Correlation among Structure, Water Peak Absorption, and

Dec 28, 2017 - Ge11.5SbxSe88.5−x (x = 5, 10, 15, 20, 25, 30) compositions labeled as s1 to s6 and then loaded into precleaned quartz ampules. The am...
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Correlation among Structure, Water Peak Absorption, and Femtosecond Laser Ablation Properties of Ge−Sb−Se Chalcogenide Glasses Min Xie,†,‡ Shixun Dai,*,†,‡ Chenyang You,†,‡ Peiqing Zhang,†,‡ Chenfeng Yang,†,‡ Wenyong Wei,†,‡ Guangtao Li,†,‡ and Rongping Wang*,†,‡ †

Laboratory of Infrared Materials and Devices, The Research Institute of Advanced Technologies and ‡Key Laboratory of Photoelectric Detection Materials and Devices of Zhejiang Province, Ningbo University, Ningbo 315211, China ABSTRACT: A series of Ge11.5SbxSe88.5−x (x = 5, 10, 15, 20, 25, 30) chalcogenide glasses were fabricated aiming at investigating the role of structure and water peak absorption in determining femtosecond laser ablation thresholds (Fth). The results indicate that the optical band gap decreases and that the Vickers hardness increases with increasing Sb content. Meanwhile, Raman spectra were measured before the femtosecond laser ablation experiment, suggesting a rise in average bond energy in the six glass samples with increasing Sb concentration. Ablation of the sample disks in air were performed with high repetition rate ultrashort laser pulses (150 fs, 1 kHz) at different wavelengths (2.86 and 4 μm) to investigate how the water peak absorption coefficient affects ablation thresholds, which reveals the dominating role of the multiphoton ionization (MPI) progress on the ablation threshold fluence. The results will be useful for photonic devices based on Ge−Sb− Se glasses applied in high-power laser operations to prevent ablation.

1. INTRODUCTION Chalcogenide glasses have great advantages owning to their attractive optical properties, which include a faster optical response time, high linear and nonlinear refractive index, better structural stability, and ultrabroad transparency window spanning from the visible to the mid-infrared.1−5 Photonic devices based on chalcogenide fibers or waveguides have attracted wide attention for applications in medical technologies,6 chemical sensors,7 atmosphere pollution monitoring,8 and infrared imaging.9 In many of such applications, the materials are frequently interacted with ultrashort-pulsed lasers. For example, the generation of both a mid-infrared and far-infrared supercontinuum based on chalcogenide fibers requires that the fibers to be pumped with femtosecond laser pulses.10,11 Meanwhile, the oscillation of the chalcogenide Raman laser requires the materials to be pumped with picosecond pulses.12 Thus, it is essential to investigate the interaction of chalcogenide glasses with ultrashort pulses. Messaddeq et al. reported that self-organized periodic structures were formed on Ge−S-based chalcogenide glass irradiated by a femtosecond laser with a wavelength of 806 nm.13 Hari et al. first studied the wavelength and pulse length dependence of the ablation thresholds in As2Se3 glass with picosecond laser pulses, and they observed a sharp increase in the ablation threshold with a decrease in the macropulse length.14 However, most reports related to the fs laser interaction with chalcogenide glass involved As-based chalcogenides;15,16 the investigation of Asfree materials on damage is still relatively lacking. Ge−Sb−Se glasses lacking As are less toxic, and they have recently been reported as a nonlinear supercontinuum gain medium and as tapered fiber materials.17−19 In a Ge−As−Se © XXXX American Chemical Society

glass system, it has been demonstrated that the glass with a mean coordination number around 2.45 has a minimum glass fragility index, and thus, structural relaxation in such a glass is less.20,21 A typical glass that has been frequently used in waveguide and fiber-based optical devices has a composition of Ge11.5As24Se64.5.22,23 We thus chose Ge11.5SbxSe88.5−x and explored the correlation among structure, water peak absorption, and femtosecond laser ablation properties. First, the transmission properties, band gap (Eopt), and Vickers hardness (Hv) of the series of Ge11.5SbxSe88.5−x glasses were measured. Meanwhile, structural analyses of the six samples were conducted through Raman spectroscopy before the femtosecond laser experiment. Through induction of an OPA system delivering 150 fs pulses with two wavelengths (4 and 2.86 μm), the correlation between the absorption of the water peak and the laser ablation properties were investigated using different techniques including the super long depth of view optical microscope and scanning electron microscope (SEM).

