Synthesis and spectroscopic studies of 4-Formyl-4′-N,N-dimethylamino-1,1′-biphenyl: The unusual red edge effect and efficient laser generation

The synthesis and photophysics of 4-formyl-4-N,N-dimethylamino-1,1-biphenyl are reported. The emission spectrum in various solvent polarities demonstrates solvatochromism, indicating that the fluorescence originates from an electronically excited species with a strong charge transfer character. The change in [ _max(absorption) – _max(emission)] varies from 1500 cm^–1 inn-heptane to as much as 7500 cm^–1 in acetonitrile. In protic solvents, the unusual excitation energy-dependent steady-state emission (red edge effect), resulting from solvent dielectric relaxation, was observed in media with a low viscosity. The large Stokes-shifted and high-yield fluorescence led to the observation of the efficient lasing action. The frequency tunability of the laser output is strongly solvent dependent, generating a new charge transfer laser dye in the blue-green region.     

A tunable light source in the 370 nm range based on an optically stabilized,frequency-doubled semiconductor laser

A tunable harmonic output power of 18 W at a wavelength of =370 nm is obtained by resonance-enhanced frequency doubling of an optically-stabilized semiconductor laser. A commercially available AlGaAs laser diode which emits a maximum power of 10 mW at =740 nm is operated in an extended-cavity configuration. Dispersion prisms are used in the extended cavity to obtain longitudinal-mode selection with low loss of optical power. The output is focussed into an optically isolated high-finesse ring resonator which contains a LiIO₃ crystal for second-harmonic generation. One potential application of this laser source is the optical excitation and laser cooling of ytterbium in an ion trap. In a related demonstration experiment, the frequency-doubled diode laser is applied to excite the =369.5 nm ² S _1/2-² P _1/2 transition of ytterbium ions in a hollow-cathode discharge.     

Initial performance of a CW chemical laser

The performance of a continuous HF chemical laser is presented in this paper. Population inversion was obtained by diffusion of H₂ into a supersonic jet containing F atoms [H₂+F HF(v)+H₁ H=–31.7 kcal/mole]. A peak power of 630 W was obtained with an F atom flow rate of 0.040 mole/sec, and the efficiency of conversion of chemical energy to laser energy was 12%. The performance of a corresponding DF laser is also given. Major laser output is from 2-1 and 1-0 transitions for both lasers. Radiation is at 2.6 to 2.9m and 3.6 to 4.0m for the HF and DF lasers, respectively. The ratio of DF to HF laser power is 0.7 under similar flow conditions.Work conducted under US Air Force Contract F04701-69-C-0066.     

Gain of a CW optically pumped far-infrared laser with sandwich resonator

The FIR laser gain of CW optically pumped HCOOH was measured and compared to theoretical calculations. Since the laser has a transversely pumped parallel-plate (or sandwich) waveguide, the pump field is the same for oscillating and amplifying operation. Therefore, true FIR-gains were measured. The theory takes into account several phenomena, that are frequently simplified when considering FIR lasers, such as the pump and the FIR beam's spatial distribution, power broadening of the pump absorption, as well as heat and molecular diffusion. As a result, good quantitative agreement between theory and measurement was achieved.     

Measurement of Optical Constants of CVD-Diamond for Short-Wavelength Far-Infrared Lasers

A diagnostic system using short-wavelength far-infrared (FIR) lasers (40–70 m in wavelength) is now being developed for high density and large volume plasmas. In the wavelength region, a CVD-diamond is the excellent materials for optical windows of the laser and the plasma vessel and beam splitters of a multichannel interferometer. To design these optical elements, the optical constants (refractive index n, absorption coefficient and transmissivity T) of the CVD-diamond have been measured precisely by using FIR lasers of 48-, 57- and 71-m in wavelength. As an example, the result for 57.1511 m light is n = 2.383(1) ± 0.002, = 0.19 ± 0.05 cm^-1 and T = 97.5 ± 1.5% at 1.023 mm in thickness.     

