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We report the first femtosecond polycrystalline Cr:ZnS oscillator with an octave-spanning spectrum. Average power of the laser is 1.4 W at the mid-IR central wavelength 2.4 μm and the pulse repetition frequency 79 MHz.

We report 45 dB single stage Cr:ZnSe amplifier operating over 2.2-2.6 μm spectral range seeded by CW Cr:ZnSe single frequency laser. The maximum output energy of 1.6 mJ was demonstrated in 25 ns pulses at 1000 Hz repetition rate.

Lasing is studied in CO2 pumped at ~4.3 µm by a tunable Fe:ZnSe laser. Gain is measured at ~10 µm at pressures ≤12 atm where gain bandwidth is broad enough for potential picosecond pulse amplification.

The 658 nm probe kinetics revealed strong bleaching and multiexponential decay of the excited level with additional relaxation via intersystem crossing. The origin of induced excited state absorption was due to some other center impurity center.

Nonlinear harmonic generation in nanostructures is one of the key topics in nanophotonics, as it allows infrared-to-visible light conversion at the nanoscale. Indeed, various nanophotonic designs have been developed to demonstrate high efficiencies for third harmonic generation (THG). However, fabrication approaches for these designs usually involve high-cost lithography-based technological steps. This work reports on the efficient THG in a free-standing Si nanowire array encapsulated into a flexible polymer membrane. High nonlinearity of Si material [Formula: see text] and light coupling with optical resonances in the nanowires yield a strong third-harmonic signal and efficient infrared ([Formula: see text] nm) to visible ([Formula: see text] nm) upconversion. The fabricated membranes demonstrate high flexibility and transparency, which make them convenient to use as infrared light visualizers.

Semiconductor nanowires (NWs) offer multiple advantages for designing novel optoelectronic devices, such as small footprint, high quantum efficiency, high nonlinear susceptibility. Gallium phosphide (GaP) is one of the attractive materials owing to its low optical absorption and high nonlinear susceptibility. However NWs should be transferred to planar substrates for optical studies, which do not allow efficient signal outcoupling. We demonstrate efficient second harmonic generation in individual GaP nanowires trapped using optical tweezers. Such vertically arranged configuration of NW allows to both efficiently generate second harmonic and to probe linear optical response using broadband light source. Such experiment allows to examine interplay between harmonic generation efficiency and NW dimensions.

Controlled growth of heterostructured nanowires and mechanisms of their formation have been actively studied during the last decades due to perspectives of their implementation. Here, we report on the self-catalyzed growth of axially heterostructured GaPN/GaP nanowires on Si(111) by plasma-assisted molecular beam epitaxy. Nanowire composition and structural properties were examined by means of Raman microspectroscopy and transmission electron microscopy. To study the optical properties of the synthesized nanoheterostructures, the nanowire array was embedded into the silicone rubber membrane and further released from the growth substrate. The reported approach allows us to study the nanowire optical properties avoiding the response from the parasitically grown island layer. Photoluminescence and Raman studies reveal different nitrogen content in nanowires and parasitic island layer. The effect is discussed in terms of the difference in vapor solid and vapor liquid solid growth mechanisms. Photoluminescence studies at low temperature (5K) demonstrate the transition to the quasi-direct gap in the nanowires typical for diluted nitrides with low N-content. The bright room temperature photoluminescent response demonstrates the potential application of nanowire/polymer matrix in flexible optoelectronic devices.

Flexible optoelectronic structures are required in a wide range of applications. Large scale modified silicone-embedded n-GaP nanowire arrays of a record 6 µm thin membranes were studied. A homogeneous silicone encapsulation was enabled by G-coating using a heavy-load centrifuge. The synthesized graft-copolymers of polydimethylsiloxane (PDMS) and polystyrene demonstrated two times lower adhesion to Si compared to standard PDMS, allowing 3 square inch area high quality silicone/nanowire membrane mechanical release, preserving the growth Si substrate for a further re-use after chemical cleaning. The 90% transparent single-walled carbon nanotubes electrical contacts to the embedded n-GaP nanowires demonstrated mechanical and electrical stability. The presented methods can be used for the fabrication of large scale flexible inorganic optoelectronic devices.

