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Impurity Cr2+ ions in the CdSe crystal are studied by tunable EPR spectroscopy in the frequency range of 70–320 GHz. The discovered Jahn–Teller tetragonal distortions of the nearest environment of the chromium ion confirm the results of recent ultrasound studies. The set of spectroscopic parameters that allows obtaining agreement between the calculates and experimental angular and field–frequency dependences of the EPR spectra is determined.

In high purity

We report the investigation of a repetitively pulsed laser based on a Fe : ZnSe crystal, cooled to a temperature of 195 K by two thermoelectric modules with a maximum cooling power of 34 W. Two pulsed synchronised Er : YAG lasers, operating in the free-lasing regime, are used for pumping. It is shown that the average laser power in a series of short pulses may exceed 20 W. An average power of 3.1 W is attained in the stationary temperature regime. The lasing spectrum is centred at a wavelength of 4.38 μm, and the total divergence angle is 5 mrad.

A method is developed to determine the symmetry properties of strains and the type of Jahn–Teller effect in crystals with impurity ions in a triply degenerate electronic T state. This method is based on a calculation of the isothermal contribution of the impurity subsystem to the elastic moduli of a crystal and the absorption and velocity of normal modes for all three possible problems, namely, T ⊗ e, T ⊗ t2, and T ⊗ (e + t2). The calculation results are compared with experimental data. The efficiency of the method is demonstrated for a CdSe:Cr2+ crystal. The CrSe4 center is found to be described in terms of the problem T ⊗ e. The parameters of the ground-state adiabatic potential are determined.

We report the graphene-based passive mode-locking in Fe:ZnSe. The laser delivers 415 mW average power output radiation at a wavelength of 4.4 μm being pumped by 7-W fiber Er:ZBLAN laser. A pulse duration of 732 fs retrieved from the first-order autocorrelation function. Besides, we observed pulsed nanosecond oscillation under continuous-wave pumping and strong amplitude modulation caused by Kerr self-focusing. A numerical simulation of the soliton propagation in a laser cavity using the nonlinear Schrodinger equation supports obtained results. Such simple and effective ultrafast mid-IR high-repetition-rate coherent sources are designed to become a valuable tool in various fields of photonics.

We demonstrate the laser performance of a Fe2+:CdTe single crystal pumped by a Q-switched Er:YAG laser at room temperature. The Fe:CdTe laser produced 2 mJ of output energy at a central wavelength of 5.55 µm with a slope efficiency of 16% with respect to absorbed pump energy. The tunability was demonstrated in the wavelength range from 5.1 to 6.3 µm.

We report, for the first time, to the best of our knowledge, a femtosecond mode-locked Fe:ZnSe laser. Passive mode locking is implemented using graphene as a saturable absorber. The laser operates at 4.4 µm with a repetition frequency of 100 MHz and 415 mW output power pumped by a fiber 7 W Er:ZBLAN laser. The pulse duration of about 732 fs is retrieved from the first-order autocorrelation function. Additionally, we observe pulsed nanosecond oscillation under continuous-wave pumping and strong amplitude modulation caused by Kerr self-focusing. This Letter fills the gap in operating regimes of Fe:ZnSe lasers and paves the way for the development of powerful ultrafast high-repetition-rate mid-IR sources for the most advanced fields of science.

In high purity C e 3 + -doped selenide glass pumped by a 4.08 µm Fe:ZnSe laser, 5.1–5.5 µm laser oscillations were observed. This is the first evidence of laser action corresponding to the 2 F 7 / 2 → 2 F 5 / 2 transition of C e 3 + ions.

The electron paramagnetic resonance (EPR) spectra of iron-doped ZnSe single crystals were studied. In addition to cubic Fe3+ and Mn2+ centers and also Fe2+, and Cr2+ centers, monoclinic Fe3+ complexes locally compensated by Cu+ ions were revealed. Some trigonal centers with a spin of 3/2 were also found and studied. The zero-field splittings of monoclinic centers were measured, and the parameters of the monoclinic and trigonal spin Hamiltonians were determined. The nature of trigonal centers was discussed.

