Optical Emission Intensity Research Articles

Mathematical Modeling of Optical Radiation Emission as a Function of Welding Power during Gas Shielded Metal Arc Welding.

Arc welding is accompanied by intense optical radiation emission that can be detrimental not only for the welder himself but also for people working nearby or for passersby. Technological progress advances continuously in the field of joining, so an up-to-date radiation database is necessary. Additionally, many literature irradiance data have been measured for a few welding currents or for parts of the optical spectral region only. Within this paper, a comprehensive study of contemporary metal active gas, metal inert gas, and cold metal transfer welding is presented covering optical radiation emission from 200 up to 2,700 nm by means of (spectro-) radiometric measurements. The investigated welding currents range from 70 to 350 A, reflecting values usually applied in industry. Based upon these new irradiance data, three mathematical models were derived in order to describe optical radiation emission as a function of welding power. The linear, exponential, and sigmoidal emission models depend on the process variant (standard or pulsed) as well as on the welding material (mild and stainless steel, aluminum). In conjunction with the corresponding exposure limit values for incoherent optical radiation maximum permissible exposure durations were calculated as a function of welding power. Typical times are shorter than 1 s for the ultraviolet spectral region and range from 1 to 10 s for visible radiation. For the infrared regime, exposure durations are of the order of minutes to hours. Finally, a validation of the metal active gas emission models was carried out with manual arc welding.

Comparison of three optical diagnostic techniques for the measurement of boron atom density in a H2/B2H6 microwave plasma

Three different optical diagnostic techniques have been used to measure the density of boron atoms in a microwave generated H2/B2H6 plasma of a diamond deposition reactor. These techniques are: optical emission intensity ratio of doublet 249.677 and 249.772 nm boron lines (OEIRD), laser induced fluorescence on 208.96 nm line (LIF) and resonance absorption on 249.772 nm line (RA) with a boron hollow cathode lamp (HCL) as light source. LIF results point to an important variation of the boron atom density with the distance from the substrate, in contradiction with OEIRD which indicates an almost constant density. RA measurements show a stronger absorption when the probe light from the HCL crosses the reactor outside the bright plasma core region than when it goes through the plasma core. This indicates a larger boron atom density in the volume surrounding the plasma core than inside it. As the plasma is not homogeneous, in the sense that the plasma induced emission is confined inside the plasma core but the ground state boron atoms are also present outside this region, the OEIRD method cannot be applied to this plasma. These experimental results question the simulation results of H2/B2H6 microwave plasma which predicted a fast decay of the boron atom density outside the plasma core.

Pulsed high-voltage discharge in air with a pressure gradient

Results of experiments on high-voltage discharges in air with a pressure gradient are presented. The experiments were carried out at the setup developed at the Institute of Applied Physics, Russian Academy of Sciences. The goal of the experiments was laboratory modeling of high-altitude atmospheric discharges―sprites and jets. The setup and diagnostic techniques are described. The experimental results include the distribution of the gas pressure in the vacuum chamber formed by means of pulsed air puffing, photographs of discharges in air with a pressure gradient, and the dependences of the discharge current and optical emission intensity on the initial conditions.

Electron kinetic energy and plasma emission diagnosis from femtosecond laser produced air plasmas

The characteristics of a plasma formed by a focused ultrashort laser in atmospheric-pressure air are studied with linear and circular pulses. We show that the ionization threshold for circular pulses is 1.36 times higher than for that linear pulses. Using an intensified CCD camera, we study plasma emission over seven orders of magnitude in a dynamic range. In spite of possessing a lower total ion number, plasmas produced by circular pulses are found to be brighter visible-light emitters than those produced by linear pulses of the same energy. This indicates that circular pulses produce plasmas with more electron kinetic energy than linear pulses and that kinetic energy plays a role in the optical emission intensity. The presence of high energy electrons is verified by demonstrating that a high-ionization-potential gas can be made to radiate more brightly by the addition of a low-ionization-potential gas even though the second gas lowers the achievable focal intensity.

