Mode-locked diode-pumped vanadate lasers operated with PbS quantum dots

The use of glasses doped with PbS nanocrystals as intracavity saturable absorbers for passive Q-switching and mode locking of c-cut Nd:Gd_0.7Y_0.3VO₄, Nd:YVO₄, and Nd:GdVO₄ lasers is investigated. Q-switching yields pulses as short as 35 ns with an average output power of 435 mW at a repetition rate of 6–12 kHz at a pump power of 5–6 W. Mode locking through a combination of PbS nanocrystals and a Kerr lens results in 1.4 ps long pulses with an average output power of 255 mW at a repetition rate of 100 MHz.     

Upconversion due to energy transfer involving Pr<Superscript>3+</Superscript> ions in pairs

Upconversion luminescence in triply ionized praseodymium-doped TeO₂–Li₂O glass using excitation at ∼590 nm into the ¹D₂ level from a dye laser pumped with the second harmonic of a pulsed Nd:YAG laser has been reported. The mechanism involved in the upconversion emission observed at ∼480 nm indicates that the most important contribution is energy transfer among praseodymium ions in pairs followed by the dipole–dipole interaction. The rate-equation model for the emission at ∼480 nm that provides direct information to determine the energy-transfer rates containing the pair of states involved in the upconversion process has been explored.     

Laser emission in Nd:YVO<Subscript>4</Subscript> channel waveguides at 1064 nm

In this work, we report 1064 nm laser emission in Nd:YVO₄ channel waveguides fabricated by carbon implantation. Typical threshold pump powers (∼808 nm) were ≥45 mW. Maximum conversion efficiency was 11.5% (29.6% slope efficiency), and up to 9 mW of signal was delivered. Sample lengths of 4 mm were sufficient to completely absorb the pump power. The special spectral characteristics of this material such as broad absorption band and superior cross sections compared to the YAG crystal makes it suitable for developing compact sources to be integrated in optoelectronic devices.     

Diode-pumped passively mode-locked Nd:CTGG disordered crystal laser

A diode-pumped passively mode-locked Nd: CTGG disordered crystal laser has been experimentally demonstrated for the first time to our knowledge. Mode locked with a semiconductor saturable absorber mirror, the laser generated 5.2 ps pulses at a repetition rate of 88 MHz. After intracavity dispersion compensation, the mode-locked pulses were shortened to 4.3 ps. Multiple emission wavelengths of the Nd:CTGG laser could be synchronously mode locked under dispersion compensation.     

CW and passive Q-switching of 1331-nm Nd:GGG laser with Co<Superscript>2+</Superscript>:LMA saturable absorber

In this paper, the output performances at 1331 nm in continuous-wave (CW) operation and the passive Q-switching regime of a Nd:Gd₃Ga₅O₁₂(Nd:GGG) laser crystal have been investigated under pumping with diode lasers. A maximum CW output power of 1.5 W was reached at an incident pump power of 7.5 W; the overall optical-to-optical efficiency and the slope efficiency with respect to the pump power were 21.5% and 19.4%, respectively. The passive Q-switching regime was achieved with Co²⁺:LaMgAl₁₁O₁₉ (Co²⁺:LMA) saturable absorber (SA) crystals. A maximum average output power of 183 mW was recorded with a Co²⁺:LMA SA with initial transmission T _i of 90%. The pulse energy was 18.7 μJ and the pulse duration was 26.1 ns, which correspond to a pulse peak power of 0.7 kW. With a Co²⁺:LMA SA with T _i=81%, the average power decreased to 131 mW. However, the pulse energy increased to 21.4 μJ, the pulse duration was 16.4 ns and the pulse peak power increased to 1.3 kW.     

Transient characteristics of green upconversion emission of Er3+ in MgO:LiNbO3 crystal: Mg threshold concentration effect

Transient characteristics of upconverted emission (560 nm) of Er³⁺ in LiNbO₃ crystals codoped with 0–7.4 mol% MgO were studied under pulse excitation at 800-nm wavelength. The results show that the transients display considerable Mg-doping-level-dependent nonexponential behavior and a clear Mg optical-damage-resistance threshold concentration effect. Below the Mg threshold concentration, the lifetime increases slightly with the increased Mg concentration. Above the threshold, however, the lifetime drops abruptly by 4–7 times and the nonexponential feature becomes more evident. It is found that each transient can be fitted by a double-exponential function contributed from isolated and clustered Er³⁺ sites. The fit parameters show that doping of MgO above the threshold concentration increases the clustered Er site concentration and the nonradiative cross relaxation probability. The Mg threshold concentration effect derived from the transients is in qualitative agreement with that from the fluorescence spectrum measured as a function of the Mg concentration. The effect of the Mg threshold concentration on the clustered Er site concentration is qualitatively explained on the basis of the microscopic defect model of MgO:LiNbO₃ and is conducted with the Mg site change around the threshold concentration.     