High-energy laser operation of a F e 2 + -doped single-crystal CdTe is demonstrated at the temperature of 77 K. Pumped with 250 µs pulses of a 4.08 µm Fe:ZnSe laser, the Fe:CdTe laser produced a record output of 0.35 J, with a slope efficiency of 44%. A further up-scaling by employing high-energy pump sources is feasible, thus facilitating material-processing applications. The laser was tunable from 4.86 to 5.37 µm. In a nonselective cavity, the laser’s central wavelength was 5.03 µm at 77 K, and shifted to 5.23 µm at 215 K. At the later operation point, reachable using a Peltier element, the laser still produced 0.15 J of output energy with an efficiency of 22%.

Temperature dependence of elastic moduli (C11 − C12 ) / 2 , C33 , and C44, the latter for the piezo-active and non-piezo-active versions, have been measured in the interval of 4 - 180 K at 28 - 262 MHz in a CdSe: Cr2+ crystal. Anomalies below 40 K have been found for all the moduli, except C33 . The interpretation of the results has been carried out involving the Jahn-Teller effect and relaxation between the equivalent distortions of the tetrahedral CrSe4 centers.

One-dimensional gratings with a period of are formed on the surfaces of CdSe and ZnSe crystals ablated by two interfering radiation beams of a nanosecond excimer KrF laser. Investigated are the dependences of the shape and depth of gratings on the energy density under irradiation by a single pulse, and on the number of pulses at a given energy density. The maximum grating depth is estimated as of the period. By forming a one-dimensional grating with a period of and a depth of on the CdSe-crystal surface, this surface becomes antireflective at a wavelength of . The surface reflectivity is reduced by 88 %. A possibility of forming two-dimensional gratings having periods of 1 and is demonstrated.

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.

The first CW single-crystal Fe:ZnSe laser optically pumped by Er:ZBLAN fiber laser with an output power of 2.1 W is reported. Temperature-dependent spectral characteristics and tuning (3.8-5.1 μm) are investigated.

Efficient gain-switched output at 3.12 μm was achieved from the Cr2+:CdSe laser pumped at 2.1 μm by the repetitively-pulsed Ho3+:YAG laser. Two lasing regimes were registered, with the output wavelength centered at 3.12 μm or 2.7 μm, depending on the concentration of Cr2+ ions, crystal temperature, and Q-factor of a resonator. A high beam quality was maintained up to 2.5 W of an output power. The maximum output power reached 5 W.

Pulse-periodic lasing at wavelengths of 2.8 and 3.3 µm is obtained in the Cr2+:CdSe single-crystal laser. In the region of 2.8 µm, the pumping conversion is 28% (more than 50% of the absorbed energy). In the region of 3.3 µm, lasing is achieved at several tunable narrow lines appropriate for using in remote lidars. The pumping conversion in this spectral region is more than 17% (more than 30% of absorbed energy).

A room-temperature laser based on a single crystal has been studied. The crystal was pumped by a nonchain electric-discharge HF laser with a light pulse FWHM of . The pump spot diameter on the crystal surface varied from 3.9 to . The slope efficiency at these values of was 37% – 39 %, respectively. The maximum laser energy was (at ) with the total efficiency with respect to the absorbed pump energy of . A further increase in the laser energy was not achieved because the crystal was destroyed at an incident pump energy and power of and , respectively. The damaged region has a form of a long narrow channel shifted from the entrance surface inside the active element.