Tuning the Optical Band Gap of Graphene Oxide By Ozone Treatment

Graphene oxide (GO) inherits high transparency, substantial conductivity, high tensile strength from its parent materials graphene. Apart from these properties, it emits fluorescence which makes it a potential material to use in optoelectronics and bio-sensing applications. In this work, we have utilized systematic ozone treatment to alter the optical band gap of single-layered graphene oxide in aqueous suspensions. Due to controlled ozonation, additional functionalization takes place in GO graphitic sheet which changes GO electronic structure. This is confirmed by the increase in vibrational transitions of a number of oxygen-containing functional groups with treatment and the appearance of the prominent carboxylic group feature at c.a. 1700 cm-1. Albeit, timed ozone induction introduces only slight change in color and absorption spectra of GO samples, the emission spectra show a gradual increase in intensity with a significant blue shift up to 100 nm from deep red to green. This large blue shift suggests an increase in optical band gap with additional functionalization introduced by ozone treatment. We utilize a semi-empirical theoretical approach to describe the effects of functionalization-induced changes. This model attributes the origins of fluorescence emission to the quantum confined sp² carbon islands in GO encircled by the functional groups. As we decrease the graphitic carbon cluster size on the GO sheet, the optical bandgap calculated via HyperChem molecular modeling increases, which supports the experimentally observed blue shifts in emission. This theoretical result is further supported by the TEM of ozone-treated samples, which shows a decreasing trend of average ordered graphitic carbon cluster size on GO sheets with treatment time. Theoretical modeling, as well as the experimental results, indicate that the optical bandgap and emission intensity of GO are alterable with controlled ozone treatment, which allows tailoring the optical properties of GO for specific applications in optoelectronics and bio-sensing.

Y2O3 wall interactions in Cl2 etching and NF3 cleaning plasmas

Time-dependent behavior of chlorine inductively coupled plasmas is presented for Si etching, following NF3-Ar plasma cleaning of a chamber coated with Y2O3. Optical emission intensities were recorded throughout the processes for Cl, O, F, Si, SiClx=1-3, SiF, and N2, as well as from added trace rare gases Xe and Ar for determination of number densities for selected species by actinometry. Time-dependent Langmuir probe measurements of ion and electron number densities and electron energy distributions were also carried out. Ex situ x-ray photoelectron spectroscopy measurements of the surface composition of Y2O3 coupon pieces after different etching and clean processes were also performed. Initially fluorinated yttria surfaces are shown to have a relatively high probability for loss (“recombination”) of Cl through formation of both Cl2 and SiClx. As etching proceeds, SiClx abstracts F from the surface and deposits Si and Cl, lowering of the heterogeneous recombination of Cl. The initially high recombination coefficient for Cl is explained by the weakening of the surface binding energy for Cl and SiClx at YFx sites, due to the highly electronegative nature of F, allowing recombination reactions forming Cl2 and SiClx to become energetically favorable.

Simultaneous electrical and optical study of spoke rotation, merging and splitting in HiPIMS plasma

To gain more information on the temporal and spatial behaviour of self-organized spoke structures in HiPIMS plasmas, a correlation between the broadband optical image of an individual spoke (taken over 200 ns) and the current it delivers to the target has been made for a range of magnetron operating conditions.As a spoke passes over a set of embedded probes in the niobium cathode target, a distinct modulation in the local current density is observed, (typically up to twice the average value), matching very well the radially integrated optical emission intensities (obtained remotely with an ICCD camera). The dual diagnostic system allows the merging and splitting of a set of spokes to be studied as they rotate. It is observed that in the merger of two spokes, the trailing spoke maintains its velocity while the leading spoke either decreases its velocity or increases its azimuthal length. In the spoke splitting process, the total charge collected by an embedded probe is conserved. A simple phenomenological model is developed that relates the spoke mode number m to the spoke dimensions, spoke velocity and gas atom velocity. The results are discussed in the context of the observations of spoke dynamics made by Hecimovic et al (2015 Plasma Sources Sci. Technol. 24 045005)

Effect of an optical emission spectrometer feedback-controlled method on the characterizations of nc-TiC/a-C:H coated by high power impulse magnetron sputtering

