We report 3.8-5.0 μm tunable three-stage double-pass Fe:ZnSe solid-state MOPA with approximately 30-fold overall amplification coefficient operating at RT and pumped by EO Q-switched Cr:Er:YSGG laser system radiation. EO Q-switched Cr:Er:YSGG MOPA with the Q-switch based on a La3Ga5SiO14 crystal providing 90 ns pulses with energy up to 350 mJ at a 3 Hz repetition rate was used as a pump source. The maximum output energies of Fe:ZnSe system in 90 ns pulses exceeded 55 mJ at 4.4 μm under 220 mJ of pumping. The pulse jitter was measured to be ⪅8 ns and was limited by the Er:Cr:YSGG laser jitter.
We report on developing two flashlamp-pumped electro-optically Q-switched Cr:Er:YSGG lasers with the Q-switch based on La3Ga5SiO14 crystal. The “short” laser cavity was optimized for applications requiring high peak power. In this cavity, the 300 mJ output energy in 15 ns pulses at 3Hz repetition rate was demonstrated under pump energy below 52 J. However, several applications, such as Fe:ZnSe pumping in a gain-switched regime, requiring longer (~ 100 ns ) pump pulse duration. We developed a 2.9 m long laser cavity capable of delivering 190 mJ of output energy in 85 ns pulses for these applications.
Current work reports comparative laser characterization of Cr:ZnS/Se polycrystalline gain media in non-selective Fabry-Perot and twisted mode cavities. It was demonstrated that the spectral output of lasers based on polycrystalline Cr:ZnS/Se in non-selective Fabry-Perot cavities is broadened to ~20-50 nm due to spatial hole burning in the gain elements. Mitigating the spatial hole burning in the same cavities via “mode twisting” results in narrow-line oscillation with linewidth narrowing to ~80-90 pm. Switching between broadened and narrow line oscillations was demonstrated by adjusting the orientation of intracavity waveplates with respect to the Brewster facilitated polarization.
We report the laser operation of spinning mirror-based mechanically Q-switched (MQS) 2940 nm Er:YAG with record 805 mJ output energy in a single 61 ns pulse and ~10 ns pulse jitter. The laser was operated at 1Hz repetition rate and 670 Hz rotational rate of the spinning mirror. The highest output energy was achieved with the use of a 300 mm long MQS Er:YAG laser cavity consisting of 70% output coupler, 7x120 mm AR coated Er(50%):YAG crystal, and spinning HR mirror. The maximum output energy was limited by the optical damage of the Er:YAG AR coatings.
We present recent results on high energy, high repetition rate 2090 nm Ho:YAG laser system resonantly pumped by the radiation of efficient Tm-doped CW fiber lasers. The laser consists of a master oscillator power amplifier (MOPA) with two additional amplification stages. We demonstrate maximum energy of 120 mJ per pulse with a 15 ns pulse width at 500 Hz, corresponding to a peak power of 8 MW. The results presented in a wide 100-1000 Hz range of repetition rates.
We report on mid-IR Fe:ZnSe master oscillator power amplifier (MOPA) laser system operating at room temperature (RT) pumped by a radiation of mechanically Q-switched Er:YAG laser operating at 2.94 m, at 3Hz repetition rate. The maximum output energy was as high as 250mJ in ~250 ns pulses. The RT gain-switched Fe:ZnSe master oscillator demonstrated tunability over 3.60-5.15 µm spectral range with a maximum output energy of ~3 mJ. The output energies of 12 (8), 34 (25) and 60 (48) mJ were demonstrated at 4.4 (4.1) µm in the 1st, 2nd and 3rd stages of amplifier, respectively, with a total pump energy of 200 mJ.
We report mechanically Q-switched 2.94 µm Er:YAG laser based on spinning mirror with 805 mJ output energy in a single 61 ns pulse at 1 Hz repetition rate and 670 Hz rotational rate of the spinning mirror. This record output energy was achieved with the use of 300 mm long MQS Er:YAG laser cavity consisted of 70% output coupler, 7x120 mm AR coated Er(50%):YAG crystal and spinning HR mirror. The maximum output energy was limited by the optical damage of the Er:YAG AR coatings.
We report a technique for generation of ultra-broadband coherent femtosecond continua in the infrared. The laser architecture is based on the Cr:ZnS–GaSe and Cr:ZnS–ZGP tandem arrangements that enable simultaneous amplification of ultrashort middle IR pulses and augmentation of pulses’ spectrum via a chain of intrapulse three-wave mixings. The first part of the tandems is based on a single-pass polycrystalline Cr:ZnS amplifier, which is optically pumped by off-the-shelf continuous wave Er-doped fiber laser and outputs 2-cycle pulses with multi-Watt average power at 80 MHz repetition rate, at the central wavelength 2.5 μm. The second stage of the tandems comprises a GaSe or ZGP crystals configured for intrapulse difference frequency generation. The Cr:ZnS–GaSe tandem has allowed us to achieve multi-octave 2–20 μm continuum with 2 W power in the range 2–3 μm and power in excess of 20 mW in the important range 3–20 μm. On the other hand, Cr:ZnS–ZGP tandem features long-wave infrared (6–12 μm) output pulses with record braking sub-Watt power level. Last but not least, Cr:ZnS–GaSe and Cr:ZnS–ZGP IR sources have small footprints and are easily convertible to the optical frequency combs with low carrier-to-envelope timing jitter.
