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Abstract
In this study, we systematically investigated femtosecond laser-induced damage characteristics of ZHBLAN (27ZrF4–25HfF4–10BaF2–5SrF2–5CaF2–4LaF3–14AlF3–10NaF), and the impact of Er3+ doping on the optical characteristics and damage characteristics of the matrix material. The glasses were subjected to a comparative analysis with respect to typical ZBLAN glasses. The sizes and morphology of damaged craters changed with the power of the 800 nm femtosecond laser. The samples were analyzed using digital and scanning electron microscopes, and their femtosecond laser-induced damage thresholds (LIDTs) were calculated by linear regression. The LIDTs decreased obviously from 612 mJ/cm2 to 532 mJ/cm2 after Er3+ was doped into the ZHBLAN glass. This is related to the absorption of Er3+ near the wavelength of the femtosecond laser and the change in the bandgap structure of the glass due to Er3+ doping. In addition, we have analyzed the electron convergence rate within the material during laser irradiation. The combination of physical models and simulation results reveals the association between the ionization rate of optical materials and the laser damage threshold.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The 3–5 µm mid-infrared (MIR) region not only contains the characteristic vibrational frequencies of molecular bonds but also is an atmospheric communication window, making MIR lasers promising tools for important applications in defense, sensing, and medicine [1–6]. Owing to the development of semiconductor lasers, the use of near-infrared laser in pumping rare earth (RE)-doped glass fibers for obtaining MIR lasers has been widely researched, and research into this method has focused on the selection of matrix glasses. Fluoride glasses are promising materials due to their excellent MIR transmission characteristics, low phonon energy, and high RE solubility, which is suitable for laser materials [7,8].
RE ions doping in matrix glass is commonly used to achieve specific spectroscopic properties required for photonic applications. In general, MIR band lasing at 2.7–3.9 µm has been successfully achieved in ZBLAN fibers because of the energy level of RE ions, such as Er3+, Dy3+, and Ho3+ [9–11]. Brierley et al. first obtained a laser output of 2.702 µm in a ZBLAN glass; the concentration of Er3+ ions was 0.1 mol%, and a 476.5 nm laser source was used [12]. Since ZBLAN glass has a relatively high phonon energy (∼580 cm−1), which leads to severe multiphonon relaxation inside the glass, the emitted laser cannot be extended to the long wavelength region [13]. The fluoroindate glass is currently used to produce laser output of over 4 µm due to its lower phonon energy (∼510 cm−1) [14]. Majewski et al. obtained 4.3µm laser emission from Dy3+-doped InF3 glass fiber by using 1.7 µm laser pumping [15]. However, the unstable components and poor thermal stability of InF3 glass have seriously hindered the application of InF3 glass fibers [16]. With the advantages of low transmission loss in the MIR band and a mature manufacturing process, the ZBLAN glass fiber remains the typical RE-doped matrix material at present.
However, the poor stability and thermal conductivity of ZBLAN glass fibers still limit power improvements needed in different fields [17]. For example, fluoride fibers would exhibit catastrophic damage under conditions of high-intensity laser beam exposure; the resulting damage may reduce device performance [18]. Therefore, the harmonization of the optical and physical and chemical properties of fluorozirconate remains a key area of research into current fluorozirconate glass systems [19]. The most common method to obtain MIR laser output is using laser-pumped RE-doped glass fibers [20,21]. However, the doping of RE ions into matrix materials promotes absorption in matrix glasses, and the effect of RE ion doping on the damage threshold of a matrix glass is rarely reported.
In this work, the laser damage characteristics of Er3+-doped fluorozirconate glasses were systematically investigated using an 800 nm femtosecond laser. The spectral characteristics, laser damage characteristics, and the effect of additional absorption on laser-induced damage to these glasses after Er3+ doping were systematically compared. In addition, the rate of electron convergence inside a material under femtosecond laser irradiation was simulated with a physical model. Variation in electron convergence rate among doped and undoped fluorozirconate glasses under the same conditions was investigated. This work provides a scientific basis for the preparation of RE-doped fluorozirconate glass fibers with high damage threshold.
