High-power mid-IR (4–5 μm) femtosecond laser system with a broadband amplifier based on Fe<Superscript>2+</Superscript>:ZnSe

Concept of a solid-state femtosecond laser system with a multigigawatt power level in the 4–5 μm range has been proposed. The system contains an ultrashort pulse seeder, a two-stage chirped pulse amplifier based on a broadband Fe²⁺:ZnSe active element with optical pumping by a YSGG:Cr:Er laser, and an output stage that provides additional nonlinear optical compression of an amplified pulse to approximately 30 fs in a dielectric CaF₂ medium with anomalous group velocity dispersion in this spectral range.     

Continuous-wave broadly tunable Cr:ZnSe laser pumped by a thulium fiber laser

We describe a compact, broadly tunable, continuous-wave (cw) Cr²⁺:ZnSe laser pumped by a thulium fiber laser at 1800 nm. In the experiments, a polycrystalline ZnSe sample with a chromium concentration of 9.5 × 10¹⁸ cm⁻³ was used. Free-running laser output was around 2500 nm. Output couplers with transmissions of 3%, 6%, and 15% were used to characterize the power performance of the laser. Best power performance was obtained with a 15% transmitting output coupler. In this case, as high as 640 mW of output power was obtained with 2.5 W of pump power at a wavelength of 2480 nm. The stimulated emission cross-section values determined from laser threshold data and emission measurements were in good agreement. Finally, broad, continuous tuning of the laser was demonstrated between 2240 and 2900 nm by using an intracavity Brewster cut MgF₂ prism and a single set of optics.     

High-power Kerr-lens mode-locked Yb:YAG thin-disk oscillator in the positive dispersion regime

We demonstrate a self-starting Kerr-lens mode-locked (KLM) Yb:YAG thin-disk oscillator operating in the regime of positive intracavity group-delay dispersion (GDD). It delivers 1.7?ps pulses at an average power of 17?W and a repetition rate of 40?MHz. Dispersive mirrors compress the pulses to a duration of 190?fs (assuming sech² shape; Fourier limit: 150?fs) at an average power level of 11?W. To our knowledge, this is the first KLM thin-disk oscillator with positive GDD. Output powers of up to 30?W were achieved with an increased output coupler transmission and intracavity GDD. We demonstrate increase of the pulse energy with increasing positive intracavity GDD, limited by difficulties in initiating mode-locking.     

High peak power Nd:YAG laser pumped by 600-W diode laser stack

The Q-switched laser with triangle slab made of Nd:YAG crystal side pumped by 600-W quasi-cw diode laser stack has been designed. The multimode (M²≈2.6) output energy of about 42 mJ was demonstrated in free running mode for110-mJ pump energy. In Q-switch experiments, the KDDP Pockels cell was placed between the slab and rear mirror in plane-plane cavity with output coupler of 84% transmission. The energy of 8 mJ in 2.1-ns pulse duration was obtained for near TEM₀₀ output beam. For passive Q-switching by means of Cr:YAG crystal of 12.6% unsaturated transmission, the energy of 5.1 mJ in 2.5-ns pulse duration was obtained for output beam close to TEM₀₀ mode.     

Octave-spanning spectrum generation in Ti:sapphire oscillator

A Ti:sapphire oscillator emitting an octave-spanning spectrum that ranges from 460 to 1000 nm is demonstrated. To our knowledge, it is the first time that a supercontinuum as a whole measured after the output coupler extending beyond 500 nm in the blue end is generated from a femtosecond Ti:sapphire oscillator. In the Ti:sapphire oscillator operating in self-Q-switched and self-mode locked mode, two chirped mirrors and a prism pair were used for dispersion compensation. These experimental results would open a new option for the construction of simple continuum sources.     

Nonlinear refractive index of RECOB (RE = Gd and La) crystals

The Z-scan technique is employed to obtain the nonlinear refractive index (n ₂) of the Ca₄REO(BO₃)₃ (RECOB, where RE = Gd and La) single crystals using 30 fs laser pulses centered at 780 nm for the two orthogonal orientations determined by the optical axes (X and Z) relative to the direction of propagation of the laser beam (k//Y// crystallographic b-axis). The large values of n ₂ indicate that both GdCOB and LaCOB are potential hosts for Yb:RECOB lasers operating in the Kerr-lens mode locking (KLM) regime.     