An optical emission spectrometer (OES) feedback-controlled high power impulse magnetron sputter deposition system was used to deposit nc-TiC/a-C:H thin films in a reactive mode. The plasma emission intensity of Ti2+ ions at the emission wavelength of 365nm was monitored and the amount of reactive C2H2 gas was precisely controlled to maintain stable emission intensity during the deposition process. By controlling the C2H2 input flow rate and different Ti2+ ion optical emission intensities, various nc-TiC/a-C:H thin films were obtained at a fixed duty cycle of 2% and repetition frequency of 500Hz at 75°C. The characteristics and mechanical properties of the obtained films were also investigated. As the OES set point was 40%, the phase changing from TiC to nc-TiC/a-C:H films was clearly identified. The highest hardness of 26GPa was obtained at the OES set point of 40%, where the film was composed of TiC. In addition, the lowest friction coefficient of 0.08 was obtained at the OES set point of 7%, which represented the nanocomposite structure of nc-TiC/a-C:H films.

Spectroscopic and Langmuir probe measurements of Ar-N2 mixture plasma in magnetic pole enhanced ICP source

Over the last decade, the plasma sources based on electromagnetic power absorption mechanisms have emerged for large scale semiconductor manufacturing. For this purpose inductively coupled plasmas (ICPs), helicons, electron cyclotron resonance (ECR) etc have been widely explored. Recently, new designs of ICPs have been proposed for the improvement of plasma processing reactors including ICPs enhanced by a ferromagnetic core and ICPs with multiple antennas for a uniform spread of plasma over a large processing area. Present paper deals with variation in plasma processing parameters of a low pressure Ar-N2 inductively coupled plasma source with magnetic pole enhancement (MaPE-ICP). Optical emission spectroscopy (OES) and an RF compensated Langmuir probe are employed to examine the trends of parameters including the electron density, electron temperature and electron energy probability functions (EEPFs). Moreover, Ar metastable fractions are also calculated from line ratio method using optical emission intensities of the Ar spectral lines. The variation in emission intensities of the selected lines and in metastable fractions can be related with electron densities in various parts of the EEPFs. Furthermore, at low RF powers the evolution in EEPFs from ‘bi-Maxwellian’ to ‘Maxwellian’ distribution with increasing Ar concentration in the mixture is also observed. The dissociation fraction of N2 determined by ‘advanced actinometry’ technique, based on Trace rare gas-actinometry (TRG-actinometry), is also reported. It is found that dissociation fraction increases with Ar concentration in the discharge as well as RF power and filling gas pressure.

Transient Hg, Ba+, Ca+ and Y+ Optical Emission Lines of a Mercury HID Lamp Exposed to X-Ray: Thermal Analysis of the Tungsten Electrode with Emissive Mixture Based on Barium, Calcium and Yttrium

A study is made on optical emission spectrum modification during X-ray exposure, in the case of a high-pressure mercury discharge lamp operated at several powers. A strong dependence on the lamp operating power for all the spectral lines intensities is observed. Erosion of the emissive mixture based on barium, calcium and yttrium compounds is evidenced by higher values recorded for spectral line intensities in all cases of X-ray exposure compared with those corresponding to non-irradiated lamps. Boltzmann type functions fitted the experimental data of measured optical emission intensities versus time for Ba+, Ca+ and Y+ lines. However, X-ray influence on plasma electron temperature is not significant. The spectroscopic data are correlated with the results of electrical measurements and of thermal analysis performed for the electrode having the emissive mixture based on barium, calcium and yttrium compounds. A qualitative explanation of the results is also given.

Insights into the mechanism of in-plasma photo-assisted etching using optical emission spectroscopy