2. Experiments
2.1 Sample preparation
On the basis of previous research [22], the composition of 27ZrF4–25HfF4–10BaF2–5SrF2–5CaF2–4LaF3–14AlF3–10NaF-0.5ErF3 named ZHBLANE, and ZBLANE (53ZrF4–20BaF2–4LaF3–3AlF3–20NaF-0.5ErF3) were melted using a conventional melt-quenching method. The fabrication process consisted of weighing 15 g of high-purity fluoride raw materials (99.99%), grinding them for a period over 20 minutes using a mortar. Then, the mixed samples were heat in platinum crucibles at 850°C for 2 hours. Finally, the molten glass was then poured onto a preheated copper plate and annealed at 240°C for 1 hour, with the aim of eliminating any residual stress within the material. Much hydroxyl in internal structures of the glasses was eliminated by performing the preparation in a glove box filled with dry oxygen. For comparison, ZHBLAN and ZBLAN matrix glass were melted in a same experimental environment. The annealed glass samples were prepared at the same size of Φ 9 mm × 2 mm and then precisely polished to prepare for subsequent optical measurements.
2.2 Characterization of samples and laser damage measurement
All the measurements of thermal, optical, and femtosecond laser damage were carried out at room temperature. The transmission spectra were acquired by a spectrometer (Lambda 950 UV/VIS/NIR,) and a Fourier transform infrared spectrometer (Nicolet 380). The measured wavelengths were 0.2–2.5 and 2.5–14 µm, respectively.
The laser radiation experimental equipment was utilized for performing a laser damage test and observing damaged areas. The setup of the laser radiation test equipment is described in Fig. 1. The femtosecond laser system was composed of a Ti: sapphire femtosecond laser (Mira 900D+) and optical parametric amplifier (OPA, Legend Elite Series). This system produced 130 fs laser pulses with a central wavelength of 800 nm and a repetition rate of 1 kHz. The laser pulse generated by the OPA system was directed through a half-wave plate and an attenuator, which were combined to regulate the laser power. An electronic shutter was used to control the exposure time precisely. Additionally, two small apertures were used to eliminate stray light and enhance the beam quality. The laser was focused on the sample surface by a 10× (NA = 0.25) objective lens. To ensure the accuracy of the injected laser power in the experiment, we tested the laser power between the objective and the aperture (actual injected power) before the damage experiment. The sample was positioned on a three-dimensional displacement platform, and laser irradiation position was precisely adjusted. A charge-couple device camera was installed behind the beam splitter to enable real-time monitoring of the sample surface during the laser damage process. Furthermore, the 3D images and morphologies of the damaged craters were analyzed using a digital microscope (VEX-1000E) and a scanning electron microscope (VEGA3SB-EasyProbe).
3. Results and discussion
3.1 Transmission spectra and bandgap of the Er-doped glasses
To determine the applicability for potential applications in MIR band, the transmission spectra of Er-doped 2 mm-thick glasses were measured. Figure 2(a) shows that the infrared cutoff edge of the Er-doped fluorozirconate glass is approximately 9 µm, and the transmittance of the glasses is about 90% in a wavelength range of 1–6 µm. As shown in Fig. 2(b), in the visible to near-infrared band, the peaks at 378, 520, 640, 800, 980, and 1530 nm are due to the excitation of the 4I15/2 ground state of Er3+ to 4G11/2, 2H11/2, 4F9/2, 4I9/2, 4I11/2, and 4I13/2 energy level [23]. The positions of the peaks of the Er-doped fluorozirconate glasses on different matrix materials are the same. Figure 2(b) shows that due to Er3+ incorporation, an additional absorption peak appears at 800 nm, which is the central wavelength of the femtosecond laser light source for the damage experiments. This peak at 800 nm can greatly promote the absorption of 800 nm laser by the material, thereby reducing the damage resistance of the material. The Burger-Lambert-Beer equation can be used to calculate the absorption coefficient (α) of the glass [24]:
where d represents the thickness of the glass, T0 is the maximum transmittance of the glass, and T is the transmittance of each wavelength. The absorption coefficient of the glass is 0.108 cm−1 at 800 nm. The weak absorption of the H2O group near 2.9 µm is attributed to the fabrication process, which was carried out within an oxygen-filled glove box.
