The terahertz range of the electromagnetic spectrum reveals important insights when studying material properties. An ultra‐broadband terahertz time‐domain spectroscopy system based on state‐of‐the‐art, high‐stability organic nonlinear optical crystals used as both THz wave generator and detector is presented. In transmission geometry, a broad spectrum exceeding 20 THz and a high dynamic range of more than 80 dB is achieved using a compact 100 MHz femtosecond laser working at telecom wavelength 1560 nm. In the normal‐incidence reflection geometry, a similar bandwidth with a dynamic range surpassing 60 dB is reported. The experimental results are supported by a complete theoretical model, which includes the pump pulse duration, THz phonon/vibrational modes of the organic crystals and optical/THz beam path optimizations. The effectiveness of the newly developed system is demonstrated by measuring pharmaceutical samples with distinct THz features in the ultra‐broadband THz range and by measuring narrow water vapor lines at a spectral resolution of 2.7 GHz (0.090 cm−1), resulting in an excellent accuracy with a frequency deviation of less than 0.05% from the reference values.
The photorefractive properties of Sn 2 P 2 S 6 crystals doped with Te and Sb in the near-infrared wavelength range up to 1064 nm are reported. The main photorefractive parameters, i.e., two-wave mixing gain, effective electrooptic coefficient, diffusion length, concentration of traps, and response time, are compared with conventional nominally pure Sn 2 P 2 S 6 . Te-doped Sn 2 P 2 S 6 shows the fastest response with the smallest decrease of the photorefractive efficiency with increasing wavelength in the near infrared. Sb doping, on the other hand, inhibits photorefraction in the near infrared. Sn 2 P 2 S 6 :Te and Sn 2 P 2 S 6 :Sb crystals both show a high two-wave mixing gain ⌫ at 633 nm, and 10 and 20 cm −1 . Te-doped Sn 2 P 2 S 6 shows a photorefractive gain of 4.5 cm −1 at 1064 nm. Response times at 1064 nm of 20 ms have been measured for the intensity 6 W/cm 2 , which is 2 orders of magnitude shorter than in Rh-doped BaTiO 3 .
We demonstrate self-pumped optical phase conjugation in Te-doped Sn2P2S6, a semiconducting ferroelectric crystal, using a 1.06 microm wavelength cw Nd:YAG laser. The photorefractive gain of this crystal has been increased to Gamma = (3.9+/- 0.4) cm-1 by Te doping. We observed self-pumped optical phase conjugation in a ring cavity scheme with phase conjugate reflectivities of more than 40 percent and a very fast phase conjugate rise time below 100ms at a light intensity of 20 W/cm2. This is more than two orders of magnitude faster than in any other photorefractive crystal, as e.g. in Rh-doped BaTiO3.
We demonstrated for what is the first time to our knowledge photorefractive two-wave mixing in a bulk ferroelectric crystal using cw light at the telecommunication wavelength 1.55 microm. In the Te-doped ferroelectric semiconductor Sn2P2S6 with absorption constant <0.1 cm(-1) at 1.55 microm, grating recording times of 10 ms and a two-beam coupling gain of 2.8 cm(-1) have been measured at 350 mW power (intensity 440 W/cm(2)) without a necessity to apply an external electric field. With a moving grating technique, a maximal gain of 6.0 cm(-1) has been obtained.
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