1. Introduction

Laser micromachining modifies or removes local target material. When combined with computer-aided-design, direct laser writing (DLW) is capable of mass-producing microphotonic devices and subsystems on metal, dielectric [1–7] and soft material platforms [8–10]. Especially, pulses laser micromachining is widely adopted for producing high quality microstructures for photonic applications. The light-matter interaction dynamics in DLW include photo-induced plasma effects (femto to picosecond scale) and slow photo-thermal process (nano to microsecond scale) [11]. The photo-induced plasma effect can lead to mechanical deformation on the target surface, and the subsequent photo-thermal effect plays a major role in the phase transition and melting-reflow process [12]. By controlling the laser dosage levels, DLW can be a lithography-free, low cost and versatile platform for manufacturing microstructures [1,2].

Chalcogenide glass (ChGs) is a special group of photonic materials with high refractive index, wide transparent window in mid-infrared and high third order nonlinearities [13]. Also, its rich photochemical processes, such as phase changing and photo-induced polymerization, provide another degree of freedom for tailoring its structure by using light [14–19]. Waveguides, photonic crystal structures and metasurfaces have been demonstrated in ChG films by dry etching, ion exposure or nanoimprinting processes [13,20–25]. The studies of DLW is mostly limited on single point dosage test [16,19,26,27], or proof-of-concept waveguide demonstration in non-phase change ChG films [17,18,28,29].

In this work, we focus on DLW patterned chalcogenide strips and waveguides by using pulsed laser. A nanosecond infrared laser is used to study the power-dependent phase change and ablation results on ChG glass (Ge₂₃Sb₇S₇₀ (GSS) and Ge₂Sb₂Te₅ (GST)). Both ablation and phase transition interactions can induce a large refractive index contrast (Δn>1) in chalcogenide glass thin film [17,23,28,30,31]. Laser parameters such as power and number of the pulse exposure are studied for their influence on the size and surface roughness of the pattern. The topology of the laser processed device is characterized by the combination of scanning electron microscopy (SEM) and surface profilometry. As the transient laser exposure creates non-uniform phase transition in the exposed area, micro-Raman spectroscopy is used to map the crystallization profile [32–34]. We show that tunable optical gratings can be achieved by using DLW and baking on Phase Change Materials (PCM) to switch between amorphous and crystalline states. DLW provides a low cost and versatile way to produce ChG PCM based microphotonic devices.

2. Results and discussion

2.1 Experimental apparatus

 Figure 1(a) shows the schematics of the confocal optical pathways. Three optical pathways are coupled onto the sample for alignment, illumination, and laser writing. A low power (< 4 mW) HeNe laser (633 nm) is used for optical alignment (red pathway). A Spectra-Physics HIPPO Nd:YAG laser (center wavelength 1064 nm, pulse duration of 10 ns, repetition rate of 10kHz-300kHz and duty cycle of 0.05%) is focused onto the sample using a long working distance NIR objective (Mitutoyo, 50×, NA=0.42)(dark red path in Fig. 1(a)). The objective reduces the diameter of the Gaussian beam from 0.6 mm to 6 µm, resulting the energy density of 88 J/cm² with 25µJ excitation energy. A collimated white light source is used to illuminate the sample collinearly with the excitation laser, and the reflected light is collected by a CCD camera for monitoring the patterned sample (yellow path in Fig. 1(a)). The sample is mounted on a motion stage (Physics Instruments) with automatic translation controls in the x, y and z directions with 50 nm spatial resolution and 1 cm traveling distance. The speed, direction and distance of stage movement can be programmed. Smooth straight lines (edge roughness 5 nm) are obtained by carefully adjusting the laser power level and the speed of the stage (Fig. 1(b-e)). It takes a few minutes to sweep the area of 40 mm² [1,2,35]. Sapphire and Titanium-coated Lithium Niobate (Ti-LN) are bulk film samples (thickness of 500 µm). The amorphous ChG thin films are deposited in a Physical Vapor Deposition chamber at room temperature. To prevent oxidation, the evaporation process does not start until the chamber pressure is under 5 × 10⁻⁶ Torr. Ge₂Sb₂Te₅ and Ge₂₃Sb₇S₇₀ powders are purchased off-the-shelf as source material (Irradiance glass. Inc.). The thickness of the Ge₂Sb₂Te₅ and Ge₂₃Sb₇S₇₀ thin films are 80 nm and 450 nm, respectively [36]. Atomic force microscopy (AFM) characterization shows the r.m.s. surface roughness of the deposited Ge₂Sb₂Te₅ and Ge₂₃Sb₇S₇₀ films are 1.19 nm and 1 nm, respectively.

