Peak Absorption Cross Section Research Articles

Continuous-wave laser operation of Yb:LuVO_4

We report on the room-temperature spectroscopic properties and continuous-wave laser operation of a new Yb:LuVO4 crystal. The peak absorption cross section for the pi-polarization is 8.42 x 10(-20) cm2 at 985 nm, and the stimulated emission cross section at 1020 nm is 1.03 x 10(-20) cm2. An output power of 0.36 W at 1041 nm was obtained with a slope efficiency of 47% by use of Ti:sapphire laser pumping. With diode pumping the output power reached 1.05 W at the highest available incident pump power of 6.8 W.

Low temperature mid-infrared cross-sections for peroxyacetyl nitrate (PAN) vapour

Abstract. Laboratory absorption spectra of peroxyacetyl nitrate (PAN, CH3C(O)OONO2) vapour have been measured in the mid-infrared range 550 cm-1 to 1400 cm-1 (18.2 to 7.14 µm) at both 250 K and 273 K, using a Fourier transform infrared spectrometer at a nominal spectral resolution of 0.25 cm-1 (unapodised). In addition, the 1600 cm-1 to 2200 cm-1 (6.25 to 4.54 µm) spectral region has been measured at 250 K. Cross-sectional data at each temperature, as well as integrated band intensities and peak infrared absorptivities for nine absorption bands of PAN in this spectral range, have been derived from a total of twelve separately measured PAN transmission spectra. A general increase in the peak absorption cross-section for all bands is noted with decreasing temperature, with cross-sectional increases in the range 6% (for the 1842 cm-1 band) and 30% (for the 991 cm-1 band) at 250K, relative to those previously reported at 295K. Differences in integrated band intensities range from -22% to +16% for the 1741 cm-1 and 991 cm-1 bands respectively over the same temperature range. These new absorption cross-sections for PAN are the first to be reported at temperatures below 295 K, allowing the possibility of improved retrievals of the atmospherically important PAN species from remotely sensed infrared spectra of the cold upper troposphere. These new cross-sectional data accompany this paper as an electronic supplement.

Absence of the enhanced intra-4f transition cross section at 1.5μm of Er3+ in Si-rich SiO2

We present measurements of the optical absorption cross section of the I15∕24→I13∕24 transition at 1.5μm of Er3+ ions embedded in SiO2 and Si-rich oxide, using cavity ringdown spectroscopy on thin films. The peak absorption cross section for Er3+ embedded in Si-rich oxide (10at.% excess Si) was found to be (8±2)×10−21cm2 at 1536nm, similar to typical values for Er embedded in SiO2. The data imply that the silicon nanoclusters incorporated in Si-rich oxide do not enhance the peak cross section of the Er3+ I15∕24−I13∕24 transition by 1-2 orders of magnitude, contrary to what has been reported in earlier work.

Optical properties of Er in Er-doped Zn/sub 2/Si/sub 0.5/Ge/sub 0.5/O/sub 4/ waveguide amplifiers

