Enhancement of OH spectra in atmospheric pressure plasma jet by femtosecond laser

Non-thermal atmospheric pressure plasma jet (APPJ) is an attractive tool for treating bio and medical samples in atmospheric pressure because of its simple configuration and low gas temperature. Various phenomena in APPJ, such as the configuration of the APPJ,^1,2 the plasma behavior of the APPJ depending on gas for sustaining plasma,^1–4 and the plasma radiation of the APPJ,^4–6 have already been investigated by many research groups. In addition, the APPJ source was studied for a variety of applications such as wound healing,^7–9 cancer treatment,^10–13 dental treatment,^14–16 sterilization,^17–19 and treatment of other delicate biomedical samples.^20–23 

When He APPJ is discharged, ambient air diffuses into He APPJ and various reactive oxygen and nitrogen species (RONS), such as hydroxyl radical (OH), nitric oxide (NO), nitric dioxide (⁠ $NO2$⁠), ozone (⁠ $O3$⁠), and oxygen atoms (O), are produced. These RONS play a key role in biomedical applications. Among RONS, hydroxyl radicals (OH) have the strongest oxidation ability and are important species in the physicochemical processes in the APPJ. Therefore, it is important to increase the OH density in APPJ. Pei and coworkers reported that a positive DC pulsed bias was more efficient in increasing OH density than a negative DC bias.²⁴ Yue and coworkers produced APPJ with multi-ring electrodes and measured OH density in APPJ while the frequency and voltage of the DC pulse changed.²⁵ Gott and coworkers measured the increase in OH density when the operation condition of APPJ, such as voltage and frequency of DC pulse, pulse width, and gas flow rate changed.²⁶ The increase in OH density was reported when the gas mixing ratio changed^27–29 and APPJ interacted with liquid samples.^30–36 These increases in OH density were obtained by the optimization of the basic operation conditions required for APPJ discharge. However, there are a few studies in that added external effects increase OH density without altering the operation condition of APPJ.

In this study, we have irradiated fs laser at He APPJ through an objective lens and produced laser induced plasma (LIP) while maintaining a fixed operation condition of He APPJ. The increase in the OH spectra could be measured. The electron temperature and density were determined using He collisional-radiative (CR) model when the fs laser interacted with He APPJ. We have discussed the mechanism of the increase in the OH spectra.

Figure 1 shows a schematic diagram of the APPJ configuration combined with a femtosecond (fs) pulsed laser. The inner and outer diameters of the quartz tube were 4 and 6 mm, respectively. A single electrode of 10 mm width and 1 mm thickness was tightly wrapped around the quartz tube, and the electrode was placed at 15 mm distance from the quartz tube. The grounded metallic target was located at 15 mm from the quartz tube. High purity He gas (>99.999%) flowed through the quartz tube, where the gas flow rate of He gas was adjusted to 3 standard liters per minute. An AC bias of a square waveform with 30% duty cycle for a negative polarity, 5 kHz frequency, and $10 kVp.p$ voltage was produced using the combination of a function generator (Keysight, 33500B) and an AC amplifier (Trek, 10/10B-HS). The AC bias was applied to the electrode wrapped around the quartz tube.

LIP was produced using a 1030-nm fs laser (Amplitude, Tango) of which the temporal pulse width was fixed at 500 fs. The fs laser beam was focused at 7 mm from the end of the quartz tube using an objective lens (Newport, 5724-B-H), of which the working distance of the objective lens was 5.9 mm and the numerical aperture was 0.5. The objective lens was placed at 5.9 mm distance from the plasma plume of the APPJ. The beam size of fs laser was adjusted to 8.0 mm in front of the objective lens. The spot size of the laser was $∼2.6 μm$ at the focal point. The LIP was produced at the center of the plasma plume of the APPJ at the photo in Fig. 1. The laser energy was measured using an optical power sensor (Coherent, LM20) behind the focal point, and the power increased up to 184 μJ. The repetition rate of fs laser was fixed at 5 kHz, which was the same frequency of AC bias for APPJ discharge. The time interval between the pulse of fs laser and AC bias for the plasma discharge was controlled by the delay generator (SRS, DG645), where the signal from the waveform generator was used as the reference signal.

Plasma radiation from the APPJ and LIP was collected using a lens module and transferred to a spectrometer through an optical fiber of which core size was $0.4 μm$⁠. The transferred plasma radiation was analyzed using a portable spectrometer (Avantes, AvaSpec-ULS4096CL-EVO) with $<0.4 nm$ optical resolution for He transition lines. The whole optical system was relatively calibrated using the standard lamp (Aventes, AvaLight-DH-BAL-CAL).