Based on the data analysis of ultrasonic experiments, a novel approach has been developed to explore Jahn-Teller effect (JTE) problems in non-cubic crystals with JT centers without involving additional experimental data beyond the information about the electronic term and crystal symmetry. Distinguished from cubic crystals, the axis of symmetry of the bulk non-cubic crystal do not necessarily coincide with those of the local impurity center, thus complicating the relation between the distortions produced by the ultrasound wave and the JTE active modes. We analysed the problem with corresponding calculations for the wurtzite-type hexagonal crystal CdSe:Cr2+, in which the chromium ion substitutes the cadmium one in the tetrahedral environment, resulting in its electronic ground state 5 T 2(e2t2 ). Experimental investigation of this system by ultrasound at frequencies of 28-105 MHz in the temperature range of 4-180 K, yields a peak in the attenuation of the ultrasound below 40 K for the normal modes related to the c 11, c 44, c 55, c 55, and c 66 elastic moduli. The peak has been interpreted as the manifestation of the JTE, similar to the one, observed in cubic crystals doped with 3d ions. However, no anomalies of attenuation have been detected for the mode related to the c 33 elastic modulus, in contradiction to the theoretical predictions based on the previous method, worked out for cubic crystals. In the new method we obtained direct relations between the deformations, related to the crystal moduli, and the local JT modes, calculated the partition functions for each of the three possible JTE problems for systems with an electronic T term, T⊗e, T⊗t2 and T⊗(e + t2) revealed how these deformations alter the vibronic energy levels responsible for the relaxations in the JT centers. It emerged that in the wurtzite crystal under consideration, only in the T⊗e problem the deformation related to the elastic moduli c 33 displaces all the vibronic energy level uniformly, without relaxation possibilities, thus supporting the new method and explaining the experimental observations.

The characteristics of a Fe:ZnSe laser pumped by a single-pulse free-running Er : YAG laser and a repetitively pulsed HF laser are presented. An output energy of 4.9 J is achieved in the case of liquid-nitrogen cooling of the Fe2+:ZnSe active laser element longitudinally pumped by an Er:YAG laser with a pulse duration of 1 ms and an energy up to 15 J. The laser efficiency with respect to the absorbed energy is 47%. The output pulse energy at room temperature is 53 mJ. The decrease in the output energy is explained by a strong temperature dependence of the upper laser level lifetime and by pulsed heating of the active element. The temperature dependence of the upper laser level lifetime is used to determine the pump parameters needed to achieve high pulse energies at room temperature. Stable repetitively-pulsed operation of the Fe2+:ZnSe laser at room temperature with an average power of 2.4 W and a maximum pulse energy of 14 mJ is achieved upon pumping by a 1-s train of 100-ns HF laser pulses with a repetition rate of 200 Hz.

We demonstrate a first-of-its-kind efficient chirped pulse amplification of broadband mid-IR (4-5 μm) femtosecond seed pulse (230 ps, 4μJ) generated in AgGaS2 based OPA driven by Cr:forsterite laser in multi-pass Fe2+:ZnSe amplifier optically pumped by solid-state Q-switched Cr:Yb:Ho:YSGG laser (2.85 μm, 30mJ, 5Hz, 0.6 J/cm2). The system delivers 1.2 mJ at pulse duration of 230 ps. Straightforward compression to 150 fs pulse is achievable with 70% efficiency using diffraction grating pair with peak power of about 6 GW. Further non-linear compression in a bulk CaF2 due to the SPM and anomalous GVD should provide the enhancement of peak power up to 20 GW. Possible routes to reach sub-TW and even TW power level in mid-IR are discussed.

We report a room temperature (RT) operation of a Fe:CdTe laser pumped by a Q-switched 2.94 μm Er:Y3Al2(AlO4)3 laser. The Fe:CdTe laser produced 2 mJ of output energy at λ=5.55 μm with slope efficiency of 16% with respect to absorbed pump energy. With the use of an intracavity prism, the spectral tuning was demonstrated in the 5.1–6.3 μm range, being the longest wavelength tuning achieved for Fe2+:II–VI lasers. The lifetime of the upper laser level was measured to be 530 ns at RT.

