Passively Q-switched Er-doped fiber laser using a semiconductor saturable absorber mirror

A passively Q-switched Er-doped fiber laser using a semiconductor saturable absorber mirror (SESAM) is demonstrated. Q-switching is observed with the output power produced at a slope efficiency of 29.4% with respect to the absorbed pump power. The maximum average output power of 8.37 mW is achieved. The pulse repetition frequency obtained can be turned from 1.72 to 7.95 kHz. The pulse energy of 17.2 nJ has been obtained at the pump power of 46.75 mW, and the pulse width is 30 μs.     

Tunable terahertz generation from one CO2 laser in a GaSe crystal

Coherent terahertz pulses have been generated at a range of 236.3-1104.5 μm (0.27-1.3 THz) by one CO₂ laser with dual-wavelength output based on collinearly phase-matched different frequency generation (DFG) in a GaSe crystal. This source has the advantages of compact and simplicity for tuning. The output power of the THz pulse and phase-matching conditions were investigated. The maximum single pulse energy of 11 nJ was generated at a frequency of 1.23 THz (243.6 μm), corresponding to a peak output power 182 mW.     

Influence of deposition pressure on structural, optical and electrical properties of nc-Si:H films deposited by HW-CVD

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited using HW-CVD technique at various deposition pressures. Characterisation of these films from Raman spectroscopy revealed that nc-Si:H thin films consist of a mixture of two phases, crystalline phase and amorphous phase containing small Si crystals embedded therein. We observed increase in crystallinity in the films with increase in deposition pressure whereas the size of Si nanocrystals was found ∼2 nm over the entire range of deposition pressure studied. The FTIR spectroscopic analysis showed that with increasing deposition pressure the predominant hydrogen bonding in the films shifts from, Si-H to Si-H₂ and (Si-H₂)_n complexes and the hydrogen content in the films was found in the range 6.2-9.3 at% over the entire range of deposition pressure studied. The photo and dark conductivities results also indicate that the films deposited with increasing deposition pressure get structurally modified. It has been found that the optical energy gap range was between 1.72 and 2.1 eV with static refractive index between 2.85 and 3.24. From the present study it has been concluded that the deposition pressure is a key process parameter to induce the crystallinity in the Si:H thin films using HW-CVD.     

Graphene-based passively Q-switched 2 μm thulium-doped fiber laser

We demonstrated stable pulses generation at 2 μm in a passively Q-switched thulium-doped fiber laser using a few layer graphene thin film. The maximum output power was 4.5 mW and the single pulse energy was 85 nJ at 53 kHz repetition rate, and the pulse width was about 1.4 μs. The pulse width and the repetition rate of the Q-switched fiber laser can be changed along with the pump power. To the best of our knowledge, this is the first report of graphene saturable absorber for passively Q-switched 2 μm fiber lasers.     

Structural and optical properties of amorphous hydrogenated silicon carbonitride films produced by PECVD

Amorphous hydrogenated silicon carbonitride thin films (a-Si:C:N:H), deposited by plasma enhanced chemical vapour deposition (PECVD) using hexamethyldisilazane (HMDSN) as monomer and Ar as feed gas, have been investigated for their structural and optical properties as a function of the deposition RF plasma power, in the range of 100-300 W. The films have been analysed by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), UV-vis-NIR spectrophotometry and atomic force microscopy (AFM). From the analysis of the FT-IR spectra it results that the films become more amorphous and inorganic as RF plasma power increases. The incorporation of oxygen in the deposited layers, mainly due to the atmospheric attack, has been evaluated by XPS and FT-IR spectroscopy. Reflectance/transmittance spectra, acquired in the range of 200-2500 nm, allow to descrive the film absorption edge for interband transitions. A relationship between the optical energy band gap, deduced from the absorption coefficient curve, and the deposition RF plasma power has been investigated. The reduction of the optical energy gap from 3.85 to 3.69 eV and the broadening of the optical absorption tail with RF plasma power increasing from 100 to 300 W are ascribed to the growth of structural disorder, while the increase of the refractive index, evaluated at 630 nm, is attributed to a slight densification of the film. The AFM analysis confirms the amorphous character of the films and shows how the deposited layers become rougher when RF plasma power increases. The wettability of the film has been studied and related to the chemical composition and to the morphology of the deposited layers.     