We report laser operation of spinning mirror mechanically Q-switched (MQS), flashlamp pumped 2.94 μm Er:YAG laser depending on the angular speed of the mirror, repetition rate, size and temperature of the gain element, pulse duration and jitter of Q-switch pulses, as well as pump pulse energy. The highest output energy of 260 mJ with a pulse duration of 150 ns was realized with the use of 7×120 mm Er(50%):YAG at 5 Hz repetition rate and 4200 rad/s angular speed of the spinning mirror. The efficiency of Q-switched operation was ~50% with respect to free-running regime. Using optical triggering, the pulse jitter was measured to be smaller than 10 ns for 160 ns Q-switched pulses. Optical triggering could be used for synchronization with mode-locked laser in chirp-pulse and regenerative amplifiers. We also report on development of room temperature gain-switched Fe:ZnSe laser pumped by a radiation of MQS Er:YAG laser. The maximum output energy of 9 mJ from Fe:ZnSe laser was demonstrated using MQS Er:YAG laser as pump source.
Iron-doped binary and ternary chalcogenide crystals are very promising for tunable solid-state lasers operating over the 3-6 μm spectral range. The most significant results have been reported for iron doped ZnSe crystals with 9.6 W output power in CW at 77K when pumped by radiation of Cr:ZnSe laser, and 1.4 J at ~150 ns pulse duration at room temperature (RT) when pumped by the radiation of HF laser. The lifetime of the upper laser level 5T2 of the Fe2+ ion in a ZnSe matrix falls with temperature from 52µs at 77 K to 370 ns at RT due to the increase of nonradiative relaxation. It allows effective laser oscillation in the gain-switched regime at RT and operation in Q-switched regime at low temperature. We report on RT gain-switched Fe:ZnSe lasers tunable over 3.60-5.15 µm pumped by radiation of mechanically Q-switched Er:YAG laser operating at 2.94 µm. The maximum output energy was measured to be 5 mJ under 15mJ of pump energy. The long upper level lifetime of Fe:ZnSe gain medium is sufficient for energy storage with pumping by radiation of free running Er:YAG lasers. We demonstrated that Q-switched regime of oscillation could be effectively utilized for Fe:ZnSe lasers. The rotating back mirror was used as a mechanical Q-switcher of a Fe:ZnSe laser. The maximum output energy in single 150 ns pulse was measured to be 3mJ which is ~25% from the theoretical limit. This approach could be attractive for development of high-energy short-pulse solid-state mid-IR systems operating over 3.6-5.0 µm spectral range
Kerr-lens mode locked lasers based on polycrystalline Cr:ZnS and Cr:ZnSe have come of age and, arguably, represent the most viable route for generation of ultra-short pulses in the range 2–3 μm. Developed designs of Kerr-lens mode locked oscillators feature high efficiency and provide access to few-cycle MIR pulses with Watt-level power in a very broad range of pulse repetition rates. However, currently available dispersive mirror coatings limit spectral coverage of these oscillators to below one octave hampering their conversion to frequency combs via frequency envelop offset frequency (fceo) control and stabilization. Supercontinuum (SC) generation using photonic waveguides is a promising approach for spectral broadening of pulsed coherent sources at low pulse energies and small footprint. Among many materials promising for this application stoichiomentric Si3N4 (SiN) holds a unique place due to its high nonlinearity, CMOS compatible fabrication process, and spectral coverage over visible-middle-infrared (MIR) range. In the current paper we experimentally demonstrate the generation of a supercontinuum spanning more than 1.5 octaves over 1.2-3.7 um range in a silicon nitride waveguide using sub-40-fs pulses at 2.35 um generated by 75 MHz Cr:ZnS laser. The coupling efficiency was about 16%, which corresponds to 0.56nJ pulse energy and 12.4 kW peak power. We also have observed that threshold for SC generation was about 50 mW of incident power that corresponds to 2.4KW peak power. The demonstrated coherent 1.5 octaves spanning bandwidth is ideal for self-referenced f-2f detection of the fceo. In addition, this represents a promising broadband coherent source for dual comb spectroscopy.
II-VI binary and ternary chalcogenides (e.g. ZnS, ZnSe; CdZnTe, ) doped with transition metal (TM) ions such as Cr, and Fe are arguably the materials of choice for effective mid-IR lasers potentially covering 1.8-9 µm spectral range. This talk summarizes progress in Cr:ZnS/Se and Fe:ZnSe laser systems, enabling a wide range of tunability (1.8-5.0µm) with output power levels of up to 140 W, as well as Fe doped ternary chalcogenides with tunability potentially extended up to 9 um.
TM:II-VI media feature a unique combination of superb ultra-fast laser capabilities with high nonlinearity enabling exceptional output characteristics of polycrystalline Cr:ZnS/Se oscillators in Kerr-Lens-Mode-Locked (KLM) regime over 2-2.6 um and effective up and down conversion of fs pulses via random phase matching (RFM). Extension of mid-IR spectral coverage to 3-8 um is demonstrated by Cr:ZnS KLM laser pumped subharmonic parametric oscillators (OPOs) based on quasi-phase matching in OP-GaAs, and RFM in polycrystalline ZnSe.
Fe:II-VI semiconductors are complimentary to Cr doped compounds and 3-8 um KLM ultrafast oscillators based on Fe doped chalcogenides are feasible. Another important feature of Fe:II-VI media is excellent energy storage capability at 77-200K (~60 µs luminescence life time) enabling efficient Q-switched regime and high energy amplification of ns and ultrafast pulses.
One of the major problems in the development of CW, gain switched, Q-switched and KLM ultrafast Fe:II-VI lasers was the absence of convenient pump sources overlapping with absorption band (2.7-4.5 um) of Fe: gain media. Potential utilization of Quantum Cascade Lasers (QCL) as pump sources of Fe:II-VI lasers will be discussed in the form QCL-solid state laser hybrid platforms as well as Fe doped active layers integrated in QCL structures.
