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Abstract
Femtosecond (fs) laser–thin film interaction is one of the most practical methods for fabricating functional nanostructures. However, the details of the interaction mechanism remain unclear. In this study, we demonstrate an abnormal ablation effect on nanofilms by using a tightly focused single fs laser pulse. After the irradiation of a single Gaussian-shaped femtosecond laser pulse, a molten micro/nanopatch at the irradiated central high-power zone is isolated from the surrounding film. The confined localized threshold effect is proposed as the main mechanism for the phase isolation. With this effect, the high refractive index dielectric Ge2Sb2Te5 crystal nanostructures can be fabricated by directed dewetting of the isolated molten micro/nanopatch on Si substrates. After the laser irradiation, the central isolated liquid through an amorphous GST film is transformed into a crystalline state after resolidification. The isolated central micro/nanopatch size can be controlled by the focused spot size and pulse energy, so that the morphologies (size, geometrical morphology, and distribution) of GST nanostructures can be flexibly modulated. Furthermore, separated solid and liquid phase states detected using spatial-temporal-resolved microscopy validates the crucial role of the confined-localized threshold effect in the dewetting effect based on the separated liquid phase.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High refractive index dielectric or semiconductor nanostructures provide a superior alternative approach to control and manipulate an electromagnetic field at nanoscale dimensions with low dissipative losses compared to the plasmonic metallic nanostructures. Based on the Mie resonance effects of the high confinement of surface phonon polaritons [1–3], the so-called all-dielectric metasurface based on high refractive index opens a door for a broad range field of biology [4,5], optics [4], and physics [6] etc. Different types of dielectric materials have been demonstrated to constitute all-dielectric metasurfaces [2,7], such as Si, Ge, and SiGe etc. Recently, the need of efficient, dynamic, and programmable functionality for the next-generation metasurface system require the tuning/switching of their dielectric properties. The phase transition between crystalline and amorphous states in dielectric nanostructures has demonstrated as a valid method for switching the Mie resonance. Going beyond, phase-change materials (PCMs) with two reversible and rapidly switchable stable phases (amorphous and crystalline states) in distinct optical and electrical properties offer better alternatives for nanophotonics because of their tunable and low-loss dielectric properties [8–10]. In particular, chalcogenide alloys, especial Ge2Sb2Te5 (GST) which is one of the most representative PCMs, shows hugely contrasting properties between amorphous and crystal phases, whose phase switching can be triggered by thermal sources, electrical, and laser pulses. Up to now, GST has been widely studied as an active medium in rewritable optical disk storage [11], non-volatile electronic memories [12–14], and reconfiguration metasurfaces [8–10] based on laser excitation. Furthermore, in contrast to nanosecond and microsecond laser pulses, it was recently demonstrated that multi-level phase states with semi-crystallized states or different crystalline depth can be achieved by femtosecond laser pulses [15]. We recently demonstrated an enhanced crystallization effect by multiple fs laser pulses irradiation [16]. These remarkable properties have led to their great potentials in the tunable all-dielectric metasurfaces applications [17,18]. In general, the size, geometrical morphology, arrangement, and phase state of GST nanostructures are crucial factors affecting their optical properties through electric and magnetic responses. Thus, precisely controlling the morphology of GST nanostructures is essential. A lithography-free and single-step technique for the controllable formation of nanostructures with ordered arrangement is desirable.
Direct ion-beam milling, multistage e-beam and nanoimprint lithography techniques are the most popular approaches for nanostructures fabrication. Nevertheless, the high cost and microscale size limitation restrict their practical applications. Therefore, various methods of dielectric nanostructures fabrication have been developed aiming to the ordered large-scale nanostructuring, such as monodisperse nanoparticle colloidal methods [19], laser ablation [7], laser induced forward/backward transfer [3], and thin film dewetting [20,21] etc. Among all these various nanofabrication approaches, thin film dewetting has demonstrated to be one of the most prevalent methods for the fabrication of functional nanostructures. However, the typical uncontrolled and spontaneous dewetting of thin films often leads to nanostructure formation on the entire substrate with random arrangements and sizes [21]. The point-by-point direct laser writing with confined affected area provides an opportunity to arrange dewetting surface structures in a desirable order. Generally, the heat effects of long-pulse laser (such as nanosecond laser) and continuous lasers are used for thin film dewetting to fabricate metallic nanostructures [22,23]. Recently, tightly focused fs laser with pulse energies below the ablation threshold has been suggested to be a promising alternative for fabricating nanostructures with various morphologies, such as micro/nanobumps [24–26] and nanoneedle/nanojets [25–27] on thin nanoscale films. However, dewetting effect–based, isolated, and well-arrayed nanoparticle (nanodome) structures are rarely obtained through the irradiation of Gaussian-shaped fs lasers. For example, Makarov et al. reported that well-ordered Au and Si nanoparticles based on the fs laser–induced direct dewetting effect can be fabricated through fs laser surface patterning of thin films [7,28]. This method is a single-step process and is based on the prepatterned micro/nanopatch mechanism caused by direct fs laser cutting with an ultrahigh irradiated pulse number to induce residual heat transfer.
