Use of carbon-containing materials for efficient high-order harmonic generation of laser radiation

Optics Communications 285 (2012) 2934–2941

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Use of carbon-containing materials for efficient high-order harmonic generation of laser radiation R.A. Ganeev a, b,⁎, P.A. Naik a, H. Singhal a, J.A. Chakera a, M. Kumar a, U. Chakravarty a, P.D. Gupta a a b

Laser Plasma Division, Raja Ramanna Centre for Advanced Technology, Indore 452 013, India Institute of Electronics, Akademgorodok, 33, Dormon Yoli Street, Tashkent 100 125, Uzbekistan

a r t i c l e

i n f o

Article history: Received 14 January 2011 Received in revised form 27 January 2012 Accepted 16 February 2012 Available online 3 March 2012 Keywords: Laser plasma Harmonic generation

a b s t r a c t A systematic study of the high-order harmonic generation (HHG) of laser radiation in various carboncontaining plasma plumes (CCPPs) is presented. The materials studied are: graphite, boron carbide, C60, polytetrafluoroethylene, polyethylene, carbon nanotubes, and soot. These studies show that CCPPs present the best plasma media for efficient lower order (from 9th to 19th) harmonic generation, while the harmonic cutoff restricted to the 29th order. The advantages of CCPPs for the harmonic generation are confirmed by comparison of the HHG conversion efficiency with that in Ag and In plasma plumes, where the highest conversion efficiency was reported. Use of two-color pump scheme allows further enhancement of harmonic yield from the CCPPs. © 2012 Elsevier B.V. All rights reserved.

1. Introduction High-order harmonic generation (HHG) of laser radiation may be considered at present to be the simplest and most efficient technique of obtaining coherent short wavelength radiation in the extreme ultraviolet (XUV) spectral range [1–4]. HHG research is being actively pursued with the commercial availability of new compact high power laser systems offering high output parameters (high energy and intensity of the pulses, and a high pulse repetition rate). HHG experiments are carried out either in gasses or from surfaces. However, the data on the generation of such radiation obtained to date with the use of the above techniques have shown a low efficiency of conversion to the XUV range (10 − 5 and below), which limits the practical use of the harmonics. The quest for ways of increasing the HHG efficiency in the XUV range has long been (and still is) one of the most topical problems in nonlinear optics. However, in majority of the cases, the efficiency of conversion to high order harmonics turns out to be insufficient for using them as real coherent short wavelength radiation sources in biology, plasma diagnostics, medicine, microscopy, photo-lithography, etc. Laser produced plasma plume can be used as a nonlinear medium for the HHG if the effect of the limiting factors (self-phase modulation, self-defocusing, and the phase mismatch induced by the abundance of free electrons in plasma plumes) is minimized [5,6]. Among the special features of HHG in laser produced plasma plumes, the foremost one is the wide range of nonlinear medium characteristics available for

⁎ Corresponding author. Fax: + 998 71 2636787. E-mail address: [email protected] (R.A. Ganeev). 0030-4018/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2012.02.034

varying the conditions of production of laser plume on the surface of a solid. One can vary the plasma parameters such as the plasma dimension, the density of ions, electrons, and neutral particles, and the degree of ionization. The use of any element of the periodic table that exists as a solid, largely extends the range of materials employed. In contrast, only four rare gasses are typically used in gas jet HHG schemes. In several cases, plasma plume gives an opportunity to realize quasi-resonance conditions leading to a sharp increase in the efficiency of single harmonic generation due to the effect of ion transitions on the nonlinear response in the spectral range in question. This effect can hardly be realized in the gas jet based HHG schemes because of a low probability that the atomic transition frequencies of the gasses coincide with the frequencies of some of the harmonics. The advantages of HHG in a plasma plume could largely be realized with the use of a low-excited and weakly ionized plasma, because the limiting processes governing the dynamics of the laser frequency conversion would play a minor role in this case. This has been confirmed by several studies concerned with high order harmonic generation in the plasma plume [7–16]. A substantial increase in the highest order of the generated harmonics, the emergence of a second plateau in the intensity distribution of highest order harmonics, the high efficiencies obtained with several plasma plumes, the realization of resonance enhancement of individual harmonics, the efficient harmonic enhancement for plasma plumes containing clusters of different materials, and other properties have demonstrated the advantages of using specially prepared plasmas for HHG [8,11,12]. In this connection, a search of new plasma medium and definition of the best plasma plumes for efficient HHG in different spectral ranges is a way for further enhancement of harmonic yield in the XUV range. We describe below the motivation of this work, which

