Simulation of Chromium Atom Deposition Pattern in a Gaussain Laser Standing Wave with Different Laser Power

One-dimensional deposition of a neutral chromium atomic beam focused by a near-resonant Gaussian standing- laser field is discussed by using a fourth-order Runge-Kutta type algorithm. The deposition pattern of neutral chromium atoms in a laser standing wave with different laser power is discussed and the simulation result shows that the full width at half maximum （FWHM） of a nanometer stripe is 115nm and the contrast is 2.5：1 with laser power 3.93mW; the FWHM is 0.8nm and the contrast is 27：1 with laser power 16mW, the optimal laser power; but with laser power increasing to 50mW, the nanometer structure forms multi-crests and the quality worsens quickly with increasing laser power.     

Broad-area diode-laser system for a rubidium Bose-Einstein condensation experiment

We report on master-oscillator power amplification using a broad-area laser diode (BAL) emitting at a wavelength of λ =780 nm. The master oscillator is an injection-locked single-mode diode laser delivering a seeding beam of 35 mW, which is amplified in double pass through the BAL up to 410 mW. After beam shaping and spatial filtering by a single-mode fibre we obtain a clean Gaussian beam with a maximum power of 160 mW. There is no detectable contribution of the BAL eigenmodes in the spectrum of the output light. This laser system is employed for operation of a ⁸⁷Rb magneto-optical trap (MOT) and for near-resonant absorption imaging in a Bose-Einstein condensation experiment. Received: 10 April 2000 / Revised version: 13 June 2000 / Published online: 2 August 2000     

Characterization of a high-power tapered semiconductor amplifier system

We have characterized a semiconductor amplifier laser system which provides up to 200 mW output after a single-mode optical fiber at 780 nm wavelength. The system is based on a tapered semiconductor gain element, which amplifies the output of a narrow-linewidth diode laser. Gain and saturation are discussed as a function of operating temperature and injection current. The spectral properties of the amplifier are investigated with a grating spectrometer. Amplified spontaneous emission (ASE) causes a spectral background with a width of 4 nm FWHM. The ASE background was suppressed to below our detection limit by a proper choice of operating current and temperature and by sending the light through a single-mode optical fiber. The final ASE spectral density was less than 0.1 nW/MHz, i.e. less than 0.2% of the optical power. Related to an optical transition linewidth of Γ/2π=6 MHz for rubidium, this gives a background suppression of better than -82 dB. An indication of the beam quality is provided by the fiber coupling efficiency of up to 59%. The application of the amplifier system as a laser source for atom-optical experiments is discussed. Received: 8 May 2000 / Revised version: 21 September 2000 / Published online: 7 February 2001     

偏斜椭圆激光驻波场作用下中性原子沉积纳米光栅结构特性分析

分析了椭圆激光驻波场的偏斜对中性原子运动过程和沉积过程的影响,对不同偏斜角度椭圆激光驻波场作用下中性铬原子沉积纳米光栅结构的特性进行了仿真研究,由仿真结果可以看出,随着偏斜椭圆形激光束偏斜角的增加,对应于不同y平面,激光驻波场汇聚中性原子所形成纳米光栅条纹的对比度不断减小、半高宽不断增大.当椭圆长短轴之比为2:1条件下,椭圆激光驻波场的偏斜角为0°时,纳米光栅的条纹半高宽为3.2 nm,条纹对比度为36:1,而当偏斜角为15°时,激光驻波场中心位置处的沉积条纹的半高宽为6.5 nm,条纹对比度为24:1,而当椭圆激光驻波场偏斜角度达到30°时,沉积条纹的单峰结构将会产生分裂,形成了双峰结构,且随着偏斜角的增加,沉积条纹的分裂越严重,纳米光栅的沉积质量越差.对于其他长短轴比例条件下的激光场亦可根据比例关系获得相应的纳米光栅沉积特性. 关键词： 原子光刻 偏斜椭圆激光驻波场 纳米光栅     

Fabrication of Nanoimprint Stamp Using Interference Lithography

Interference lithography is used to fabricate a nanoimprint stamp, which is a key step for nanoimprint lithography. A layer of chromium in thickness of about 20 nm is deposited on the newly cleaned fused silica substrate by thermal evaporation, and a layer of positive resist in thickness of 150nm is spun on the chromium layer. Some patterns, including lines, holes and pillars, are observed on the photoresist film by exposing the resist to interference patterns and they are then transferred to the chromium layer by wet etching. Fused silica stamps are fabricated by reactive ion etching with CHF3/O2 as etchants using the chromium layer as etch mask. An atomic force microscope is used to analyse the pattern transfer in each step. The results show that regular hole patterns of fused silica, with average full width 143nm at half maximum （FWHM）, average hole depth of 76nm and spacing of 450nm, have been fabricated. The exposure method is fast, inexpensive and applicable for fabrication of nanoimprint stamps with large areas.     

