Au Nanoplasma as Efficient Hard X-ray Emission Source - ACS

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Au nanoplasma as efficient hard X-ray emission source Frances Camille P. Masim, Matteo Porta, Wei-Hung Hsu, Mai Thanh Nguyen, Tetsu Yonezawa, Armandas Balcytis, Saulius Juodkazis, and Koji Hatanaka ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00692 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 22, 2016

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Au nanoplasma as ecient hard X-ray emission source †

Frances Camille P. Masim,

Tetsu Yonezawa,

‡Research ¶Division

¶

‡

Matteo Porta,

Armandas Bal£ytis,

§, k

¶

Wei-Hung Hsu,

Saulius Juodkazis,

‡ §

Mai Thanh Nguyen,

and Koji Hatanaka

∗,‡

Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan

of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Hokkaido 060-8628, Japan

§Nanotechnology

Facility, Center for Micro-Photonics, Swinburne University of Technology, Victoria 3122, Australia

kInstitute

of Physics, Center for Physical Sciences and Technology, Vilnius, LT-02300, Lithuania

E-mail: [email protected]

Abstract Hard X-ray generation up to 15 keV (∼ 1Å) from Au nanoparticle suspensions was systematically investigated for dierent particle diameters ranging from 10 to 100 nm with a narrow size distribution of ±2%. Scaling law of X-ray generation is close to a 6-photon process before the onset of saturation for excitation by 40 fs laser pulses with central wavelength of 800 nm. This is consistent with bulk plasmon excitation at λbulk ' 138 nm. The longitudinal E-eld component due to nanoparticle focusing is p

responsible for the excitation of the longitudinal bulk plasmon. The proposed analysis †

Au nanoplasma as ecient hard X-ray emission source 1

ACS Paragon Plus Environment

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also explains X-ray emission from water breakdown via an electron solvation mechanism at ∼6.2 eV. The most ecient emission of X-rays was observed for 40±1 nm diameter nanoparticles which also had the strongest UV extinction. X-ray emission was the most ecient when induced by pre-chirped 370±20 fs laser pulses and exhibited the highest intensity at a negative chirp.

Keywords: femtosecond laser, intense laser, chirp, pulse width dependence, gold nanoparticle, size dependence, X-ray

The interaction of intense ultrashort laser pulses with condensed matters has widespread applications, including the production of high-energy electrons, ions, and photons. 1 At higher intensities approaching ∼ 1020 W/cm2 (in vacuum), particle acceleration beams for electrons and protons up to MeV energies were demonstrated using peta-watt lasers. 2 Intense ultrashort laser pulses is accompanied by highly-ionized solid density plasma that oers the design of micron-sized sources of ultrashort X-ray 3 and THz waves 4 in areas of lithography and time-resolved diraction. 5 Recent developments of compact and ecient hard X-ray sources manifested great contributions in biological and medical elds as well as practical applications. X-ray sources derived from solid targets such as at surfaces of metals 6 and transparent glasses 7,8 have superior X-ray yields in the hard X-ray spectral region. 9 Methods to enhance the X-ray yield by utilizing the design of structured and layered 10 surfaces such as sub-λ gratings 11 as targets for X-ray generation were also reported. Liquid targets have recently been considered as replacements for solid targets for practical X-ray sources due to the possibility to control the X-ray spectra via choices of solute contents of the micro-jet 12 or droplet irradiated by high intensity laser pulses at lower ∼ 1015 W/cm2 irradiance. 13 Recent experimental studies on liquid targets were extensively reported showing various aspects of femtosecond laser-induced X-ray generation using CsCl aqueous solutions from the viewpoints of electron temperature dependence on laser intensity, solute components and concentration, 14 X-ray emission eciency as a function of laser chirp, 15 and X-ray 2