There is a growing need for compact, efficient integrated waveguide optical amplifiers for use in optoelectronic communication. Zn/sub 2/Si/sub 0.5/Ge/sub 0.5/O/sub 4/ (ZSG) doped with Er (ZSG-Er) is a promising new host material due to the high concentration of Er that can be incorporated and the high optical activity of the incorporated Er. In this paper, the absorption and emission cross sections of Er in ZSG-Er (to the authors' knowledge, for the first time) are measured both through photoluminescence spectra and direct gain and absorption measurements. Peak absorption and emission cross sections are about 3/spl times/10/sup -24/ m/sup 2/ from a Landenburg-Fuchtbauer analysis of the photoluminescence spectra, comparable to measurements on other oxide-based glass amplifiers. The population statistics of the excited Er level, along with the excited-state lifetime, are determined through a novel frequency-domain method in which the spontaneous emission power at 1550 nm is measured as a function of frequency under a modulated 980-nm input. The determined lifetime of 2 ms is comparable to the 2.3 ms measured using a conventional pump-probe technique. The novel analysis technique yields the population statistics of the excited Er atoms and the lifetime of the excited Er state under given pumping conditions independent of the unknown and variable coupling in and out of the waveguide. This method predicts zero net gain at 70 mW, about what is observed. Comparison of calculated gain and absorption based on Er density and measured cross sections with measured gains suggest that only about 20%-30% of the Er in the material is optically active. A 4.7-cm-long sample demonstrated a signal enhancement of /spl sim/13dB. Cavity characteristics were measured using an analysis of coherent reflection under no pumping. The facet reflectivity was determined to be 0.27, and the scattering/absorption loss was 1.05/cm, for a total distributed loss of 1.65/cm in a 4-cm cavity. These losses, compared with an estimated achievable gain of 0.25/cm under full inversion, suggest that optically pumped lasing at this concentration is not possible. Measurements of both the cross sections and population statistics, compared with actual gain and absorption properties, give insight into the contribution of the Er dopant under different conditions and can be used to model and improve rare-earth-based amplifiers.

Quantitative absorption spectroscopy of residual water vapor in high-purity gases: pressure broadening of the 139253-μm H_2O transition by N_2, HCl, HBr, Cl_2, and O_2

We determined the respective pressure-broadening coefficients of HCl, HBr, Cl2, and O2 (expressed relative to that of the reference gas N2) for the (v1,v2,v3)J(Ka,Kc) = (0,0,0)3(0,3) --> (1,0,1)2(0,2) rovibrational transition of H2 16O that occurs at 1.39253 microm. The experiment used a continuous-wave cavity ring-down spectroscopy analyzer to measure the peak absorption losses as a function of added moisture concentration. The measured pressure-broadening coefficients for HCl, HBr, Cl2, and O2 are, respectively, 2.76, 2.48, 1.39, and 0.49 times that of the N2 pressure-broadening coefficient, and detection limits for water vapor range from 0.22 nmol mol(-1) for O2 matrix gas to 2.3 nmol mol(-1) for HBr matrix gas. The degradation of the detection limit (relative to the N2 matrix gas) is ascribed to a pressure-broadening-induced reduction in peak absorption cross section and to elevated background loss from the matrix gas.

High-resolution absorption cross sections of formaldehyde at wavelengths from 313 to 320 nm

Absorption cross sections have been measured for the 2(0)(2)4(0)(3) and 2(0)(3)4(0)(1) vibrational bands of the [formula: see text] electronic transition of formaldehyde in the wavelength range 313-320 nm. Accurate values are of considerable importance for atmospheric monitoring and to understand the photochemistry of this compound. The 0.10 cm(-1) FWHM wavenumber resolution of the experiments is determined by the bandwidth of the ultraviolet laser used, and is a factor of 10 or more higher than any previously reported data. The absorption cross section data are thus obtained at a spectrometer resolution close to the Doppler broadening limit at 294 K of 0.07 cm(-1) FWHM, for isolated rotational lines, but lifetime broadening effects contribute a further approximately 0.5 cm(-1) of width. Our spectral resolution is thus higher than required to resolve the sharpest spectral features and, as a consequence, the cross sections peak at greater values than previous studies of these structured rovibronic bands conducted at much lower spectrometer resolutions. Previous data can be quantitatively reproduced by convolution of the newly obtained spectra with lower-resolution instrument functions. Pressure broadening of regions of the spectra in the presence of up to 500 Torr of N2 is examined and the effects on peak absorption cross sections are very small. The influence of reduced temperature on the spectrum is also explored through experimental measurements and spectral simulations.

