Efficient white-light continuum generation in

Efficient white-light continuum generation in transparent solid media using ~250 fs, 1053 nm laser pulses T. Imran, G. Figueira Grupo de Lasers e Plasmas - Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Av. Rovisco Pais 1049-001 Lisbon, Portugal Abstract. We report white-light continuum generation in solid-state media (fused silica and sapphire) using seed pulses centered at 1053 nm and at a repetition rate of 10 Hz. We have investigated the influence of different parameters, such as changing the focal position and the energy of the incident pulse within the medium to obtain optimal white-light continuum. Preliminary results indicate that for intense laser pulses, waist position inside the media and input energy are crucial for high efficiency white-light continuum generation over the wavelength range 400-1100 nm. It was also found that pulses centered at 1053 nm generate a flatter spectrum, with higher white-light continuum efficiency. Such a flat response over a broad bandwidth in the continuum has the potential to be efficiently compressed to shorter durations. Keywords: White-light continuum, Femtosecond laser pulses, Fused silica glass, Sapphire PACS: 42.65.Jx

Introduction The propagation of short laser pulses through an optical medium can lead to considerable temporal and spatial broadening , which can extend from the ultraviolet to the infraredand is the spectral representation of the spatio-temporal modification of the pulse in the medium [1,2]. When the incident laser pulse is of femtosecond duration, the spectral broadening manifests itself in light that emerges out of the medium as a white light disk surrounded by a distinct, concentric, rainbow-like pattern. The central, low divergence part of the output beam is referred to as the “white-light continuum” or “supercontinuum”, and it can be readily separated by placing an appropriate diameter iris or aperture after the optical medium, or making observations in the far field regime. The primary process responsible for the physical phenomena of supercontinuum generation is self-focusing, which causes the pulse to compress in space, resulting in a corresponding increase in the peak intensity [3]. The other dominant processes, which are responsible for starting the mechanisms leading to spectral broadening are self phase modulation (SPM) [4], self-steepening [5], and parametric four-photon mixing [4]. But the physics governing supercontinuum production is not properly understood despite its intrinsic interest and its utility in contemporary schemes for the generation of ultra-short pulses [6]. It is now generally accepted that self-focusing clearly plays the role of the initiator of the sequence of processes that lead to white-light continuum generation [7-9]. When the laser power that is incident on a medium exceeds the critical power required for self2 focusing, Pc r = λ 0 , a catastrophic collapse of laser energy occurs at a finite π n 0 n 2 (BELOW) TO BE INSERTED ON THE FIRST PAGE CREDIT 2LINE OFthe EACH distance. Here n0 and n2 denote linearPAPER and nonlinear refractive indices of the medium respectively, and λ0 is the center wavelength of laser light. Experimentally, the CP1228, Light at Extreme Intensities—LEI 2009, edited by Dan Dumitras © 2010 American Institute of Physics 0-7354-0771-8/10/$30.00

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threshold power for the continuum generation coincides with the calculated critical power for self-focusing. During self-focusing, because of self phase modulation , an ultrashort pulse broadens spectrally, and a temporal rearrangement of the new frequency components is suffered, due to group velocity dispersion (GVD). This causes a reduction of peak power during self-focusing, such that a higher input power is required for the onset of catastrophic self-focusing, critical power to initiate the continuum is higher for ultrashort pulses than for long pulses [15,16]. Furthermore, the band gap of the material exerts a strong influence on white-light light generation. There appears a certain band gap threshold of 4.7 eV below which optical media do not generate whitelight continuum, and above which spectral bandwidth of the continuum is increased with band gap [8]. Since its first observation, supercontinuum generation has been demonstrated in a variety of materials, including solids, liquids, and gases [1113,18,19]. Its unique characteristics make the supercontinuum an ideal broadband light source for applications such as femtosecond time-resolved spectroscopy, optical pulse compression for the generation of short pulses, seed pulse for optical parameteric amplifiers, and biomedical applications [20-26]. We have recently initiated experiments at our laser facility [27] on white-light continuum studies using high intensity, femtosecond laser pulses through solid material like fused silica and sapphire. Most of the experimental study in this area has been done using conventional Ti:sapphire-based femtosecond laser systems, working at ~800 nm and higher repetition rates [9, 28-30]. In this paper, to our best knowledge, we report the first ever demonstration of supercontinuum generation in solid material (fused silica, sapphire), pumped with ~250 fs pulses at 1053 nm, 10 Hz repetition rate. We probe the dependence of intensity and spectral distribution of white light generation on physical focusing conditions. The results that we present here find applications in optimizing supercontinuum production using various solid materials and in investigating the possibilities of pulse compression for generating ultrashort pulses.

