Picosecond pulse duration laser–material interactions are extremely complex and much less studied than the physics of femtosecond and nanosecond ablation. Additionally, multimode laser beam structure can inhibit robust analysis and comparisons of effects at any pulse duration. To address these gaps, single-pulse laser ablation of Al, Si, Ti, Ge, and InSb in air and Ge in vacuum was studied using low-transverse order Gaussian beams at a 1064 nm wavelength and 28 ps pulse duration. Crater depths of 0.4 to 6.3 μm and volumes of up to 4000 μm3 were measured using a laser confocal microscope. Crater depths plateau with increasing fluence and are slightly higher for Ge in vacuum than in air. Crater volume increases linearly with fluence for all materials in air. In vacuum, the volume of material above the surface was less than in air and increased at a lower rate with increasing fluence. The ratio of volume above the surface (due to melt flow and redeposition) to volume below the surface plateaus for all materials to ∼0.7 in air and 0.4 for Ge in vacuum. The ablation efficiency, defined as atoms removed per incident photon, was higher at low fluences and decreased to ∼0.004 for all materials at higher fluences. Simulations using the directed energy illumination visualization tool showed that bulk melt flow out of the crater caused by the evaporation recoil pressure dominated at higher fluences. Plateauing of crater depth with fluence was caused by melt reflow into the crater, which effects smaller crater widths more than larger ones, as evidenced by comparing multimode results with TEM00 simulations. Recondensation of evaporated material was identified as the main difference between craters formed in air versus vacuum, and the Knudsen layer jump conditions in DEIVI were modified to account for an estimated ≈20 % recondensation rate. The simulations showed a resulting reduction in evaporation, which created less recoil pressure, driving less melt out of the crater. The results of this study help elucidate mass removal mechanisms in the picosecond pulse duration regime and their dependence on laser and environmental characteristics.
Much progress has been made in power scaling mid-infrared lasers based on transition metal ions such as Cr2+, Co2+, and Fe2+ in zinc and cadmium chalcogenides. Still, the exploration of the physics of the devices is incomplete. In this work, we analyze absorption spectra from Fe2+ in several binary and ternary hosts at low temperatures. We examine the effect of host ion size and mass on the zero-phonon energy of these spectra and further develop our previous model for the upper state lifetime of Fe2+ in these materials. The effect of the relative disorder in the crystalline environment on the lasing characteristics of Cr:ZnS, Cr:ZnSe, Fe:ZnSe, and Fe:CdMnTe laser devices is explored. We show that increasing disorder of the crystal host is easily observed in broadening of the absorption spectra and the spectral linewidth of the laser output, and the reduction in the portion of the emission spectrum accessible for mode locking. Practical design guidelines for laser devices are developed.
Laser ablation of aluminum, silicon, titanium, germanium, and indium antimonide at 1064 nm in ambient laboratory air with pulse durations ranging from 100 ps to 100 μs has been characterized with optical microscopy. Highly focused spots of 10 μm yields fluences of 0.004 to 25 kJ / cm2 and irradiances spanning 4 × 106-1014 W / cm2. Single pulse hole depths range from 84 nm to 147 μm. A one-dimensional thermal model establishes a set of nondimensional variables for hole depth, fluence, and pulse duration. For pulse durations shorter than the radial diffusion time, the hole depth exceeds the thermal diffusion length by a factor of 1 to 30 for more than 90% of the data. For pulses longer than this critical time, transverse heat conduction losses dominate and holes as small as 10 − 3 times the thermal diffusion depth are produced. For all cases, the ablation efficiency, defined as atoms removed per incident photon, is 10 − 2 or less, and is inversely proportional to volume removed for pulse durations less than 100 ns. At high fluences, more than 10 to 100 times ablation threshold, explosive boiling is identified as the likely mass removal mechanism, and hole depth scales approximately as fluence to 0.3 to 0.4 power. The power-law exponent is inversely proportional to the shielding of the laser pulse by ejected material, and shielding is maximum at the 1-ns pulse duration and minimum near the 1-μs pulse duration for each material. Using the thermal scaling variables, the high-fluence behavior for each material becomes strikingly similar.
ZnSe doped with Cr2+ was analyzed by EDS, XPS and Micro-Raman spectroscopy techniques. EDS and XPS
analysis revealed that chromium concentration is more than 2% and there are additional impurities, Ga, Ti, and Ta.
EDS measurements did not reveal any variation in chromium concentration when a line scan was performed over a
200 μm distance. XPS analysis indicated that the sample surface is inhomogeneous. Photoluminescence was
acquired by exciting the sample with 325 nm laser beam. Photoluminescence revealed charge transfer bands.
