Terahertz optical properties of potassium titanyl phosphate crystals

Abstract

This paper studies the terahertz optical properties of nonlinear potassium titanyl phosphate crystals with different conductivities in the spectral range of 0.2 to 2.6 THz. The observed properties are characterized by several absorption lines lying along different optical axes which represent the relevant potassium sublattice phonon modes. The peculiarities of these absorption lines are attributed to the structural order of potassium ions.

Time-domain spectroscopy of KTiOPO4 in the frequency range 06–70 THz

“…It may seem that the projection onto the c-direction, from such a small tilt, is insignificant. However, we observe a similar increase in absorption, from A 1 modes in the 1-2 THz region, as reported in [16]. This could explain the small discrepancy in absorption between reflection and transmission measurements, since this sample tilt was not present in the transmission measurements.…”

“…Our measured absorption peaks compare well with the measurements in [11,16,30,[32][33][34]. The peak around 1.7 THz (≈60 cm −1 ) has, in addition to these measurements, been observed for both flux grown KTP by [11,16,32] and for hydrothermally grown KTP by [33]. This peak has only been observed for A 1 symmetry and c excitations.…”

“…Note that Vivekanandan [32] used pulverized KTP so all directions were excited simultaneously. This peak has been suggested to be either part of the Ti-O vibrations responsible for many of the <200 cm −1 peaks [33] or from a crystal sublattice [16]. The broad feature we observe around 1 THz was also reported by [16] and they speculated that it had also been observed by [34].…”

“…In this frequency regime KTP appears to be a good option by e.g. using it's quasi-phase matching properties in optical parametric amplifiers [16,17], or as envisaged in [18], pumped with a dual wavelength fiber laser for direct THz conversion through the beat frequency [19]. Recently, THz generation using parametric amplification in KTP was observed [20], and very recently Wu et al demonstrated generation of >100 kW peak power from a KTP THz parametric oscillator [21].…”

“…A study by Watson in 1991 [33], compared flux grown KTiOAsO 4 (KTA) with hydrothermally grown KTP to provide insight into the importance of the Ti − O bond in the Raman spectra. Later studies focusing on the THz frequency region using femtosecond lasers for long wavelength radiation reported KTP absorption for 0.1 − 1.5 THz [34], 0.5 − 2.0 THz [35] and 0.2 − 2.6 THz [16]. These studies were all performed in transmission and therefore limited to ca 2.5 THz due to the strong increase in absorption for higher frequencies and low amplitude at higher frequencies afforded by standard THz sources.…”