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intensity enhancemnent under double pulse excitation. 16 A wide range of control parameters over light-matter interaction exist in the case of ultrashort laser pulses, some of which, are specic to the ultrashort laser pulses: spatial and temporal chirp. Polarization, optical nonlinearities of irradiated materials, dierent contributions of E-eld components due to peculiarities of focusing at the near-eld, provide ways to enhance electron temperature and concentration for brighter X-ray emission as briey introduced below. It has been demonstrated that the resonant absorption mechanism is responsible for hard X-ray generation in the case of metal-doped glasses 7 and water jets 15,16 irradiated by ∼ 150 fs pulses. By introducing a negative chirp and choosing p-polarized pulses, X-ray generation is enhanced by almost an order of magnitude. 15 For 5-6 times shorter pulses, the multi-photon ionization (MPI) becomes comparable to or more ecient than impact ionization (IMP), and even higher electron densities and temperatures are reached. 17,18 In the case of X-ray generation from wide-bandgap undoped dielectrics such as pure silica, sapphire, or water with high dielectric strength against dielectric breakdown, the polarization dependent rates of MPI become important, especially for short sub-50 fs pulses. Surface breakdown intensity p cir lin /σN , where σN is the corresponding N of silica and sapphire scales as I cir /I lin ∝ 1/ N σN photon absorption cross section dening the rate of the ionization. 19 The linear polarization is more ecient in ionization as compared with circular, this trend is observed up to an irradiance of ∼ 30 TW/cm2 . 19 However, once the plasma is formed, the electron quiver energy is likewise polarization dependent and becomes twice larger for the

circular

polarization as compared with

linear

ac-

cording to εosc ∝ (1+α2 ), where α = 0, 1 for linear and circular polarization, respectively. 17,18 The IMP and MPI rates are dependent on the εosc as wmpi ∝ εN osc and wimp ∝ εosc ; hence, more ecient ionization is expected for circularly polarized pulses. 17 The chirp eect at high irradiance inuences the ionization rates, especially when photon energy is close to the fundamental absorption of a target material. By introducing a negative

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chirp with shorter wavelength (a larger photon energy) at the leading part of the pulse, ionization rates can be signicantly enhanced due to εN osc dependence of wmpi . Then, the trailing red-shifted (in wavelength) part of the pulse will contribute more eciently to the ionization and heating of plasma due to the wimp ∝ λ2 scaling. 15,17 Creation of practical Xray emitting sources depends on understanding the eects of temporal and spatial chirp, 20 polarization, as well as nonlinear phenomena in delivery of ultra-short laser pulses for Xray generation at high irradiance conditions, especially when more complex targets such as colloidal suspensions and layered materials are used. Plasmonic nanoparticles and nanostructures have been demonstrated to accelerate electrons which enables the fabrication of miniaturized particle accelerators 21 and X-ray sources. 5 Ultrashort laser irradiation of, e.g., gold nanoparticles leads to the subsequent conversion of absorbed energy into high enegry electrons, ions, and X-rays. 2225 A surface plasmon resonant excitation in gold is crucial for the ecient absorption and generation of extreme atomic charge states. 23,24 Optical properties of gold nanoparticles such as large absorption cross section and spectral selectivity (visible to near-infrared ranges) based on surface plasmon resonance permits them to have potential applications in biomedical eld. Here, it is shown that the most ecient X-ray emission was observed from Au nanoparticles of r ' 20 nm radius under irradiation of negatively pre-chirped pulses which were approximately 8 times longer compared with the shortest 45 fs pulses. Chirp can be efciently used to augment the X-ray emission by a factor of two over the intensity values observed using bandwidth limited excitation pulse duration. Power scaling of X-ray emission implies bulk plasmon generation in gold nanoparticles and absorption being the driving mechanism of X-ray emission. Numerical modelling of optical extinction of nanoparticles separated scattering and absorption contributions and conrmed the presence of a strong E-eld component perpendicular to the surface, which is required to excite longitudinal bulk plasmons.

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Ti: Sapphire laser (800 nm)

Pump 1-mm solution jet

Au3+

90°

X

Y

Off-axis parabolic mirror

15 cm

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Z

3D stage

d=Φ Geiger counter

Computer

Figure 1: Schematic diagram of the experimental setup for hard X-ray generation from Au nanoparticle suspensions. Femtosecond laser pulses: pulse duration t0 = 40 fs, central wavelength λ = 800 nm and pulse energy from Ep = 0.1 mJ to 1 mJ at 1 kHz repetition rate were irradiated onto the front surface of the 1-mm solution jet.

