A microchip-laser-pumped DFB-polymer-dye laser

A miniaturized, high repetition rate, picosecond all solid state photo-induced distributed feedback (DFB) polymer-dye laser is described by applying a passively Q-switched and frequency-doubled Cr⁴⁺:Nd³⁺:YAG-microchip laser (pulse width Δτ=540 ps, repetition rate ν=3 kHz, pump energy E_pump=0.15 μJ) as a pump source. A poly-methylmethacrylate film doped with rhodamine B dye serves as active medium. The DFB-laser pulses are temporally and spectrally characterized, and the stability of the thin polymer/dye film at high repetition rates is analyzed. The shortest DFB-laser pulses obtained have a duration of 11 ps. After the emission of 350000 pulses the intensity of the DFB-laser output has decreased by a factor of two and the pulse duration has increased by a factor of 1.2. For single DFB-laser pulses of 20-ps duration the spectral bandwidth is measured to be Δλ=0.03 nm, which is only 0.005 nm above the calculated Fourier limit assuming a Gaussian profile for the temporal shape of the pulses. Coarse wavelength tuning of the DFB laser between 590 and 619 nm is done by turning the prism. Additionally, a fine tuning of the DFB-polymer-laser wavelength is achieved by changing the temperature of the polymer/dye layer (=-0.05 nm/°C) in the range from 20 to 40 °C. Received: 1 March 2001 / Revised version: 23 May 2001 / Published online: 18 July 2001     

Mid-infrared laser-induced grating experiments of C2H4 and NH3 from 0.1–2 MPa and 300–800 K

Laser-induced thermal gratings (LITG) were generated in mixtures of ethylene and ammonia in nitrogen using mid-infrared laser radiation from a grating-tuned, low-pressure, pulsed (5 ms pulse width) CO₂ laser, and read out with a continuous wave Nd:YLF laser. The LITG signal intensity was measured as a function of pressure (0.1–2 MPa) and temperature (300–800 K, at 0.1 and 1 MPa) by tuning the laser to the accidental coincidences of the 10P(14) and 10R(6) emission lines with molecular absorption transitions of C₂H₄ and NH₃, respectively. Comparisons are made with theoretical predictions for the grating efficiency from a simple thermalization model. A theoretical comparison of the temporal LITG signal response for three excitation pulse shapes – a delta function, a realistic pulse, and a square wave is presented. Furthermore, it is shown that for NH₃, most of the decrease of the LITG signal intensity with increasing temperature is due to the corresponding decrease in fractional molecular absorption of the pump beam radiation. The diagnostic capabilities of the mid-infrared LITG experiment is demonstrated for spatially resolved ethylene measurements with long laser pulses in a premixed stoichiometric CH₄–air flame at atmospheric pressure. Received: 17 March 2000 / Revised version: 23 March 2000 / Published online: 13 September 2000     

Kinetics of photon generation and propagation in a scintillation layer

Experimental results of an investigation of the kinetics of photon generation and propagation in a scintillation layer (n-terphenyl in polystyrene) are presented in this paper for the case of relativistic μ-meson passage through the layer. The duration of the photon emission Δt_em has been measured as a function of the scintillation layer thickness (l=0.05–0.5 m). The total duration of the photon emission and exit from the scintillation layer Δt_tot measured with a side coating having diffuse light reflection coefficient ρ=0.9 and 0.95 is given. A kinetic model of the photon generation and propagation in the scintillation layer is described. Satisfactory agreement of the expected values of Δt_em and Δt_tot with the exeeriment is indicated. Ivanovo State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 73–77, June, 1999.     