 

Fig. 1. (a) Schematics of direct laser writing setup. Three light sources are coupled onto the chip for alignment (633 nm c.w. laser), illumination (Tungsten bulb) and laser writing (1064 nm, pulsed). The sample is mounted on a piezo controlled xyz linear translation stage. (b) Optical image of example patterning results on GST, (c) GSS, (d) Sapphire and (e) Ti-LN.

Download Full Size | PDF

2.2 Power and exposure-time dependent linewidth

Theoretically, the spatial resolution (R) can be expressed as [37]: R = (k×λ)/NA, where λ = 1064 nm is the laser center wavelength, NA = 0.42 is the numerical aperture of the objective, and k is a coefficient depending on process-related factors and material properties (empirically ranging from 0.25 to 1) [37]. The upper limit of R is derived to be 2.53 µm as k=1. In practice, larger feature sizes are usually observed due to thermal or plasma diffusion. Long thermal diffusion length and high laser power leads to larger feature sizes but smoother edges due to the reflow process. Using materials with lower thermal conductivity or applying laser pulses with higher pulse energy, higher repetition rate or longer exposure time also lead to larger feature sizes. The resolution can be further reduced by refining the laser power steps, replacing a laser with shorter wavelength, or using an objective with larger NA.

With a fixed laser repetition rate of 15 kHz, we adjust the laser power level and exposure time (stage movement speeds) to achieve small feature size/linewidth with smooth edges. The laser power levels are set in the range between 0 to 0.8 mJ/pulse. The stage translation speeds are between 1 mm/s and 12 mm/s. The same set of direct laser writing experiments are carried out on both dielectric single crystalline materials (Sapphire, LN) and semiconductor amorphous phase change materials (GSS and GST). We use a surface profilometer (Dektak XT, Bruker) to measure the linewidth profiles of laser inscription. By using a stylus sensor which has controllable scanning speed and stress force, the profilometer could obtain accurate surface profiles by mechanically contacted measurement. At a fixed linear translation speed of 5 mm/s, the feature size is observed to be strongly dependent on laser power. As shown in Fig. 2(a), the linewidth expands more than an order of magnitude (from 6 µm to 80 µm) in GST as the laser power doubles (from 40% to 80%). The minimal linewidth depends on the melting temperature and thermal diffusion length of the target material. Shortening the exposure time can also reduce the feature size (Fig. 2(b)). It is noted that the exposure time (t) is inversely related to the stage translation speed (s): s = R/t. As the translation speed increases from 1mm/s to 12 mm/s, a 15% reduction in linewidth is observed in GST while ∼40% shrinkage of the linewidth is observed in GSS, LN and sapphire. The laser pulse energy is set to 0.4 mJ/pulse. In Fig. 2(c), the cross-section profiles of single line inscriptions with increasing powers and a constant stage speed of 5 mm/s for GSS samples demonstrate that line width increases with an increasing power. For the nanosecond pulsed laser, when the laser energy is over a certain threshold that the laser heating the material’s over its melting temperature, photo-thermal induced ablation dominates in this process. The ablation produces a thermal-affected area that may leads to melting and re-deposition of the removed materials [38]. And the shockwaves induced in this process forms the thermally induced defects such as cracks and notches (Fig. 2(c)(d)) [27,39]. As the laser energy increases, the thermal ablation effect will bring a higher thermal transfer that induces more significant melting, re-deposition and shockwaves which leads to larger notch width (Fig. 2(c)(d)) [39]. The depth of written inscriptions on GSS is about 400 nm, which is close to the thickness of deposited film. At the same power levels, the GST notch width is much wider, as shown in Fig. 2(d). The translation speed is kept at 5 mm/s for both measurements. Summarizing the results in Fig. 2, we conclude that reducing the laser power and increasing the translation speed can reduce the feature size. Faster stage translation speed has a more significant influence on reducing the edge roughness. The smallest feature of GST has been achieved to 6 µm, sharp edges, good edge roughness and highest sensitivity to laser power. As the laser writing could create small and smooth features on GST samples, we choose GST as our main material for studying the laser material interactions for different regimes based on varying laser dosage levels. This may lay a fundamental step towards chalcogenide applications such as laser-induced phase change, reversible photonic memories and tunable metasurfaces based on GST.

