Determination of the dielectric function of porous silicon by high-order laser-harmonic radiation

Porous-silicon reflectance has been determined over a large energy range, from 1 eV to 16 eV, by combining a NIR/visible/UV spectrometer with a new VUV light source as laser-harmonic radiation. The porous-silicon dielectric function was deduced from reflectance measurements by Kramers–Kronig analysis. We point out that, for the first time, laser harmonics have been applied in the optical characterization of materials as a new and suitable alternative to synchrotron radiation. Received: 9 January 2001 / Accepted: 28 April 2001 / Published online: 20 June 2001     

VUV laser photoionization of laser-stimulated desorbed species

We report here first results about single-photon VUV laser photoionization of desorbed species from a silicon surface irradiated by a pulsed and tunable UV laser (290-300 nm). The combination of VUV photoionization at 10 eV with laser-induced surface desorption offers a largely non-destructive and sensitive method for quantitative analysis. Indeed it allows mass spectrometry measurements with uniform sensitivity and without breaking the chemical bonds in the probed species. The energy of the VUV photons (9.91 eV) is above the ionization limits of a number of molecules and fragments. Moreover, adjustment of the delay between the desorbing and the probe lasers allows the measurement of the time-of- flight distribution of the ejected species. Data extracted from these measurements are fundamental for a better understanding of laser-surface interaction phenomena.     

VUV 157 nm laser ablation of composite structures

We report on the laser ablation of composite prismatic structures using a vacuum ultraviolet (VUV) 157 nm F₂ laser. Polycarbonate and CR-39 substrates have been intentionally seeded with silver wires and silicon carbide whiskers respectively. The seed particles remain attached to the underlying substrate after laser ablation, forming composite silver-polycarbonate and silicon carbide-CR-39 interfaces. Strong optical absorption at 157 nm in the polymeric substrates allows precise control over the depth between the base of the substrate and composite interface. The surface roughness of the as-received seed particles has a significant effect on the final surface quality of the ablated structures. The textured surface on the silicon carbide whiskers is resolved on the walls of the ablated structures. This is in contrast to the composite structures formed using silver wires, which have a comparatively smoother surface.     

Developing VUV spectroscopy for protein folding and material luminescence on beamline 4B8 at the Beijing Synchrotron Radiation Facility

The new 4B8 beamline provides UV–VUV light in the wavelength range from 360 to 120 nm. It uniquely enables two kinds of spectroscopy measurements: synchrotron radiation circular dichroism spectroscopy and VUV excited fluorescence spectroscopy. The former is mainly used in protein secondary structure studies, and the latter in VUV excited luminescent materials research. Remote access to fluorescence measurement has been realised and users can collect data online. Besides steady‐state measurements, fluorescence lifetime measurements have been established using the time domain method, while a laser‐induced temperature jump is under development for protein folding dynamics using circular dichroism as a probe.     

小分子在真空紫外波段从量子态到量子态的光解离动力学

本文主要描述小分子在真空紫外波段(VUV,6-20 eV)光解离动力学的最新实验和理论研究进展.得益于基于商业化激光器的真空紫外光源技术,以及离子速度成像、高分辨氢原子-里德堡态标记-飞行时间测量和VUV-VUV泵浦-探测等方法的发展,研究人员现在可以对很多小分子在真空紫外波段的光解离动力学进行量子态到量子态层面的测量和研究,本文重点综述H_2(D_2,HD),CO,N_2,NO,O_2,H_2O(D_2O,HOD),CO_2,N_2O以及一些多原子分子在真空紫外波段光解离动力学的最新研究进展.这些小分子在真空紫外波段的光解离在天体化学以及大气化学中有着非常重要的应用.分子吸收一个VUV光子以后,通常会被直接激发到比较高的电子激发态,解离过程会涉及到多个电子态势能面之间的复杂非绝热相互作用.在实验上对解离截面等参数进行从量子态到量子态层面的精细测量对于深入了解这些复杂的势能面之间的相互作用有非常重要的意义.最近建成的大连相干光源是目前世界上唯一一台在真空紫外波段工作的自由电子激光,具有脉冲能量高、扫描范围宽(50～150 nm)等优越的性能,它的建成必将会大大促进小分子真空紫外光解离研究的发展.     

