thin film with wide band gap (such as Ta2O5 and Nb2O5) will be measured, and more different substrates will be analyzed.
Funding
National Key Research and Development Program of China (2018YFE0115900); National Natural Science Foundation of China (11874369, 52002271, U1831211); Strategic Priority Research Program of the Chinese Academy of Sciences (XDB1603), CAS special research assistant project.
Acknowledgments
This work was supported by the National Key R&D Program of China (Grant No. 2018YFE0115900), National Natural Science Foundation of China (Grant No. 11874369, U1831211 and 52002271), and Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB1603), CAS special research assistant project.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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