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http://iopscience.iop.org/issue/1347-4065/56/9

Japanese Journal of Applied Physics

Table of contents

Volume 56 Number 9, September 2017

Illumination intensity dependence of the performance of interdigitated back contact heterojunction silicon solar cells and the effects of front surface field 090301 Hiroshi Noge, Hideyuki Takagishi, Kimihiko Saito and Michio Kondo

Mirror-based polarization-insensitive broadband vertical optical coupling for Si waveguide 090302

Akihiro Noriki, Takeru Amano, Daisuke Shimura, Yosuke Onawa, Hironori Sasaki, Hiroki Yaegashi, Koji Yamada, Hidetaka Nishi, Tai Tsuchizawa, Masahiko Mori and Yoichi Sakakibara

Weak temperature and bias dependence of spin accumulation in epitaxial Co

70

Fe

30

/MgO

tunnel contacts to p-type Ge 090303

Kun-Rok Jeon, Byoung-Chul Min and Seoung-Young Park

Design of slotted high quality factor photonic-crystal nanocavities embedded in electro-optic polymers

Masahiro Nakadai, Ryotaro Konoike, Yoshinori Tanaka, Takashi Asano and Susumu Noda Effects of surface morphology on the ionic capacitance and performance of perovskite solar

cells 090305

Hong Liu, Chun-Jun Liang, Hui-Min Zhang, Meng-Jie Sun, Jing-Jing Liang, Xue-Wen Zhang, Chao Ji, Ze-Bang Guo, Ya-Jun Xu and Zhi-Qun He

Surface smoothing of poly(methyl methacrylate) film by laser induced photochemical etching

JoonHyun Kang, Song-ee Lee, Joon-Suh Park, Young-Hwan Kim and Il Ki Han 090306

Design of compact surface optical coupler based on vertically curved silicon waveguide for

high-numerical-aperture single-mode optical fiber 090307

Yuki Atsumi, Tomoya Yoshida, Emiko Omoda and Youichi Sakakibara

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Regular Papers

Semiconductors, dielectrics, and organic materials

First-principles calculation of electron–phonon coupling at a Ga vacancy in GaN 091001 Takeshi Tsujio, Masato Oda and Yuzo Shinozuka

Interface trap of p-type gate integrated AlGaN/GaN heterostructure field effect transistors

Kyu Sang Kim 091002

Improved hysteresis in a normally-off AlGaN/GaN MOS heterojunction field-effect transistor with a recessed gate structure formed by selective regrowth 091003 Satoshi Nakazawa, Nanako Shiozaki, Noboru Negoro, Naohiro Tsurumi, Yoshiharu Anda, Masahiro Ishida and Tetsuzo Ueda

Crystal defects observed by the etch-pit method and their effects on Schottky-barrier-diode

characteristics on β-Ga

2

O

₃

091101

Makoto Kasu, Takayoshi Oshima, Kenji Hanada, Tomoya Moribayashi, Akihiro Hashiguchi, Toshiyuki Oishi, Kimiyoshi Koshi, Kohei Sasaki, Akito Kuramata and Osamu Ueda

Towards an understanding of hot carrier cooling mechanisms in multiple quantum wells Gavin Conibeer, Yi Zhang, Stephen P. Bremner and Santosh Shrestha 091201 Photonics, quantum electronics, optics, and spectroscopy

Enhanced light extraction efficiency of micro-ring array AlGaN deep ultraviolet light-

emitting diodes 092101

Gabisa Bekele Fayisa, Jong Won Lee, Jungsub Kim, Yong-Il Kim, Youngsoo Park and Jong Kyu Kim

111 sun concentrator photovoltaic module with wide acceptance angle that can efficiently operate using 30-min intermittent tracking system 092301 Nawwar Ahmad, Yasuyuki Ota, Kenji Araki, Kan-Hua Lee, Masafumi Yamaguchi and Kensuke Nishioka

Precise characterization of focused vortex beams 092501 Takahiro Saito, Yoko Takeo and Hidekazu Mimura

Highly efficient red-emitting BaMgBO

₃

F:Eu

³⁺

,R

⁺

(R: Li, Na, K, Rb) phosphor for near-UV excitation synthesized via glass precursor solid-state reaction 092601 Kenji Shinozaki and Tomoko Akai

Observation of applied voltage response of dye-doped liquid crystal by optical measurement

of real and imaginary parts of complex refractive index 092602

Moritsugu Sakamoto, Kenta Bannai, Kohei Noda, Tomoyuki Sasaki and Hiroshi Ono

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Device physics

Evaluation of AlGaN/GaN metal–oxide–semicondutor high-electron mobility transistors with plasma-enhanced atomic layer deposition HfO