Fig. 2. (a) Transmission spectra with 2 mm-thick Er-doped fluorozirconate glasses in a range of 0.25–9 µm. (the inset images are the photographs of corresponding glasses). (b) The absorption spectroscopy of the ZHBLANE glasses in the 200–1800nm range (the inset image shows the absorption coefficient of the glass at 800 nm).
The optical bandgap (Eg) of the glass can be calculated with the Tauc equation [25]. According to the equation, the transmittance of a glass is inversely proportional to the intrinsic absorption of the glass. In particular, a glass with high transmittance and low intrinsic absorption has a large Eg.
where α is the linear absorption coefficient, hυ is the energy of the incident photon, and B is a constant related to the extent of the band tail. Additionally, m is a coefficient that controls electron transition. The direct and indirect transition bandgaps can correspond to m of 0.5 and 2, respectively. Using the indirect transition process (m = 2) is appropriate in calculating the optical bandgap for amorphous materials (fluoride glasses) [25].The x-axis intercept of the fitting line (αhυ0.5 = 0) is equal to the optical bandgap after the (αhυ)0.5 and hυ (Tauc’s plots) are plotted [26]. As shown in Fig. 3(a), the Eg values of the ZHBLAN and ZBLAN glasses are 5.76 and 5.20 eV, respectively. Figure 3(b) shows that Eg decreases after Er3+ are doped into the matrix glass, and the Eg of the ZHBALNE glasses (5.28 eV) is higher than that of the ZBLANE glass (4.82 eV). The damage threshold of ZHBLANE glass is higher than that of ZBLANE glass. The Eg of the optical glass can be approximately estimated by measuring their ultraviolet cutoff wavelength [27]. As indicated in the inset, after doping with Er3+, the UV absorption edges of ZHBLAN and ZBLAN red-shifted from 201.7 nm to 212.1 nm and 231.8 to 235.1 nm, respectively. Both Tauc formula calculations and changes in the position of the UV absorption cut-off edge confirm that Er3+-doping the matrix glass reduces the Eg.
Fig. 3. (a)#and (b): the bandgap of undoped and doped fluorozirconate glasses (the inset shows the position of the UV-absorbing cutoff edge of fluorozirconate glass).
3.2 Femtosecond laser-induced damage characteristics and damage morphology analysis
During the laser damage test, the “S-on-1” regime was utilized, which involved directing S-pulses towards the samples [28]. Previous experimentation has demonstrated that the laser power employed during the test was insufficient to induce damage to the glass samples. However, owing to a cumulative effect, the area of the damage crater expands nonlinearly with increasing number of pulses number, and the expansion leads to a small calculated LIDT value [29]. Therefore, it is recommended to use above 20 mW of laser power with 100 pulses number to achieve the laser damage. In order to avoid the randomness of the experiment, three groups of experiments were repeated with different power at different positions of the glass.
The fluorozirconate glasses were placed in a displacement platform to maintain the glasses at the same horizontal position. The laser spot was consistent throughout the experiment This consistency is extremely important for laser damage morphology. All glasses were irradiated by laser with an average power of 20, 25, 30, and 35 mW. Figure 4(a) shows the digital microscopy image of the ZHBLANE glass damage morphology at varying power. As injected laser power increases, the energy density of each point is sufficient to cause laser damage. Therefore, as laser power density increases, the size of a damage crater gradually increases and gradually approaches a circular shape close to the shape of the spot.
