We demonstrate intense high-order harmonic generation from plasma that is created from different carbon targets. We obtain high-order harmonic energy in the multi-microjoule range for each harmonic order from the 11th to the 17th harmonic. By analyzing the target morphology and the plasma composition, we conclude that the intense harmonics from the bulk carbon targets originate from nanoparticles target.
© 2011 OSA
Since the demonstration of high-order harmonics with temporal duration of a few hundredattoseconds , there has been increasing interest from the scientific community to use harmonics as a source of attosecond light pulses. The extremely short-pulse nature of high-order harmonics have opened the door to various new applications, and have spawned a new science that is now called “attoscience” and “attophysics”. In the past years, several interesting applications have emerged in attosecond science, such as the quantum stroboscope . This work has shown that one could watch the movement of the electron wave packet during ionization of an atom/molecule using attosecond pulses, by varying the delay between the attosecond pulse train and the ultrashort infrared pulse. Encouraged by the success in uncovering new science at the attosecond timescale, scientists are further exploring high-order harmonic generation (HHG) in the extreme ultraviolet (XUV) range and its related science.
A more direct method for studying the attosecond dynamics of electron systems is to use intense attosecond harmonics to excite the system. This is especially promising for C60 fullerenes, where the surface plasmon resonance (SPR) lies in the XUV range (, 10 nm full width at half maximum (FWHM)) [3,4], which is readily accessible using high-order harmonics. Driven by such and similar needs, there have been efforts to realize harmonic sources with higher intensity and higher energy. For example, it has been proposed to use an aperiodically corrugated hollow-core fiber to modulate the intensity of the fundamental laser pulse along the direction of propagation, resulting in multiple quasi-phase-matched high harmonic emissions at the cut-off region. This technique shows that the yield of selected harmonics is increased equally by up to two orders of magnitude Compared with no modulation . Other works have demonstrated techniques to increase the conversion efficiency of high-harmonic generation by using waveguide  and new focusing geometries [7, 8].
It has been shown that low-density plasma produced from solid targets is an efficient medium to generate harmonics. We have demonstrated HHG from lowly ionized laser plasmas for more than fifteen targets, ranging from Al () to Bi () . The energy of a single harmonic order obtained from these solid targets is measured to be in the sub-microjoule range. Further, we have demonstrated that the 13th harmonic from indium plasma is extremely efficient, with conversion efficiency of 10−4 from the pump laser to the harmonic . However, harmonics from indium plasma is quasi-monochromatic (that is, the 13th harmonic is 100 times more intense than all other harmonic orders), thus making it difficult to generate attosecond pulses. Therefore, there is a need to search for methods to efficiently generate intense harmonics over a broader spectral range.
Other targets that could increase the harmonic yield are nanoparticle targets. In particular, we have shown that by using indium oxide nanoparticles  or C60 film , we could obtain intense harmonics between the wavelength ranges of 50 - 90 nm. The energy in each of these harmonic orders was measured to be a few μJ, which is sufficient for many applications. However, the problem of using nanoparticles or film target is the shot-to-shot instability and the rapid decrease of the harmonic intensity, due to inhomogeneity of the particle density and rapid change in the morphology of these targets. This negative feature of harmonics from such targets prevents us from using these intense harmonic sources in applications, or to perform attosecond measurements, which require stable harmonics. In this paper, we report highly efficient and stable high-order harmonics generated from laser ablation plasma of various graphitic carbon targets, and compare them with those produced from C60 nanoparticles or C60 film.
2. Experimental details
Harmonic generation from ablation plasma requires two laser pulses: first, a long (sub-nanosecond) prepulse is used to create the plasma, and a second intense, short (femtosecond) main pulse is used to generate the harmonics. These two high-power Ti:sapphire laser pulses (prepulse and main pulse) need to be temporally synchronized, and their characteristics should be independently controlled to study and optimize the conditions for efficient harmonic generation. Therefore, we used the 10 Hz, multi-TW, 35 fs Ti:sapphire laser beam line of the Advanced Laser Light Source (ALLS) . The output of this beam line was configured into two beams before compression, with each beam having a maximum energy of 200 mJ and pulse duration of 210 ps. Each beam has a variable energy controller, which can independently vary the pulse energy using a computer. One of the two beams is sent to a vacuum compressor, which compresses the 210 ps pulse to 35 fs, with maximum energy of 150 mJ. We will refer to the two beam lines used in this work as Line #1(sub-nanosecond prepulse; 210 ps duration, 200 mJ maximum energy) and Line #2 (femtosecond main pulse; 35 fs duration, 150 mJ maximum energy).