In this study, we demonstrate that the fs laser quasi-cutting process can be realized based on the laser threshold effect of a tight focused single-shot ultrashort laser pulse with the Gaussian shape in a single step. Crystalline GST nanostructures with controllable morphologies can be fabricated through single-shot fs laser pulse irradiation. The size of a central molten spot size plays a crucial role in the resulting dewetting nanostructures. Single GST micro/nanostructure with a diameter range of nanometer to micrometer can be flexibly fabricated using specific tight focus spots and pulse energies. The confined localized threshold effect during fs laser–material interactions is considered as the main mechanism resulting in the formation of controllable single or disordered GST nanostructures. Strong lateral temperature gradients can be induced based on the tight fs laser pulse focusing condition, promoting the subsequent structural modulations. Upon the tight focused Gaussian-shaped fs laser pulse irradiation, a circular molten patch of GST formed at a pulse energy higher than the melting threshold can be obtained. Based on the dewetting effect driven by the thermocapillary force, the isolated molten GST patch is transformed into crystalline GST nanostructures on the central irradiated zone with increased height due to hydrodynamic instability. The experimental results obtained through an imaging technique with ultrafast spatiotemporal resolution strongly validate with the proposed mechanism. Furthermore, randomly distributed GST nanoparticles and quasi-concentric circular-distributed GST nanostructures, including nanodisks and nanoparticles, can be obtained in an irradiated area by changing the size of the confined molten GST area.
2. Experimental setup
2.1. Femtosecond laser fabrication
Experiments are performed using a Ti:sapphire fs laser system (spitfire ace, Spectra-Physics) that provided a Gaussian shaped pulse with a central wavelength of 800 nm, a pulse duration of 35 fs, and a repetition rate of 1 kHz. The 40-nm-thick films of amorphous GST is deposited on Si (111) substrates through magnetron sputtering from a high-purity stoichiometric target, and AFM is used to verify the film thickness. The sample is mounted on a computer-controlled six-axis moving stage (M-840.5DG, PI, Inc.). The pulse number delivered to the sample is controlled by a fast mechanical shutter (SH05, Thorlabs) synchronized with the laser repetition rate. The single-shot fs laser pulse is focused on the sample surface. Different types of focusing objectives are employed to change the size of the focused spot size on the sample surface. The 20× objective (NA = 0.45) is used to obtain a small focal spot size (width at the waist defined by 1/e2 point) around 4.4 µm, and a relatively large-sized focused spot around 55 µm is achieved by using an achromatic lens (f = 100 mm). A variable neutral density filter is used to change the irradiation pulse energy. Figures 1(a)–(c) illustrate the formation of crystal GST micro/nanostructures. The area of circular micro/nanoscale molten GST is initially separated from the surrounding film by using the single-shot fs laser pulse based on the melting threshold [Fig. 1(a)], similar to a laser quasi-cutting process. Subsequently, isolated molten GST [Fig. 1(b)] shrinks because of the Rayleigh instability. After solidification, the molten area at the center of the irradiated spot is transformed into crystal micro/nanostructures [Fig. 1(c)]. Figures 1(d) and 1(e) illustrate typical nanostructures obtained based on the threshold effect. Depending on the pulse energy irradiated on the thin films of amorphous GST, different molten areas can be induced. At low pulse energy, the molten area is not completely separated from the surrounding area, leading to the formation of partly shrunken nanodisk structures [Fig. 1(d)]. At high laser pulse energy, the entire area of the thin film of amorphous GST is completely separated from the surrounding thin film above the melting threshold, leading to the formation of shrunken nanoparticles due to the surface tension [Fig. 1(e)]. As an example of the formation of crystal GST nanoparticles with a large area, Fig. 1(f) presents a dark-field microscopic image of well-arrayed nanodisks (left) and nanoparticles (right) structures.