Investigation on femtosecond laser irradiation energy in inducing hydrophobic polymer surfaces

This study investigates the use of ultrashort femtosecond laser pulses to induce hydrophobic properties on PMMA surfaces. The modification of surface wetting property exhibits a strong dependence on the amount of energy deposited on the PMMA surface. A simple equation has been deduced from the laser parameters to express the energy deposition. It was revealed that water contact angle (WCA) of more than 120°, with a maximum of around 125°, could be achieved when the total energy deposited per unit area on the PMMA surface ranged from 600 J/cm² to 900 J/cm² at an energy deposition rate of around 50 J/cm²/s. Beyond this range, WCA reduced with increasing amount of energy deposition. Furthermore, with higher energy deposition rate or higher laser fluence, total energy required to induce hydrophobic surfaces was reduced. Under different energy deposition, the quantity of polar groups or non-polar groups induced was responsible for the changes in WCA and thus the different surface hydrophobicity.     

Pulsed laser deposition and characterization of nitrogen-substituted SrTiO3 thin films

Nitrogen-substituted cubic perovskite-type SrTiO₃ thin films were deposited in a one-step process using pulsed reactive crossed beam laser ablation (PRCLA) and RF-plasma assisted pulsed laser deposition (RF-PLD). Both techniques yield preferentially oriented films on SrTiO₃(0 0 1), LaAlO₃(0 0 1) and MgO(0 0 1) substrates with the unit cell parameters within 0.390(5) < a < 0.394(9) nm. The nitrogen content is higher in films deposited by PRCLA (0.84-2.40 at.%) as compared to films deposited by RF-PLD with nitrogen plasma (0.10-0.66 at.%). PRCLA with an ammonia gas pulse leads to a higher nitrogen content compared to the films grown with a nitrogen gas pulse, while films deposited by RF-PLD with ammonia plasma reveal only minor nitrogen contents (<0.10 at.%). The amount of the incorporated nitrogen can be tuned by adjusting the deposition parameters. Films deposited by PRCLA have a lower roughness of 1-3 nm compared to 12-18 nm for the films grown by RF-PLD. PRCLA yields partially reduced films, which exhibit electronic conductivity, while films deposited by RF-PLD are insulating. There is also a pronounced influence of the substrate material on the resistivity of the films deposited by PRCLA: films grown on SrTiO₃ substrates exhibit a metallic-like behaviour, while the corresponding films grown on MgO and LaAlO₃ substrates reveal a metal-to-semiconductor/insulator transition. Nitrogen incorporation into the SrTiO₃ films results in an increased optical absorption at 370-500 nm which is associated with N(2p) localized states with the energy about 0.7 eV higher than the valence band energy in strontium titanate. The optical band gap energies in the studied N-substituted SrTiO₃ films are 3.35-3.40 eV.     

Reflective carbon nanotube as the saturable absorber for mode-locked 1064 nm laser

With a reflective single-walled carbon nanotube as the saturable absorber, a laser diode-pumped passively mode-locked Nd:YVO₄ laser at 1064 nm was realized for the first time. The pulse duration of 12 ps was produced with a repetition rate of 83.7 MHz. The peak power and the single pulse energy of the mode-locking laser were 1.28 kW and 15.4 nJ, respectively.     

Enhancement of terahertz wave difference frequency generation based on a compact walk-off compensated KTP OPO

A compact, walk-off compensated dual-wavelength KTP OPO near the degenerate point of 2.128 μm pumped by a Nd:YAG pulsed laser is employed as the pump for terahertz (THz) source based on difference frequency generation (DFG) in a GaSe crystal. Coherent THz radiation that is continuously tunable in the range of 81-1617 μm (0.186-3.7 THz) is achieved. An enhancement of 76.7% in average for the THz energies at different wavelengths is realized using the walk-off compensated KTP OPO than the common one. Using a 8 mm-long GaSe crystal, the maximum output THz pulse energy is 48.9 nJ with the peak power of 11 W, corresponding to the energy conversion efficiency of 5.4 × 10^− 6 and the photon conversion efficiency of about 0.09%.     

A detailed study through the focal region of near-threshold single-shot femtosecond laser ablation nano-holes in borosilicate glass

A detailed study of the morphology of nano-craters drilled in borosilicate glass by single shot femtosecond laser ablation near the ablation threshold has been performed by scanning electron microscopy, atomic force microscopy and scanning electron microscopy imaging after focused ion beam sectioning. The influence of the numerical aperture (NA = 0.4 and 0.8), the pulse energy (16 nJ < E_p < 600 nJ) and the position of the specimen surface into the focal region were systematically investigated, leading to nanometric or micrometric scales in every spatial dimension. The nanocrater’s size is not restricted by the diffraction limit but determined by the laser pulse stability and the material properties. If the beam is focused inside the glass, two craters are drilled, shaping very distinct morphologies. Their dimensions have been studied in details and different relationships have been proposed for the evolutions of the depths and of the various diameters of these craters as functions of the pulse energy, the numerical aperture and the position of specimen surface in the beam-material interaction region. It is suggested that the long, thin conical profile with very high aspect ratio of the secondary craters is due to a spontaneous reshaping of the beam which transforms the incoming Gaussian pulse into a Gaussian-Bessel pulse. As proposed in the developed model the geometry of the second craters seems to be connected with the one of the main craters.     