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was focused on the analysis of HHG in the carbon-containing plasma plumes (CCPPs). It was observed during HHG studies in gas jets that lighter atoms generate harmonics at higher order but with lower efficiency [17]. In the case of plasma plume based HHG, this feature has not been observed. Instead, it is found that the highest harmonic orders are produced in the case of heavier ions (like Ag), while lighter ions (like C) showed higher conversion efficiency for mid order harmonics [18]. These observations are related to single color pump HHG experiments. In the case of twocolor pump, the highest orders of even harmonics were also observed in the case of heavier ions (Ag, 38th harmonic) [15]. Thus, the investigation of light material containing plasma plume (in particular CCPP) is a reasonable goal for improving the harmonic yield in the 40 nm– 100 nm spectral range. Such carbon containing materials like graphite, boron carbide, polyethylene, Teflon, soot, fullerenes, and carbon nanotubes can, at appropriate experimental conditions, provide an excellent plasma medium for enhanced harmonic generation in aforementioned spectral range. The use of the various ions has been studied by Kubodera et al. [19] to provide important information on their role in the HHG. Those studies were carried out using a KrF laser at an intensity of 10 17 W cm − 2, and the maximum harmonic order observed, in particular from carbon plasma, was the 15th order. The propagation effects in the high order harmonic generation of short pulse KrF laser radiation in carbon vapor and low charged carbon plasma have been studied in Ref. [6]. It was been found that, under those experimental conditions, the high order harmonics are generated mainly from neutral atoms. In a plasma plume, the sum-frequency generation of high order harmonics is suppressed by an unfavorable positive phasemismatch. Noncollinear phase-matched high order harmonic generation by difference–frequency mixing in plasmas is also discussed in Ref. [6]. They suggested that the observed anomalous continuous growth of the fifth harmonic intensity with the plasma length originates from the noncollinear phase-matched difference–frequency mixing and defocusing of the laser radiation. The generation of up to the 27th harmonic (λ = 29.5 nm) of Ti: sapphire laser from the prepulse produced carbon plasma has been reported by Ganeev et al. [20]. A gradual decrease in the intensity of harmonics was observed instead of the standard plateau observed for high harmonics appearing in the case of metal plasma plumes under similar conditions. The harmonic generation appeared to be efficient in the case of singly ionized plasma when the higher charge ion concentration was negligible. The conversion efficiency for the 19th harmonic was measured to be 10 − 6. Previous studies [20] have shown some peculiarities distinguishing the carbon plasma plume from other species used for the frequency conversion of the femtosecond beam propagating through the plasma plume. In particular, together with a steep decrease of each subsequent harmonic, the HHG from carbon plume has shown a decrease in its cutoff energy from the linear dependence between the cutoff harmonic and second ionization potential observed in earlier studies in various plasma plumes [18]. Both of these peculiarities distinguishing the carbon plume from other samples may be attributed to the specific properties of C atoms and ions, or creation of specific conditions of plasma formation on the carbon-containing targets. An alternative to the previous phase matching approaches widely probed in gas jet based HHG studies is to search for media (in particular, carbon containing plasma plumes) possessing those properties, which would allow an increase in the HHG efficiency. In this connection, the multiparticle CCPPs, such as fullerenes and carbon nanotubes (CNTs), can be considered as an attractive nonlinear medium for the HHG. The nonlinear optical parameters responsible for low order harmonic generation in fullerenes have been analyzed in Refs. [21,22] and [23,24] for second and third order respectively. High values of nonlinear optical susceptibilities were obtained in those studies (χ(3) (−3ω;ω,ω,ω) = 2 × 10− 10 esu and χ(3) (−2ω;ω,ω,0) = 2.1 × 10 − 9 esu for the C60

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films at the wavelength of λ = 1064 nm). Previous studies of fullerenes have also demonstrated generation of the fifth harmonic [25]. Note the absence of reports on higher order harmonics in fullerenes until recent studies where the application of laser ablation of fullerene containing targets allowed the production of plasma plumes containing sufficient amount of particles for efficient conversion of the short laser pulses (i.e. with duration of a few tens of fs) in the XUV range [14,16,26]. CNTs have remarkable electronic and optical properties due to their particular structure combining one-dimensional solid-state characteristics with molecular dimensions. The nonlinear optical studies of CNTs have been limited so far to second and third harmonic generation of the incident laser radiation [27–29]. Third harmonic generation from solid samples of carbon nanotubes has been studied experimentally, using ultrashort pulses generated by a Cr:Forsterite laser, at a wavelength of 1250 nm. Those studies have shown an unusual nonperturbative behavior of the third harmonic yield, for relatively low input laser fields at an intensity ~ 10 10 W cm − 2. Second and third harmonic generation in single-walled CNT films, using the fundamental 1064 nm radiation from a Q-switched Nd:YAG laser, has also been investigated [30]. The reported measurements were performed both on commercially available CNTs and on samples of CNTs grown with a catalyst-free method. Third harmonic generation was observed in both samples, while second harmonic generation was observed only in the sample grown by catalyst-free method. The above works reveal the low order nonlinear optical properties of nanotubes. However, no experimental observation of harmonic generation above third order in CNT medium has been reported so far. In this paper, we systematically analyze the CCPPs produced on various targets as the nonlinear media for efficient harmonic generation. The paper is organized as follows. In Section 2, we describe the experimental setup. In Section 3, we describe the targets used for efficient HHG. In Section 4, we show the advantages of these media being applied for above nonlinear optical process. The two-color pump scheme (i.e. fundamental + second harmonic pulses) used for further growth of harmonic yield, both for odd and even harmonic generation, is described in Section 5. In Section 6, a comparison of the harmonic yields from various CCPPs has been made. The application of nanostructured carbon containing targets (fullerenes and carbon nanotubes) for plasma formation and HHG is discussed in Sections 7 and 8. The discussion of results obtained is presented in Section 9. Investigations into HHG in laser produced CCPPs are summarized in Section 10.

