Femtosecond laser irradiation of amorphous Ge2Sb2Te5 thin films initiates reversible phase transitions. The amorphization and crystallization of Ge2Sb2Te5 thin films were experimentally and theoretically confirmed. Electron and lattice temperatures kinetics during laser pulse duration were evaluated by two-temperature models calculations and experimental data. The dynamical changing of optical properties have been taking into account. Temperatures and cooling rates, which are necessary to initiate phase transitions by IR laser pulses with subpicosecond duration. The observed results open perspectives for improvement of Ge2Sb2Te5 nanophotonical devices.

Ge2Sb2Te5 based devices attract the attention of researchers due to wide opportunities in designing phase change memory. Herein, we studied a possibility to fabricate periodic micro- and nanorelief at surfaces of Ge2Sb2Te5 thin films on silicon oxide/silicon substrates under multi-pulse femtosecond laser irradiation with the wavelength of 1250 nm. One-dimensional lattices with periods of 1250 ± 90 and 130 ± 30 nm were obtained depending on the number of acted laser pulses. Emergence of these structures can be explained by plasmon-polariton generation and laser-induced hydrodynamic instabilities, respectively. Additionally, formation of the lattices whose spatial period is close to the impacted laser wavelength can be modelled by considering the free carrier contribution under intensive photoexcitation. Raman spectroscopy revealed both crystallization and re-amorphization of the irradiated films. The obtained results show a possibility to fabricate rewritable all-dielectric data-storage devices based on Ge2Sb2Te5 with the periodic relief.

The possibility of manufacturing silicon nanoparticles by picosecond laser fragmentation of silicon microparticles in water is analysed. It is shown that for fragmentation duration of 40 min, the dependence of the average sizes of particles on the initial mass concentration of the micropowder varied in the range of 0.5 – 12 mg mL −1 is nonmonotonic, with the maximum average size of 165 nm being achieved at a concentration of 5 mg mL −1 . To explain the obtained result, the simulation of propagation of a focused laser beam in a scattering suspension of silicon microparticles is performed for their different mass concentrations. It is demonstrated that at concentrations not exceeding 5 mg mL −1 , fragmentation occurs in the paraxial region of the beam when it propagates deep into the cuvette with a suspension, while at higher concentrations it occurs primarily in the superficial layer owing to strong extinction. Calculations results allow the experimental features of the formation of silicon nanoparticles to be explained. Spectrophotometry measurements on suspensions of nanoparticles obtained at the initial concentration of microparticles of 12 mg mL −1 are compared with the theoretical estimates of the absorption and scattering coefficients obtained in the framework of the Mie theory. Measured optical properties indicate the potential of using fragmented nanoparticles as scattering and/or absorbing contrast agents in optical imaging of biological objects.

Ge2Sb2Te5 (GST225) looks to be a promising material for rewritable memory devices due to its relatively easy processing and high optical and electrophysical contrast for the crystalline and amorphous phases. In the present work, we combined the possibilities of crystallization and anisotropic structures fabrication using femtosecond laser treatment at the 1250 nm wavelength of 200 nm thin amorphous GST225 films on silicon oxide/silicon substrates. A raster treatment mode and photoexcited surface plasmon polariton generation allowed us to produce mutually orthogonal periodic structures, such as scanline tracks (the period is 120 ± 10 μm) and laser-induced gratings (the period is 1100 ± 50 nm), respectively. Alternating crystalline and amorphous phases at the irradiated surfaces were revealed according to Raman spectroscopy and optical microscopy studies for both types of structures. Such periodic modulation leads to artificial optical and electrophysical anisotropy. Reflectance spectra in the near infrared range differ for various polarizations of probing light, and this mainly results from the presence of laser-induced periodic surface structures. On the other hand, the scanline tracks cause strong conductivity anisotropy for dc measurements in the temperature range of 200–400 K. The obtained results are promising for designing new GST225-based memory devices in which anisotropy may promote increasing the information recording density.

