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We demonstrate the capability to control the ripple periodicity on polycrystalline ZnO films by applying temporally delayed femtosecond double pulses. It is shown that there is a characteristic pulse separation time for which one can switch from low- to high- spatial-frequency ripple formation. Results are interpreted based on the relation of the characteristic delay time with the electron-phonon relaxation time of the material. Our results indicate that temporal pulse shaping can be advantageously used as a mean to control the periodic nanoripples’ formation and thus the outcome of laser assisted nanofabrication process, which is desirable for the applications of nanopatterned transparent semiconductors.
©2013 Optical Society of America
1. Introduction
Femtosecond pulsed lasers have received considerable attention as tools for materials structuring at micro- and nanoscale. Among the distinct advantages of fs laser sources are the rapid energy delivery, the minimization of the total energy deposition to the material and hence the reduction of the heat-affected zone [1,2]. As a consequence, fs lasers allow precise removal of the material, while the optimal interplay between the laser and material parameters allow the fabrication of features with dimensions beyond the diffraction limit [3,4]. In particular, excitation of solid surfaces in air or liquid media by a fs laser beam close to ablation threshold may lead to periodic surface nanostructures in the form of near or subwavelength ripples (also called laser-induced periodic surface structures, LIPSS) [5–7]. LIPSS have been extensively studied and observed in many types of materials, including metals [8], semiconductors [9], ceramics [10] and polymers [11] and have revealed new interesting properties for optical, electronic, wetting and biological applications [12–14]. The periodicities reported are either close to or multiple times lower than the laser beam wavelength, corresponding to low-spatial-frequency (LSFR) and medium/high spatial-frequency (M/HSFR) ripples, respectively. It is shown that the most critical parameters affecting ripples’ periodicity are the laser energy and number of pulses [15].
At the same time, there are other emerging possibilities of ultrafast laser-based modification techniques which may be exploited for expanding the breadth and novelty of applications. In particular fs pulsed irradiation induces fast energy deposition into the electronic system, temporally decoupled from the relaxation to the lattice and subsequent material modification. Hence it allows a realistic attempt towards understanding the fundamental mechanisms of the interaction. The use of temporally designed pulses appears in this context is a feasible way to take advantage of the materials specific response and to determine an optimal energy coupling required for controlling the outcome of processing [15–17]. For instance, a desirable effect in laser processing applications is to control the surface morphology by modulating the energy deposited into the material. Temporally shaped pulses provide the ability to temporally control such energy delivery. Indeed, various experimental studies have been performed to explore the effect of temporally shaped pulses on the morphology of metallic [18–21] and dielectric surfaces [22–24]. Although nanoripples’ formation and related mechanisms have been extensively studied under single-pulse irradiation conditions, the physical processes occurring upon irradiation with simple pulse shapes, as well as the consequences to the surface morphology have been rarely investigated. Specifically, the influence of temporal control of laser energy delivery on the nanoripples’ formation is yet to be explored.
In this work, we present an effective way of controlling the formation of High-spatial-frequency (HSFR) or Low-spatial-frequency (LSFR) ripples on ZnO surfaces via irradiation with double fs laser pulses, tailored on a sub-picosecond time scale. It is shown that by proper choice of pulse separation, one can switch from HSFR to LSFR formation and in this way can control the final surface nanomorphology. Exploring novel techniques to fabricate and control nanostructures on ZnO is of high technological interest, considering the numerous applications of nanopatterned ZnO surfaces, inexpensive transistors [25,26] for disposable electronics and low-cost LEDs [27], thin-film batteries [28] and ITO replacement for displays and photovoltaic panels [29]. Apart from the importance for practical applications, it is concluded that our results may provide further insight on the mechanism of nanoripples’ formation, since the characteristic delay range in which the switch of ripple periodicity is observed is possibly related to the electron-phonon relaxation time of the material.
2. Experimental
Irradiation experiments were performed with a femtosecond Ti:Sapphire laser system operating at a wavelength of 800 nm and a repetition rate of 1 kHz. The pulse duration was set to 80 fs, measured by a means of cross correlation techniques. A 4f pulse shaping configuration using a Spatial Light Modulator (SLM) was used to filter the fourier spectrum of the laser pulses and create double pulse sequences with pulse separations varying from 0 to 7 ps. The repetition rate as well as the number of the double pulse sequences were controlled by a pockels cell. The pulse trains at the output of this cell were properly focused normally onto the ZnO surface giving a spot diameter of 15μm. All irradiation experiments were performed with the sample placed inside a chamber evacuated down to a pressure of 10−2 mbar. The number of laser pulses and fluences used ranged from 15 – 1000 and 0.55 – 4.5 J/cm2, respectively. Field emission scanning electron microscopy (SEM) was used for the morphological characterization of the irradiated areas.