2. Experimental scheme To create the plasma plume, a prepulse was split from the uncompressed Ti:sapphire laser (energy 20 mJ, duration 210 ps, central wavelength 800 nm, and pulse repetition rate of 10 Hz) and was focused on a target placed in a vacuum chamber by using a plano-convex lens (focal length f = 500 mm) (see Fig. 1). After some delay, the femtosecond pulse (main pulse, energy 45 mJ, duration 48 fs, central wavelength 800 nm, bandwidth 20 nm) was focused on the plasma plume from

Fig. 1. Experimental setup for HHG in laser produced plasma plume. FL, focusing lenses; SHC, second-harmonic crystal; T, target; S, slit; CM, cylindrical mirror; FFG, flat field grating; ZOS, zero order beam stop; MCP, micro-channel plate; CCD, charge coupled device.

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the orthogonal direction using a plano-convex lens (f = 500 mm). The Rayleigh length of the focused radiation and plasma sizes were 0.4 and 0.5 mm respectively. To optimize the harmonic yield, different delays were used between the heating prepulse and the main pulse. The XUV spectrum was analyzed using an XUV spectrometer, detected by a microchannel plate, and recorded by a CCD camera. For second harmonic (SH) generation, a beta barium borate (BBO) crystal (1 mm thick, type-I phase matched) was inserted between the focusing lens and the plasma plume, so that, after frequency upconversion in the crystal, the emerging laser field consisted of both the SH and fundamental laser pulses. The SH conversion efficiency at these conditions was measured to be 8%. The polarizations of SH and fundamental fields were orthogonal. These polarization conditions were used in our HHG experiments with CCPPs. The confocal parameters of the two beams were the same, as the SH conversion was carried out after the focusing lens. At these conditions, the two pulses had good spatial and temporal overlap. More details of experimental setup are described in our previous work [15]. 3. Description of the targets used The targets used in these studies were all carbon containing materials like graphite, boron carbide, polyethylene, polytetrafluoroethylene (Teflon), soot, fullerenes, and carbon nanotubes. i) Atomic carbon is a very short-lived species and, therefore, carbon is stabilized in various multi-atomic structures with different molecular configurations (allotropes). Among the allotropes of carbon (graphite, diamond, and amorphous carbon), graphite was used. The physical properties of carbon vary widely with the allotropic form. All the allotropic forms are solids under normal conditions but graphite is the most thermodynamically stable one. The different forms of carbon include the hardest naturally occurring substance: diamond, and also one of the softest known substances: graphite. Moreover, it has an affinity for bonding with other small atoms, including other carbon atoms, and is capable of forming multiple stable covalent bonds with such atoms. As a result, carbon is known to form almost 10 million different compounds. Carbon also has the highest sublimation point of all elements. Carbon sublimes in a carbon arc, which has a temperature of about 5800 K. Thus, irrespective of its allotropic form, carbon remains solid at temperatures higher than the highest melting point metals such as tungsten and rhenium. Although thermodynamically prone to oxidation, carbon resists oxidation more effectively than elements such as iron and copper that are weaker reducing agents at room temperature. ii) Boron carbide (B4C) is an extremely hard chemical material used in tank armor, bullet-proof vests, and numerous industrial applications. With a hardness of 9.3 on the Moh's scale, it is one of the hardest materials known, behind cubic boron nitride and diamond. iii) Polyethylene is the most widely used plastic. It is a thermoplastic polymer consisting of long chains of the monomer ethylene. Polyethylene contains the chemical elements carbon and hydrogen and is created through polymerization of ethylene and contains long \CH2\ chains. iv) Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications and is also known also as Teflon. PTFE is a fluorocarbon solid, as it is a high molecular weight compound, consisting wholly of carbon and fluorine. v) Soot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles. Soot, as an airborne contaminant in

the environment, has many different sources but they are all the result of some form of pyrolysis. They include soot from internal combustion engines, power plant boilers, hog-fuel boilers, ship boilers, central steam heat boilers, waste incineration, local field burning, house fires, forest fires, fireplaces, furnaces, etc. vi) Once considered exotic, fullerenes are now-a-days commonly synthesized and used in research. Fullerenes have a graphitelike structure, but instead of purely hexagonal packing, they also contain pentagons (or even heptagons) of carbon atoms, which bend the graphene sheet into spheres, ellipses, or cylinders. The optical and nonlinear optical properties of the fullerenes have not yet been fully analyzed and represent an intense area of research in nanomaterials. The fullerenes are fairly large molecules formed completely of carbon bonded trigonally, forming spheroids (the best-known and simplest is the soccerball-shaped structure C60, which is a subject of present study). vii) CNTs are structurally similar to buckyballs, except that each atom is bonded trigonally in a curved graphene sheet that forms a hollow cylinder. Carbon nanotubes (also known as buckytubes) are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-todiameter ratio of up to 1.32 × 10 8:1, which is significantly larger than for any other material. These cylindrical carbon molecules have novel properties that make them potentially useful in many applications in nanotechnology, electronics, optics and other fields of material science. They exhibit extraordinary strength and unique electrical properties, and are efficient thermal conductors. Nanotubes are members of the fullerene structural family. The ends of a nanotube might be capped with a hemisphere of the buckyball structure. The diameter of a nanotube is on the order of a few nanometers, while they can be up to 18 cm in length. Nanotubes are categorized as single-walled nanotubes and multi-walled nanotubes. The bulk samples of graphite, boron carbide, Teflon, and polyethylene cut to the size of 4 × 4 × 2 mm 3 for use, and soot, C60 and CNT powders were glued to the target holders. 4. Harmonic generation from the CCPPs In this section, we present some special features of CCPPs as the source for efficient lower order harmonic generation using the single color (λ = 800 nm) pump. The heating prepulse intensity on the target surface was in the range of 10 10 W cm − 2. The plasma temperature at such intensity was estimated to be approximately 14 eV [31], and the particle density was assumed to be ~10 17 cm − 3. The ionization potentials of C atoms and singly charged ions are 11.25 eV and 24.38 eV, respectively. Thus, such plasma could mainly consist of excited neutral atoms (C) and singly ionized carbon (C +), which was confirmed by spectral measurements. Fig. 2 shows the HHG spectrum obtained in soot plasma plume. The harmonic cutoffs (Hc) in any CCPP did not differ much from each other. The general feature was that they varied in a narrow range, between 23rd and 29th harmonics, and did not depend much on the experimental conditions. The “plateaulike” shape of the highest harmonics does not exactly represent the approximately equal intensity harmonics, as was previously reported in the case of some metal containing plasma plumes [18]. Each subsequent harmonic order was approximately 1.5 times less intense than previous order. On increasing the prepulse intensity (Ipp > (3–5) × 10 10 W cm − 2), the plasma lines started appearing in the spectra. The harmonics disappeared after the generation of strong C 2 + and C 3 + lines at higher prepulse intensities. The influence of driving pulse duration on the HHG efficiency in graphite plasma was studied. Since this plasma has proven to be most