Figure 1 is a schematic diagram of the experimental setup. To create the ablation plasma, we focused the prepulse from Line #1 on to a target placed in a vacuum chamber, using a plano-convex lens (focal length f = 150 mm). The focal spot diameter of the prepulse beam on the target surface was adjusted to be roughly 600 μm. The intensity I pp of the sub-nanosecond prepulse on the target surface was varied between to . After a 24 ns delay, the femtosecond main pulse (energy to 25 mJ, pulse duration , 800 nm central wavelength, 40 nm bandwidth at full-width at half maximum (FWHM)) from Line #2 was focused on the plasma from the orthogonal direction using an MgF2 plano-convex lens ().
The harmonic spectrum was spectrally dispersed by a homemade spectrometer with flat-field Hitachi grating (1200 lines/mm). The spectrum was then detected by a micro-channel plate (MCP) with phosphorus screen readout, and is finally recorded by a charge coupled device (CCD) camera. The ultraviolet (UV) spectrum of the plasma was analyzed using a spectrometer (SpectraPro500i, Acton Research Corp.) and recorded by a time-resolved CCD camera (DH501-18F-01, Andor Technology).
3. Experimental results
The targets used in this experiment are the following. The C60 film is a fullerene film on a glass substrate. The films are grown by evaporating C60 powder (99% purity, MER Corporation) in a resistively heated oven at 600 °C. The effusive beam of C60 molecules is deposited onto a glass substrate maintained at liquid nitrogen temperature. The thickness of the film used in the experiment was about a few microns. The C60 nanoparticle is a fullerene powder (98% C60; 2% C70; Alfa Aesar), which was mixed with epoxy, yielding a solid target with an inhomogeneous distribution of fullerene nanoparticles. The laser pulse from Line #1 is focused on the target to create low-ionized plasma. A temporal delay line of 24 ns and 75 ns is respectively introduced in Line #2, and the compressed femtosecond laser pulse irradiates the plasma to generate the harmonics. The main pulse focus is about 7 mm far from the plasma. We estimated his intensity to be around 2x1014W.cm−2 at the interaction point with the plasma. Figure 2 shows the harmonic spectrum obtained from the different carbon targets.
We were able to generate intense harmonics on all solid carbon targets used. We have also noted that unlike other solid targets, it is very easy to generate harmonics from plasma produced on the carbon target. We observe that for the mainpulse energy varying from 8 to 15 mJ the harmonic intensity is increasing. Then saturate for the pulse energy between 15 and 20 mJ and finally decreasing for higher pulse energy.
Next, we compare in Fig. 3 the harmonic spectra obtained from bulk carbon target to those from C60 fullerene target.
High-order harmonics (HH) produced from the ablation plume of bulk carbon targets and C60 fullerene exhibit plateau-like harmonic spectrum up to the 25th order. We observed that the intensity of the harmonic spectrum obtained from bulk carbon target is comparable or slightly higher than those obtained using C60 fullerene targets.
We measured the harmonic energy generated from carbon plasma by using the following method. First, we generated the third harmonic of Ti:sapphire laser radiation (Line #2) with two nonlinear crystals (BBO, type I and II) collinearly placed one after another. The energy of generated third harmonic (266 nm) was measured using an energy meter. Then the third harmonic beam was sent to a vacuum monochromator (Acton, VM502) and converted to a visible light using the layer of sodium salycilate deposited on the glass plate. The radiation from this plate was collected by a photomultiplier tube (PMT), and the signal was recorded on an oscilloscope. The signal from PMT was calibrated by the energy of third harmonic measured using an energy meter. Then we compared the third harmonic generated from the plasma using the “monochromator + PMT” scheme and the calibrated oscilloscope signal, which allowed us to measure the energy of the third harmonic generated from the plasma. The energy of other high-order harmonics was determined by comparison with the reference third harmonic signal taking into account the spectral response of the monochromator and sodium salycilate. To determine the energy of the higher harmonics that were out of the shortest wavelength range of the measurements by this monochromator (80 nm), we used the above-described homemade XUV spectrometer to measure the harmonic spectrum, thus extrapolating the harmonic energy measurements to the higher-order harmonics. By using this calibration method, we estimated the energy of the 13th and the 15th harmonic generated from the bulk carbon plasma to be in the range of microjoule with about 5% uncertainty.