Various mechanisms have been proposed to explain photo-assisted etching (PAE) of Si, including photogenerated carrier-mediated etching, photon-stimulated desorption, and photon-induced damage (breaking of Si–Si bonds) caused mainly by vacuum ultraviolet photons irradiating the substrate. In this study, the authors use optical emission spectroscopy to gain an insight into possible in-plasma PAE mechanisms. Emissions from Cl, Si, SiCl, and Ar were recorded as a function of power while etching p-Si in Cl2/Ar Faraday-shielded inductively coupled plasmas at a pressure of 60 mTorr with no substrate bias. Under these conditions, ion-assisted etching was negligible and PAE was dominant. The Si:Ar optical emission intensity ratio, ISi/IAr (proportional to the etching rate of Si), increased substantially with power. Accounting for the contribution to this signal from the dissociation of SiClx (x = 1–4) etch products, the residual increase in the emission indicated that the PAE rate also increased with power. Time resolved emissions were also recorded in a pulsed plasma where power was modulated between 500 and 300 W. ISi/IAr was found to modulate with the instantaneous power. This rules out the photon-induced damage mechanism since, if this mechanism was dominant, the ISi/IAr signal would not be modulated.

Laser-induced breakdown spectroscopy of liquid solutions: a comparative study on the forms of liquid surface and liquid aerosol.

Liquid surface and liquid aerosol as the traditional liquid forms for laser-induced breakdown spectroscopy (LIBS) and inductively coupled plasma (ICP), respectively, have been used to analyze chromium (Cr) and cadmium (Cd) elements using LIBS in a liquid solution. The spectral differences, the effects of laser energy and laser frequency, the accumulated number of laser pulses, gate delay time, and the quantitative analyses for a liquid surface and a liquid aerosol were compared. The results showed that the liquid surface demonstrated a lower plasma threshold, higher optical emission intensity, and higher single-to-noise ratio. Moreover, the relative standard deviations (RSDs) of the intensities of the liquid aerosol are better than those of the liquid surface. Furthermore, the results of the quantitative analyses of Cr I 357.86 nm and Cd I 361.05 nm of the liquid surface are close to those of the liquid aerosol. The limit of detections of Cr and Cd of the liquid surface were 2.764 and 86.869 μg/mL, which were close to those of liquid aerosol, 2.847 μg/mL of Cr and 97.635 μg/mL of Cd. For both the liquid surface and liquid aerosol, the coefficient of determination R2 of the calibration curve for Cr and Cd were above 0.99, and the average RSDs of Cr and Cd of the liquid surface were 0.027 and 0.054, which were similar to the 0.020 of Cr and 0.042 of Cd of the liquid aerosol. These results suggest that both the liquid surface and aerosol have similar detection abilities for water quality monitoring.

Lyα and C iii] emission inz= 7–9 Galaxies: accelerated reionization around luminous star-forming systems?

We discuss new Keck/MOSFIRE spectroscopic observations of four luminous galaxies at z~7-9 selected to have intense optical line emission by Roberts-Borsani et al. (2016). Previous follow-up has revealed Lyman-alpha in two of the four galaxies. Our new MOSFIRE observations confirm that Lyman-alpha is present in the entire sample. We detect Lyman-alpha emission in COS-zs7-1, confirming its redshift as z=7.154, and we detect Lyman-alpha in EGS-zs8-2 at z=7.477, verifying a tentative detection presented in an earlier study. The ubiquity of Lyman-alpha in this sample is puzzling given that the IGM is likely significantly neutral over 7<z<9. To investigate this result in more detail, we have initiated a campaign to target UV metal emission in the four Lyman-alpha emitters as a probe of both the radiation field and the velocity offset of Lyman-alpha. Here we present the detection of intense CIII] emission in EGS-zs8-1, a galaxy from this sample previously shown to have Lyman-alpha at z=7.73. Photoionization models indicate that an intense radiation field and low metallicity are required to reproduce the intense CIII] and optical line emission. We argue that this extreme radiation field is likely to affect the local environment, increasing the transmission of Lyman-alpha through the galaxy. Moreover, the centroid of CIII] indicates that Lyman-alpha is redshifted from the systemic value by 340 km/s. This velocity offset is larger than that seen in less luminous systems, providing an additional explanation for the transmission of Lyman-alpha emission through the IGM. Since the transmission is further enhanced by the likelihood that such systems are also situated in the densest regions with the largest ionized bubbles, the visibility of Lyman-alpha at z>7 is expected to be strongly luminosity-dependent, with the most effective transmission occurring in systems with intense star formation.