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efficient medium for generation of lowest harmonics (i.e., 9th to 15th orders), it is of interest to know the range of pulse duration above which the harmonic yield diminishes considerably. In these experiments, the energy of the laser radiation remained unchanged at different pulse durations. Initially (i.e., at chirp-free conditions, t = 48 fs), the laser was focused at a distance of 5 mm before the plasma plume. To keep the same laser intensity inside the plasma volume, the decrease in intensity due to increased the pulse duration was compensated by moving the focal plane toward the plasma area. The pulse duration was varied by changing the distance between the gratings in the compressor stage. Fig. 3 shows the spectral curves of the harmonic distribution obtained in the cases of laser pulses of different duration (in the range of 48 fs and 15 ps). One can see that the increase of pulse duration above few picoseconds leads to complete disappearance of the harmonics. The use of longer pulses leads to the appearance of plasma lines, without any harmonics. These experiments show that optimization of HHG requires an appropriate range of pulse duration, above which various impeding processes start playing a decisive role.

5. Two-color pump of carbon containing plasmas—the efficient technique for enhancement of harmonic spectrum

Harmonic intensity (arb. units)

Insertion of a nonlinear crystal (BBO) in the path of the driving radiation led to the appearance of a second harmonic wave, which enabled study of the joint action of the two waves during harmonic generation in CCPP. In spite of the influence of group velocity mismatch and walk-off

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of the fundamental and the second harmonic radiation in the BBO crystal, the calculations show that the ordinary 800 nm pulse and the extraordinary 400 nm pulse still have sufficient temporal overlap after leaving the crystal, especially in the case of chirped pulses. The BBO crystal was inserted at appropriate position after the focusing lens. It was observed that enhanced odd and even harmonic generation takes place despite considerable difference between the two pump intensities (Iω: I2ω = 12:1), and without any precise temporal and spatial matching of the two pumps. In our previous work [15], we had used a KDP crystal (1 mm thick), which also had the same conditions of partial temporal overlap of the fundamental and SH waves after leaving the crystal. However, in that case, the conversion efficiency in KDP crystal was considerably less (2%) due to smaller nonlinear susceptibility and narrower angular and spectral phase matching. Even at those conditions of the high ratio between the two pump intensities (50:1), one could observe the advantages of two-color pump scheme with regard to the single color pump in different metal containing plasma plumes. The stronger SH wave in the present studies allowed study of new features of the HHG in the plasma plumes. Note that the transverse length of the plasma plume in these experiments did not exceed 0.5 mm, and the Rayleigh length of the focused radiation was less than the plume size. The two-color pump, which allowed the generation of odd and even harmonics in the CCPPs, was used. The fullerenes and CNTs were used as the targets to analyze the influence different combinations of carbon in various ablated materials on the harmonic generation in XUV range at variable focusing and chirp conditions. Here we show that the advantages of two-color pump are emphasized in the case of multiparticle containing plasmas. As an example, Fig. 4 presents the comparison of single- and two-color pumps of two different plasmas. These studies show that a noticeable enhancement of odd harmonics was observed in the two-color pump case for CNT and C60 plasmas, together with relatively strong even harmonic generation. It may be noted that, in some of gas jet HHG experiments, the use of two color driving pulses led to the enhancement of harmonic efficiency, extension of harmonic cutoff, and generation of single attosecond pulses [32–34]. In our experiments with CCPPs, we did not observe any extension of harmonic cutoff, which remained approximately same for both pump schemes (i.e. single- and two-color pump), and mostly depended on the conditions of excitation of the C-rich target. The maximum observed harmonics in both these schemes were in the range of 29th harmonic for the 48 fs driving pulses. An enhancement of the HHG efficiency in multiparticle plasma was observed in the two-color case compared to the single color 800 nm pump (Fig. 4a and b). The enhancement factor in that case was in the range of × 2 to ×4 depending on the harmonic order and plasma species. Even harmonics of the fundamental radiation up to the 18th order were obtained in these studies. The analysis of the influence of SH intensity on the spectrum of harmonics showed that the latter can be considerably modified depending on the pulse energies of two pumps. Fig. 5 shows the variations of the harmonic spectra obtained from the boron carbide plasma plume at different energies of SH pulses. The relative values of the SH energy varying from 220 arb. units (curve 1) to 5 arb. units (curve 2) presented in this picture are calibrated with respect to the energy of fundamental pulse (3000 arb. units). The SH pulse energy was changed by rotation of the half-wave plate inserted in front of the focusing lens. One can see that the increase of SH energy led to drastic change of the harmonic pattern from containing only the odd harmonics (curve 2) to the spectrum containing odd and even orders (curve 1).