Spectroscopy and continuous-wave diode-pumped laser action of Yb^3+:YVO_4

The growth, spectroscopic properties, and laser performance of Yb:YVO4 crystal with laser diode pumping are reported. A peak absorption cross section of 7.4 x 10(-20) cm2 at 985 nm, a radiative lifetime of 0.25 ms, and a stimulated-emission cross section of 1.25 x 10(-20) cm2 at 1008 nm for pi polarization were determined for the Yb3+ ions in YVO4. Continuous-wave laser action of Yb:YVO4 at 1020-1027 nm was demonstrated with an output power of 610 mW and a slope efficiency of 49%.

Spectroscopic study of double sodium-yttrium fluoride crystals doped with erbium Na0.4Y0.6F2.2:Er3+: I. Intensities of luminescence spectra and kinetics of luminescence

Na0.4Y0.6F2.2:Er3+ (NYF:Er) crystals with an erbium concentration as high as 100 at. % (Na0.4Er0.6F2.2) were grown by the Bridgman-Stockbarger method. The optical spectra were investigated at low (6 K) and room temperatures. It is shown that the absorption spectrum of NYF:Er crystals contains wide bands (790–801 and 965–980 nm) corresponding to the emission range of laser diodes. The peak absorption cross section σa for the band peaked at λ=970.4 nm is 0.15×10−20 cm2. On the basis of the analysis of the absorption and luminescence spectra at low (6 and 12 K) temperatures, the structure of the Stark splitting of erbium levels was determined as a structure of quasi-centers for which Stark components are inhomogeneously broadened. The oscillator strengths of the transitions from the ground state of erbium to excited multiplets were calculated from the absorption spectra measured at T=300 K, and the intensity parameters Ωt were determined by the Judd-Ofelt method: Ω2=1.65×10−20 cm2, Ω4=0.56× 10−20 cm2, and Ω6=1.01×10−20 cm2. These values of the intensity parameters were used to calculate the probabilities of radiative transitions and the branching ratios. The rates of multiphonon nonradiative transitions in NYF: Er were estimated. The luminescence decay kinetics for radiative levels of erbium ions upon their selective excitation by nanosecond laser pulses was studied. The intracenter lifetimes of radiative levels of erbium ions were determined from the luminescence kinetics upon selective ion excitation by low-intensity light in a sample with a low erbium concentration (0.5%). It is demonstrated that, with an increase in temperature from 6 to 300 K, luminescence from the 4 G 11/2, 2 G(H)9/2, and 4 F 9/2 levels is quenched as a result of multiphonon nonradiative transitions. Luminescence from the 4 I 9/2 level is quenched only insignificantly with increasing temperature, and no quenching of luminescence from the 4 I 11/2 and 4 I 13/2 levels is observed. The spectra of steady-state luminescence of NYF:Er(0.5–15%) crystals were investigated upon broadband excitation by UV and UV-visible lamp light and selective time-resolved laser excitation. It is shown that low-lying levels of erbium ions separated by an energy gap smaller than 2500 cm−1 are populated via cascade mechanisms. On the basis of the results obtained, it is concluded that NYF:Er 3+ crystals are promising candidates for active media of tunable diode-pumped lasers.

New ultraviolet absorption cross-sections of BrO at atmospheric temperatures measured by time-windowing Fourier transform spectroscopy

The UV absorption cross-section spectra of the atmospherically important radical BrO have been determined using the recently developed technique of time-windowing Fourier transform spectroscopy (TW-FTS). The absorption spectra of the A2Π3/2–X2Π3/2 band system were recorded in the flash photolysis of a gaseous mixture of Br2 and O3. The bromine-photosensitized decomposition of O3 was observed at five different temperatures between 203 and 298K. The absolute UV absorption cross-section was determined from the time-dependent observation of reactant and product absorptions and by a kinetic analysis of the BrO behavior. The integrated UV absorption cross-section of BrO was, within the accuracy of the measurements, constant over the temperature range studied, as expected from spectroscopic considerations. For the (7, 0) vibrational band at 29540cm−1 (338.5nm), the peak absorption cross-sections were determined to be 2.19±0.23 at 298K, 2.23±0.23 at 273K, 2.52±0.26 at 243K, 2.75±0.29 at 223K, and 3.03±0.31 at 203K (all in units ×10−17cm2permolecule, at a spectral resolution of 3.8cm−1, with error intervals of 2σ). Further, vibrational constants and the dissociation limit for the electronic state A2Π3/2 were derived. The A←X dissociation energy was determined to be D0=35240±160cm−1 or 421.6±1.9kJ/mol.