Fig. 4. (a) Microscopic images; (b) scanning electron microscopy images; (c) 3D modes of 100 pulses damaged craters in ZHBLANE glass generated with increased laser power.
The influence of laser power on the morphology of damage craters were analyzed by scanning electron microscopy, and the internal morphology of the damage craters was observed. Figure 4(b) shows that the rings of ablation marks appear inside the damage craters due to the influence of pulse number. In addition, significant cracks form around the damage craters due to the ultrahigh peak power of the femtosecond laser. The 3D image of the damage craters can be drawn by 3D scanning performed on a microscope, as shown in Fig. 4(c). The depth of damaged craters increases from 26.27 µm to 54.79 µm as laser energy density increases in the spot.
3.3 Laser-induced damage threshold of Er-doped glasses
The laser-induced damage threshold of the fluorozirconate glasses with and without Er3+ doping was studied at various laser power levels under 100 pluses. During the measurements, the incident pulse energy (Ein) was calculated based on the average laser power (Pavg) and the repetition rate of the laser (R). Ein was calculated as follows:
The diameters of the damaged craters (D) were accurately measured with an optical microscope. The beam of the laser presents a Gaussian distribution profile, which allowed for measurement of the beam waist radius (ω0) defined as 1/e2 of peak intensity. Additionally, D2 was expressed as a function of ω0, Ein, and pulse energy threshold (Eth). The function can be written as follows [30]: The Eth of the glasses at D2 = 0 was determined by linear regression analysis. Equation (4) demonstrates that the slope of the linear fit corresponds to the laser beam radius (ω0), which was measured to be approximately 23 µm in this experiment. The LIDTs of the fluorozirconate glasses at 800 nm wavelength were calculated by plotting the square of the crater diameter against the logarithm of pulse energy. Finally, the Eth is converted into the laser-induced damage threshold (Fth) through the following formula [31]: In order to avoid the contingency of the experiment, three groups of experiments were carried out at different positions of the same glass, and the LIDT values were calculated respectively. The LIDT of ZHBLANE glass calculated three groups are 542, 518 and 537 mJ/cm2, as is shown in Fig. 5(a), and the average LIDT is 532 mJ/cm2 with about 8% flotation. As shown in Fig. 5(b), the LIDT of other glasses is calculated separately in the same way. The corresponding Fth are 494, 416, 612, and 532 mJ/cm2, as the test sequence for glass (ZBLAN, ZBLANE, ZHBLAN, and ZHBLANE) under the same laser conditions.Fig. 5. (a) The relationship between the square of damage pit diameter of ZHBLANE glass and the logarithmic form of InE; (b) The LIDT of doped and undoped fluorozirconate glasses.
The results show that when Er3+ ions are introduced, the material exhibits significant absorption of laser energy in the 800 nm band, owing to the energy transition of Er3+:4I15/2→4I9/2. Therefore, the absorption capacity of the material to the laser is improved, and the glass surface is more likely to form damage. Additionally, the rich energy level structure of Er3+ ions bring about changes in the valence band energy level (bonding state) and the conduction band energy level (anti-bonding state) within the glass. Specifically, as the energy level of the bonding state increases and that of the anti-bonding state decreases, the band gap of the glass decreases, leading to a reduction in the strength of the chemical bonds within the glass. As a result, the laser damage resistance of glass is worse.
4. Theoretical simulation research on femtosecond laser damage
Optical damage and ablation during the radiation of high-power femtosecond lasers in prepared glasses upon interaction between laser and materials have been explored [32–34]. Figure 6 shows physical changes involved in ablation at different timescales. On the femtosecond and picosecond timescales, Part of the energy absorbed by electrons is transferred to the lattice through electron-phonon coupling. This stage is called the carrier excitation phase [35,36], where the valence band electrons absorb laser energy continuously delivered to the conduction band [37]. Within a few nanoseconds, a pressure or shock wave propagates away from the laser focal volume. This stage is called the thermal phase. The surface structure of a material changes as the thermal energy diffuses out of the focal volume within a few microseconds [38]. The mechanism of femtosecond laser damage is primarily related to the accumulation of conduction band electrons (CBEs), as opposed to heat accumulation, which is different from nanosecond or picosecond laser damage.