Figure 2 shows the emission spectra of He flow and the APPJ when fs laser was irradiated and LIP was produced, where the laser energy was 184 μJ and the exposure time of the spectrometer was adjusted to 200 ms. The snapshots on the right side represent the photos for He APPJ without fs laser, LIP, and He APPJ with fs laser, respectively. When He APPJ was produced without fs laser, typical He APPJ spectra outside the quartz tube were measured in Fig. 2(a), where a strong emission of O, $N2$⁠, $N2+$⁠, and He $33S−23P$ (706.5 nm) transition line was clearly observed.^37–40 The other He transition lines were difficult to distinguish from the background level. When fs laser was irradiated at pure He gas flow without the discharge, LIP was produced and the strong emission for He and O transition lines were measured in Fig. 2(b), and the highest peak was the $33D−23P$ transition with 587.6 nm wavelength among He transition lines. The peak height of OH (⁠ $A2Σ+−X2Π$⁠) and N₂ transition lines were almost similar to the background level. Figure 2(c) shows the spectra of H, He, O, $N2$⁠, $N2+$⁠, and OH, when the fs laser was irradiated at He APPJ and LIP was produced as like the photo in Fig. 1. The peak height of OH transition line was larger than the sum of the heights in Figs. 2(a) and 2(b) and the height was clearly enhanced by the interaction of He APPJ and fs laser. The other significant variation of the APPJ spectra was not observed except where the LIP was produced.

Figure 3(a) shows the peak height of OH transition line while changing the time interval between the guided streamer in He APPJ and the laser pulse at the measurement point when the laser energy was fixed at 176 μJ. The hatched lines in the figure indicate the fluctuation in the peak height of the APPJ without fs laser. A positive time interval means that the guided streamer arrived earlier than the laser pulse at the measurement position. For negative time interval (the laser pulse arrived before the guided streamer of APPJ), the height of OH transition line remained almost constant. A noticeable increase in the OH transition line became evident at around 0 μs, and the maximum appeared at 2 μs time interval. The height returned to the peak height of OH transition without fs laser after 6 μs. This means that the interaction between the guided streamer in He APPJ and fs pulsed laser caused the increase in the peak height of OH transition line.

Figure 3(b) shows the increase in OH transition line as the light energy of fs laser changed while keeping the pulse width of 500 fs and the time interval of +2 μs. The laser energy changed from 14 μJ (0.56  $PW/cm2$⁠) to 184 μJ (7.4  $PW/cm2$⁠). When the light energy was less than 110 μJ, the peak height of He APPJ spectra was almost the same as that of the spectra without fs laser, where the spectra changed slightly depending on the operating conditions of the APPJ. As the light energy of fs laser increased, the peak height of the OH transition line increased starting from 114 μJ (4.6  $PW/cm2$⁠) while the optical breakdown appeared. In addition, the increasing speed of the OH peak height was faster at higher energy of fs laser, showing a stiff slope in more than 176 μJ. This means that the focused fs laser beam above 176 μJ produced a strong enough intense light field to generate the OH molecules remarkably.

In the APPJ, the guided streamer was produced inside the quartz tube and it propagated outside of the quartz tube. The discharging gas was mainly composed of He inside the quartz tube. However, ambient air penetrates the plasma plume of He APPJ outside the quartz tube, and He and air are mixed. The further away the plasma plume is from the quartz tube, the more air penetrates the plume. These particle densities of the positive ions, negative ions, and neutral particles were studied by Petrova and coworkers as the fraction of air changed in He APPJ.⁴¹ In their study, neutral He atoms existed uniformly throughout the whole region of the APPJ and electrons were the most dominant negative charges regardless of the fraction of air in He APPJ. The most dominant positive ions are $N+$ and $He2+$ even if there was only a trace amount of air (⁠ $∼10−6$ fraction of air) in the APPJ. As the fraction of air increased, the density of $N+$⁠, $He2+$⁠, and $He+$ rapidly decreased and the most dominant positive ion changed to $O2+$ in higher than $10−3$ fraction of air. Thus, the dominant species of the positive ions in He APPJ changed depending on the fraction of air. In addition, the most dominant neutral particles were He atoms in the ground state. The second dominant particles were oxygen atoms and molecules at the fractions below $10−5$⁠, and were molecular nitrogen at the fractions above $10−5$⁠. These compositions of He APPJ were experimentally confirmed, and the dominant species were neutral He atoms, nitrogen molecules, and water outside the quart tube.⁴² Thus, the spectra involved in these species should be observed and the spectrum in Fig. 2(a) is typical He APPJ spectra outside the quartz tube. The similar spectra of He APPJ were reported.^37–40 In addition, the spectra of LIP without He APPJ can be easily understood because fs laser interacted with pure He gas. However, it was difficult to understand the OH enhancement by the composition of the He–air mixture, and an additional explanation was needed.