Results and Discussion Characterization of Au nanoparticles X-ray generation and measurements were carried out as shown in Fig. 1. For the study of X-ray emission and establishing its power scaling dependence, Au nanoparticle suspensions with narrow size distributions were used. Mono-dispersed colloidal suspensions with < 2% diameter distribution were used for smaller nanoparticles up to 70 nm in size and < 6% for the larger ones. The extinction spectra of suspensions of various size Au nanoparticles is shown in Fig. 2 (a). Strong losses at the UV spectral range do not have a resonant character and are dominated by the

d

-to-sp Au interband transitions. On the other hand,

the plasmonic resonance region around 520 - 600 nm depends on the nanoparticle size. 26 The strongest relative contribution at the UV wavelengths of 200-300 nm was observed for the 40-50 nm diameter nanoparticles which proved to facilitate the most ecient generation of Xrays. The resonant peak positions in extinction spectra were calculated using experimentally

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Au

1.25 1.00 0.75

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50 nm

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seed: 13 nm step 1-10 : 1 19 nm 2 25 3 31 4 39 5 45 6 54 7 71 8 83 9 105 10 131

seed 400

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(a) Simulation max (nm)

step 5

Extinction

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625 600 1:1 line 575 550 525

1000

Wavelength (nm)

525 550 575 600 625

Experimental max (nm)

Figure 2: (a) Experimental extinction spectra of Au nano-sphere suspensions at dierent growth stages (steps from 1 to 10); a 1:3 (sample:water) dilution was used in all spectroscopically investigated samples as compared to the solution used for X-ray generation. Insets show TEM images of 45 and 71-nm-diameter nanoparticles (steps 5 and 7, respectively). The central wavelength of the fundamental (800 nm) and second harmonics (400 nm) of the fslaser excitation are marked. The average diameters of Au nanoparticles were calculated from TEM images for the dierent steps of growth. (b) Numerically simulated extinction spectra of spherical Au nanoparticles; the diameters were determined experimentally by TEM imaging (see, (a)). Simulations were carried our using Lumerical FDTD Solutions software. determined nanoparticle sizes for the step 1-10 generations of the colloidal suspensions. Figure 2(b) shows correlation between the experimental and numerical peak positions of the plasmonic resonance. In each case modeling was carried out for a single isolated spherical nanoparticle. Good correspondence between numerical and experimental results indicates that nanoparticle solutions had no agglomerated clusters nor had signicant neighboring nanoparticle interaction eects. This also corroborates that dispersions were mono-dispersed in terms of nanoparticle size.

Power scaling of X-ray generation Figure 3 shows the power scaling of X-ray generation from 30-nm-diameter Au nanoparticle suspension as well as HAuCl 4 aq solution and water for reference. Strong increase of X-ray intensity is observed for lower pulse energies below 0.1 mJ, with subsequent saturation. The six photon processes can explain the initial steep power scaling which would correspond to a 133 nm wavelength. For the electron density in gold Ne = 5.90×1022 cm−3 the bulk plasmon 6


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1000

X-ray intensity (arb. units)

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Au; d = 30 nm 3+ HAu Cl4 solution H2O

100

10

1

0.1 0.1

1

Pulse energy (mJ)

Figure 3: Intensity of X-ray emission vs pulse energy for r = 30 nm radius Au nanoparticle colloidal suspensions, HAuCl 4 aq solution and distilled water (data from dierent experiments). Laser pulses of tp = 40 fs duration and λ = 800 nm central wavelength were focused with an objective lens of N A = 0.2 numerical aperture; insets show images of the solutions.

Figure 4: X-ray intensity vs Au nanoparticle diameter ( 2r). Au concentration was 1.2×10−3 M in all samples. Horizontal error bars represent the standard deviation of particle diameter and vertical error bars represent the X-ray intensity measurements performed three times between 30 min intervals. Pulse energy was Ep = 0.5 mJ. frequency of ωp =

q

Ne e2 me m∗ ε0

corresponds to the wavelength λbulk = 138 nm, considering that p

the eective electron mass is m∗ = 1, me and e are the free electron mass and charge, respectively. Usually, the eective electron mass in metals is 20-30% lower due to periodicity of the lattice. The 6-photon process, indeed, is probable for excitation of the bulk plasmon, which is a longitudinal wave and cannot be directly excited by transverse E-eld of the incident light. However, the bulk plasmon can be excited by the longitudinal component of the light eld which is comparable with the total eld for the 30-50 nm diameter particles. 27 The resonat absorption, which is the dominant mechanism of X-ray generation in solution 7