Nuclear signatures in high-order harmonic generation from laser-driven muonic atoms

High-order harmonic generation from muonic atoms exposed to intense laser fields is considered. Our particular interest lies in effects arising from the finite nuclear mass and size. We numerically perform a fully quantum mechanical treatment of the muon-nucleus dynamics by employing modified soft-core and hard-core potentials. It is shown that the position of the high-energy cutoff of the harmonic spectrum depends on the nuclear mass, while the height of the spectral plateau is sensitive to the nuclear radius. We also demonstrate that gamma-ray harmonics can be generated from muonic atoms in ultrastrong VUV fields, which have potential to induce photonuclear reactions.     

Study of High-Order Harmonic at VUV/XUV

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Thermal annealing dependence of some optical properties of plasma-modified porous silicon

The photoluminescence and reflectance of porous silicon (PS) with and without hydrocarbon (CH_x) deposition fabricated by plasma enhanced chemical vapour deposition (PECVD) technique have been investigated. The PS samples were then, annealed at temperatures between 200 and 800 °C. The influence of thermal annealing on optical properties of the hydrocarbon layer/porous silicon/silicon structure (CH_x/PS/Si) was studied by means of photoluminescence (PL) measurements, reflectivity and ellipsometry spectroscopy. The composition of the PS surface was monitored by transmission Fourier transform infrared (FTIR) spectroscopy. Photoluminescence and reflectance measurements were carried out before and after annealing on the carbonized samples for wavelengths between 250 and 1200 nm. A reduction of the reflectance in the ultraviolet region of the spectrum was observed for the hydrocarbon deposited polished silicon samples but an opposite behaviour was found in the case of the CH_x/PS ones. From the comparison of the photoluminescence and reflectance spectra, it was found that most of the contribution of the PL in the porous silicon came from its upper interface. The PL and reflectance spectra were found to be opposite to one another. Increasing the annealing temperature reduced the PL intensity and an increase in the ultraviolet reflectance was observed. These observations, consistent with a surface dominated emission process, suggest that the surface state of the PS is the principal determinant of the PL spectrum and the PL efficiency.     

High-Resolution Infrared-Vacuum Ultraviolet Photoion and Pulsed Field Ionization-Photoelectron Methods for Spectroscopic Studies of Neutrals and Cations

扫描单模红外光参量振荡(IR-OPO)激光器的波长激发待测分子,并用固定波长的真空紫外(VUV)激光器光电离(PI)探测被红外激发的分子,可获得高灵敏度的中性多原子分子的高分辨红外光谱. 这种方法(IR-VUV-PI)基于真空紫外光电离作为探测,可分辨样品中的不同成分,因此适用于对含同位素,自由基,络合物等通常为非纯的样品进行红外光谱研究. 高分辨的IR-VUV-PI谱可实现对分子单一振转态的选择,在选态的基础上进行真空紫外脉冲场电离零动能光电子谱(VUV-PFI-PE)的研究,可得到高分辨振转解析的光电子谱.被研究的分子包括一卤代甲烷(CH3X(X=Br,I)),乙烯(C2H4),丙炔(C3H4)等. 实验表明,采用高分辨的单模红外光参量振荡器代替先前使用的低分辨红外光参量振荡器可显著的提高IR-VUV-PI和IR-VUV-PFI-PE谱的信噪比. 并进一步讨论了采用IR,VUV,分子束三束同轴以提高IR-VUV-PI和IR-VUV-PFI-PE谱灵敏度的方案.     