Time-resolved UV absorption of polyimide

The 355-nm transient absorption of polyimide thin films has been measured following excitation withsubablative, 24-ps long, 355-nm laser pulses. The 355-nm absorption increases by 25% following 355-nm, 20 mJ/cm² excitation and recovers with a fast time constant 34 ps, and a slow time constant which is much longer than 6 ns. The data are fitted by a three-level rate equation model incorporating the temperature dependence of the ground state absorption coefficient. The fast component is attributed to the decay ofS ₁ and the slow component results from increased ground state absorption caused by a laser-induced temperature rise. The nonlinear intensity dependence is attributed to excited state (S ₁) absorption. These results indicate the importance of considering the dynamic absorption in modelling ablation.     

Ultra-short efficient laser-driven hard X-ray source operated at a kHz repetition rate

Ultra-short bursts of hard X-ray radiation are generated by interaction of high-contrast femtosecond laser pulses with a jet of liquid Ga. The X-ray emission shows a strong dependence on the angle of incidence and polarization of the laser beam, consistent with the processes of resonant absorption and vacuum heating. As much as 60% of the total X-ray emission consists of [_Ga]K radiation (9.22–9.25 keV) with a photon flux of 6×10⁹ photons/(ssrad). Using this novel X-ray source, static diffraction patterns from a GaAs(111) crystal were recorded with an acquisition time of 2 s. PACS 52.38.Ph; 52.59.Px; 52.50.Jm     

A diode-laser pump source with small focus diameter for end- pumped systems

A pump source with eight angular multiplexed laser diodes is built. The diode laser emission is collimated and focused using relatively inexpensive aspherical lenses. The maximum output power is 6.3W at 970nm. The overall transmittance of the beam shaping optics is T=85%. The electrical, temporal and spectral properties are studied. The variances of the spatial intensity distribution are r0x=99m and r0y=85m. The beam propagation factors are M2x=38 and M2y=112. The high intensity in the focus makes this system ideal for end pumping of solid state lasers.     

The absorption of pulsed CO2-laser radiation by ethylene at total pressures from 25 to 3000 Torr

The absorption of pulsed CO₂-laser radiation by ethylene has been measured at total pressures from 25 to 3000 Torr, using the P(12) and P(14) lines in the 10.6 m band, with incident fluences from 0.1 to 0.7 J/cm². Marked deviations from the Beer-Lambert absorption law were observed, with the effective absorption coefficient varying with pressure, fluence, absorption path-length and the addition of non-absorbing gas. Pressure broadening of the rotational lines of the ethylene absorption spectrum was shown to be the major cause of these deviations, together with lesser effects which can be attributed to the rise in temperature of the absorbing gas during the laser pulse.     

Tunable CW laser oscillation of NdP5O14 at 1.3 μm

Low-threshold tunable cw laser oscillation is achieved at the⁴ F _3/2⁴ I _13/2 transition of NdP₅O₁₄. Continuous tunign over 1.8% of the central wavelenght is possible with 55 mW of absorbed pump power. At gain line maximum near 1.32 , the output power exceeded 1 mW. Pump threshold as low as 8 mW was observed. Reliable cw operation of the 1.3 m NdP₅O₁₄ laser pumped by a 15 mW AlGaAs laser diode is demonstrated.     

Millimeter wave propagation through a chiral medium model of human trunk

That millimeter wave propagation through a chiral medium of human trunk has been discussed by obtaining the electromagnetic filed, absorbent power, specific absorption rate, temperature field and their distribution in a human trunk model with plane strati calate homogeneous tissues under a normal incidence plane wave. The chiral medium is described electromagnetically by the constitutive relationsD=E+B andB=H+E. The constants, and ate real and have values that are fixed by the size, the shape, and the spatial distribution of the elements that collectively compose the medium. Also, the principle of thermal therapeutics for millimeter wave is discussed preliminaryly.     

