Harmonic generation in the discrete spectral region of xenon using broadband femtosecond laser pulses

The conversion of 34-fs Ti:sapphire laser pulses into the wavelength region 105–210 nm has been studied in xenon for laser intensities up to 5×10¹³ W/cm². A strongly structured, pressure-dependent emission spectrum is observed. Radiation is detected in regions expected for the 5th and 7th harmonics but also in regions in between. In the resonance region (<147 nm), self-phase-modulation processes in the conversion medium together with phase-matched, efficient harmonic generation in negative-dispersive spectral regions explain the observations. Broadband emission is detected in the resonance-free, positive-dispersive spectral region 155–210 nm. Emission from the xenon dimer is superimposed on a direct 5th-harmonic signal. PACS 32.70.-n; 32.80.-t; 42.65.Ky; 42.65.Re     

Inspection of periodically poled waveguide devices by confocal luminescence microscopy

Using confocal defect-luminescence microscopy, we investigated Er³⁺-doped periodically-poled LiNbO₃ waveguide devices and found that the Er³⁺ ion and vibrational Raman modes can be used effectively as probes to inspect and characterize the devices with high spatial resolution, non-destructively, and free of topographical artifacts. The technique is based on scanning over the sample, while simultaneously measuring and evaluating the emission or Raman spectra in terms of their moments (i.e., area, average emission wavelength, and spectral width). The contrast is observed even in tempered samples because of differences in intrinsic electric (strain) fields and defect arrangements that occur in regions inside and outside the waveguide as well as in regions in which domains have been inverted, or which are as grown. Evaluating different parts of the Er³⁺ spectra and/or using different moments, different aspects of the device can be highlighted and regions inside and outside the waveguide, in which domains have been inverted or not inverted, as well as domain walls, can be identified. Raman spectroscopy is less sensitive, but it still can identify the domain wall regions. PACS 42.64.Wi; 42.65.Yj; 77.84.Dy     

High-harmonic generation and plasma radiation from water microdroplets

This work studied the emission of XUV radiation from water microdroplets under excitation with either a single or a pair of intense femtosecond laser pulses (Ti:Sa, 80 fs, 10¹⁴ W/cm² , 800 nm, 1 kHz). When varying the delay between the two pulses a transition from pure incoherent plasma emission to coherent high-harmonic generation was observed. Under optimized conditions high-harmonic radiation up to the 27th order was obtained. PACS 33.80.-b; 39.10.+j; 42.65.ky; 52.50.Jm     

Carrier concentration dependence of optical Kerr nonlinearity in indium tin oxide films

Optical Kerr nonlinearity (n₂) in n-type indium tin oxide (ITO) films coated on glass substrates has been measured using Z-scans with 200-fs laser pulses at wavelengths ranging from 720 to 780 nm. The magnitudes of the measured nonlinearity in the ITO films were found to be dependent on the carrier concentration with a maximum n₂-value of 4.1×10^-5 cm²/GW at 720-nm wavelength and an electron density of N_d=5.8×10²⁰ cm^-3. The Kerr nonlinearity was also observed to be varied with the laser wavelength. By employing a femtosecond time-resolved optical Kerr effect (OKE) technique, the relaxation time of OKE in the ITO films is determined to be ∼1 ps. These findings suggest that the Kerr nonlinearity in ITO can be tailored by controlling the carrier concentration, which should be highly desirable in optoelectronic devices for ultrafast all-optical switching. PACS 42.65.An; 42.65.Hw; 78.40.Fy     

Characterization of second and third order optical nonlinearities of ZnO sputtered films

