Laser on porous silicon after oxidation by irradiation and annealing

Stimulated emission has been observed from oxide structure of silicon when optically excited by 514 nm laser. The photoluminescence (PL) pulse has a Lorentzian shape with a full width at half maximum (FWHM) of 0.5-0.6 nm. The twin peaks at 694 nm and 692 nm are dominated by stimulated emission which can be demonstrated by its threshold behavior and transition from sub-threshold to linear evolution in light emission. The gain coefficient from the evolution of the peak-emission intensity as a function of the optically pumped sample length has been measured. The oxide structure was fabricated by laser irradiation and annealing treatment on silicon. A model for explaining the stimulated emission has been proposed in which the trap states of the interface between oxide of silicon and porous nanocrystal play an important role.     

Silicon donor and Stokes terahertz lasers

Two types of lasers based on hydrogen-like impurity-related transitions in bulk silicon operate at frequencies between 1 and 7 THz (wavelength range of 50-230 μm). These lasers operate under mid-infrared optical pumping of n-doped silicon crystals at low temperatures (<30 K). Dipole-allowed optical transitions between particular excited states of group-V substitutional donors are utilized in the first type of terahertz silicon lasers. These lasers have a gain ∼1-3 cm⁻¹ above the laser thresholds (>1 kW cm⁻²) and provide 10 ps-1 μs pulses with a few mW output power on discrete lines. Raman-type Stokes stimulated emission in the range 4.6-5.8 THz has been observed from silicon crystals doped by antimony and phosphorus donors when optically excited by radiation from a tunable infrared free electron laser. The scattering occurs on the 1s(E)→1s(A₁) donor electronic transition accompanied by an emission of the intervalley transverse acoustic g-phonon. The Stokes lasing has a peak power of a few tenths of a mW and a pulse width of a few ns. The Raman optical gain is about 7.4 cm GW⁻¹ and the optical threshold intensity is ∼100 kW cm⁻².     

Infrared luminescence from spark-processed silicon

The infrared (IR) photoluminescence (PL) emission of spark-processed silicon (sp-Si) was investigated. A broad and strong room temperature PL peak in the 945 nm (1.31 eV) spectral range was observed when sp-Si was excited with an argon laser. This peak is different from the PL commonly reported for anodically etched porous silicon and other silicon-based materials. The PL intensity increases substantially after annealing sp-Si between 350 and 500 °C in air after which it decreases again. The PL wavelength is observed to peak at 1010 nm by annealing sp-Si near 450 °C. It was further found that the most efficient PL occurs for a Si/O ratio of 0.3, for a small spark gap of about 1 mm, and for spark-processing times in the 15-60 min range.A model for the IR PL is proposed which mirrors that for visible PL. Specifically, it is proposed that the electrons which have been pumped by the laser from the ground state into a broad quasi-absorption band (or closely spaced absorption lines between 1.7 and 2.3 eV) revert back to lower IR levels at 1.31 eV by a non-radiative transition from where they revert radiatively to the ground state by emitting the observed 945 nm light.     

Nano-laser on silicon quantum dots

A new conception of nano-laser is proposed in which depending on the size of nano-clusters (silicon quantum dots (QD)), the pumping level of laser can be tuned by the quantum confinement (QC) effect, and the population inversion can be formed between the valence band and the localized states in gap produced from the surface bonds of nano-clusters. Here we report the experimental demonstration of nano-laser on silicon quantum dots fabricated by nanosecond pulse laser. The peaks of stimulated emission are observed at 605 nm and 693 nm. Through the micro-cavity of nano-laser, a full width at half maximum of the peak at 693 nm can reach to 0.5 nm. The theoretical model and the experimental results indicate that it is a necessary condition for setting up nano-laser that the smaller size of QD (d < 3 nm) can make the localized states into band gap. The emission energy of nano-laser will be limited in the range of 1.7-2.3 eV generally due to the position of the localized states in gap, which is in good agreement between the experiments and the theory.     