Fig. 1. (a)–(c) Formation of single crystalline GST nanostructure from an amorphous GST film through single-shot fs laser irradiation; the inset in (b) is the SEM image of the relatively large central patch. (d) GST nanodisk produced at a pulse energy of 0.8 nJ (5.2 mJ/cm2). (e) SEM image of the well-arrayed GST nanoparticles formed at a pulse energy of 2.1 nJ (13.8 mJ/cm2). (f) Dark-field microscopy of the arrayed GST nanodisk (left) and nanoparticle (right).
The morphology of GST micro/nanostructures is investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). A commercial Raman system is employed for the detection of the phase state of GST micro/nanostructures. Furthermore, an optical fiber spectrometer connected to a dark-field microscope is used for the optical characterization of crystal GST nanostructures.
2.2. Temporal-spatial-resolved pump–probe imaging
The reflectivity evolution of the irradiated GST thin film surface is detected using a reflective-type pump–probe experimental system, which is consistent with that used in our previous work [16]. The relatively high pulse energy combined with a large-sized focused spot is selected because of the restriction of image and spatial resolution in the reflective-type pump–probe technique. An achromatic lens with focusing length f = 150 mm at a 45° incident angle is selected to focus the pump pulse on the GST film surface. The focal spot size of the pump laser pulse under normal irradiation condition is around 70 µm. The output fs laser pulse is split into pump and probe pulses. The pump pulse is used to excite the sample surface. A frequency-doubled probe pulse with a specific time delay (τ) is focused on the excited sample surface to capture the images of the irradiated surface. The transient reflected optical images are recorded using a charge-coupled device (CCD).
3. Results and discussions
3.1 Directed dewetting of the thin films of amorphous GST through a Gaussian-shaped fs laser pulse based on the confined-localized threshold effect
The spontaneous dewetting of an as-deposited thin film has been demonstrated as a promising method to fabricate nanoparticles efficiently induced by thermal annealing or laser irradiation which has adaptability no matter on noble metal or semiconductor thin films. Generally, the formation of semiconductor dewetting nanoparticles are accompanied with phase transitions, such as crystalline-amorphous transformation or even composition segregation. Futhermore, studies have demonstrated that a single nanoparticle can be formed by heating the isolated micro/nanoscale patch of thin films when their width-to-height ratio satisfies certain conditions [28,29]. Thus, the formation of a single nanoparticle can be controlled by modulating the size of the heated molten patch. In this research, the size of the focused spot and pulse energy are deliberately controlled to change the area of molten GST.
Figures 2(a)–(d) present the SEM images of GST nanostructures fabricated using single-shot fs laser pulse irradiation focused by the 20× objective (∼4.4 µm focal spot size) with increased pulse energies. This focusing condition enables the formation of nanodisks and nanoparticles with controllable sizes. Typically, three distinct patterns of surface micro/nanostructure fabrication can be obtained. In the first regime, a nanodisk structure [red dashed line presented in Fig. 2(a)] can be fabricated at a low pulse energy of 0.8 nJ (5.2 mJ/cm2). The nanodisk is characterized by a thin GST film ring around it (area between blue and red dashed lines), as demonstrated through profile measurement shown in Fig. 3(a). When the pulse energy increases slightly to 1 µJ, the surrounding circular ring of the GST film become thinner [Fig. 2(b)] and the grown nanodisk develops with smaller diameter and higher height, as illustrated in the AFM image [Fig. 3(a), the red line]. Nanodisks with a diameter of approximately 450 ± 50 nm can be produced using the pulse energies of 0.8–1.5 nJ (5.2–9.8 mJ/cm2). The following results are obtained for nanodisk formation with increased pulse energy: 1) the nanodisk diameter decreases, and 2) the height (distance between the substrate and the surface of the circular ring area) of the thinner ring of the GST film decreases while its width (distance between the outer thinner and inner GST rings) increases. This evolution process is verified using AFM profile measurement [Fig. 3(a)]. After a further increase in the pulse energy at 1.6 nJ (10.5 mJ/cm2), the isolated GST nanoparticle [Fig. 2(c)] surrounded by the Si substrate is obtained at the center of the irradiation zone. Moreover, the fabricated GST surface structure is analyzed using energy dispersive X-ray spectroscopy (EDX) to verify the nanoparticle composition. Figure 2(f) presents the EDX mapping of Ge, Sb, and Te distribution. The complete coverage of Ge, Sb, and Te on the central nanoparticle confirms the complete overlapping of the elements of initial thin films. When the pulse energy increased to 2.4 nJ (15.7 