We measured the second and third order optical nonlinearity of zinc oxide, grown on glass substrates by the ion beam sputtering technique. Second and third harmonic generation measurements were performed by means of the rotational Maker fringes technique for different polarization configurations, thus allowing the determination of all non-zero components of the second order susceptibility at three different fundamental beam wavelengths, i.e., 1064 nm, 1542 nm and 1907 nm. The dispersion of the nonlinear optical coefficients has been evaluated, while the nonlinear optical coefficients were found to range between 0.9 pm/V and 0.16 pm/V for d₃₃, 0.53 pm/V and 0.08 pm/V for |d₁₅|, 0.31 and 0.08 pm/V for |d₃₁|, with increasing wavelength. Finally, one third order susceptibility, χ_ijkl ⁽³⁾, has been determined by third harmonic generation measurements at a fundamental wavelength λ=1907 nm and a value for χ₃₃₃₃ ⁽³⁾ of 185×10^-20 m²/V² has been found. PACS 42.65.An; 42.65.Ky; 42.70.Nq     

Two-photon-absorption of frequency converter crystals at 248 nm

The two-photon-absorption coefficient of KDP, BBO, LTB, and CLBO crystals has been determined from the measurement of the intensity dependent transmission through long samples. The intensity of the sub-picosecond KrF excimer laser pulses on the samples was varied from 0.2–80 GW/cm². The linear absorption of the samples was determined by using a low intensity, long pulse KrF laser. The first-principle simulations to the experimental data show a TPA value of 0.48 cm/GW for KDP, 0.5 cm/GW (o-ray) and 0.34 cm/GW (e-ray) in BBO, 0.22 cm/GW in LTB and 0.53 cm/GW in CLBO. PACS 78.20.Ci; 42.65.Ky; 42.65.Yj     

Generation of attosecond pulses with ellipticity-modulated fundamental

We have studied experimentally and theoretically high-order harmonic generation using a laser field with a time-dependent ellipticity. We show that the harmonic emission can be confined into a narrow temporal window, in which the fundamental polarization is quasi-linear. This allows a single attosecond pulse (200 as) with a fundamental field obtained from 10 fs pulse to be generated. PACS 42.65.Ky; 42.65.Re; 32.80.Wr     

Modified GaSe crystal as a parametric frequency converter

Physical and optical properties of GaSe, doped with indium and sulphur has been discussed. Generation of tunable mid-infrared radiation by the technique of second harmonic generation (SHG) of tunable CO₂ laser radiation has been demonstrated. Conversion efficiency of SHG in GaSe:In and GaSe:S has been compared. PACS 42.65.k; 42.65.ky     

All-solid-state continuous-wave yellow laser based on intracavity frequency-doubled self-Raman laser action

We report continuous-wave yellow emission from a compact Nd:YVO₄ self-Raman laser with intra-cavity frequency-doubling in LBO, pumped by a 4.5 W high-brightness diode laser. A maximum yellow output of 140 mW was observed with an overall optical (diode-to-yellow) conversion efficiency of 4.4%, and with a high beam quality (M²∼1.2). A variety of different resonator configurations were investigated in order to achieve low threshold, highest output powers and efficiency, and to investigate amplitude stability. PACS 42.55.Xi; 42.55.Ye; 42.65.Ky     

Accurate time delay determination for femtosecond X-ray diffraction experiments

Intensity correlations and noise reduction are observed and characterized in the broadband supercontinuum generated by spatio-temporal solitons propagating in air, i.e., in filamentation of ultrashort laser pulses. Large correlations and reduction of the laser noise are observed already at the first steps of the filamentation process, while further propagation results in cascaded χ⁽³⁾ broadening processes and yield complex correlation maps. The spectral range yielding an optimal laser noise reduction of 3.6 dB is found to cover 10 nm around the fundamental wavelength. PACS 42.65.Jx; 42.65.Tg; 42.65.Ky; 42.50.Lc     