₂

/AlN date dielectric for RF power applications Yu Sheng Chiu, Quang Ho Luc, Yueh Chin Lin, Jui Chien Huang, Chang Fu Dee,

Burhanuddin Yeop Majlis and Edward Yi Chang 094101

Numerical simulations of the electrical transport characteristics of a Pt/n-GaN Schottky diode Fayçal Bouzid, Fortunato Pezzimenti, Lakhdar Dehimi, Mohamed L. Megherbi and

Francesco G. Della Corte 094301

Nanoscale science and technology

Sub-10 nm, high density titania nanoforests–gold nanoparticles composite for efficient

sunlight-driven photocatalysis 095001

Viet V. Tran, Oanh T. T. Nguyen, Chi H. Le, Tuan A. Phan, Ban V. Hoang, Thang D. Dao, Tadaaki Nagao and Chung V. Hoang

Zero percolation threshold in electric conductivity of aluminum nanowire network fabricated by chemical etching using an electrospun nanofiber mask 095002 Keisuke Azuma, Koichi Sakajiri, Takashi Okabe, Hidetoshi Matsumoto, Sungmin Kang, Junji Watanabe and Masatoshi Tokita

Thermoelectric elastomer fabricated using carbon nanotubes and nonconducting polymer 095101 Jeong-Hun Choi, Cheol-Min Hyun, Hyunjin Jo, Ji Hee Son, Ji Eun Lee and Ji-Hoon Ahn Terahertz spectroscopy of graphene complementary split ring resonators with gate tunability Satoru Suzuki, Yoshiaki Sekine and Kazuhide Kumakura 095102 Electrical characterization and microwave application of polyacrylonitrile/carbon nanotube-

based carbon fibers 095103

Eiichi Sano, Takehito Watanuki, Masayuki Ikebe and Bunshi Fugetsu

Novel band structures in germanene on aluminium nitride substrate 095201 Miaojuan Ren and Mingming Li

Crystal growth, surfaces, interfaces, thin films, and bulk materials

Atomistic study of comblike structure on the MoO

₂

/Mo(110) surface by scanning tunneling microscopy and density functional theory calculations 095501 Arifumi Okada, Shinsuke Hara and Masamichi Yoshimura

Epitaxy of Si

1−x

C

x

via ultrahigh-vacuum chemical vapor deposition using Si

2

H

6

, Si

3

H

8

, or

Si

4

H

10

as Si precursors 095502

Sangmo Koo, Hyunchul Jang and Dae-Hong Ko

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Reduction in number of crystal defects in a p

⁺

Si diffusion layer by germanium and boron cryogenic implantation combined with sub-melt laser spike annealing 095503 Atsushi Murakoshi, Tsubasa Harada, Kiyotaka Miyano, Hideaki Harakawa, Tomonori Aoyama, Hirofumi Yamashita and Yusuke Kohyama

Improvement of gas hydrate preservation by increasing compression pressure to simple

hydrates of methane, ethane, and propane 095601

Masato Kida, Yusuke Jin, Mizuho Watanabe, Tetsuro Murayama and Jiro Nagao

First-principles study on adsorption structure and electronic state of stanene on α-alumina

surface 095701

Masaaki Araidai, Masashi Kurosawa, Akio Ohta and Kenji Shiraishi

Single crystal elasticity of gold up to ~20 GPa: Bulk modulus anomaly and implication for a

primary pressure scale 095801

Akira Yoneda, Hiroshi Fukui, Hitoshi Gomi, Seiji Kamada, Longjian Xie, Naohisa Hirao, Hiroshi Uchiyama, Satoshi Tsutsui and Alfred Q. R. Baron

Plasmas, applied atomic and molecular physics, and applied nuclear physics

Measurement of linear polarization in the HeI 2

¹

P–3

¹

D emission line in an electron cyclotron

resonance discharge plasma 096101

Takanori Higashi, Taiichi Shikama, Tatsuya Teramoto, Akira Ueda and Masahiro Hasuo Low pressure micro-Joule picosecond laser-induced breakdown spectroscopy and its prospective applications to minimally destructive and high resolution analysis 096201 Syahrun Nur Abdulmadjid, Eric Jobiliong, Maria Margaretha Suliyanti, Marincan Pardede, Hery Suyanto, Koo Hendrik Kurniawan, Tjung Jie Lie, Rinda Hedwig, Zener Sukra Lie, Indra Karnadi, Erning Wihardjo, May On Tjia and Kiichiro Kagawa