II-VI chalcogenides (e.g. ZnSe/S) doped with transition metal (TM) ions such as Cr, and Fe are arguably the materials of choice for fabrication of effective mid-IR gain media. TM:II-VI materials feature a favorable blend of laser spectroscopic parameters: a four-level energy structure, absence of excited state absorption, close to 100% quantum efficiency of fluorescence (for Cr doped II-VI media), broad mid-IR vibronic absorption and emission bands. This talk summarizes progress in fabrication of high quality Cr:ZnS/Se and Fe:ZnS/Se by cation vacancy and cation interstitial enhanced post growth thermal diffusion. We also describe recent breakthrough on recrystallization and effective doping of ZnS ceramics under hot isostatic pressing resulting in a large cm-scale monocrystalline domains formation and an increase of the Fe diffusion coefficient by three orders of magnitude.
We report recent advances in high-power Cr:ZnS/Se and Fe:ZnSe laser systems, enabling a wide range of tunability (1.8-5.0µm) with output power levels of up to 140 W near 2500 nm, 32 W at 2940 nm, and 35 W at 4300 nm with corresponding optical efficiencies of 62%, 29%, and 35%.
Current improvements of output characteristics of polycrystalline Cr:ZnS/Se oscillators in Kerr-Lens-Mode-Locked (KLM) regime are reported: up to 2 W output power at 75-1200 MHz repetition rate, up to 2 cycle pulse duration (16 fs) with efficiency of 20-25% with regards to Er-fiber laser pump power. The effects of efficient up-conversion of mid-IR fs pulses in the laser medium as well as supercontinuum generation are demonstrated.
Further extension of mid-IR spectral coverage to 3-8 m is demonstrated by Cr:ZnS KLM laser pumped degenerate (subharmonic) parametric oscillators (OPOs) based on based on quasi-phase matching in orientation-patterned gallium arsenide, and random phase matching in polycrystalline ZnSe.
In this paper, prospects of using diamond with NV− centers as a gain medium have been studied. Spectroscopic characterization of NV− centers in diamond as well as absorption saturation and pump-probe experiments have been carried out. Absorption and emission cross-sections were estimated to be 2.8 × 10-17 cm2 and 4.3 × 10-17 cm2 at the maximum of absorption and emission bands, respectively. It was observed from emission spectra under pulse excitation that some NV− are photoionized to NV0 centers with ZPL at 575 nm. Room temperature luminescence lifetime of NV− centers was measured to be 12ns, which is close to the previously reported lifetime in bulk diamond (~13ns). Saturated transmission was only about 11% of calculated values even at energy fluence much higher than the saturation flux. Two excited state absorptions (ESAs) with different relaxation times (“fast-decay” and “slow-decay with relaxation times of ~500 ns and several tens of microseconds, respectively) were revealed in transmission decay kinetics at 632 nm. Kinetics of transmission at 670 nm was dominated by “slow-decay” ESA process. Kinetics of dk/k0in shorter wavelength were strongly dominated by “fast-decay” ESA process. These results definitively indicate that stimulated emission of NV− centers is suppressed by photoionization and ESAs and the possibility of diamond lasers based on NV− centers is low.
Middle-infrared (mid-IR) lasers enabling a wide range of scientific, medical, technological, and defense related applications continue to enjoy a strong demand. Transition metal (TM) doped II-VI chalcogenides are appealing mid-IR gain medial providing direct access to 1.8-6 μm spectral range. . II-VI chalcogenides are available in single crystal and in polycrystalline forms. With respect to single crystals, polycrystalline gain elements fabricated by postgrowth thermal diffusion of TM impurities in II-VI hosts feature better optical quality and enable superior laser characteristics. Despite significant progress in post-growth thermal diffusion technology, there are still some difficulties associated with the diffusion of certain TM ions in certain II-VI hosts. Specifically, the diffusion length Fe in ZnS during ~1 month annealing at 950°C is of the order of 0.1 mm. In this work, enhancement of diffusion coefficient under Hot Isostatic Pressing, at temperature and pressure of 1350°C and 2000 atm, and effect of these extreme conditions on the overall optical quality of the crystal were studied. The high temperature was applied to increase the diffusion rate, and the high pressure was needed to suppress strong sublimation and sphalerite - wurtzite phase transition at elevated temperatures. Under these conditions, the diffusion coefficient Fe in ZnS was enhanced by 5500 times as compared to standard diffusion processes carried out at 950°C. It was also demonstrated that the grain size had grown from ~30μm to ~5.5mm, which is believed to be another reason for efficient diffusion besides the elevation of temperature. The XRD patterns were measured such that the X-ray beam falls on a single grain. The XRD patterns showed only peaks characteristic to single crystals with zinc blende structure. Lasing characterization was performed to investigate the optical quality of the crystal. Slope efficiencies of 23.2% and 15.4% were obtained for TM11 and TM00 modes of operation, respectively. The emission of the laser was demonstrated to be in the 3840-3920 nm.
This paper summarizes recent improvements of output characteristics of polycrystalline Cr:ZnS/Se master oscillators in Kerr-Lens-Mode-Locked regime. We developed a flexible design of femtosecond polycrystalline Cr:ZnS and Cr:ZnSe lasers and amplifiers in the spectral range 2–3 μm. We obtained few-optical-cycle pulses with multi-Watt average power in very broad range of repetition rates 0.08–1.2 GHz. We also report on efficient nonlinear frequency conversion directly in the polycrystalline gain elements of ultra-fast lasers and amplifiers. In this work we also report on recent progress in spinning ring gain element technology and report to the best of our knowledge the highest output power of 9.2 W Fe:ZnSe laser operating in CW regime at 4150nm.