Wavelength (nm) Fig. 3. Dependences of the spectral distribution of the harmonics in graphite plasma plume on the pulse duration of the driving radiation. (1) 48 fs, (2) 600 fs, (3) 2 ps, (4) 5 ps, (5) 12 ps, (6) 15 ps. The peak at λ = 63.6 nm appearing on the curves (4) and (5) is a plasma line from the excited singly charged carbon ion. The curves presented in this and other figures are moved with regard to each other for better visibility and comparison.

6. Comparison of the HHG conversion efficiencies in various types of carbon- and metal containing plasmas In this section, we present the results of comparative studies of different CCPPs, as well as show the comparison of these harmonic spectra with those obtained from silver and indium plasma plumes.

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Wavelength (nm) Fig. 4. Comparison of the harmonics generation from a) fullerene plasma plume, and b) carbon nanotube plasma plume, in the cases of single color pump (thin curves) and two-color pump (thick curves) schemes.

The latter two have been shown earlier to be the most efficient nonlinear media for generation of harmonics in the range of 15–25 nm (Ag), and strongest ever single (13th) harmonic on the plateau region (In) [18]. A comparison of the harmonic intensities obtained at identical experimental conditions in the cases of three CCPPs (graphite, polyethylene, and PTFE) is shown in Fig. 6. Again, the harmonic cutoffs were extended in these experiments only up to the 27th order. One can see here very strong and broad lower order harmonics, together with extended lobes on the shorter wavelength sides of harmonics. The

origin of the extended lobes has been studied in Ref. [15] and is attributed to the influence of self-phase modulation of the driving femtosecond pulse during propagation through the plasma plume. The harmonic spectrum from the indium plasma plume (curve 4) is also shown. The intensity of 13th harmonic from indium plasma plume considerably exceeds that of other neighboring harmonics due to closeness with resonance transition possessing high oscillator strength. Its efficiency in the plateau range has been reported to be the highest among the solid targets. However, as seen from Fig. 6, the HHG efficiency for the harmonics from C-rich plasmas is comparable to that of the 13th harmonic from the indium plume. We address below the issue of measurement of HHG conversion efficiency in the CCPPs, since this point deserves more clarification. The calibration of HHG conversion efficiency was done as follows. The low-order harmonics were analyzed using a monochromator (Acton, VM502) with sodium salycilate scintillator followed by a photomultiplier tube. At first, this detector system was calibrated using the 3rd harmonic of 800 nm radiation of known energy, generated in the nonlinear crystals. This was compared with the 3rd harmonic generated from silver plume. This calibration gave the absolute conversion efficiency for the 3rd harmonic generating from plasma plume to be 5 × 10 − 4. Using this monochromator, the energy of the low-order harmonics (up to 70 nm) was measured. The energy of each of the next low-order harmonics from Ag plasma was observed to decrease gradually by a factor of 4 to 6. The absolute conversion efficiency for the 9th harmonic (88.4 nm) was measured to be 3 × 10 − 6. The same 9th harmonic signal was then observed using the “extreme ultraviolet spectrometer + MCP + phosphor + CCD” detection system described earlier in the experimental setup section. It was found that, starting from 11th harmonic, the spectrum demonstrates a plateau pattern. The conversion efficiency in the plateau region was calculated to be 1 × 10 − 6, taking into account the absolute conversion efficiency of 9th harmonic. By improving the harmonic output in the plateau region by optimizing the heating pulse energy, time delay between the prepulse and the main pulse, the targetbeam distance etc., a conversion efficiency of 4 × 10 − 6 could be achieved. A further increase of harmonic output by a factor of 2 was observed in loosely focused geometry (f/40, 355-mm focal length lens) for the femtosecond main beam. In that case, the conversion efficiency was measured to be 8 × 10 − 6. From this calibration and the comparative measurements of the harmonics from carbon and silver plasmas, one can estimate the HHG conversion efficiency in CCPPs to be ~ 2 × 10 − 5.

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Wavelength (nm) Fig. 5. Harmonic spectra from B4C plasma plume using two color pump in the case of (1) phase matching polarization of the fundamental radiation for the second harmonic generation in BBO crystal and (2) rotation of the polarization of fundamental radiation on 80° out from the phase matching conditions.

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Wavelength (nm) Fig. 6. Comparison of the harmonics obtained at identical conditions of experiment. (1) graphite, (2) polyethylene, (3) PTFE, and (4) indium plasma plumes.