Corrugated neat thin-film conjugated polymer distributed-feedback lasers

Wave-guided thin-film distributed-feedback (DFB) polymer lasers are fabricated by spin coating a PPV-derived semiconducting polymer, thianthrene-DOO-PPV, onto oxidised silicon wafers with corrugated second-order periodic gratings. The gratings are written by reactive ion beam etching. Laser action is achieved by transverse pumping with picosecond laser pulses (wavelength 347.15 nm, duration 35 ps). The DFB-laser surface emission and edge emission are analysed. Outside the grating region the polymer film is used for comparative wave-guided travelling wave laser (amplified spontaneous emission (ASE)) studies. The pump pulse threshold energy density for wave-guided DFB-laser action (4–9 μJ cm^-2) is found to be approximately a factor of two lower than the threshold for wave-guided travelling wave laser action. The spectral width of the DFB laser (down to Δλ_DFB≈0.07 nm) is considerably narrower than that of the travelling wave laser (Δλ_TWL≈14 nm). The DFB-laser emission is highly linearly polarised transverse to the grating axis (TE mode). Only at high pump pulse energy densities does an additional weak TM mode build up. The surface-emitted DFB-laser radiation has a low divergence along the grating direction. For both the DFB lasers and the travelling wave lasers, gain saturation occurs at high excitation energy densities. Received: 7 January 2002 / Revised version: 15 February 2002 / Published online: 14 March 2002     

High-pulse-energy high-efficiency mid-infrared generation based on KTA optical parametric oscillator

A high-energy, high-efficiency mid-infrared KTA OPO at 3.47 μm intracavity pumped by a Nd:YAG laser is presented. The maximum output energy is 31 mJ at the repetition rate of 10 Hz with a V-shaped cavity, corresponding to the absolute optical-to-optical conversion efficiency of 4.76% from the diode and the photon conversion efficiency of 87% from the fundamental to mid-infrared energy. The pulse width is 5.8 ns at the maximum output energy and the peak power reaches higher than 5 MW. The line width of the mid-infrared wave is about 1.1 nm (or 0.9 cm⁻¹ in wave number). The output energy demonstrates good stability. To our knowledge, these are the highest pulse energy and conversion efficiency of mid-infrared OPOs using bulk nonlinear crystals in the 3–5 μm range.     

Low-threshold mid-infrared optical parametric oscillation in periodically poled LiNbO3 synchronously pumped by a Ti:sapphire laser

We report the generation of tunable high-repetition-rate optical pulses in the mid-infrared using synchronously pumped parametric oscillation in periodically poled LiNbO₃ (PPLN). Using a Kerr-lens-mode-locked Ti:sapphire laser as the pump source and a PPLN crystal incorporating grating periods of 21.0–22.4 μm, we have achieved wavelength conversion in the -–4 6μm spectral range in the mid-infrared. The use of a semi-monolithic cavity design and hemispherical focusing has permitted pulse generation in the strong idler absorption region of PPLN, resulting in a simple, compact, all-solid-state configuration with a pump power threshold as low as 17 mW and mid-infrared idler powers of up to 64 mW at 9% extraction efficiency. Signal output powers of up to 280 mW at 35% extraction efficiency are available over the -–1.004 1.140μm spectral range at 80.5 MHz and pulse repetition rates at harmonics of the fundamental frequency up to 322 MHz have also been obtained. Received: 5 December 2000 / Revised version: 23 January 2001 / Published online: 27 April 2001     

The low temperature spectral weight transfer problem in Kondo insulators

In this paper we address the problem of the spectral weight transfer in Kondo insulators (KI). We employ the X-boson approach for the periodic Anderson model, in the U →∞ limit. We calculate the two energy gaps of the system analytically: the indirect gap, Δ_ind = E_g ≃ E_mir, present in the density of states, and the direct one Δ_dir, associated with the minimum energy necessary to produce inter-band transitions. We find that the optical behavior of the system is governed by two energy scales: one of low frequency, characterized by E_g ≃ E_mir, in the mid-infrared region (MIR), which is a reminiscent of the heavy fermion E_mir peak, that appears in Kondo insulators as a broad maximum in the MIR region and that controls the low temperature transport properties, the gap opening in optical conductivity and the formation of the Drude peak, at ω = 0, in the intermediate temperature range. The other energy scale appears at high frequencies, and is characterized by the direct gap Δ_dir. According to our results, this peak controls the anomalous redistribution of spectral weight in the optical conductivity. We apply the theory in order to study the Kondo insulator FeSi, and we calculate the optical conductivity of the system and the spectral weight transfer in the optical conductivity.     