2. EXPERIMENTAL METHODS 2.1. Glass Preparation. Glass samples were fabricated using the conventional melt-quench method. High-purity Ge, Sb, and Se precursors (5N) were weighed following Ge11.5SbxSe88.5−x (x = 5, 10, 15, 20, 25, 30) compositions labeled as s1 to s6 and then loaded into precleaned quartz ampules. The ampules were evacuated to 10−7 Torr at 120 °C Received: November 4, 2017 Revised: December 28, 2017 Published: December 28, 2017 A

DOI: 10.1021/acs.jpcc.7b10894 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

are attributed to the residual H2O and CO2 pollution in the glasses, respectively. Meanwhile, thinner (micron level) glass samples were fabricated by a hot pressing method to measure the optical band gap (Eopt) of the glasses. Raman spectra of these hot-pressed samples are almost identical to those of the bulk glasses, indicating that these hot-pressed films have the same structure as the bulk glasses. Therefore, the measured Eopt values for the films can be considered to be the same as those for the bulk glasses. Table 1 lists the Eopt values for the bulk glasses based on the classical Tauc equation.24 Figure 1b shows the evolution of Eopt as a function of Sb content in Ge11.5SbxSe88.5−x glass samples. It can be seen that Eopt decreases with increasing Sb content. Generally, the optical band gap is determined by differences in the electronic states between the bottom of the conduction band and the top of the valence band. The outer electron orbits of the Sb atoms are far from the nuclei, and the corresponding valence electron energies are always higher. With Sb atoms, whose atomic radii are bigger, gradually replacing Se atoms, the top of the valence band energy of glass increases.25 Furthermore, the ionic polarization of Sb ion is higher than that of Se ion,25 suggesting that Sb atoms contribute more to the valence band. Thus, the valence electrons are more likely to be excited to the conduction band, leading to a decrease in the optical band gap. 3.2. Compactness of Glassy Networks. Hardness is a parameter reflecting the overall strength of the glass chemical bonds. Materials with greater chemical bond strength are less susceptible to laser radiation. Meanwhile, small material surface defects, such as cracks, will further affect photoionization and lower the damage threshold.26 Microhardness is related to the glass structure, mean coordination number (MCN), and the type of chemical bonds.27,28 Figure 2 shows the dependence of Sb concentration between the Vickers hardness and MCN in the six Ge11.5SbxSe88.5−x glass samples. For samples s1 to s6, the values of the Vickers hardness and MCN are increasing monotonically with increasing Sb concentration. Meanwhile, the hardness varies monotonically with mean coordination number in a certain interval, which can be verified in reference.29 3.3. Structural Properties. Figure 3 shows Raman spectra and their decompositions for the six Ge11.5SbxSe88.5−x glass samples. For s1 to s4, the lattice vibrations for the samples is nearly the same, extending from 140 to around 350 cm−1, while that for s5 and s6 is extending from 100 to around 350 cm−1, indicating the similarity in mass and bond forces among these three elements. The general features of these spectra include: (i) the vibration at 203 cm−1 and shoulder at 218 cm−1, which are usually attributed to the corner-sharing (CS) and edgesharing (ES) GeSe4/2 tetrahedra, respectively;30 (ii) the peaks at ∼160 and 170 cm−1 are associated with Sb−Sb bonds and stretching modes of Ge−Ge bonds in the ethane-like (ETH) structure,31 respectively; (iii) the band peak at around 195 cm−1 is due to the stretching mode of Sb−Se in SbSe3/2 pyramids; (iv) the decomposed peak at around 256 cm−1 is ascribed to the vibration of Se−Se bonds in Se chains or Se rings.31,32 To understand the evolution of these different structural units in Ge11.5SbxSe88.5−x glasses, we decomposed these Raman spectra, and the relative contribution of each structural unit (defined as the ratio of the integrated area of each structural unit to that of the whole Raman spectrum) is shown in Figure 4. It can be found that the number of Ge−Se bonds remains almost constant until the Sb concentration reaches 25% and then drops drastically, while that of Sb−Se bonds continues to