Nonlinear properties of phototropic media on the basis of CuxSe nanoparticles in quartz glass

Energy and kinetic characteristics of the induced transparency in quartz sol-gel glasses containing copper selenide nanoparticles of different stoichiometry are studied. The dependence of the nonlinear optical properties of the glass samples on the chemical composition of copper selenide particles, which gives rise to an additional absorption band in the near-infrared region and determines its spectral position, is established. It is found that the time of relaxation of the induced transparency increases and the peak absorption cross section decreases as the absorption maximum shifts to the low-energy spectral region.

Infrared Laser Absorption Measurements of the Kinetics of Propargyl Radical Self-Reaction and the 193 nm Photolysis of Propyne

Infrared laser absorption spectroscopy is used to investigate recent discrepancies in measurements of the propargyl radical self-reaction rate coefficient and product formation in 193 nm photolysis of propyne. The cross section of the propargyl radical is measured relative to that of HCl using the Cl + propyne reaction, yielding a peak absorption cross section (assuming Doppler-limited line shapes) of (1.9 ′ 0.4) x 10 - 1 8 cm 2 for the P(12) line of the υ 1 fundamental at 296 K. The rate coefficient for the propargyl radical (CH 2 CCH) self-reaction is determined by modeling the infrared absorption of the propargyl radical formed in the 193 nm photolysis of propargyl chloride (HCCCH 2 Cl) and propargyl bromide (HCCCH 2 Br), using a more precise literature determination of the pressure-broadened absorption cross section. The propargyl self-reaction rate coefficient so obtained, (3.9 ′ 0.6) x 10 - 1 1 cm 3 molecule - 1 s - 1 , is consistent with several recent measurements but disagrees significantly with the one previous infrared absorption determination. Both the propargyl radical and acetylene (HCCH) are observed in the 193 nm photolysis of propyne (CH 3 CCH). The propargyl is formed promptly following the UV photolysis pulse, and the magnitude of the signal is unaffected by the addition of O 2 . The observed propargyl signal is consistent with direct CH 2 CCH formation in the 193 nm photolysis of propyne and appears inconsistent with formation by secondary reactions of the 1-propynyl radical (CH 3 CC). The observed CH 2 CCH yield per 193 nm photon absorbed is 0.49 ′ 0.10.

The singlet–triplet spectroscopy of 1,3-butadiene using cavity ring-down spectroscopy

The T1←S0 absorption spectrum of gas-phase 1,3-butadiene (C4H6) has been investigated over the region from 20 500 to 23 000 cm−1 using cavity ring-down spectroscopy. Resolved vibrational structure and partially resolved rotational structure have been observed for the first time in the gas phase. The T1←S0 origin transition is located at 20 777 cm−1, with a peak absorption cross section of 2.5×10−26 cm2/molecule. Vibronic bands appear 249, 491, 1166, and 1617 cm−1 above the origin. This structure is observed on top of a rising background whose absolute magnitude and wavelength dependence is quantitatively accounted for as Rayleigh scattering. Using the recent calculations of Brink et al. [J. Phys. Chem. A 102, 6513 (1998)] as a guide, the bands 491, 1166, and 1617 cm−1 above the origin can be assigned as totally symmetric fundamentals, while the band 249 cm−1 above the origin is the first overtone of the bg symmetry CH2 torsion (calculated at 129.6 cm−1) of a planar T1 excited state. The rotational band contour of the origin transition shows several sharp band-heads that appear in doublets with a splitting of 2 cm−1. Only part of this structure can be accounted for as a single vibronic band. The possible explanations for the remaining band heads and the observed Franck–Condon intensities are discussed in terms of the shape of the T1 potential energy surface.