We report on CW, acousto-optically tuned Cr:ZnS mid-IR laser operating over 1890 - 2785 nm spectral range with 40% slope efficiency with respect to the pump power in the maximum of tuning curve. The tuning range over 1890 - 2785 nm was demonstrated by tuning the radio frequency (RF) applied to a TeO2 acousto-optical filter between 30.3 MHz and 20.12 MHz. A maximum output power of 2.8 W was achieved at 2.23 μm under 10.7 W of pump power. The linewidth of the Cr:ZnS laser measured at central wavelength was < 0.8 nm.
Progress in fabrication and mid-IR lasing of Cr and Fe thermal-diffusion and radiation enhanced thermal diffusion doped II-VI binary and ternary polycrystals is reported. We demonstrate novel design of mid-IR Fe:ZnSe and Cr:ZnSe/S solid state lasers with significant improvement of output average power up to 35W@4.1 μm and 57W@2.5 μm and 20W@2.94 μm. We report significantly improved output characteristics of polycrystalline Cr:ZnS/Se lasers in gain-switched regime: 16 mJ at 200 Hz, pulse duration 5 ns with tunability over 2400-3000 nm as well as Kerr-Lens-Mode-Locked regime in terms of average power (up to 2 W), peak power and pulse energy (0.5 MW and 24 nJ, respectively), and pulse duration (less than 29 fs).
We report a novel design of CW Cr2+:ZnS/ZnSe laser systems and demonstrate record output powers of 27.5 W at 2.45 μm and 13.9 W at 2.94 μm with slope efficiencies of 63.7% and 37.4%, respectively. Power scaling of ultra-fast Cr2+:ZnS/ZnSe Kerr mode-locked lasers beyond 2 W level, as well as the shortest pulse duration of 29 fs, are also reported. New development of Fe:ZnSe laser with average output power > 35 W at 4.1 μm output wavelength and 100 Hz pulse repetition rate (PRR) was achieved in a nonselective cavity. With intracavity prim selector, wavelength tunability of 3.88-4.17 μm was obtained with maximum average output power of 23 W. We also report new results on Tm-fiber pumped passively and actively Q-switched Ho:YAG laser systems. High peak power actively Q-switched Ho:YAG laser demonstrates stable operation with pulse energy > 50 mJ, 12 ns pulse duration, and 100-1000 Hz PRR which correspondents to more than 4 MW peak power. The actively Q-switched Ho:YAG laser system optimized for high repetition rate delivers 40 W average output power at 10-100 kHz PRR. The Ho:YAG laser with passive Q-switcher demonstrates constant 5 mJ output energy from 200 Hz to 2.23 kHz PRR with optical slope efficiency with respect to Tm-fiber laser of ~43%.
We report on study of gamma radiation-enhanced thermal diffusion of Transition Metal and Rare Earth ions into IIVI semiconductor crystals. ZnSe and ZnS samples with of iron thin film deposited on one facet were sealed in evacuated quartz ampoules at 10-3 Torr. The crystals were annealed for 14 days at 950°C under γ-irradiation from 60Co source. The irradiation dose rates of 43.99 R/s, 1.81 R/s were varied by distance between 60Co source and furnaces. For comparison, the samples were also annealed without irradiation at the same temperature. The spatial distributions of transition metal were measured by absorption of focused laser radiation at 5T2-5E mid-IR transitions of iron ions. In addition, samples of ZnSe were similarly sealed in evacuated quartz ampoules in the presence of Praseodymium metal and annealed at 950°C under 43.99 R/s and 0 R/s and the diffusion lengths and Pr concentrations were compared. The γ-irradiation results in better intrusion of the iron ions from the metal film and increase of the diffusion length at ~25%, while Praseodymium diffusion is dramatically enhanced by γ-irradiation during the annealing process.
Recent efforts have demonstrated efficient Cr2+ :II-VI chalcogenide (e.g. ZnSe, ZnS) broadly tunable (1.9-3.3μm) lasers under direct intra-shell Cr2+ optical excitation. We report on the spectroscopic study of Cr2++:ZnSe/ZnS under visible excitation into the charge transfer band of Cr2+ ions. Polycrystalline samples prepared by thermal diffusion method were studied. Middle-infrared (mid-IR) photo-luminescence (PL) of Cr2+ ions was compared under continuous wave (CW) direct 1532nm (5T2→5E) excitation and under 532nm excitation into charge transfer band. The quantum yield of Cr:ZnSe mid-IR luminescence under CW green excitation was estimated as close to 100% at room temperature. To estimate Cr excitation rate via charge transfer band under short pulse excitation, mid-IR PL kinetic measurements were performed with the use of 532nm picosecond and nanosecond pumping. Mid-IR PL kinetics of Cr:ZnSe under pulsed green excitation exhibit a relatively slow growth reaching a peak at ~5-10μs for nanosecond and picosecond excitations, respectively, while PL kinetics in Cr:ZnS reveal shorter measured rise time (<1μs) limited by the response time of the detector. This rise of the PL intensity under 532nm pulsed excitation implies that 5E population continues to grow after the excitation pulse due to slow relaxation processes from higher-lying excited levels of Cr2+ to the upper laser level 5E. At the same time for nanosecond excitation the excited level is pumped at a rate faster than it is depleted and, hence, it is reasonable to expect that the population of the 5E level could be inverted. For laser experiments we used 5ns radiation from BBO based optical parametric oscillator tunable over 450-700nm. Cr:ZnSe lasing at 2.5μm induced by 2+→1+→2+ ionization transitions of chromium under visible excitation was achieved.