Quasi-phase matching in LiNbO3 using nonlinear coefficients in the XY plane

We report quasi-phase-matched second-harmonic generation in periodically poled lithium niobate (PPLN), where both fundamental and second-harmonic waves are ordinary waves. It provides a lower-limit value for d₂₂ of 1.1 pm/V. The measured temperature and wavelength bandwidth of the second-harmonic signal are in good agreement with the theoretical predictions. Since the d₂₂=d_YYY nonlinear coefficient of LiNbO₃ changes its sign as a result of electric field periodic poling along the Z direction, we deduce that all tensor components of the second-order susceptibility χ⁽²⁾ of trigonal 3m crystals are reversed, thereby expanding the quasi-phase-matching possibilities in these crystals. Furthermore, it enables the realization of all-optical processes based on the nonlinear coefficients in the XY plane, such as all-optical polarization rotation in PPLN, as well as multipartite entanglement experiments based on simultaneous phase matching using different elements of χ⁽²⁾ in a single LiNbO₃ crystal. PACS 42.65.-k; 42.65.Ky; 42.65.Lm     

Spectroscopy and self-frequency doubling of the 4F3/2?→?4I13/2 laser channel in the La2CaB10O19:Nd3+ bi-functional crystal

Nd_xLa_2-xCaB₁₀O₁₉ crystals with different Nd³⁺ concentrations were grown by the high-temperature flux method and oriented for frequency doubling of 1340 nm wavelength radiation. The room temperature spectroscopy (absorption, emission and lifetimes) of the ⁴ F _3/2 → ⁴ I _13/2 Nd³⁺ laser channel was investigated. We have clearly shown that two inequivalent Nd³⁺ centers in the La³⁺ and Ca²⁺ positions are responsible for the spectroscopic properties. Self-frequency doubling can be obtained from these two centers at 661.1 and 671.5 nm. PACS 42.55.f; 42.65.Ky     

Electrically actuated elastomers for electro–optical modulators

We demonstrate the technique of cross-polarized wave (XPW) generation at 1 μm with a 310-fs optical parametric chirped-pulse amplification system at the millijoule level. We show an improvement of the temporal contrast by three orders of magnitude with an energy transmission of 22%. Additionally, we report that XPW generation preserves the beam spatial quality and shortens the pulse duration by a factor of 3^1/2, resulting in a peak power transmission of ∼35%. PACS 42.65.Re; 42.65.Yj; 42.70.Mp     

Colour emission tunability in Ho3+–Tm3+–Yb3+ co-doped Y2O3 upconverted phosphor

The frequency upconversion (UC) emission throughout the visible region from the Y₂O₃:Ho³⁺?CTm³⁺?CYb³⁺ co-doped phosphors synthesized by using low temperature combustion process upon excitation with a diode laser operating at 980?nm have been presented. The colour emission tunability in co-doped phosphor has been observed on increasing the pump power and seen by the naked eyes. The tunability in colour emission has also been visualized by CIE chromaticity diagram. The variation in UC emission intensity of the ¹G₄????³H₆ (Tm³⁺) and ⁵F₃????⁵I₈ (Ho³⁺) transitions lying in the blue region has been monitored with increase in the pump power and marked that their ratio can be used to determine the temperature. The developed phosphor has been used to record fingerprints. The observed most intense visible colour emission from the developed material may be used for photodynamic therapy and as an alternative of traditional fluorescent biolabels.     

Giant nonlinear optical response of nanoporous anatase layers

A giant nonlinear optical response has been observed for nanoporous layers of titanium dioxide (anatase) under picosecond laser excitation with photon energy below the gap. At excitation intensity of 10 MW/cm² the nonlinear refractive index variation at the wavelength of 1064 nm corresponds to ⁽³⁾=2×10^-5 esu which is six orders of magnitude higher than the respective value of bulk TiO₂. This effect is explained by resonant excitation of electronic states of defects at the developed surface of anatase nanoparticles. PACS 42.65.-k; 73.20.At; 78.67.Bf; 81.07.Bc     

Light-induced absorption in reduced congruent and near-stoichiometric Er:LiNbO3 crystals