A Planck-scale axion and SU(2) Yang–Mills dynamics: present acceleration and the fate of the photon

From the time of CMB decoupling onwards we investigate cosmological evolution subject to a strongly interacting SU(2) gauge theory of Yang–Mills scale, Λ ∼ 10^-4 eV (masquerading as the U(1)_Y factor of the SM at present). The viability of this postulate is discussed in view of cosmological and (astro-) particle physics bounds. The gauge theory is coupled to a spatially homogeneous and ultralight (Planck-scale) axion field. As first pointed out by Frieman et al., such an axion is a viable candidate for quintessence, i.e. dynamical dark energy, being associated with today’s cosmological acceleration. A prediction of an upper limit for Δt_mγ=0, the duration of the epoch stretching from the present to the point where the photon starts to be Meissner massive, is obtained: Δt_mγ=0∼2.2 billion years.     

Fe-site assignments in Y1Ba2Cu3O7 revisited

A new symmetric Fe³⁺ site (labelled E) (δ=0.28(2) mm/s and Δ=0.15 mm/s) is observed in the R₁Ba₂Cu₃O₇ family of superconductors for R=Y and Eu. The decay and growth of site E are established in two seemingly unrelated experiments, one as a function of room temperature aging for R=Y, and the other as a function of oxygen loading at elevated temperatures and pressures for R=Eu. In each experiment, the results show that site E is genetically related to the well documented chainsites A(Δ=1.9 mm/s), C(Δ=1.1 mm/s) and D(Δ=1.6 mm/s). We propose that site E represents a quasi-octahedral chain-site having pairs of O(1), O(4) and O(5) nearestneighbor oxygen sites. The two remaining doublets, B(Δ≈0.4 mm/s) and B′(Δ=0.78 mm/s) represent Fe³⁺ dopant sites present in the all-important CuO₂ planes, possessing coordination numbers of 6 and 5, respectively.     

Weak-pulse transparency enhancement in an optically dense three-level medium induced by a 2π pulse in a neighboring transition (V-scheme)

The coherent V-configuration interaction between an optically dense resonantly-absorbing three-level medium (neon) and two ultrashort superradiance pulses with converging wave fronts is investigated experimentally and theoretically. Both separate and combined propagation of pulses with wavelengths λ ₁=614.3 nm (strong field, θ ₁⩾π) and λ ₃=594.5 nm (weak field, θ ₃≈/20) are studied. For propagation of a separate strong-field pulse, supertransparency of the absorbing medium was observed, which is associated with the generation of a soliton-like pulse at the difference frequency (Δν≈1700 MHz) and the dispersion-diffraction stabilization effect. Under these conditions a weak-field pulse is completely absorbed. Combined propagation of the pulses leads to novel effects. A below-threshold pulse (weak field) was observed to pass through the absorber while interacting coherently with a strong-field pulse at a neighboring transition. It is shown theoretically that absorption of the weak pulse is reduced for two reasons: first, as a result of incoherent transparency of the resonance transition caused by emptying of the lower level by the field of the strong pulse, and second, as a result of coherent transfer of polarization between the upper levels via the two-photon processes. When the conditions for combined propagation are met, the latter mechanism ensures inversionless amplification of a weak pulse over a wide band of frequencies. In this case, the gain can even exceed the linear absorption coefficient in absolute value. A difference in propagation velocities of the weak and strong pulses was recorded experimentally, along with a shift in the carrier frequency of the weak field towards the red (≈600 MHz). A mechanism for transfer of phase modulation from a strong pulse to a weak pulse via the common lower level is discussed theoretically. Zh. éksp. Teor. Fiz. 113, 71–88 (January 1998)     

Tunable mid-infrared,high-energy femtosecond laser source for glyco-protein structure analysis

We have developed a 6–12 μm mid-infrared (MIR) femtosecond laser source for glyco-protein structure analysis. The MIR femtosecond laser pulses are generated by a differential frequency generation (DFG) configuration with a combination of Ti:sapphire based regeneratively amplified femtosecond laser pulses (780 nm, 160 fs, 1 mJ) and a β-BaB₂O₄ (BBO) based optical parametric amplifier (OPA). The MIR pulse energy exceeds 4.5 μJ, where a glyco-protein molecule has resonant absorption lines due to the vibrational–rotational transitions. The pulse width is estimated to be less than 1 ps according to the cross correlation measurement between the two OPA output pulses. Using the MIR femtosecond laser pulses, we demonstrated photo-dissociation of the sialyl Lewis X (sLeX) proton added ion, which is the first time to the best of our knowledge. PACS 42.65.Re; 42.62.-b; 42.60.-b; 42.65.-k; 87.50     