Cobalt doped II-VI wide band semiconductors (e.g. ZnSe, ZnS, CdSe) are promising media for infrared (IR) laser applications. They could be utilized as effective passive Q-switches for cavities of Alexandrite as well as Nd and Er lasers operating over 0.7-0.8, 1.3-1.6, and ~2.8 μm spectral ranges. We report spectroscopic characterization of Co:ZnSe and Co:ZnS crystals. Absorption cross-sections were measured for 4A2(F) → 4T1(P), 4A2(F) → 4T1(F), and 4A2(F) → 4T2(F) transitions with maximum absorption at 768(726), 1615(1500), 2690(2740) nm for ZnSe(ZnS) crystals, respectively. The calculated absorption cross-sections of the above transitions were estimated to be 64(56)×1019, 7.5(7.8)×1019, and 0.52(0.49)×1019 cm2 for ZnSe(ZnS) crystal hosts. In addition to the above applications the cobalt ions could be utilized for excitation of Fe2+ ions via resonance energy transfer process. Tunable room temperature lasing of Fe 2+ doped binary and ternary chalcogenides has been successfully demonstrated over 3.5-6 μm spectral range. However, II-VI lasers based on Fe2+ active ions don’t feature convenient commercially available pump sources (e.g. some Fe doped crystal hosts require pump wavelengths longer than 3 μm). Therefore, the process of energy transfer from Co2+ to Fe2+ ions could enable utilization of commercially available visible and near-infrared pump sources. We report a spectroscopic characterization of iron-cobalt co-doped ZnS and ZnSe crystals over 14-300K temperature range. Mid-IR laser oscillation at 3.9 μm(3.6 μm) via energy transfer in the Co:Fe:ZnSe (Co:Fe:ZnS) co-doped crystals was demonstrated under cobalt excitation at 4A2(F) → 4T1(P) (~0.7μm) and 4A2(F) → 4T1(F) (~1.56 μm) transitions.
We report on spectroscopic characterization of laser active powders based on iron doped II-VI ternary and quaternary semiconductors for mid-IR laser applications. Iron doped Cd1-x MnxTe, Cd1-x MnxS, Cd1-xMnxSe, Cd0.5Mn0.5S0.5Se0.5 , Cd1-xZnxTe compounds with x=0.5-0.25, were prepared by using thermo diffusion technique. The starting binary powders were mixed in the appropriate molar ratios, sealed in evacuated (10-3 Torr) quartz ampoules, and annealed at 800-1000oC for several days. Samples composition, integrity, and grain size were characterized by micro-Raman and Xray diffraction and revealed a variation of the crystal field parameters depending on powder composition. Fe2+ photoluminescence was characterized by spectral band position (normalized with respect to the detection platform spectral sensitivity) and lifetime at different temperatures, enabling calculation of the absorption and emission crosssections. Practical utility of the developed powders was demonstrated by a room temperature random lasing of iron doped Cd0.5Zn0.5Te powders over 5620-6020 nm spectral range pumped by a 2.94 μm radiation of a Q-switched Er:YAG laser. In summary, the following has been accomplished: (1) It was demonstrated that laser active Fe2+ doped ternary and quaternary II-VI materials can be produced by simple annealing of the commercially available binary powders omitting expensive and complicated crystal growth processes; (2) It is possible to effectively shift PL of Fe2+ in II-VI host materials towards shorter or longer wavelength by varying composition, type and amount of the second cation in ternary II-VI materials; (3) Major spectroscopic characteristics of Fe2+ doped II-VI ternary and quaternary compounds were obtained and their practical utility for mid-IR lasing demonstrated.
Power scaling of mid-infrared laser systems based on chromium and iron doped zinc selenide (ZnSe) and zinc sulfide (ZnS) crystals is being advanced through the integration of surface relief anti-reflection microstructures (ARMs) etched directly in the facets of the laser gain media. In this study, a new ARMs texture fabrication process is demonstrated for polycrystalline ZnSe and ZnS material that results in a significant increase in pulsed laser damage resistance combined with an average reflection loss of less than 0.5% over the wavelength range of 1.9-3.0μm. The process was utilized to fabricate ARMs in chromium-doped zinc selenide (Cr2+:ZnSe) materials supplied by IPG Photonics and standardized pulsed laser induced damage threshold (LiDT) measurements at a wavelength of 2.09μm were made using the commercial testing services of Spica Technologies. It was found that the pulsed LiDT of ARMs etched in ZnSe and Cr2+:ZnSe can match or even exceed the level of a well-polished surface, a survivability that is many times higher than an equivalent performance broad-band thin-film AR coating. The results also indicate that the ARMs plasma etch process may find use as a post-polish damage mitigation technique similar to the chemical immersion used to double the damage resistance of fused silica optics. ARMs etched in Cr2+:ZnSe were also evaluated by IPG Photonics for survivability under continuous wave (CW) laser operation at a pump laser wavelength of 1.94μm. Catastrophic damage occurred between power levels of 400-500 kilowatt per square centimeter for both as polished and ARMs textured samples indicating no reduction in CW damage resistance attributable to surface effects.
