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We present spectral measurements that uncover the detailed dynamics of the supercontinuum generation in fused silica, YAG, and LiF crystals when pumped with 100 fs pulses with a central wavelength of 2.3 μm, which falls into the range of anomalous group velocity dispersion of the media. The multi-octave supercontinuum spectra, which span from the ultraviolet to the mid-infrared, are recorded by means of high-dynamic range measurements over the entire wavelength range covered by the supercontinuum radiation. The spectral dynamics versus the input pulse energy uncovers different mechanisms of spectral broadening, which are characterized by the occurrence of specific but well-distinguished spectral signatures and which are interpreted in terms of plasma-induced and anomalous group velocity dispersion-induced compression of the driving pulse.
© 2016 Optical Society of America
1. INTRODUCTION
The first demonstration of white light generation with femtosecond laser pulses in a bulk medium [1] revealed that a considerable improvement of pulse-to-pulse reproducibility and spatial uniformity of broadband radiation benefits from the short duration of the pump pulse. Since then, the femtosecond supercontinuum (SC) has become recognized as an indispensable tool for many applications, ranging from time-resolved spectroscopy to few optical cycle pulse generation.
The SC generation with femtosecond laser pulses in various solid-state dielectric media in the range of normal group velocity dispersion (GVD) was widely studied experimentally, see, e.g., [2–5], and its physical mechanism is fairly well understood in the framework of femtosecond filamentation [6]. In a normally dispersive nonlinear medium, the catastrophic self-focusing is halted by the pulse splitting, which produces wavelength-shifted leading and trailing subpulses, which are responsible for redshifted and blueshifted spectral broadenings, respectively [7–9].
Advances in the near- and mid-infrared ultrashort-pulse laser sources, which are exclusively based on optical parametric frequency conversion, provided access to experimentally investigate femtosecond filamentation and supercontinuum generation phenomena in the range of anomalous GVD of wide bandgap dielectric media. The first experiments performed in this spectral range demonstrated notable extension of the SC spectra in fused silica [10,11]. These early findings facilitated the discovery of a qualitatively new filamentation regime, where simultaneous compression in space and time leads to formation of quasi-stationary self-compressed objects: spatiotemporal light bullets [12–16]. Such filamentation conditions yielded multi-octave SC spectra with unprecedented wavelength coverage, as generated in wide bandgap dielectric crystalline materials, such as YAG, sapphire, and [17–22], in water [23], in various types of glasses: fused silica [24–27], BK7 [28], fluoride (ZBLAN) [29], tellurite [30,31], and chalcogenide glass [32] as well as in birefringent nonlinear crystals, such as DAST [33] and beta-barium borate (BBO) [34]. The numerical simulations suggest that even broader SC spectra could be obtained in sodium chloride and potassium iodide crystals [35]. By tuning the wavelength of the input pulses deeply into the mid-infrared, the ultrabroadband mid-infrared SC was generated by accessing the zero and anomalous GVD range of semiconductor media, such as GaAs [36,37] and ZnSe [38].
More recently, filamentation of intense femtosecond laser pulses with a central wavelength of 3.9 μm in air demonstrated a notable enhancement of the SC spectrum by odd-harmonics generation [39]. Such SC generation regime was also accessed in a thin solid-state medium ( crystal) under tight focusing condition and uncovered a different scenario of the spectral broadening, which originates from the plasma-induced compression of the driving pulse and subsequent spectral broadening of the individual harmonics peaks via cross-phase modulation, resulting in generation of odd-harmonics-enhanced SC, whose spectrum spans over more than four octaves [40].
In this paper, we investigate the dynamics of mid-infrared laser pulse-induced spectral broadening and multioctave SC generation in fused silica, yttrium aluminium garnet (YAG), and lithium fluoride (LiF). By increasing the input pulse energy, we capture different mechanisms of spectral broadening, which originate either from plasma-induced or GVD-induced compression of the driving pulse and which are clearly distinguished by the occurrence of specific spectral features.