Femtosecond laser structuring opens for chalcogenide semiconductor Ge2Sb2Te5 new perspectives in photonics applications due to wide change of its structural and optical properties in such processing. We studied laser-induced modification of amorphous Ge2Sb2Te5 thin films on silicon substrates. The investigations show that periodic relief formation is accompanied by phase transitions to the fcc crystalline phase and back. Furthermore, the irradiated Ge2Sb2Te5 samples demonstrate optical transparency in the near infrared region. The examined structures are interesting for further studies as a base of new memory devices which may possess optical anisotropy and be integrated into fiber optics applications.

It was shown that the ablation of meso- and microporous silicon layers by picosecond laser pulses in water and ethanol leads to the formation of nanosilicon suspensions with a diameter of less than 100 nm. It has been demonstrated that the use of porous silicon allows to reduce the thresholds of laser ablation and increase the concentration of particles in suspensions in comparison with the case of crystalline silicon ablation.

The paper discusses the possibility of manufacturing silicon nanoparticles, which are suitable for contrasting biological tissues imaged by optical coherence tomography, by femtosecond laser ablation of porous silicon in various liquids. The manufactured nanoparticles are characterised by average sizes of 87, 112, and 102 nm for cases of ablation in water, ethanol, and liquid nitrogen, respectively, as well as a relatively narrow size distribution, which provides additional advantages for subsequent delivery into biological tissues. Electrochemical etching, which results in the formation of layers of porous silicon, allows the yield of ablation products to be increased several-fold by lowering the ablation threshold, thereby increasing the light scattering efficiency of the prepared suspensions compared with the case of using crystalline silicon as targets. The possibility of obtaining high-contrast images of a biotissue phantom based on an agar gel with embedded nanoparticles is shown. The magnitude of the contrast depends on the liquid used for ablation and correlates with the values of the reduced scattering coefficient of the studied suspensions.

Phase transitions and periodic surface modification in amorphous Ge 2 Sb 2 Te 5 thin films on multilayer substrate were revealed as a result of the samples irradiation by femtosecond laser pulses with the wavelength of 1250 nm. Raman spectroscopy revealed partial crystallization in the treated samples. Calculations and analysis of scanning electron and atomic-force microscopy data showed that formation of the periodic surface structures is related to photoinduced surface plasmon-polariton excitation and depends on laser radiation fluence. The obtained results are useful for design and fabrication of new promising data-storage and polarization optics devices.

Femtosecond laser annealing of thin-film multilayered structures based on amorphous silicon and germanium were studied. The original samples were synthesized via plasma-enhanced deposition on glass substrate. Scanning electron microscopy revealed formation of periodic surface structures in the irradiated films. Raman spectra analysis revealed crystallization of amorphous germanium as a result of femtosecond laser pulses action, as well as fluence-dependent mixture of the germanium and silicon layers at absence of crystallization of the amorphous silicon layers.

The processes of femtosecond laser annealing of thin-film multilayer structures based on amorphous silicon and germanium produced by plasma-chemical deposition on a glass substrate have been studied. The formation of periodic structures on the surface of irradiated films has been detected by the scanning electron microscopy method. Analysis of Raman spectra has shown that amorphous germanium crystallizes and a mixing of germanium and silicon layers depending on the pulse energy density occurs in the absence of amorphous silicon layers crystallization as a result of exposure to femtosecond laser pulses.

Picosecond pulsed laser ablation of meso- and microporous silicon layers in water and ethanol leads to the formation of nanosilicon suspensions with particle diameters below 100 nm. It is established that the use of porous silicon targets allows the laser ablation threshold to be reduced and the nanoparticle concentration increased as compared to the ablation of crystalline silicon.