 

Fig. 2. Cross sectional profiles of crystalline and amorphous materials at increasing power levels. (a) Width of laser-removed notch versus laser power, at the translation speed of 5 mm/s. The average pulse number per step along the line is 30. (b) Width profile of different materials under different speeds but fixed laser pulse energy of 0.4 mJ/pulse. Error bars represent standard deviation (s.d.) for three measurements of width profiles. (c) Width profile characterization of laser inscriptions for GSS, (d) GST at different power levels. The bumps outside of the notch are formed by laser melt and redeposition.

Download Full Size | PDF

2.3 Transition from phase change to laser ablation in thin film GST

Figure 3 presents data on the transition from the phase change to the laser ablation regimes for laser interacted with thin film GST. We consider three laser pulse energy regimes (nonthermal melting, thermal phase melting, and ablation) [27,39]. At lower laser pulse energies, the light-matter interaction leads to a phase transition in GST that enables high-contrast photonic patterns. At lower laser pulse energies (level I in Fig. 3(a)), a partial crystalline phase transition is observed on the top surface of an a-GST thin film. Somewhat higher laser pulse energies (level II in Fig. 3(a)) can melt the crystalline GST such that the reflow process leads to the cross-section profile schematically depicted in the figure [39]. At the highest pulse energies (level III in Fig. 3(a)), the GST can be totally ablated near the center of the beam [27]. We studied the topology and the chemical composition of the different regions of the patterned GST after exposure to different laser pulse energy levels. Regions labelled ii or iii in Fig. 3 are crystalline states, while regions labeled i are amorphous GST. Figure 3(b) shows a perspective view of the laser-machined GST thin film after exposure to pulse energy level I. The nanocrystalline GST strip is ∼8 µm wide. Figure 3(c-d) are the microRaman measurement of laser-written GST after exposure to laser pulse energy level II. Three typical Raman peaks are observed for a-GST (black curve in Fig. 3(c)). The dotted and solid red curves represent c-GST in the reflow (ii) and middle region (iii) of the laser exposed area. Typical Raman peaks for the metastable face centered cubic (fcc) state of c-GST are observed in the ii and iii regions, while the stable hexagonal close packed (hcp) lattice structure is only observed in region iii [25]. The transition from the amorphous state to the crystalline fcc state takes place around 150°C, while the more stable hcp state starts to appear at temperatures above 220°C [25]. The Raman intensity profile of the fcc peak indicates thicker c-GST on the side than in the middle region (Fig. 3(d)). The optical images in Fig. 3(e) show that at laser pulse energies higher than 32 μJ, the c-GST in the middle is totally removed, meaning that 32 μJ is the threshold for entering the laser ablation regime (level III). This observation is consistent with prior work [27].

 

Fig. 3. Laser induced phase transitions and ablation on GST. (a) Schematics of GST state evolution under low (I), medium (II) and high (III) laser pulse energy dosage. The dark grey, light gray and blue areas represent crystalline, as-deposited amorphous GST and silica substrate respectively. (b) SEM imaging of laser induced phase transitions on GST at pulse energy level I indicated in (a). (c) Normalized Raman spectra of GST sample exposed to laser pulse energy at level I. Region ii and iii are c-GST and region i is a-GST. (d) Raman fcc peak of c-GST (Red solid square dots) and cross-sectional profile in the z direction (red curve) versus position along the x axis. The GST profile is obtained at the translation speed of 5 mm/s and laser pulse energy of 30 μJ. (e) Laser induced ablation on GST at higher power levels (II & III).