A new method for producing point holograms

A new method is proposed for producing point holograms by utilising highly correlated, extended sources rather than point-like sources in the recording process. The theory of this method is presented. Its feasibility is demonstrated experimentally. A multiple imaging experiment is described which utilises this kind of hologram. Advantages and limitations of holograms of extended sources are discussed.List of Symbols R complex amplitude of the reference wavefront in the hologram plane - S complex amplitude of the signal wavefront in the hologram plane - D complex amplitude of the reconstructing wavefront in the hologram plane - T complex amplitude of the reconstructed wavefront in the hologram plane - r complex amplitude of the wavefront illuminating the reference transparency - s complex amplitude of the wavefront illuminating the reference transparency - d complex amplitude of the wavefront illuminating the reference transparency - t complex amplitude of the wavefront illuminating the reference transparency - p amplitude transmittance of the reference transparency - amplitude transmittance of the signal transparency - (-function) amplitude transmittance of the reconstructing transparency - amplitude transmittance of the reconstructed transparency - P _r ,P _s propagation functions at distancesz _r ,z _s , respectively (equation 3) - x, y co-ordinates in the source plane - , co-ordinates in the hologram plane - z _r ,z _s distances of the reference, signal source from the hologram plane - X _r ,y _r co-ordinates of the reconstructing source (-function) - a, h shift co-ordinates in the autocorrelation integral (equation 12) - f _r,f _s Fourier transform operator at distances z_r,z _s (equation 7a) wavelength - k=2/ wavenumber - * conjugate complex - (*) convolution     

74% Slope efficiency from a diode-pumped Yb3+:LiNbO3:MgO laser crystal

An optimization of the laser action performance from a diode-pumped Yb³⁺-doped LiNbO₃:MgO crystal has been carried out. In this sense, efficient laser action at 1.06 m when pumping with a fiber-coupled laser diode at 980 nm has been demonstrated, achieving laser slope efficiencies as high as 74%. The influence of output mirror transmittance on both pumping threshold and laser slope efficiency has been investigated, and the parameters of relevance in laser dynamics (emission cross section and optical losses) have been determined. Under the experimental conditions leading to maximum slope efficiency, the pump power at threshold was 300mW, and the pump-to-laser conversion efficiency was 40%. PACS 42.55.Xi; 42.55.Rz; 42.60.Lh     

Millimeterwave spectra of some superionic conductors

Using a quasi-optical continuously tunable spectrometer in the frequency range of 40–90 GHz (=7.5-3.3 mm) the absorption spectra of the superionic conductors Na-alumina, Ag-alumina,-AgI and CuTeBr have been measured at various temperatures. In the measured spectral range all the materials showed a monotonic increase in the absorption as a function of frequency. These results are discussed in the light of existing theoretical models.Alfred P. Sloan Foundation Fellow     

Study of the Influence of the Excitation Laser Power Density on the Intermediate-Field Electric Modulation Component of the GaAs Photoreflection E 0-Signal

The influence of the excitation laser power density L on the amplitude A and characteristic time constant of the intermediate-field electric modulation component of photoreflection spectra measured in the region of the E ₀ fundamental transition of GaAs is studied. Crystalline samples with the charge carrier concentration n 10¹⁶ cm^–3 are investigated for L = 100 W/cm²–1 W/cm². A logarithmic dependence of the electric modulation signal on the excitation laser power density has been established for all examined samples. It is demonstrated that the observed change in the characteristic time constant does not have any noticeable effect on the dependence A(L).     

Polarization dependence on the holographic recording in spiropyran-doped polymers

We report on the photoinduced anisotropy in the holographic recording in spiropyran doped polymers as a result of the photoizomerization of spiropyran into merocyanine. Photoizomerization is achieved with UV polarized light ( = 355 nm) from the third harmonic of Q-switched Nd:YAG laser. After UV illumination a strong and broad polarization-dependent absorption peak centered 600 nm appears. Absorption holograms were recorded with low power He–Ne lasers with 2.2% output diffraction efficiency for polarization parallel to the polarization of the UV laser. The extremely long lifetime of the merocyanine states (12 days in the dark) and their high resolution makes this material promising as an optical memory element.     