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Small sized nanoparticles are an attractive alternative to the monoparticles for harmonic generation, since they demonstrate the local-field-induced enhancement of the nonlinear optical response of the medium. This feature has been used for enhancement of the low order harmonics in the vicinity of surface plasmon resonances (SPR) of nanoparticles [35]. Another possible mechanism that can enhance the harmonic efficiency is the increase of recombination cross section of accelerated electron and parent particle in the last stage of the three-step model of HHG. C60 can be chosen as a nonlinear medium for harmonic generation because a) it is highly polarizable, b) it is stable against fragmentation in intense laser fields due to very large number of internal degrees of freedom, which leads to the fast diffusion of the excitation energy, c) it exhibits giant plasmon resonance at ~ 20 eV, d) it has large photoionization cross section, and e) multi-electron dynamics is known to influence ionization and recollision that are central to HHG process. The saturation intensities of different charge states of C60 are higher compared to isolated atoms of similar ionization potential [36]. To study the high-order nonlinearities through HHG, the C60 powder (98% C60, 2% C70, Alfa Aesar) was mixed with epoxy and fixed on to glass substrates. High harmonics in C60 or in any complex multi-electron system will have two contributions—the usual harmonics generation process and the physical mechanisms that lead to enhancement of harmonics (9th–15th in C60) around the frequencies at which the system displays collective electron oscillations (SPR = 20 eV in C60 with a full width at half maximum of ~10 eV). The intensities of the pump pulse and driving pulse are crucial for optimizing the HHG from C60. Increasing the intensity of the driving pulse did not lead to an extension of the cutoff for the fullerene plume, which is a signature of HHG saturation in the medium. Moreover, at relatively high femtosecond laser intensities, one observes a decrease in the harmonic output, which can be ascribed to phase mismatch as a result of higher free electron density. A similar phenomenon is observed when the pump pulse intensity on the surface of fullerene-rich targets is increased above the optimal value for harmonic generation. This reduction in harmonic intensity can be attributed to phenomena such as the fragmentation of fullerenes, an increase in free electron density, and selfdefocusing. The measurements showed no significant change in the intensities of the harmonics generated in C60 for the delays ranging between 15 ns and 88 ns. The stability of C60 molecules against ionization and fragmentation is of particular interest, especially for their application as a medium for HHG. The structural integrity of the fullerenes ablated off the surface should be intact until the driving pulse arrives. Hence, the pump pulse intensity becomes a sensitive parameter. At lower intensities the concentration of the clusters in the plume would be low, while at higher intensities one can expect fragmentation. C60 has demonstrated both direct and delayed ionization and fragmentation processes and is known to survive even in intense laser fields, due to the large number of internal degrees of freedom. The fullerene powder directly glued on the glass surface could survive for a longer time during interaction with pump pulse radiation. Our analysis of harmonic spectra at these conditions showed that harmonics could be observed during approximately 100 shots on the same spot. The appearance of crater on the fullerene containing targets also induced the change of optimal conditions for the HHG in laser plasma. One can note that for other solid carbon-containing targets the stability of harmonics became considerably better, and the stable HHG was observed during few thousand shots, without changing the position of ablated spot. The analysis of harmonic spectra from C60 plasma plumes (Fig. 7) shows the preferential conditions when two-color pump was applied for efficient odd and even harmonics generation (compare curves 1

and 2). We were able to generate the lowest observable even harmonic (i.e. 10th), whose intensity was equal to that of the lower (9th) harmonic. Note that the ratio of applied pumps was 12:1. These studies showed the way for manipulation of HHG parameters by relatively weak and orthogonally polarized field. No harmonics were observed in the case of circularly polarized driving laser radiation (curve 3), as expected. Most of the experiments were carried out at the conditions when femtosecond radiation was focused before the plasma plume. However, the HHG efficiency in fullerene plasma was approximately two times stronger in the case of the focusing of laser radiation “after” the plasma plume compared with the focusing “before” the plasma plume. Modulation of the harmonic spectra from fullerene plasma was studied by: a) introducing chirp by changing the distance between the gratings in the compressor, b) phase modulation of the fundamental radiation by introducing a 10 mm thick BK-7 glass plate between the focusing lens and plasma plume, and c) phase modulation during propagation of intense laser radiation through the fullerene containing plasma. In all of these cases, a broadening of the harmonic bandwidth and a shift of the central wavelength toward the blue or red sides of the spectrum was observed. The study of the harmonic spectra at different chirps in the laser pulse showed a suppression of the blue side lobes for both negatively and positively chirped pulses. This suppression is perhaps due to a decrease in intensity of the laser pulse when chirped, leading to a corresponding decrease in the SPM of the laser pulse propagating through the C60 plasma. In absence of SPM, these harmonics show only blue or red shift depending on whether the pulse has negative or positive chirp.

8. Efficient HHG in carbon nanotubes When intense laser fields are applied to the physical objects such as atomic systems and solid state systems like metallic clusters, and in particular CNTs, traditional perturbation theory is no longer suitable, and many phenomena, including HHG, cannot be understood by perturbative methods [37]. In the case of CNTs, so far, only theoretical studies on the HHG have been reported. The generation of high order harmonics from the single-walled CNTs interacting with a bichromatic laser field (fundamental and its second harmonic) has been theoretically investigated in Ref. [38], where the nonlinear motion of electrons in metallic carbon nanotubes driven by intense laser fields has been studied and the induced current spectrum was

Harmonic order 80000

Harmonic intensity (arb. units)

7. Efficient HHG in fullerenes

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9

11

13

15

17

19 (3) (2)

60000

40000

20000 (1)

0

90

80

70

60

50

40

Wavelength (nm) Fig. 7. Harmonic spectra from the C60 plasma plume for (1) two color pump, orthogonal polarizations, (2) single color pump, (3) single color pump, circular polarization.