mJ/cm2), the GST diameter decreases and height increases. The AFM results indicate that when the pulse energy increased to 2.8 nJ (18.4 mJ/cm2) [Fig. 2(d)], the nanoparticle height decreases and its dimeter continued decreasing [Fig. 3(b)]. We attribute the diameter and height decrease to the jetting effect as the pulse energies increase [30]. The spattered GST nanoparticles marked by the red arrow shown in Fig. 2(d) also confirms the jetting dynamics. Thus, the second regime of GST nanoparticle is developed on the Si substrate with a pulse energy of approximately 1.5–2.8 nJ (9.8–18.4 mJ/cm2), where nanoparticles are surrounded by a bumped circle ring [Figs. 2(c) and (d)]. In these focusing condition [Figs. 2(a)–(d)], the corresponding isolated molten GST micro/nanopatch is measured from 900 to 1000 nm, and the corresponding ξ ranges from 22.5 to 25. Furthermore, according to our experimental results, the ordered single GST nanostructure fabrication probability is around 100%. With further increase in the pulse energy to 3 nJ (19.7 mJ/cm2), a third typical concentric microstructure with a GST circular micropatch decorated by random GST nanoparticles in the central area is found [Fig. 2(e)]. As the pulse energy increases, the central molten area increases, which breaks the ordered single dewetting nanostructure formation condition (ξ≈53), leading to the formation of a resolidificated micropatch structure. The crystalline phase state of the central GST circular micropatch is confirmed by the Raman measurement. Furthermore, according to the AFM profile measurement shown in Fig. 3, the three raised typical GST surface structures are surrounded by a bumped peripheral rim. Two factors, phase transition and materials transport, are included in the GST nanostructures height variation. The phase transition from amorphous state to crystalline state lead to the thickness reduction. However, this reduction is counterbalanced by materials transport effect under fs laser induced thin film dewetting process, leading to the height of the GST nanostructures higher than the original deposited GST thin film surface. Moreover, the molten GST material shrinks radially outwards in the fs laser irradiation area, which is driven by the surface tension that is considered as the main mechanism for its formation.
Fig. 2. GST nanostructures produced through the single-shot fs laser pulse with various pulse energies: (a) 0.8 nJ (5.2 mJ/cm2), (b) 1 nJ (6.5 mJ/cm2), (c) 1.6 nJ (10.5 mJ/cm2), (d) 2.8 nJ (18.4 mJ/cm2), and (e) 3 nJ (19.7 mJ/cm2). (f) EDX elemental mapping of GST nanoparticle presented in (c). The red arrow shown in (d) is provide to assist the reader in locating the splattered nanoparticles.
Fig. 3. (a) AFM profile of GST nanodisk obtained at the pulse energies of 0.9 nJ (5.9 mJ/cm2), 1 nJ (6.5 mJ/cm2), and 1.3 nJ (8.5 mJ/cm2), respectively. (b) AFM profile of GST nanostructure evolution with increased pulse energy.
The evolution of fabricated surface structures indicates that the irradiation pulse energy of the fs laser pulse plays a crucial role in the morphology control of GST micro/nanostructures. The horizontal molten area obtained on the thin film of amorphous GST increases with an increase in the pulse energy due to the threshold effect; thus, the isolated molten micro/nano-patch area increases. The vertical film of amorphous GST does not melt completely at low pulse energies; partially molten GST is accumulated at the center of the thin film surface and shrunk. Therefore, the relatively low surface tension of an unseparated molten area leads to first regime of nanodisk formation on the surface of the GST thin film. The complete vertical layer of the GST thin film melt, resulting in an increased molten area at high pulse energies. Even under this condition, the width-to-height ratio satisfies single nanostructure formation. The separation and melting of the GST film area caused by the melting threshold is accompanied by the dewetting effect, leading to the contraction of a single nanoparticle structure at the center of the irradiation zone on the substrate surface. However, the larger isolated area of the central molten circular patch with a range of width-to-height ratios that do not satisfy single nanostructure formation similar to the whole thin film dewetting upon thermal annealing, leading to the fabrication of random surface nanostructures, which is related to Rayleigh instability [21,22,31]. Therefore, for the third regime, instead of the complete melting in the high-energy area treated with the Gaussian-shaped fs laser pulse, a circular micropatch with nanoparticles in the central irradiated zone is obtained. Figure 4 presents the fabricated nanostructures with different morphologies and their corresponding dark-field optical microscope images. Dark-field images indicate that the isolated nanostructure exhibits a color change from blue to yellow, revealing that the variations in the excitation wavelengths of Mie resonance related to the nanostructure diameter [7,32].