for 2 h to remove surface moisture from raw materials and then sealed. They were heated to 850 °C and kept at this temperature for 20 h in a rocking furnace to ensure homogeneity. After the ampules were removed from the rocking furnace, they were quenched in water. Then, the resulting glass boule was subsequently annealed below the glass transition temperature Tg for 6 h and then slowly cooled to room temperature. Following the annealing process, the glasses were sectioned to form disks with a thickness of approximately 1 mm. The disks were polished to high optical quality to meet the requirements for further measurements. 2.2. Property Measurements. The optical absorption spectra of six Ge11.5SbxSe88.5−x glass samples were recorded in a range from 400 to 2500 nm with a spectrophotometer (Lambda 950 UV−vis−NIR, PerkinElmer). The IR transparency spectra were measured by Fourier transform infrared spectroscopy (FTIR) (Nicolet 380, Thermo Scientific) at a range from 2.5 to 12 μm. The Vickers hardness was measured by a microhardness tester (MH-30, Everone). Raman spectra were obtained by utilizing a confocal micro Raman spectrometer (InVia, Renishaw) with a 785 nm excitation laser ranging from 100 to 700 cm−1 at an average power of 0.05 mW. The femtosecond laser experimental configuration consisted of a Ti:sapphire femtosecond laser (Mira900D+, Coherent) and an optical parametric amplifier (OPA) (Legend Elite+ OperA Solo, Coherent) to emit 150 fs pulses of at different central wavelengths of 4 and 2.86 μm with a repetition of 1 kHz. The laser beam was directed perpendicular to the sample surface by a lens of CaF2 with 75 mm focal length, forming an almost circular beam spot on the surface. The spot size was measured by a spectral quality analyzer (PY-III-HR-C-A, Spiricon). At a given number of pulses (NOP = 10 000), the experimental conditions were kept the same except for the laser power, which was controlled by the beam attenuator. In order to ensure the accuracy of the experiment, laser irradiation was repeated three times at different sample positions. Furthermore, we observed and analyzed morphological characteristics with a scanning electron microscope (SEM) (VEGA3SB-EasyProbe, Tescan) and an optical microscope (VEX-1000E, Keyence) with the view of super long depth. All the experiments were taken at room temperature.

3. RESULTS AND DISCUSSION 3.1. Optical Absorption and Band Gap Properties of Glass Samples. Figure 1a shows the transmission spectra of six Ge11.5SbxSe88.5−x glass samples. It is clear that they have similar transmittance properties in a spectral range from 2.6 to 11 μm. From the inset of Figure 1a, it is found that there are two main absorption bands located at 2.86 and 4.31 μm, which

Figure 1. (a) Transmission spectra versus wavelength and (b) optical band gap versus Sb content in Ge11.5SbxSe88.5−x glass samples. B

DOI: 10.1021/acs.jpcc.7b10894 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Characteristic Data of Glass Samples s1−s6a label

composition

Eopt (eV)

Hv (kg·mm−2)

MCN

MPI

Fth (4 μm) (mJ/cm2)

s1 s2 s3 s4 s5 s6

Ge11.5Sb5Se83.5 Ge11.5Sb10Se78.5 Ge11.5Sb15Se73.5 Ge11.5Sb20Se68.5 Ge11.5Sb25Se63.5 Ge11.5Sb30Se58.5

1.70 1.65 1.61 1.55 1.50 1.24

98.16 109.86 119.36 127.32 130.60 135.12

2.28 2.33 2.38 2.43 2.48 2.53

6 6 6 5 5 4

201.42 185.46 175.29 165.76 148.42 121.87

a Glass composition, optical band gap (Eopt), Vickers hardness (Hv), mean coordination number (MCN), the order of multiphoton ionization (MPI,) and laser ablation threshold fluences Fth at the wavelength of 4 μm.

Figure 2. Vickers hardness and mean coordination number versus Sb content in Ge11.5SbxSe88.5−x glasses.

increase monotonically. Starting from glasses with sufficient Se concentration (s1−s5), Se is preferentially bonded with Ge. The number of Ge−Se bonds remains almost constant in Serich glasses and then decreases with further decreasing Se concentration. The increasing tendency of Sb−Se bonds can be assigned to the broken Se rings and long Se chains that form again with increasing Sb content. Meanwhile, the number of Se−Se bonds keeps decreasing as more Sb is added into the glasses, which is ascribed to the consumption of Se atoms. The number of homopolar bonds, like Ge−Ge, continues to increase, while the number of Sb−Sb bonds is zero in the glasses with lower Sb content but then begin to appear with further increase of Sb concentration. It is also evident that, with increasing Sb content, Ge−Ge bonds are formed before Sb−Sb bonds.30 As Se is replaced by Sb in Ge11.5SbxSe88.5−x glass samples, the amount of Sb−Se bonds increases, while that of Se−Se bonds decreases, resulting in a rise in average bond energy.25 3.4. Femtosecond Laser Ablation. For femtosecond laser ablation with narrow pulse duration and high peak powers in dielectric materials, laser energy is absorbed by the materials via a nonlinear process, and this promotes electrons from the valence band to the conduction band.33 The generally accepted mechanism of laser ablation in the short-pulse regime is the generation of seed electrons by photoionization, either via