Threshold characteristics of ultraviolet and near infrared nanosecond laser induced plasmas

The present contribution compares the energy absorption, optical emission, temperature, and fluid dynamics of ultraviolet (UV) λ = 266 nm and near infrared (NIR) λ = 1064 nm nanosecond laser induced plasmas in ambient air. For UV pulses at the conditions studied, energy absorption by the plasmas increases relatively gradually with laser pulse energy starting at delivered energy of E ∼ 8 mJ. Corresponding measurements of plasma luminosity show that the absorption of UV radiation does not necessarily result in visible plasma emission. For the NIR induced plasmas, the energy absorption profile is far more abrupt and begins at ∼55 mJ. In contrast with UV, the absorption of NIR radiation is always accompanied by intense optical emission. The temperatures of both types of plasma have been measured with Rayleigh scattering thermometry (at times after the Thomson signal sufficiently diminishes). The UV plasmas can attain a wider range of temperatures, including lower temperatures, depending on the pulse energy (e.g., T ∼ 400–2000 K for E ∼ 7–35 mJ at Δt = 10 μs after the pulse) while the NIR plasmas show only hotter temperatures (e.g., T ∼ 12 000 K for E = 75 mJ at Δt = 10 μs after the pulse) as is consistent with the literature. Differences in the fluid dynamics for UV versus NIR pulses are shown with Schlieren imaging. The contrast in the UV and NIR plasma threshold behavior is attributed to differing roles of avalanche ionization and multiphoton ionization as is also illustrated by a simple numerical model.

Rayleigh Scattering Measurements of Heating and Gas Perturbations Accompanying Femtosecond Laser Tagging

Planar Rayleigh scattering and Rayleigh scattering polarimetry are applied toward measurements of molecular dissociation, thermometry, and the visualization of fluid dynamics in the afterglow of a femtosecond laser plasma under conditions relevant to the femtosecond laser tagging diagnostic. The generation of shock waves and a high-temperature region are observed, with a corresponding temperature increase of (, ) and (, ) produced with a 175 mm focusing lens. The spatial profile of the temperature rise along the beam is found to scale linearly with the optical emission intensity. The transverse width of the emission region is also observed to correlate with increased temperature. Rayleigh polarimetry measurements show a high dissociation fraction of up to (, ). Modeling of three-body recombination into the state using the measured dissociation fraction is able to reproduce the emission intensity time history between 1 and . Local heating is found to play a key role in the observed trends in the spatial profile, width, intensity, and temporal evolution of femtosecond laser tagging at laser intensities near or above .

Is it possible to deduce the ground state OH density from relative optical emission intensities of the OH(A2Σ+-X2Πi) transition in atmospheric pressure non-equilibrium plasmas?—An analysis of self-absorption

The measurement of absolute densities of reactive species and radicals such as OH is of growing interest for many plasma applications. In this paper, we extend the use of a self-absorption model for atomic emission spectroscopy to molecular emission spectroscopy. The proposed analysis of self-absorbed molecular emission spectra is a simple and inexpensive method to determine OH(X) densities and rotational temperatures compared to laser induced fluorescence. We compare the recorded absolute OH density in a non-equilibrium diffuse atmospheric-pressure RF glow discharge by this method with broadband UV absorption considering a number of rotational lines with J′ ⩽ 6.5, the detection limit of the line integrated OH(X) density with this method is of the order of 2 × 1019 m−2. The accuracy of the density is sensitive to the rotational temperature of the OH(A) state and the non-equilibrium rotational population distribution.

Transient change in the shape of premixed burner flame with the superposition of pulsed dielectric barrier discharge

We investigated the transient phenomena in a premixed burner flame with the superposition of a pulsed dielectric barrier discharge (DBD). The length of the flame was shortened by the superposition of DBD, indicating the activation of combustion chemical reactions with the help of the plasma. In addition, we observed the modulation of the top position of the unburned gas region and the formations of local minimums in the axial distribution of the optical emission intensity of OH. These experimental results reveal the oscillation of the rates of combustion chemical reactions as a response to the activation by pulsed DBD. The cycle of the oscillation was 0.18–0.2 ms, which could be understood as the eigenfrequency of the plasma-assisted combustion reaction system.