We report the effect of magnetic field induced quantum tunneling and relaxation transitions between orthogonal configurations in polyatomic systems where no tunneling is expected. Typical situations of this kind occur in molecular systems and local centers in crystals in ground and excited electronic $T$ states, subject to the $T\ensuremath{\bigotimes}e$ problem of the Jahn-Teller effect, where the wave functions of the three tetragonally distorted configurations are orthogonal. A detailed microscopic theory of this effect shows how the magnetic field violates the orthogonality between the latter allowing for tunneling and relaxations, which decrease in strong fields due to the induced decoherence. The novel effect is demonstrated experimentally as a big, sharp peak in ultrasound attenuation by ${\mathrm{Cr}}^{2+}$ centers in ZnSe:${\mathrm{Cr}}^{2+}$ in the magnetic field $B=0.15$ T at the temperature below 8 K. It may influence a variety of magnetic, electronic, and photonic properties of any system in an electronic $T$ state.

Laser oscillator based on Cr2+:CdSe single crystal pumped by radiation of Tm3+:Lu 2 O 3 -ceramic laser was created and investigated. Repetitively-pulsed oscillations at the wavelength of 2.92 μm with bandwidth of 80 nm were demonstrated. The output power was up to 3W at 15–30 kHz repetition rate with the pulse duration of ∼40–300 ns in the good-quality beam.

Energies of over 10 J and efficiencies of over 44% have been demonstrated in single-shot operation of liquid nitrogen cooled single-crystalline Fe:ZnSe laser at 4100-nm wavelength.

An output power of 0.85 W with a differential efficiency with respect to absorbed pump power of 55.4% is achieved for a Cr2+:CdSe laser with a wavelength of 2.6 µm under optical pumping with a semiconductor disk laser with a wavelength of 1.98 µm.

We realized laser generation on a Fe2+:ZnTe crystal for first time. The crystal was pumped at room temperature by 40 ns pulses of an Er:YAG laser operating at a wavelength of 2.94 μm in the Q-switching mode. The output energy of the Fe2+:ZnTe laser was 0.18 mJ at a slope efficiency of 2.4% with respect to absorbed pumping energy. We achieved tuning of the Fe2+:ZnTe laser generation wavelength within the range of 4.35–5.45 μm using a prism-dispersion cavity.

Continuous tuning of the lasing wavelength from 2350 to 3450 nm in the cw Cr2+:CdSe laser is demonstrated. It is shown that the laser efficiently operates at a wavelength of 3.28 µm, which is promising for the methane optical frequency standard. The single-frequency mode of the laser with a lasing linewidth not exceeding 60 MHz is implemented in this spectral range.

In this Letter, we report a compact and robust coherent source, operating in mid-infrared (IR) and based on the Fe:ZnSe chalcogenide gain medium, optically pumped by an Er:ZBLAN fiber laser. The output power of 2.1 W with a 59% slope efficiency with respect to the absorbed pump power at liquid nitrogen cooling is achieved. We show that strong re-absorption at a high pump power and iron ion doping concentrations leads to a continuous tuning of central wavelength from 4012 to 4198 nm. The robustness of a high-power Er:ZBLAN fiber laser combined with prominent spectroscopic properties of Fe:ZnSe media pave the way for the development of a reliable tunable continuous-wave mid-IR sources for scientific and industrial purposes.

Repetitive-pulse generation of the diode-pumped Er:YAG laser (λ =2 .94 μm) in the free mode and in the mode of cavity passive Q-switching was achieved using a Q-switch based on the Fe 2+ :ZnSe crystal. When using pump pulses 3 ms long, the pulse-average output power of the Er:YAG laser in the free generation mode was 0.5 W. In the passive Q-switching mode, giant pulses 180 ns long with an energy of 3 μJ were obtained. DOI: 10.3103/S1068335610060035

Seeded free growth method with physical transport was used for preparation of large-size II—VI single crystals uniformly doped by transition metals directly during the growth. The grown crystals possess small intrinsic losses. Based on these crystals new results on development of mid-IR lasers were achieved. With the CdSe:Cr crystal pumped at the room temperature (RT) by a continuous wave (CW) 1.908-μm thulium fiber laser, output laser power at 2.6 μm was increased up to 1.7 W. CW lasing from the ZnSe:Fe crystal was achieved using the 2.97-μm CW Cr 2+ :CdSe laser as a pump source. In this case the output power as high as 0.2 W was obtained at 4.1 μm and T=80 K. Broad spectral tuning of the Cr 2+ :CdSe (2.26-3.61 μm) and Fe 2+ :ZnSe (3.95-5.05 μm) lasers was achieved at RT in the pulsed operation. The RT laser action from CdS:Cr and CdSe:Fe was demonstrated for the first time. The output power of the CW Cr +2 :CdS laser was 0.81 W. The pulsed Fe 2+ :CdSe laser was tuned in the 4.6-5.9 μm spectral range.