Fig. 4. (a)–(d) Fabricated GST nanostructures with different sizes and the pulse energies of 0.9 nJ (5.9 mJ/cm2), 1.7 nJ (11.1 mJ/cm2), 2.3 nJ (15.1 mJ/cm2), and 2.5 nJ (16.4 mJ/cm2), respectively. (e)–(h) Corresponding dark-field optical images of GST nanostructures presented in (a)–(d). All the images share the same scale bar in (a)–(d) and (e)–(h), respectively.
3.2 Modulation of dewetted GST micro/nanostructures based on the control of the focused spot size
Based on the aforementioned analysis and experiment results, the diameter of dewetted GST micro/nanostructures could be modulated by changing the pulse energies. This modulation is based on the confined-localized threshold effect and can be used to determine the isolated patch area on the central zone through the irradiation pulse energy. Furthermore, to prove the modulation effect, we fabricated GST nanostructures through single-shot fs laser pulse irradiation with a relatively large-sized focused spot (about 55 µm) by using an achromatic lens (f = 100 mm) under loosely focusing condition. Figures 5(a)–(d) illustrate the changes in GST nanostructures with different pulse energies under large focusing spot size condition (∼55 µm). The isolated GST nanoparticle [Fig. 5(a)] surrounded by the bumped circular rim and randomly distributed GST nanoparticles [Figs. 5(b)–(d)] are fabricated in the central irradiation area. The results of EDX mapping [Fig. 5(e)] indicated a clear separation of Si and GST elements, thus characterizing the fabricated GST nanoparticles embedded on the Si substrate. The regime of the single GST nanoparticle can be formed at relatively low pulse energies of 1–1.5 µJ (42.1– 63.2 mJ/cm2). The results of EDX mapping confirm the elemental composition of the fabricated nanoparticle. The experimental results demonstrate that the focusing condition plays an important role for the single GST nanoparticle formation probability. Unlike to the tightly focusing condition, the morphologies (including the geometrical morphology and size) of the formed single GST nanostructure remains nearly unchanged when the pulse energies changes. The geometrical morphology remains a nanoparticle structure, and the diameter of the isolated nanoparticles is around 1.7 ± 0.2 µm. Furthermore, the probability of the GST nanoparticle fabrication keeps around 50%∼70% when the pulse energy changes, and the value of ξ remains around 137 in this loosely focusing condtion. According the threshold effect, the area of cutting isolated molten area increases with the increase of the irradiated pulse energy, which is similar to the trend observed in the thermal annealing induced thin film dewetting, leading to the formation of randomly arranged nanoparticles [Figs. 5(b)–(d)]. As shown in Fig. 5(b), the central GST micropatch (marked by the red dashed line) exhibits randomly distributed nanoholes with pulse energy of 2 µJ (84.2 mJ/cm2). These nanoholes are formed mainly because of the sputtering of melted materials caused by fingering instability in the non-complete dewetting stage [21]. During the nanoscale thin film nanostructuring by fs laser irradiation, the jetting dynamics is anther typical effect induced by the cavitation bubbles near the free liquid surface [30,33,34]. Thus, the jetting effect during the fs laser induced melting process is proposed as the main factor for the surrounding nanoparticle formation. When the GST thin film melts completely, the pressure and local temperature gradient may have induced the partial vaporization of the film and lead to the ejection of melted materials. The nanoparticles distributed near the GST micropatch can be regarded as an evidence of jetting effect. Furthermore, as expected, with higher pulse energies, the whole spontaneous random nanoparticles are fabricated in the central area [Figs. 5(c) and (d)] due to the Rayleigh-Plateau instability [21], which is similar to that in the 4.4 µm focusing condition. This is mainly because of the breakup of the single dewetting nanostructure formation condition. The elementary composition of central micro/nanostructures is confirmed by the EDX and Raman measurements.
