We report on a self-guided microwave surface-wave induced generation of ~60 μm diameter and 6 cm-long column of argon-plasma confined in the core of a hollow-core photonic crystal fiber. At gas pressure of 1 mbar, the micro-confined plasma exhibits a stable transverse profile with a maximum gas-temperature as high as 1300 ± 200 K, and a wall-temperature as low as 500 K, and an electron density level of 1014 cm−3. The fiber guided fluorescence emission presents strong Ar+ spectral lines in the visible and near UV. Theory shows that the observed combination of relatively low wall-temperature and high ionisation rate in this strongly confined configuration is due to an unprecedentedly wide electrostatic space-charge field and the subsequent ion acceleration dominance in the plasma-to-gas power transfer.
© 2013 OSA
The last two decades have triggered dramatic progress in controlling and confining light at the micrometre and nanometre scales through the development of a myriad of micro- and nano-structured photonic devices and materials such as photonic crystals  and photonic crystal fibers , thus enabling highly integrated photonic functionalities. However, the active material phase-states of these devices are either in solid-phase or in gas-phase . Subsequently, their band-gap energies limit their operational optical spectral-range to below those of near UV , and their refractive indices to values higher than 1. One route that could extend these to ranges that no other solid-state or gas-phase materials would allow is to incorporate plasma-phase in their active materials. The plasma-state features can be used, for example to laser-emit in the range of UV to the soft x-ray where electrical discharge-induced plasma is hitherto the sole material for laser-action at wavelengths shorter than 300 nm [5,6]. One of the existing photonic structures that could be a candidate for plasma micro-confinement is hollow-core photonic crystal fiber (HC-PCF)  and its integrated form photonic microcells . The ability of HC-PCF in confining light and gas together over a micrometric modal area and a meter-scale interaction length enabled the demonstration in gas-phase media of several coherent and nonlinear optical phenomena . It would thus be desirable to extend gas-filled HC-PCF performances to ionised gases, as it would trigger, for example, the advent of the long hoped for compact and integrated UV-XUV laser sources and would provide a photonic platform whereby micro-plasmas could be generated and harnessed with the same degree of control which we currently have with the other matter phases.
This in turn, requires the vision and development of a system that could generate and micro-confine stable gas-plasma within a photonic structure without any damage or intrusion, whilst exhibiting both high electron density (>1011 cm−3) and high power-density coupled to the plasma (>10 kW.cm−3). Recent attempts using conventional electrode excitation schemes [9,10] illustrate the challenge of such an endeavour and the need for an alternative excitation approach. Indeed, Shi et al.  placed electrodes inside hollow capillaries to produce a longitudinal direct-current (DC) excitation, but failed to produce stable DC breakdown at tube diameter smaller than 60 μm. On the other hand, Ji et al.  successfully generated atmospheric-pressure dielectric barrier discharges (DBD) in polymer vapour-filled capillaries with diameters as small as 30 μm. However, the non-homogeneous spatial-concentration of the DBD on the inner surface, and the low power density involved (estimated to kW.cm−3) disqualifies it as a viable technique to generate and sustain uniform micro-plasmas that preserve the integrity of the host structure  whilst sufficiently sustaining high power density to efficiently ionise inert gases. Finally, it is noteworthy that none of these works produces plasma within a real photonic structure.
Here we report on a new means of generating microwave plasma in a photonic dielectric structure based on HC-PCF that combines stability, high ionization rate and unprecedentedly large microwave power densities. The results show a rather counter-intuitive situation whereby a high level of power densities (~0.1 MW.cm−3) and electron densities (~1014 cm−3) co-exist with a relatively low plasma gas-temperature. The temperature level, however, is sufficiently high to be comparable to the transformation temperature of HC-PCF silica material, yet exhibits a steep transverse distribution that preserves the structural integrity of the HC-PCF. In this novel scheme a 6 cm-long argon-plasma column is generated and confined using low power (~10s W) microwave source in the ~100 μm diameter core of a Kagome-lattice HC-PCF. The plasma generation scheme is electrode-free and relies on exciting a self-guiding microwave surface-wave (SW) whose maximum electric-field intensity, in a similar manner to a plasmonic wave, is localised at the interface between the argon-filled HC-PCF core and the nanometric thick silica core-surround. The configuration exhibits an ultra-large electrostatic space-charge field, which enables the confinement of micro-plasma (~1300 K gas temperature, and more than 10 eV electron energy) within a cross area as small as 60 μm diameter, with no damage to the HC-PCF. This in turn, and despite a relatively low microwave pump power, favours unprecedentedly strong emission of several Ar+ lines in the visible and near UV and their guidance within the fiber-core.