Reduction of the wavelength in on-chip light circuitry is critically important not only for the sake of keeping up with Moore's law for photonics but also for reaching toward the spectral ranges of operation of emerging materials, such as atomically thin semiconductors, vacancy-based single-photon emitters, and quantum dots. This requires efficient and tunable light sources as well as compatible waveguide networks. For the first challenge, halide perovskites are prospective materials that enable cost-efficient fabrication of micro- and nanolasers. On the other hand, III-V semiconductor nanowires are optimal for guiding of visible light as they exhibit a high refractive index as well as excellent shape and crystalline quality beneficial for strong light confinement and long-range waveguiding. Here, we develop an integrated platform for visible light that comprises gallium phosphide (GaP) nanowires directly embedded into compact CsPbBr

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.

The neutral vacancy (GR1) centers are the intrinsic lattice defects in diamond with interesting optical properties. In the present work, we performed optical characterization of the GR1 centers in CVD-grown bulk diamond to explore their laser feasibility. The non-linear optical measurements reveal efficient saturable absorption of ground level of the centers under ns-pulsed excitation at 632 nm. The transmission changes from 55% at small incident intensity to 84% at the saturated level. The experimentally measured data was fitted with equations representing two limiting cases and obtained the saturation parameters, namely saturation fluence (6.8 mJ/cm2) and saturation intensity (6.4 MW/cm2), at 633 nm for ground state absorption. The intensity of saturation 6.4 MW/cm2 obtained after fitting the experimental data is two orders of magnitude smaller than that at 694 nm documented in the literature for GR1 centers in natural diamond. The calculation gives essentially the same value of absorption cross-section (⁓4.5 × 10-17 cm2) at 633 nm for both approximations. We also estimated the radiative lifetime and quantum yield as about 8.5 ns and 13%, correspondingly. The cross-sections were estimated for transition 1E ↔ 1T2. The emission cross-section was obtained to be ⁓9 × 10-17 cm2 near the maximum of emission band. Such a high absorption cross-section and fast recovery of ground state in combination with high concentration of the centers can provide high optical gain.

There is strong demand for effective gain materials for the 3.0-3.9 μm spectral range not nicely covered by Cr:ZnSe and Fe:ZnSe amplification bands. We characterized, Fe:ZnAl2O4 ceramic sample, Fe:MgAl2O4 and Fe:InP single crystals as promising laser materials for this mid-IR spectral range. In all crystals, the absorption bands corresponding to 5E↔5T2 transition of Fe2+ ions in the tetrahedral sites were measured. In addition, absorption band of Fe2+ ions in the octahedral sites were observed in Fe:ZnAl2O4 sample with maximum absorption cross-section at ~1.0 μm. From the absorption measurements, the radiative lifetime of Fe:MgAl2O4 was calculated to be 60 μs. Saturation absorption of Fe2+ ions in Fe:ZnAl2O4 was studied using Ho:YAG@2.09 μm and Er:YAG@2.94 μm lasers. Saturation measurements were taken up to energy density of 2 J/cm2 and showed no saturation of absorption. This can be explained by a fast non-radiative (

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.

The influence of hydrogen plasma treatment on electrical and optical properties of the vertical GaN nanowires (NWs)/Si heterostructures synthesized via plasma assisted molecular beam epitaxy is studied. The effect of the treatment is thoroughly studied via variation of the passivation duration. Photoluminescence (PL) investigation demonstrates that the passivation affects the doping of the GaN NWs. The samples were processed as photodiodes with top transparent electrode to obtain detailed information about the n-GaN NWs/p-Si heterointerface under illumination. Electron beam induced current (EBIC) measurements demonstrated the absence of the potential barriers between active parts of the diode and the contacts, indicating ohmic behavior of the latter. I-V characteristics obtained in the dark and under illumination show that hydrogen can effectively passivate the recombination centers at the GaN NWs/Si heterointerface. The optimum passivation duration providing improved electrical properties is found to be 10 minutes within taken passivation regimes. It is demonstrated that the longer treatment causes degradation of the electrical properties. The discovered phenomenon is discussed in details.