The enhancement of spectra by the interaction between the guided streamer of the APPJ and LIP by the pulsed laser is almost the same as the enhancement in double-pulse laser-induced breakdown spectroscopy (DP-LiBS). There are two enhancement schemes of DP-LiBS, such as the pre-ablation scheme by change of environment of the sample and the reheating scheme of the heating electrons inside plasma by the intense light field, such as inverse bremsstrahlung (IB).^43–45 Therefore, it is necessary to analyze the plasma parameters of He APPJ interacted with fs laser to determine the dominant scheme for OH enhancement.

Among plasma diagnostic methods, a collisional-radiative (CR) model is a useful tool to analyze processes in plasma and to determine the electron temperature and density of plasma. The He CR model has been investigated by many researchers and its accuracy was evaluated at various plasmas.^46–59 In addition, we have developed the He CR model, have applied the CR model to He APPJ to determine the electron temperature and density of He APPJ, and have evaluated its accuracy in our previous work.^60–65 In our He CR model, the energy levels of the principal quantum number $n≤26$⁠, angular momentum quantum numbers $L≤3$ in $n≤7$⁠, and all spin quantum number $n≤10$ were considered.^60,63 In addition, electron impact excitation/de-excitation processes, electron impact ionization processes, spontaneous radiative emission processes, three-body recombination processes, radiative recombination processes, dielectronic recombination processes, and heavy-particle collision processes between He, $He+$⁠, $He2$⁠, and $He2+$ more than 25 categories were included.⁶³ In addition, the radiation trapping effect (RTE) was considered because RTE is one of the key processes in He plasmas.^{49–53,60,61,63} The considered whole processes and atomic data were explained in detail in our previous work.^60,63

The guided streamer of He APPJ was quite fast and it passed through the measurement region in $∼3μs$⁠. The plasma radiation was observed in a short time, and the plasma radiation was absent at other most time of a period of AC bias. Thus, the measured spectra were averaged values of plasma radiation, and the measured peak intensity of He spectra was underestimated. However, the transition line intensity ratio (LIR) between different transition lines remains constant because the decay rates of the excited states of He atom were less than $1μs$ and the background light was eliminated as shown in Fig. 2. Plasma parameters of electron temperature and density were determined by analyzing the spectra of He APPJ with the He CR-model.

The He CR-model gives the population densities of the excited states of He, and the CR model alone cannot determine the electron temperature and density of He plasma. When the CR model is combined with the transition line intensity ratio (LIR) method, electron temperature and density can be determined.^50,60,62 In the experiment, the absolute intensities of transition lines can change due to the operation condition of the plasma, such as the discharging power, gas pressure, and plasma radius, despite the same electron temperature and density. However, the intensity ratio of transition lines cannot change for the same electron temperature and density.⁶⁶ In our experiment, four LIR of 667.8 nm (⁠ $31D−21P$⁠)/728.1 nm (⁠ $31S−21P$⁠), 728.1 nm/706.5 nm (⁠ $33S−23P$⁠), 501.6 nm (⁠ $31P−21S$⁠)/706.5, and 587.6 nm (⁠ $33D−23P$⁠)/706.5 nm of He atoms were selected. The electron temperature and density of He APPJ interacting with fs laser was determined using the He CR model combined with the LIR method, and Fig. 4 shows the electron temperature and density as the fs laser energy increased.

When He APPJ was discharged, ambient air diffused into the APPJ and OH molecules in He APPJ were produced from water molecules (H₂O) in the air. Table I represents the OH production processes in He APPJ. OH molecules are produced by the electronic collisional dissociation processes (R1–R3) and the heavy particle collisional dissociation processes with different atoms and molecules (R4–R12). One of the reactants is the product of the dissociation of H₂O and O₂ in the heavy particle collisional dissociation processes. In addition, the dissociative rate coefficients are less than $10−20 m3/s$ for two body collisional processes (R5–R8) and $10−43 m3/s$ for three body collisional processes (R9–R12).