Polarized spectral properties of Pr3+ ions in NaGd(MO4)2 crystal

A Pr³⁺:NaGd(MoO₄)₂ single crystal has been grown by the Czochralski method. The polarized absorption spectra, polarized fluorescence spectra, and fluorescence decay curves of the crystal were measured at room temperature. The Judd–Ofelt intensity parameters Ω _t,q (t=2,4,6) and the spectroscopic parameters related to the ³ P ₀ and ¹ D ₂ multiplets were obtained and analyzed. The decay mechanisms for the ³ P ₀ and ¹ D ₂ multiplets were also discussed.     

Spectroscopic properties of Nd<Superscript>3+</Superscript>:CaNb<Subscript>2</Subscript>O<Subscript>6</Subscript> crystal

The absorption spectra, fluorescence spectrum and fluorescence decay curve of Nd³⁺ ions in CaNb₂O₆ crystal were measured at room temperature. The peak absorption cross section was calculated to be 6.202×10⁻²⁰ cm² with a broad FWHM of 7 nm at 808 nm for E//a light polarization. The spectroscopic parameters of Nd³⁺ ions in CaNb₂O₆ crystal have been investigated based on Judd-Ofelt theory. The parameters of the line strengths Ω _t are Ω ₂=5.321×10⁻²⁰ cm²,Ω ₄=1.734×10⁻²⁰ cm²,Ω ₆=2.889×10⁻²⁰ cm². The radiative lifetime, the fluorescence lifetime and the quantum efficiency are 167 μs, 152 μs and 91%, respectively. The fluorescence branch ratios are calculated to be β ₁=36.03%,β ₂=52.29%,β ₃=11.15%,β ₄=0.533%. The emission cross section at 1062 nm is 9.87×10⁻²⁰ cm².     

Small inclusions in smectic C* free standing films

The creation and the coalescence of inclusions has recently been observed in smectic C* freely suspended films at the temperature near the smectic C*-cholesteric phase transition. A small finite anchoring energy permits to describe small inclusions by analytical approximate solutions of dipolar or quadrupolar type. Using the proposed solutions our model enables us to discuss the coalescence of smallest inclusions and their observed growth. This work was supported by the research project AV0Z1-010-914 and by Grant No. 202/02/0840 of the Grant Agency of the Czech Republic.     

Characterization of 1.06 μm optical vortex laser based on a side-pumped Nd:GdVO<Subscript>4</Subscript> bounce oscillator

We directly produced for the first time a high-power 1.06 μm vortex mode from a diode-pumped Nd:GdVO₄ bounce amplifier. A maximum output of 17.8 W was achieved for a pump power of 55 W. The corresponding optical efficiency from the diode to the output was above 30%.     

On the chaining dynamics of inclusions in SmC* free standing films

A simplified model of an inclusion represented by (+1)-wedge disclination and an accompanying hyperbolic defect ((−1)-wedge disclination) in smectic C* free standing films is used to describe the early stage of the ordering process of inclusions into chains. The elastic interaction between inclusions and their associated hyperbolic defects is used to discuss the dynamics observed experimentally during the inclusion chaining when inclusions are at distances much larger than their radii. This work was also supported by Grant No. 202/02/0840 of the Grant Agency of the Czech Republic and by the research project AV0Z1-010-914.     

Random lasing in porous scattering medium

The spectrum behavior evolution and the threshold of random lasing depending on the way of photon walk randomization in an active random medium were investigated. The following three ways of photon walk randomization were implemented: multiple light scattering by corundum and silica particles embedded into a solid polymer solution of dye (astrafloxin), multiple light reflection at sub-millimeter extensive air pores (mean diameter 200 μm) produced in the medium, and the combined action of both these effects. The most effective lasing is observed in the case of an active medium with air pores and scattering particles in the interpore space. Such a combined porous scattering medium acts as a network of dielectric waveguides transmitting effectively the random light. This spatial structure of the random active medium significantly increases the photon path in the medium, thereby promoting photon multiplication due to stimulated emission. In this combined medium the random lasing reveals the narrowest spectrum, the lowest threshold, and the highest density of spectral energy in the spectrum maximum.     