OPEN ACCESS

Simple introduction of carboxyl head group with alkyl spacer onto multiwalled carbon

nanotubes by solution plasma process 096202

Shimpei Nemoto, Tomonaga Ueno, Anyarat Watthanaphanit, Junko Hieda, Maria Antoaneta Bratescu and Nagahiro Saito

Device processing, fabrication and measurement technologies, and instrumentation Thermally enhanced formation of photon-induced damage on GaN films in Cl

₂

plasma

096501 Zecheng Liu, Atsuki Asano, Masato Imamura, Kenji Ishikawa, Keigo Takeda, Hiroki Kondo, Osamu Oda, Makoto Sekine and Masaru Hori

Spotlights 2017

Properties of quarter-wavelength coaxial cavity for triode-type thermionic RF gun 096701

Konstantin Torgasin, Kenta Mishima, Heishun Zen, Kohei Yoshida, Hani Negm, Muhamed

Omer, Toshiteru Kii, Kazunobu Nagasaki, Kai Masuda and Hideaki Ohgaki

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Cross-disciplinary areas

Concentration dependence of cation-induced electrohydrodynamic flow passing through an

anion exchange membrane 097201

Ayako Yano, Hiroki Shirai, Moino Imoto, Kentaro Doi and Satoyuki Kawano

Experimental study on the motion of droplets excited by Lamb waves on an inclined non-

piezoelectric substrate 097301

Jun Zhu, Wei Liang and Gaohui Li

Theoretical model of lossy acoustic bipolar cylindrical cloak with negative index

metamaterial 097302

Yong Y. Lee and Doyeol Ahn

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Low pressure micro-Joule picosecond laser-induced breakdown spectroscopy and its prospective applications to minimally destructive and high resolution analysis

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Low pressure micro-Joule picosecond laser-induced breakdown spectroscopy and its prospective applications to minimally destructive and high resolution analysis

Syahrun Nur Abdulmadjid¹, Eric Jobiliong², Maria Margaretha Suliyanti³, Marincan Pardede⁴, Hery Suyanto⁵, Koo Hendrik Kurniawan⁶*, Tjung Jie Lie⁶, Rinda Hedwig⁷, Zener Sukra Lie^6,7, Indra Karnadi⁸, Erning Wihardjo⁸, May On Tjia^6,9, and Kiichiro Kagawa^6,10

1Department of Physics, Faculty of Mathematics and Natural Sciences, Syiah Kuala University, Darussalam, Banda Aceh 23111, NAD, Indonesia

2Department of Industrial Engineering, University of Pelita Harapan, Lippo Village, Tangerang 15811, Indonesia

3Research Center for Physics, Indonesia Institute of Sciences, Kawasan PUSPIPTEK, Serpong, Tangerang Selatan 15314, Banten, Indonesia

4Department of Electrical Engineering, University of Pelita Harapan, Lippo Village, Tangerang 15811, Indonesia

5Department of Physics, Faculty of Mathematics and Natural Sciences, Udayana University, Kampus Bukit Jimbaran, Denpasar 80361, Bali, Indonesia

6Research Center of Maju Makmur Mandiri Foundation, Kembangan, Jakarta Barat 11630, Indonesia

7Department of Computer Engineering, Bina Nusantara University, Jakarta 14810, Indonesia

8Department of Electrical Engineering, Krida Wacana Christian University, Jakarta 11470, Indonesia

9Physics of Magnetism and Photonics Group, Faculty of Mathematics and Natural Sciences, Bandung Institute of Technology, Bandung 40132, Indonesia

10Fukui Science Education Academy, Fukui 910-0804, Japan

*E-mail: kurnia18@cbn.net.id

Received January 23, 2017; revised May 18, 2017; accepted June 7, 2017; published online August 2, 2017