Characterization of high-quality MBE-grown GaN films on intermediate-temperature buffer layers

High-quality GaN thin films are grown by rf-plasma assisted molecular beam epitaxy. The quality of the GaN epitaxial layer is significantly improved by using an intermediate-temperature GaN buffer layer (ITBL) in addition to a conventional 20-nm-thick low-temperature buffer layer. The GaN epitaxial layers demonstrate systematic improvements in the electron mobility increasing from 82 cm² V^-1 s^-1, for films grown with just the low-temperature buffer layer, to about 380 cm² V^-1 s^-1 for films grown with an ITBL of thickness 800 nm. The photoluminescence also indicates systematic improvements in the intensity and the full-width-half-maximum with the use of ITBL. Photoreflectance spectra are measured from the GaN films. Detailed analyses of the excitonic transition energy demonstrate that the residual strain relaxes rapidly with the use of ITBL, which is attributed to the observed improvements in the mobility and the PL spectra. Received: 30 November 2000 / Accepted: 4 December 2000 / Published online: 9 February 2001     

Efficient and compact intracavity-frequency-doubled Nd:GdVO4/KTP laser end-pumped by a fiber-coupled laser diode

A compact and efficient diode-pumped intracavity-frequency-doubled Nd:GdVO₄/KTP green laser is demonstrated with a flat–flat cavity design. With a 1.3 at. % Nd³⁺-doped GdVO₄ crystal and pumped at the weak-absorption peak of 806 nm, the second-harmonic output power at 532 nm was measured to be 1.95 W at an incident pump power of 8.4 W, corresponding to an optical conversion efficiency of 23.2%. The output characteristic at the fundamental wavelength of 1.063 μm was investigated with two different pump wavelengths. More than 4.5-W output power was generated when the laser was pumped at 806.2 nm. Received: 26 July 2000 / Revised version: 18 September 2000 / Published online: 7 February 2001     

Temperature-tuned difference-frequency mixing in periodically poled KTiOPO4

4 (PPKTP). We generated 12 μW of radiation tunable around 1.6 μm by difference-frequency mixing of the outputs of a frequency-doubled Nd:YLF laser at 523 nm (240 mW) and a tunable Ti:sapphire laser near 760 nm (340 mW). A temperature tuning rate of 0.73 nm/°C for the generated wavelength and a FWHM temperature acceptance bandwidth of 6.9 °C cm was observed. The effective d₃₃ coefficient was estimated to be ∼5 pm/V. Received: 02 September 1998     

Spatial beam profiles of femtosecond X-ray pulses generated by laser Compton scattering from a low-emittance electron beam

Polarization-dependent spatial beam profiles of femtosecond X-ray pulses generated by a laser Compton scheme were measured. The X-ray pulses were generated by the interaction at an angle of 90° between 100-fs laser light and a 3-ps, 3π-mm mrad electron beam. The polarization of the laser light was linear in two different directions, either parallel or perpendicular to the electron beam axis. The measured profiles showed good agreement with theoretical results. Received: 5 July 2002 / Revised version: 17 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +81-424/684477, E-mail: msf_yorozu@shi.co.jp     

Efficient generation of blue light by frequency doubling of a Nd:YAG laser operating on 4 F 3/24 I 9/2 transitions

Efficient room-temperature operation of ⁴ F _3/2⁴ I _9/2 transitions in diode-end-pumped Nd:YAG lasers at 946 nm and 938.5 nm is reported. 7.0-W continuous-wave output power at 946 nm and 3.9 W at 938.5 nm have been obtained. An analytical model has been developed for the quasi-three-level laser including the influence of energy-transfer upconversion. Frequency doubling of these transitions in periodically poled KTP generated blue light at 473 nm and 469 nm. Both single-pass extra-cavity as well as intracavity schemes have been investigated. Received: 31 July 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +46-8/750-5430, E-mail: stefan.bjurshagen@acreo.se     

Laser gain measurements at 193 nm in a small discharge cell containing ArF excimer laser gas mixtures

Spatial and temporal gain profiles as well as the peak net gain at 193 nm have been measured in X-ray preionized discharges excited by a single pulse electrical system working in the charge transfer mode. Ar- and F₂-containing laser gas mixtures with He or Ne as a buffer gas have been used. With a pumping pulse duration of ∼ 100 ns (FWHM) and a specific peak power deposition of ∼ 1 MW cm^-3 bar^-1 in a gas mixture containing F₂ : Ar : He (0.1%:5%:94.9%), at 2 bar total pressure, a very high peak net gain coefficient of ∼ 30% cm^-1 was measured in the gas discharge. The FWHM of the gain waveform was ∼ 60 ns. PACS 42.55.Lt; 42.60.Lh; 52.80.-s     