Diode-pumped laser with Yb:YAG single-crystal fiber grown by the micro-pulling down technique

Laser emission obtained from an Yb:YAG single-crystal fiber directly grown by the micro-pulling down technique is demonstrated for the first time. We achieved 11.2 W of continuous wave (CW) output power at 1031 nm for 55 W of incident pump power at 940 nm. In the Q-switched regime, we obtained pulses as short as 17 ns, for an average power of 2.3 W at 2 kHz corresponding to an energy of 1.15  mJ. In both cases, the M ² factor was 2.5. This single-crystal fiber showed performance similar to a standard rod elaborated by the Czochralski method. The potential of Yb³⁺-doped single-crystal fibers is presented for scalable high-average and high-peak-power laser systems.     

Filamentation control in the temperature gradient argon gas

A novel technique of controlling the evolution of the filamentation was experimentally demonstrated in an argon gas-filled tube. The entrance of the filament was heated by a furnace and the other end was cooled with air, which resulted in the temperature gradient distribution along the tube. The experimental results show that multiple filaments are merged into a single filament and then no filament by only increasing the temperature at the entrance of the filament. Also, the filament can appear and disappear after increasing the local temperature and input pulse energy in turn. This technique offers another degree of freedom to control the filamentation and opens a new way for multi-mJ level monocycle pulse generation through filamentation in the noble gas.     

Wavelength-independent all-optical synchronization of a Q-switched 100-ps microchip laser to a femtosecond laser reference source

We present a Q-switched microchip laser emitting 1064-nm pulses as short as 100 ps synchronized to a cavity dumped femtosecond laser emitting 800-nm pulses as short as 80 fs. The synchronization is achieved by presaturating the saturable absorber of the microchip laser with femtosecond pulses even though both lasers emit at widely separated wavelengths. The mean timing jitter is 40 ps and thus considerably shorter than the pulse duration of the microchip laser.     

Laser-assisted local patterning of ZnO-based spots for mirror-less lasing

ZnO is known as one of the best materials for the implementation of the random lasing effect, associated with mirror-less laser emission in a simultaneously amplifying and highly scattering medium. Normally, the fabrication of this medium requires a rather complicated procedure of deposition and thermal treatment of ZnO-based films on some specific substrates, yielding wurtzite-orientation ZnO nanocrystals. We demonstrate a rapid synthesis of highly efficient ZnO-based random lasing spots on a piece of Zn by employing the phenomenon of laser-induced air breakdown. Being ignited near the surface of a Zn target, plasma of the air breakdown serves as a local reactor to locally transform its properties and thus form a film of well-packed 20–40 nm ZnO nanospheres. Exhibiting extremely high amplification and scattering, this medium is capable of generating the random lasing effect within the exciton-based photoluminescent band.     

Fs-Laser structuring of ridge waveguides

Thin films made by PLD from Er:ZBLAN and Nd:Gd₃Ga₅O₁₂ are micro machined to form optical wave guiding structures using Ti:sapphire and Yb:glass fiber laser radiation. For the manufacturing of the ridge waveguides grooves are structured by ablation using femtosecond laser radiation. The fluence, the scanning velocity, the repetition rate, and the orientation of the polarization with respect to the scanning direction are varied. The resulting structures are characterized using optical microscopy and scanning electron microscopy. Damping and absorption coefficients of the waveguides are determined by observing the light scattered from the waveguides due to droplets in the thin films and the surface roughness of the structured edges. To discriminate between damping due to droplets and the structured edges, damping measurements in the non-structured films and the structured waveguides are performed. Ridge waveguides with non-resonant damping losses smaller than 3 dB/cm are achieved. Due to the high repetition rate of the Yb:glass fiber laser, the manufacturing time for one waveguide has been decreased by a factor of more than 100 compared to earlier results achieved with the Ti:sapphire laser.     

Room-temperature diode-pumped Yb<Superscript>3+</Superscript>-doped LiYF<Subscript>4</Subscript> and KYF<Subscript>4</Subscript> lasers

We present a comparative analysis on the growth, the spectroscopic features, and the cw laser action of room-temperature Yb(5%):LiYF₄ (YLF) and Yb(10%):KYF₄ (KYF) crystals. Optical slope efficiencies of 33% and 52% have been demonstrated for Yb:YLF and Yb:KYF crystals, respectively. A remarkable wide wavelength tunability from 1.01 to 1.07 μm has been obtained for both laser crystals.     

Temperature dependence of photoconductivity and persistent photoconductivity of single ZnO nanowires

Photoelectrical properties of single ZnO nanowires have been investigated using photocurrent–voltage characteristics measurements varying with excitation photon energy and temperature. It is found that persistent photoconductivity (PPC) exists, and the PPC decreases with decreasing temperature. The temperature dependence of the PPC effect indicates that thermally activated return of electrons from shallow traps is responsible for the PPC phenomenon. The photosensitivity is found to be linear with the applied voltage, and it increases with decreasing temperature. A temperature dependence of photoconductivity gain was introduced to explain the experimental results.