Abstract
Optically active phonon modes in ferroelectrics such as potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA) in the ~7–20 THz range play an important role in applications of these materials in Raman lasing and terahertz wave generation. Previous studies with picosecond pulse excitation demonstrated that the interaction of pump pulses with phonons can lead to efficient stimulated Raman scattering (SRS) accompanying optical parametric oscillation or amplification processes (OPO/OPA), and to efficient polariton-phonon scattering. In this work, we investigate the behavior of infrared OPAs employing KTP or KTA crystals when pumped with ~800-nm ultrashort pulses of duration comparable to the oscillation period of the optical phonons. We demonstrate that under conditions of coherent impulsive Raman excitation of the phonons, when the effective χ(2) nonlinearity cannot be considered instantaneous, the parametrically amplified waves (most notably, signal) undergo significant spectral modulations leading to an overall redshift of the OPA output. The pump intensity dependence of the redshifted OPA output, the temporal evolution of the parametric gain, as well as the pump spectral modulations suggest the presence of coupling between the nonlinear optical polarizations PNL of the impulsively excited phonons and those of parametrically amplified waves.
Highlights
Active phonon modes in ferroelectrics such as potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA) in the ~7–20 THz range play an important role in applications of these materials in Raman lasing and terahertz wave generation
Nonlinear optical (NLO) frequency conversion techniques have become an indispensable tool in fundamental research and industrial applications to generate coherent laser emission away from the frequencies provided by laser gain media
The OPA layout is based on the common design employing single-filament white-light continuum (WLC) as the seed, generated by focusing a few μJ of the 800-nm pulses into a 5-mm thick sapphire plate[1]
Summary
Active phonon modes in ferroelectrics such as potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA) in the ~7–20 THz range play an important role in applications of these materials in Raman lasing and terahertz wave generation. One of the fundamental reasons behind the successful application of various NLO crystals–such as beta-barium borate (BBO), lithium borate, potassium titanyl phosphate (KTiOPO4, KTP) and many others–in frequency conversion schemes, is the instantaneous nature of the NLO response, most importantly the 2nd-order susceptibility χ(2) Electrons in these materials respond to the electric field oscillations of the interacting waves instantaneously, as long as the electronic susceptibilities χ(i) are not coupled to any resonance in the NLO material. Φ is the phase offset (“lag”) between the nonresonant and resonant responses, and ωj is the frequency of a certain NLO-active resonance mode Taking advantage of this enhancement, various χ(2)-based schemes have been employed in spectroscopy of interfaces, in which either one of the exciting beams, or the generated (second-harmonic or sum-frequency) beam, is in resonance with the vibrational or electronic mode(s) at the interface. Science for almost 30 years, the coupling of resonances in NLO crystals to the electronic polarizability–which typically is assumed to be negligible–as well as its implications for the efficiency of the frequency conversion devices, still need to be investigated in greater detail
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