Fig. 5. Fabricated GST surface structures at the pulse energies of (a) 1 µJ (42.1mJ/cm2), (b) 2 µJ (84.2 mJ/cm2), and (c) 2.3 µJ (96.8 mJ/cm2), respectively. (d) Magnification of disordered distributed nanoparticles presented in (c). (e) EDX elemental mapping of GST nanoparticles presented in (a).
Dewetted nanostructures are formed through the solidification of the melted materials. According to studies, amorphous-to-crystalline phase change can be induced by applying an electric field, a laser pulse, or temperature gradients. To characterize the detailed crystalline structure of the fabricated GST nanostructures, Raman spectra of different locations are measured. Figure 6(a) reveals that the as-deposited GST thin film (area B) exhibits a broad peak covering 100–180 cm−1 with two overlapping bands peaking at approximately 123 and 150 cm−1, in which the broad band appearing at approximately 150 cm−1 is considered the main characteristic peak of amorphous GST attributed to the stretching vibration of amorphous Te-Te bonds [35,36]. For large GST nanoparticle with diameter of 1.7 µm fabricated in the central laser-irradiated area, the Raman spectrum is highly modified with two sharp peaks at approximately 105 and 160 cm−1, confirming the fs laser–induced crystallization on the GST nanoparticle [35,37]. In particular, peaks near 110 cm−1 corresponds to the vibration of a heteropolar bond in tetrahedral GeTe4 or pyramidal SbTe3 [37,38]. The band peak near 160 cm−1 can be ascribed to Sb-Sb vibrations in (Te2)Sb-Sb(Te2) or (TeSb)Sb-Sb(Te2) [37], which is considered A1g vibration mode. Because the laser power decreases from the center to the edge of the irradiated zone, the surface crystallization on area A [Fig. 6(a)] is highly different from that on the nanoparticle structure. In this case, peaks can be observed near 125 and 141 cm−1. The Raman peak appearing at 125 cm−1 corresponds to the A1 vibration mode of the GeTe4-nGen (n = 0, 1, 2, 3) tetrahedron with an apex angle [35]. The peak that appears at 141 cm−1 can be attributed to the vibrations of Te-Te bonds [39]. Figure 6(b) presents the Raman spectra of the GST nanoparticle with diameter of 450 nm. For the nanoparticle with a smaller diameter, Raman peaks shift to higher wave numbers located at 115 and 165 cm−1. The peak shift indicates changes in the bond length and angles [35,40], and the enhancement of Mie resonances could also be considered as one of the factors [41]. Recently, a fs laser–printing technique was used to demonstrate the effect of the size of printed nanoparticles on crystalline structure, and the nanoparticle nuclear and cooling period during crystallization with different diameters is considered the main cause of phase state differences [32]. A new peak appearing at 135 cm−1 is observed for the smaller nanoparticle, which may be attributed to the segregation of the Te crystalline phase due to the overmelting effect caused by the high irradiation energy [42]. The corresponding Raman spectra of the GST nanoparticle confirms that the Te crystalline-phase state is different from the initial amorphous-phase state.
Fig. 6. (a) Raman spectra of the different locations of the fabricated GST surface structure. The inset is the SEM image of the fabricated GST surface structure. (b) Raman spectra of the GST nanoparticle with smaller diameter. Insets shown in (a) and (b) exhibit SEM images of the fabricated GST surface structure, the scale bar is 5 µm, and 500 nm, respectively.