Femtosecond laser micromilling of Si wafers

Femtosecond laser micromilling of silicon is investigated using a regeneratively amplified 775 nm Ti:Sapphire laser with a pulse duration of 150 fs operating at 1 kHz repetition rate. The morphological observation and topological analysis of craters fabricated by single-shot laser irradiation indicated that the material removal is thermal in nature and there are two distinct ablation regimes of low fluence and higher fluence with logarithmical relations between the ablation depth and the laser fluence. Crater patterns were categorized into four characteristic groups and their formation mechanisms were investigated. Femtosecond laser micromilling of pockets in silicon was performed. The effect of process parameters such as pulse energy, translation speed, and the number of passes on the material removal rate and the formation of cone-shaped microstructures were investigated. The results indicate that the microstructuring mechanism has a strong dependence on the polarization, the number of passes and laser fluence. The optimal laser fluence range for Si micromilling was found to be 2-8 J/cm² and the milling efficiency attains its maximum between 10 and 20 J/cm².     

Optical properties of silicon microcolumn grown by nanosecond pulsed laser irradiation

We have investigated the optical properties of silicon pillars formed by cumulative nanosecond pulsed excimer laser irradiation of single-crystal silicon in vacuum created under different repetition rates. The changes in optical characteristics of silicon pillar were systematically determined and compared as the number of KrF laser shots was increased from 1 to 15,000.The results show that silicon pillar PL curves exhibit a blue band around 430 nm and an ultraviolet band peaking at 370 nm with the vanishing of the green emission at 530 nm. A correlation between the intensity of the blue PL band and the intensity of the Si-O absorption bands has been exploited to explain such emission, whereas, the origin of the ultraviolet band may be attributed to different types of defects in silicon oxide.     

Pulsed Nd:YAP laser at 1432 nm pumped with high power laser diode

LD end pumped Nd:YAP laser operation in the eye safe spectrum region at 1432 nm is reported. With pump energy of 256 mJ, maximum linearly polarized output of 26 mJ is obtained. The optical-to-optical overall efficiency is around 10%, and the slope efficiency is around 18%. The laser beam operates with spiking mode with a total emission period of less than 300 µs at 10 Hz. The stimulated emission cross section is estimated at around 0.85 × 10^− 20 cm².     

Field emission measured from nanostructured germanium and silicon thin films

We have prepared nanostructured thin films of germanium and silicon. The films were grown by an ion beam sputtering technique followed by a rapid annealing step using an electron beam annealer. The annealing temperature is a comparatively low 500 °C, resulting in well defined nano-islands on the film surface. Electron field emission has been measured from the surfaces under high vacuum. The threshold electric field value for significant current flow was measured as 2.5 V μm⁻¹ for a silicon thin film which is comparable to other silicon technologies. A value of 0.5 V μm⁻¹ for a germanium thin film represents an order of magnitude improvement for related germanium nanostructured systems.     

Surface damage morphology investigations of silicon under millisecond laser irradiation

The surface damage morphologies of single crystal silicon induced by 1064 nm millisecond Nd:YAG laser are investigated. After irradiation, the damage morphologies of silicon are inspected by optical microscope (OM) and atomic force microscope (AFM). The plasma emission spectra of the damaged region are detected by the spectrometer. It is shown that surface oxidation and nitridation have occurred during the interaction of millisecond laser with silicon. In addition, the damage morphologies induced by 2 ms and 10 ns pulse width laser are compared. The damage morphology obtained by 2 ms laser is an evident crater. Three types of damage morphologies are formed at different laser energy densities. The circular concentric ripples are found surrounding the rim of the crater. The spacing of the ripples is 15 ± 5 μm. Two types of cracks are observed: linear crack and circular crack. The linear crack is observed in the center of the damaged region which propagates to the periphery of the damaged region. The circular crack is located at the rim of the crater. The damage morphology induced by 10 ns laser is surface layer damage. The periodic linear waves are generated due to the interference between the incident beam and the scattered beam. The spacing of the ripples is 1.54 μm which is close to the incident laser wavelength 1.064 μm. The linear crack is located at the center of the damaged region. Furthermore, for the same laser energy density, the dimension of the damaged region and the crater depth induced by 2 ms laser are greater than that of 10 ns laser. It indicates that the damage mechanism under millisecond pulse laser irradiation is strongly different from the case of nanosecond pulse laser.     