Figure 4. Relative numbers of the bonds derived from the simulation of the above Raman spectrum.

multiphoton ionization (MPI) or tunnel ionization (TI), with subsequent avalanche ionization (AI) and laser energy absorption by dense plasma.34 Once the density of plasma in the conduction band exceeds the critical density, irreversible ablation occurs on the surface of dielectric materials.35,36 In this experiment, laser ablation was performed in the “S-on1” regime, which means that the pulses with a number of S are radiated at the same position on the surface of the sample. We chose two laser wavelengths (2.86 and 4 μm) to investigate the correlation between the absorption of water peak and the laser ablation properties. All glass samples were irradiated by 150 fs multipulses (10 000 pulses) with a decreasing average power ranging from 33 to 15 mW. To quantitatively analyze the effect

Figure 3. Typical decomposed Raman spectra of Ge11.5SbxSe88.5−x glass series. C

DOI: 10.1021/acs.jpcc.7b10894 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 5. 3D images of beam distribution profile at 2.86 μm (a) and at 4 μm (b).

of the water peak on the ablation threshold, we measured the spot distribution of the laser light at the same beam intensity at 2.86 and 4 μm, as shown in Figure 5a,b, respectively. It can be found that the beam profiles follow a similar Gaussian distribution, except for a slightly higher relative intensity and a smaller spot radius at 2.86 μm. Clearly, compared to 2.86 μm (photon energy about 0.43 eV), more photons are required to excite electrons from the valence band to the conduction band than for the 4 μm laser wavelength, corresponding to a lower photon energy (about 0.31 eV). The ablation of glass samples was first determined by measuring the diameter of the ablated spot D as a function of the incident peak fluence, F. The beam waist radii w0 can be obtained from the Gaussian shape of the beam profile above (defined as 1/e2 of peak intensity), corresponding to 50 μm (at 2.86 μm) and 60 μm (at 4 μm), respectively. Hence, the incident peak fluence F at the center of Gaussian beam was calculated as37 F=

2Epulse πw02

=

Table 2. Relevant Parameters of Ge11.5SbxSe88.5−x Glass Samples at Water Peak and the Ablation Threshold Fluences at the Wavelength of Water Peak Absorption

(1)

where Epulse is the total pulse energy, Pavg is laser average power, and R is the repetition rate of the laser. Material removal (ablation) takes place when the laser energy density (fluence, F) overtakes a material ablated threshold. Then the ablation threshold fluence Fth is obtained from the series and their associated square of crater diameters (D2) by a least-squares-fit from the relation38 D2 = 2w02(ln F − ln Fth)

absorption coefficients (cm−1)

Eopt (eV)

MPI

Fth (2.86 μm) (mJ/cm2)

s1 s2 s3 s4 s5 s6

0.088 0.180 0.151 0.129 0.099 0.120

1.70 1.65 1.61 1.55 1.50 1.24

4 4 4 4 4 3

173.76 163.37 154.62 147.08 136.17 125.37

To further understand the correlation among power, composition, and ablation thresholds at a wavelength of 4 μm, the morphological characteristics of craters in six samples are analyzed by SEM. Figure 7 shows the images of the overall ablation craters of the s1, s4, and s6 glass samples irradiated by different laser powers ranging from 33 to 15 mW. From the line of view, the ablation spots are gradually narrowed with a reduction in the laser fluence. Once the decreasing fluence is less than Fth of the sample, the ablation crater no longer occurs, since the provided energy is insufficient to stimulate the electron transition. Meanwhile, the corresponding column depicts the morphological characteristics of s1, s4, s6 with the same laser fluence. It can be found that the extent of ablation morphology is aggravated as the Sb concentration increased, corresponding to a lower ablation threshold, which is in agreement with the ablation threshold trend at 4 μm in Table 1. This observation indicated that the addition of Sb content would decrease the resistance of glass from damage. On one hand, the optical band gap of the six samples decreases with increasing Sb content, which means less orders of photon absorption are required to excite the electrons transited from the valence band to the conduction band. On the other hand, while Se is replaced by Sb in Ge11.5SbxSe88.5−x glass samples, the hardness and average bond energy increase. Taking the above factors into account, the progress of MPI is of greater importance in the ablation threshold. Hence, a higher Sb concentration corresponded to a lower ablation threshold in the sample glasses. Meanwhile, unique morphological features of the craters were observed at different power conditions. At a comparatively low fluence of 265.26 mJ/cm2 (the last column), a uniformly ablated, rough crater but that features a bottom could be seen as well as ripple structures in the border region (s1). Meanwhile, many bubbles were also observed at the center of ablation craters (s4), attributed to the formation and explosion of ejected molten material during laser radiation. At a laser fluence of 583.57 mJ/cm2 (the first column), volcano-like structures were observed within the ablated region. It can be explained by the formation of not completely ejected material, which is redeposited at the crater walls when the crater depth exceeds a certain value. In general, ablation progresses include amorphization, melting, recrystallization, nucleated vaporization, and finally ablation.40

