High-Harmonic Generation in an Optical Fiber Functionalized with Zinc Oxide Thin Films

Abstract

High-order harmonic generation (HHG) in semiconductor thin films from ultrashort mid-infrared laser drivers holds the potential for the realization of integrated sources of extreme ultraviolet light. Here, we demonstrate solid-state HHG in zinc oxide thin films synthesized by the radiofrequency reactive magnetron sputtering process directly on the cleaved facets of optical fibers. Harmonics 3 to 13 of the radiation from a fiber-based laser system delivering 500 kW, 96 fs pulses at 3130 nm are produced in the thin film and guided along the fiber. A proper choice of the laser wavelength and fiber material allows for filtering out the mid-IR pump laser and achieving the HHG mode selection. The possibility to nanostructure the fiber exit by, e.g., focused ion beam milling paves the way to an increased control over the HHG spatial mode.

1. Introduction

The middle-wave infrared (mid-IR) range is gaining significant momentum across a whole range of applications, from highly sensitive biological and chemical sensors for homeland security applications and industrial and environmental monitoring to advanced astronomy applications such as planet hunting. Ultrashort, high-intensity, pulses in the mid-IR are particularly interesting for the production of spatially coherent UV radiation through the process of high-order harmonic generation in rare-gas atoms because the harmonic cut-off energy scales as ${{\lambda }_p}^2$ where ${\lambda }_p$ is the wavelength of the laser driver. In HHG experiments, the generation of the mid-IR radiation is often based upon an optical parametric amplifier or a difference frequency generation stage pumped by a near-infrared femtosecond laser. These highly complicated systems operate at low repetition rates and therefore do not provide sufficient flux desired for many applications such as metrology and imaging. In 2011, for the first time, Ghimire et al. put into evidence HHG from a bulk semiconductor crystal when electrons undergo either intra-band or inter-band processes [1]. Above-bandgap harmonics down to ~130 nm in ZnO were observed for laser field strength approaching the critical field strength of the material (0.6 V·Å⁻¹). The fact that such a high field strength does not inflict damage to the material is attributed to the small multiphoton absorption cross-section for the long mid-IR wavelength laser used, associated with a pulse duration shorter than the electronic relaxation time scales. This highlights another advantage of using an ultrashort mid-IR laser driver for HHG, but this time, in solid-state materials. The necessary field strength, corresponding to focused intensities of ~1 TW·cm⁻², is two orders of magnitude lower than that required for gas-phase HHG. This dramatic reduction in ultrafast laser intensity opens the door to HHG from oscillator-like laser systems, and especially from fiber-based laser systems in the mid-IR. Along this line, we reported recently on above-bandgap HHG in ZnO from a low-energy laser system operating at the wavelength of 2.1 µm and delivering 120 fs 8 nJ sech-shaped pulses at a repetition rate of 20 MHz through a single-mode optical fiber [2]. Above-bandgap harmonics (H₇ and H₉ in our experiment, limited by the detection scheme) were obtained by propagating the laser field in microcones patterned in ZnO bulk samples by focused ion beam (FIB) milling. The lateral confinement in the microcone contributed to enhancing the laser field by a factor of 20, to intensities above 1 TW·cm⁻² (Ref. [2]), sufficient to produce above-bandgap harmonics. The possibility of solid-state sample structuration is a major advantage over gas-phase media. Not only can it be used for field enhancement, but it also opens interesting prospects in the control of harmonics properties such as the generation of beams carrying orbital angular momentum [3]. Apart from the possibility to structure the material by FIB milling or reactive-ion etching techniques, for example, current semiconductor synthesis methods, such as magnetron sputtering [4], allow for the synthesis of semiconductor thin films on almost any surface, relaxing constraints on the sample used for harmonic generation. HHG was recently demonstrated in ZnO thin films epitaxially grown on sapphire substrates [5]. The authors observed that the yield for a harmonic order higher than 9 is actually similar in bulk samples and thin films. This observation was attributed to the very short absorption length of these harmonics. With such a short absorption length, the harmonics generated along propagation in the bulk sample are inevitably reabsorbed in the generation medium itself. Only the very last tens of nanometers effectively produce harmonics that can be collected out of the generation medium, thus justifying the use of thin films for solid-state HHG.