We report the first room temperature gain switched hot-pressed Fe:ZnSe ceramic laser pumped by radiation of 2.94 μm mechanically Q-switched Er:YAG laser. The maximum output energy was 1.5 mJ (90 ns) with 15% slope efficiency.

Engineering of nonlinear optical response in nanostructures is one of the key topics in nanophotonics, as it allows for broad frequency conversion at the nanoscale. Nevertheless, the application of the developed designs is limited by either high cost of their manufacturing or low conversion efficiencies. This paper reports on the efficient second-harmonic generation in a free-standing GaP nanowire array encapsulated in a polymer membrane. Light coupling with optical resonances and field confinement in the nanowires together with high nonlinearity of GaP material yield a strong second-harmonic signal and efficient near-infrared (800-1200 nm) to visible upconversion. The fabricated nanowire-based membranes demonstrate high flexibility and semitransparency for the incident infrared radiation, allowing utilizing them for infrared imaging, which can be easily integrated into different optical schemes without disturbing the visualized beam., ACS Nano, 14 (8), ISSN:1936-0851, ISSN:1936-086X

We report on saturation of GR1 centers absorption resulting in ~70% inversion. Probe kinetics revealed strong bleaching of ground level followed by relaxation to metastable level with branching ratio ~0.5.

We report the mechanically Q-switched 2.94 µm Er: YAG laser with a record 350 mJ output energy at 3 Hz repetition rate with 160 ns pulse duration and ~10 ns pulse jitter for fine synchronization of pulses.

We report on room temperature Fe:ZnSe MOPA system tunable over 3.6-5.2 µm pumped by radiation of Q-switched 2.94 µm Er:YAG laser. The maximum output energy was measured to be 60mJ under 200mJ of pump energy.

We report mid-IR frequency comb with 3.25 W average power and spectrum spanning 60 THz near 2.4 pm. We stabilized the offset frequency of the comb with accumulated phase error of 75 mrads.

Time- and frequency-resolved measurements of resonant nonlinear refraction of mid-IR radiation by CO and CO2 gas reveal a fast optical nonlinearity that is dominated by powerbroadening resulting in sign reversals of the nonlinear refractive index.

We report optical characterization of Fe:ZnSe hot-pressed ceramics and demonstrate first room-temperature gain-switched lasing of this new ceramic medium. The maximum output energy was measured to be 0.7 mJ at 3.5% slope efficiency.

We give an overview of recent experimental results on mid-IR femtosecond lasers and optical frequency combs based on TM:II-VI media providing access to few-cycle pulses with Watt-level power over 2–20 µm spectral range.

Gain dynamics in a 10 pm CO2 amplifier pumped by a 4.3 pm Fe:ZnSe laser are studied. Measured gain coefficients reached 30%/cm, indicating potentially high efficiency for short pulse amplification.

Enabling broad tunability, high peak and average power, ultrashort pulse duration, and all known modes of laser operation—transition-metal (TM)-doped II–VI chalcogenides are the materials of choice for direct lasing in the mid-IR. The host materials feature broad infrared transparency, high thermal conductivity, low phonon cutoff, low optical losses, and are available as either single crystals or polycrystalline ceramics. Doped with TM ions, these media exhibit a four-level energy structure, the absence of excited state absorption, as well as broad absorption and emission bands. Doped single-crystals of high optical quality are difficult to grow; however, the advent of postgrowth diffusion doped ceramics has resulted in significant progress in laser development. Here, we summarize recent experimental laser results on Cr and Fe doped II–VI chalcogenides providing access to the 1.8–6 μm spectral range with a high (>60%) efficiency, multi-Watt-level [140 W in continuous wave (CW)] output powers, tunability of >1000 nm, short-pulse (