2. Experiments on microplasma ignition in Kagome-latticed HC-PCF
The principle of the experiment is schematically illustrated in Fig. 1. An inert gas (here argon) filled HC-PCF  is inserted in a non-intrusive fashion into a microwave cavity called surfatron . The surfatron is a microwave cavity comprising two co-axial metallic cylindrical tubes. This cavity exhibits a gap at a position where the field is at its maximum, and which acts as an impedance-matching coupler between the cavity field and the one that propagates along the optical fiber (Fig. 1(b)). Under these conditions, the scenario of the generation of stable plasma within the fiber core is as follows. The high intensity of the electromagnetic wave-field at the gap of the cavity creates gas breakdown in the fiber section residing in the gap. The subsequent plasma creates the necessary conditions for the formation of a tandem of a sub-diffraction self-guided surface-microwave and self-sustained extended plasma whereby the field intensity is highly localised at the core-surround interface between the plasma-filled core and the fiber cladding .
Figure 1(c) schematically shows the typical transverse distribution of the plasma particles. As observed, the neutral plasma region (NPR) is confined within a central area surrounded by a positively-charged layer termed the space-charge sheath . The layer results from a charge separation between the highly-mobile electrons and the slower positive ions as both species move towards the wall. The net positive charge within this layer creates an electrostatic potential that decelerates the electrons and accelerates the ions beyond the Bohm sound speed . This dynamic turns the space-charge sheath into an electrical gap barrier that self-consistently confines the NPR with respect to the dielectric wall. Unlike previous work , we will show that the sheath-layer and subsequently the fluid-dynamics of the plasma are strongly affected by the highly reduced HC-PCF core-radius. Furthermore, to our knowledge, there is no report of SW microwave plasma generated in a capillary with a diameter less than 500 μm or in a HC-PCF . The HC-PCF used in our experiments is a 19-cell core defect with a Kagome-lattice cladding (see SEM picture on Fig. 2(a)). This fiber exhibits a 100 µm core diameter and has been fabricated by the well-known stack and draw process. Such a large fiber-core facilitates the ignition and maintenance of the microwave plasma. The fiber guides via inhibiting coupling (IC) mechanism . Unlike the photonic bandgap guiding HC-PCF, IC guiding HC-PCF can exhibit fiber-core sizes as large as 100 μm, and secondly, it guides over a spectral range that spans from UV to mid-IR . The transmission spectrum has been measured using a white-light source (see Fig. 2(b)). Several low-loss transmission bands are identified going from 400 nm to 1700 nm. One notes that the UV low-loss spectral windows have not been measured due to the limitation of our optical spectrum analyser.
Figures 3 and 4 summarise theoretical and experimental results of the SW generation of a ~6 cm-long argon-plasma column inside a ~100 μm core diameter HC-PCF, with no damage to the dielectric structure of the fiber and with a high ionisation rate (~10−2). Figure 3(a) shows the 3D electromagnetic finite-element-method (FEM) simulation results of the overall field distribution for a constant electron density of 4 × 1014 cm−3 and a constant electron-neutral collisional rate of 2.4 × 109 s−1 estimated at the experimental pressure of 1 mbar. The surfatron cavity is being excited at 2.45 GHz with a continuous microwave power of 45 W. This cavity was designed through an iterative process between FEM simulations and experimental results. Similarly, the value of ne has been chosen to be close to the one deduced from the experimental conditions. The field distribution readily shows the vacuum resonant mode in the surfatron cavity and the SW mode that extends outside the cavity. Figure 3(b1) shows a close-up of the SW field-magnitude distribution. The field maximum intensity is localised at the interface between the fiber-core and the cladding, with a propagation constant β ~300 m−1 and an attenuation coefficient α ~20 m−1. The SW exhibits similar properties to those of plasmonic waves with the salient difference that in addition to a transverse field component, whose intensity is localised at the interface core-cladding (Fig. 3(b2)). The field comprises also a longitudinal component with phase-shift relative to the transverse components, and whose maximum intensity is in the plasma region (Fig. 3(b3)). The latter is the necessary field that maintains the plasma along the SW propagation length, whilst the transverse component (with magnitude ~10 times larger) enables the SW propagation and subsequently the plasma column formation. Hence, most of the total intensity of microwave is localised at the interface being carried by the transverse component of the field. This is illustrated in Fig. 3(c), which shows the transverse profile of the field magnitude at z = 3.15 mm (Fig. 3(c1)) and z = 7.32 mm (Fig. 3(c2)) (origin position at the gap). Furthermore, we use the propagative model described in  to deduce the evolution of ne, and subsequently of the propagation constant β and the attenuation coefficient α, as a function of the propagation length over the whole plasma column . Experimentally, the plasma length that extends outside the cavity away from the gap was measured to be 4 cm (see Fig. 4(a)). In turn, using the above-mentioned propagative model, and an electron-neutral collisional rate of 2.4x109 collision/s estimated for the experimental pressure of 1 mbar, the evolution of ne, β and α along the plasma column is deduced (see Fig. 3(d)), giving the electron density at the gap position (z = 0) to be ne(0) = 4 × 1014 cm−3. The 1014 cm−3 electron density-level obtained here with the CW microwave excitation at 1 mbar gas pressure confirms a very effective power-transfer to the plasma, yielding an ionization degree of ~10−2 assuming a gas temperature of 1000 K, which is close to the experimentally measured value (see below). Such an ionization rate is several orders of magnitude higher than the ~10−6 in a DC excitation [20,21] (taking ne ~1012 cm−3 for gas at 33 mbar pressure and room temperature) and the ~10−5 in the micro-DBD (taking ne ~1014 cm−3 for gas at atmospheric pressure and room temperature) .