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(b)

2

Au nanoparticles 3+ HAu Cl4

1

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Laser pulse energy (mJ)

Figure 5: (a) X-ray emission spectrum and air transmission. (b) Temperature of electrons, Te , deduced from the black body t of emission from the suspensions of Au colloidal particles 2r = (30 − 40) nm and 2.5 mM solution of HAuCl 4 aq. 15

, is due to the longitudinal component of E-eld and directly couples light into absorbed

energy in the case on nanoparticles. The power scaling of X-ray generation in water and HAuCl 4 aq solution (Fig. 3) exhibits a slope consistent with a 4-photon process. Interestingly, the 4-photon absorption would correspond to the ∼ 6.2 eV energy which was for a long time assumed as the band gap energy of water and would be in accordance with water breakdown. However, recently it was demonstrated that this energy corresponds to the creation of a solvated electron complex out of the valence band in pure water 28 while the bandgap of the water is ∼ 9.5 eV. The slope of the 4-photon process in water and HAuCl 4 aq solution is consistent with breakdown ionisation of water at the irradiation conditions used.

X-ray intensity dependence on Au particle size and temporal chirp A systematic study of X-ray emission dependence on particle size is shown in Fig. 4. For the diameters 2r = 40 nm the strongest X-ray emission was observed in the saturated region

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75

50

25 -10000

-5000

0

5000

10000

2

 (fs )

Figure 6: Chirp eect: X-ray intensity vs. the second order dispersion Φ2 [fs2 ]; Φ1 ≡ 0. Insets depict chirped pulse irradiating a gold nanoparticle. Pulse duration resulting in the strongest X-ray emission at Φ2 = ±6000, fs2 is tp = 372 fs while the shortest pulse was tp = 45 fs at Φ2 = 0. of the power dependence (Fig. 3). The solution jet is irradiated at 1 kHz repetition rate. The Geiger counter is working in the single photon counting mode and has saturation rate at 1000 counts/sec. In our experiments, the counter head was covered by an aluminummade iris with a 1-mm hole to eliminate saturation and was used at a typical 70 counts/sec rate (Fig. 4). Figure 5(a) shows the X-ray emission spectrum from a solution jet of Au nanoparticle colloidal suspension. Characteristic X-ray lines of Au L-alpha (9.7 keV) or Au L-beta (11.4 keV) were not clearly observed due to low concentration of Au atoms at

1.2 × 10−3 mol/L concentration. Monochromatic X-ray emission lines were dominant at a higher concentration such as in the case of 6.5 mol/L CsCl aqueous solution. 12 A broad spectrum was observed with a tail up to 15 keV ( ∼ 1Å). The intensity of X-ray emission measured in air is strongly absorbed at the low energy soft X-ray spectral region. However, when presence of hard X-rays with photon energy E > 7 eV is substantial, the black body emission tting of cummulative bremsstrahlung and thermal emission allows to extract the electron temperature Te from the Planck's formula:

I(ν, T ) =

1 2hν 3 , 2 c exp(hν/kT ) − 1

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(1)

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|Ex|

xy plane

|Ey|

|Ez|

|Ex|

xz plane

|Ez|

λ = 215 nm

305 nm

405 nm

530 nm

800 nm

k

40 nm

Ex

Extinction -15 2 cross-section (10 m )

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d = 45 nm

4 y

2

Ex x z

Absorption

0 200

400

600

800

1000

Wavelength (nm)

Figure 7: The calculated absorption and total extinction spectra of a 45 nm diameter Au nano-sphere in water. Schematic E-eld distributions for the dierent components illustrate the features of light localisation at several characteristic spectral locations. The longitudinal Ez eld component in xz- and xy-planes are important for excitation of bulk plasmons; the incident eld is Ex = 1, k is the propagation direction of light. where T is the absolute temperature of the emitter, h is the Planck constant, ν = c/λ is the frequency of the electromagnetic radiation, c is speed of light in vacuum, λ is the wavelength, and an emissivity ≡ 1 is assumed for the black body conditions. The results of the tting using Eqn. 1 are shown in Fig. 5(b). Electron temperatures Te ' 2.5 keV are reached for the Au nanoparticle dispersion at the most ecient X-ray emission conditions. An order of magnitude lower emission of X-rays were observed in a 2.5 mM concentration aqueous solution of HAuCl4 . Inuence of the temporal chirp on X-ray emission is presented in Fig. 6. When Φ2 '