The characteristics of an Fe:ZnSe laser thermoelectrically cooled to 220 K are described. Output energy of 7.5 J and optical-to-optical efficiency of 30% have been demonstrated in single-shot operation at 4.3 μm with a 2.94 μm Er:YAG pump laser. By using an intracavity prism, continuous tuning from 3.75 to 4.82 μm has been obtained at output energy up to 3.1 J.

The efficient lasing of a Cr2+:CdSe single crystal pumped by 1.94-μm, 300-μs pulses from a Tm:YAP laser was obtained. The Cr2+:CdSe laser with a nonselective resonator emitted up to 17 mJ at a wavelength of ~2.65 μm with the quantum slope efficiency of 63% with respect to the absorbed pump energy. The absorption coefficient of the Cr2+:CdSe crystal at the laser wavelength did not exceed 0.045 cm-1. By using a resonator with a dispersion prism, the laser wavelength was continuously tuned in the spectral range from 2.26 to 3.61 μm.

A Fe2+:ZnSe laser emitting in the spectral range between 3.77 and 5.05 μm is used for the first time for intracavity laser spectroscopy. The intracavity absorption spectra of methane in the 4.1-μm region are recorded at different instants of lasing (0–80 μs) and the dynamics of spectral holes during lasing is investigated. It is found that an intracavity absorption signal linearly increases with time during lasing in the time range from 0 to 80 μs, which provides the effective absorption length up to 24 km.

The specially designed ZnSe–ZnTe superlattice (SL), type-II SL, has the light up-conversion effect that SL emits a photoluminescence (PL) peak under the irradiation of the excitation light with the lower energy than the PL peak. The excitation intensity dependence of the energy of up-converted PL peak obeys the characteristic of type-II SL. The PL peak intensity has the super-linear dependence on the excitation intensity. The five kinds of samples in this work showed these phenomena. The model to explain these situations has been proposed.

A modified vapour phase contact-free method to grow homogeneous single crystals of II-VI compounds doped by transition metals is presented. Single crystals of ZnSe:Cr, ZnSe:Fe, ZnSe:Co, ZnSe:Ni, CdSe:Cr, ZnTe:Cr, ZnS:Fe and ZnS:Mn with doping level up to 1019 cm–3 have been grown. Efficient lasing at about 2.5 and 4 µm with ZnSe:Cr and ZnSe:Fe crystals respectively has been achieved. Dependence of Fe2+:ZnSe laser characteristics on temperature is presented in more detail. A possibility of using Cr2+:ZnSe laser in intracavity laser spectroscopy is demonstrated. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The long period ZnSe/ZnSxSe1–x (x = 0.2–0.4) MQW structures for the electron beam longitudinally pumped laser were grown by metalorganic vapour phase epitaxy on ZnSSe substrates with 5–13° misorientation from (001) to (111)A crystal plane. The best structures with up to 40 QWs had mirror like surface and high intense cathodoluminescence at room temperature (RT). The substrates were cut from ZnSSe single crystals obtained by seeded physical vapour transport. Based on the obtained structures, the etalons were fabricated for electron beam pumping. Lasing at 445 and 460 nm was achieved at T = 80 K and RT, respectively. Output power of the laser was about 1 W at T = 80 K and much less at RT. Possible reasons are discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The photoluminescence (PL) spectra of the type II ZnSe–ZnTe superlattices with dual-subband structures have two emission peaks. The PL peak at the higher energy side appears by the excitation even with the light, with the middle energy between the PL peaks. The emission energy of the up-converted light shows dependence similar to the case of direct excitation (high-energy excitation) on the excitation intensity. The emission energy increases with increase in the excitation intensity. The up-converted and the direct excited emissions are due to the same state in the superlattice.