Effect of the ceramic grain size and concentration on the dynamical mechanical and dielectric behavior of poly(vinilidene fluoride)/Pb(Zr<Subscript>0.53</Subscript>Ti<Subscript>0.47</Subscript>)O<Subscript>3</Subscript> composites

Laser-induced backside wet and dry etching (LIBWE and LIBDE) methods were developed for micromachining of transparent materials. Comparison of these techniques is helpful in understanding the etching mechanism but was not realized due to complications in setting up comparable experimental conditions. In our comparative investigations we used a solid tin film for dry and molten tin droplets for wet etching of fused-silica plates. A tin–fused-silica interface was irradiated through the sample by a KrF excimer laser beam (λ=248 nm, FWHM=25 ns); the fluence was varied between 400 and 2100 mJ/cm². A significant difference between the etch depths of the two investigated methods was not found. The slopes of the lines fitted to the measured data (sl_LIBDE=0.111 nm/mJ cm⁻², sl_LIBDE=0.127 nm/mJ cm⁻²) were almost similar. Etching thresholds for LIBDE and LIBWE were approximately 650 and 520 mJ/cm², respectively. To compare the dependence of etch rates on the pulse number, target areas were irradiated at different laser fluences and pulse numbers. With increasing pulse number a linear rise of depth was found for wet etching while for dry etching the etch depth increase was nonlinear. Secondary ion mass spectroscopic investigations proved that this can be due to the reconstruction of a new thinner tin-containing surface layer after the first pulse.     

Optical induction of 3D rotational symmetry refractive lattices by combined interferometric-mask method in doped LiNbO3 crystals

A novel combined interferometric–mask method for the formation of micro- and nanometric scale three-dimensional (3D) rotational symmetry quasi-crystalline refractive lattice structures in photorefractive materials is demonstrated experimentally. The method is based on micrometric scale spatial modulation of the light by amplitude mask in the radial directions and along the azimuthal angle and the use of counter-propagating beam geometry building up Gaussian standing wave, which defines the light modulation in the axial direction with half-wavelength periodicity. 3D intensity pattern can be represented as numerous mask-generated 2D quasi-periodic structures located in each anti-node of the standing wave. The formed 3D intensity distributions of the optical beams can be imparted into the photorefractive medium thus creating the micro- and sub-micrometric scale 3D refractive index volume lattices. The used optical scheme allows also the formation of 2D lattices by removing the back-reflecting mirror. 2D and 3D refractive lattices were recorded with the use of 532 nm laser beam and rotational symmetry mask in doped lithium niobate crystals and were tested by the probe beam far-field diffraction pattern imaging and direct observation by phase microscope. The formed rotational symmetry 3D refractive structures have the periods of 20–60 μm in the radial directions, 60 μm along the azimuthal angle and half-wavelength 266 nm in the axial direction.     

Electronic band structures of low-dimensional adsorbate systems: Rare-gas adsorption on transition metals

Angle-resolved photoemission data are dis-cussed for five different Xe adlayers which exhibit electronic structures of different dimensionalities. Xe adsorption on Ni (110)-(1 × 2)-3Hand the (×) R30^° Xe layer on Ru (001) reveal two-dimensional (2D) Xe-derived band structures that are characteristic for hexagonal rare-gas layers. Different Xe 5p dispersion widths on Ni and on Ru are found due to the difference in the Xe-Xe nearest-neighbor distance. For three rare-gas systems (two different Xe coverages on hydrogen-modified Pt (110)-(1 × 2)-H and Kr step decoration on a Pt (997) surface) true one-dimensional (1D) band structures are found. For Xe step adsorption on Pt (997), electronic localized (0D) behavior is observed due to an enlarged Xe-Xe separation. The qualitative differences of the band structures in the case of 2D, 1D and 0D rare-gas systems are demonstrated and are explained by the different dimensionalities of the various structures. Received: 3 August 2000 / Accepted: 4 August 2000 / Published online: 7 March 2001     

Inscription of holographic gratings using circularly polarized light: influence of the optical set-up on the birefringence and surface relief grating properties