Fig. 6. Time scale of physical phenomena related to the interaction between femtosecond laser and glass.
The interaction between a material and femtosecond laser consists of three parts. First, photoionization excites electron leap from valence band to conduction band, and the laser energy is absorbed in the material by the nonlinear absorption effects (including multiphoton or tunneling effects), resulting in the production of seed electrons. Then, the seed electrons induce avalanche ionization leading to the generation of additional CBEs. Finally, when the density of the CBEs reaches a critical value, the generated plasma has a natural frequency resonant with the laser, and then irreversible optical damage occurs on the surfaces of the material.
Therefore, we simulated the ionization rate. Photoionization is the beginning of laser damage, and the size of photoionization rate (Wpi) can characterize the electron convergence rate and can also reflect the ability of damage resistance. The most common expression for Wpi is as follows [39,40]:
The Wpi can be calculated by Eq. (6) according to the parameters of the femtosecond laser and the matrix glass material. The general rate of photoionization is a function of electric field, as is shown in Fig. 7(a). The Wpi of ZHBLAN glasses is significantly faster when Er3+ were incorporated into the matrix glass, and Wpi gradually tends to saturate when the electric field is over 300 MV·cm−1. In addition, Eqs. 6 and 9 show that the ionization rate has a direct relationship with the bandgap of a glass and decreases when a matrix glass has a wide band gap. Table 1 summarizes the bandgap values and damage thresholds for the glasses. The glass with a wide bandgap is more resistant to laser damage under the same conditions.
Fig. 7. (a) Ionization rate calculation of doped and undoped ZHBLAN glasses; (b) Keldysh parameter of the femtosecond laser in relation to laser energy on ZHBLAN glass.
Table 1. Band gap of undoped/doped ZHBLAN glasses and corresponding damage thresholds
Further, to assess whether tunnel ionization or multiphoton ionization plays a leading critical role in photoionization, we introduced the Keldysh parameter γ. At a low laser frequency and strong electric field (γ < 1.5), the photoionization rate mainly depends on tunnel ionization. Conversely, when γ > 1.5, multiphoton ionization dominates the ionization process [29]. Figure 7(b) shows the Keldysh parameter as a function of laser energy fluence for 800 nm femtosecond laser in the optical glasses (Eq. (7)). Photoionization is a multiphoton process when laser energy is at the damage threshold (612 mJ/cm2) during the damage experiment. Therefore, photoionization is mainly caused by the multiphoton process when laser energy is lower than 24.15 J/cm2. Otherwise, it is caused by the tunnel ionization.
5. Conclusion
In summary, we investigated the laser damage characteristics of novel ZHBLAN glass and analyzed the effect of Er3+ doping on its characteristics. The results show that the LIDT of ZHBLAN glass decreases from 612 mJ/cm2 to 532 mJ/cm2 after the incorporation of 0.5 mol Er3+. Absorption spectroscopy indicates that decreased LIDTs of glass are caused by two factors: Er3+ absorption at the laser central wavelength and a narrowed band gap that leads to faster ionization. Increase in laser power aggravated the damage to craters and considerably changed the morphological characteristics of the craters. At a high laser power (approximately 35 mW), the shape of a damaged crater became circular. In addition, we analyzed the rate of electron convergence inside the material during laser irradiation. The combination formula and simulation results showed that increase in bandgap decelerated electron convergence and enhanced damage resistance. This study provides scientific guidance for the fabrication of novel ZHBLAN glass fibers for photonic devices.
Funding
National Natural Science Foundation of China (62090062, 62090063, 62090064, 62090065); Key R&D Program of Ningbo City (2022Z208); K. C. Wong Magna Fund in Ningbo University.
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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