Method of power density determination in microwave discharge, sustained in hydrogen–methane gas mixture

We present the results of studying a continuous microwave discharge maintained at a frequency of 2.45GHz in a CVD reactor based on a cylindrical resonator excited at the TM013 mode. The discharge was ignited in hydrogen and a gaseous mixture of hydrogen and methane and was studied by the method of optical emission spectroscopy. Density of atomic hydrogen and gas temperature were measured, as well as the spatial distribution of both optical emission intensity of the plasma and intensity of the Hα line of atomic hydrogen. The main parameters of the discharge were calculated numerically using the two-dimensional self-consistent model of the discharge. Basing on the obtained results, we proposed a method for high-precision experimental determination of the plasma volume and calculation of the specific energy contribution to the plasma, i.e., microwave power density in plasma (MWPD), with minimal errors. According to the calculations, in the experiment performed, the microwave power density in the plasma varied from 50 to 550W/cm3 as the gas pressure increased from 80 to 350Torr. The method allows one to perform unified MWPD calculations in different CVD reactors and to compare diamond film deposition regimes.

Doping concentration dependence of the photoluminescence spectra of n-type GaAs nanowires

In this letter, the photoluminescence spectra of n-type doped GaAs nanowires, grown by the metal organic chemical vapor deposition method, are measured at 4 K and 77 K. Our measurements indicate that an increase in carrier concentration leads to an increase in the complexity of the doping mechanism, which we attribute to the formation of different recombination centers. At high carrier concentrations, we observe a blueshift of the effective band gap energies by up to 25 meV due to the Burstein-Moss shift. Based on the full width at half maximum (FWHM) of the photoluminescence peaks, we estimate the carrier concentrations for these nanowires, which varies from 6 × 1017 cm−3 (lightly doped), to 1.5 × 1018 cm−3 (moderately doped), to 3.5 × 1018 cm−3 (heavily doped) as the partial pressure of the disilane is varied from 0.01 sccm to 1 sccm during the growth process. We find that the growth temperature variation does not affect the radiative recombination mechanism; however, it does lead to a slight enhancement in the optical emission intensities. For GaAs nanowire arrays measured at room temperature, we observe the same general dependence of band gap, FWHM, and carrier concentration on doping.

(Invited) Basic Properties of h-BN Epilayers

III-nitride wide bandgap semiconductors have been at the center stage of the semiconductor R & D effort over the last two decades. Photonic/electronic devices based on III-nitrides, including UV/blue/green/white light emitting diodes (LEDs), violet/blue laser diodes (LDs), UV detectors, and high power/temperature transistors, have been successfully realized and commercialized. Among the members of the III-nitrides, BN is the least studied and understood. BN exists in three crystalline forms (hexagonal, cubic and wurtzite). The sp2-bonded hexagonal form (h-BN) is the most stable phase when synthesized at any temperature under ambient pressure. Its extraordinary physical properties, such as ultra-high chemical stability, thermal conductivity, electrical resistivity, and band gap (Eg ~ 6.5 eV) make h-BN the material of choice for many emerging applications. Due to its close lattice match to graphite, h-BN is being recognized as the most suitable substrate/dielectric/separation layer for 2D electronics and optoelectronics. As such, much efforts in the synthesis of h-BN have been largely focused on mono- or a few layers of h-BN in the past. In this presentation, I will summarize recent progress in the growth of wafer-scale h-BN epilayers with a few um in thickness by metal-organic chemical vapor deposition (MOCVD). The basic properties of h-BN epilayers will be discussed in comparison with other nitride semiconductors, in particular AlN. H-BN shows the unique qausi-2D nature with exceptionally high density of states (DOS) and large exciton binding energy (around 740 meV), which results in high optical absorption and emission intensity. P-type conduction in h-BN and diode behaviors in the p-n junction structures consisting of p-type h-BN and n-type Al-rich AlGaN heterostructures have been demonstrated, which could potentially address the intrinsic problem of low p-type conductivity in Al-rich AlGaN for DUV photonic devices. The potential applications of h-BN for deep UV emitters and detectors, 2D templates/dielectric/separation layers, and solid-state neutron detectors will also be discussed.