Development of ultra-short pulse VUV laser system for nanoscale processing

We have developed intense vacuum ultraviolet (VUV) radiation sources for advanced material processing, such as photochemical surface reactions and precise processing on a nanometer scale. We have constructed a new VUV laser system to generate sub-picosecond pulses at the wavelength of 126 nm. A seed VUV pulse was generated in Xe as the 7th harmonic of a 882-nm Ti:sapphire laser. The optimum conversion was achieved at the pressure of 1.2 Torr. The seed pulse will be amplified by the Ar₂^*\mathrm{Ar}_{2}^{*} media generated by an optical-field-induced ionization Ar plasma produced by the Ti:sapphire laser. We have obtained a gain coefficient of g=0.16 cm⁻¹. Our developing system will provide VUV ultra-short pulses with sub-μJ energy at a repetition rate of 1 kHz.     

Generation of femtosecond vacuum-ultraviolet pulses

High power femtosecond pulses in the Vacuum Ultra Violet (VUV) have been generated through the nonlinear interaction of femtosecond KrF pulses with xenon and argon gas. Under near resonant two photon excitation of xenon by a femtosecond KrF laser, parametric four wave mixing processes lead to VUV pulses at 147 and 108 nm with pulse energies in the 10 µJ range. Tuning is demonstrated by mixing the KrF pulse with a 500 fs dye laser pulse at 497 nm, resulting in 165 nm emission. In argon, a three photon resonance leads to third harmonic generation at 83 nm and micro joule level pulses near 127 nm generated by a six wave mixing process. Since the spectra of the VUV pulses show an ionization-induced blue shift with increasing KrF laser intensity, the VUV pulses can be shown to have temporal duration less than the pulse width (450 fs) of the KrF laser. Blue shifting of the third harmonic of the KrF laser in argon is dominated by a reduction in the neutral gas density rather than by an increase in the electron density.     

Photoconductivity and photoemission studies of diamond irradiated by ultrashort VUV pulses

We investigated relaxation of free charge carriers in pure crystalline diamond exposed to VUV irradiation of high order harmonics of femtosecond Ti:Sa laser in the spectral range 17–32 eV. Electron–hole pairs, possessing a significant kinetic energy, are generated in the material via direct interband transitions, relaxation of which is monitored by means of induced conductivity in the bulk and photoemission from the surface of the material. The experimental data provided by these complementary techniques are compared and discussed in terms of the competition between ionization and conductivity looking for evidences of multiplication of free charge carriers due to impact ionization. PACS 42.65.Re; 72.20.Jv; 72.40.+w; 79.60.-i     

Reflectancevs. Electroreflectance measurements on arsenic-doped silicon crystal

Summary Reflectance and electroreflectance measurements from 2.5 to 5.5 eV, performed on arsenic heavily doped silicon samples, are reported and discussed. Silicon crystals implanted with arsenic up to a fluence of 10¹⁷ cm⁻² were laser irradiated and a free-carrier concentration of the order of 10²¹cm⁻³ was reached. The reordering of the system is studied in detail at different initial conditions. A comparison between reflectance and electro-reflectance appears to be quite useful to determine the behaviour of the heavily doped semiconductor. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Experimental determination of optical constants of MgF2 and AlF3 thin films in the vacuum ultra-violet wavelength region (60-124 nm), and its application to optical designs

The investigation of some solar radiations of interest for astrophysicists requires optics in the 80-130 nm vacuum ultra-violet spectral range (VUV). In this domain, where both transmittance and reflectance of most materials are very low, the measurement of optical constants is specifically difficult, and optical data are consequently often either inexistent or uncertain. Reliable modelling of optical components for VUV, like polarizing multi-layered mirrors, necessitates prior measurement of complex indices of the thin films involved in the coating. Fluorides like MgF₂ or AlF₃ are among the rare materials capable to contribute to multi-layer mirrors in the VUV.We have determined optical constants of thin films of these two materials by using a two media reflectance method at normal incidence and a graphical determination particularly suited to this VUV region, which we presented in a previous paper. Optical constants are determined in the range 60-124 nm with 2 nm step, and are compared to existing data. On the basis of these measured indices, polarizing mirrors for λ₁ = 121.6 nm or λ₂ = 103.2 nm have been modelled and fabricated. Their reflectance measured versus incidence angle by using monochromatized synchrotron radiation at the above wavelengths is found in agreement with the calculated predictions.     