A novel two-stage technique to fabricate silicon nanoparticles is reported. At the first stage, silicon nanowire arrays are formed by metal-assisted chemical etching. At the second stage, the nanoparticles are produced by pulsed laser ablation of the silicon nanowire targets in water and ethanol. The fabricated particles have relatively small mean size in the range of 24 – 45 nm depending on the used buffer liquid. The ablation threshold of the silicon nanowire arrays is 2 – 11 times smaller than that for crystalline silicon targets. Owing to the achieved parameters, the proposed technique is more efficient in comparison with traditional approaches of mechanical milling of silicon nanowires and laser ablation of crystalline silicon. Raman spectroscopy study revealed crystalline structure of the fabricated silicon nanoparticles. The properties of the produced nanoparticles indicate their high potential in biophotonics.

Silicon nanoparticles with the size from 50 nm to 300 nm depending on used targets were formed by the picosecond laser ablation technique of porous silicon and silicon microparticles in water. Raman spectra analysis of the produced nanoparticles revealed a slight presence of the amorphous phase (10 – 12%) in them for the case of ablation of micro- and mesoporous silicon layers and almost complete crystallinity as a result of laser fragmentation of the silicon micropowders in water. The results can be helpful for further use of the considered nanoparticles in applications of photonics and biomedicine.

Sequential use of electrochemical etching and picosecond laser ablation in ethanol and liquid nitrogen allows fabrication of silicon particles with size smaller than 100 nm and high level of crystallinity. Fabricated ensembles of nanoparticles exhibit effective photoluminescence with emission peaks located within biotissue optical transparency window, thus being promising as contrasting agents for bioimaging.

Silicon nanoparticles with sizes of 50–300 nm have been formed via picosecond laser ablation of porous silicon and silicon microparticles in water using various targets. Raman spectroscopy has revealed the presence of a low amount of the amorphous phase in the obtained particles (10–12%) prepared from micro- and mesoporous silicon layers and almost a zero degree of crystallinity in the case of laser fragmentation of silicon micropowders in water. The results are promising for further application of the nanoparticles in photonics and biomedicine.

Silicon photonics now successfully combines achievements of the most highly developed semiconductor and optics technologies [1]. In particular, silicon nanowire (SiNW) arrays are attracting more and more interest from researchers because of their great potential from this point of view [2]. In our work light propagation in SiNW layers is studied via Raman scattering, third-harmonic generation and cross-correlation function measurements. The studied SiNW arrays are produced by the metal-assisted chemical etching [3] and characterized by a wire diameter of 50–100 nm and a layer thickness ranging from 0.2 to 16 μm. These structures are mesoscopic for light in the visible and near infrared ranges. The Raman signal increases monotonically with layer thickness increases at a 1.064 μm pump wavelength. The Stokes component for SiNW arrays with a thickness larger than 2 μm exceeds that for crystalline silicon by more than an order. At the mentioned thicknesses, an increase is also registered for the third-harmonic signal, one that is up to fourfold greater than that for crystalline silicon for a 1.25 μm pump wavelength. Measurements of cross-correlation functions for the scattered photons evidence the significant photon lifetime increase in the SiNW layers at their thickness increase. This fact can be connected with multiple scattering inside the studied mesoscopic structures and the increase of the interaction length for the Raman and third-harmonic generation processes. The obtained results allow to consider the SiNW arrays as effective media for enhancement of optical and nonlinear optical processes, in particular, as matrices for optical sensors. Acknowledgements: The work was supported by the Russian Foundation for Basic Research grant no. 15-29-01185.

Silicon nanowire (SiNW) arrays formed by metal-assisted chemical etching of boron low-doped crystalline silicon (c-Si) substrates were studied regarding enhanced efficiency of optical interactions in them. Along with high diffuse reflection of the structures under study, Raman and third-harmonic signals exhibited an order of magnitude growth compared to c-Si response in the near-infrared spectral region. Changes in the orientation dependencies of the third-harmonic intensity with SiNW length increase promoted the explanation of the optical peculiarities of SiNW arrays. Cross-correlation of the scattered radiation function evidenced that the effects could be explained by enhanced photon lifetime inside mesoscopic structure of SiNW arrays due to multiple scattering. Hence a correlation between the photon lifetime and the Raman and third-harmonic efficiency in SiNWs was revealed.