Download Full Size | PDF

2.4 Laser patterned chalcogenide grating

Having demonstrated direct laser patterning of GST, we now utilize the technique to fabricate a planar photonic device for a proof-of-concept. A one dimensional grating with micrometer scale feature size is defined on a GST-on-glass substrate using the direct laser patterning technique introduced above. The reflection spectra of the grating are measured by FTIR [40,41]; the method is schematically depicted in Fig. 4(a). Unpatterned thin film GST samples before (a-GST) and after baking (c-GST) (Fig. 4(b)) are characterized for comparison with laser patterned grating samples (Fig. 4(c)(e)). For unpatterned thin film GST samples, the reflection after baking to create a crystalline state (c-GST) is larger than before baking (a-GST) [15]. An optimized laser pulse energy (25 ∼26 μJ) and stage speed (10 mm/s) are chosen for reducing the feature size on a-GST samples (laser pulse energy level II as depicted in Fig. 3(a)). Linewidths of 6 µm and 9 µm gap were formed under laser exposure pulse energies of 25 and 26 μJ, respectively (Insets of Fig. 4(c)). We keep the GST strip width to be 8 μm. The two samples exhibit similar reflection spectra for the first order diffraction peak with a maximum reflection at the wavelength around 1.7 μm. The reflection spectra show higher reflectivity for gratings with larger lattice constant (Fig. 4(c)). The results align with the numerical predictions (Fig. 4(d)). The simulated cross-sectional mode profile around 1.7 μm shows that the highest electric field intensity is located around the side edge of the a-GST strips.

 

Fig. 4. Intensity of first order diffraction versus wavelength of input laser. (a) Schematic of FTIR setup for angled reflection measurements. The incident angle θ_i= 5°. The reflection angle θ_r=25°. (b) Measured reflection spectrum of thin film a-GST and c-GST samples. (c) Measured and (d) simulated reflection first-order diffraction spectra of the laser inscribed ChG grating (a-GST) has the lattice constant of 17 µm (red curve) and 14 µm (blue curve). The GST strip width is 8 µm for both devices. The laser energy levels are 25∼26 μJ (level II in Fig. 3(a)). Insets in (c): microscope images of correspondent device (Scale bar: 10 μm). Inset in (d) Cross sectional optical mode profile near the edge of the grating. (e) Measured and (f) simulated first-order diffraction spectra before and after GST phase change for the grating fabricated with laser energy of 25 μJ. Insets in (e): Cross sectional schematics of the laser-patterned GST grating before (red curve) and after (blue curve) baking at 200°C. The light and dark grey represent a- and c-GST region, respectively.

Download Full Size | PDF

Phase change material-based optical devices can be tuned through thermal post processing. An ellipsometer (J.A. Woolam, M-2000VI) is used to characterize the UV-vis-NIR refractive index spectra of an as-grown a-GST thin film and the c-GST after baking above the phase transition temperature. After baking at 200°C, the thickness of the a-GST film reduces from 80 nm to 73 nm [42–45], and the refractive index near 1 µm increases from 4.1 + 0.42i to 5.05 + 1.54i (Δn>1 at higher wavelength beyond 1 μm) (Fig. 5 in the Appendix). After baking beyond the phase transition temperature, the GST thin film is fully crystallized (top insets of Fig. 4(e)). The reduced thickness and increased refractive index lead to higher diffraction efficiency (Fig. 4(e-f)).

3. Conclusion

In this work, we demonstrate that a nanosecond pulsed sub-bandgap (infrared) laser can directly pattern high contrast photonic features through both laser ablation and phase change processes. At an optimized pulse energy, faster translation speed or less overlap between adjacent exposure spots lead to smaller feature sizes and smoother edges. We demonstrate the fabrication of features as small as 6 µm. The linewidths of the direct laser patterning lines are compared across dielectric and semiconductor platforms. By controlling the exposure dosage (laser pulse energy and exposure time), three distinguishable regimes of laser inscription results are observed in GST. We then use this mask-free and chemical-free process to pattern a set of GST gratings. We verified the tunability of first-order diffraction intensity through control over the line spacing and phase transition of GST. The results demonstrate a simple and straightforward micro-fabrication method for high-contrast planar photonics on dielectric and semiconductor materials that are difficult to remove with conventional chemical etching process. The results can thus be applied to the further development of these materials for applications such as metasurfaces, diffractive grating, or filters.

Appendix

GST phase change characterization

We measure the refractive index spectra of a-GST (before baking) and c-GST (after baking) thin film (prepared in house) by an Ellipsometer in the range of UV-vis-NIR. After baking at 200°C (beyond the phase transition temperature), a large refractive index contrast (Δn>1) in the near infrared range has been observed (Fig. 5).

 

Fig. 5. Refractive index spectra of GST before and after baking at 200°C.

Download Full Size | PDF

Funding

Air Force Office of Scientific Research (FA9550-18-1-0300); National Aeronautics and Space Administration (80NSSC17K0526); Shandong governmentsponsored study abroad program scholarship (201801050).

Acknowledgments

The authors acknowledge Patrick Sohr, Yong Wang, Prof. Stephanie Law for assistance on FTIR measurement, and technical support from Advanced Material Characterization Laboratory at the University of Delaware.