Thermal transport in melting and recrystallization of amorphous and polycrystalline Si thin films

The liquid-solid interface motion and temperature history of thin Si films during excimer laser annealing are observed by in situ experiments combining time-resolved (~1 ns) electrical conductance, optical reflectance/transmittance at visible and near-IR wavelengths and thermal emission measurements. For laser energy densities causing partial melting, the maximum temperature remains close to the melting point of amorphous silicon (a-Si). When complete melting occurs, substantial supercooling is observed, followed by spontaneous nucleation. These phase transformations are consistent with the recrystallized poly-Si morphologies. It is also found that the melting of poly-Si occurs close to the melting point of crystalline silicon. This temperature is higher than the melting point of a-Si by about 100-150 K.     

Fabrication of black multicrystalline silicon surface by nanosecond laser ablation

The optimal regimes for uniform texturing of a multicrystalline silicon (mc-Si) surface by pulsed laser radiation have been determined. The morphology and reflectance spectra of the texturized mc-Si have been studied. The laser-texturized mc-Si samples with reflectance of 2?C3% over a wide spectral region have been produced. The influence of subsequent chemical etching on the reflective properties of the texturized surface has been analyzed.     

Brewster’s angle method for absorption coefficient measurement of high-resistivity silicon based on CW THz laser

A simple method for measuring the absorption coefficient of low absorbing materials using CW THz laser was put forward. The method was based on transmittance measurements at the Brewster’s angle for p-polarized light, where the reflectance would be minimal, so interference caused by multiple reflections in the sample would be eliminated. Numerical simulations were carried out to evaluate errors in the proposed method. An experiment was also made to measure the absorption coefficient of high-resistivity Czochralski silicon at 118.83 μm by the method. Based on CW THz laser, the method offers a convenient way to measure the material absorption coefficient and has a low cost, so it shows promising application prospects.     

Specular reflectance of silicon and germanium at variable angles of incidence in the infrared region

The results of systematic measurements of specular reflectances of both single as well as double surface polished silicon and germanium at glancing angles of incidence from 10° to 70° in the infrared (IR) region of 2.5–25 μm have been discussed. The experimental results have been explained by comparison with the theoretical reflectance data obtained using Fresnel's theory.     

Direct creation of black silicon using femtosecond laser pulses

Using a direct femtosecond laser surface structuring technique, an array of equally spaced parallel nanostructure-textured microgrooves on silicon was produced that causes a dramatic reduction of the treated silicon reflectance. The processed area appears velvet black at all viewing angles. Throughout the visible region, the reflectance of the blackened surface is less than 5%. The antireflection effect of the processed surface also extends to the mid-infrared wavelength range. Furthermore, this technique has a potential in reducing silicon reflectance at terahertz frequencies and even in millimeter wavelength range.     

Experimental determination of optical constants in the vacuum ultra violet wavelength region between 80 and 140 nm: A reflectance versus thickness method and its application to ZnSe

The 80-120 nm spectral range is a key domain for solar physics. Below 105 nm solids do not transmit light and the reflectance of available mirrors is particularly low which makes optical measurements specifically difficult. Optical constants of the materials may consequently be unavailable or unreliable.We present here a two media reflectance method at normal incidence suited to this VUV range, in which the variable is not the incidence angle but the thickness of the top layer made of the material to be analyzed. The real (n) and imaginary (k) parts of the complex index are directly and graphically determined in the (n, k) plane as the common intersection point of isoreflectance curves corresponding to samples different only in the thickness of the top layer.The method is tested and illustrated with ZnSe films evaporated on Al covered float glass substrates. In the literature, the reflectance magnitudes measured on ZnSe crystals differ strongly from an author to the other, leading to discrepant data for ZnSe in the VUV domain.We obtain precise and reliable values of (n, k), which fit the experimental values determined on freshly cleaved ZnSe crystals by J.L. Freeouf and the theoretical values calculated from the electronic band structure of ZnSe by John P. Walter and Marvin L. Cohen, but strongly differ from the optical constants selected by E.D. Palik in his tables.