A time-resolved spectroscopic study is performed by using 125–500 micro-Joule (µJ) ps laser focused directly without the aid of microscope on a Cu plate sample in a variety of low-pressure ambient gases including air, helium and argon. It is shown that the ultrashort µJ laser-induced low- pressure plasma in Ar ambient gas exhibits the typical characteristics of shock wave plasma responsible for the thermal excitation and sharp emission of the analyte atoms. It is found that the highest signal to background (S/B) ratio of about 100 is achieved in 1.3 kPa argon ambient gas and detected with optical multichannel analyzer (OMA) gate delay of 1 ns and gate width of 50 µs. The emission spectra obtained from pure Zn sample show the effective suppression of the ionic emission with ablation energy around and below 500 µJ. The experimental setup is successfully applied to Cr analysis with low detection limit in steel. In particular, its application to C analysis in steel is demonstrated to resolve the long standing problem of overlapping contributions from the neutral and ionic Fe emission. It is further found that an element of high excitation energy such as fluorine (F) can be clearly detected from a non metal teflon sample. Further, its application to alluminum sample containing various concentrations of Mg, Ca, Fe, and Si impurity elements clearly displays the existence of linear calibration lines promising for quantitative analyses in certain dynamical ranges. Finally, in view of the tiny crater sizes of less than 10 µm diameter created by the very low ps laser energy, this technique is promising for micrometer resolution mapping of elemental distribution on the sample surface and its depth profiling.

1. Introduction

The application of laser-induced breakdown spectroscopy (LIBS) for practical and rapid spectrochemical analysis has in the last decade found its way into ever expanding areas of material testing and investigation in industries and research laboratories. Its rising popularity is largely due to improvements in the spectroscopic sensitivity and accuracy^1,2)as well as the technical advances of the ablation lasers.^3,4)Addition- ally, explorations of alternative experimental setups such as a variety of double pulse conﬁgurations⁵^–¹⁰⁾as well as more favorable ambient gases of lower gas pressures¹¹^–¹³⁾ have also been carried out for improvements of the spectral quality. In particular, taking advantage of the delayed He assisted excitation (HAE) mechanism using low pressure ambient He gas has been demonstrated to promise further LIBS applications demanding extra high resolution and extra low limits of detection (LOD).^14,15)

In another direction of recent developments, a number of studies have been conducted taking advantage of the readily available ultrashort pulse lasers, such as picosecond (ps) and femtosecond (fs) lasers which offer the benefit of effectively delivering high power density of irradiation pulses on the target surface at much lower laser energy output without producing the undesirable long heating effect.¹⁶^–²⁰⁾ Further, melting of sample and material redeposition around the crater are minimized and making possible the realization of precision ablation with crater sizes of tens of µm.^21,22)With

further reduced crater size, this is potentially useful for high spatial resolution mapping of elemental distribution. The use of such pulses were also reported to lower ablation or gas breakdown threshold.^23,24) In the earlier applications of the ultrashort pulses in LIBS, atmospheric ambient air is commonly used with mJ pulse energies are employed,^22,25,26) that generally exhibit the advantage of low background with demonstrated potential application for quantitative analysis.

Further improvement of spectral resolution and enhancement of detection limit was reported for the special case of Hg analysis in gas phase in low pressure ambient air.²⁷⁾In a more recent reports^20,28) the dynamic characteristics of emission induced by ns and ps Nd:YAG laser from metal samples in low pressure He ambient gas were shown to exhibit typical features of shock-wave-induced emission. Additionally, the use of ps laser was shown to give distinctly better spectral quality of the H impurity emission at low output laser energy of 1 mJ and 300 µJ along with considerably smaller crater size of 10 µm compared to 50 µm created by the ns laser.

In the mean time, the development of the so-called µLIBS has been reported for LIBS measurement of elemental distribution with micrometer spatial resolution. The results of its applications have been reported for high-resolution microscopic surface mapping of elemental distributions on conductive and non-conductive samples,²⁹⁾ biological samples,³⁰⁾ silicone,³¹⁾ and microscopic mapping of rare earth elements in nuclear wastes.³²^–³⁴⁾ In particular Wang et al.³⁴⁾ reported the result of their microscopic surface mapping of Japanese Journal of Applied Physics56, 096201 (2017)

https://doi.org/10.7567/JJAP.56.096201 REGULAR PAPER

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the rare earth elements in nuclear waste glass-ceramic using ns Nd:YAG laser operated at 266 nm with 2 mJ output energy delivering 400 µJ ablation pulse energy on the target. With the single-shot crater size of less than 7 µm, the resulted map is shown to attained a spatial resolution of about 10 µm. Most recently, the µLIBS was extended into a hybrid LIBS-Raman microanalysis tool oﬀering the possibility of high spatial mapping of elemental and molecular compositional distributions.³⁵⁾ It is now understood that the much needed high spatial resolution in µLIBS can only be achieved with corre- spondingly small crater size created by considerable reduction of laser pulse energy without compromising the resulted emission intensity and spectral quality.

