Laser Written Photonic Waveguides and Beam Shaping
The wave guiding structures are created by femtosecond laser pulses, which are tightly focused inside the bulk material. Due to the nonlinear absorption processes these lead to a permanent refractive index modification after relaxation of the plasma in the focal point. Under suitable irradiation conditions, the refractive index change is positive, allowing direct fabrication of guiding structures.
Using the combination of beam shaping with femtosecond laser writing we demonstrated a novel method to create waveguide coupler and splitter devices in fused silica. The major advantage of the beam shaping method is the possibility to create complex devices in a single sweep by simultaneously writing two or more waveguides with changing separation.
Using the combination of beam shaping with femtosecond laser writing we demonstrated a novel method to create waveguide coupler and splitter devices in fused silica. The major advantage of the beam shaping method is the possibility to create complex devices in a single sweep by simultaneously writing two or more waveguides with changing separation.
Time-trapping towards manipulation of single-photon wavepackets
In this project we are going to demonstrate experimentally the manipulation of femtosecond long, weak wavepackets utilizing a new way of trapping ultrashort pulses, which was recently theoretically suggested https://doi.org/10.1103/PhysRevLett.123.243905. A soliton propagating in a hollow-core fiber creates a refractive index potential capable of trapping a much weaker pulse or even single photons. This trapping allows a propagation of the weak pulse, which is not affected by dispersive broadening anymore. Such propagation would otherwise be impossible, since unlike the soliton, a strong pulse that preserves its shape in frequency and time through nonlinear effects, a weak pulse experiences no such effects. Furthermore, the trapping allows for the manipulation of the trapped weak pulse by manipulating the soliton trap. As the trapping also increases the coupling between photons, our approach can be utilized for the realization of all-optical quantum gates in the future.
In the experiment, the pulses for the soliton trap at 800nm and 50fs are delivered by an optical parametric amplifier (OPA). The weak pulse at 1600nm will be obtained by the downconversion of a small part of the OPA output. Before the pulses are coupled into a 3m long hollow-core fiber, it is necessary for the trapping mechanism to work that the fundamental mode of the pulses is converted into higher order spatial modes. This is done by diffractive optical elements (DOEs).
Third harmonic generation in dielectric layers
Generating ultrashort pulses in the ultraviolet (UV) spectral range remains challenging due to a lack of suitable laser materials. By contrast, the near-infrared (NIR) range benefits from well-established laser gain media, including Ti:sapphire and Yb- and Nd-based materials. Nonlinear optical processes can be used to bridge the gap between the NIR and UV spectral ranges by converting NIR laser radiation into UV. Third harmonic generation (THG) is a particularly attractive method as it enables direct frequency conversion from NIR to UV in a single step. However, direct THG typically has low conversion efficiency, only a few percent for Yb-based wavelengths, and negligible efficiency in the Ti:sapphire range. These poor efficiencies are due to difficulties in phase matching and the strong absorption of UV wavelengths in conventional nonlinear crystals. To overcome this limitation, cascaded THG is often employed, whereby the conversion is divided into two successive second-order nonlinear processes. While this approach increases the overall efficiency, it also significantly increases the system's complexity and sensitivity to alignment errors.
The Ultrafast Research Group is investigating dielectric layer systems, known from HR-mirrors or anti-reflection coatings, for their potential in THG applications. The conversion to the third harmonic in a single dielectric layer with a thickness of a few hundred nanometres is typically limited to efficiencies of around 10⁻⁶. In collaboration with our partners at the Laser Zentrum Hannover, we have demonstrated that custom-designed dielectric layer systems, known as frequency tripling mirrors (FTMs), can achieve significantly higher THG efficiencies. At the fundamental wavelength of 800 nm, where phase matching prevents THG in nonlinear crystals, FTMs with a total layer thickness of just 3.5 µm have achieved conversion efficiencies of up to 3.5% in experiments.
These results highlight the promise of dielectric layer systems as an alternative to conventional crystal-based nonlinear optics for efficiently generating UV pulses.