2. EXPERIMENTAL SETUP AND MATERIAL PROPERTIES
The experiment was performed using a commercial optical parametric amplifier (OPA) (Topas-Prime, Light Conversion Ltd.), pumped by an amplified Ti:sapphire laser system (Spitfire-PRO, Newport-Spectra Physics). The idler pulse from the OPA with a central wavelength of 2.3 μm, duration of 100 fs, and an energy up to 50 μJ served as the pump.
The pump beam was spatially filtered, suitably attenuated, and focused by an lens into the focal spot of 30 μm FWHM diameter (). The focal spot was located inside the nonlinear medium; its position was found experimentally so as to achieve the spectral broadening with the lowest input pulse energy. A 10 mm long ZnSe crystal was used to compensate the dispersive broadening of the pump pulse in the signal-idler beam separation optics as well as in other optical elements in the beam path. As the nonlinear media, we used thin samples (of 3 mm thickness) of three different wide-bandgap nonlinear dielectric materials: UV-grade fused silica, YAG, and LiF, whose relevant linear and nonlinear optical parameters are listed in Table 1. The wavelength of the pump pulse (2.3 μm) was chosen so as to fall into the range of anomalous GVD of all three investigated materials.
Table 1. Linear and Nonlinear Parameters of UV-Grade Fused Silica, YAG, and LiFa
The input pulse energy was varied by using a neutral metal-coated gradient filter (NDL-25C-2, Thorlabs Inc.). High dynamic range spectral measurements were performed with a home-built scanning spectrometer with Si and PbSe detectors, operating in the and spectral ranges, respectively. The output spectra were collected into the slit of the spectrometer using a pair of Al-coated concave mirrors. The estimated angular acceptance of the spectrometer was from the beam propagation axis, so the spectrometer captures the on-axis part of the SC and only a fraction of the conical emission. The measured spectra were corrected to sensitivity functions of the detectors and transmission of the bandpass optical filters used in the measurement. Finally, the spectra from each detector were slightly scaled to achieve consistency in the overlap region.
3. RESULTS AND DISCUSSION
A. Fused Silica
Fused silica is a versatile material used for a variety of applications in linear and nonlinear optics, so there is no wonder that, to date, a large fraction of the SC generation experiments in the range of anomalous GVD were performed in that nonlinear medium [10,11,18,24–27,43,44]. These studies uncovered a number of general features, which characterize the SC spectrum. First of all, the angular distribution of the conical emission takes a specific shape, which consists of multiple elliptical structures around the carrier wavelength (in the region of anomalous GVD) and a V-shaped tail in the visible spectral range (in the region of normal GVD) [25,43,44]. Second, the angle-integrated SC spectrum shows an intense blueshifted peak in the visible range [18,24,25], which is identified as an axial component of the conical emission and whose blueshift increases with increasing the wavelength of the driving pulse [24,25]. However, to date and to the best of our knowledge, the entire spectral extent of the SC generated with mid-infrared laser pulses in fused silica was thus far never measured.
Figure 1 presents the spectral dynamics versus the input pulse energy, which uncovers two distinct phases of the spectral broadening, which are characterized by the occurrence of specific spectral features and which are attributed to different mechanisms of pulse compression.
Fig. 1. Spectral broadening in fused silica versus the input pulse energy. Insets on the right show the visual appearance of the output beam in the far field at various stages of spectral broadening. The dimensions of the insets in the following figures are of the same scale.