ZnO thick films with thickness up to 4μm were deposited onto Corning glass (1737F) in an Alcatel D.C. magnetron system using a 99.999% pure metallic zinc target of 15 cm diameter. The base pressure of the ultra-high vacuum (UHV) chamber was below 5x10−7 mbar while during the deposition the pressure was 8x10−3 mbar and the substrate temperature at 27 °C. All the films were grown at a constant plasma current of 0.45 A. In order to avoid thickness non-uniformities, the distance between the target and substrate was set to 20 cm. The film’s thicknesses were measured using an AlphaStep profilometer.
3. Results and discussion
3.1. Effect of laser fluence on ripple periodicity
Experiments have been performed on two separated ZnO films with thickness of 1.7 and 3.2 μm, however similar results were obtained in both cases. Here we focus on the experiments performed using seven different fluences, at a constant number of 15 pulses. Figures 1(a) and 1(b) show the SEM images of the spots obtained upon irradiation of the surface using two fluences, one close to the damage threshold and the other well above, being 0.55 J/cm2 and 0.92 J/cm2, respectively. Three distinct ripple periodicities are clearly observed on the spot obtained at the higher fluence (Fig. 1(b)), while their periodicity decreases from the spot center towards the outer periphery. The respective values, calculated via a Fourier transform algorithm, are ~650nm (Low Surface Frequency Ripples-LSFR), ~221nm (Medium SFR-MSFR) and ~160nm (High SFR-HSFR). On the contrary, LSFRs are absent on the spot obtained when irradiating with the lower fluence (Fig. 1(a)). Figure 1(c) summarizes the fluence dependence of the ripples’ period indicating that while HSFRs and MSFRs are generated at all fluences, LSFRs disappear below a critical fluence that is close to the damage threshold of the material. It should be noted that for fluences above ~1.5 J/cm2, the LSFRs disappear from the spot center while only MSFR and HSFR are visible around the spot.
Fig. 1 SEM images of the ZnO spots obtained upon irradiation (a) close to the damage threshold using a fluence of 0.55 J/cm2 and (b) above the damage threshold using a fluence of 0.92 J/cm2. (c) Graph of the ripples period versus the irradiation fluence.
The physical mechanism behind the formation of LSFR is attributed to the optical interference of the incident laser beam with the excited surface plasmon at the solid air interface [6,29]. Surface plasmons, are traveling waves of collective longitudinal oscillating electrons at optical frequencies that can be excited at the interface of a material-dielectric. An effective coupling of the incident electromagnetic radiation with the plasmon oscillation, leads to the periodic modulation of the absorbed intensity. This interference effect propagates perpendicularly to the electric field and is followed by a spatially and periodically modulation of energy deposition, generating finally the LSFR.
Two are the dominant mechanisms proposed and reviewed below for the creation of subwavelength ripples (M/HSFR). The first one is attributed to the generation and interference of a second harmonic wave [5] while the second is a grating-assisted surface plasmon-laser coupling effect [6,9]. For high laser fluencies Ne (here fluence higher than ~0.9 J/cm2), the excited carrier density can reach above 10−21 cm−3 and as a result, the refractive index of ZnO changes significantly. As a result, the initial transparent material turns into one with a metallic character. As a consequence, the surface reflectivity of the ZnO film increases and starts to significantly absorb the incident radiation. In this case the formation of LSFR occurs. On the other hand, for carrier densities in the range of 10−20 < Ne < 10−21, (fluence of 0.5 – 0.8 J/cm2), the peak intensity is high enough to generate a second harmonic wave on the material surface. The interference of this wave with the surface electromagnetic wave scattered at 400 nm gives rise to the HSFR with periodicity close to ~200nm. The second mechanism is attributed to a grating coupling effect. R.W. Wood [30] was the first to propose coupled surface plasmon polaritons using the grating configuration. In particular, incident radiation at an angle θ with respect to the normal plane of the surface can be scattered from the grating, increasing or decreasing the component of its wave vector by integer multiples of the grating wave vector kg (kg = 2π /Λ). This gives rise to diffracted orders which lead to the formation of HSFR. When the fluence is high, the strong thermal effects are dominating over the grating-coupling effect and the generation of LSFR occurs due to the interference mechanism. In contrast for fluences when nonthermal ablation effects take place, the grating coupling effect becomes dominant and assists the generation of HSFRs. Accordingly, the above mechanism is able to describe the generation of ripples with periodicities multiple times lower than the laser wavelength.
3.2. Pulse separation dependence ripple periodicity
A proper way to control the flow of laser energy deposition on the material is the use of temporally shaped laser pulses.
A simple pulse shape is created by splitting the initial Gaussian pulse in two equivalent pulses of the half intensity/same duration, separated by a delay time τd. Figure 2 presents an example of SEM images of different spots attained, following irradiation with 15, 20 and 50 sequences of two identical pulses of 1.1 J/cm2 at various τd. The respective spots obtained after irradiating the surface with 15, 20 and 50 double laser shots of 0.55 J/cm2 fluence are also shown which are almost identical to those obtained using no pulse separation (Fig. 1(a)). It is clear that by using a fluence of 1.1 J/cm2 for single or double pulse irradiation with small time delays (τd < 500 fs), all three types of ripples are generated. On the contrary, after double-pulse irradiation with τd > 500 fs the LSFRs disappear from the spot center, being replaced by MSFR and HSFR at the periphery.
