Abstract
Optical properties of lithium triborate (LBO) in the terahertz regime (0.2–2 THz) were characterized using broadband terahertz time-domain spectroscopy. The frequency dependence of refractive index and absorption coefficient of the LBO crystal was experimentally investigated over the temperature range of 77–297 K, which the experimental results indicated that LBO has very low optical absorption coefficient at terahertz frequencies especially for the beam polarization along the crystal’s principal dielectric axis X. Moreover, a giant birefringence was observed, and the refractive index difference between the axis X and Z gradually decreased with decreasing temperature, which is attributed to the behavior of the TO phonon modes of B1 and B2 symmetries at low frequencies at different temperatures. As potential applications, LBO can be exemplarily used as terahertz wave shapers, beam splitters, terahertz wave plates, circular polarizers and other polarization devices.
Highlights
In recent years, terahertz wave has received extensive attention and reveals potentials in numerous applications, in homeland security and fundamental research[1,2,3]
We investigated the optical properties of lithium triborate (LBO) in the terahertz regime (0.2–2 THz) using a broadband terahertz time-domain spectrometer (THz-TDS)
Our experimental results showed that LBO has a giant birefringence and low optical absorption coefficient especially for the wave polarizing along the X-axis which makes it suitable for terahertz wave plates, wave shapers, beam splitters, circular polarizers and other polarization devices
Summary
Terahertz wave has received extensive attention and reveals potentials in numerous applications, in homeland security and fundamental research[1,2,3]. The lithium triborate (LBO) crystal[15] is widely used for optical frequency doubling, optical parametric oscillator (OPO), optical parametric amplification (OPA)[16], and other nonlinear optical processes due to its extremely low linear optical absorption, large birefringence, moderate nonlinear coefficients, and high damage threshold (45 GW · cm−2 for 1 ns pulses at 1.064 μm)[17]. It is nonhygroscopic and chemically stable with good mechanical properties and has a high melting point of 1107 K18. We analyzed the experimental results from the perspective of crystal phonon modes with Kurosawa relation and the classical pseudo-harmonic phonon model and demonstrated the validity of our results
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