Femtosecond Irradiation Research Articles

Formation of Kerr waveguide and microgratings on a germanium surface under the action of femtosecond radiation of the mid-IR range

A new nonlinear physical model of the formation of ordered structures of a surface topography of a semiconductor by ultrashort pulses of linearly polarized laser radiation has been proposed. The model is based on the formation of the dynamic near-surface waveguide due to the optical Kerr effect, interference with the part of waveguide modes, and the formation of microgratings on the surface, which are oriented by polarization of radiation. The model is confirmed by experimental data.

Study of the formation of thermochemical laser-induced periodic surface structures on Cr, Ti, Ni and NiCr films under femtosecond irradiation**Presented at the Fundamentals of Laser Assisted Micro- and Nanotechnologies (FLAMN-16) International Symposium (Pushkin, Leningrad oblast, 27 June to 1 July 2016).

The formation of femtosecond laser-induced periodic surface structures (LIPSS’s) on Cr, Ti, Ni and NiCr films (with different Cr contents) is investigated. It is established that thermochemical LIPSS’s with periods of 950, 930 and 980 nm are formed, respectively, on the surfaces of titanium, chromium, and nichrome (with a chromium content of 20%); however, thermochemical LIPSS’s are not formed on the surfaces of nickel and nichrome with a low chromium content, although Raman data indicate that oxidation occurs in all cases. A weakly ordered ablated structure with a period of 250 – 300 nm is found to be formed on oxidised areas of thermochemical LIPSS’s in the case of chromium and nichrome (80/20). Experimental data on selective etching of thermochemical LIPSS’s on titanium and chromium films are presented.

A Storage Ring Based Free-Electron Laser for Generating Ultrashort Coherent EUV and X-ray Radiation

Generation of ultrashort coherent radiation pulses in the extreme ultraviolet (EUV) and x-ray regime is of remarkable interest in the synchrotron radiation user community. In this work, a novel technique is proposed for directly imprinting strong coherent microbunching on the electron beam with very small laser-induced energy spread. Theoretical analysis and numerical simulations demonstrated that this technique can be used for the generation of megawatt-scale level, fully temporal coherent femtosecond EUV and soft x-ray radiation pulses at a storage ring light source.

Effect of scanning velocity on femtosecond laser-induced periodic surface structures on HgCdTe crystal

In this paper, a femtosecond laser line-scanning irradiation was used to induce the periodic surface microstructure on HgCdTe crystal. Low spatial frequency laser induced periodic surface structures of 650–770nm and high spatial frequency laser induced periodic surface structures of 152–246nm were respectively found with different scanning speeds. The evolution process from low spatial frequency laser induced periodic surface structures to high spatial frequency laser induced periodic surface structures is characterized by scanning electron microscope. Their spatial periods deduced by using a two-dimensional Fourier transformation partly agree with the predictions of the Sipe-Drude theory. Confocal micro-Raman spectral show that the atomic arrangement of induced low spatial frequency laser-induced structures are basically consistent with the crystal in the central area of laser-scanning line, however a new peak at 164cm−1 for the CdTe-like mode becomes evident due to the Hg vaporization when strong laser ablation happens. The obtained surface periodic ripples may have applications in fabricating advanced infrared detector.

Trapped electronic states in YAG crystal excited by femtosecond radiation

The excitation of an electronic subsystem of an yttrium aluminum garnet by 800 nm femtosecond radiation was studied theoretically and experimentally. The spatio-temporal dynamics of the refractive index (n) inside the beam waist was explored by means of the pump–probe interferometric technique with a submicron resolution. The observed increase in n indicated the formation of bound electronic states relaxed for $${\sim}150$$ ps. We showed that the experimental data agreed with the computational simulation based on the numerical solution of the nonlinear Schrodinger equation only if these transient states were considered to arise from a direct light-induced process but not from the decay of radiatively generated free-electron–hole pairs.

Kerr nonlinearity effect on femtosecond pulse radiation filamentation in air

The conditions for filamentation of femtosecond pulse laser radiation when focusing in air are studied experimentally and theoretically. A good agreement between experimental and calculated results is shown if neglecting the filament plasma. It is shown that the Kerr nonlinearity plays a fundamental role in the generation, existence, and cessation of a filament at a small numerical aperture (NA ≤ 2.15 × 10–3). The Kerr effect first leads to the beam self-focusing and generation of a filament, and at the final stage, to radiation defocusing and a sharp decrease in its axial intensity due to the beam wavefront distortions. In the case of aberration focusing, a spatial quasi-soliton is formed after a visible filament due to the balance between Kerr self-focusing and diffraction spreading. The quasi-soliton is a source of the directional white supercontinuum.