Rapid thermal annealing of electrically-active defects in virgin and implanted silicon

Chemical etching of single-crystalline (100)Si induced by pulsed laser irradiation at 308, 423, and 583 nm has been investigated as a function of the laser fluence and C1₂ pressure. Without laser-induced surface melting, etching requires Cl radicals which are produced only at laser wavelengths below 500 nm. With low laser fluences ((308 nm)<100 mJ/cm²) etching is non-thermal and based on direct interactions between photocarriers and Cl radicals. For fluences which induce surface melting ((308 nm)>440 mJ/cm²) etching is thermally activated. In the intermediate region both thermal and non-thermal mechanisms contribute to the etch rate.     

Saturated absorption and nonlinear refraction of silicate glasses doped with silver nanoparticles at 532 nm

The nonlinear optical characteristics of silicate glasses doped with silver nanoparticles are investigated by Z-scan technique using second harmonic radiation of picosecond Nd:YAG laser (=532 nm, =55 ps). The real and imaginary parts of third-order nonlinear susceptibility of silver-contained glasses were measured. It was found that the sign of Im ⁽³⁾was negative due to saturated absorption and the sign of Re ⁽³⁾ was changed from negative (self-defocusing) to positive (self-focusing) with growth of laser radiation intensity. The mechanisms responsible for saturated absorption and nonlinear refraction are discussed.     

λ=2537 Å line from a low-pressure mercury discharge lamp emission profile and line absorption by a gas containing a mercury vapor

By measuring the emission-line intensity of Hgi =2537 Å (6³ P6¹ S) from a low-pressure mercurcury lamp, we have determined the dependence of the upper-level population on the discharge current and the mercury vapor density. From the radial profile of the intensity the spatial distribution function of the population has been determined to be the zero-th order decay mode for the trapped radiation. Line absorption by mercury vapor in a cell has been measured with various mercury densities and Lorentzian widths. The results are consistent with the upper-level population distribution as determined above. On the basis of these findings we calculate the emission-line profile and its change during the absorption in the absorption cell. The amount of absorption at an arbitrary depth of the absorption cell is calculated, and the optimum cold-spot temperature of the lamp of 40–50°C is suggested for the maximum absorption under the typical condition of the photo-CVD experiment.     

Light source and geometrical requirements for the optimization of optical anemometry signals

The criteria for the design of optical arrangements for laser anemometry are formulated for reference-beam, two-beam and single-beam modes of operation. The dependence of useful light intensity upon optical path-length difference and number of axial laser modes is calculated. Laser power requirements are evaluated and the dependence upon band-pass filtering is quantitatively assessed. A new two-channel integrated-optical unit, with light-path compensation, and embodying the proposed design criteria, is described.Nomenclature c velocity of light - D _1/e diameter of laser beam at 1/e-point - d half distance separating the beams leaving the lens - d _m effective diameter of measuring control volume - d _ph diameter of aperture in front of photo-multiplier - d _r waist diameter of focused reference beam - d _s waist diameter of scattering light beam - d _l waist diameter of fucused light beam - F f-number of lens - scattering function introduced in Mie's theory - f signal frequency - f bandwidth of filter - f _D Doppler line width of laser radiation - f _G effective bandwidth of gain envelope of a laser - f _M frequency difference between two adjacent axial modes - h Planck constant (6.6256×10^–34J sec) - K 2/ wave number - L cavity length of laser - l _m length of measuring control volume - m total number of axial modes of laser - M magnification (a/b) - N _fr number of fringes in crossing region of the two light beams - N _ph number of fringes seen by photomultiplier - N _s number of photons scattered per particle passage - N _e number of electrons leaving photo cathode per unit time - P _l total light power emitted by laser - P _s total light power scattered by particle - Q _scat scattering coefficient - R distance from particle centre to point on plane of observation - r _p radius of scattering particle - u velocity component perpendicular to fringe pattern - _fr distance between fringes inside measuring control volume - _c efficiency of light collecting system - _q overall quantum efficiency of photodetector - g _q coordinate measuring angle from the optical axis - half angle between wave fronts - wave length of light - frequency of incident light wave - _e standard deviation of electron flux - transit time of scattering particle - solid angle - d solid angle element This paper was presented at the Conference on Electro-optic Systems in Flow Measurement, Southampton, September 1972.