In the input pulse energy range of 1.1–1.9 μJ (the power below ), we observe a weak third harmonic (TH) peak centered at 767 nm. The TH generation is a simplest third-order nonlinear process; however, due to large phase mismatch, the TH peak is relatively weak [18,27] and hence is often overlooked in the SC generation experiments, where the dynamic range of the detection apparatus is typically limited to 3 or 4 orders of magnitude. Thereafter, in the input pulse energy range of 2.0–2.7 μJ (), an almost symmetric spectral broadening around the carrier wavelength along with dominant blueshifted spectral broadening of the TH peak is observed. The TH spectrum extends into the visible spectral range and becomes detectable by the naked eye, appearing as a faint red spot, as shown in the inset of Fig. 1. These spectral features are presented in more detail by plotting the lineouts of the spectra in Fig. 2(a). The spectral broadening around the carrier wavelength could be attributed to plasma-induced compression of the driving pulse, where plasma generation results in blow-up of the trailing part of the pulse, in analogy with recent observations in [40]. The blueshifted spectral broadening of the TH peak could be explained by the difference of the group velocities of the driving and TH pulses: because the driving pulse travels with the largest group velocity, the slower TH pulse shifts toward its trailing front, as a result experiencing large spectral blueshift via cross-phase modulation. Along with spectrally broadened TH, we also capture a weak spectral peak at 425 nm, which is almost unresolved in Fig. 1 but quite clearly present in Fig. 2(a). That spectral peak corresponds to the fifth harmonic, which is generated by the cascaded four wave mixing between the fundamental and TH frequencies [40,45].
Fig. 2. Characteristic spectra in fused silica, originating from (a) plasma-induced pulse compression and (b) GVD-induced pulse compression and formation of the light bullet. A step-wise change in the background is due to different noise levels of the detectors.
An explosive spectral broadening is observed with the input pulse energy of 2.8 μJ (corresponding to ), which yields an ultrabroadband SC extending from the ultraviolet (310 nm, at the intensity level) to the mid-infrared (3.75 μm at the intensity level). The SC spectrum features the characteristic intense blueshifted peak, which is centered at 430 nm, while the TH peak becomes quickly masked by much more intense SC emission. Such a broad spectrum is produced by formation of the light bullet, which emerges from simultaneous compression in space due to self-focusing and in time due to opposite effects of self-phase modulation and anomalous GVD [12–16], which is further referred to as the regime of GVD-induced pulse compression.
Interestingly, with further increase of the input pulse energy, the central wavelength of the blue peak continuously shifts to longer wavelengths, for instance, to 570 nm at 5.2 μJ, as depicted in Fig. 2(b). The wavelength shift of the blue peak is easily detected by visual means, as evident from the change of the color appearance of the output beam from blue to green, as shown in the inset of Fig. 1, suggesting that the phase matching condition, which yields the position of the blue peak is intensity-dependent. At the same time, we observe gradual shrinking of the mid-infrared part of the SC spectrum. Such peculiar spectral behavior is also observed in other investigated nonlinear media (as will be shown below) and could be attributed to the periodic “breathing” nature of propagation of the light bullet [15].
B. YAG
YAG is an attractive nonlinear material because of its high nonlinearity, wide transparency range, and high optical damage threshold. In particular, owing to its large nonlinear index of refraction, the SC is generated at reasonably low input pulse energies, as reported with mid-infrared laser pulses with carrier wavelengths of 2 μm [18] and 3 μm [17].
Figure 3 presents the dynamics of the spectral broadening in YAG versus the input pulse energy. In general, in the input pulse energy range of 0.7–2.0 μJ () the spectral dynamics in YAG is qualitatively similar to those observed in fused silica and captures both phases of spectral broadening associated with plasma-induced and GVD-induced pulse compression, respectively, as highlighted in more detail in Fig. 4. Figure 4(a) illustrates the characteristic output spectra in the early phase of spectral broadening, associated with plasma-induced compression of the driving pulse. The TH generation at lower input pulse energy is followed by spectral broadenings around the carrier wavelength and the TH peak, which overlap with the input pulse energy of 1.0 μJ. However, owing to very large phase mismatch for TH generation as due to large dispersion of the crystal, the detected TH peak is much weaker than that in fused silica and hence is not observed by visual means.
Fig. 3. Spectral broadening in YAG versus the input pulse energy. Insets on the right show the visual appearance of the output beam in the far field at various stages of spectral broadening.
Fig. 4. Characteristic spectra in YAG, originating from (a) plasma-induced pulse compression and (b) GVD-induced pulse compression and formation of the light bullet.
