Irradiation with ultrashort laser pulses with a duration of no more than a few units of picoseconds causes not only relatively quick photoinduced phase transitions in thin ChVS films, but also (under certain conditions) the formation of surface gratings with periods that are close to the wavelength of the structuring laser pulses [70, 71] or even several times smaller than their wavelength [72, 73].

As a rule, the formation of such laser-induced periodic surface structures (LIPSSs) is determined by the excitation of surface plasmon polaritons by femtosecond laser pulses [71, 73]. A surface electromagnetic wave photoinduced in this way can interfere with the incident radiation and lead to the appearance of a resulting standing wave, which induces both the periodic modulation of phase transitions in the surface layer and (if the ablation threshold is exceeded) the formation of a periodic relief on the micron and submicron scales. In this case, the irradiated surface should not be necessarily a metal for the generation of plasmon polaritons: the sufficient concentration of free charge carriers during the process of LIPSS formation is provided by the generation of free charge carriers in the field of high-power femtosecond laser pulses [74].

The theory of LIPSS formation began to be actively developed in the early 1980s by the group of Professor Sipe [75], as well as by Emel’yanov et al. [76, 77] and other scientists. It was shown that the period and orientation (perpendicular or parallel to the polarization of the structuring laser pulses) of appearing structures depended on the values of the complex dielectric constants of the environment and the surface layer during irradiation. In turn, according to the Drude theory, the latter value directly depends on the value of the laser-pulse energy fluence absorbed by the surface, which was reflected in [71, 78], where it is shown that the energy characteristics of incident laser pulses are correlated with the period and the orientation of appearing LIPSSs through the concentration of free-charge carriers and the value of the complex dielectric constant of the surface layer during irradiation.

Despite the advances achieved through the development of femtosecond-laser technology in the formation of LIPSSs on different surfaces of metals, semiconductors, and dielectrics, the interest of scientists in such structures has been stimulated by challenges of finding ways to miniaturize memory elements in the last decade. In particular, the classical recording of information by a focused beam of light is spatially limited by the diffraction limit, λ/2n, where λ is the light wavelength and n is the refractive index of the medium carrying the information. The decrease in the λ value and work with UV radiation involve significant difficulties in information encoding and decoding and makes these related technologies much more expensive than technologies used in the variant with visible or near-infrared radiation. At the same time, the creation of regions with periodic modulation of the relief on wavelength and subwavelength scales using laser radiation makes it possible to increase the volume of information encoded in one memory cell (voxel) without reducing the wavelength of the structuring radiation as a result of artificial anisotropy [79–81], which is determined by mechanical stresses in the selected direction or by periodic modulation of the dielectric constant [82]. In this way, the direction of the anisotropy axis and value of birefringence and dichroism, which is determined by the polarization and energy fluence of the structuring laser pulses, respectively, currently make it possible to encode up to 8 bits of information (instead of 1 bit) in a voxel of fixed volume without reducing the wavelength [83].

However, the above-listed works describe experiments either with thin films of amorphous silicon, in which LIPSSs and regions appear in the nanocrystalline phase after irradiation, or with sodium borosilicate and quartz glasses. In the former case, the practical use of these materials as a basis for information carriers is significantly limited by the relatively low contrast of optical properties of amorphous and crystalline silicon; in the latter case, it is highly limited by the impossibility of rewriting information. The use of ChVSs makes it possible to largely avoid these limitations and simultaneously allow the formation of anisotropic regions with LIPSSs, which has been demonstrated in studies with thin films of GST225 over the past 5 years.

Thus, the change in the type of substrates on which thin films are applied [71, 84] and regulation of the energy, number, and wavelength of laser pulses [72, 85] make it possible to control the LIPSS morphology: gratings can be oriented parallel or perpendicular to the polarization and their period is approximately equal to or several times lower than the wavelength of incident radiation λ. The typical image of LIPSSs on a thin GST225 film is shown in Fig. 3.

Scanning electron microscope (SEM) microphotograph (130 nm thick) of a GST225 film on a thermally oxidized silicon substrate irradiated with 750 laser pulses (135 fs, 1250 nm, 0.1 J/cm2). The light horizontal band corresponds to the reamorphized region along the path of the irradiating laser beam with a Gaussian profile in its central part. The direction of the electric-field strength is indicated by an arrow [71].