With the current synthesis method, it becomes possible to synthesize semiconductor thin films directly onto the cleaved facets of optical fibers, opening the possibility to produce and “guide” the harmonic radiation through an optical fiber. This opens new opportunities for characterization of materials, for example, where the ultraviolet excitation beam can be “guided” to a particular spatial region of the sample, mostly as a fiber probe but for ultraviolet or even extreme ultraviolet radiation. Here, we combine the advantage of these new manufacturing possibilities together with the precise control of the mid-IR laser properties propagating through the film to leverage solid-state HHG and to develop a source of ultrafast radiation in the UV available at the tip of an optical fiber. To this aim, we first developed a fiber-based laser system delivering 500 kW, 96 fs pulses at 3130 nm. Then, the laser beam was launched into a silica fiber, the tip of which has been coated with a 400 nm thick film of ZnO. Using this experimental configuration, high-order harmonics up to order 13 were produced in the thin film and guided along the silica fiber down to the detector. The silica fiber absorbs the residual pump light at 3130 nm and allows us to achieve the HHG mode selection. We also report on experimental evidence of the non-perturbative character of the high-order harmonic generation in the ZnO film even at moderate laser intensities.

2. Laser System

The schematic of the set-up used to generate and deliver high-order harmonics from a mid-IR fiber laser is depicted in Figure 1.

Our study starts from a chirped pulse laser system delivering 1 µJ 760 fs pulses at a 1 MHz repetition rate at a wavelength of 1965 nm. Solid-state HHG cannot be obtained with such picosecond pulses. We had to decrease the duration down to approximately 100 fs while achieving, at the same time, hundreds of kW peak power. Furthermore, the longer the wavelength of the laser driver, the easier it is to obtain HHG because, as stated in the introduction, the laser-induced damage threshold of the semiconductor material increases with wavelength. We have already demonstrated that the exploitation of soliton dynamics in very large mode area optical fibers is well suited to achieve these three requirements in a relatively simple setting [6,7,8]. We have shown that 100-fs class pulses with megawatt peak power can be obtained in the mid-IR by nonlinear frequency conversion of near-IR picosecond pulses in fibers. Along this line, we have developed a nonlinear stage (Figure 1) based on soliton dynamics in very large mode area fibers to reach a wavelength of $\lambda =3130 \mathrm{n}\mathrm{m}$ suitable for HHG in thin films of ZnO. We have chosen this wavelength because the laser photon energy ($E_{\mathrm{L}}=0.4 \mathrm{e}\mathrm{V}$) is well below the bandgap energy, making multiphoton ionization negligible. This wavelength is also comparable to those used in other works [1,5]. According to our previous experiments, a field strength inside the material of ~0.1 V·Å⁻¹ is necessary to generate HHG. This corresponds to a focused intensity of ~0.3 TW·cm⁻². To this aim, the laser beam was launched into a quasi-single-mode silica fiber with a 77 µm core diameter to exacerbate the multisoliton fission process and the soliton self-frequency shift (SSFS) effect [9,10] at a very high energy level, without the onset of supercontinuum generation. The exacerbation of these solitonic effects in a very large mode area fiber leads to the generation of femtosecond pulses with high energy (tens to hundreds of nanojoules) and ultrashort duration (in the range of 70–120 fs), which are tunable in wavelength by the intrapulse stimulated Raman scattering and easily separable from each other by means of bandpass filters. In our experimental set-up, and as a result of the multisolitonic fission process exacerbated in a 40 cm long piece of very large mode area fiber, the first ejected soliton was both compressed to 90 fs at the megawatt peak power level and shifted to 2250 nm. However, the frequency shift obtained at the output of the first stage is too small to reach the mid-IR range. Indeed, the shift decelerates and eventually stops because of dissipative effects (the infrared absorption edge and the bend loss in our case) and because of a decrease in the fiber nonlinearity at longer wavelengths due to the increase in its effective mode area. To reach 3130 nm, we have added a nonlinear stage where the SSFS effect occurs in a fluoride glass fiber. In particular, we have selected a single-mode fluoride glass fiber developed at Le Verre Fluoré, France [8]. This fiber exhibits a wide transparency window in the mid-R up to approximately 4 µm, a large core (26 µm in diameter), a low numerical aperture (0.075), a low nonlinear Kerr index (n₂ = 2.1 × 10⁻²⁰ m²·W⁻¹), and anomalous dispersion at 2250 nm. All these features contribute to the generation of high-energy solitons, tunable in wavelength from 2230 nm up to 3130 nm. It is worth noting that the second fiber must have a higher nonlinear coefficient than the previous one in order to accelerate the Raman-induced frequency shift, hence the smaller core diameter for the fluoride fiber. It is, however, very difficult to manufacture fluoride fiber with an NA lower than 0.075. As a consequence, the normalized spatial frequency V = 2.67 leads to a slightly multimode behavior for this fiber at the pump wavelength of 2250 nm. Nevertheless, by carefully adjusting the launching conditions, this fiber was found to be effectively single-mode at 2250 nm. Furthermore, the stimulated Raman scattering effect acts as a nonlinear spatial filter and cleans the spatial shape of the most redshifted soliton, ending up with a Gaussian beam at 3130 nm.