Spectroscopic Study of Neodymium-Doped Sodium–Yttrium Double Fluoride Nd3+:Na0.4Y0.6F2.2 crystals

Nd3+:Na0.4Y0.6F2.2 (Nd3+:NYF) crystals are grown by the Stockbarger–Bridgman method for a stoichiometric mixture prepared by the solid-phase method and containing neodymium up to 20 at. %. The absorption spectrum of Nd3+:NYF crystals exhibits bands located in the emission region of laser diodes. The peak absorption cross section of the 796.8-nm band is σ a = 0.96 × 10–20 cm2 and the bandwidth is Δλ = 17.5 nm. The most intense luminescence band is located at 1.05 μ m and the radiative time of the 4F3/2 level is τ0 = τexp ~ 960 μ s. It is shown that the 2P3/2 and 4D3/2 levels of Nd3+:NYF crystals are also radiative with lifetimes τ exp equal to ~110 and 9.5 μ s, respectively. However, these radiative transitions are partially quenched due to nonradiative relaxation. The intensity parameters Ω t are determined by the Judd–Ofelt method to be Ω2 = 1.18 × 10–20, Ω4 = 1.55 × 10–20, and Ω 6 = 2.85 × 10–20 cm 2. Using these parameters, the probabilities of radiative transitions and branching ratios are calculated, and the probabilities of nonradiative transitions are estimated. A conclusion is made that Nd3+:NYF crystals are promising as active media for diode-pumped tunable lasers, in particular, up-conversion-pumped lasers.

Diode-laser absorption measurements of hydrazine and monomethylhydrazine

Absorption measurements of pure hydrazine, N 2H 4, and monomethylhydrazine, CH 3N 2H 3 (MMH), were recorded in an optical absorption cell (3 passes, 50 cm/pass) at low pressure (0–35 Torr) using a tunable external-cavity diode laser (ECDL), operating in the range from 6350 to 6650 cm -1 (1.49 to 1.58 μm). Peak absorption cross-section measurements of hydrazine and MMH, as determined from fixed wave-length absorption measurements, were found to be α e( ν)=0.163±0.008 cm -1atm -1 at 6495 cm -1 ( λ=1539.6 nm) and α e( ν)=0.152±0.003 cm -1atm -1 at 6560 cm -1 ( λ=1524.4 nm), respectively.

Assessing the Contribution of the Lowest Triplet State to the Near-UV Absorption Spectrum of HOCl

The near-UV absorption spectrum of HOCl is examined using laser induced fluorescence to monitor OH fragments resulting from photodissociation. The measured variation in OH photofragment yields along with an analysis of their Doppler line shapes reveals the presence of a weak absorption feature centered at 380 nm arising from excitation to the lowest excited triplet electronic state of HOCl. Although the peak absorption cross section for the band is modest, σ(380 nm) = 4 × 10-21 cm2 , its long-wavelength tail extends down to 480 nm. Present estimates suggest that as a result of absorption by the triplet state, the instantaneous photochemical lifetime of stratospheric HOCl will be shortened by 10−20% and the average abundance reduced by 11−12% relative to previous estimates.

Development of a narrow-band, tunable, frequency-quadrupled diode laser for UV absorption spectroscopy

A compact, lightweight, low-power-consumption source of tunable, narrow-bandwidth blue and UV radiation is described. In this source, a single-longitudinal-mode diode laser seeds a pulsed, GaAlAs tapered amplifier whose ~860-nm output is frequency quadrupled by two stages of single-pass frequency doubling. Performance of the laser system is characterized over a wide range of amplifier duty cycles (0.1-1.0), pulse durations (50 ns-1.0 mus), peak currents (</=14 A), and average currents (</=2.0 A). The capabilities and limitations of this source are discussed. We recorded high-resolution, Doppler-limited absorption spectra of nitric oxide and sulfur dioxide near 215 nm; the SO(2) spectrum was found to have significantly more structure and higher peak absorption cross sections than previously reported.