It is important to note that the appropriate parameters of pulse irradiation in the transparency region of GST225 with λ from 800 to 2000 nm make it possible to produce LIPSSs with clearly defined subwavelength periods Λ obeying the law Λ = λ/2n [72], where the values of the refractive index are n ≈ 4 in the given spectral range for amorphous GST225 [33], and provide periodic modulation on the nanoscale.

At the same time, reversible “amorphous–crystalline” GST225 phase transitions and the formation of different types of LIPSSs by varying the energy inside the laser beam, polarization, and wavelength of laser pulses make it possible to carry out rewriting of the surface gratings and information encoded in these gratings [71, 72] (Fig. 3).

The periodic alternation of regions in the amorphous and crystalline phases inside the formed LIPSSs [72, 85, 86] (so-called phase-change gratings) determine a clearly defined the form anisotropy [82] in the studied structures based on GST225 and, consequently, the anisotropy of their optical and electrical properties. An attempt at a comprehensive analysis of this topic was carried out in [87]. The measured reflection spectra in the near-IR range show that the values of the reflection coefficients can differ by up to 4%, depending on the polarization of the incident light in the presence of LIPSSs in the irradiated film (Fig. 4), and the behavior of these spectral dependences is in good agreement with the simulation results within the generalized Bruggeman model for LIPSS-simulating nanocomposite laminar media [88, 89].

Reflection spectra of a 200-nm-thick GST225 film on a thermally oxidized silicon substrate irradiated with 240 laser pulses (135 fs, 1250 nm, 0.2 J/cm2). The direction of polarization of light incident at an angle of 13° to the normal line is compared with the scanning direction of the surface-modifying laser beam. The orientation of the LIPSS is orthogonal to the scanning direction [87].

The anisotropy of the conductivity of GST225 films that were considered in the same work is much more pronounced and can reach 5 orders of magnitude for measurements in the temperature range of 200–400 K at a direct current applied in two mutually orthogonal directions in the sample plane. This significant contrast is explained by the periodic alternation of GST225 regions in the form of parallel lines in the crystalline and amorphous phases, when the conductivity proves to be maximum with current applied along the lines as a result of the effective transfer of charge carriers through crystalline channels and the amorphous regions act as barriers in the orthogonal direction.

As a result, along with reversible phase transitions, the existence of LIPSSs in GST225 films irradiated with femtosecond laser pulses opens up new horizons for the use of these structures with artificial optical and electrophysical anisotropy as rewritable memory elements sensitive to the polarization of incident light and current applied in the film plane.

Such LIPSSs, when produced with high quality using femtosecond laser pulses, also demonstrate the properties of the diffraction grating. Thus, the work [90] experimentally demonstrated the possibility of recording two-dimensional structures (with linear dimensions of 1 mm in length and about 50 μm in width) on amorphous GST225 films during irradiation with femtosecond pulses; they consisted of 50 parallel high-quality alternating amorphous ridges and crystalline cavities. The period of such structures corresponded to the irradiation wavelength and the relief height was only 8 nm, which suggests the formation of an almost two-dimensional structure. This type of region was actually a two-phase binary periodic structure, which can be used as a diffraction grating. The study experimentally demonstrated the formation of a diffraction pattern upon the reflection of light and determined the efficiency of diffraction orders for the TM and TE modes, when incident waves are charcterized by an electric-field-strength vector lying in the plane of light incidence and ortho-gonally to it, respectively. It was concluded that the relative intensity of diffraction orders could be used to determine the topography of the formed LIPSSs and that this approach allowed for direct optical control of the laser writing process in situ.

Various types of surface structures can also be formed in arsenic sulfide (As2S3) films: surface one-dimensional gratings, ring-shaped concentric structures, and nanopillars [73] (Fig. 5). The formation of a certain type of structures and their period and orientation are determined by the number and energy density of laser pulses acting on the material.

SEM images of an As2S3 film irradiated with 2 (a), 10 (b), 20 (c), and 50 (d) laser pulses with a wavelength of 800 nm and an energy of 6.2 mJ/cm2. The direction of the electric-field strength is indicated by an arrow [73].