±6000 fs2 the strongest emission was observed as compared with bandwidth limited pulse

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Effective integrated extinction cross-section (arb. units)

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3

Total extinction Scattering Absorption

2 1 0 0

20

40

60

Nanoparticle radius (nm)

Figure 8: The FDTD simulated extinction, absorption, and scattering cross sections of dierent size spherical Au nanoparticles, integrated over the 200 nm ≤ λ ≤ 1000 nm spectral span shown in Fig. 7 and normalized to the corresponding geometrical cross-sections; σex = σsc + σab [cm2 ]. duration of t0 = 45 fs. The Φ = 6000 fs2 correspond to a pulse duration tp ' 372 fs. Negative chirp is slightly more ecient in X-ray generation as was earlier observed in jets. 15

Bulk plasmon excitation in Au nanoparticles Frequencies of the surface plasmon polariton and the bulk plasmon are related as ωSP = √ ωb / 2. For the λ = 0.8 µm wavelength (ω = 2πc/λ) radiation used the 6-photon process, which matches the bulk plasmon energy in gold, corresponds to ~ωb = 9.2 eV, whereas surface plasmon polariton energy is ~ωSP = 6.5 eV. These plasmon excitations are dened by the electronic properties of gold and are independent from the particle size and shape which dene the resonances at visible wavelengths (see, Fig. 2). The energy values of 9.2 and 6.5 eV match the plasma waves in gold and the energy required to create a solvated electron in water, respectively. 28 The observed power law of X-ray emission suggests that the light energy absorption pathway is via the bulk plasmon excitation for gold colloidal nanoparticles and the breakdown of pure water or solution (Fig. 3). Earlier studies of Au colloidal particles in water under exposure by 400 nm/150 fs laser pulses showed that nanoparticles with radii r = (20 − 25) nm require the smallest uence to be disintegrated. At this size, nanoparticles absorb enough energy for dissintegration (explosion) whereas for the smaller nanoparticles melting occurs. It is noteworthy, that such size does not correspond to the resonant plasmon band (see, Fig. 2). It was shown 11


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recently that explosion of a gold nanoparticle proceeds via electron ejection from its surface. 22 This is very similar to the formation of a solvated electron in water at the initial stage of water breakdown. For the electron ejection, an E-eld component perpendicular to the surface of a Au nanoparticle is essential. Earlier demonstration of the X-ray generation via resonant absorption of the longitudinal E-eld component, 15 photo-catalysis 27,29,30 and ablation 31 perpendicular to the surface are consistent with the proposed conjecture that the longitudinal eld component plays a key role in absorption and electron heating resulting in the X-ray generation from the Au colloidal particle suspension. Even though a strong incident Ex component is perpendicular to the surface at the equator of a nano-sphere, it creates a strong reection and energy is not absorbed, while the longitudinal component Ez is absorbed 32 (hence, the resonant absorption).

E-eld distribution on single Au nanoparticle To reveal the absorption contribution in the total extinction due to a gold nano-sphere in water, the cross sections were calculated, where σex = σsc + σab (Fig. 7). Furthermore, spatial distributions of E-eld components around particle were extracted. For the linearly polarised Ex ≡ 1 beam incident on the nano-sphere with the optimum size 2r = 45 nm for X-ray generation (see, Fig. 4), the total extinction has a contribution of absorption that is signicantly larger than scattering (Fig. 8). The spectrum and volume integrated cross sections, normalized to the geometrical cross-sections of the Au particles are shown in Fig. 8. The largest values of an integral absorption over the range of visible wavelengths mark the size region of nanoparticles for the most ecient X-ray generation. The insets in Fig. 7 show maps of the E-eld components, including the longitudinal eld Ez in the xz- and xy-planes, for the beam propagating along z-axis ( k k z). The light penetration depth into the particle is limited to the skin depth, however, there are peculiar spectral positions for the Ez component for a particular particle size (see λ =

305 nm in Fig. 7) where absorption is slightly smaller yet light eld penetration is substantial. 12