In view of the benefits of employing low pressure ambient gas and short laser pulses described earlier, it is desirable to combine those benefits with low ablation energies (µJ) and investigate the possible further enhancement in the spectral quality and the reduction of crater size without weakening the induced shock wave plasma and compromising the spectral quality. For that purpose, this experiment is undertaken to investigate and verify the generation of effective shock wave plasma for the required thermal excitation process in low pressure ambient gases using µJ ps laser. The prospect for minimally destructive and high resolution spectrochemical analysis is demonstrated using a variety of samples.

2. Experimental procedure

The experimental setup employed in this experiment is basically the same as the one shown in our previous reports.^21,22,27) The ps 1,064 nm Nd:YAG laser (Ekspla PL 2143, 20 ps, maximum energy of 30 mJ) is operated in the Q-switched mode at a repetition rate of 10 Hz with the laser output energy varied from 125 to 500 µJ yielding a minimum ablation power density of around 10 TW=cm² at the center of the irradiated spot. The plasma is generated by focusing the ps laser beam on the surface of the target using a lens of 70 mm focal length. The spectral measurement of the secondary plasma emission is carried out by employing an optical multichannel analyzer (OMA; Andor I+Star intensi- fied CCD 1,024 × 256 pixels) of 0.012 nm spectral resolution at 500 nm. This system is attached to the output port of a spectrograph (McPherson 2061 with 1,000 mm focal length f=8.6 Czerny Turner configuration) which has its input port connected to an opticalfiber. The opposite end of thefiber is inserted into the chamber for the collection of the plasma emission. The OMA is operated with 1 ns gate delay and 50 µs gate width.

A copper plate (Rare Metallic, 99.99%), which is known to have very strong and sharp emission lines is used as the sample in the experiment for investigating the induced plasma characteristics and the pressure dependent emission intensities while a zinc sample is used for the measurement of its ionic emission lines. For the demonstration of practical application, both steel samples (JSS 154) containing Cr 2.04%, pure iron sample (Rare Metallic, 99.999%, thickness of 0.4 mm), standard aluminium sample (Rare Metallic, 99.99%, thickness of 0.2 mm), and a commercially available Teflon sample are used. The sample chamber is a small, vacuum-tight metal chamber of 11 × 11 × 12.5 cm³, which is evacuated with a vacuum pump and thereafterfilled with one of the selected ambient gases (air, Ar, and He) at certain pressure. For examining the stability of peak emission intensity during increasing number of laser shots, the sample is irradiated at thefixed position and the resulted incremental depths per pulse are measured using a digital microscope (Keyence VHX-700).

3. Results and discussion

The ﬁrst part of this study is devoted to verify the shock- wave plasma generation. Figure 1 shows the photos of plasmas generated by irradiating a Cu target with ps laser pulses of 125, 250, and 500 µJ in ambient Ar gas of 1.3 kPa.

Despites their relatively small sizes, all the plasmas exhibit the hemispherical shape and consist of the tiny primary plasma at the target surface and the much larger secondary plasma growing out of the primary plasma. These structural characteristics are typical of shock wave induced plasmas reported previously using ns and ps lasers of 1 mJ laser pulse.14,15,20,28)The dynamical shock wave characteristics of the generated plasma is further verified by measuring the intensity time profiles from Cu target irradiated by 125 µJ ps laser. Figure 2 shows the results for Cu I 510.5 nm and Cu I 521.8 nm emission lines. The emission intensities of both lines are seen to increase rapidly reaching their maxima within 2 µs, which are followed by slow decays before vanishing after 10 µs. This dynamical behavior represents respectively the characteristic thermal excitation stage and the subsequent cooling stage in the shock wave plasma excitation process. The plasma temperatures are estimated by assuming the local thermodynamic equilibrium (LTE) state characterized by the electron number density above a certain limit.²⁶⁾ In our case, the figure estimated from the Stark broadening effect is between 10¹⁶–10¹⁷cm⁻³, which clearly satisfies the validity of LTE, and allows the temperature of 125 µJ

1.1 mm 1.6 mm

500 µJ

2.8 mm 250 µJ

Fig. 1. (Color online) Photograph of the micro-plasma generated by using Cu target in ambient Ar gas at 1.3 kPa with the laser energies set at 125, 250, and 500 µJ.

Jpn. J. Appl. Phys.56, 096201 (2017) S. N. Abdulmadjid et al.

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Cu plasma to be estimated from the intensity ratio of the two emission lines on the basis of Boltzmann distribution law. The resulted temperature estimates of peak emission at 2 µs is about 7500 K, which decreases to 6500 K at 10 µs.

