V. I. Konov

Mode-locked 1.93 μm thulium fiber laser with a carbon nanotube absorber

We report a ring-cavity thulium fiber laser mode locked with a single-wall carbon nanotube absorber used in transmission. A carboxymethyl cellulose polymer film with incorporated carbon nanotubes synthesized by the arc discharge method has an absorption coinciding with in the amplification bandwidth of a Tm-doped fiber. This laser is pumped by an erbium fiber laser at 1.57 μm wavelength and produces a 37 MHz train of mode-locked 1.32 ps pulses at 1.93 μm wavelength with an average output power of 3.4 mW.

D.c. arc plasma deposition of smooth nanocrystalline diamond films

We report on chemical vapour deposition growth of nanocrystalline (grain size 30–50 nm) diamond films of 100 nm to 2 μm thickness in methane-rich mixtures. A d.c. arc discharge in CH4H2Ar gas mixtures with a methane percentage CH4(CH4 + H2) varied from 10% to 100% was used for diamond deposition on Si substrates seeded with ultrafine (5 nm) diamond particles. The films obtained were characterized by scanning electron microscopy, X-ray diffraction, Raman spectroscopy and X-ray excited Auger electron spectroscopy. Remarkably well-crystallized diamond films were produced even in hydrogen-free gas mixtures. Raman spectra confirmed the nanocrystalline structure of all the films examined. The film hardness measured with a nanoindenter was in the range 70–85 GPa typical for diamond, the highest values corresponding to 100% methane content. The films were very smooth with surface roughness Ra < 20 nm. The essential improvement in surface smoothness was obtained by means of laser-induced disintegration of coalesced seeding particles.

Bone-ablation mechanism using CO2 lasers of different pulse duration and wavelength

Bone ablation using different pulse parameters and four emission lines of 9.3, 9.6, 10.3, and 10.6 μm of the CO2 laser exhibits effects which are caused by the thermal properties and the absorption spectrum of bone material. The ablation mechanism was investigated with light- and electron-microscopy at short laser-pulse durations of 0.9 and 1.8 μs and a long pulse of 250 μs. It is shown that different processes are responsible for the ablation mechanism either using the short or the long pulse durations. In the case of short pulse durations it is shown that, although the mineral components are the main absorber for CO2 radiation, water is the driving force for the ablation process. The destruction of material is based on explosive evaporation of water with an ablation energy of 1.3 kJ/cm3. Histological examination revealed a minimal zone of 10–15 μm of thermally altered material at the bottom of the laser drilled hole. Within the investigated spectral range we found that the ablation threshold at 9.3 and 9.6 μm is lower than at 10.3 and 10.6 μm. In comparison the ablation with a long pulse duration is determined by two processes. On the one side, the heat lost by heat conduction leads to carbonization of a surface layer, and the absorption of the CO2 radiation in this carbonized layer is the driving force of the ablation process. On the other side, it is shown that up to 60% of the pulse energy is absorbed in the ablation plume. Therefore, a long pulse duration results in an eight-times higher specific ablation energy of 10 kJ/cm3.

177fs erbium-doped fiber laser mode locked with a cellulose polymer film containing single-wall carbon nanotubes

A mode-locked soliton erbium-doped fiber laser generating 177fs pulses is demonstrated. The laser pumped by a 85mW, 980nm laser diode emits 7mW at 1.56μm at a pulse repetition rate of 50MHz. Passive mode locking is achieved with a saturable absorber made of a high-optical quality film based on cellulose derivative with dispersed carbon single-wall nanotubes. The film is prepared with the original technique by using carbon nanotubes synthesized by the arc-discharge method.

Raman and photoluminescence investigations of nanograined diamond films

Abstract Size-induced transformations in the Raman scattering and photoluminescence spectra of two nanocrystalline diamond materials (an ultrafine explosive powder and the continuous nanograined films) were registered. A narrow peak dominating in the spectrum of 5 nm-size diamond powder appeared to be low-frequency shifted (−11 cm −1 ) and broadened with respect to a single crystal. The peculiarity of the Raman spectra of the nanograined diamond films was a presence of two wide bands at 500 cm −1 and 1140 cm −1 , corresponding to TA- and TO-like phonons in the spectrum of an amorphous diamond. The photoluminescence spectra of both materials demonstrated a strong temperature independent band with a maximum near 2.2 eV. Increasing the crystallite size led to the weakening of this band simultaneously with the appearance of a well-crystallized diamond peak at 1332 cm −1 in the Raman spectrum. Measurements were performed at room and liquid nitrogen temperatures.