The present paper sheds light on the interfacial structural transformation taking place upon epitaxial growth of ferromagnetic 3d metal compounds – Co, Ni and CoFeB – on the surface of magnetically ordered iron oxides – Fe3O4, Fe2O3 and Y3Fe5O12. The reducing conditions due to the excess of neutral metal atoms at the surface cause reduction of the Fe3+ ions of the underlying iron oxide layer to the Fe2+ state and subsequent recrystallization of the subsurface region to an MeO-like rock salt crystal structure. The corresponding change of the lattice symmetry can be readily recognized by in situ high-energy electron diffraction reciprocal-space mapping and ex situ X-ray diffraction techniques. From an analysis of diffraction and X-ray absorption data a tentative model of the structural transformation is proposed, based on penetration of Me atoms into the near-surface region of the iron oxide lattice. Once a few nanometre thick oxidized layer is formed, further growth of pure metallic film with native crystal structure takes place. An important observation made is that the oxidation/reduction effects do not occur when 3d metals are deposited onto more stable MgO and Gd3Ga5O12 surfaces. This circumstance makes it appropriate to propose these materials for use as buffer layers to prevent formation of antiferromagnetic monoxide transition regions in multilayered magnetic structures. The presented results are supposed valuable for development of hybrid ferromagnetic heterostructures attractive for various spintronic applications.

In this work we have studied volt-ampere characteristics of single core-shell GaN/InGaN/GaN nanowire. It was experimentally shown that negative differential resistance effect can be obtained in the studied heterostructure. On the base of numerical calculation results the model describing negative differential resistance phenomenon was proposed. We assume this effect to be related with strong localization of current flow inside the nanowire and emergence of Gunn effect in this area.

In this paper we study growth of quasi-one-dimensional GaN nanowires (NWs) and nanotube (NT)-like nanostructures on Si(111) substrates covered with a thin AlN layer grown by means of plasma-assisted molecular beam epitaxy. In the first part of our study we investigate the influence of the growth parameters on the geometrical properties of the GaN NW arrays. First, we find that the annealing procedure carried out prior to deposition of the AlN buffer affects the elongation rate and the surface density of the wires. It has been experimentally demonstrated that the NW elongation rate and the surface density drastically depend on the substrate growth temperature, where 800 °C corresponds to the maximum elongation rate of the NWs. In the second part of the study, we introduce a new dopant-stimulated method for GaN nanotube-like nanostructure synthesis using a high-intensity Si flux. Transmission electron microscopy was used to investigate the morphological features of the GaN nanostructures. The synthesized structures have a hexagonal cross-section and possess high crystal quality. We propose a theoretical model of the novel nanostructure formation which includes the role of the dopant Si. Some of the Si-doped samples were studied with the photoluminescence (PL) technique. The analysis of the PL spectra shows that the highest value of donor concentration in the nanostructures exceeds 5∙1019 cm−3.

In this report, we demonstrate that the use of a GaN seeding layer prepared by droplet epitaxy prior to the growth of epitaxial GaN on Si (111) can lead to the formation of oriented arrays of Y-shaped nanoislands (nanotripods) and nanowires and affects the surface density of the nanostructures. From the transmission electron microscopy studies, it is shown that at least some of the seeding islands have a cubic zinc-blende (ZB) GaN structure, and their {111} facets act as the nucleation centers for further growth of GaN nanorods with a wurtzite (WZ) structure. It is also demonstrated that even if the Ga droplets are deposited on the silicon surface prior to the nitridation, a silicon nitride interlayer between silicon and GaN will be inevitably formed in the further growth process. The density and the position of the seeding centers can be controlled with growth parameter variation during the droplet epitaxy; thus the technique proposed and studied in this report can be used for the preparation of site- and density-controlled arrays of nanostructures.