Light-induced absorption (LIA) in thermally reduced congruent and near-stoichiometric Er:LiNbO₃ crystals, which have different cuts of crystal (X- and Y-cut) and different doping levels of Er³⁺ (0.2, 0.4, 0.6 and 1.0 mol %), has been studied within continuous-wave light intensity by using polarized 632.8 nm beam as probe light and polarized 488 nm beam as pump light. Effects of thermal reduction condition, pump intensity, Li/Nb ratio and Er-doping level on the LIA are summarized and discussed. Studies on spectral characteristics of both linear absorption and light-induced absorption show that photo-ionization of bipolarons into small polarons plays a predominant role in the LIA process. The role of the Er³⁺ ion in the LIA is investigated by studying the doping level and the thermal reduction effects on the absorption and emission properties of Er³⁺. The results show the possibility that the Er³⁺ ion directly participates in the light-induced charge transport is small. It affects the LIA only via the bypassing part of pump intensity to excite near-infrared emissions of Er³⁺. The thermaleffect on the LIA is demonstrated. A two-center model with inclusion of the thermal contribution can qualitatively explain two major LIA characteristics that include the appearance of a slow stage in the LIA procedure and the unsaturation behavior of the LIA coefficient in high pump intensity regime. In addition, the pump depletion effect on the LIA characteristics is discussed. The possibility of pump beam fanning arising from light-induced scattering is ruled out experimentally. PACS 42.70.Hj; 42.70.Ln; 42.70.Mp; 42.65.Hw; 81.05.-t     

Yellow nanosecond sum-frequency generating optical parametric oscillator using periodically poled LiNbO<Subscript>3</Subscript>

Nanosecond yellow light has been generated through simultaneously phase matched sum-frequency generation and optical parametric oscillation in a periodically poled LiNbO₃ crystal. 300 mW of yellow light at a wavelength of 586 nm has been generated from 1.3 W of laser power from a Q-switched Yb:YAG laser operating at 1031 nm. The conversion efficiency of the device is 23%. PACS 42.65.Ky; 42.65.Yj     

All-optical stabilization of carrier-envelope phase by use of difference frequency generation with seeded amplification of colored conical emission

We stabilized the carrier-envelope phase of pulses emitted by a femtosecond regenerative amplifier through difference frequency generation between pump and seeded amplification of colored conical emission. Seeded amplification of colored conical emission was induced by modulational instability in the second harmonic generation with a supercontinuum injected and amplified. As a consequence, it inherited the origin phase of the pump pulse. After difference frequency with the pump pulses, the generated tunable idler pulses were carrier-envelope phase stabilized, which was verified with a simple and robust spectral interference setup. PACS 42.65.Re; 42.65.Yj; 42.25.Kb     

Optical investigation of the emission lines for Eu and Tb ions in the GaN powder host

Gallium nitride (GaN) doped by Eu³⁺ and Tb³⁺ ions have been synthesized using powder technology. The emission and absorption spectra have been obtained by using photoluminesence technique and correlated with the local structural environments. The room temperature yellow emission from GaN grains as well as from Eu³⁺ and Tb³⁺ ions has been observed for nano- as well as for microGaN grains. Additionally, for GaN:1%Eu³ micrograins the blue emission from GaN nanocrystals has been observed.     

Dual mode emission and harmonic generation in ZnO-CaO-Al2O3: Er nano-composite

Er³⁺ doped ZnO-CaO-Al₂O₃ nano-composite phosphor has been synthesized through combustion method and its emission and harmonic generation properties have been studied. The X-ray diffraction and thermal analysis techniques have been used to prove the dual phase (ZnO and CaO-Al₂O₃) nature of the phosphor. The phosphor has shown up-conversion emission on near-infra-red (976 nm) excitation and down-conversion emission on 355 nm excitation in presence of Er³⁺ and thus behaves as a dual mode phosphor. On excitation with 976 nm diode laser, material shows color tunability (calcination of composite material at different temperatures). Formation of ZnO nanocrystals on heat treatment of as-synthesized sample has shown its characteristic emission at 388 nm and also the energy transfer from ZnO to Er³⁺ ions. The low temperature emission measurements have been carried out and the results have been discussed. Phosphor has shown strong second harmonic generation (SHG) at 532 nm on 1064 nm and at 266 nm on 532 nm excitation.