Ablation of CVD diamond with nanosecond laser pulses of UV-IR range

Etch rates of CVD diamond upon irradiation by nanosecond (5‐9 ns) pulses at three diVerent wavelengths 1078, 539 and 270 nm at laser fluences in the range 1‐1000 J/cm2 were measured. A Nd:YAP laser system operated at first, second and fourth harmonics was used in the ablation experiments. Both shallow (<15 microns) and through holes were etched in a 95-mm thick free-standing diamond film grown by microwave plasma CVD. The ablation rate was found to be wavelength-independent, this result being ascribed to surface blackening caused by amorphization/graphitization as confirmed by Raman analysis. The maximum etch rate approached 600 nm/pulse. The etch rate depended on the crater depth, which was ascribed to the eVect of laser‐plasma interaction inside the deep channel. The possibility of cutting trenches of high aspect ratio has been demonstrated. In a separate experiment, a batch of thin diamond films diVering in thermal conductivity (k=2‐5 W/cmK ) was ablated with a KrF excimer laser (l=248 nm). No dependence of ablation rate on film quality was observed, which could be explained assuming grain boundaries to be the main source of thermal resistance.

Smoothening of diamond films with an ArF laser

Abstract Polycrystalline diamond films of various microstructures were modified by large-area ArF laser irradiation. The modified film surfaces were characterized employing electron microscopy, mechanical profilometry and optical reflectometry. Laser irradiation was found to smooth the surface finish over a wide range of spatial frequencies covering microroughness imposed by the individual crystallites as well as waviness with periods exceeding the crystal sizes. No smoothening was achieved, however, for chemically non-uniform diamond films. The results can be rationalized in terms of laser-induced surface evaporation. Resistivity measurements revealed the simultaneous formation of a highly stable conductive surface layer upon laser irradiation.

UV laser polishing of thick diamond films for IR windows

Abstract Diamond has very suitable properties for infrared (IR) window applications. The rugged surface of a free-standing 320 μm thick diamond film grown by chemical vapour deposition (CVD) is polished with the UV light of an ArF excimer laser (λ=193 nm). The angle of incidence is 85° and irradiation is subsequently performed at three planar directions around the perpendicular axis. Scanning electron microscopy (SEM) is used to visualise the surface before and after the polishing procedure. With a profilometer (tip radius 5 μm) the improvement of the average roughness of the surface is measured. Polishing is analysed in dependence on the size of surface irregularities by means of fast Fourier transformation (FFT) of the signal of the profilometer. Surface roughness measurements are also performed on the polished surface with an atomic force microscope (AFM). With IR transmittance measurements the optical quality is analysed over a wavelength range of 2.5 to 25 μm before and after polishing. The oxidative removal of the laser induced graphitic layer is studied with optical spectroscopy techniques.

Starting mechanisms and dynamics of bubble formation induced by a Ho:Yttrium aluminum garnet laser in water

The starting mechanisms and dynamics of laser-induced bubble formation at a submerged fiber tip in distilled water were experimentally investigated using pressure measurements and fast flash videography. A fiber guided Ho:YAG laser operating in the free running (τ=200 μs) and Q-switched (τ=45 ns) mode at a wavelength of λ=2.12 μm was used as a light source. It is shown that the beam profile at the distal fiber tip (multimode fiber d=300 μm) exhibits hot spots that result in an inhomogeneous temperature distribution in the heated water volume. Depending on the laser irradiance, three different bubble formation processes are distinguished: bubble formation by heating, by rarefraction (cavitation), and by a combination of these two processes. For laser irradiances of less than 0.5 MW/ cm2 bubble formation takes place at temperatures near the critical point of water (T=280 °C). A rapid decrease in the threshold temperature for bubble formation was found for laser irradiances between 0.5 and 1 MW/cm 2. At lase...

Laser ablation of metals and ceramics in picosecond–nanosecond pulsewidth in the presence of different ambient atmospheres

Abstract Ablation tests of AlN, Si3N4, SiC, Al2O3 ceramics, steel and aluminum have been carried out in vacuum, air and argon atmospheres using UV (270 nm), visible (539 nm) and IR (1078 nm) picosecond (100÷150 ps) and nanosecond (6÷9 ns) laser pulses. Ablation rate dependencies have been measured in the range of laser energy densities varied from (2÷5)×101 J/cm2 to (5÷10)×103 J/cm2. Peculiarities of laser ablation processes at different wavelengths, pulsewidths and ambient gases are discussed. In particular, the efficiencies of laser ablation in picosecond and nanosecond regions are compared. The scanning electron microscope (SEM) pictures of high quality microstructures, deep and narrow cuts and holes produced in ceramics with typical size of tens microns and aspect ratio as high as 20, are demonstrated.