The spectral superbroadening, which is associated with GVD-induced self-compression of the driving pulse and formation of the light bullet, is observed with an input pulse energy of 1.05 μJ, corresponding to the input pulse power of . The widest SC spectrum is measured with the input pulse energy of 1.1 μJ and spans from 350 nm to 3.8 μm, with the blueshifted peak centered at 620 nm, as shown in Fig. 4(b). Notice the apparent differences in the spectral intensities obtained in the plasma-induced and GVD-induced pulse compression regimes and a wide angular distribution of the visible part of the SC produced in the latter, as shown in the inset of Fig. 3 (the angular dimensions of the insets in Figs. 1 and 3 are to scale).
Thereafter with further increasing the input pulse energy up to 2.0 μJ, we observe a narrowing of the SC spectrum due to the redshift of the blue peak and gradual shrinking of the mid-infrared part. At the same time, in the input pulse energy range of 1.2–2.0 μJ, the SC spectrum shows a development of two distinct spectral humps around 1 μm and 1.7 μm [as shown in more detail in Fig. 4(b)], whose origins are unclear, and which progressively shift toward longer wavelengths with increasing the input pulse energy.
A second boost of the SC spectrum, which is expressed by spectrally broadened and intensified blue peak, as well as the intensified mid-infrared portion of the spectrum, is observed with the input pulse energy above 2.1 μJ. However, the spectral intensity of the SC in the 0.9–1.5 μm range becomes notably reduced and shows a development of an interference pattern, which indicates filament refocusing and splitting of the light bullet after a secondary nonlinear focus [27]. Finally, with the input pulse energy above 4 μJ (), a very clear interference pattern across the visible and near-infrared spectral range develops as a result of the filament breakup into two filaments, which also produce the interference fringes in the far field pattern, as illustrated in the inset of Fig. 3.
C. LiF
Although LiF has the largest bandgap among the solid-state dielectric media, formation of long-living color centers resulting from irradiation by intense laser pulses is often considered as a major drawback to its potential applications in nonlinear optics and SC generation in particular. However, recent experiments demonstrate that the color centers only weakly modify the UV portion of the SC spectrum on the long-term time scale [46]. Color centers are also shown to produce intense luminescent tracks in the sample under mid-infrared excitation, which nevertheless do not alter the filamentation process in general [47].
Therefore, we first investigated the impact of color center formation on the SC spectrum. The time evolution of the SC spectrum was recorded in the ultraviolet–near-infrared spectral range (200–900 nm) using a fiber spectrometer (QE65000, Ocean Optics), which operated in a single-shot regime. Figure 5 shows the modification of the SC spectrum as a function of the number of laser shots, as recorded with a fixed input pulse energy of 10.2 μJ.
Fig. 5. (a) Time evolution of the ultraviolet-near infrared part of the SC spectrum in LiF as recorded with the input pulse energy of 10.2 μJ. (b) Examples of the individual SC spectra after a different number of laser shots.
Formation of the color centers starts almost immediately, after just a few tens of laser shots, as manifested by the changes of the SC spectrum. More specifically, the SC spectrum shows slight shrinking of its ultraviolet edge, a redshift of the blue peak, an enhancement of the spectral intensity around 800 nm and the occurrence of a prominent spectral dip whose central wavelength is exposure time-dependent. These spectral modifications exhibit rapid evolution with increasing number of laser shots, and the most dramatic change of the SC spectrum is recorded after 800 laser shots, as shown in more detail in Fig. 5(b). Interestingly, thereafter the SC spectrum starts to broaden again, and, after approximately 5000 laser shots, the shape of the SC spectrum eventually stabilizes and remains unchanged over the rest of the measurement time. The final SC spectrum in a modified crystal shows a marked increase of the spectral intensity in the visible spectral range and even broader ultraviolet extension as compared with that measured in an unmodified crystal, as shown in Fig. 5(b).
These findings suggest that, although the color center formation alters the SC spectral shape, the process of spectral broadening and SC generation in a modified volume of the crystal remains almost unaffected. This is verified by studying the dynamics of the spectral broadening in a modified crystal, which captures the above-discussed phases of the spectral broadening, associated with plasma-induced and GVD-induced pulse compression, as shown in Fig. 6.