D. M., T. K and N. A. are supported by AFOSR Young Investigator Program (FA9550-18-1-0300). A. S. is supported by NASA Early Career Faculty program (80NSSC17K0526). C. C. is supported by Shandong government sponsored study abroad program scholarship (201801050).

Disclosures

The authors declare no conflicts of interest.

References

1. R. R. Gattass and E. Mazur, “Femtosecond laser micromachining in transparent materials,” Nat. Photonics 2(4), 219–225 (2008). [CrossRef]  

2. F. Chen and J. R. V. de Aldana, “Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining,” Laser Photonics Rev. 8(2), 251–275 (2014). [CrossRef]  

3. A. Zoubir, M. Richardson, C. Rivero, A. Schulte, C. Lopez, K. Richardson, N. Hô, and R. Vallée, “Direct femtosecond laser writing of waveguides in As₂S₃ thin films,” Opt. Lett. 29(7), 748–750 (2004). [CrossRef]  

4. F. Christensen and M. Müllenborn, “Sub-band-gap laser micromachining of lithium niobate,” Appl. Phys. Lett. 66(21), 2772–2773 (1995). [CrossRef]  

5. S. Daniel and J. Peña, “Study of the wavelength dependence in laser ablation of advanced ceramics and glass-ceramic materials in the nanosecond range,” Materials 6(11), 5302–5313 (2013). [CrossRef]  

6. S. Ben Yoo, B. Guan, and R. Scott, “Heterogeneous 2D/3D photonic integrated microsystems,” Microsyst. Nanoeng. 2(1), 16030 (2016). [CrossRef]  

7. X. Zheng, B. Jia, H. Lin, L. Qiu, D. Li, and M. Gu, “Highly efficient and ultra-broadband graphene oxide ultrathin lenses with three-dimensional subwavelength focusing,” Nat. Commun. 6(1), 8433 (2015). [CrossRef]  

8. E. Zgraggen, I. Soganci, F. Horst, A. La Porta, R. Dangel, B. Jan Offrein, S. Snow, J. Young, B. Swatowski, C. Amb, O. Scholder, R. Broennimann, U. Sennhauser, and G. Bona, “Laser direct writing of single-mode polysiloxane optical waveguides and devices,” J. Lightwave Technol. 32(17), 3036–3042 (2014). [CrossRef]  

9. M. Deubel, G. von Freymann, M. Wegener, S. Pereira, K. Busch, and C. Soukoulis, “Direct laser writing of three-dimensional photonic-crystal templates for telecommunications,” Nat. Mater. 3(7), 444–447 (2004). [CrossRef]  

10. M. Farsari and B. Chichkov, “Materials processing: two-photon fabrication,” Nat. Photonics 3(8), 450–452 (2009). [CrossRef]  

11. X. Liu, D. Du, and G. Mourou, “Laser ablation and micromachining with ultrashort laser pulses,” IEEE J. Quantum Electron. 33(10), 1706–1716 (1997). [CrossRef]  

12. B. Rethfeld, D. S. Ivanov, M. E. Garcia, and S. I. Anisimov, “Modelling ultrafast laser ablation,” J. Phys. D: Appl. Phys. 50(19), 193001 (2017). [CrossRef]  

13. B. J. Eggleton, B. Luther-Davies, and K. Richardson, “Chalcogenide photonics,” Nat. Photonics 5(3), 141–148 (2011). [CrossRef]  

14. M. Wuttig and N. Yamada, “Phase-change materials for rewriteable data storage,” Nat. Mater. 6(11), 824–832 (2007). [CrossRef]  

15. M. Wuttig, H. Bhaskaran, and T. Taubner, “Phase-change materials for non-volatile photonic applications,” Nat. Photonics 11(8), 465–476 (2017). [CrossRef]  

16. L. Liu, X. Zheng, X. Xiao, Y. Xu, X. Cui, J. Cui, C. Guo, J. Yang, and H. Guo, “Femtosecond laser ablation and photo-induced effects of As₄₀S₆₀, Ga_0.8As_39.2S₆₀ and Ga_0.8As_29.2Sb₁₀S₆₀ chalcogenide glasses,” Opt. Mater. Express 9(9), 3582–3593 (2019). [CrossRef]  