The polarization properties of the optical set-up used for holographic recording of diffraction gratings on azopolymer thin films are analyzed. The state of polarization of circularly polarized light is fully analyzed after reflection on a mirror at various incidences (Lloyd-mirror set-up). The Stokes analysis is performed using a photopolarimeter and the phase shift, the ellipticity and the azimuth orientation are compared with those calculated from Fresnel formulae. At large angles of incidence, an initially right circularly polarized (RCP) beam becomes elliptically polarized with an azimuth of nearly +45°. From these results, holographic diffraction gratings are recorded on an azobenzene-containing polymer thin film using (i) co- and contra-circularly polarized beams and (ii) a right circularly polarized beam interfering with a +45° linearly polarized light beam. Using Jones-matrix formalism, the polarization states of the diffracted orders from the birefringence (Δn) and the surface-relief (2Δd) gratings are derived and compared with experimental measurements. Finally, the induced local birefringences and surface-relief amplitudes are discussed in connection with atomic force microscopy measurements. The diffraction efficiencies obtained under the (+45°+RCP) and (LCP + RCP) (where LCP = left circularly polarized) configurations are thus compared and discussed. Received: 5 October 2001 / Revised version: 26 November 2001 / Published online: 17 January 2002     

Flat layered structure and improved photoluminescence emission from porous silicon microcavities formed by pulsed anodic etching

Single-mode, highly directional and stable photoluminescence (PL) emission has been achieved from porous silicon microcavities (PSMs) fabricated by pulsed electrochemical etching. The full width at half maximum (FWHM) of the narrow PL peak available from a freshly etched PSM is about 9 nm. The emission concentrates in a cone of 10° around the normal of the sample, with a further reduced FWHM of ∼5.6 nm under angle-resolved measurements. Only the resonant peak is present in such angle-resolved PL spectra. No peak broadening is found upon exposure of the freshly prepared PSM to a He-Cd laser beam, and the peak becomes somewhat narrower (∼5.4 nm) after the PSM has been stored in an ambient environment for two weeks. At optimized etching parameters, even a 4-nm FWHM is achievable for the freshly etched PSM. In addition, scanning electron microscopy (SEM) plane-view images reveal that the single layer porous Si formed by pulsed current etching is more uniform and flatter than that formed by direct current (dc) etching, demonstrated by the well-distributed circular pores with small size in the former in comparison with the irregular interlinking pores in the latter. The SEM cross-section images show the existence of oriented Si columns of 10 nm diameter along the etching direction within the active layer, good reproducibility and flat interfaces. It is thus concluded that pulsed current etching is superior to dc etching in obtaining flat interfaces within the distributed Bragg reflectors because of its minor lateral etching. Received: 7 March 2001 / Accepted: 23 July 2001 / Published online: 30 October 2001     

激光干涉光刻法制作100 nm掩模

 介绍了一种利用激光干涉光刻技术得到特征图形,并通过离子束刻蚀将图形转移到铬层上,从而获得掩模的方法。针对掩模透光率以及对干涉图形对比度可能产生影响的两个参数分别进行了数值仿真,从而证明此方法的可行性和参数的优化选择。自搭干涉光刻实验系统,用257 nm的激光光源实现光刻,得到特征尺寸为100 nm的图形,再经过离子束刻蚀,最终得到周期200 nm、线宽100 nm的掩模。     

Multiwatt, tunable, diode-pumped CW Yb:GdCOB laser

We demonstrate that Yb-doped Ca₄GdO(BO₃)₃ (GdCOB) crystals are suitable for the development of high-power diode-pumped lasers emitting at around 1.04 μm. A 15%-doped Yb:GdCOB crystal was longitudinally pumped with a cw fiber-coupled diode emitting 10 W at 976 nm. While 5.2 W of diode power was absorbed, we obtained 3.2 W of 1043-μm laser light, with a beam quality factor M² equal to 3, and 2.5 W in a diffraction-limited beam. Furthermore, the laser is continuously tunable between 1018 and 1086 nm. Thermal effects have been investigated with a Shack–Hartmann wavefront analyser: although thermal lensing is not negligible, it does not affect the performance of the laser with the resonator design we used. Received: 1 August 2000 / Revised version: 18 September 2000 / Published online: 21 February 2001     

Formation of conical microstructures upon laser evaporation of solids

The formation and development of the large-scale periodic structures on a single crystal Si surface are studied upon its evaporation by pulsed radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The development of structures occurs at a high number of laser shots (∼10⁴) at laser fluence of 1–2 J/cm² below optical breakdown in a wide pressure range of surrounding atmosphere from 1 to 10⁵ Pa. The structures are cones with angles of 25, which grow towards the laser beam and protrude above the initial surface for 20–30 μm. It is suggested that the spatial period of the structures (10–20 μm) is determined by the capillary waves period on the molten surface. The X-ray diffractometry reveals that the modified area of the Si substrate has a polycrystalline structure and consists of Si nanoparticles with a size of 40–70 nm, depending on the pressure of surrounding gas. Similar structures are also observed on Ge and Ti. Received: 12 February 2000 / Accepted: 28 March 2000 / Published online: 20 June 2001