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Hence, such anti-resonances in extinction and Ez longitudinal component contribute to the absorption and energy delivery from the light pulse to a nanoparticle via resonant absorption (an oscillating dipole aligned with the linear polarization of the exciting beam would increase reection and reduce absorption 32 ). Due to its longitudinal character, this Ez eld can excite a bulk plasmon. The Ez component in the xy-plane reveals its strong presence inside the nanoparticle which is expected to cause substantial absorption. Generation of hard X-rays from a 2.5 keV temperature plasma described here is an ecient process in this part of EM spectrum. Conversely, for 1 THz (300 µm) radiation the Eqn. 1 predicts a 4.6 × 1011 times smaller radiance as calculated for the black body heated to 5 keV (with peak emission at 2.46). The emitted photon number ratio, between X-rays and T-rays at correspondingly 2.46 and 300 µm wavelength is, however, signicantly smaller

3.8 × 105 . Hence, terahertz emission from breakdown plasmas can be intense enough for use in applications, 33,34 however, in the case of water solutions a back-scattering geometry has to be used for the T-ray source due to strong water absorption.

Conclusions Hard X-ray emission up to 15 keV ( ∼ 1Å) from aqueous solution jets of 30-50 nm diameter Au nanoparticles is demonstrated to follow a 6-photon process consistent with bulk plasmon excitation in gold. Formation of the longitudinal component of the incident light eld Ez (z is the propagation direction k = 2π/λ) required for excitation of bulk plasmon is due to near-eld focusing by gold colloidal nanoparticles. Electron temperatures corresponding to the black body radiation of 2.5 keV produce a bright hard X-ray source which can be used in air conditions. Temporal chirp of 45 fs pulses provides a tool to control X-ray intensity. This mechanism is additionally corroborated by the X-ray generation scaling power law for water and is consistent with electron ejection from water molecule and formation of solvated electron at ∼ 6.5 eV. 28

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Creation of high temperature plasma fragmentation of colloidal particles in solution opens rich possibilities to investigate temporal dynamics of the breakdown on sub-wavelength scales and at short time windows where modeling is already predicting dierent scenarios to be cross checked and validated experimentally. 22,28 Laser-driven explosions with high temperature black body radiation can provide not only hard X-ray sources but also ecient THz emitters 4,33,34

. Further studies are strongly required for directionality control for such EM-radiation

to be used in practical applications.

Methods Experimental setup for X-ray detection and measurements Femtosecond laser pulses (Mantis, Legend, Elite, HE USP, Coherent, Inc.) with pulse duration of t0 = 40 fs, central wavelength of λ = 800 nm, repetition rate of 1 kHz, and horizontally-polarized were used for X-ray generation. The pulse energy was typically ranged from Ep = 0.1 mJ to 1 mJ. It was adjusted by a half-wave plate and a polarization beam splitter (both from Edmund Optics), which were inserted into the optical path right after the regenerative amplier before the compressor in the laser system. Focusing was carried out with an o-axis parabolic mirror (47-097, Edmund Optics) with an eective focus length of 50.8 mm and the angle of incidence of 90 degrees, the numerical aperture NA = 0.25. It was tightly focused in air onto a 1-mm diameter Au nano-colloid solution jet. The local laser power after being focused tightly to the solution jet was estimated to be ∼ 1.41×1014 W/cm2 or more when the laser power was 1 W (a geometrical focus is considered to be of a 1.22λ/N A diameter). The incident angle of the laser pulse was normal to the solution jet. The solution ow rate is approximately 180 mL/min, while the laser pulse width, the focus size, and the repetition rate are 40 fs, a few tens of ∼ µm, and 1 kHz, respectively. Therefore, every single laser pulse irradiates the fresh surface of the solution jet. A Geiger counter (SS315, Southern Scientic) was used for X-ray measurements. The major gas component of the 14


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Geiger counter is helium ( > 95%, ∼0.5 bar) with some halogens as quenching agent. The input window material is mica (1.6 - 2 mg/cm 2 ) and its thickness is ∼6 µm. The distance between the laser focus (the X-ray point source) and the Geiger counter was kept constant at 15 cm in the entire experiment. The solid angle of the detector was 2.23 × 10−3 steradians. All the experiments were carried out in air under atmospheric pressure (1 atm) at room temperature (296 K). Therefore, it is certain that the Geiger counter detects only X-ray, not