Electromagnetic pulse amplification in a Cherenkov laser

An investigation is made of the amplification of a Gaussian electromagnetic pulse in a Cherenkov waveguide laser for the cases of long and short waveguides. It is shown that in the first case, the concept of a characteristic pulse duration τ ₀ can be introduced. It is established that when the pulse duration is short (τ<τ ₀) the gain is determined only by its spectral width, and the amplification process leads to a change in the pulse envelope. It is shown that in a short waveguide pulse amplification can be achieved without any change in shape. Zh. Tekh. Fiz. 69, 79–83 (April 1999)     

Mid-infrared ZGP OPO pumped by near-degenerate narrowband type-I PPKTP parametric oscillator

The total pulse energy of the signal and idler in a near-degenerate type-I periodically poled KTiOPO₄ (PPKTP) optical parametrical oscillator (OPO) was spectrally confined within a 2 nm spectral bandwidth at 2.13 μm. This was achieved by using a volume Bragg grating as the output coupler. Both the signal and the idler from the PPKTP OPO were then simultaneously used to pump a mid-infrared ZnGeP₂ (ZGP) OPO. The 2 nm bandwidth was narrower than the ZGP crystal acceptance bandwidth and, thus, made efficient conversion in the second OPO possible. A total slope efficiency of 10% from 1.06 μm to the 3.5–5 μm region was demonstrated, generating 250 μJ in the mid-IR with only 3.6 mJ of 1.06 μm pump energy. This corresponds to a Nd:YAG pump to mid-IR conversion efficiency of 7%. PACS 42.65.Yj; 42.72.Ai; 42.40.Eq     

Generation of broadly tunable picosecond mid-infrared laser and sensitive detection of a mid-infrared signal by parametric frequency up-conversion in MgO:LiNbO3 optical parametric amplifiers

Picosecond optical parametric generation and amplification in the near-infrared region within 1.361-1.656 μm and the mid-infrared region within 2.976-4.875 μm is constructed on the basis of bulk MgO：LiNbO 3 crystals pumped at 1.064 μm.The maximum pulse energy reaches 1.3 mJ at 1.464 μm and 0.47 mJ at 3.894 μm,corresponding to a pumpto-idler photon conversion efficiency of 25%.By seeding the hard-to-measure mid-infrared radiation as the idler in the optical parametric amplification and measuring the amplified and frequency up-converted signal in the near-infrared or even visible region,one can measure very week mid-infrared radiation with ordinary detectors,which are insensitive to mid-infrared radiation,with a very high gain.A maximum gain factor of about 7 脳 10 7 is achieved at the mid-infrared wavelength of 3.374 μm and the corresponding energy detection limit is as low as about 390 aJ per pulse.     

Generating power of a thermoelectric generator under periodically alternating temperature gradients

The thermo-emf ΔV and thermoelectric current ΔI generated by imposing a temperature gradient alternating at a period of T on a thermoelectric (TE) generator were measured as a function of t, where t is the lapsed time and 1/T was varied from 0 to 1/30 s^-1. A TE generator was sandwiched between two Peltier modules connected in series. The alternating temperature gradients were produced by imposing an alternating voltage V on two Peltier modules, where V was varied from 1.0 to 3.7 V. Both ΔV and ΔI generated by the TE generator oscillate at a period of T but their amplitudes tend to increase monotonically with an increase of V. The effective thermo-emf ΔV_eff and current ΔI_eff calculated from ΔV and ΔI increase abruptly with an increase of 1/T and have a local maximum at 1/T=1/120 or 1/240 s^-1. The generating power ΔW_eff(=ΔV_effΔI_eff) tends to increase proportionally with an increase of input power W_input, owing to the increase in the temperature difference. The rate of ΔW_eff to W_input at 1/T=1/240 s^-1 reached approximately 3.2 times as large as that obtained for the steady temperature gradient corresponding to 1/T=0 s^-1. It was thus found that the generating power of the TE generator operating under the temperature gradient alternating at an optimum period is remarkably increased compared to that of a TE generator working under a conventional steady temperature gradient. PACS 72.15.Jf; 84.60.Rb; 85.30De     

Synthesis and Characterization of New Fluorene-Based Singlet Oxygen Sensitizers

The synthesis, photophysical characterization, and determination of singlet oxygen quantum yields (Φ_Δ) for a class of fluorene derivatives with potential application in two-photon photodynamic therapy (PDT) is reported. It has been demonstrated that these compounds possess the ability to generate singlet oxygen (¹O₂) upon excitation. A photochemical method, using 1,3-diphenylisobenzofuran (DPBF) as ¹O₂ chemical quencher, was employed to determine the singlet oxygen quantum yields (Φ_Δ) of the fluorene-based photosensitizers in ethanol. Φ_Δ values ranged from 0.35 to 0.75. These derivatives may have potential application as two-photon photosensitizers when pumped via two-photon excitation in the near-IR spectral region.     