To date, nanoscale dielectric and plasmonic systems with a nonlinear response are of great interest to researchers. This is due to a wide range of their potential applications in nonlinear optical converters and optical communication systems. The fundamental problem of nanoscale frequency converters is the low efficiency of nonlinear optical generation. The reason for this is that the main mechanism for increasing the efficiency of nonlinear signal generation via phase matching is not available at scales smaller than the wavelength. Here, we experimentally investigate the generation of the second optical harmonic in hybrid GaP/Au nanoparticles resonantly enhanced with plasmonic and Mie resonances. Using dark-field spectroscopy, it is shown that nanoantennas support a series of optical resonances in the visible range, the spectral position of which is in good agreement with the numerical simulation. We measured the second harmonic generation spectrum, with sharp resonances which is in accordance with linear scattering. Finally, the dependence of the second harmonic optical signal on the polarization is measured.

Optical porperties of highly-strained gallium phosphide nanowires were investigated via polarized Raman spectroscopy. 5% elastic strain was created in individual nanowire lying on nickel substrate by the means of atomic force microscopy. Micro-Raman mapping along the nanowire cross section in parallel and perpendicular polarization was carried out. Strain-induced effects on transverse optical mode position and shape were analyzed. The pronounced splitting of the mode due to high level of strain was observed. It was found that in parallel polarization the mode shape is sensitive to the position of the pumping spot which can be attributed to enhanced light-nanowire coupling effects.

Nonlinear harmonic generation in nanostructures is one of the key topics in nanophotonics, as it allows infrared-to-visible light conversion at the nanoscale.his work reports on the efficient third-harmonic generation in a free-standing Si nanowire array encapsulated into a polymer membrane. High nonlinearity of Si material X 31112.45 ≈ 10 –19m2 /V 2 and light coupling with optical resonances in the nanowires yield a strong third-harmonic signal and efficient infrared (1200 — 2000nm) to visible (400 — 666nm) upconversion. The fabricated membranes demonstrate high flexibility and semitransparency, which makes them convenient to use as visualizers.

In this work we investigate effects of the crystal phase, twinning defects and shell formation on the work function distribution over the surface of axially heterostructured GaP/GaPAs/GaP nanowires via frequency-modulated Kelvin probe force microscopy. Analysis of experimental data is supported by theoretical model based on density functional theory and Schrodinger–Poisson calculations. The study revealed that the work function of pure (110) ZB and ( 11 2 ¯ 0 ) WZ GaP is 4.34 eV and 4.20 eV respectively. The work function is found to be sensitive to the lattice twinning period which can additionally shift it between the value of WZ and ZB. Formation of a sub-monolayer thick GaPAs shell results in drastic increase of the work function (up to 4.75 eV). Thus, in this work we introduce several techniques for manipulating GaP nanowire work function. The ability to distinguish different crystal phases of the same material by means of Kelvin probe microscopy is shown for the first time.

Wurtzite GaP nanowires are interesting for the direct bandgap engineering and can be used as templates for further growth of hexagonal Si shells. Most wurtzite GaP nanowires have previously been obtained with Au catalysts. Here, we show that long (∼500 nm) wurtzite sections are formed in the top parts of self-catalyzed GaP nanowires grown by molecular beam epitaxy on Si(111) substrates in the droplet consumption stage, which is achieved by abruptly increasing the atomic V/III flux ratio from 2 to 3. We investigate the temperature dependence of the length of wurtzite sections and show that the longest sections are obtained at 610 °C. A supporting model explains the observed trends using a phase diagram of GaP nanowires, where the wurtzite phase is formed within a certain range of the droplet contact angles. The optimal growth temperature for growing wurtzite nanowires corresponds to the largest diffusion length of Ga adatoms, which helps to maintain the required contact angle for the longest time.