Absorption and emission cross sections of Er^3+ in Al_2O_3 waveguides

Al(2)O(3) slab waveguide films were doped with erbium using ion implantation to a peak concentration of 1.5 at. %. Prism coupling measurements show absorption caused by (4)I (15/2) ?(4)I (13/2) intra-4f transitions in Er(3+) with a maximum at 1.530 mum of 8 dB/cm. The Er(3+) absorption cross section is determined as a function of wavelength. We used the McCumber theory to derive the emission cross section spectrum from the absorption results, which we then compared with the Er(3+) photoluminescence spectrum. The peak absorption and emission cross sections are found to be 6 x 10(-21) cm(-2). The results are used to predict the optical gain performance of an Er-doped Al(2)O(3) optical amplifier that operates around 1.5 mum.

Investigation of OH absorption cross sections of rotational transitions in the band under atmospheric conditions: Implications for tropospheric long‐path absorption measurements

The accuracy of tropospheric hydroxyl radical measurements by long‐path absorption spectroscopy is ultimately limited by the uncertainty of the effective OH absorption cross sections. The latter were determined from calculated spectra for the Q1(2), Q1(3), and P1(1) rotational lines of the OH transition at 308 nm. The calculations took into account Doppler broadening, measured data of the collisional broadening of OH by air molecules, and instrumental line broadening effects. The calculated spectra were compared with OH absorption spectra measured in a flow reactor near room temperature at 1013 hPa. Excellent agreement between calculated and measured OH spectra was achieved using the collision‐broadening parameters determined recently by Leonard (1990). The effective absorption cross sections at the line center (peak absorption cross sections) calculated for the Voigt line shape at 300 K and 1013hPa are: , , . The full width at half maximum of the spectral lines is 2.5 pm, and the shape factor a is 1.41. Calculations of the effective absorption cross section under different atmospheric conditions demonstrate a strong pressure dependence for all lines. At 2800 m altitude (720 hPa) the peak absorption cross section of the OH Voigt lines is about 35% higher than at sea level. The temperature dependence in an interval of ±20 K around room temperature (at 1013 hPa) is only small (±3% for the Q lines and ±6% for the P1(1) line). The estimated total uncertainty of the effective absorption cross section is 8%.

Zero-phonon line as the foundation stone of high-resolution matrix spectroscopy, persistent spectral hole burning, single impurity molecule spectroscopy

The purely electronic zero-phonon line (ZPL) - the optical analog of the Mössbauer-γ-resonance line - is a remarkable feature of the low temperature impurity absorption and luminescence spectra of solids. The narrowness and high peak absorption cross section of ZPLs has opened new horizons for high resolution spectroscopy of impurity molecules (centers). The newest and most spectacular achievement is single impurity molecule spectroscopy. Persistent spectral hole burning (PSHB), serving as a tool to control by means of illumination the absorption coefficient, and bound to it, via the Kramers-Kronig relations, the refraction index, turns inhomogeneous broadening of ZPLs into a useful feature and opens a variety of applications including PSHB optical data storage and processing. Space-frequency and space-time domain holographies are developed. The latter comprises the causality relations between events subject to holography and opens new principal possibilities in optics.

Fundamentals of single-molecule spectroscopy in solids

Single-molecule spectroscopy in solids depends upon several physical phenomena, notably the extreme narrowing of zero-phonon optical transitions at low temperatures which leads to high peak absorption cross-sections and inhomogeneous broadening. The factors influencing the signal-to-noise ratio for fluorescence excitation detection are reviewed, and recent single-molecule experiments are briefly listed.