Characterization of laser-generated silicon plasma

A study of visible laser ablation of silicon, in vacuum, by using 3 ns Nd:YAG laser radiation is reported. Nanosecond pulsed ablation, at an intensity of the order of 10¹⁰ W/cm², produces high non-isotropic emission of neutrals and ionic species. Mass quadrupole spectrometry, coupled to electrostatic ion deflection, allows estimation of the energy distributions of the emitted species from plasma. Neutrals show typical Boltzmann-like distributions while ions show Coulomb-Boltzmann-shifted distributions depending on their charge state. Time-of-flight measurements were also performed by using an ion collector consisting of a collimated Faraday cup placed along the normal to the target surface. Surface profiles of the craters, created by the laser radiation absorption, permitted to study the ablation threshold and ablation yields of silicon in vacuum. The plasma fractional ionization, temperature and density were evaluated by the experimental data. A special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated at the non-equilibrium plasma conditions. The angular distribution of the neutral and ion species is discussed.     

Tunable erbium doped fiber ring laser using fiber Bragg grating-assisted add-drop filter

A stable and low costless tunable erbium doped fiber ring laser using fiber Bragg grating-assisted add-drop filter is proposed and demonstrated. A stable laser output is obtained with a 4 nm tuning range. The power fluctuation, full-width at half maximum and SMSR are measured to be less than 0.50 dB, smaller than 0.015 nm and better than 55 dB in this tuning range.     

Nanocrystal and interface defects related photoluminescence in silicon-rich Al2O3 films

In this study, silicon nanocrystal-rich Al₂O₃ film has been prepared by co-sputtering a silicon and alumina composite target and subsequent annealing in N₂ atmosphere. The microstructure of the film has been characterized by infrared (IR) absorption, Raman spectra and UV-absorption spectra. Typical nanocrystal and interface defects related photoluminescence with the photon energy of 1.54 (IR band) and 1.69 eV (R band) has been observed by PL spectrum analysis. A post-annealing process in oxygen atmosphere has been carried out to clarify the emission mechanism. Despite the red shift of the spectra, enhanced emission of the 1.69 eV band together with the weak emission phenomenon of the 1.54 eV band has been found after the post-annealing. The R band is discussed to originate from silicon nanocrystal interface defects. The IR band is concluded to be a coupling effect between electronic and vibrational emissions.     

Tunable, μs-pulsed ytterbium fiber laser system with a linewidth below 2.7 GHz

We report on a widely tunable, pulsed laser system with narrow spectral linewidth based on a continuous wave ytterbium fiber oscillator, a pulse shaper and a power amplifier stage. The system is tunable from 1055 nm to 1085 nm and provides a maximum pulse energy of 155 μJ with a pulse duration of 1-5 μs. The linewidth is less than 2.7 GHz over the whole tuning range.     

Low threshold, actively Q-switched Nd:YVO4 self-Raman laser and frequency doubled 588 nm yellow laser

We reported an actively Q-switched, intracavity Nd³⁺:YVO₄ self-Raman laser at 1176 nm with low threshold and high efficiency. From the extracavity frequency doubling by use of LBO nonlinear crystal, over 3.5 mW, 588 nm yellow laser is achieved. The maximum Raman laser output at is 182 mW with 1.8 W incident pump power. The threshold is only 370 mW at a pulse repetition frequency of 5 kHz. The optical conversion efficiency from incident to the Raman laser is 10%, and 1.9% from Raman laser to the yellow.     

Surface modification of a WTi thin film on Si substrate by nanosecond laser pulses

Interaction of a nanosecond transversely excited atmospheric (TEA) CO₂ laser, operating at 10.6 μm, with tungsten-titanium thin film (190 nm) deposited on silicon of n-type (1 0 0) orientation, was studied. Multi-pulse irradiation was performed in air atmosphere with laser energy densities in the range 24-49 J/cm². The energy absorbed from the laser beam was mainly converted to thermal energy, which generated a series of effects. The following morphological changes were observed: (i) partial ablation/exfoliation of the WTi thin film, (ii) partial modification of the silicon substrate with formation of polygonal grains, (iii) appearance of hydrodynamic features including nano-globules. Torch-like plumes started appearing in front of the target after several laser pulses.     

Field emitters with low turn on electric field based on carbon fibers

Field emitters of vertical carbon fibers on a silicon substrate are fabricated by catalytic chemical vapor deposition. After an ageing process of 150 min, field emission measurement of the fibers is carried out in a vacuum chamber with a base pressure of 5.0 × 10⁻⁴ Pa. The experimental results display that field emission performance of the carbon fibers depends strongly on the vacuum level during the experiments. After the field emission measurement, damage to the carbon fiber field emitters is observed from the scanning electron microscopic images.     