Efficient second harmonic generation (SHG) in nanophotonic designs based on all-dielectric nanostructures demands materials with large values of the quadratic nonlinear susceptibility, low dissipative losses, and high refractive index. One of the best materials meeting all these parameters is gallium phosphide (GaP). However, second-order nonlinearity requires high crystallinity and morphology quality of the GaP layer grown for further lithographic processing. Here we develop a method to fabricate high-quality crystalline GaP metasurfaces, which demonstrate pronounced linear and nonlinear optical properties. Direct growth of a GaP layer on a sapphire substrate tackles the previous problem of wafer bonding, because of high optical contrast between fabricated resonant nanoparticles and the substrate. As a result, the fabricated GaP metasurface supports bound state in continuum mode with an experimental quality factor around 100 yielding a strong enhancement of SHG in narrow spectral range. We believe that the developed approach will become a versatile platform for nonlinear all-dielectric nanophotonics.

The development of growth techniques aimed at the fabrication of nanoscale heterostructures with layers of ferroic 3d metals on semiconductor substrates is very important for their potential usage in magnetic media recording applications. A structural study is presented of single-crystal nickel island ensembles grown epitaxially on top of CaF2/Si insulator-on-semiconductor heteroepitaxial substrates with (111), (110) and (001) fluorite surface orientations. The CaF2 buffer layer in the studied multilayer system prevents the formation of nickel silicide, guides the nucleation of nickel islands and serves as an insulating layer in a potential tunneling spin injection device. The present study, employing both direct-space and reciprocal-space techniques, is a continuation of earlier research on ferromagnetic 3d transition metals grown epitaxially on non-magnetic and magnetically ordered fluorides. It is demonstrated that arrays of stand-alone faceted nickel islands with a face-centered cubic lattice can be grown controllably on CaF2 surfaces of (111), (110) and (001) orientations. The proposed two-stage nickel growth technique employs deposition of a thin seeding layer at low temperature followed by formation of the islands at high temperature. The application of an advanced three-dimensional mapping technique exploiting reflection high-energy electron diffraction (RHEED) has proved that the nickel islands tend to inherit the lattice orientation of the underlying fluorite layer, though they exhibit a certain amount of {111} twinning. As shown by scanning electron microscopy, grazing-incidence X-ray diffraction (GIXD) and grazing-incidence small-angle X-ray scattering (GISAXS), the islands are of similar shape, being faceted with {111} and {100} planes. The results obtained are compared with those from earlier studies of Co/CaF2 epitaxial nanoparticles, with special attention paid to the peculiarities related to the differences in lattice structure of the deposited metals: the dual-phase hexagonal close-packed/face-centered cubic lattice structure of cobalt as opposed to the single-phase face-centered cubic lattice structure of nickel.

Chromium doped II-VI semiconductors (such as ZnSe and ZnS) feature broad mid-IR emission in the 2-3 μm spectral range due to intershell transitions of the Cr2+ ions. These materials show much promise for development of a tunable, electrically-pumped, mid-IR laser source. For integration into a mid-IR active multilayered structure, the ternary alloy ZnSxSe1-x is an attractive waveguiding material due to its lattice-matching ability and lower index of refraction with respect to the Cr2+:ZnSe active material. Epitaxial growth of each layer is desired to achieve the electronic and optical properties necessary for successful integration into a lasing device, so a study was conducted on the effects of sulfur content and growth temperature on the crystal quality of the resulting thin films. Several films of ZnSxSe1-x were deposited by pulsed laser deposition (PLD) using a 248 nm KrF excimer laser source at varying growth temperatures and with various compositional parameters onto (100) GaAs substrates. The samples were analyzed via x-ray diffraction (XRD) and energy dispersive x-rays (EDX) to investigate the crystal quality and elemental content of the films for device integration. Film-substrate epitaxy was achieved and upper bounds to the defect density were calculated for several regimes of compositional parameter and growth temperature. From all samples produced, the lowest defect density of 2.2 x 1010 cm-2 was observed for the x=0.06 film grown at 450°C, while the lowest lattice mismatch between the substrate and epilayer of 0.059% was observed for the x=0.02 film grown at 450°C.