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analyzed. The effect of variation of the intensity of the applied laser fields on electron current density and high order harmonic generation has been investigated. Numerical calculations have shown that, with application of the bichromatic laser field, both odd and even harmonics can be generated. In the present study, single-walled CNT powder was used as the target for laser ablation. Laser ablation technique was used to produce a plasma plume containing CNTs. Most experiments were carried out using CNT powder glued on glass substrates using a fast drying glue. The analysis of the morphology of the CNT powder and the deposited CNT debris was carried out using a transmission electron microscope. The morphological characteristics of the targets prior to laser ablation were analyzed and compared with the ablated material debris deposited on a copper grid and carbon film. The diameter of CNTs was 3–6 nm, with length varying from 0.3 to 10 μm. The debris of the plasma plume was studied at various pump pulse intensities. At a pump pulse intensity in the range of 3 × 10 9–2 × 10 10 W cm − 2, CNTs were observed to be deposited. No harmonics were observed during ablation of the glass substrate alone, without CNTs. The above-described morphology and HHG results indicate the ability of CNTs to survive a strong excitation and also that the HHG originates from unfragmented nanotubes. From the above observations, one can conclude that CNTs are responsible for HHG. In the case of CNT targets, an extended cutoff with harmonics up to the 29th order was observed. Most of our studies were carried out at driving laser intensities below 7 × 10 14 W cm − 2. It may be noted that the harmonics were observed at higher pulse intensities as well. However, in that case, the harmonic spectra were different compared to those at moderate excitation of CNT containing targets. This indicates that, at high excitation of CNT plasma, the harmonic generation takes place in a mixture of neutral and ionized nanotubes, as well as fragmented CNT clusters, carbon atoms, and ions. It is difficult to define the relative output of harmonics from these plasma components without the time-offlight spectroscopy analysis. It is found experimentally that the intensity of the harmonics from CNT-rich plumes was stronger compared to those generated from plasma rich in single particles, created on the surface of bulk metal targets under the same experimental conditions. 9. Discussion Laser ablated carbon plasmas have been intensively examined during the last few years to define plasma conditions for the synthesis of C structures with unique properties, in particular, the fullerenes and the carbon nanotubes [39,40]. The physical properties of the plasma plume, such as species concentrations and temperature, directly affect the properties of the material being formed. Successful synthesis is strongly dependent on the formation of atomic and molecular species with required chemistry and energy. For selection of optimal plasma conditions, a detailed understanding the basic physical and chemical processes governing the ablation plume composition and reliable methods for controlling of the relative amounts of plume species are needed. It has been shown previously that an efficient harmonic emission is observed only in the case when singly ionized and neutral carbon lines appeared from the plume [20]. Time resolved spectra of the carbon plasma have been previously recorded by setting the gate width to 50 ns and laser focusing on the target surface at a laser irradiance of 1.2 × 10 10 W cm − 2 [41]. Stark broadening of the spectral lines in the plasmas results from the collisions with the charged species leading to both, a broadening of the line and a shift in the peak wavelength. The maximum intensity of the spectral lines is reached after a characteristic time, depending on the observation location, and it illustrates the most populated section of laser-induced plasma. It has been found that the intensity of C emission lines decreases sharply in the early stage of the plasma.