2Pavg Rπw02

glass sample

(2)

From the above formula, the ablation thresholds for s1 and s6 at a wavelength of 4 μm can be obtained by plotting the square of crater diameters (D2) versus the logarithmic form of pulse fluence,39 as shown in Figure 6. All other samples have been investigated with the above-mentioned method, and the obtained values for ablation thresholds are compiled in Tables 1 and 2.

Figure 6. Squared ablation diameter D2 as a function of the logarithmic form of peak fluence ln F for s1 (black triangles) and s6 (red squares) glass samples at the wavelength of 4 μm. D

DOI: 10.1021/acs.jpcc.7b10894 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 7. SEM images (3000×) of s1, s4, and s6 glass samples after exposure to a decreasing power at 4 μm. Note that each row corresponds to a sample, and each column describes the morphological characteristics of s1, s4, s6 with the same laser fluence.

Figure 8. Ablation morphologies of six Ge11.5SbxSe88.5−x glasses under optical micrographs (500×) and SEM images (3000×) irradiated with a lower average power at the wavelength of 2.86 μm.

In order to quantitatively study how the absorption coefficient affects the ablation threshold at the position of water peak, the related parameters were listed in Table 2. For the s1 sample, the absorption coefficient at the position of water peak is smallest, while possessing the highest ablation threshold. It may be attributed to the small value of the absorption coefficient. For the s2 to s6 samples, the ablation thresholds at the position of the water peak decrease with increasing Sb concentration, which is in agreement with the tendency of ablation thresholds at the nonabsorption peak (4 μm). It is also consistent with the characteristics of ablation craters at a low power, as shown in Figure 8. It can be found the extent of ablation was gradually exacerbated with the addition of Sb content, especially for the s6 sample, which exhibited a molten state in the center of the pit, suggesting an apparent lower ablation threshold compared to others. In a comparison of the two tables, it is clear that the ablation thresholds at 2.86 μm are almost less than that of 4 μm, except for the s6 sample. Obviously, it can be explained by the beam distribution profiles; namely, a smaller wavelength means a smaller beam spot and more concentrated energy distribution. Furthermore, absorption coefficients at the position of water peak can slightly enlarge ablation thresholds. That is the reason why ablation threshold is larger than that of 4 μm for s6.

correlations between structure and water peak absorption in determining the femtosecond laser ablation threshold fluences. The optical band gap Eopt decreases with increasing Sb content. By contrast, the Vickers hardness and average bond energy increased in Ge11.5SbxSe88.5−x glass samples. With different wavelengths of 2.86 and 4 μm, the specific ablation threshold fluences were calculated according to the experimental data. Compared to ablation thresholds at 4 μm, the values at 2.86 μm were in general relatively smaller, apart for partial values of Fth, which were slightly enlarged, attributing to the absorption of water peak. All six samples follow a similar decreasing trend in ablation thresholds at wavelengths of 2.86 and 4 μm, namely decreasing with increasing Sb content, which suggested the dominating role of multiphoton ionization (MPI) on laser ablation. The results are helpful for the use of Ge−Sb−Se chalcogenide glasses in high-power laser operations.

4. CONCLUSIONS In summary, we have investigated the properties of a series of Ge−Sb−Se chalcogenide glasses aimed at figuring out the

Shixun Dai: 0000-0001-5085-0933



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.D.) *E-mail: [email protected] (R.W.) ORCID Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acs.jpcc.7b10894 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C



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ACKNOWLEDGMENTS This work was partially supported by the National Natural Science Foundation of China (grant nos. 61435009, 61377099). It was also sponsored by the K.C. Wong Magna Fund in Ningbo University.



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DOI: 10.1021/acs.jpcc.7b10894 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b10894 J. Phys. Chem. C XXXX, XXX, XXX−XXX