Spectroscopy and lasing of cryogenically cooled Yb, Na:CaF2

Absorption, photoluminescence and cw-lasing properties of a novel Na⁺-codoped Yb³⁺:CaF₂ laser crystal are investigated in the temperature range from 10 K to 290 K. Cryogenic cooling leads to the disappearance of the ground-state absorption in the spectral region above 1000 nm and a substantial increase of emission and absorption cross-sections. A particular advantage of the Yb³⁺, Na⁺-codoped CaF₂ crystal lies in the possibility of a direct pumping in the vicinity of the zero phonon line while nearly perfectly avoiding an overlap with the stimulated emission. Further advantages of the low-temperature operation are demonstrated by achieving a close to the theoretical limit slope efficiency of 92% in a cw-laser operation with an output coupler of 28%. By seeding stretched pulses from a femtosecond Yb fiber oscillator into a cryogenically cooled DPSS Yb³⁺,?Na⁺:CaF₂ regenerative amplifier, we obtain >3-mJ pulses at a 1-kHz repetition rate with a spectral bandwidth exceeding 12 nm. The pulses are compressed with a single grating compressor to 173 fs as verified by SHG FROG. Shaping of the spectral amplitude of the seed and active control of the higher-order phase is shown to be crucial for obtaining sub-200-fs pulses at multi-mJ energies.     

Enhanced refractive index well-confined planar and channel waveguides in KTiOPO<Subscript>4</Subscript> produced by MeV C<Superscript>3+</Superscript> ion implantation with low dose

We report on the fabrication and characterization of planar and channel waveguides in KTiOPO₄ crystals by 6.0 MeV C³⁺ ion implantation with the dose of 1×10¹⁴ ions/cm². The dark mode spectroscopy of the planar waveguide was measured using a prism coupling arrangement. An increase of the both n _x and n _y refractive indices induced by the annealing after implantation is believed to be responsible for waveguide formation. The bright near-field intensity distribution of the transverse-electric and transverse-magnetic modes in the annealed channel waveguide was collected and studied by end-coupling method.     

Absorption coefficient of infrared laser window materials

The absorption coefficient of a number of potential i.r. laser window materials has been measured in the region of multiphonon absorption. Absorptions as low as 0·004 cm^?1 have been detected by using thick samples and a differential technique with a dual-beam spectrophotometer. The absorption coefficients of LiF, CaF₂, BaF₂, SrF₂, MgF₂, Al₂O₃, KCl, NaCl and KBr all decrease exponentially with increasing wavenumber. The slope of the exponential bears a nearly constant relationship to the longitudinal optical phonon frequency, consistent with a multiphonon process whose strength depends on the number of phonons generated. Absorption data are also presented for Si, Ge, GaAs: Cr, GaAs: Fe, ZnSe and CdTe.     

Study of Nd2+ absorption in x-irradiated CaF2, SrF2, BaF2 crystals

The absorption spectra of x-irradiated alkaline-earth fluoride (CaF₂, SrF₂, BaF₂) crystals doped with Nd³⁺ ions have been investigated. X-irradiation results in creating the absorption bands of inter-configuration 4fⁿ–4fⁿ⁻¹–5 d¹ transitions of Nd²⁺. The charge reduction of the neodymium by irradiation is not temperature-stable and the ions reoxidation (Nd²⁺ → Nd³⁺) occurs under heating to 570 K in CaF₂, 520 K in SrF₂ and 470 K in BaF₂.     

Synthesis and characterization of ZnxHg1?xSeyS1?y quantum dots

This article describes the synthesis of highly water-soluble Zn _x Hg_1−x Se _y S_1−y quantum dots (QDs) in aqueous solution through a simple photo-assisted reaction between ZnSe QDs and mercury(I) nitrate dihydrate [Hg₂(NO₃)₂·2H₂O]. In order to deduce the optimal synthesis conditions, we varied several parameters, including the concentrations of mercaptosuccinic acid (MSA) and Hg₂(NO₃)₂·2H₂O, the illumination time, and the reaction temperature. When irradiated at temperatures below 80 °C, the ZnSe QDs reacted with the S²⁻ ions formed rapidly from MSA and the Hg²⁺ ions formed from Hg₂ ²⁺ ions to form Zn _x Hg_1−x Se _y S_1−y QDs through a process of photo-etching and surface combination. Under different conditions, we prepared a series of Zn _x Hg_1−x Se _y S_1−y QDs that emit fluorescence at the maximum wavelengths ranging from 405 to 760 nm. Inductively coupled plasma-mass spectrometry and transmission electron microscopy/energy dispersive spectrometry revealed that the content of Hg in the Zn _x Hg_1−x Se _y S_1−y QDs was greater when the synthesis was conducted at higher temperature. The Zn_0.88Hg_0.12Se_0.44S_0.56 QDs exhibit improved photostability than crude ZnSe QDs and possess long lifetimes (τ₁ ~ 38 ns and τ₂ ~ 158 ns).     