The pulse of interest at 3130 nm was selected by a bandpass filter with a 500 nm bandwidth centered at 3000 nm and characterized by means of a home-built mid-IR frequency-resolved optical gating system based on second-harmonic generation (SHG-FROG). Independent measurement of the spectrum was carried out by means of a grating-based spectrometer (Yokogawa AQ6376) spanning from 1500 nm to 3400 nm. Results are shown in Figure 2 for a slightly shorter central wavelength of 3050 nm.

The average power of the pulse train was measured to be 55 mW, which corresponds to 55 nJ of energy contained in the pulse at a 1 MHz repetition rate. The pulse FWHMI duration was deduced to 96 fs from the reconstruction of the FROG trace, indicating that the instantaneous power reaches 0.5 MW at 3050 nm. The fiber length was selected to avoid any dissipative effect, which otherwise would lead to some elongation of the pulse via dispersive effects. Similar duration and peak power are thus expected up to 3130 nm with this custom-designed fluoride fiber. All these parameters make this pulsed source highly suited to HHG in solid-state materials. It is worth noting that the conversion efficiency between the CPA and the output of the fluoride fiber is only 5%, which seems to be very low. However, the dramatic temporal compression experienced in the nonlinear fibers yields 500 kW peak power, which is sufficient to observe HHG.

3. Thin Film Synthesis

In a second step, 400 nm zinc oxide films were deposited onto the cleaved facet of several silica fibers. We have selected ZnO as a generation material because of its large bandgap ($E_{\mathrm{G}}=3.37 \mathrm{e}\mathrm{V}$), relatively high nonlinear index (n₂ = 5 × 10⁻¹⁹ m²·W⁻¹) and our long-term experience in synthesis of ZnO thin films [11]. It is worth noting that ZnO stands as a legacy material for solid-state HHG studies since the seminal report by Ghimire et al. [1]. Using this material allows us to compare our results to those obtained by other groups or to those obtained by us but in different settings.

The films were obtained using a radio frequency (RF) reactive magnetron sputtering process from a Zn target (99.99% purity, 3-inch diameter) in an Ar + O₂ reactive atmosphere. A custom-designed device, compatible with ultra-high vacuum environments, enables the deposition of oxide material onto the cleaved sections of multiple fibers. The device shields the fibers within a compartment during plasma exposure, presenting only their cleaved facets for material deposition. Prior to deposition, the chamber was evacuated to a base pressure of 10⁻⁶ mbar. Subsequently, a sputtering gas mixture of Ar/O₂ (37 sccm Ar and 25 sccm O₂) was introduced into the chamber using a mass flow controller. ZnO deposition is carried out at room temperature under a pressure of 10⁻² mbar with a plasma power of 300 W. Under the specified power and pressure conditions, we have achieved a deposition rate of ZnO on the substrate surface of 0.67 nm/s, leading to a film thickness of 400 nm for 10 min of ZnO deposition.

To investigate the structural properties of the deposited oxide, ZnO thin films were also fabricated on 10 mm × 10 mm fused silica substrates under similar conditions, replicating those employed for deposition on fibers (identical target-to-substrate distance and synthesis parameters). Figure 3 presents the X-ray diffraction (XRD) pattern and a scanning electron microscopy (SEM) cross-sectional image of the deposited layers on a silica substrate. The ZnO XRD pattern exhibits characteristic peaks for the wurtzite crystal phase. The diffractogram primarily shows (002) and occasionally (004) reflections, suggesting a preferential growth orientation along the c-axis normal to the substrate. Consistent with the XRD pattern, ZnO films deposited on a silica substrate display a characteristic columnar growth morphology as shown in the SEM image.