Fig. 2 SEM images of different spots for irradiation with 15, 20 and 50 sequences of two identical pulses of 1.1 J/cm2, at various time delays.
Similar to Fig. 1, the corresponding dependence of the different ripple periodicities observed on τd is presented in Figs. 3(b) and 3(c) for zero pulse and 1 ps time delay, indicating that one can transit between LSFRs and MSFR/HSFRs via proper variation of the pulse time separation.
Fig. 3 The corresponding dependence of different ripple periodicities on pulse delay time. Switch between LSFRs and MSFR/HSFRs occurs via proper variation of the pulse separation time. (a) Spots irradiated with a fluence of 0.55 J/cm2 and zero pulse delay. (b), (c) fluence of 1.1 J/cm2 with and zero pulse and 1 ps time delay respectively. (d) Ripples period vs pulse delay for 15 laser pulses and fluence of 1.1 J/cm2. Around 500 fs periodicity changes from LSFR to MSFR.
The characteristic time delay of around 500 fs as shown in graph of Fig. 3(d), for which the change from LSFR to M/HSFR occurs, may possibly be related to the electron-phonon relaxation time (τe-ph) of the material. Experimental studies on the ultrafast dynamics of crystalline ZnO films reported carrier cooling times of ~0.8 ps [31] and 1 ps [32]. Furthermore, theoretical studies [32] predict that τe-ph ~0.7 ps. The above values are close to the characteristic τd for which the switching of the ripple periodicity occurs in our experiments. In particular, for τd < τe-ph the second pulse arrives before the energy absorbed by the first pulse, is transferred to the lattice. Considering that material modification takes place after the energy is transferred to the lattice, the final ripple profile should be similar with that of the single pulse of the same total fluence of 1.1 J/cm2. Therefore, according to the fluence dependence of the ripple periodicity presented in the previous section, LSFR are generated. On the other hand, for τd > τe-ph the second pulse arrives when the energy of the first pulse is already transferred from the electronic system to the lattice. Accordingly, M/HSFR are generated due to the action of the first pulse, which has the half intensity compared to that corresponding zero τd, while the second pulse interacts with the already formed ripple profile.
The above effect may be exploited for tailoring the outcome of fs laser nanopatterning of ZnO. This is presented in Fig. 4 showing two scan lines fabricated at zero (Fig. 4(a)) and 1 ps (Fig. 4(b)) delay times, respectively. In this case, the sample was scanned at a constant velocity, so that the effective number of pulses per spot to be equal to 200. In accordance to the switching effect described above, the periodicity of the respective nanopatterns changes from ~650nm to ~220nm. This further demonstrates the control over different nanopatterns’ formation upon changing the pulse separation which can expand the breadth and novelty of applications of nanopatterned ZnO surfaces. Experiments are currently in progress to further explore the properties of large area ZnO surfaces, which are nanopatterned using different time delays.
Fig. 4 Two scan lines fabricated at (a) zero and (b) 1 ps delay times. The periodicity of the respective nanopatterns changes from ~650nm to ~220nm.
Finally, the surface modification attained at the highest τd, is very similar to that induced by a single pulse of the half fluence, not only in terms of the ripple characteristics but also in terms of the affected area. Indeed, a gradual decrease of the modified area is observed upon increasing the pulse separation, indicating that for long delay times, the material responds as if it receives only half of the fluence. This is indicated in Fig. 5 presenting the LSFR, HSFR + MSFR and total spot areas as a function of τd. Furthermore, it is clear that for τd < 500 fs, the LSFR area decreases at the expense of that of HSFR + MSFR while the total spot area gradually decreases upon increasing pulse separation. A spot area decrease has been previously studied for certain semiconductors and metals both experimentally [33,34] and theoretically [19].
4. Conclusions
In summary, we reported the capability to control the periodicity of nanoripples formed on polycrystalline transparent ZnO layers via irradiation by temporally delayed fs double laser pulses. Exploring novel techniques to fabricate and control nanostructures on thin films is of high technological interest, considering the numerous applications of nanopatterned semiconducting surfaces. It is also postulated that apart from the importance for practical applications, our results may provide further insight into the mechanism of nanoripples’ formation. Our results indicate the use of temporally shaped ultrafast pulses as an additional route for controlling and optimizing the outcome of laser structuring which can pave the way for sophisticated materials processing.
Acknowledgments
This work was supported by the Integrated Initiative of European Laser Research Infrastructures LASERLAB-II (Grant Agreement No. 228334). The authors acknowledge Ms. Aleka Manousaki for her support with the Scanning Electron Microscope.
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