RF-PACVD of water repellent and protective HMDSO coatings on bell metal surfaces: Correlation between discharge parameters and film properties

Hexamethyldisiloxane (HMDSO) films have been deposited on bell metal using radiofrequency plasma assisted chemical vapor deposition (RF-PACVD) technique. The protective performances of the HMDSO films and their water repellency have been investigated as a function of DC self-bias voltage on the substrates during deposition. Plasma potential measurements during film deposition process are carried out by self-compensated emissive probe. Optical emission spectroscopy (OES) analyses of the plasma during deposition reveal no significant change in the plasma composition within the DC self-bias voltage range of −40 V to −160 V that is used. Raman and X-ray photoelectron spectroscopy (XPS) studies are carried out for film chemistry analysis and indicate that the impinging ion energy on the substrates influences the physio-chemical properties of the HMDSO films. At critical ion energy of 113 qV (corresponding to DC self-bias voltage of −100 V), the deposited HMDSO film exhibits least defective Si-O-Si chemical structure and highest inorganic character and this contributes to its best corrosion resistance behavior. The hardness and elastic modulus of the films are found to be bias dependent and are 1.27 GPa and 5.36 GPa for films deposited at −100 V. The critical load for delamination is also bias dependent and is 11 mN for this film. The water repellency of the HMDSO films is observed to be dependent on the variation in surface roughness. The results of the investigations suggest that HMDSO films deposited by RF-PACVD can be used as protective coatings on bell metal surfaces.     

Morphological and optical properties of silicon thin films by PLD

Silicon thin films have been prepared on sapphire substrates by pulsed laser deposition (PLD) technique. The films were deposited in vacuum from a silicon target at a base pressure of 10⁻⁶ mbar in the temperature range from 400 to 800 °C. A Q-switched Nd:YAG laser (1064 nm, 5 ns duration, 10 Hz) at a constant energy density of 2 J × cm⁻² has been used. The influence of the substrate temperature on the structural, morphological and optical properties of the Si thin films was investigated.Spectral ellipsometry and atomic force microscopy (AFM) were used to study the thickness and the surface roughness of the deposited films. Surface roughness values measured by AFM and ellipsometry show the same tendency of increasing roughness with increased deposition temperature.     

High-power tunable terahertz generation from a surface-emitted THz-wave parametric oscillator based on two MgO:LiNbO3 crystals

A high-powered tunable terahertz wave (THz-wave) has been parametrically generated via a surface-emitted THz-wave parametric oscillator (TPO) pumped by a multi-longitudinal-mode Q-switched Nd:YAG laser. The effective parametric gain length was enlarged by employing two MgO:LiNbO₃ crystals. The tunable THz-wave radiation from 0.8 to 2.8 THz was realized via varying phase-matching angle between the pump wave and the Stokes wave. The maximum THz-wave radiation was 173.9 nJ/pulse at 1.7 THz as the pump energy was 82 mJ, corresponding to an energy conversion efficiency of about 2.12 × 10⁻⁶ and a photon conversion efficiency of about 0.035%. The first-order, the second-order and the third-order Stokes waves were observed during the experiments.     

Deposition of aluminum nitride thin film on Si(1 1 1) by KrF excimer laser and its characterizations

We present the deposition of aluminum nitride (AlN) thin film by KrF excimer laser sputtering and the study of the effects of substrate temperature and laser fluences. Deposition rate of AlN thin film at 0.3 Å/pulse has been achieved with laser fluence of 1500 mJ/cm² and at substrate temperature of 250 K, and this shows the enhancement of the deposition rate at low substrate temperature. Surface morphology of the deposited films is characterized by atomic force microscopy (AFM). In addition, the electrical performance of the MIS devices with AlN thin films prepared in this experiment has been characterized.     

Effect of deposition time on structural, electrical, and optical properties of SnS thin films deposited by chemical bath deposition

The effect of deposition time on the structural, electrical and optical properties of SnS thin films deposited by chemical bath deposition onto glass substrates with different deposition times (2, 4, 6, 8 and 10 h) at 60 °C were investigated. The obtained films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and optical absorption spectra. All deposited films were polycrystalline and had orthorhombic structure with small crystal grains. Their microstructures had changed with deposition time, and their compositions were nearly stoichiometric. Electrical parameters such as resistivity and type of electrical conduction were determined from the Hall Effect measurements. Hall Effect measurements show that obtained films have p-type conductivity and resistivity values of SnS films have changed with deposition time. For allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, band gap values varied in the range 1.30-1.97 eV, 0.83-1.36 eV, 0.93-1.49 eV and 0.62-1.23 eV, respectively.     