At these temperatures, the plasma thermal energies are more than adequate to excite the ablated Cu atoms in the two stages of shock wave plasma excitation process. We have thus demonstrated that shock wave plasma can indeed be eﬀectively generated even with 125 µJ ps laser pulses.

The second part of this study is conducted to investigate the maximum reduction of crater size produced by the the lowest energy of ps laser pulses. Figure 3 shows the crater produced on the Cu target by 100 successive shots of 125 µJ ps laser pulses. The resulted crater has a diameter of 32.8 µm and approximately depth of 2.5 µm. Thus the crater created by a single irradiation pulse is expected to be less than 3 µm wide and 25 nm deep which is less than half the size reported earlier³⁴⁾ and certainly more than adequate for its application to microscopic mapping of elemental distribution on the sample surface as well as profiling of elemental distribution beneath the surface. For this application, the plasma stability is examined by measuring the emission intensities with the target rotated at 1 rpm during the laser irradiation. Figure 4 shows the spectra of Cu I 521.8 nm, Cu I 515.3 nm, and Cu I 510.5 nm obtained from 10 series of measurements with 10 accumulated shots conducted to yield the average spectral data presented in the figure. For each series, thefluctuation is estimated to be around 6% which is

certainly low enough for producing a credible compositional mapping on the sample surface and in depth proﬁling. This is further examined by measuring the series of Cu emission lines produced with repeated irradiations up to 200 laser shots in 1.3 kPa ambient Ar gas. The result presented in Fig. 5 show that the emission intensities per shot decrease by about 30, 50, and 60% after 100, 150, and 200 shots, respectively.

This intensity variation is understood as the consequence of increasing crater depth induced plasma confinement effect as reported previously.¹⁵⁾ Nevertheless, the intensity ratio between Cu I 521.8 nm and Cu I 510.5 nm emission lines remains approximately constant during the first 150 shots as shown in the inset of Fig. 5. This indicates that the plasma temperature does not change significantly during the laser drilling process. With further development, the result signifies the possibility of its application to the depth profiling of elemental distribution in the sample.

The following experiment is carried out to investigate the diﬀerent spectral qualities of Cu emission spectra produced in diﬀerent ambient gases at various gas pressures.

In particular, the benefit of Ar ambient gas is examined as it was reported previously to offer the desired favorable condition.²⁰⁾Figure 6(a) shows the spectra of Cu lines produced by 125 µJ ps laser pulses in ambient Ar gas at different gas pressures of 1.3 kPa, 13 kPa, and 1 atm. The resulted spectra of typical Cu I 510.5 nm, Cu I 515.3 nm, and Cu I 521.8 nm emission lines are all seen to have very low backgrounds, and very narrow spectral profiles with about 0.1 A line widths at 1.3 kPa. By increasing the gas pressure, the emission lines become considerably weaker. Figure 6(b) shows the plot of pressure dependent intensities of those emission lines in ambient air and argon gas. The close-up views of the detailed variations at low pressures are displayed in the insets. In all cases, the emission intensities practically vanish at 100 kPa.

Meanwhile the highest intensities are all attained at 1.3 kPa in both ambient gases, with the emission intensities produced in Ar ambient gas being consistently much higher than those produced in air.

Recalling the advantages of ambient He gas employed in previous studies as mentioned in the introduction,^14,15,20)it is necessary to investigate whether the shock wave plasma induced with the very low ps laser energy can produce suﬃcient thermal energy for the excitation of the He atom which is known to have high excitation energies for its

Fig. 2. Time proﬁles of Cu I 510.5 nm and Cu I 521.8 nm emission intensities induced by 125 µJ ps laser in 1.3 kPa Ar ambient gas.

Diameeter = 32.88 µm

Fig. 3. (Color online) Crater produced by 100 shots of 125 µJ ps laser on Cu target in ambient Ar gas at 1.3 kPa.

Fig. 4. Cu emission spectra produced by 10 series of measurements with 125 µJ ps laser irradiation on rotating target in 1.3 kPa ambient Ar gas. The data presented in theﬁgure are the averages obtained from 10 accumulated laser shots in each measurement.