17. M. Kang, L. Sisken, J. Cook, C. Blanco, M. C. Richardson, I. Mingareev, and K. Richardson, “Refractive index patterning of infrared glass ceramics through laser-induced vitrification,” Opt. Mater. Express 8(9), 2722–2733 (2018). [CrossRef]  

18. T. Gretzinger, S. Gross, M. Ams, A. Arriola, and M. J. Withford, “Ultrafast laser inscription in chalcogenide glass: thermal versus athermal fabrication,” Opt. Mater. Express 5(12), 2862–2877 (2015). [CrossRef]  

19. T. Gu, H. Jeong, K. Yang, F. Wu, N. Yao, R. D. Priestley, C. E. White, and C. B. Arnold, “Anisotropic crystallization in solution processed chalcogenide thin film by linearly polarized laser,” Appl. Phys. Lett. 110(4), 041904 (2017). [CrossRef]  

20. C. Ríos, P. Hosseini, R. A. Taylor, and H. Bhaskaran, “Color Depth Modulation and Resolution in Phase-Change Material Nanodisplays,” Adv. Mater. 28, 4720–4726 (2016). [CrossRef]  

21. K. Dong, S. Hong, Y. Deng, H. Ma, J. Li, X. Wang, J. Yeo, L. Wang, S. Lou, K. B. Tom, K. Liu, Z. You, Y. Wei, C. P. Grigoropoulos, J. Yao, and J. Wu, “A Lithography-Free and Field-Programmable Photonic Metacanvas,” Adv. Mater. 30(5), 1703878 (2018). [CrossRef]  

22. A. V. Pogrebnyakov, J. A. Bossard, J. P. Turpin, J. D. Musgraves, H. J. Shin, C. Rivero-Baleine, N. Podraza, K. A. Richardson, D. H. Werner, and T. S. Mayer, “Reconfigurable near-IR metasurface based on Ge₂Sb₂Te₅ phase-change material,” Opt. Mater. Express 8(8), 2264–2275 (2018). [CrossRef]  

23. Y. Zou, L. Moreel, H. Lin, J. Zhou, L. Li, S. Danto, J. D. Musgraves, E. Koontz, K. Richardson, K. D. Dobson, R. Birkmire, and J. Hu, “Solution Processing and Resist-Free Nanoimprint Fabrication of Thin Film Chalcogenide Glass Devices: Inorganic–Organic Hybrid Photonic Integration,” Adv. Opt. Materials 2(8), 759–764 (2014). [CrossRef]  

24. P. Pitchappa, A. Kumar, S. Prakash, H. Jani, T. Venkatesan, and R. Singh, “Chalcogenide phase change material for active terahertz photonics,” Adv. Mater. 31(12), 1808157 (2019). [CrossRef]  

25. P. Guo, J. Burrow, G. Sevison, A. Sood, M. Asheghi, J. Hendrickson, K. Goodson, I. Agha, and A. Sarangan, “Improving the performance of Ge₂Sb₂Te₅ materials via nickel doping: Towards RF-compatible phase-change devices,” Appl. Phys. Lett. 113(17), 171903 (2018). [CrossRef]  

26. Q. Zhang, H. Lin, B. Jia, L. Xu, and M. Gu, “Nanogratings and nanoholes fabricated by direct femtosecond laser writing in chalcogenide glasses,” Opt. Express 18(7), 6885–6890 (2010). [CrossRef]  

27. C. H. Chu, C. D. Shiue, H. S. Cheng, M. L. Tseng, H. P. Chiang, M. Mansuripur, and D. P. Tsai, “Laser-induced phase transitions of Ge₂Sb₂Te₅ thin films used in optical and electronic data storage and in thermal lithography,” Opt. Express 18(17), 18383–18393 (2010). [CrossRef]  

28. D. Mao, M. Chen, N. Augenbraun, A. Soman, X. Ma, T. Kananen, M. Doty, and T. Gu, “Micromachining of chalcogenide waveguides by picosecond laser,” CLEO: Science and Innovations SF3O. 5 (2019).