We have shown in our previous work that nanoparticles and films of C60 would generate harmonics that are more intense than those obtained from solid targets . The disadvantage of using nanoparticle and film targets is the instability of the harmonic signal, which considerably varies shot-by-shot, and even disappears after a few laser shots if the target is not move. On the contrary, we have found for the first time that carbon bulk targets can generate intense harmonics, with intensity comparable to the nanoparticle or C60 film targets. Further, the harmonic intensity using bulk carbon target remains stable for several minutes, even without moving the target position. By creating the plasma during 5 minutes on the same place of the solid carbon target the harmonic signal generated doesn’t decrease more than 10%. The one from nanoparticles decreased more than 90% after few second. Figure 4 shows the variation of the harmonic spectra as a function of time.
To further understand the mechanism for this stable and intense harmonic generation, we first studied the composition of the target used. For this, Fig. 5 represents a scanning electron microscope (SEM) image of the target. We also performed X-ray photoelectron spectroscopy (XPS) measurement of the target to determine its chemical composition. Figure 6 is the XPS measurement.
From this figure, we see that the peak that corresponds to carbon is lower than the others peaks. However, the data shows that the target is composed of 33% carbon. The other peaks observed on the curve are due to, among others, oxygen from air contamination. The carbon peak is small because of the low efficiency of the detector for particles with low energy. The SEM image and the XPS curve shows that the bulk carbon target is made of multiple layers of graphite.
The question that arises now is why bulk carbon targets result in such intense harmonics, while for most other solid targets, the harmonic intensity is weaker, with less than 200 nJ energy per pulse. To understand the particularity of the carbon plasma, we analyzed the ablated material debris deposited on silicon substrates that were placed close to the ablation plume. Figure 7 shows the scanning electron microscope image of the plasma deposition.
The SEM image reveals that the plasma created from bulk carbon target contains nanoparticles with size varying between 100 nm to 300 nm. We therefore suspect that during the interaction of prepulse with the carbon target, nanoparticles are formed in the plasma and are then pumped by the main pulse to induce the generation of harmonics. To determine the nature of these nanoparticles, Fig. 8 shows the XPS measurements of the plasma deposition.
On the curve, we note the presence of several peaks that signify the existence of different elements in the plasma deposition. The measurement reveals the presence of elements such as oxygen and nitrogen coming from the air contamination of the sample, and silicon, which is the substrate used for deposition. However, the XPS measurement shows that the target is mainly composed of carbon. This study shows that nanoparticles that are present in the deposition are formed by atoms of carbon. We therefore believe that when the prepulse is focused on the carbon target, it forms plasma composed of carbon nanoparticles. These nanoparticles then interact with the main pulse to generate the harmonics.
We also noted that unlike most other solid targets, the highest harmonic order obtained in carbon target does not exceed 21. According to the cut-off law defined by the three-step model, we suggest that these harmonics are generated by neutral atoms, rather than ion as in the case of other solid targets.
Our experiments using the bulk carbon target reveal two experimental observations, that is, a low cutoff (25th order or less) and high harmonic intensity. From these observations, we suggest that the origin of these harmonics is similar to what has been described for nanoparticles targets . That is, the presence of nanoparticles in the plasma deposition suggests that neutral atoms of nanoparticles are the main source of intense harmonics from this bulk carbon. An explanation for intense harmonic generation from nanoparticles could be the higher concentration of neutral atoms due to the presence of nanoparticles. Unlike single atoms and ions, whose density quickly decreases due to plasma expansion, nanoparticles retain densities that are close to its solid state. Combined with the higher harmonic efficiency of neutral atoms compared with their ions, the neutral atoms within the nanoparticle could generate high-order harmonic efficiently.
In this paper, we presented our results on the generation of intense and stable harmonics from various bulk carbon targets. We obtained intense harmonics for wavelengths ranging from 47 nm to 70 nm. From energy calibration, the energy in each of these harmonics was estimated to be a few μJ. The experimental observations, which include low cut-off and the presence of dense nanoparticles in the plasma deposition, suggest that neutral atoms from nanoparticle are the main source for intense harmonics from this bulk carbon. Our results demonstrate the possibility of generating high intensity and stable source of coherent short wavelength radiation using bulk carbon target. This source could be used for example to induce two-photon ionization of helium with a wavelength of 89 nm as demonstrated by Kobayashi et al in reference .
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