Control of material removal of fused silica with single pulses of few optical cycles to sub-picosecond duration

Surface ablation of a dielectric material (fused silica) by single femtosecond pulses is studied as a function of pulse duration (7–450 fs) and applied fluence (F _th<F<10F _th). We show that varying the pulse duration gives access to high selectivity (with resolution ∼10 nm) for axial removal of matter but does not influence the transverse ablation selectivity, which only depends on the normalized applied fluence F/F _th. The ablation efficiency is shown to be inversely dependent on the pulse duration and saturates with respect to the applied fluence earlier at ultra-short pulse durations (≤30 fs). The deduced optimal fluence F _opt corresponding to the highest ablation efficiency for each pulse width defines two regimes of laser application. Below F _opt, the removed material depth can be accurately adjusted in a large range (∼40–200 nm) as a function of the applied fluence and the morphology of the ablated pattern almost reproduces the Gaussian beam distribution. Above F _opt, the material removal depth tends to saturate and the morphology of the ablated pattern evolves to a top-hat distribution. The coupled evolution of depth and morphology is related to the dynamics of formation of dense plasma at the surface of the material, acting as an ultra-fast optical shutter.     

Mass shift and width broadening of ρ-mesons produced in heavy ion collisions

The mass shift Δm_ρ and width broadening ΔΓ_ρ of ρ-mesons produced in heavy ion collisions is estimated using general formulae which relate the in-medium mass shift of a particle to the real part of the forward scattering amplitude Re f(E) of this particle on constituents of the medium and ΔΓ to the corresponding cross section. It is found that the mass increases by some tens of MeV but, more importantly, the width becomes large, increasing by several hundred MeV at beam energies of a few GeV·A and by twice that amount at beam energies of about a hundred GeV·A. Received: 28 July 1998     

Microwave radiation of the atmosphere induced by a pulsed gamma source

The model problem of the perturbation of the atmosphere by an isotropic gamma source (pulse width Δτ ≤ 100 ns) situated at various altitudes (0 ≤ h ≤ 100 km) and assumed to be a “point” source was solved. The action of emitted gamma quanta caused the formation of a spatial region in the atmosphere containing highly excited atoms and molecules, the emission from which was recorded over a certain microwave range (0.8–1.0 GHz). The amplitudes of the electric component intensity of the field of noise Rydberg radiation over the range specified were calculated. Rydberg radiation duration, type, and the degree of polarization were estimated. The shape of the emission line and the character of broadening of signals received by two receivers situated at an altitude of H ₁ = 20000 km and on the surface of the Earth (H ₁ = 0 km) were analyzed. During measurements, both receivers were situated on the axis perpendicular to the surface of the Earth.     

The effect of strong electrical field on the conductivity of proton solid electrolytes NaHSO4 and KHSO4

The influence of passing of high voltage pulse discharges through proton solid electrolytes NaHSO₄ and KHSO₄ and corresponding melts on their conductivities has been investigated. We have discovered the phenomenon of high voltage activation of the investigated electrolytes which is seen by a relative increase of the conductivity. This relative conductivity increase Δσ_rel reaches 754 % (at U=2.8 kV and 181°C) and 218 % (at U=2.0 kV and 200°C) for the solid NaHSO₄ and KHSO₄, respectively. For molten NaHSO₄ and KHSO₄ Δσ_rel is considerably higher and reaches 613% (at U=1.4 kV and 207°C) and 572 % (at U=2.2 kV and 232°C), respectively. We established the “memory” effect which was seen in maintaining of the excess conductivity for a long period of time. The process of relaxation of the excess conductivity of these electrolytes has been studied. The relaxation times of the non-equilibrium charge carriers in these electrolytes varies within the range of (1.9–7.8).10⁴s.