Diamond-like phase formation in an amorphous carbon films prepared by periodic pulsed laser deposition and laser irradiation method

Diamond-like carbon (DLC) films were fabricated by pulsed laser ablation of a liquid target. During deposition process the growing films were exited by a laser beam irradiation. The films were deposited onto the fused silica using 248 nm KrF eximer laser at room temperature and 10⁻³ mbar pressure. Film irradiation was carried out by the same KrF laser operating periodically between the deposition and excitation regimes. Deposited DLC films were characterized by Raman scattering spectroscopy. The results obtained suggested that laser irradiation intensity has noticeable influence on the structure and hybridization of carbon atoms deposited. For materials deposited at moderate irradiation intensities a very high and sharp peak appeared at 1332 cm⁻¹, characteristic of diamond crystals. At higher irradiation intensities the graphitization of the amorphous films was observed. Thus, at optimal energy density the individual sp³-hybridized carbon phase was deposited inside the amorphous carbon structure. Surface morphology for DLC has been analyzed using atomic force microscopy (AFM) indicating that more regular diamond cluster formation at optimal additional laser illumination conditions (∼20 mJ per impulse) is possible.     

Characteristics of wave-breaking-free operation in a passively mode-locked all-fiber ring laser based on AS-Yb-PBGF

A passively mode-locked all-fiber ring laser is constructed based on all-solid Yb-doped photonic bandgap fibers (AS-Yb-PBGF), which provides the laser gain and anomalous dispersion simultaneously. Without any other dispersion compensators, the cavity is simple, compact, and maintenance- and alignment-free. The characteristics of wave-breaking-free operation in the laser are investigated by means of numerical simulations using the split-step Fourier method. The approximately parabolic pulses can be generated and evolved for maintaining the normal group-velocity dispersion intracavity, when the length of the single-mode fiber is between 2 and 4 m. The obtained highest energy of the output pulse is up to 2.27 nJ with the pulse duration of 8.61 ps, corresponding to the time-bandwidth product of 10.05. By dechirping out of cavity, the pulse duration is 0.70 ps.     

Characterization of polymer thin films obtained by pulsed laser deposition

The development of laser techniques for the deposition of polymer and biomaterial thin films on solid surfaces in a controlled manner has attracted great attention during the last few years. Here we report the deposition of thin polymer films, namely Polyepichlorhydrin by pulsed laser deposition. Polyepichlorhydrin polymer was deposited on flat substrate (i.e. silicon) using an NdYAG laser (266 nm, 5 ns pulse duration and 10 Hz repetition rate).The obtained thin films have been characterized by atomic force microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and spectroscopic ellipsometry.It was found that for laser fluences up to 1.5 J/cm² the chemical structure of the deposited polyepichlorhydrin polymer thin layers resembles to the native polymer, whilst by increasing the laser fluence above 1.5 J/cm² the polyepichlorohydrin films present deviations from the bulk polymer.Morphological investigations (atomic force microscopy and scanning electron microscopy) reveal continuous polyepichlorhydrin thin films for a relatively narrow range of fluences (1-1.5 J/cm²).The wavelength dependence of the refractive index and extinction coefficient was determined by ellipsometry studies which lead to new insights about the material.The obtained results indicate that pulsed laser deposition method is potentially useful for the fabrication of polymer thin films to be used in applications including electronics, microsensor or bioengineering industries.     

Ytterbium femtosecond fiber laser without dispersion compensation tunable from 1015 nm to 1050 nm

We report on a widely tunable ytterbium fs-fiber laser without dispersion compensation. The all-normal dispersion laser contains a spectral filter for wavelength tuning and for generating additional amplitude modulation to support the nonlinear polarization evolution as mode-locking mechanism. By tilting the interference filter the center wavelength of the laser can be tuned from 1015 nm to 1050 nm with a pulse energy up to 2.0 nJ. The pulses can be dechirped externally to 108 fs.     

Efficient second-harmonic generation of ultrashort pulses with simultaneous quasi-phase matching and group velocity matching in periodic 90° domain structures of potassium niobate

Triple-line-shaped electrodes were devised to fabricate a periodically poled 90° domain structure of KNbO₃ (PP90KN) of 20.25 μm period with uniform periodicity and reproducibility. Using the fundamental pulse of 141 fs duration at 1619 nm wavelength, an efficient broadband type-I quasi-phase-matched second-harmonic generation was realized in an 8.3 mm long PP90KN by satisfying both phase matching and group-velocity-matching in the telecommunication L-band at room temperature. Second-harmonic pulses showed no significant temporal and spectral distortion and had a pulse duration of about 144 fs, which agreed well with the theoretical value of 142 fs. The conversion efficiency of the second-harmonic generation was 28.7% at the fundamental pulse energy of 0.92 nJ, which is about 60% of the theoretical value.