Spatially resolved Raman spectroscopy evaluation of residual stresses in 3C-SiC layer deposited on Si substrates with different crystallographic orientations

Raman scattering studies were performed on hot-wall chemical vapor deposited (heteroepitaxial) silicon carbide (SiC) films grown on Si substrates with orientations of (1 0 0), (1 1 1), (1 1 0) and (2 1 1), respectively. Raman spectra suggested that good quality cubic SiC single crystals could be obtained on the Si substrate, independent of its crystallographic orientation. Average residual stresses in the epitaxially grown 3C-SiC films were measured with the laser waist focused on the epilayer surface. Tensile and compressive residual stresses were found to be stored within the SiC film and in the Si substrate, respectively. The residual stress exhibited a marked dependence on the orientation of the substrate. The measured stresses were comparable to the thermal stress deduced from elastic deformation theory, which demonstrates that the large lattice mismatch between cubic SiC and Si is effectively relieved by initial carbonization. The confocal configuration of the optical probe enabled a stress evaluation along the cross-section of the sample, which showed maximum tensile stress magnitude at the SiC/Si interface from the SiC side, decreasing away from the interface in varied rate for different crystallographic orientations. Defocusing experiments were used to precisely characterize the geometry of the laser probe in 3C-SiC single crystal. Based on this knowledge, a theoretical convolution of the in-depth stress distribution could be obtained, which showed a satisfactory agreement with stress values obtained by experiments performed on the 3C-SiC surface.     

Strong violet luminescence from ZnO nanocrystals grown by the low-temperature chemical solution deposition

The luminescence properties of zinc oxide (ZnO) nanocrystals grown from solution are reported. The ZnO nanocrystals were characterized by scanning electron microscopy, X-ray diffraction, cathodo- and photoluminescence (PL) spectroscopy. The ZnO nanocrystals have the same regular cone form with the average sizes of 100-500 nm. Apart from the near-band-edge emission around 381 nm and a weak yellow-orange band around 560-580 nm at 300 K, the PL spectra of the as-prepared ZnO nanocrystals under high-power laser excitation also showed a strong defect-induced violet emission peak in the range of 400 nm. The violet band intensity exhibits superlinear excitation power dependence while the UV emission intensity is saturated at high excitation laser power. With temperature raising the violet peak redshifts and its intensity increases displaying unconventional negative thermal quenching behavior, whereas intensity of the UV and yellow-orange bands decreases. The origin of the observed emission bands is discussed.     

Continuous-wave broadly tunable Cr:ZnSe laser pumped by a thulium fiber laser

We describe a compact, broadly tunable, continuous-wave (cw) Cr²⁺:ZnSe laser pumped by a thulium fiber laser at 1800 nm. In the experiments, a polycrystalline ZnSe sample with a chromium concentration of 9.5 × 10¹⁸ cm⁻³ was used. Free-running laser output was around 2500 nm. Output couplers with transmissions of 3%, 6%, and 15% were used to characterize the power performance of the laser. Best power performance was obtained with a 15% transmitting output coupler. In this case, as high as 640 mW of output power was obtained with 2.5 W of pump power at a wavelength of 2480 nm. The stimulated emission cross-section values determined from laser threshold data and emission measurements were in good agreement. Finally, broad, continuous tuning of the laser was demonstrated between 2240 and 2900 nm by using an intracavity Brewster cut MgF₂ prism and a single set of optics.     

1.54 μm room temperature emission from Er-doped Si nanocrystals deposited by ECR-PECVD

In this work, silicon nanocrystals (Si-nc) embedded in a silicon-rich silicon oxide (SRSO) matrix doped with Er³⁺ ions for different erbium and silicon concentrations have been deposited by electron-cyclotron resonance plasma-enhanced chemical-vapor-deposition (ECR-PECVD) technique. Their optical properties have been investigated by photoluminescence (PL) and reflectance spectroscopy.Room temperature emission bands centered at ∼1.54 and at 0.75 μm have been obtained for all samples. The most intense emission band at ∼1.54 μm was obtained for samples with concentrations of 0.45% and 39% for erbium and silicon, respectively. Moreover, it has been found that the broad emission band centered at ∼0.75 μm for all samples shows a very strong interference pattern related to the a specific sample structure and a high sample quality.