Experimental setup The experiments were performed using the front-end of the multi-terawatt, hybrid Ti:sapphire-Nd:glass L2I laser system [27], consisting of a Ti:sapphire Kerr-lens modelocked oscillator (Mira, Coherent Inc.) that delivers a ~ 150 fs, 76 MHz pulse train with pulse energy of 4 nJ at 1053 nm. Pulses are then stretched to ~900 ps in an Offner grating stretcher and directed into a homemade Ti:sapphire regenerative amplifier, operating at 10 Hz, raising their energy to ~ 2 mJ. After compression in a double-pass grating setup we obtain ~ 250 fs, mJ-level pulses. In a typical experiment (Figure. 1(a)), a pulse energy of the order of ~ 200 µJ, is focused into a few-mm (10 mm and 15 mm) thick fused silica and sapphire plate to generate white-light continuum, which is detected by a CCD camera. We measured the laser energy, incident on the solid media, using an energy meter. The same energy meter was used to measure the energy of the entire white-light continuum. The parameters of the incident laser light were changed, by using lenses of different focal lengths (300 mm to 600 mm) and by optimizing the focal plane inside the media. The resulting white-light continuum spectrum is detected by a broadband spectrometer (Ocean Optics HR2000+). The white-light continuum beam has central white-light part, which is surrounded by colored rings. These rings do not extend continuously from the central white-light, but occur as distinct rings of red, green, and blue light, often separated from the central white light beam and from each other by dark rings. The higher frequency light appears

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in the outermost ring, and the lower frequency light forms the ring nearest the white light continuum, as shown in Figure. 1(a) (inset). The white-light continuum after the solid media is made to pass through an iris to cut off the outer unwanted portion of the white-light continuum beam. A flip type mirror mount is used after the solid media (sapphire and fused silica plates) to send the focused beam to the spectrometer to study and characterize the continuum spectrum. Flat, octave spanning spectra (400 nm to ~ 1200 nm) can be seen in Figure. 1(b). A power meter is used in close proximity to the material under study to estimate the efficiency of the process. This experimental setup later on may be applied to the extracavity pulse compression, to generate the short laser pulses [32].

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Figure 1. (a). Schematic to generate and characterize the white-light continuum generation and CCD image of the continuum. (b). Seed beam, white-light continuum spectra of sapphire and fused silica.

Experimental Results and Discussion We discuss here the results of white-light generation in sapphire and fused silica plates. In our experiments laser pulses of ~250 fs duration are used. Such pulses undergo temporal broadening when propagating through long media, reducing the light intensity. In using the fact that SPM is known to be inversely proportional to the pulse duration [1], free electron generation is expected to contribute to SPM, particularly enhancing the blue spectral region [13]. In contrary, our measurements indicate that the free electron contribution to SPM is larger in the case of 250 fs pulses at 1053 nm than for the conventional laser systems of 800 nm center wavelength. In our measurements, we have optimized the continuum generation, for a lens of given focal length, the focal plane inside the sample and input power so as to obtain the flattest possible spectrum of white-light with high conversion efficiency. At low incident power, when the beam power approximately equals the critical power ( P > Pcr ), a single filament is formed within the crystal (Fig. 1(a)). Two or more filaments are observed along the direction transverse to laser propagation at higher incident powers considerably exceeding the critical power for self focusing. This multiple filamentation occurs as there is beam break up due to modulation instability [14] of spatial modes. Accordingly, some special features of the white-light continuum spectrum generated at 1053 nm can be described as follows: (i) Rresidual peak is observed at the pump wavelength . In fact, in the resulting spectrum, seed beam (pump wavelength, 1053 nm) can be minimized by using the

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appropriate filter to obtain the more flatter spectrum, as opposed to the 800 nm cases where the spectra build up, asymmetrically around the pump wavelength [13,17,31]. (ii) We have observed, on all the recorded continuum spectra, a broad peak centered at about 600 nm which is not observed in the case of 800 nm centered wavelength beam [31]. The basic mechanism responsible for the white-light continuum generation at 1053 nm is same as that for 800 nm, that is the free electron enhanced self-phase modulation process[4]. This scenario can also account for the increased spectral band-width of the continuum based on its band gap dependence [8]. Note, for a given material band gap, a higher maximum intensity is expected at a longer pump wavelength and this is causing, in turn, a larger nonlinear effect and thus a larger broadening of the continuum. In Figure. 2 we show the white-light spectra from sapphire and fused silica that were obtained using different focal length lenses while keeping the input laser energy constant. In the case of the shortest possible focal length lens, we obtained poor continuum and the highest conversion efficiency (>80%), although this was accompanied by damage to the medium. Longer focal length lens (400 mm and 500 mm) yielded somewhat lower conversion efficiencies, typically around