A new mechanism of neutron acceleration is discussed and studied experimentally in detail for cold neutrons passing through the accelerated perfect crystal with the energies close to the Bragg one. The effect arises due to the following reason. The crystal refraction index (neutron-crystal interaction potential) for neutron in the vicinity of the Bragg resonance sharply depends on the parameter of deviation from the exact Bragg condition, i.e. on the crystal-neutron relative velocity. Therefore the neutrons enter into accelerated crystal with one neutron-crystal interaction potential and exit with the other. Neutron kinetic energy cannot vary inside the crystal due to its homogeneity. So after passage through such a crystal neutrons will be accelerated or decelerated because of the different energy change at the entrance and exit crystal boundaries.

The present study, for the first time, reports the successful consolidation of chromium doped zinc sulfide (ZnS) polycrystalline infrared (IR) transparent ceramics (maximum transmittance of 67% at 11.6 μm) via hot pressing under vacuum. The phase composition of the as-sintered Cr2+:ZnS ceramics was determined to be primarily cubic ZnS sphalerite, with a minor amount of hexagonal ZnS wurtzite. Scanning electron microscopy (SEM) observation of the sintered ceramics showed a highly consolidated microstructure. IR absorption and photoluminescence measurements revealed that the Cr2+ ions are tetrahedrally coordinated within the ZnS host lattice.

We present time- and frequency-resolved measurements of self-focusing and self-defocusing of 4.3–4.8 μm Fe:ZnSe laser pulses tuned near rovibrational transitions of CO 2 And CO. We estimate the resonant nonlinearity of the air at 4.3 μm and discuss the novel nonlinear optics of simple molecules.

We use time- and frequency-resolved measurements of self-focusing and self-defocusing of 4.3-\ensuremath{\mu}m radiation in ${\mathrm{CO}}_{2}$ gas to study the resonant nonlinear optical response in the vicinity of individual rovibrational lines. Measurements over a range of lines on the $4P$ and $4R$ branch of the 000-001 transition indicate that the nonlinear response of ${\mathrm{CO}}_{2}$ is strongly affected by power broadening, resulting in a sign reversal of the nonlinearity as compared to that of a saturable two-level system.

The role of Si (111) substrate surface preparation and buffer layer composition in the growth, electronic and optical properties of the GaN nanowires (NWs) synthesized via plasma-assisted molecular beam epitaxy is studied. A comparison study of GaN NWs growth on the bare Si (111) substrate, silicon nitride interlayer, predeposited AlN and GaO x buffer layers, monolayer thick Ga wetting layer and GaN seeding layer prepared by the droplet epitaxy is performed. It is demonstrated that the homogeneity and the morphology of the NW arrays drastically depend on the chosen buffer layer and surface preparation technique. An effect of the buffer and seeding layers on the nucleation and desorption is also discussed. The lowest NWs surface density of 14 μm-2 is obtained on AlN buffer layer and the highest density exceeding the latter value by more than an order of magnitude corresponds to the growth on the 0.3 ML thick Ga wetting layer. It is shown, that the highest NWs mean elongation rate is obtained with AlN buffer layer, while the lowest elongation rate corresponds to the bare Si (111) surface and it is twice as lower as the first case. It is found, that use of AlN buffer layer corresponds to the most homogeneous NWs array with the smallest length dispersion while the least homogeneous array corresponds to the bare Si substrate. Vertically aligned GaN NWs array on the wide bandgap GaO x semiconductor buffer layer grown by plasma-enhanced chemical vapor deposition demonstrates its potential for electronic applications. Photoluminescence (PL) study of the synthesized samples is characterized by an intense optical response related to the excitons bound to neutral donors. The highest PL intensity is obtained in the sample with AlN buffer layer.