Ionisation response in semiconductor structures exposed to the X-ray radiation of a femtosecond laser-plasma source

The possibilities of applying a femtosecond laser-plasma source of X-ray radiation for modelling the effect of single nuclear particles based on the principle of equivalent charge generation are analysed. The parameters of femtosecond X-ray radiation for the experimental modelling of individual radiation effects are validated. The experimental setup forming the X-ray radiation is described. The results of comparative ionisation response modelling in simple electronic devices using the FLUKA and FEANT codes are presented.

A tunable XUV monochromatic light source based on the time preserving grating selection of high-order harmonic generation**Project supported by the National Natural Science Foundation of China (Grants Nos. 11627807, 11127403, and 11474130), the National Basic Research Program of China (Grant No. 2013CB922200), and the Natural Science Foundation of Jilin Province of China (Grant No. 20160101332JC).

We accomplish a laboratory facility for producing a femtosecond XUV coherent monochromatic radiation with a broad tunable spectral range of 20 eV–75 eV. It is based on spectral selected single-order harmonics from intense laser driven high harmonic generation in gas phase. The time preserving for the selected harmonic radiation is achieved by a Czerny–Turner type monochromator designed with a conical diffraction grating mount for minimizing the time broadening caused by grating diffraction and keeping a relatively high diffraction efficiency. Our measurement shows that the photon flux of the 23-order harmonic (H23) centered at 35.7 eV is photons/s approximately with a resolving power . This source provides an ultrashort tunable monochromatic XUV beam for ultrafast studies of electronic and structural dynamics in a large variety of matters.

Correlated signals of the accumulated photon echo on CdSe – CdS quantum dots

The possibility of an observation of correlated signals of the accumulated photon echo under a three-photon excitation of a sample, which is an ensemble of semiconductor nanoparticles of “core – shell” type, by a femtosecond radiation of Ti:Sa laser is discussed. The phase matching conditions of such signals are obtained.

Controllable Si (100) micro/nanostructures by chemical-etching-assisted femtosecond laser single-pulse irradiation

In this study, silicon micro/nanostructures of controlled size and shape are fabricated by chemical-etching-assisted femtosecond laser single-pulse irradiation, which is a flexible, high-throughput method. The pulse fluence is altered to create various laser printing patterns for the etching mask, resulting in the sequential evolution of three distinct surface micro/nanostructures, namely, ring-like microstructures, flat-top pillar microstructures, and spike nanostructures. The characterized diameter of micro/nanostructures reveals that they can be flexibly tuned from the micrometer (∼2 μm) to nanometer (∼313 nm) scales by varying the laser pulse fluence in a wide range. Micro-Raman spectroscopy and transmission electron microscopy are utilized to demonstrate that the phase state changes from single-crystalline silicon (c-Si) to amorphous silicon (a-Si) after single-pulse femtosecond laser irradiation. This amorphous layer with a lower etching rate then acts as a mask in the wet etching process. Meanwhile, the on-the-fly punching technique enables the efficient fabrication of large-area patterned surfaces on the centimeter scale. This study presents a highly efficient method of controllably manufacturing silicon micro/nanostructures with different single-pulse patterns, which has promising applications in the photonic, solar cell, and sensors fields.

Suppressing the memory state of floating gate transistors with repeated femtosecond laser backside irradiations

We demonstrate that infrared femtosecond laser pulses with intensity above the two-photon ionization threshold of crystalline silicon induce charge transport through the tunnel oxide in floating gate Metal-Oxide-Semiconductor transistor devices. With repeated irradiations of Flash memory cells, we show how the laser-produced free-electrons naturally redistribute on both sides of the tunnel oxide until the electric field of the transistor is suppressed. This ability enables us to determine in a nondestructive, rapid and contactless way the flat band and the neutral threshold voltages of the tested device. The physical mechanisms including nonlinear ionization, quantum tunneling of free-carriers, and flattening of the band diagram are discussed for interpreting the experiments. The possibility to control the carriers in memory transistors with ultrashort pulses holds promises for fast and remote device analyses (reliability, security, and defectivity) and for considerable developments in the growing field of ultrafast microelectronics.