4. High-Order Harmonic Generation

For HHG experiments, we searched for the experimental configuration leading to the longest wavelength (3130 nm) and the highest energy (55 nJ). The pump spectrum is shown in Figure 4a. Then, the beam from the fluoride fiber was collimated and focused onto the ZnO-coated facet of a silica fiber, as shown in Figure 1. The resulting harmonic spectrum was plotted in Figure 4b. With this experimental procedure, the harmonics were generated down to approx. 200 nm and guided along the fiber while the residual pump beam was absorbed by the silica material. This experimental arrangement therefore facilitates the detection of the harmonics, which are collected by means of a UV-grade silica fiber and directed towards a UV-VIS-NIR spectrometer. The diameter at 1/e² in intensity of the focused beam was estimated to be 12.6 μm, resulting in a vacuum peak intensity of approximately 0.45 TW·cm⁻² and a field strength of 0.13 V·Å⁻¹ in the ZnO sample (refractive index n = 1.9). The dashed vertical line in Figure 4b marks the bandgap edge of the material, highlighting the fact that above-bandgap harmonics H₉, H₁₁, and H₁₃ were generated in the ZnO thin films and detected with our relatively simple detection arrangement. In Figure 4b, for proper visual identification of the harmonics, the highest-order harmonics H₁₁ and H₁₃ were magnified by a factor of 3 and 30, respectively.

It is worth noting that the harmonic spectra are well defined, contrary to what can be observed in bulk samples. This peculiarity of HHG in thin films has been reported in [5], where the authors studied HHG in ZnO thin films epitaxially grown on sapphire substrates. They have attributed this effect to the absence of spectral broadening of the pump in the thin film. Indeed, in bulk materials, the femtosecond pump travels over hundreds of microns in the generation medium where it experiences spectral broadening. This spectrally broadened pump pulse favors the emission of spectrally broad harmonics. In our experiment, the pump pulse does not undergo any spectral broadening since it is focused directly onto the generation medium supported by the fiber. The harmonic spectra are therefore very well defined, which makes them more easily selectable by means of band-pass filters.

We also note that the silica material exhibits a loss level of approx. 3 dB/m at 200 nm. In our proof-of-concept experiment, the length of the piece of silica fiber used to guide the ultraviolet light was limited to a few tens of cm by the device used for thin film deposition. Although the power loss experienced by the highest harmonics is certainly not negligible, it is not the limiting factor in our experiments. The detector itself is limited to approx. 190 nm and does not allow us to detect harmonics higher than H₁₃.

The strength of the highest harmonics, H₇ to H₁₃, was plotted in Figure 5 as a function of the infrared laser intensity from 0.1 to 0.32 TW·cm⁻². Harmonic 7 to 11 are well approximated by a power-law function, $I^{4.5}$, which clearly indicates the non-perturbative character of the HHG process in the ZnO film even at moderate laser intensities. The non-perturbativity is even more pronounced for H₁₃, showing a power-law function fit scaling as $I^{2.6}$.

5. Conclusions

High-order harmonic generation and guidance by means of a functionalized optical fiber have been achieved. The process is based on magnetron sputtering, a mature semiconductor deposition process that has been used to fabricate ZnO thin film directly at the tip of an optical fiber. When excited by 55 nJ 96 fs pulses from a mid-infrared fiber laser operating at 3130 nm, the ZnO films produced up to the 13th harmonic of the fundamental laser frequency. The harmonics were generated at the input of the fiber and guided all along the fiber. We have thus demonstrated both high-order harmonic generation and waveguiding in a single optical fiber. The non-perturbative character of the generated harmonics is attested by the power-law dependence of the harmonic intensity. The originality of our work lies in the fact that the harmonics are guided in the fiber, which allows us to filter out the mid-IR laser to achieve the HHG mode selection. HHG has also been tested with a ZnO thin film at the exit of the fiber. In this case, the harmonic generation is equally efficient, but the residual mid-IR light was remaining and had to be filtered out by an external spectral filter. Another perspective is the possibility to nanostructure the fiber output by reactive-ion etching or focused ion beam milling to achieve further control over the HHG spatial mode and/or to reduce the pulse energy required.