29. T. Gu, J. Gao, E. E. Ostroumov, H. Jeong, F. Wu, R. Fardel, N. Yao, R. D. Priestley, G. D. Scholes, Y. Loo, and C. B. Arnold, “Photoluminescence of Functionalized Germanium Nanocrystals Embedded in Arsenic Sulfide Glass,” ACS Appl. Mater. Interfaces 9(22), 18911–18917 (2017). [CrossRef]  

30. K. Chaudhary, M. Tamagnone, X. Yin, C. M. Spägele, S. L. Oscurato, J. Li, C. Persch, R. Li, N. A. Rubin, L. A. Jauregui, K. Watanabe, T. Taniguchi, P. Kim, M. Wuttig, J. H. Edgar, A. Ambrosio, and F. Capasso, “Polariton Nanophotonics using Phase Change Materials,” Nat. Commun. 10(1), 4487 (2019). [CrossRef]  

31. R. Haglund, D. Hewak, S. Ramanathan, and J. J. Hu, “Feature issue introduction: Optical Phase Chang Materials,” Opt. Mater. Express 8(9), 2967–2969 (2018). [CrossRef]  

32. W. Qiu, C. Cheng, R. Liang, C. Zhao, Z. Lei, Y. Zhao, L. Ma, J. Xu, H. Fang, and Y. Kang, “Measurement of residual stress in a multi-layer semiconductor heterostructure by micro-Raman spectroscopy,” Acta Mech. Sin. 32(5), 805–812 (2016). [CrossRef]  

33. L. Ma, X. Fan, and W. Qiu, “Polarized Raman spectroscopy–stress relationship considering shear stress effect,” Opt. Lett. 44(19), 4682–4685 (2019). [CrossRef]  

34. C. Xu, T. Xue, W. Qiu, and Y. Kang, “Size Effect of the Interfacial Mechanical Behavior of Graphene on a Stretchable Substrate,” ACS Appl. Mater. Interfaces 8(40), 27099–27106 (2016). [CrossRef]  

35. M. Malinauskas, A. Žukauskas, S. Hasegawa, Y. Hayasaki, V. Mizeikis, R. Buividas, and S. Juodkazis, “Ultrafast laser processing of materials: from science to industry,” Light: Sci. Appl. 5(8), e16133 (2016). [CrossRef]  

36. J. Michon, S. Geiger, L. Li, C. Goncalves, H. Lin, K. Richardson, X. Jia, and J. Hu, “3D integrated photonics platform with deterministic geometry control,” Photonics Res. 8(2), 194–201 (2020). [CrossRef]  

37. S. Okazaki, “Resolution limits of optical lithography,” J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 9(6), 2829–2833 (1991). [CrossRef]  

38. F. Wang, Z. Wang, D. Mao, M. Chen, Q. Li, T. Kananen, D. Fang, A. Soman, X. Hu, C. Arnold, and T. Gu, “Light Emission from Self-Assembled and Laser-Crystallized Chalcogenide Metasurface,” Adv. Opt. Mater. 8(8), 1901236 (2020). [CrossRef]  

39. K. Phillips, H. Gandhi, E. Mazur, and S. K. Sundaram, “Ultrafast laser processing of materials: a review,” Adv. Opt. Photonics 7(4), 684–712 (2015). [CrossRef]  

40. D. Wei, C. Harris, and S. Law, “Volume plasmon polaritons in semiconductor hyperbolic metamaterials,” Opt. Mater. Express 7(7), 2672–2681 (2017). [CrossRef]  

41. D. Wei, C. Harris, C. Bomberger, J. Zhang, J. Zide, and S. Law, “Single-material semiconductor hyperbolic metamaterials,” Opt. Express 24(8), 8735–8745 (2016). [CrossRef]  

42. G. Sosso, S. Caravati, R. Mazzarello, and M. Bernasconi, “Raman spectra of cubic and amorphous Ge₂Sb₂Te₅ from first principles,” Phys. Rev. B 83(13), 134201 (2011). [CrossRef]  

43. P. Němec, V. Nazabal, A. Moreac, J. Gutwirth, L. Beneš, and M. Frumar, “Amorphous and crystallized Ge–Sb–Te thin films deposited by pulsed laser: Local structure using Raman scattering spectroscopy,” Mater. Chem. Phys. 136(2-3), 935–941 (2012). [CrossRef]  

44. T. Nonaka, G. Ohbayashi, Y. Toriumi, Y. Mori, and H. Hashimoto, “Crystal structure of GeTe and Ge₂Sb₂Te₅ meta-stable phase,” Thin Solid Films 370(1-2), 258–261 (2000). [CrossRef]  

45. V. Bragaglia, B. Jenichen, A. Giussani, K. Perumal, H. Riechert, and R. Calarco, “Structural change upon annealing of amorphous GeSbTe grown on Si (111),” J. Appl. Phys. 116(5), 054913 (2014). [CrossRef]