Plan de Gestión Integral de Residuos Sólidos (PGIRS)

Femtosecond lasers have many applications ranging from the measurement of ultrafast phe- nomena to nonlinear material processing. Two different types of sources are readily available, conventional femtosecond sources operating at a repetition rate around 100 MHz but deliver- ing pulses of up 10 nJ and amplifier systems delivering pulses above theµJ level but operating at kHz repetition rate. The main limitation for increasing the pulse energy that can be directly achieved from conventional Ti:sapphire oscillators have been the onset of instabilities such as multipulsing and cw-generation due to high intensities in the laser medium. By operating the laser in the positive dispersion regime, heavily chirped picosecond pulses can be generated in the laser cavity. The long pulse duration inside the laser allows decreasing the intracavity in- tensity due to pulse stretching. This reduces pulse instabilities substantially and offers a very good alternative for energy scalability. Since the average power for a laser can be increased only with additional pump power, which is limited, in order to increase the pulse energy at a given pump power a reduction of the laser repetition rate is required.

In the framework of this thesis we used a Herriott-type multi-pass cavity for decreasing the repetition rate of the laser below 70 MHz. This approach allowed to extend the cavity length while leaving the operating point of the laser invariant. Operating the laser in the posi- tive dispersion regime allowed the development of a pure-Kerr lens mode-locked Ti:sapphire oscillator at 10.7 MHz repetition rate that delivers pulse energies up to 200 nJ [26] using a 10 W pump laser. Careful control of the intracavity dispersion by means of chirped mirrors [5] allowed for the generation of pulses as short as 26 fs (spectral width≈80 nm). This was the first system operating in the positive dispersion regime in which dispersion compensation with chirped mirrors was done. A few other experiments in this regime, in which dispersion compensation was done entirely with an intracavity prism pair, reported much narrower spec- tra (up to≈40 nm) [24, 37, 38]. Chirped mirrors allow to almost arbitrarily change the shape of the intracavity dispersion, what can not be done in systems that rely on intracavity prisms for dispersion compensation. The experiments presented showed that the exact shape of the intracavity dispersion strongly influences mode-locking efficiency and stability as well as the spectral shape of the generated pulses. Three main spectral shapes (parabolic-like,Π-like and M-like) were clearly identified by varying the average intracavity dispersion. Best compress- ibility was achieved for parabolic-like spectra. These results are in agreement with numerical simulations based on the distributed complex cubic-quintic Ginzburg-Landau model [10].

Pulses with an energy up to 0.5µJ and sub-50-fs duration were realized by reducing the repetition rate of the laser to 2 MHz by using a 10 W pump laser. This yields a peak power as high as 10 MW. To achieve and sustain stable mode-locked operation at this repetition rate the beam spot size in the crystal was increased and an saturable Bragg reflector was used to assist starting of mode-locked operation. Further energy increase can be achieved with additional

pump power. Pump lasers delivering more than 18 W are now commercially available. Careful dispersion optimization in a conventional Ti:sapphire oscillator operating in the positive dispersion regime allowed to generate 60 nJ pulses at 70 MHz repetition rate. The generated pulses have a spectral width supporting pulse compression down to 33 fs. The output average power was>4 W using a 18 W pump laser (demo-version Verdi V18 provided by Coherent GmbH). In a system in which dispersion compensation was done by chirped mirrors and intracavity prisms 50 nm central wavelength tunability was demonstrated, without introducing considerable loss of output power.

The high-energy output achieved from chirped pulse oscillators can be used to seed an amplifier in a single or double pass geometry. This would allow for an additional increase in pulse energy. For example, if the amplifier is seeded with the 5 MHz chirped pulse oscillator presented in chapter 3, a pulse energy of more that 1.5µJ can be expected for a reasonable am- plification factor of 5. Such a system could be very attractive for ultrafast electron diffraction experiments that are currently done at kHz repetition rate.

The 10.7 MHz system described in chapter 3 was used for bulk material modification. 27- fs 26-nJ pulses from this laser were used to fabricate a new type of pearl-chain waveguides. At energies below 26 nJ smooth waveguides have been written. Pearl-chain waveguides have higher refractive index contrast and can be used for the fabrication of couplers and small- radius bended structures. One could think of using a pearl-chain waveguide as a pre-form for a diameter-modulated channel. Such a channel (in the shape of a capillary), being filled with a noble gas, already allowed the demonstration of dramatic enhancement in the yield of high harmonics generated with intense femtosecond pulses [73].

By locking a 76-MHz chirped pulse oscillator to an evacuated enhancement cavity it was possible to generate 7.8 µJ pulses with a pulse duration of 55 fs. With this system, high harmonics were generated in a Xenon gas-jet. To couple the harmonic radiation out of the cavity a≈600-µm-thick sapphire plate was inserted in the cavity in a similar configuration as reported by [14] and [32]. Radiation up to the 9th harmonic was observed on a phosphor screen after diffraction from a MgF2-coated aluminium grating. The low efficiency of the detection system limited the harmonics observed so far. A potential limitation for upscaling the intracavity intensity is the non-linear response of the sapphire plate used for out-coupling the harmonics. This effect was clearly observed in the experiments described in chapter 6 and has been previously reported [32, 108]. Therefore, it is clear that in order to further increase the intracavity intensity, alternative output-coupling methods have to be applied. The output-coupling method should allow for efficient extraction of the high harmonic radiation while maintaining high finesse of the cavity. A number of possible configurations have been proposed by K. Moll and J. Ye and are briefly discussed in chapter 6.