α-ray or β -ray. Optimization of the focal region placement was carried out with 3D mechanical stage for the highest yield of X-ray emission. X-ray emission spectroscopy was performed using a solidstate detector (XR-100CR, PX2CR, Amptek). Numerical modeling of extinction spectra and light eld enhancement were carried out using nite dierence time domain (FDTD) software package (FDTD Solutions, Lumerical Solutions Inc.). A uniform 0.5 nm 3D mesh was used, permittivity of gold was tabulated in software according to results reported by Johnson and Christy. 35

Femtosecond laser pulses: pre-chirping 2

Pulses can be described by a Gaussian temporal envelope of the form E(t) = E0 e−2 ln 2(t/tp ) cos(ωt+

βt2 ), where ω is the cyclic frequency of light, tp is the pulse duration at the full-width at half p maximum (FWHM), β [1/fs2 ] is the linear chirp, and E0 = 2I0 /(cε0 n) is the eld amplitude, n is the refractive index, c is speed of light, t is time, I0 = 2Iav is the peak intensity which is twice larger than the average, Iav for the Gaussian, and ε0 is the permittivity of vacuum. The instantaneous cyclic frequency ωins (t) = ω0 + 2βt, where β > 0 corresponds to the positive chirp with trailing high frequency components. Pulse pre-chirping is implemented to increase pulse duration up to ten times using a 1D array of liquid crystal cells (FemtoJock, Biophotonics Solutions, Inc.). For the slow frequency ω varying spectral phase

ϕ(ω), it can be expanded into the Taylor series around the central frequency ω0 with the few rst terms as ϕ(ω) = ϕ(ω0 ) + Φ1 ω +

Φ2 2 ω 2!

15

+ ..., where ϕ(ω0 ) is the absolute phase of


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the pulse in time domain, the rst derivative ϕ0 (ω0 ) = Φ1 is the group delay (GD) which denes a shift of envelope in time domain, ϕ00 (ω0 ) = Φ2 [fs2 ] is the group delay dispersion (GDD) or the second order dispersion which denes the chirp in time domain. Duration of q p 2 the time broadened pulse at FWHM is dened as tp = t0 1 + [4 ln 2Φ2 /t20 ] ≡ t0 1 + β 2 where t0 = 45 fs is the shortest spectral bandwidth limited pulse duration. For this study higher orders as well as Φ1 were set to zero. The pulse energy of Ep = 0.1 mJ corresponds to the eld strength E0 = 4.23 × 1011 V/m, which is much higher than the breakdown of dry air at 6 × 106 V/m. This complicates the determination of the actual uence and irradiance on the nanoparticle due to breakdown of air and water jet with colloidal particles as discussed in detail in the Sec. Discussion. The power scaling of X-ray emission is used instead to reveal the mechanism of X-ray generation rather the incident uence/irradiance onto a nanoparticle.

Synthesis of Au nano-spheres Au nano-sphere colloidal suspensions with atomic concentration of 1.2 × 10−3 mol/L were prepared via citrate reduction method 36,37 wherein seeded growth synthesis of nanoparticles with diameters of 10 to 100 nm. Mono-dispersed suspensions of Au nanoparticles were used for X-ray generation experiments. Figure 2 shows optical extinction, cumulative scattering and absorbtion losses, spectra measured in transmission.

Author information Corresponding author Email (K. Hatanaka): [email protected]

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Notes The authors declare no competing nancial interest.

Acknowledgment S.J. is grateful for partial support via the Australian Research Council DP130101205 Discovery project. FDTD simulation work was performed on the swinSTAR supercomputer at Swinburne University of Technology. T.Y. acknowledges the support of Murata Foundation.

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λ = 215 nm

305 nm

40 nm

Ex x

z

arb. units) -15

y

405 nm

530 nm

0.4

800 nm 0.2


Graphical TOC Entry Extinction cross-section (10

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70

60

50

40

Absorption

0 0.0 200

400

600

800

25

50

75

100

Size of Au nanoparticle (nm)

1000

Wavelength (nm)

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Au Nanoplasma as Efficient Hard X-ray Emission Source - ACS

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