Currently, a majority of Volumetric Bragg gratings (VBG) use photorefractive glasses with a transmission band between
0.3 and 2.7 μm. We have proposed and realized VBGs for mid-IR spectral range based on LiF color center crystals
(LiF:CC). γ-irradiated LiF:CC crystals feature strong absorption bands in the visible and near-IR spectral range, where
selective color center photo-bleaching allows for the LiF refractive index modification. The absence of active absorption
in LiF:CCs at wavelengths longer than 1.3 μm results in a VBG that is stable under mid-IR irradiation. Our calculations
predict that ~60% diffraction efficiency over 1-6 μm spectral range could be realized in ~1 cm long VBG. To verify this
calculation, we fabricated periodic structures in LiF:CC crystals with 24 and 12 μm spacings by CCs photo-bleaching
using femtosecond Ti:sapphire laser pulses. Periodic structures exhibit diffraction in multiple orders in the Raman-Nath
regime at 0.532, 0.632, and 1.56 μm. The first order diffraction efficiencies were stronger for a visible radiation due to a
bigger refractive index variations and additional amplitude modulation. The demonstrated diffraction at 1.56 μm is a
clear manifestation of a phase grating in LiF and serves as a proof of feasibility of these LiF:CC crystals for mid-IR
VBG applications.
Compact solid-state laser systems based on chromium doped II-VI semiconductor materials (ZnS, ZnSe, CdSe) with
tunability over 2-3.6 μm, output power exceeding 10W, and efficiency up to 70% were demonstrated recently. A further
increase of the output power requires a thorough thermal management of the active element. Fiber geometry of gain
element is very promising among other different approaches to control beam quality and thermal lensing. The proposed
transition metal doped ZnS:ZnSe/As2S3:As2Se3 composite materials with index matching of II-VI and V-VI components
represent a new way for design of mid-infrared laser active fibers. Chalcogenide glasses have wide transparency range in
mid-IR, enable fiber geometry, and their refractive index can be varied from n=2.1 to n=2.5 matching refractive index of
ZnS (n=2.26) and ZnSe (n=2.44) crystals and eliminating scattering losses. The II-VI compounds provide a tetrahedral
coordination of the chromium ions required for mid-IR lasing. We report the first mid-IR laser active
Cr:ZnSe/As2S3:As2Se3 composites fabrication and room-temperature lasing at 2.4 μm. The Cr:ZnSe/As2S3:As2Se3 composites were prepared by annealing of the appropriate compounds under vacuum and by casting and drying of
Cr:ZnSe microparticles suspension in As2S3:As2Se3 propylamine solution. All samples demonstrated mid-IR
photoluminescence typical for Cr2+ ions in ZnSe host. High optical gain and low passive losses in Cr:ZnSe/As2S3:As2Se3 composite material were demonstrated in random lasing experiments.
We report an optimization of Fe:ZnSe crystals fabrication, as well as a fourfold increase of the output energy of the
gain-switched middle-infrared Fe:ZnSe laser pumped by the radiation of Q-switched Cr:Er:YSGG (2.8μm) laser. Lasing
was studied over 236-300K temperature range. In Fabry-Perot cavity with 18% OC reflectivity the maximum output
energy reached 4.7 mJ @ 4.3μm and 3.6 mJ @ 4.37μm at 236K and 300K, respectively and was limited only by
available pump energy. Threshold was about 8 mJ and was practically unchanged over studied temperature range. The
laser slope efficiencies decreased from 19% to 16 % with an increase of temperature from 236 to 300K.
Middle infrared (mid-IR) chromium-doped zinc selenide (Cr:ZnSe) bulk lasers have attracted a lot of attention due to
their unique combination of optical and laser properties facilitating a wide range of potential scientific, industrial, and
medical applications. Utilization of thin film waveguide geometry enabling good thermal management and control of
beam quality is a viable pathway for compact chip-integrated optical laser design. Cr:ZnSe thin films are also promising
as saturable absorbers and mode-lockers of the cavities of solid state lasers operating over 1.3-2.1 μm. We recently
reported the first successful demonstration of mid-IR Cr:ZnSe planar waveguide lasing at 2.6 μm under gain-switched
short-pulse (5 ns) 1.56 μm excitation as well as the passive Q-switching of the cavity of a fiber-pumped Er:YAG laser
operating at 1645 nm using a highly doped Cr:ZnSe thin film. PLD grown Cr:ZnSe waveguide were fabricated on
sapphire substrates (Cr:ZnSe/sapphire) with chromium concentration of 1018-1019 cm-3. Further development of mid-IR
lasing in the Cr:ZnSe planar waveguide under continuous wave excitation were investigated. In addition, deposition of
Cr:ZnSe-based thin film structures on n-type GaAs substrates were also investigated for possible mid-IR
electroluminescence.
The anti-EGFR antibody, cetuximab, was labeled with IRDye 800CW fluorescent dye and conjugated to gold nanorods
(GNR). GNR with aspect ratio of ~ 4 and plasmon resonance peak at ~785 nm were fabricated for use in these
experiments. The IRDye:cetuximab:nanorod conjugate treatment with NIR light selectively heated the GNR and was
sufficient to treat cancers. Excitation induced fluorescence of the IRDye 800CW enabling real-time imaging. We
characterized and optimized the parameters for the conjugation of the GNR to cetuximab to facilitate active targeting of
the nanorods to the site of the tumor. This combination of selective targeting, imaging, and photothermal treating of
malignant cells is a viable approach for a variety of squamous cell carcinomas.
The objective of this work was to develop a compact and efficient Tm-fiber-Ho:YAG, hybrid laser passively Q-switched
by Cr:ZnSe saturable absorber. We used a folded semi-hemispherical 10 cm long cavity with a plane output coupler and
a 0.5 m concave high reflector. In these experiments we studied the performance of two high optical quality Cr:ZnSe
crystals as saturable absorbers with initial transmissions of 93.9% and 70% at 2.1 μm. With the 93.9% transmission
crystal, passive Q-switching was realized with a maximum output power of 5 W, pulse energy of 0.5 mJ, pulse duration
of 150 ns, and Q-switched-to-CW-mode extraction efficiency of 60%. With the 70% transmission crystal, passive Qswitching
was achieved with a 75% Q-switched-to-CW-mode extraction efficiency, pulse energy of 3 mJ, and duration
of 7ns. The laser demonstrated sustained damage-free, TEM000 operation with 0.5 MW of peak power showing promise
for applications requiring high-peak-power, diffraction-limited beams, and single-frequency regimes of operation.