Further information on THG in dielectric mirrors can be found here:
https://opg.optica.org/oe/fulltext.cfm?uri=oe-31-12-19309
Dispersive Wave Generation in Kagome Fibre
Pulses lasting only a few optical cycles are among the shortest events that humans can generate. Optical parametric chirped pulse amplifiers (OPCPAs) are often used to generate high-intensity, few-cycle pulses. These amplifiers are based on nonlinear effects and therefore possess unique properties that cannot be realised with conventional laser amplifiers. Firstly, the parametric nature of the amplification process enables energy to be transferred directly from the pump beam to the seed beam without storing energy in the nonlinear crystal, which significantly reduces thermal effects. Secondly, OPCPAs support a much wider gain bandwidth than typical laser amplifiers and are unaffected by gain narrowing. Finally, the nonlinear interaction enables tuning of the gain range over a wide spectral range - from ultraviolet (UV) to mid-infrared.
The UV spectral range is of particular interest due to its relevance for various applications, including time-resolved pump-probe spectroscopy, chemical reaction control, semiconductor inspection and photoionisation mass spectrometry. However, OPCPAs are subject to significant limitations in the UV range, primarily due to the poor transmission properties of commonly used nonlinear crystals. One promising approach to overcome this is to use alternative nonlinear techniques to generate UV light, such as four-wave mixing in gas-filled hollow-core fibres with a Kagome lattice structure. We have recently demonstrated that combining the high UV transmission of noble gases with the low-loss guidance and high damage threshold of Kagome lattice fibres enables the creation of a powerful, tunable UV source. This source can convert few cycle supporting spectra generated by OPCPAs into UV light. Furthermore, combining these two techniques has the potential to generate extreme UV light below 100 nm, with a conversion efficiency that is several orders of magnitude higher than that of existing methods.
Further information on OPCPAs and four-wave mixing in hollow-core fibres can be found here:
https://doi.org/10.15488/19409
https://www.oejournal.org/oea/article/doi/10.29026/oea.2023.220046
https://opg.optica.org/josab/fulltext.cfm?uri=josab-28-12-A11
Two-color field Terahertz generation
When a laser field is asymmetric in time, it drives free electrons in a preferential direction, creating a net transient current. This current emits low-frequency THz radiation, a process know as “zero-order” Brunel radiation [1]. Unlike high-harmonic generation, the electrons do not need to recombine with the parent ion, so mainly lower-order harmonics are produced. By combining two laser colors with controlled phase and amplitude, the asymmetry can be enhanced, allowing efficient and tunable THz generation. In our laboratory, we use the 2060 nm DROPO together with its thin-disk pump laser 1030 nm as a synchronized two-color source to drive Brunel radiation. By controlling the relative phase, amplitute, and polarization of the two pulses, we optimize the THz emission and broaden its spectrum.
References:
1. I. Babushkin, S. Skupin, A. Husakou, C. Köhler, E. Cabrera-Granado, L. Bergé, and J. Herrmann, “Tailoring terahertz radiation by controlling tunnel photoionization events in gases,” New J. Phys 13, 123029 (2011). DOI 10.1088/1367-2630/13/12/123029
all-optical attoclock
The all-optical attoclock [1] enables the study of electron dynamics on attosecond time scales using intense optical fields delivered by femtosecond laser systems. It is based on Brunel radiation [2], which is generated when a strong laser pulse ionizes atoms and subsequently accelerates the released electrons, giving rise to radiation at the driving frequency and its odd harmonics. For an elliptically polarized driving field, the polarization state of the emitted Brunel harmonics is highly sensitive to the electron tunneling dynamics through the Coulomb barrier, thereby allowing detailed insight into the underlying light–matter interaction.
[1] I. Babushkin et al., Nat. Phys., vol. 18, pp. 417–422 (2022), https://doi.org/10.1038/s41567-022-01505-2.
[2] F. Brunel, J. Opt. Soc. Am. B, vol. 7, pp. 521-526 (1990), https://doi.org/10.1364/JOSAB.7.000521.