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metastable excited states. Figure 7 shows the spectra generated in He surrounding gas at 1.3 kPa. As shown in Fig. 7(a), there is no sign of He I 388.8 nm emission line produced at laser energy of 125 µJ. However, as the energy is increased to 500 µJ, a small signal of the He I 388.8 nm

emission line becomes clearly observable as shown in Fig. 7(b) along with He I 587.6 nm in a separate spectral range not covered in the ﬁgure. This indicates that the He atoms in the surrounding He gas are not excited at ablation energy of less than 500 µJ. Instead, the He atoms only (a)

(b)

Fig. 6. (Color online) Typical emission spectrum of Cu irradiated by 125 µJ ps laser in 1.3 kPa Ar ambient gas (a) and pressure dependent of typical Cu emission lines in 1.3 kPa ambient Ar gas and air (b).

Fig. 5. Variations of peak intensities of Cu I 510.5 nm, Cu I 515.3 nm, and Cu I 521.8 nm emission lines with increasing number of laser shots using 125 µJ ps laser in 1.3 kPa ambient Ar gas.

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functions as a damping medium. In other words, the HAE mechanism is not expected to work and oﬀer the aforemen- tioned advantages in this experiment.

Another issue that needs to be addressed is the influence of ions produced initially in the plasma. It is well known that the strong shock wave generated by high energy laser leads to the formation of high-temperature plasma with the production of ions and electrons followed by the rapid recombination process subsequently. The ionic emission lines produced from may unnecessarily complicate the associated emission from the ablated elements. It is important to find out the amount of ions produced in the µJ ps laser-induced low- pressure plasma and how fast the recombination processes take place or how much the emission is affected by the ablation energy. For this purpose, a Zn sample is used which is known to produce the atomic line of Zn I 481.0 nm as well as ionic lines of Zn II 491.1 nm and Zn II 492.4 nm in conventional LIBS,^12,14,15) and all those emission lines can be accommodated in one spectral window of our detection system. Figure 8(a) shows Zn I 481.0 nm, ionic Zn II 491.1 nm, and Zn II 492.4 nm emission lines from the plasma generated by 500 µJ ps laser. Despite the small size of the plasma, it is seen that all those lines are clearly detected with the ionic emission showing generally much lower intensities than the atomic emission intensity. Figure 8(b) displays the pulse energy dependent intensities of the three emission lines.

The small ionic lines are shown with 10× ampliﬁcation in the extended spectral region of shorter wavelengths. The Zn I emission intensity exhibits a fast initial rise to its maximum at 500 µJ and abruptly levels oﬀbeyond that. On the other hand, the Zn II emission intensities are negligible below 200 µJ and only begin to increase monotonously with increasing laser energy (>200 µJ) which is clearly related with the relatively higher Zn ionization energy of 7.7 eV. The energy dependent variation of Zn I=Zn II intensity ratio plotted in Fig. 8(c) clearly shows that the recombination processes take place rapidly and the contribution of the ionic emission can

virtually be neglected at ps laser output energy of less than 500 µJ in 1.3 kPa ambient Ar gas.

Given the favorable results described above, it is desirable to apply the low-pressure µJ ps LIBS technique to LIBS analyses of samples with diﬀerent elemental compositions for its further assessment. One of these is the Cr analysis using a standard JSS 154 steel sample. Figure 9 shows the typical spectrum of sharp Cr emission lines with high S=B ratios of 60 obtained from 10 laser shots of 125 µJ ps laser energy. The detection limit of better than 200 ppm is estimated from this spectrum using the conventional criterion of three times the noise level in the vicinity of the signal divided by the signal intensity. This detection limit is well below the standard limit required for Cr analysis in the steel industry. The same experimental condition is subjected to more severe test by applying it to the more demanding analyses of C in steel due to the problem of simultaneous occurrence of the C I 247.8 nm, Fe II 247.857 nm, Fe I 247.8541 nm, and Fe I 247.8573 nm emission lines commonly found using ICP and conven- (a)

(b)

Fig. 7. Spectra produced in He ambient gas at a pressure of 1.3 kPa and ps laser energy of (a) 125 µJ and (b) 500 µJ.

(a)

(b)

(c)

Fig. 8. (a) Spectra of Zn I 481.0 nm, Zn II 491.1 nm, and Zn II 492.4 nm emission lines produced by using 500 µJ ps laser. (b) Plots of the energy dependent intensity variations of neutral Zn and ionic Zn emission lines detected in ambient Ar gas at 1.3 kPa. (c) Energy dependent intensity ratio between the Zn I 481.0 nm and Zn II 491.1 nm.