We report methods of fabrication and laser-spectroscopic characterization of mid-IR gain media based on micron size
Cr2+:ZnSe/ZnS powders, as well as Cr2+:ZnSe/ZnS doped fluorocarbon polymer films, and perfluorocarbon liquids. All
samples demonstrated strong mid-IR luminescence over 2000-3000nm spectral range under optical 1700nm excitation.
The random lasing of the doped liquids and polymer films was realized with pump energy density of 100 and 15mJ/cm2,
respectively. Previously we have demonstrated mid-IR electroluminescence of Cr:ZnSe with n-conductivity provided by
thermal diffusion of Al and Zn. However, the formation of conductivity was accompanied by compensation of the Cr2+
optical centers and relatively weak chromium electroluminescence. In this paper we report study of the Cr2+
compensation in the crystals co-doped with donor and acceptor impurities. Optical and electrical characterization of
Cr:ZnSe crystals with Ag, Cu, Al, In, and Zn co-dopants were studied to optimize mid-IR electroluminescens of the Cr2+
ions. The best results were obtained with p-conductive Ag:Cr:ZnSe samples featuring a low 600 Ωcm resistivity. First
mid-IR electroluminescence in presumable p-type Ag:Cr:ZnSe was demonstrated, which could prove valuable for
developing laser diodes that function in this spectral region.
We report a simple method for fabricating transition metal (TM) doped II-VI powders with average size of about 10-
20μm as well as room temperature mid-infrared (2-3 μm) random lasing based on Cr2+-doped ZnSe and ZnS powders
prepared without crystal growth stage under optical intra-shell excitation of chromium. Fabrication of Cr2+-doped ZnSe
and ZnS powders involved two simple stages. At the first stage, pure ZnSe, (ZnS) and CrSe, (CrS) (with a concentration
of Cr2+ ion 2×10-19 cm-3 and 5 × 10-19 cm-3 for ZnSe and ZnS respectively) chemicals with an average grain size of 10μm were uniformly mixed by means of a mechanical shaker. At the second stage the obtained ZnSe/CrSe mixture was sealed into evacuated (~10-4 Torr) quartz ampoule and annealed either at 1200°C for 15 minute, or 1000°C for 3 days. In
the case of ZnS/CrS mixtures the annealing was performed in evacuated quartz ampoule at 1000 °C for 14 days. After
annealing, under 1560 nm excitation, the powders demonstrated room temperature middle-infrared luminescence of Cr2+
similar to Cr2+ emission in bulk ZnSe and ZnS. Moreover, the output-input characteristic clearly demonstrated the
threshold-like behavior of the output signal with the threshold pump energy density of ~44.5 mJ/cm2 ~7.46 mJ/cm2, and 63.6 mJ/cm2 for Cr:ZnSe annealed for 15 min, 3 days, and Cr:ZnS respectively.
Here we report a new method for transition-metal (TM) doped II-VI Quantum Dots (QD) fabrication and first mid-IR (2-3 μm) lasing
at 77K of Cr2+:ZnS QD powder (~ 27 nm grain size). Cr2+:ZnS nanocrystalline dots (NCDs) were prepared using laser ablation. The mid-IR photoluminescence (PL) and lasing were studied. The dependence of PL spectrum profile on pump energy demonstrated a threshold behavior accompanied by the appearance of a sharp stimulated emission band around 2230 nm. The stimulated emission band is shifted to the longer wavelength with respect to the spontaneous emission and corresponds to the peak of the Cr:ZnS gain spectrum. This was also accompanied by a considerable lifetime shortening.
PbWO4 crystals with Ho concentration ranging from 0.2% to 4% were grown by the Czochralski method. Polarized
optical absorption, emission and kinetics of fluorescence were studied over 20-300K temperature and 0.2-8&mgr;m spectral
ranges. Stimulated Raman scattering in PbWO4 crystal was studied under 1.6 and 2.0 &mgr;m excitation.
Evanescent wave cavity ring-down spectroscopy (EW-CRDS) is used to observe the adsorption isotherm for hemoglobin (Hb) from controlled urine samples to assess the potential for rapid diagnosis in hemoglobinuria. The absorbance of Hb at 425 nm is monitored using an alexandrite laser-pumped, room temperature, LiF:F color-center pulsed laser. A minimum absorbance detection level of 2.57×10–4 is achieved, corresponding to a minimum detectable concentration of Hb in urea of 5.8 nM. A multilayered Hb biofilm is formed, and a minimum of eight layers are required to model the adsorption isotherm, allowing for cooperative binding within the layers and extending 56 nm into the interface. A binding constant for Hb to silica 18.23±7.58×106 M is derived, and a binding constant for Hb to Hb in subsequent layers is determined to be 5.631±0.432×105 M. Stoichiometric binding coefficients of 1.530±0.981 for layer one and 1.792±0.162 for subsequent layers suggest that cooperative binding both to the silica surface and between the layers of the biofilm is important.