Type B epitaxy of Ge on CaF2(111) surface

It was known experimentally that type B orientation, which is rotated 180° about the [111] axis, dominated the heteroepitaxial growth of Ge(111) on a CaF₂(111) substrate at an elevated temperature. We performed first principles calculations using density functional theory to determine the energetics of the Ge(111)/CaF₂(111) interface and found that the type B orientation of the Ge film is most likely a result of a direct bonding between Ge atoms and Ca²⁺ at the CaF₂ surface with the top F^? layer depleted. Our theoretical prediction is supported by our X-ray diffraction experiments on {111} < 121> biaxially textured Ge/CaF₂ samples.     

Effect of annealing in N2 atmosphere on net acceptor concentration in ZnSe:N grown by MOCVD

Annealing effect on net acceptor concentration in ZnSe:N is investigated. ZnSe:N homo-epitaxial layer was grown at 823 K by MOCVD using ammonia (NH₃) as a dopant source. Photoluminescence (PL) spectra measured on as-grown layer exhibited the strong deep donor–acceptor pair (D^dAP) emission and the weak I₁^N emission line. In order to enhance the activation of nitrogen in ZnSe epitaxial layer, sample was annealed at the 823 K in nitrogen (N₂) and hydrogen (H₂) atmosphere. Only the annealing in nitrogen atmosphere increased I₁^N emission intensity indicate the activation of nitrogen acceptor. And net acceptor concentration was estimated to be 3 × 10¹⁷cm⁻³ by C–V measurements. This activation mechanism is interpreted as hydrogen is released from N–H bonds during annealing in nitrogen atmosphere.     

Ultrafast laser performance of Yb3 +: Sc2SiO5 crystal with a single-walled carbon nanotube absorber

Single-walled carbon nanotube (SWCNT) absorber fabricated by vertical evaporation is used in passively mode-locked Yb^3 +:Sc₂SiO₅ (Yb:SSO) ultrafast laser for the first time. The performance of Yb:SSO ultrafast laser with pulse width as short as 880 fs is studied and the average output power is 712 mW. To our knowledge, this is the highest output power of femtosecond lasers with SWCNT-SAs reported. In addition, we firstly demonstrate a passively mode-locked picosecond Yb:SSO laser without inserting any dispersion compensation device. The pulses width is as short as 5.4 ps and the output power is 940 mW.     

The Ag+ induced solution–liquid–solid growth, photoluminescence and photocatalytic activity of twinned ZnSe nanowires

Cubic ZnSe nanowires with periodically alternating twins along the wire growth direction are synthesized in the ZnCl₂–Na₂SeO₃–AgNO₃–ethylenediamine (EN)-ethylene glycol (EG)-polyvinyl–pyrrolidone (PVP) solvothermal system at 180°C for 12 h. The twinned ZnSe nanowires have diameters of 75±10 nm and lengths of >10 micrometers, and grow along 〈111〉 direction. The role of AgNO₃ in the formation of ZnSe nanowires was investigated, and an Ag⁺ induced solution–liquid–solid growth mechanism is also proposed to account for the conversion of microspheres assembled from ZnSe nanocrystallites into ZnSe nanowires. Compared with ZnSe microspheres, the as-prepared twinned ZnSe nanowires exhibit stronger band edge emissions of the wurtzite- and zinc-blende-structured ZnSe and lower deep defect related emission, and their photocatalytic ability is weaker than that of ZnSe microspheres. The results suggest that this simple, mild, one-step solution approach to fabricate ZnSe nanowires may be employed for the synthesis of other selenium compounds with one dimensional nanostructures, and provides opportunities for both fundamental research and technological applications.     

The low-energy electron spectrum from the EC-decay of 57Co: 0 eV up to 15 keV

The low-energy electron spectrum from the ⁵⁷Co decay has been examined in the region from 0 up to 15 keV at instrumental resolution ranging from 2 to 15 eV. Two electrostatic spectrometers and radioactive sources prepared by vacuum evaporation of ⁵⁷Co onto Al foils were utilized. Relative intensities of the main spectrum components have been obtained as follows: (TSE+LLX+Shake-off)/LMM/KLL/KLM/KMM/K−14.4/L−14.4/MN−14.4=116±12/51±4/59.7±1.8/15.2±0.4/1.15±0.07/49.6±1.5/5.05±0.15/0.79±0.02 where TSE means “true secondary electrons”. Absolute and relative energies of the LMM, KLL, KLM, and KMM Auger transitions in Fe have also been determined, as well as their relative intensities with the exception of the LMM lines, the shapes of which were strongly distorted due to the inelastic electron scattering and probably also chemical effects. From the measured conversion electron lines of the 14.4 keV M1 transition in ⁵⁷Fe, a transition energy of 14412.8±0.8 eV and the E2 admixture less than 8×10⁻⁶ were derived. Relative intensities of both the KL_2,3(M_4,5N₁) Auger line group and the M_4,5N₁−14.4 conversion line were found to be lower by about 30% for the “oxide” state of decaying ⁵⁷Co atoms than for the “metallic” state. Pronounced broadenings of narrow spectrum lines have been observed as a consequence of the oxidation of the ⁵⁷Co sources in the laboratory atmosphere. Natural widths for most of the KLL, KLM, and KMM Auger lines and those of the K, L₁, L₂, L₃, M₁, M₂, M₃ and N₁ atomic levels in ⁵⁷Fe were also determined.     