3.3 Determination of the temporal and spatial evolution of dewetting induced by the single-shot fs laser pulse on the surface of GST thin films by using the pump–probe technique
Ultrafast time-resolved pump–probe microscopy has been considered a superior technique, in terms of validity, for recording the temporal and spatial evolution of phase changes induced by a single-laser pulse [16,43,44]. To reveal the amorphous-to-crystalline phase change dynamics of GST nanoparticles during fs laser dewetting, spatial-temporal-resolved reflective pump–probe imaging is employed to investigate crystallization evolution induced through single-shot fs laser pulse irradiation. The pump pulse energy is 2 µJ (51.9 mJ/cm2), which is higher than the GST film ablation threshold for single-shot fs laser irradiation. Figures 7(a)–(d) illustrates the spatial-temporal-resolved images of the fs laser dewetting dynamics of the GST thin film obtained after single-shot fs laser pulse irradiation. The transient reflectivity of the irradiated area exhibites typical evolution similar to that reported in our previous study [16]. In the complete irradiated area, reflectivity increased before 400 fs because of the formation of a dense electron plasma at an excited surface [45,46]. Subsequently, the reflectivity in the central area of the irradiated zone decreases with an increase in the delay time [red dashed line marked in Figs. 7(a)–(d)], and the outer ring region exhibites a higher reflectivity [area between red and blue dashed lines in Figs. 7(a)–(d)]. Figure 7(g) presents spatially resolved reflectivity distribution along the long axis of an elliptical focus area with different time delays. At τ = 100 fs, an increased flat-top emerges with the reflectivity higher than the reflectivity of the initial unexposed surface. By contrast, when τ = 400 fs, the reflectivity of the complete area remains higher than that of the initial unexposed surface, while with a concave profile due to the decreased reflectivity value emerges on the central region. The decreased reflectivity of the central area become lower than that of the unexposed surface, which indicates a strong absorption caused by a superheating liquid-phase state [46–48]. The corresponding fabricated structures is characterized through SEM [Fig. 7(e)]. An elliptical ablated structure surrounded by the bumped peripheral rim is obtained on the fs laser–irradiated area. Two distinguished morphologies are obtained, and randomly distributed nanoparticles located on the central irradiated area are fabricated based on the dewetting effect. In addition, as indicated in Figs. 7(e) and (f) reveals a bright elliptical ring characterized by a layer of a smooth structure layer on the edge of the irradiated area, which demonstrates a modified crystallization-phase change. Furthermore, the Raman spectra of the two typical structures are measured to characterize their phase states [Figs. 7(h) and (i)]. Characteristic peaks measured in these two structures validate the presence of a crystalline phase in dewetted GST nanoparticles and the modified structure. The transient reflectivity obtained at τ = 100 fs is compared with that of the final fabricated surface structures. The strong absorption area [red dashed line in Fig. 7(d)] exhibits the same size as the complete crystalline structure combined with the modified and dewetted GST nanoparticle area [Fig. 7(e)]. This effect indicates that GST crystallization is dominated by strong absorption based on the resolidification of liquid states. The ablated and modified region are distinguished by the ablation and crystallization thresholds due to the threshold effect. A specific phase marked with the area between red and blue dashed lines [Fig. 7(d)] indicates a solid-phase condition [16]. This specific area corresponds to the recovery of the final state [Fig. 7(e)] to the initial amorphous-phase state. In this case, the dividing line is determined by the melting threshold of the GST thin film. Thus, we can conclude that nanostructure formation is attributed to the confined localized threshold effect, which separate the specific phase state from the surrounding thin film and substrate, leading to the dewetting effect.
Fig. 7. (a)–(d) Temporal-spatial-resolved optical images of the surface of irradiated GST thin films at different time delays after excitation with the pump pulse. The pulse energy of the pump pulse is 2 µJ (51.9 mJ/cm2). (e) SEM image of surface structures fabricated through pump pulse irradiation. (f) Magnification of (e) showing the modified ring and disordered nanoparticles. (g) Reflectivity measured as a function of the distance at the central area of the irradiated GST surface for time delays of τ = 0.4, 7, 20, and 100 ps. Raman spectra of (h) centrally distributed GST nanoparticles and (i) modified area marked in (f); insets present the magnified SEM images of corresponding structures.
4. Conclusion
In summary, this study proposes an effective method for manipulating GST nanostructures based on the fs laser–induced dewetting effect using amorphous GST thin films. Crystalline GST nanostructures with ordered and disordered arrays can be fabricated by controlling the size of focused spots. The confined-localized threshold effect is proposed as the main mechanism for the formation of the nanostructure at the center of the fs laser–irradiated zone. Based on this effect, the morphologies of GST nanostructures can be flexibly modulated by controlling the size of the isolated micro/nanopatch determined using the specific threshold. Isolated solid and liquid phases stimulated through the single-shot fs laser pulse is demonstrated using the spatial-temporal-resolved pump–probe technique. This simple and well-controlled fs annealing approach can be employed for well-ordered and disordered large-area high-index dielectric nanoparticles with considerable potential in the low-loss retuning of all-dielectric metasurfaces.
Funding
Research Foundation from Ministry of Education of China (6141A02033123); Beijing Municipal Commission of Education (KM201910005003); Beijing Natural Science Foundation (3194045); Initiative Postdocs Supporting Program (BX20180041); National Natural Science Foundation of China (NSFC) (51805014); National Key R&D Program of China (2018YFB1107401).
Disclosures
The authors declare no conflicts of interest.
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