In addition, the reactions of R13–R18 represent the reaction of OH production involved in He⁺, $He2+$⁠, and He^*, which stands for neutral He atom in a metastable state. The rate coefficients of these reactions are higher than those of R1–R3, except for the reaction of R14. However, there existed many different positive ions in He APPJ. Among these positive ions, He⁺ and $He2+$ were mainly distributed at the edge of the plasma plume.⁷⁵ The spatial distribution of He^* was similar to those of He⁺ and $He2+$⁠, where these distribution characteristics were explained in detail in Ref. 76 In addition, the most dominant negative charge and neutral particle were electron and He atoms in the ground state. Therefore, electrons and He atoms in the ground state abundantly existed in the entire region of He APPJ, and the density of He⁺, $He2+$⁠, and He^* were less than 1/10 000 at the center of the plasma plume compared with electrons. In addition, fs laser was focused on the center of the plasma plume, and the intense laser light field can easily heat electrons in the center of the plasma plume. Thus, the electronic collisional dissociation processes should be the most dominant process for the production of OH molecules. The more the electron temperature and density increased, the more OH molecules produced. Therefore, as the light energy of fs laser increased, electron temperature and density increased, and OH density simultaneously increased.

The increasing behavior of OH peak intensity was more similar to that of electron temperature than electron density. A high energy of more than 5 eV is required to produce OH molecules by the dissociation of H₂O (R1). Energy over 9 eV is needed to produce the excited OH(A). The wavelength of fs laser was 1030 nm, and the corresponding photon energy was not sufficient to dissociate H₂O directly. Thus, H₂O does not absorb the light field of fs laser and does not dissociate with OH molecules. Hot electrons are needed for H₂O to dissociate to OH molecules inside the APPJ. When the intense light field of fs laser was irradiated at He APPJ, many electrons were heated by IB, hot electrons dissociated H₂O to OH(X) molecules in the ground state, and OH(X) was excited to OH(A) by the electronic collisional process. In addition, when the high-energy laser of $>176μJ$ was irradiated, the electron temperature was heated over ∼6.5 eV. The portion of hot electrons of >9 eV increased to more than 42% in LIP when the Maxwellian electron energy distribution was considered. Thus, the more the intensity of fs laser increased, the more electrons were heated by IB, the more the density of OH molecules increased by the electronic collisional dissociation process, and the faster the increasing rate of OH peak height. In addition, the increase in OH density by fs laser can be achieved without the change of the operation condition of He APPJ, such as AC voltage, frequency, duty cycle, gas flow rate, and gas composition. When the increasing method of OH density by fs laser is combined with the other method, OH density can be increased more in He APPJ. The position of LIP by fs laser can easily move to a different position and add at multiple positions simultaneously because the optical system for LIP is independent of the plasma discharge system and the plasma volume of LIP was small. Therefore, additional studies are needed to extend the OH enhancement region and to optimize the operation condition of fs laser.

We have configured the He APPJ system combined with the fs laser and the objective lens. The LIP was produced inside He APPJ by fs laser, and the spectra of the He APPJ were measured. As the light energy of fs laser increased, the peak height of the OH transition line was enhanced in the laser energy up to more than 114 μJ. When the time interval between the guide streamer and the laser pulse was 2 μs, the highest enhancement was observed at the position of LIP. The enhancement was observed in the range of 0–6 μs time interval. At the highest light energy of 184 μJ, the peak height of the OH transition was enhanced by ∼1.5 times compared to the case of the absence of fs laser. Plasma parameters were diagnosed using the He CR model for atmospheric pressure. The electron temperature and density were 1.6 eV and $∼5×1011 cm−3$⁠, respectively, when the optical breakdown was not observed inside He APPJ. The electron temperature and electron density increased up to 7.2 eV and $∼1.7×1014 cm−3$⁠, respectively, by the interaction of fs laser and He APPJ.

OH molecules are supplied from the dissociation of H₂O in ambient air, where the dissociation of H₂O needs much energy higher than 5 eV. Thus, hot electrons or photons with short wavelength were required to increase the population density of OH molecules in the APPJ. However, the AC bias with 1–100 kHz frequency was usually adopted to produce APPJ, and there is a limit to producing hot enough electrons in the APPJ by controlling the operation conditions of the APPJ. However, the strong light intensity by fs laser can produce hot electrons inside the APPJ by the inverse bremsstrahlung process. Our results suggest that the re-heating technique using fs laser can be a good candidate to enhance the population density of OH molecules apart from the existing method.

This work was supported by Samsung Display Co., Ltd. and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2020R1A2C2006108).

The authors have no conflicts to disclose.

Wonwook Lee: Conceptualization (lead); Formal analysis (equal); Funding acquisition (lead); Investigation (equal); Methodology (equal); Project administration (lead); Supervision (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal). Tuyen Ngoc Tran: Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Visualization (equal). Juil Hwang: Data curation (equal); Investigation (equal); Methodology (equal). Kwang-Geol Lee: Methodology (equal); Resources (equal); Supervision (equal); Validation (equal). Hyungsik Kim: Methodology (equal); Resources (equal). Woohyun Jung: Methodology (equal); Resources (equal). Kisang Lee: Methodology (equal); Resources (equal). Cha-Hwan Oh: Supervision (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.