Influence of induced colour centres on the frequency – angular spectrum of a light bullet of mid-IR radiation in lithium fluoride

The influence of the occurrence of a structure consisting of long-lived colour centres, formed in an LiF crystal upon filamentation of femtosecond mid-IR radiation, on the supercontinuum characteristics is investigated. With an increase in the number of incident pulses, the length and transverse size of the structure of colour centres induced in LiF increase, and the supercontinuum spectrum in the short-wavelength region is markedly transformed due to the occurrence of the waveguide propagation regime, absorption, and scattering of radiation from the newly formed structure of colour centres. Under these conditions, the intensity of the anti-Stokes wing decreases by two orders of magnitude after several tens of pulses. Spectral components arise in the visible range, the angular divergence of which increases with increasing wavelength.

Dual-scale nanoripple/nanoparticle-covered microspikes on silicon by femtosecond double pulse train irradiation in water

Novel dual-scale structures were obtained by femtosecond double pulse train (subpulse delay Δt>0ps) one-step irradiating silicon in water. The dual-scale structures consist of microspikes of ∼2μm width and ∼0.5μm height, and nanoripples with a mean period of 146nm or nanoparticles with a mean diameter of 90nm which entirely cover on the microspikes, for linearly polarized or circularly polarized femtosecond laser respectively. The formation of dual-scale structures involves the following processes: (1) Continuously laser energy deposited at femtosecond to picosecond timescales within silicon surfaces and central regions, will result in enhanced capillary waves and thinner melted silicon layers. Hence, the microspikes can be induced at laser fluences below ablation threshold; (2) Later (>500–800 pulses), a mass of debris and bubbles produced will lead to the remarkably and uniformly scattering or shielding of subsequent incident laser energy. Hence, the nanostructures can be induced. The novel structures exhibit high-sensitive surface enhanced Raman scattering with an enhancement factor of 108 for Rhodamine 6G detecting. Besides, the novel structures have application potentials in improving the silicon hydrophobicity, antireflection, etc.

Watt-level mid-infrared radiation via self-seeded difference-frequency generation from a pre-chirp managed femtosecond Yb-fiber amplifier.

We obtained over 1 W average power at ∼3550 nm wavelength via self-seeded difference-frequency generation (DFG) through a 5 cm long periodically poled MgO-doped lithium niobate crystal. The pump and signal sources are derived from the identical pre-chirp managed femtosecond Yb-fiber amplifier with sub-100-fs pulse duration and 84 MHz repetition rate for simple synchronization. This result is believed to be among the highest-average-power, femtosecond mid-infrared radiation obtained via DFG.

Laser-induced periodic surface structures on zinc oxide crystals upon two-colour femtosecond double-pulse irradiation

In order to study the temporally distributed energy deposition in the formation of laser-induced periodic surface structures (LIPSS) on single-crystalline zinc oxide (ZnO), two-colour double-fs-pulse experiments were performed. Parallel or cross-polarised double-pulse sequences at 400 and 800 nm wavelength were generated by a Mach–Zehnder interferometer, exhibiting inter-pulse delays up to a few picoseconds between the sub-ablation 50-fs-pulses. Twenty two-colour double-pulse sequences were collinearly focused by a spherical mirror to the sample surface. The resulting LIPSS periods and areas were analysed by scanning electron microscopy. The delay-dependence of these LIPSS characteristics shows a dissimilar behaviour when compared to the semiconductor silicon, the dielectric fused silica, or the metal titanium. A wavelength-dependent plasmonic mechanism is proposed to explain the delay-dependence of the LIPSS on ZnO when considering multi-photon excitation processes. Our results support the involvement of nonlinear processes for temporally overlapping pulses. These experiments extend previous two-colour studies on the indirect semiconductor silicon towards the direct wide band-gap semiconductor ZnO and further manifest the relevance of the ultrafast energy deposition for LIPSS formation.

Quinoline-Derived Two-Photon Sensitive Quadrupolar Probes.

