Towards a setup for narrowband terahertz generation through difference-frequency mixing of chirped ultrashort pulses

Abstract

Terahertz radiation is a useful tool for inducing lower-energy excitations in matter, where it can be used to cause structural changes through direct interaction with the crystal lattice. Sub-picosecond optical pulses can be used to generate intense broadband terahertz pulses through optical rectification in nonlinear crystals, reaching peak electric fields of several MV/m and conversion efficiencies as high as a few percent. For certain applications it may be desirable to only generate terahertz frequencies within a narrow band. One such way, using broadband optical pulses, is through chirped-pulse beating, where two linearly chirped pulse copies are delayed relative each other in time, causing them to beat. This results in an output optical pulse with an intensity envelope modulated at a single beat frequency. When incident on a nonlinear crystal lacking inversion symmetry, such a modulated pulse generates difference-frequencies only within a narrow band, centered around the beat frequency of the intensity modulation.

This scheme for narrowband terahertz generation was implemented in the project, and an electro-optic sampling setup was built to characterize the temporal profile of the terahertz pulses. In the project, terahertz radiation was generated using the organic crystal DSTMS pumped with 1500 nm light from an optical parametric amplifier. Pulse characterization was done using the electro-optic crystals ZnTe and GaP with a frequency-doubled 750 nm probe pulse. Chirp and delay was implemented using a transmission grating chirp filter and a Michelson interferometer. Few-cycle terahertz pulses were successfully generated and sampled with electro-optic sampling. The pulse stretching and the delay between pulse copies were implemented, but narrowband terahertz pulses were not sampled with the electro-optic sampling within the time frame of the Master thesis.

Popular Abstract

Matter is built up by atoms, consisting of a positively charged nucleus surrounded by a ”cloud” of negatively charged electrons. Charged particles are affected by electric forces. An electromagnetic wave consists of a coupled time-varying electric and magnetic field, where the electric field exerts and electric force on the charged nucleus and the electrons, so that electromagnetic waves will interact with matter. How energetic an electromagnetic wave is depends on its frequency, where higher frequencies correspond to higher energies.

Light can be used to induce changes in matter, since the electric field in the light wave interacts with the charged particles. The kind of interaction that occurs depends on how energetic the light is. Light in the optical region, which is light that we can see, often interacts with the electrons in an atom, giving them energy and moving them to higher energy states. Lower-energy electromagnetic waves, around terahertz frequencies, can cause molecules to rotate or vibrate. When many atoms are ordered periodically in a crystal, the atoms and molecules can vibrate relative each other only in certain ways, called vibrational \textit{modes}. Vibrational modes can be excited with electromagnetic waves with terahertz frequencies and through this it is possible to change the structure of a material. The change in the material can later be probed by another light pulse to see how the vibrations decay in the crystal.

If the pulse used to excite the crystal contains many frequencies, it is not possible to control what vibrations are excited, but all vibrations that match a frequency in the pulse can be excited. Each vibrational mode then decays into other vibrational modes, and there is no way to tell which parts of the decay process is attributed to which original vibrational mode. If the crystal was instead excited with a pulse consisting of fewer frequencies, ideally few enough to only cover one vibrational mode in the crystal, it would be possible to investigate the decay processes in the material for a specific vibrational mode.

The goal of this project was to generate such a terahertz pulse, that consists of few frequencies. An infrared (1500 nm) pulse was used to pump a nonlinear crystal. When the intensity in the pulse is high and sent through a nonlinear crystal, the frequencies in the pulse can mix, so that other frequencies are emitted from the crystal. For the crystal and pulse used in the project, the frequencies that overlap in time are mixed by being subtracted with each other. This way, terahertz frequencies can be generated if the difference frequency between each mixing frequency pair is in the terahertz range. If the pump pulse frequencies overlap in time, the generated terahertz pulse will consist of many frequencies since many frequencies can mix. In order to generate a terahertz pulse that contains fewer frequencies, the frequencies in the pump pulse can be displaced linearly in time, so that they don't overlap anymore. If the pulse is then overlapped with an identical pulse copy, that arrives at a slightly different time and has its frequencies displaced in the same way, only a few frequencies from each pulse will overlap at a specific time, so that only specific pairs of frequencies can be subtracted in the crystal. The difference between the frequencies in each pair will be centered around a constant difference frequency, so that difference frequencies are generated only within a narrow band.

• author: Clementsson, Isa
• supervisor: Jörgen Larsson; Eric Nilsson
• organization: Atomic Physics; Department of Physics
• course: PHYM01 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• publication/series: Lund reports on atomic physics (LRAP)
• report number: LRAP594
• language: English
• id: 9140122
• date added to LUP: 2023-10-18 11:22:40
• date last changed: 2023-10-18 11:22:40