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

Recent progress in iron doped II-VI chalcogenide laser materials enabled important advancements in room temperature high energy, high power laser systems operating over 3.5-6.0 um. However, a lack of efficient and convenient pump sources for direct pumping of Fe(2+) ions limits possible applications of these materials. One viable option is using readily available pump sources to excite iron centers via Forster-Dexter energy transfer between transition metal ions. This paper reports on the characterization of iron-chromium and iron-cobalt energy transfer in Fe:Cr:ZnSe and Fe:Co:ZnSe co-doped crystals. The kinetics photoluminescence and spectroscopic measurements at 5T2-5E chromium and iron transitions indicated an efficient resonance energy transfer between ions even at room temperature. It was demonstrated that an energy transfer rate in Fe-Cr centers could be shorter than the upper level lifetime of Fe(2+) ions in ZnSe with total TM ions concentration larger than 10^19 cm^-3. Therefore, this mechanism can serve as an effective pump pathway for Fe lasing. Analysis of the dipole-dipole coupling between Fe(2+) and Cr(2+) ions demonstrated the for the shortest distance between iron and chromium ions in ZnSe host, the energy transfer rate is smaller than 1 ns. The absence of excited state absorption in Fe:Cr:ZnSe host make this materials more attractive in comparison with Fe:Co:ZnSe where Fe lasing due to excited state absorption of Co(2+) ions was limited only to low (

We report detailed measurements of resonant self-focusing and self-defocusing of a continuously tunable Fe:ZnSe laser operating within the 4P-branch of the CO2 molecule. We determine the dispersion of this resonant nonlinearity near a rovibrational transition.

We report on the spectroscopic characterization, absorption saturation, and excited-state dynamics of GR1 centers in synthetic diamonds. The non-linear optical measurements reveal an efficient bleaching of the GR1 center’s ground level under ns-pulsed 633 nm excitation. The maxima of absorption and emission cross sections were estimated to be 4.5 × 10−17 cm2 and 9 × 10−17 cm2 at 630 nm and 780 nm, respectively. The radiative lifetime of the excited level was estimated to be 8.5 ns. The 658 nm probe kinetics uncovered relaxation of 1T2 excited level going predominantly to a metastable state with a lifetime of 220 μs. An induced absorption detected with the use of a highly concentrated diamond sample could be due to up-conversion and photoionization processes in additional impurity-vacancy center with absorption at the short-wavelength tail of GR1. The results presented here indicate that synthetic diamonds with GR1 centers could serve as near infrared gain media or passive Q-switchers for laser cavities over the 633–750 nm spectral range. Optimization of the center concentration and the crystal parameters are required to minimize the induced absorption at the lasing wavelengths.

We report, to the best of our knowledge, the first super-octave femtosecond polycrystalline Cr:ZnS laser at the central wavelength 2.4 µm. The laser is based on a non-polarizing astigmatic X-folded resonator with normal incidence mounting of the gain element. The chromatic dispersion of the resonator is controlled with a set of dispersive mirrors within one third of an optical octave over 2.05–2.6 µm range. The resonator’s optics is highly reflective in the range 1.8–2.9 µm. The components of the oscillator’s output spectrum at the wavelengths 1.6 µm and 3.2 µm are detected at –60 dB with respect to the main peak. Average power of few-cycle Kerr-lens mode-locked laser is 1.4 W at the pulse repetition frequency 79 MHz. That corresponds to 22% conversion of cw radiation of Er-doped fiber laser, which we used for optical pumping of the Cr:ZnS oscillator.

We report on a RT gain-switched Fe:ZnSe master oscillator power amplifier (MOPA) system tunable over 3.8–5.0 µm pumped by radiation of Er:YAG laser operating at 2.94 µm. The mechanically Q-switched Er:YAG laser with output energy up to 220 mJ was used as a pump source for a master oscillator and three-stage power amplifier. The maximum output energies in 200 ns pulses exceeded 60, 56, and 48 mJ at 4.4, 4.3, and 4.1 µm, respectively, under 220 mJ of pump energy. The extraction energy efficiencies were measured to be 25, 30, and 40% at the first, second, and third stages, respectively.