Quadrupolar probes derived from 8-dimethylamino-quinoline (8-DMAQ) having a pegylated fluorene core were prepared and studied under "one-photon" (λ=365 nm) and "two-photon" (TP) (λ=730 nm) irradiation conditions. Compound 1 a was identified as the most efficient probe by UV activation that showed sequential release of acetic acid as a model. Although the probe showed high two-photon absorption it stayed inert under femtosecond irradiation conditions. Fast and selective photolysis was observed, however, by using picosecond irradiation conditions with a remarkably high TP uncaging cross-section (δu =2.3 GM).

Chemical composition of two-photon oxidized graphene

Chemical composition of two-photon oxidized single-layer graphene is studied by micrometer X-ray photoelectron spectroscopy (XPS). Oxidized areas with a size of 2 × 2 μm2 are patterned on graphene by tightly focused femtosecond pulsed irradiation under air atmosphere. The degree of oxidation is controlled by varying the irradiation time. The samples are characterized by four wave mixing (FWM) imaging and Raman spectroscopy/imaging. Micrometer-XPS is used to study local chemical composition of oxidized areas. XPS imaging shows good contrast between oxidized and non-oxidized areas. Gradual oxidation is observed from growth of signals attributed mainly to hydroxyl (COH) and epoxide (COC) groups with a minor contribution of carboxylic groups. The highly oxidized areas are characterized by ∼40% of carbon atoms involved in hydroxyl and ∼25% in epoxide groups. The Raman peaks of oxidized graphene are relatively narrow indicating less disordered material than what is typically observed for graphene oxide.

Interaction of High-Intensity Femtosecond Radiation With Gas Cluster Beam: Effect of Pulse Duration on Joint Terahertz and X-Ray Emission

This paper studies the phenomenon of joint generation of terahertz (THz) and X-ray radiation in the argon nanocluster jet under the action of high-power femtosecond laser pulse in both the single-color and dual-color regimes. It was discovered that in a gas cluster beam the pulse duration affects the properties of THz and X-ray emission differently. For the same given total energy of optical pulse in the dual-color excitation regime of cluster medium, more than a five times increase of THz radiation power was observed in comparison with the single-color regime, while the conversion efficiency to the argon X-ray K-line reached 7 $\times\, {\text{10}}^{\mathbf -{\text{6}}}$ and remained unchanged. The possibility of separation of contributions of different beam components into the THz signal was demonstrated experimentally, using contributions from clusters and nonclustered gas as an example. We suggest an interpretation of experimental results based on a theoretical model of cluster ionization that self-consistently predicts the level and dynamics of ionization and electron temperature in the clusters.

Chemical etching mechanism and properties of microstructures in sapphire modified by femtosecond laser

Sapphire, with extremely high hardness, high-temperature stability and wear resistance, often corroded in molten KOH at 300 °C after processing. The fabrication of microstructures on sapphire substrate performed by femtosecond laser irradiation combined with KOH solution chemical etching at room temperature is presented. It is found that this method reduces the harsh requirements of sapphire corrosion. After femtosecond irradiation, the sapphire has a high corrosion speed at room temperature. Through the analysis of Raman spectrum and XRD spectrum, a novel insight of femtosecond laser interaction with sapphire (α-Al2O3) is proposed. Results indicated that grooves on sapphire surface were formed by the lasers ablation removal, and the groove surface was modified in a certain depth. The modified area of the groove surface was changed from α-Al2O3 to γ-Al2O3. In addition, the impacts of three experimental parameters, laser power, scanning velocities and etching time, on the width and depth of microstructures are investigated, respectively. The modified area dimension is about 2 μm within limits power and speed. This work could fabricate high-quality arbitrary microstructures and enhance the performance of sapphire processing.

Intramolecular vibrational dynamics of ν1mode in (CF3)2C=C=O molecule induced by resonant IR femtosecond radiation

Intramolecular dynamics of the vibrational levels (up to v = 5) of the ν 1 mode in (CF 3 ) 2 CCO molecule, induced via multiphoton selective excitation of this mode by resonant femtosecond IR radiation, was studied. The times of intramolecular vibrational energy redistribution ( IVR ) of each vibrational level v to the remaining modes of the molecule were measured. In accordance with the theory predictions the decrease of IVR time with increasing v is observed. A sharp reduction of the IVR time (up to 1 ps) at a wavelength of 2130 cm −1 of transition v=3 → v=4 was found. It was shown that with a negative chirp of a femtosecond radiation pulse the population of high-lying vibrational levels of ν 1 modes significantly increases.