Figure 4(a) shows a photograph capturing the fluorescence of the plasma column outside the cavity and scattered off the side of the HC-PCF. The fiber is ~80 cm long with its end-tips being attached to two twin chambers for gas loading and flow-control. The argon pressure near the surfatron gap is set to 1 mbar. The surfatron is placed at mid-position of the fiber length and is driven by a CW microwave generator. The surfatron has a tunable cavity length to adjust its resonance frequency, and a gap-distance of 2 mm. On resonance the microwave power coupling efficiency exceeds 90%. This allowed the minimisation of the microwave field mismatch induced heating, which causes fiber damage. Figure 4(b) shows the recorded spectrum as it is transmitted through the HC-PCF. The output profile of the transmitted emission at 750 nm (Fig. 4(b) right-hand-side inset) exhibits a Bessel-like profile indicating that the spectrum is guided dominantly in a single mode fashion. A distinctive feature of this spectrum compared to those generated in conventional capillaries with a much higher microwave power  is its strong emission of Argon ion (Ar II) in the range of 350 nm to 500 nm. The unusually large ratio of the Ar II line intensities over those of Ar I lines (range: ~700–800 nm) is indicative of a very strong ionisation rate and/or a large microwave power density . In particular, our excitation system yields power densities of ~0.1 MW.cm−3 (45 W applied to a 100 μm fiber-diameter to produce a 6 cm-long plasma). Furthermore, the plasma remains stably running over hours with no measurable damage on the fiber.
The successful confinement of such extreme levels in power and electron densities with no damage to the host structure raises questions about the gas temperature level and profile.
The gas temperature of the NPR was deduced using spectroscopic diagnostics  by fitting the measured ro-vibrational OH band, coming from traces of water adsorbed on the fiber core inner-wall to calibrated spectra. The wall temperature (i.e. the temperature at the HC-PCF outer-jacket) was measured with a thermal probe. The measurements near the gap give a gas temperature of 1300 ± 200 K in the plasma center, and a wall temperature of 500 ± 20 K. This high NPR gas temperature and strong gradient is explained in Fig. 5 which summarises the results obtained numerically using a self-consistent fluid-type model and describes the dynamics of the plasma charged particles and the energy transfer from the SW field to the electrons and to the neutral gas in our experimental conditions [26,27].