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tional LIBS.^36,37) Figure 10(a) displays the emission spectrum of JSS 150 steel sample containing 0.4% C. Clearly the emission line at 247.8 nm could be designated as the combined contribution of those four lines. For the purpose of a credible C analysis, either a detection system of ultra high resolution of 0.0001 nm is required, or alternatively the exclusion of those three Fe contributions can be veriﬁed. The second choice is pursued by repeating the same measurement on a pure iron sample (Rare Metallic, 99.999%). The resulted spectrum presented in Fig. 10(b) clearly shows no sign of the Fe I and Fe II emission lines, implying that the C I 247.8 nm emission line in Fig. 10(a) is indeed free from the inter- ference of Fe emission. Therefore C analysis of steel sample

may reliably be performed with the experimental setup proposed in the study. Admittedly, the crucial evidence on the absence of the Fe I and Fe II emission lines from the spectrum of a pure Fe sample measured in this experiment is in need of a thorough explanation in a separate experiment specially dedicated to address this problem.

The viability of this µJ LIBS technique described in this study is further demonstrated by its application to halogen analysis which is often difficult to detect using conventional LIBS. This is related to the high excitation energy of halogen gases, particularly the fluorine (F) which is known to have for instance, the F I 685.6 nm emission line associated with 14.4 eV excited state, F I 687.0 nm emission line associated with 14.4 eV excited state, F I 690.2 nm line associated with excited state of 14.5 eV and F I 690.9 nm line associated with excited state of 14.4 eV. The resulted spectrum measured from a sample using 10 successive shots of 500 µJ ps laser is presented in Fig. 11. It is shown that all the F emission lines exhibit very sharp spectral profiles with relatively low background. The concurrent C I 247.8 nm emission line is also detected with the appropriate spectral window but not shown in Fig. 11 although its intensity is estimated about 0.5 of the F I 685.6 nm intensity.

Having exhibited the excellent spectral performance of this special LIBS technique, it is desirable to investigate its possible application for quantitative analyses. This is carried out by measuring the emission spectra of the standard alluminum samples (Showa Denkoo 1000 Alloy Standards) containing various concentrations of Mg, Cu, Si, and Fe elements. The intensities of Mg I 285.2 nm, Cu I 327.4 nm, Si I 288.1 nm, and Fe I 371.9 nm emission lines are plotted with respect to the corresponding elemental concentrations in Fig. 12. It is seen that the emission intensities of each elements exhibit roughly linear relations with the associated elemental concentrations, suggesting their feasible use as the calibration lines for quantitative analysis of the corresponding elements in the Al sample for the corresponding limited ranges with the shortest calibration range for Si.

4. Conclusions

We have shown in this study that an eﬀective shock wave induced plasma emission can be readily generated at 125 µJ ps laser using 1.3 kPa ambient Ar gas. The excellent spectral quality is demonstrated along with largely reduced crater size which makes it possible for µm surface mapping

Fig. 9. Spectrum of typical Cr emission lines detected from a standard JSS 154 steel sample containing 2.04% Cr produced by 10 successive shots of 125 µJ ps laser in 1.3 kPa ambient Ar gas.

(a)

(b)

Fig. 10. Emission spectra produced by 125 µJ ps laser in 1.3 kPa ambient Ar gas from (a) low alloy steel (JSS 150) sample containing 0.4% of C showing the C I 247.8 nm emission line and (b) pure iron (Rare Metallic, 99.999%) sample showing no sign of Fe II 247.857 nm, Fe I 247.8541 nm, and Fe I 247.8573 nm emission lines.

Fig. 11. Emission spectrum produced by 500 µJ ps laser in 1.3 kPa ambient Ar gas from teﬂon sample showing the four F emission lines with low background.

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and depth proﬁling of elemental distributions of metal samples. The excellent performances for spectral analyses are consistently exhibited by its application to metal and non metal samples, in particular in C analysis of steel and F analysis of teﬂon. The possibility for quantitative analysis in certain ranges of concentrations is also demonstrated by its application to alluminum sample containing a number of impurity elements. Finally, the tiny crater size created in this experiment makes it possible to develop the application for micro mapping of elemental distribution at µm spatial resolution.

Acknowledgement

This work was partially supported through a Basic Research Grant in Physics, The Academy of Sciences for the Developing World, Third World Academy of Sciences (TWAS), under contract No. 060150 RG=PHYS=AS= UNESCO FR:3240144882.

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Fig. 12. Plots of intensity concentration relations based on Mg I 285.2 nm, Cu I 327.4 nm, Si I 288.1 nm, and Fe I 371.9 nm emission lines from alluminum samples containing various concentrations of Mg, Cu, Si, and Fe impurities.