A modified vapour phase contact-free method to grow homogeneous single crystals of solid solutions (SS) of II–VI compounds is presented. Single crystals of Cd(S, Se), Zn(S, Se), (Zn, Cd)Se, (Zn, Cd)S, Zn(S, Te), Zn(Se, Te), and Cd(S, Te) up to 55 mm in diameter were grown. Different applications of the crystals are discussed.

ZnSe-ZnTe superlattices (SL's) with dual subband structures have been succesfully grown on ZnSe substrates. Those SL's show the so-called light up-conversion effect. The efficiency of the up-conversion has been improved by about 30 times compaired to the SL structures grown on usual GaAs substrate. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Propagation of nonequilibrium acoustic phonons in ZnTe single crystals at helium temperature was studied with phonons being both optically generated and produced by a metal-film heater. Experiments with [110] ZnTe allowed for the detection of the arrival of longitudinal and transverse acoustic phonons, which are particularly well resolved when generated by a heated metal film. A bolometer also detected emission of ZnTe material with a time constant τ ∼ 90 ns, which can apparently be attributed to the luminescence of oxygen complexes, because the lifetimes of other complexes and bound excitons are substantially shorter.

Seeded chemical and physical vapour transport with H 2 and He, respectively, was used to grow twin-free ZnS x Se 1-x (x < 0.15) single crystals with diameter of 50-55 mm and height up to 15 mm. Use of a source with polycrystalline ZnSSe having a composition gradient along the furnace axis, allowed to improve the homogeneity of alloy composition. The change of x per 1 cm along the growth direction in the grown crystals was as small as 1.5%. The etch pit density was (3-5) x 10 4 cm -2 . Specific resistance of ZnSSe wafers was decreased up to 0.06 Ω cm by annealing in liquid Zn:Al.

The possibility to increase the reflectivity of ZnS/ZnSe distributed Bragg reflectors (DBR) grown on GaAs(100) and ZnSe(100) substrates was studied by using metallorganic chemical vapour deposition (MOCVD) technique. As a result of growth optimization, a maximum reflectivity of the DBR mirror on GaAs substrate as high as 99% was achieved at a wavelength of 478 nm. The maximum reflectivity of the 20-pair ZnS/ZnSe DBR stack on transparent ZnSe substrate was 91% at wavelength of 495 nm while the transmission coefficient was 5% at maximum reflectivity. Scattering by surface roughness limited the increase of the ZnS/ZnSe DBR mirror reflectivity in this case. The ZnCdSe/ZnSe QW structure grown on the ZnS/ZnSe DBR mirror showed intense low temperature cathodoluminescence.

Electron beam longitudinally pumped laser based on 15 ZnCdSe/ZnSe QW periodic-gain structure grown by molecular beam epitaxy on ZnSe(001) substrate was studied. An output power of 0.3 W was achieved. The laser wavelength was in 518-536 nm range, being at the short wavelength side of the QW emission line at low excitation level. Such unusual feature was explained by the participation of excited QW levels in the creation of the optical gain.

The mid-IR cw tunable solid state two-mode Cr 2+ :ZnSe laser with intracavity methane cryocell was developed. The laser was applied for sub-Doppler spectroscopy of (ν1+ν4) vibrational-rotational band of methane and observation of narrow resonances of saturated dispersion at λ = 2.36 μm. The new technique of low pressure methane gas cooling was used instead of liquid nitrogen "jacket" design applied in our previous work. Parameters of saturated dispersion resonances were estimated in 77-300 K temperature range. The experiments confirmed that laser with the new "dry cooled" methane cell has prospectives for reaching a short- term frequency stability at the level of 10 -15 - 10 -16 and can be used as a compact device.