Based on the results of the analysis of reflection from the irradiated surface using the “pump–probe” method, the following scenario for the evolution of the appearing structures was proposed in [73]. During irradiation by several pulses, the interference of excited plasmon polaritons with incident radiation leads to the formation of surface gratings with a period close to the wavelength of the structuring radiation (800 nm) and an orientation perpendicular to its polarization (Fig. 5a). An increase in the number of pulses (starting from 10) leads to additional redistribution of the electromagnetic field of incident radiation on the formed relief, thereby additionally leading to the formation of LIPSSs oriented along the polarization with a period of about 200 nm (Fig. 5b) and nanopillars (Fig. 5c). A further increase in the number of pulses to 50 also leads to the formation of concentric rings with a period of about 700 nm, which result from incident radiation reflected from the surface of the formed crater (Fig. 5d).

We recently obtained similar LIPSSs in thin films of arsenic selenide (As2Se3) as a result of their structuring by radiation at the second optical harmonic frequency from a Satsuma femtosecond laser (Amplitude Systems) (515 nm, 300 fs, 0.1 µJ). Irradiation with 200–800 laser pulses leads to the formation of LIPSSs with a period of about 180 nm, which are oriented parallel to the polarization of the structuring radiation (Figs. 6a and 6b). The increase in the number of pulses in the center of the crater involves the formation of additional orthogonal gratings with an average period of 460 nm (Fig. 6c).

SEM images of an As2Se3 film with a thickness of 840 nm on a substrate with a conductive sublayer (Cr/SiO2/Si), irradiated with 200 (a), 800 (b), and 1200 (c) laser pulses. The direction of the electric-field strength is indicated by an arrow.

The simultaneous existence of several types of LIPSSs in the same crater is most likely determined by the inhomogeneous intensity distribution along the profile of the Gaussian laser beam used for irradiation. Different intensities lead to an uneven spatial distribution of the concentration of photoinduced charge carriers in the surface region, which, in turn, determines the variation in the values of the complex dielectric constant and conditions for the excitation of surface-plasmon polaritons, which are responsible for the formation of the subwavelength surface relief [71, 74, 78].

We note that the above-listed structures simultaneously coexist within the ablation crater; i.e., so-called hierarchical structures are formed, which significantly expands the possibilities of designing metasurfaces based on thin As2S3 films.

In addition to the classical plasmon-polariton mechanism of LIPSS formation, it is also technologically possible to implement other approaches to significantly reduce their periods. Thus, the work [91] describes experiments to reduce the period of the formed LIPSSs in As2S3 films by 2 times via rotation of the sample by 90° at the second stage of its irradiation in the raster scanning mode with a laser beam, which, according to calculations, leads to resonant localization of the electromagnetic field along the ridges of the grating formed as a result of the first scanning and to the accompanying ablation of the material from these regions.

A similar effect of ablation from the ridge of the As2S3 surface grating was observed in [92], but without changing the scanning strategy with femtosecond laser pulses (Figs. 7a and 7b).

Atomic force microscopy-based microphotographs (a, c) and profilograms (b, d) of the surface of an As2S3 film irradiated with 10 femtosecond laser pulses (100 fs, 800 nm) with energies of 0.1 (a, b) and 0.4 mJ (c, d) [92].

The formation of nanocavities with an average diameter of 300 nm, periodically placed in the formed gratings, was also recorded (Figs. 7c and 7d). The formation of such nanocavities in ChVSs is presumably due to microexplosions and the relatively low thermal conductivity of As2S3, which causes increased heating of the material in certain areas corresponding to the maxima of the electromagnetic-field distribution during irradiation.

As a result, our analysis of all types of appearing hierarchical structures suggests that, during the formation of LIPSSs, it is sometimes necessary to take into account not only the model of plasmon-polariton excitation, but also the effects of local heating on nanoscales under the action of laser pulses.

Despite significant advances in the formation of LIPSSs in ChVSs such as GST225, As2S3, and As2Se3, it still cannot be said that the diversity of these structures is inherent in a wider range of ChVSs due to lack of works confirming this assumption to date. This is partly due to differences in the absorption spectra, thermal conductivity, and whole electronic subsystems in different ChVSs. Technologically, this means that the production of LIPSSs with the necessary quality for use in applications requires the selection of an individual set of parameters (wavelength, energy density, and number of structuring laser pulses), as well as scanning strategies and laser-beam focusing conditions, for each specific material. Nevertheless, the progress in the development of laser technology and related nanostructuring technologies gives hope for new results on the formation of LIPSSs for a wider range of ChVSs in the near future.