Fig. 6. Spectral broadening in LiF versus the input pulse energy. Insets on the right show the visual appearance of the output beam in the far field at various stages of spectral broadening.
An apparently intense TH peak at 767 nm is readily detected even in the range of low input pulse energies, which correspond to the input pulse power range well below the critical power of self-focusing. Thereafter the plasma-induced pulse compression produces spectral broadenings around the carrier wavelength and TH; the blueshift of the latter makes the TH radiation easily observable by the naked eye, as illustrated in the inset of Figs. 6 and 7(a). In the input pulse energy range of 6–8 μJ, alongside the spectrally broadened TH, we capture a relatively strong peak of the fifth harmonic, which experiences spectral broadening as well. Eventually, the spectral broadenings around the carrier, the third and fifth harmonic frequencies overlap, resulting in the so-called odd-harmonics-enhanced supercontinuum [40].
Fig. 7. Characteristic spectra in LiF, originating from (a) plasma-induced pulse compression and (b) GVD-induced pulse compression and formation of the light bullet.
With an input pulse energy of 8.5 μJ () an explosive spectral superbroadening due to GVD-induced pulse compression is observed. The broadest SC spectrum, which continuously covers the wavelength range from 290 nm in the ultraviolet to 4.0 μm in the mid-infrared, as estimated at the intensity level, is measured within an input pulse energy range of 9–13 μJ [see also Fig. 7(b)]. Because the upper wavelength limit of our spectrometer is 4.0 μm, and there is still an appreciable signal of the SC, the wavelength of the mid-infrared cut-off edge of the SC could be extrapolated at 4.3–4.4 μm. The generated SC spectrum exhibits an intense blueshifted peak, which is centered at 380 nm. With further increase of the input pulse energy, a slight redshifting of the blue peak along with a slight shrinking of the mid-infrared side of the SC spectrum is observed, as shown in more detail in Fig. 7(b). However, the effect of spectral shrinking in the case of LiF is less pronounced as compared with measurements in fused silica and YAG.
4. CONCLUSION
In conclusion, we investigated the dynamics of spectral broadening and supercontinuum generation in the range of anomalous GVD in fused silica, YAG, and LiF nonlinear crystals. High dynamic range measurements were performed over a wide spectral range, demonstrating the generation of ultrabroadband, multioctave SC, which continuously covers the wavelength range from the ultraviolet to the mid-infrared, namely, from 310 nm to 3.75 μm (3.6 octaves) in fused silica, from 350 nm to 3.8 μm (3.4 octaves) in YAG and from 290 nm to 4.3 μm (3.9 octaves) in LiF.
The dynamics of the spectral broadening versus the input pulse energy was found to be similar in all investigated nonlinear media, despite the differences of their relevant linear and nonlinear parameters. The spectral diagnostics uncovered two different phases of spectral broadening, which were characterized by the occurrence of specific spectral signatures and which were interpreted in terms of plasma-induced and anomalous group velocity dispersion-induced compression of the driving pulse. More specifically, the plasma-induced pulse compression, which occurs due to blow-up of the trailing part of the pulse by free electron plasma, produces spectral broadening around the carrier and third (or even fifth, as observed in LiF) harmonic frequencies. Eventually, these spectral broadenings overlap, producing a low-intensity broadband radiation in the visible–mid-infrared spectral range. In contrast, the group velocity dispersion-induced pulse compression, which originates from the opposite effects of self-phase modulation and anomalous GVD and leads to formation of the light bullets, produces an intense multioctave SC emission, whose spectrum spans from the ultraviolet to the mid-infrared with a characteristic blue peak located in the visible spectral range. We also show that the formation of the color centers in LiF crystal alters the SC spectral shape; however, the SC generation process in a modified volume of the crystal remains almost unaffected.
Finally, our measurements demonstrate that, under given operating conditions (the input pulsewidth, the length of the nonlinear medium, and the focusing condition), there exists an optimum input pulse energy, which produces the broadest SC spectrum, allowing us to optimize the practical setups for SC generation with mid-infrared laser pulses.
Funding
Research Council of Lithuania (APP-8/2016).
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