Abstract-Simultaneous generation and compression of broadband terahertz pulses in aperiodically poled crystals

Koustuban Ravi and Franz X. Kärtner

Fig. 1 Schematic of outlined approach : An aperiodically poled crystal with second-order susceptibility χ(2)(z) for the difference-frequency generation of different terahertz frequencies at different locations and a non-uniform sequence of pulses (blue) are required. The idea exploits the difference in optical and terahertz group velocities to offset the terahertz propagation time tTHz by the time at which it gets generated. (a) The pulse of duration τ0 (green outline) is phase matched at z = −L/2 (green-shaded area) to generate a terahertz wavelet ETHz,1 of angular frequency Ωg(z = −L/2) = 2/τ0 (purple trace) at a time trel = −LΔnc−1/2, relative to the center of the overall pump envelope. The terahertz wavelet born here reaches z = 0 at tTHz = LnTHz c−1/2. (b) The central pulse in the sequence with duration τ1 (green outline) is phase matched at z = 0 (green-shaded area) to generate a terahertz wavelet ETHz,1 with Ωg(z = 0) = 2/τ1(red trace). The previously generated terahertz radiation has reached z = 0 at precisely the same time as the newly generated trace, leading to coherent growth of the total field ETHz,total (black-dashed). (c) The pulse with duration τ2 is phase matched at the end of the crystal. While the overall pump envelope slips through the terahertz field, there is always a pulse in the sequence which is overlapped with the terahertz field. This pulse generates a terahertz angular frequency Ωg(z) that is precisely phase-matched at z. The length of the crystal here is arbitrary, hence the terahertz pulse remains compressed throughout.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-5-6580

We introduce a technique to generate compressed broadband terahertz pulses based on cascaded difference-frequency generation. The approach employs a non-uniform sequence of pump pulses in aperiodically poled crystals. The pump-pulse format and poling of crystals conceived are such that the emergent terahertz pulse is already compressed. The method circumvents pump-pulse distortions that result from non-collinear approaches and the need for external compression. While capable of generating even single-cycle pulses, it is particularly efficient for the generation of pulses with few to tens-of-cycles duration. For instance, calculations accounting for cascading effects predict conversion efficiencies in the few percent range for cryogenically-cooled lithium niobate. The focused electric fields are ≫ 100 MV/m in free space.

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