KEYWORDS: Crystals, Laser crystals, Laser induced fluorescence, Color centers, Tunable lasers, Color center lasers, Laser stabilization, Absorption, Current controlled current source, Pulsed laser operation
The first room temperature stable tunable color center (CC) distributed feedback (DFB) laser is described. The laser utilizes stabilized F2+** centers in LiF (LiF:F2+**) as a gain medium. Tunable DFB lasing was achieved in the near IR region (882 - 962 nm) with a lasing linewidth of less than 0.2 cm-1. The lasing threshold was found to be 1.2 mJ, while the slope efficiency with respect to pump energy was found to be as high as 3%.
A high level of fluorescence background signal rejection was achieved for solid and powder samples by using a combination of simple low-resolution spectrograph and ultrafast intensified/gated CCD camera. The unique timing characteristics of CCD camera match exceptionally well characteristics of Ti:sapphire oscillator allowing fast gated light detection at a repetition rate of up to 110 MHz, making this approach superior in terms of duty cycle in comparison with other time-resolved Raman techniques. The achieved temporal resolution was about 150 ps under 785 nm Ti:sapphire laser excitation. At an average excitation power up to 300 mW there was no noticeable sample damage observed. The strong Hexobenzocoronane (HBC) fluorescence with a lifetime about 2.1 ns was efficiently rejected and Raman spectrum revealed. The combination of spectrometer and ultrafast gated CCD camera allows simultaneous study of spectral and temporal characteristics of emitted light for the fluorophores with a fluorescence lifetime in nanosecond range. It is particularly important in biomedical spectroscopy, since the majority of endogenous fluorophores has a relatively short lifetime of about 1-5 ns. This capability opens an exciting possibility to build a universal instrument for solving multitask problems in applied laser spectroscopy.
Organic materials with large two-photon absorption are desired for numerous photonics applications, such as optical limiting,upconversion lasing, three-dimensional data storage,and photodynamic therapy. Stilbazolium derivatives are interesting two-photon absorbers for these applications. In this work,the nonlinear transmissivities of trans -4-[4-(dimethylamino)styryl ]-1-methylpyridinium iodide (DASPI),trans -4-(4-aminostyryl)-1-methylpyridinium iodide (ASPI),trans -4-[2-(1-methylpyrryl)vinyl ]-1-methylpyridinium iodide (MPVPI),trans -4-(2-pyrrylvinyl)-1-methylpyridinium iodide (PVPI),and trans -4-styryl-1-methylpyridinium iodide (SPI)at 800,850,900,950 and 1000 nm have been studied respectively using 21 ps laser pulses. The two-photon induced fluorescence of these compounds at 872,900,and 940 nm has also been investigated.All of these compounds exhibit two-photon absorption and two-photon induced up-converted fluorescence in the near-IR wavelengths,and the two-photon absorption cross section and
two-photon induced fluorescence intensity vary with the wavelength and with the chemical structure changes. These preliminary results suggest that it is possible to increase the two-photon absorption cross-sections by proper structure modifications
Organic materials with large excited state and/or two-photon absorption are desired for numerous device applications, such as optical limiting, two-photon upconversion lasing, three-dimensional data storage, and two-photon photodynamic therapy. Dialkylamino styryl thiazolium/benzoxazolium compounds are interesting excited state and two-photon absorbers for these applications. In this work, the nonlinear transmissivity of trans-2-[p-(N-methyl-N-(hydroxyethyl)amino)styryl]-N-methylthiazolium iodide (MHAST), trans-2-[p-(N-ethyl-N-(hydroxyethyl)amino)styryl]-N-methylthiazolium hexafluorophosphate (EHAST), trans-2-[p-(N-methyl-N-(hydroxyethyl)amino)styryl]-N-methylbenzoxazolium iodide (MHASBO) at 532 nm has been studied using 6 ns laser pulses. The two-photon induced fluorescence of these compounds at 940 nm has also been investigated. At 532 nm, both MHAST and EHAST exhibit reverse saturable absorption, however, MHASB exhibit saturable absorption. At 940 nm, all of these compounds exhibit two-photon induced up-conversion fluorescence, and the fluorescence intensity varies when the chemical structure changes. These preliminary results suggest that the nonlinear absorption (excited state absorption or two-photon absorption) characteristics of these compounds vary at different wavelengths and vary when their chemical structure changes.
Sergey Mirov, Robert Pitt, Alex Dergachev, Wonwoo Lee, Dmitri Martyshkin, Olga Mirov, Jeremy Randolph, Lawrence DeLucas, Christie Brouillette, Tasoltan Basiev, Yurii Orlovskii, Olimkhon Alimov, Ivan Vorob'ev
A novel experimental set-up using laser-induced breakdown spectroscopy (LIBS) for environmental analyses of heavy metals is described in this paper. It is based on state-of-the-art spectroscopic equipment, advanced detectors, and laser atomizers: a 0.75 m spectrometer ARC-750, intensified TE- cooled 256 X 1024 CCD camera, probe with fiber optic guide for signal transportation, and Nd:YAG laser plasma atomizers with two different methods for sample delivery. In the first method the liquid solution containing the atoms to be investigated is drawn into the chamber of the nebulizer. The mixture passes through the nozzle, accompanied by argon gas along with formed aerosol, and enters the plasma plume, which is generated by the laser spark in argon. The second method is based on direct generating of the plasma in the water jet of a continuously circulating sample. LIBS testing of samples containing Al, Cd, Cu, Fe, Pb, Zn, and Cr ions was compared with results using atomic absorption spectrophotometry. Initial indications showed good agreement between these two methods. Detection levels of less than 100 ppb were observed for copper and chromium. The described spectroscopic system exhibits high sensitivity, accumulation of luminescence spectrum in real time; and high dynamic range for concentrations detection from 100 ppb to 1000 ppm.
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