3. Dynamics and confinement of surface-wave microplasmas discharges
Figures 5(a) and 5(b) show the transverse profiles, when the core-radius is 50 μm, for the electron mean energy and its associated plasma potential (Fig. 5(a)), and for the charge particle densities (Fig. 5(b)). The results reveal the electron mean-energy reaching a staggering figure of ~10.5 eV, consistent with an axis-to-wall potential difference of ~45 V , and associated with a space-charge field which is found to be 100 times higher than the microwave electric field. Concomitantly, the results show that the plasma effective region (i.e. the NPR) is localised within ~60 μm diameter area in the center of the 100 μm diameter fiber-core, thus surrounded by a very wide space-charge sheath relative to the host-structure size. These distinctive features are mainly a direct result of the micro-scaled fiber-core as is illustrated in the exponential-like increase of the plasma potential maximum (Fig. 5(a) inset) and the sheath-width (Fig. 5(b) inset) with the radius decrease. For example, the sheath-width increases from less than 1% of the core radius to ~40% when the radius is dropped from 1 mm to 50 μm respectively (Fig. 5(b) inset). As a consequence, most of the highly-mobile electrons are confined within the NPR whereas the ions are strongly accelerated towards the core-surround. This dynamic explains both the strong ionization implied by the experimentally observed enhanced Ar II line intensities and the gas-temperature gradient between the center of the fiber and its wall. The latter is corroborated by the calculated gas temperature transverse profiles for different representative combinations of radius and electron density (Fig. 5(c)). Firstly, the figure shows NPR gas-temperatures in the range ~950 K to ~1600 K, when ne is in the range of 1.3x1014 cm−3 to 5x1014 cm−3, which is in qualitative agreement with the spectrally measured value of ~1300 ± 200 K at 4x1014 cm−3. Secondly, the on-axis gas temperature increases with the electron density as confirmed by results obtained at r0 = 1 mm for ne = 2.2x1013 cm−3 (at 0.9 W.cm−1 power per unit length transferred from the plasma to the gas) and ne = 1.3x1014 cm−3 (at 8.2 W.cm−1). However, the on-axis gas temperature decreases with the core-radius, as shown by results obtained at ne = 1.3x1014 cm−3 for r0 = 1 mm (at 8.2 W.cm−1 power per unit length) and r0 = 50 μm (at 1.4 W.cm−1). The implied decrease of thermal plasma-to-gas power transfer upon core-radius reduction is explained in Fig. 5(d). The figure compares the transverse profiles of the gas power-density gain associated with ion acceleration (and subsequent collisions with neutrals) and electron-neutral collisions, calculated for ne = 1.3x1014 cm−3. Unlike in conventional SW discharges where electron-neutral collisions control the plasma-to-gas power transfer [26,27], here the results show that the power transfer is overpowered by ion acceleration. The latter exhibits a 200-fold enhancement in power density when the radius is decreased from 1000 μm to 50 μm to reach several 10 kW.cm−3 in the NPR, (see Fig. 5(d)). Furthermore, because of the large mean free paths of the ions (typically ~100 μm), this ion-acceleration-induced collisional ion-to-neutral heat transfer is sensibly reduced, which explains the smaller on-axis gas temperature at 50 μm core-radius. Finally, it is noteworthy to highlight that because of the mean-free path of one the plasma constituents (here the ions) exceeds the geometrical transverse dimensions of the hosting structure, the fluid model provides a qualitative picture, and that in order to take into account the complicity of the plasma dynamics in such a highly confined geometry a full kinematics model is required.
In conclusion, generation and confinement of stable microwave gas-plasma in photonic dielectric microstructure such as HC-PCF has been demonstrated. This original and compact platform could in particular trigger the emergence of highly compact lasers in the UV-XUV spectral range or of plasma-state photonic crystal structures. Finally, the unique combination of extremely high microwave power densities and ionization degrees with relatively low temperatures indicates that the present plasma could host novel fundamental phenomena both in nonlinear optics and plasma physics.
The authors thank the PLATINOM platform for helping in the fibre fabrication. Work supported by Agence Nationale de la Recherche (grant ASTRID UV-FACTOR), PHOTOSYNTH, Σ LIM Labex Chaire and Fundaçao para a Ciência e a Tecnologia (Project Pest-OE/SADG/LA0010/2011).
References and links
1. J. D. Joannopoulos, P. R. Villeneuve, and S. Fan, “Photonic crystals,” Solid State Commun. 102(2-3), 165–173 (1997). [CrossRef]
3. F. Benabid, “Hollow-core photonic bandgap fibres: new guidance for new science and technology,” Philos. Trans. R. Soc. London, Ser. A 364, 3439–3462 (2006).
4. R. DeSalvo, A. A. Said, D. J. Hagan, E. W. Van Stryland, and M. Sheik-Bahae, “Infrared to ultraviolet measurements of two-photon absorption and n2 in wide bandgap solids,” IEEE J. Quantum Electron. 32(8), 1324–1333 (1996). [CrossRef]
5. H. Bergmann, U. Stamm, D. Basting, and G. Marowsky, “Principles of Excimer Lasers,” in Excimer Laser Technology (Springer Berlin Heidelberg, 2005).
6. J. J. Rocca, V. Shlyaptsev, F. G. Tomasel, O. D. Cortázar, D. Hartshorn, and J. L. A. Chilla, “Demonstration of a Discharge Pumped Table-Top Soft-X-Ray Laser,” Phys. Rev. Lett. 73(16), 2192–2195 (1994). [CrossRef] [PubMed]
7. F. Benabid, P. J. Roberts, F. Couny, and P. S. Light, “Light and gas confinement in hollow-core photonic crystal fibre based photonic microcells,” J. Eur. Opt. Soc. Rapid Publ. 4, 09004 (2009).