The appearance of a large amount of C 2 + and C 3 + ions resulted in a considerable decrease in the nonlinear response of the medium. The spectrum was collected during a single shot of a picosecond pulse, without further excitation by the femtosecond pulse, to provide rough information about the plasma components prior to interaction with the main beam. These measurements were time integrated, so one could not say exactly which plasma components existed at the moment of the propagation of the femtosecond beam through the plume. One can note that the relatively high ionization potential of C atoms (11.25 eV) can cause a smaller concentration of free electrons up to high levels of the excitation of carbon targets. This allows one to use relatively dense plasma (~10 17 cm − 3) containing mostly neutral atoms. In accordance with Ref. [41], the electron density of plasma does not exceed 7 × 10 16 cm − 3 at different ranges of excitation of the carbon target (Ilaser ~ 1.2 × 10 10 W cm − 2). One interesting aspect of the harmonics generation in carbon plasma plume is the absence of a pronounced plateau-like distribution of the harmonic intensities up to the cutoff region. It has been pointed out previously that neither low- nor high-order harmonics demonstrate the plateau behavior. While for low-order harmonics (up to ~11th order) this behavior seems to be satisfying the perturbative model of frequency conversion [42], the higher orders demonstrate analogous decrease for each next order. It may be noted that both low orders [20] and high orders ([20] and the present study) show a decrease of each next harmonic by a factor of 3 to 4. The HHG in other plasma plumes shows even faster decrease of the intensity of the low harmonics, thus giving less intense higher orders in the spectral range where the nonperturbative theory predicts a plateau distribution of the harmonics. Such a behavior is not clearly understandable and may be attributed to the higher values of nonlinear susceptibility for loworder nonlinear optical processes. The important issue in the present study is why the CCPPs generate intense harmonics. We have presented above a few observations of this phenomenon in various CCPPs and now give some assumptions, which could explain the high harmonic yield from CCPPs. These assumptions are summarized below. 1) Carbon targets allow easier generation of relatively dense plasma and corresponding phase matching conditions for lower order harmonics; 2) First ionization potential of carbon is high enough to prevent appearance of high concentration of free electrons compared to the other plasma plumes; 3) Neutral carbon atoms dominate in the carbon plume before the interaction with the short laser pulse; and 4) The phase mismatch due to free electrons is small and the effective laser intensity that the medium can experience is higher, since the laser radiation defocusing effects are negligible. Below we explore a possible explanation for our experimental observation that even solid carbon containing targets like Teflon, graphite show HHG comparable to that from fullerenes, and also show a HHG spectrum which is typical of nanoparticles (like that of fullerenes/ CNTs). This could be related to the possibility of cluster formation during laser ablation. In that case, the cluster-containing plasma will be an efficient medium for harmonic generation. Previously, it has been proven in the case of metal nanoparticle-containing plasma plumes [43] that clusters demonstrate enhanced conversion efficiency compared to plasmas produced on the surfaces of bulk metal targets. In particular, the fullerenes have been shown to be very attractive plasma medium for low-order harmonic generation [14,16]. If fullerene formation takes place during ablation of carbon-containing targets, the strong harmonic yield from all these CCPPs under investigation could be attributed to quantum confinement-induced enhancement of the nonlinear optical response of the fullerenes. To prove this assumption, the debris from carbon-containing ablated targets, in particular graphite target was analyzed. Fig. 8 shows the image of graphite debris deposited on the glass substrate after laser ablation at conditions similar to those when efficient HHG was observed

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Fig. 8. (a) Atomic force microscope image of the deposited debris during laser ablation of graphite; (b) Histogram of the cluster size distribution of the above debris deposited over the substrate.

from laser plasmas. One can clearly see the cluster formation on the deposited surface (Fig. 8a). Fig. 8b shows the histogram of nanoparticles created during laser ablation of graphite. The mean size of clusters was 0.7 nm, which coincides with the sizes of the fullerenes. Although, to prove that fullerene formation does take place during plasma ablation of carbon-containing targets, more studies like time-of-flight measurements of debris, may be needed, the above measurements do give some clue for the explanation of the observed superior properties of the CCPPs over other plasma plumes, and their similar fullerene-like HHG spectra. 10. Conclusions In conclusion, the CCPPs produced on various targets (graphite, boron carbide, C60, PTFE, polyethylene, carbon nanotubes, and soot) have been systematically analyzed as the nonlinear media for efficient harmonic generation of laser radiation. The advantages of these media for the HHG process have been demonstrated. The twocolor pump scheme has been used for further improvement of the harmonic yield, due to odd and even harmonic generation. Comparison of harmonic yields from various CCPPs and metal plasma plumes has been made. The use of nanostructured carbon containing targets (fullerenes and carbon nanotubes) for plasma formation and HHG enabled further increase of harmonic yield from CCPP. The advantages of CCPPs for the HHG are confirmed by comparison of HHG conversion efficiency with regard to the Ag and In plasmas. These studies show that, presently CCPPs are the best media for efficient lower order (from 9th to 19th) harmonic generation, while the harmonic cutoff in these plasma plumes is restricted to ~ 29th order. We have discussed some possible reasons leading to the observed enhanced harmonic yield from carbon-containing plasmas. Acknowledgments One of authors (R. A. Ganeev) gratefully acknowledges the support from the TWAS-UNESCO Associateship Scheme. He also thanks the Raja Ramanna Centre for Advanced Technology for financial support and invitation to carry out this work. References [1] E.A. Gibson, A. Paul, N. Wagner, R. Tobey, D. Gaudiosi, S. Baskus, I.P. Christov, A. Aquila, E.M. Gullikson, D.T. Attwood, M.M. Murnane, H.C. Kapteyn, Science 302 (2003) 95. [2] S. Kazamias, D. Douillet, F. Weihe, C. Valentin, A. Rousse, S. Sebban, G. Grillon, F. Auge, D. Hulin, P. Balcou, Physical Review Letters 90 (2003) 193901. [3] G.D. Tsakiris, K. Eidmann, J. Meyer-ter-Vehn, F. Krausz, New Journal of Physics 8 (2006) 19. [4] B. Dromey, M. Zepf, A. Gopal, K. Lankaster, M.S. Wei, K. Krushelnick, M. Tatarakis, N. Vakakis, S. Moustaizis, R. Kodama, M. Tampo, C. Stoeckl, R. Clarke, H. Habara, D. Neely, S. Karsch, P. Norreys, Nature Physics 2 (2006) 456. [5] Y. Akiyama, K. Midorikawa, Y. Matsunawa, Y. Nagata, M. Obara, H. Tashiro, K. Toyoda, Physical Review Letters 69 (1992) 2176. [6] W. Theobald, C. Wülker, F.R. Schäfer, B.N. Chichkov, Optics Communications 120 (1995) 177. [7] R.A. Ganeev, M. Suzuki, M. Baba, H. Kuroda, T. Ozaki, Optics Letters 30 (2005) 768.

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