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Backward and Forward Nonlinear Optical Processes in Potassium Titanyl Phosphate

Time: Fri 2023-06-02 14.00

Location: Kollegiesalen, Brinellvägen 8, Stockholm

Language: English

Subject area: Optics and Photonics Physics

Doctoral student: Kjell Martin Mølster , Laserfysik

Opponent: Associate Professor Stefano Taccheo, Politecinco di Torino, Department of Electronics and Telecommunications (DET)

Supervisor: professor Fredrik Laurell, Laserfysik, Fysik; Valdas Pasiskevicius, Fysik, Laserfysik; Ulf L. Österberg, Laserfysik

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QC 2023-05-08


Second-order nonlinear optical processes generate radiation across a broad spectrum, including UV and the far-infrared. In this thesis, new light sources in the near(NIR) and mid infrared (MIR) using three-wave mixing geometries in periodically poled potassium titanyl phosphate (PPKTP) is studied.

Collinear three-wave mixing is of three types: forward, counter, and backward, defined from propagation directions. Backward mixing is achievable with quasi-phasematching (QPM) - in contrast to birefringent phase matching which only can phasematch co- and counter- propagating geometries. Focus has been on optical parametric oscillators (OPOs), backward wave optical parametric oscillators (BWOPOs), and backward second harmonic generation (BSHG). OPO and BWOPO sources are attractive due to the need for stable, narrowband, and tunable MIR radiation in nanosecond stand-off spectroscopy. The work also pushes the boundaries of nonlinear optics, where lack of efficient sources has limited research on backward processes.Type-0 forward ns-OPOs have broad bandwidth, especially close to degeneracy, due to identical polarization at the parametric wavelengths and moderate dispersion. Probing greenhouse gases, such as CO2, requires narrow bandwidths. Type-2 ns-OPOs with orthogonal polarizations can be used as these have a much-reduced acceptance bandwidth. We provide the design parameters for the QPM grating periodicities in type-2 DFG, and, demonstrate that simple type-2 PPKTP ns-OPOs produce transform-limited mJ pulses with a good beam profile. These type-2 PPKTP OPOs can find use as parametric source for narrowband spectroscopy. Moreover, the type-2 OPOs are also useful for studying other narrow acceptance processes.

We used BWOPOs to efficiently generate mJ of nanosecond pulses at 5 kHz repetition rate, corresponding to an average power of 5.65 W, with tunable forward pulses (1-to-1.001 Hz/Hz) and bandwidths ranging from 300 to 400 MHz. A BWOPO bandwidth as small as 274 MHz was obtained when pumped with a high M2 transform-limited laser. We show that the BWOPO is insensitive to temperature variations and is tuned at 2.48 GHz/K. The BWOPO can serve as a pump-tunable spectroscopy source using a single-pass crystal with reduced complexity.

From the above, and earlier studies, PPKTP devices are found to be attractive as components for space-borne precision MIR spectroscopy. To study PPKTP's radiation hardness, we conducted experiments, using Type-0 OPO thresholds and transmission spectroscopy, on the long-term changes in linear and nonlinear properties of PPKTP when exposed to low- to mid-energy protons (10 and 60 MeV). The dosages and energies were similar to those in low earth orbit over a five-year period. The results showed no changes in nonlinearity or transmission.

We also demonstrated the first efficient, purely backward χ(2) process, i.e., backward second-harmonic generation. The QPM period was Λ = 317 nm. The crystal generated frequency doubled pulses at 1154 nm with an efficiency of 18.7 % and energy of tens of microjoules. A narrowband type-2 ns-OPO was used as pump to measure the temperature bandwidth and confirm the quadratic output/input relationship in the undepleted pump regime. The significant difference in spectral acceptance between BSHG and second harmonic generation (SHG) in PPKTP  - calculated to δλBSHG = 53 pm and δλSHG = 13.2 nm, respectively - is confirmed using a ps-BWOPO pump. The temperature tuning was 17.1 pm/K. This work establishes the feasibility of backward optical parametric amplification.  

We extended our research on potassium titanyl phosphate's (KTP) properties to the infrared and THz region by studying the transmission along the x and y crystallographic axis directions using THz pump-probe spectroscopy in reflection. Finally, backward THz polariton scattering was observed in Type-2 OPOs, along with polariton coupling into THz comb-lines when using an unpoled KTP as an off-axis oscillator.