Temperature and concentration quenching of mid-IR photoluminescence in iron doped ZnSe and ZnS laser crystals

Concentration dependences of the mid-IR kinetic of luminescence at ⁵E?⁵T₂ transition in Fe:ZnSe and Fe:ZnS laser samples were studied in 14–300 K temperature ranges. Radiation lifetime in Fe:ZnSe samples measured using low doped samples with iron concentration 0.1×10¹⁸ cm^?3 was estimated to be 57 μs. The magnetic susceptibility for higher doped (C_Fe=38 and 112×10¹⁸ cm^?3) Fe:ZnSe samples was found to consist of a paramagnetic Curie–Weiss behavior arising from the weakly interacting Fe²⁺ ions and a diamagnetic ZnSe contribution plus a temperature-independent, field-dependent contribution possibly arising from very small amounts of aggregated Fe.     

Saturable absorbers for passive Q-switching of erbium lasers emitting in the region of 3 μm

The phototropic properties of Fe:ZnSe, Co:ZnSe, and Co:ZnS single crystals have been investigated. It is shown that these crystals can be used to advantage as the saturable absorbers in solid-state erbium lasers emitting in the region of the 3-μm range. The absorption cross sections of the ground states of the Co²⁺ ion in the ZnSe (σ_GSA = 11·10⁻²⁰ cm²) and ZnS (σ_GSA = 5.6·10⁻²⁰ cm²) crystals and of the Fe²⁺ ion in the ZnSe (σ_GSA = 50·10⁻²⁰ cm²) crystal at λ = 2.79 μm were determined experimentally. It has been established that the above-indicated crystals in the excited state absorb light weakly. The use of these crystals as passive Q switches made it possible to realize, for the first time, the regime of Q-switching of a Cr,Er:YSGG laser emitting at a wavelength of 2.79 μm. Single pulses with an energy of 60 mJ and a duration of 170 nsec were obtained in the regime of passive Q-switching. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 747–751, November–December, 2005.     

High power modelocking of a stigmatic bounce geometry laser using a nonlinear mirror

We report the highest average power modelocking of a novel bounce geometry laser using the nonlinear mirror (NLM) technique. A diode-side pumped Nd:YVO₄ slab lasing at 1064 nm in a spatially stigmatic bounce-oscillator configuration was modelocked using a type-I phase-matched BiBO nonlinear crystal (NLC) in a NLM arrangement, producing 12 W of self-starting, continuous-wave (CW) modelocking. The system produced a pulse duration of 14 ps at a repetition rate of 110 MHz with high long-term stability and excellent beam quality, with an M ²<1.2 and highly circular beam profile. An investigation was made of the sensitivity of modelocking as a function of air dispersion, by varying the distance between the NLC and output coupler. No optical damage was observed throughout the investigation, even when the system was operated in Q-switched modelocking regime. The system exploits the advantages of NLM modelocking for sustaining high average and peak powers without the onset of optical damage.     

The charge states’ conversion of the activator ions in CaF2:Eu nanophosphors

According to stationary X-ray-excited luminescence spectra and thermally stimulated luminescence spectra of CaF₂:Eu nanophosphors, it was found that Eu³⁺?→?Eu²⁺ conversion can occur during thermal annealing of fine-grained (d?=?25?nm) nanoparticles in the 200–800°C range, which is accompanied by an increase in their size within 40–189?nm. An important role of the exciton mechanism of Eu²⁺ luminescence excitation was revealed according to the temperature dependence of X-ray-excited luminescence spectra of CaF₂:Eu nanoparticles of 114?nm size. The maximum of the X-ray-excited luminescence light output of CaF₂:Eu nanophosphors in the Eu²⁺ ions’ emission band was traced out at 400–500°C annealing temperature and at the size of nanoparticles of 114–180?nm. The subsequent growth of the annealing temperatures, particularly in the 800–1000°C range, causes the reduction of X-ray-excited luminescence light output because of the increment of lattice defects’ concentration due to a sharp increase in the size of nanoparticles and their agglomeration.