8. F. Benabid and J. P. Roberts, “Linear and nonlinear optical properties of hollow core photonic crystal fiber,” J. Mod. Opt. 58(2), 87–124 (2011). [CrossRef]
9. X. Shi, X. B. Wang, W. Jin, and M. S. Demokan, “Characteristics of Gas Breakdown in Hollow-Core Fibers,” IEEE Photonics Technol. Lett. 20(8), 650–652 (2008). [CrossRef]
10. L. Ji, D. Liu, Y. Song, and J. Niu, “Atmospheric pressure dielectric barrier microplasmas inside hollow-core optical fibers,” J. Appl. Phys. 111(7), 073304 (2012). [CrossRef]
11. Y. P. Raizer, “Gas Discharge Physics,” (Springer, New York, 1991).
12. F. Benabid, J. C. Knight, G. Antonopoulos, and P. S. J. Russell, “Stimulated Raman Scattering in Hydrogen-Filled Hollow-Core Photonic Crystal Fiber,” Science 298(5592), 399–402 (2002). [CrossRef] [PubMed]
13. M. Moisan, C. Beaudry, and P. Leprince, “A Small Microwave Plasma Source for Long Column Production without Magnetic Field,” IEEE Trans. Plasma Sci. 3(2), 55–59 (1975). [CrossRef]
14. M. Moisan, C. M. Ferreira, Y. Hajlaoui, D. Henry, J. Hubert, R. Pantel, A. Ricard, and Z. Zakrzewski, “Properties and applications of surface-wave produced plasmas,” Rev. Phys. Appl. (Paris) 17(11), 707–727 (1982). [CrossRef]
15. M. A. Lieberman, “Dynamics of a collisional, capacitive RF sheath,” IEEE Trans. Plasma Sci. 17(2), 338–341 (1989). [CrossRef]
16. H. Schluter and A. Shivarova, “Travelling-wave-sustained discharges,” Phys. Rep. 443(4-6), 121–255 (2007). [CrossRef]
17. Z. Zakrzewski, “Conditions of existence and axial structure of long microwave discharges sustained by travelling waves,” J. Phys. D Appl. Phys. 16(2), 171–180 (1983). [CrossRef]
18. F. Couny, F. Benabid, P. J. Roberts, P. S. Light, and M. G. Raymer, “Generation and photonic guidance of multi-octave optical-frequency combs,” Science 318(5853), 1118–1121 (2007). [CrossRef] [PubMed]
19. C. M. Ferreira, “Modelling of a low-pressure plasma column sustained by a surface wave,” J. Phys. D Appl. Phys. 16(9), 1673–1685 (1983). [CrossRef]
20. L. L. Alves, “Fluid modelling of the positive column of direct-current glow discharges,” Plasma Sources Sci. Technol. 16(3), 557–569 (2007). [CrossRef]
21. M. Lieberman and A. J. Lichtenberg, “Principles of plasma discharges and materials processing,” (John Wiley, New Jersey, 2005).
22. H. E. Wagner, R. Brandenburg, K. V. Kozlov, A. Sonnenfeld, P. Michel, and J. F. Behnke, “The barrier discharge: basic properties and applications to surface treatment,” Vacuum 71(3), 417–436 (2003). [CrossRef]
23. C. Boisse-Laporte, A. Granier, E. Bloyet, P. Leprince, and J. Marec, “Influence of the excitation frequency on surface wave argon discharges: Study of the light emission,” J. Appl. Phys. 61(5), 1740–1746 (1987). [CrossRef]
24. Z. Rakem, P. Leprince, and J. Marec, “Characteristics of a surface-wave produced discharge operating under standing wave conditions,” Rev. Phys. Appl. (Paris) 25(1), 125–130 (1990). [CrossRef]
25. C. O. Laux, T. G. Spence, C. H. Kruger, and R. N. Zare, “Optical diagnostics of atmospheric pressure air plasmas,” Plasma Sources Sci. Technol. 12(2), 125–138 (2003). [CrossRef]
26. L. L. Alves, S. Letout, and C. Boisse-Laporte, “Modeling of surface-wave discharges with cylindrical symmetry,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 79(1), 016403 (2009). [CrossRef] [PubMed]
27. J. Gregório, P. Leprince, C. Boisse-Laporte, and L. L. Alves, “Self-consistent modelling of atmospheric micro-plasmas produced by a microwave source,” Plasma Sources Sci. Technol. 21(1), 015013 (2012). [CrossRef]