Micro-Raman study revealed the LO, TO and 2TA modes at 252, 205 and 140 cm-1. Under 488 or 514.5 nm
excitation background luminescence was predominant due to excitation of Cr2+ electrons into the conduction band.
However, 632.8 nm laser excitation revealed, strong Raman signals. Raman data were acquired by exciting the
sample on the grain boundary and inside the domain. The ratio of LO and TO peak intensities changed randomly
when data were acquired from different points on the grain boundary indicating the presence of random strain in the
material. When Raman data were acquired from different points on the sample surface for comparison, it revealed
that the LO mode was distorted as well as broadened whereas the TO mode intensity increased. This was due to the
presence of local modes induced by the sample inhomogeneity and the interaction of the holes with the LO mode.
We report the first demonstration of a gain-switched chromium-doped zinc selenide channel waveguide laser. The guided-wave structure was produced by ultrafast laser inscription and exhibited output pulse energies up to 12 μJ . The laser exhibited narrow spectral output with a linewidth less than 1 nm. The beam quality was measured to be M2 ≤ 7 with a highly multimode output profile. The laser had a maximum slope efficiency of 9.8% and no deleterious thermal effects were observed up to an average pump power of 3.3 W .
Fe:ZnSe lasers have been pumped by several types of diode-pumped solid state laser, including Cr:Er:YSGG (2800 nm),1 Cr:CdSe (2970 nm),2 and Er:YAG (2698 nm,3 2936 nm4). None of these sources has exceeded 1.5 W of true continuous-wave (CW) output power. In this work, we report demonstration of a CW Fe:ZnSe laser pumped by a 10 W Er:Y2O3 laser emitting at 2740 nm,5 which had not been previously attempted. The Er:Y2O3 pump laser was characterized with respect to propagation losses, beam quality, mode size, and pointing stability. It was determined that the limit of output power from the Fe:ZnSe laser was limited by the output stability of the pump laser. The Fe:ZnSe laser operated with < 22% slope efficiency and 800 mW output power was achieved at approximately 4050 nm.
In this paper, we report on building and testing a Cr:ZnSe gain-switched amplifier pumped by a Q-switched Ho:YAG laser and seeded by a continuous wave (CW) tunable Cr:ZnSe laser. A 0.5%-doped, Brewster-cut Ho:YAG rod in an actively Q-switched, folded cavity produced 250 μJ pump pulses at 2.09 μm with pulse widths on the order of 400 ns. The seeded single-pass Cr:ZnSe amplifier exhibited output pulse energy as high as 3.8 μJ at 2.45 μm while pumped at a 10 kHz repetition rate. The gain-switched process showed a peak gain of 380 and an extraction efficiency of 1.5%. The system was tunable from 2160 nm to 2560 nm and had gain of 200 over a 400 nm range.
We have achieved ≥ 840 mW continuous-wave (CW) output power from iron-doped zinc selenide (Fe:ZnSe).1 The beam quality was measured to be M2 ≤ 1.2. The laser exhibited a slope efficiency of 47% with no thermal roll-off at maximum output power. Various dichroic mirrors and other spectral filters were inserted into the cavity to discretely tune the output of the laser from 3843 nm to 4337 nm. Demonstration of arbitrary discrete tuning shows that, in principle, Fe:ZnSe is capable of efficient continuously-tunable CW lasing over nearly 500 nm of bandwidth.
In this paper, we report record nanosecond output energies of gain-switched Cr:ZnSe lasers pumped by Q-switched
Cr:Tm:Ho:YAG (100 ns @ 2.096 μm) and Raman shifted Nd:YAG lasers (7 ns @ 1.906 μm). In these experiments we
used Brewster cut Cr:ZnSe gain elements with a chromium concentration of 8x1018 cm-3. Under Cr:Tm:Ho:YAG
pumping, the first Cr:ZnSe laser demonstrated 3.1 mJ of output energy, 52% slope efficiency and 110 nm linewidth
centered at a wavelength of 2.47 μm. Maximum output energy of the second Cr:ZnSe laser reached 10.1 mJ under H2 Raman shifted Nd:YAG laser pumping. The slope efficiency estimated from the input-output data was 47%.
We demonstrate a high-power (7.5 W) polycrystalline Cr2+:ZnSe CW laser system utilizing an astigmaticallycompensated
Kogelnik-configuration master oscillator and a normal-incidence slab power amplifier demonstrating over
2X gain. Experimental results are compared with an improved theoretical model of amplification in this type of system.
We used finite element software to model the time dependence of thermal lensing and temperature rise in a Cr2+-doped zinc selenide thin disk for pulsed pumping. Two cases, chopped cw and Q-switched pumping, were considered. The model agrees well with experimental results for the chopped pumping case but does not directly agree with Q-switched pumping because the time delay between absorption and heat transfer to the host material is not accounted for in our model.