Ultra‐broadband and high‐dynamic‐range THz time‐domain spectroscopy system based on organic crystal emitter and detector in transmission and reflection geometry

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

A Thick Film Sensing Element for True RMS Meter

A sensing element for true RMS meter was realized in thick film technology. The temperature gradient along a substrate was measured by thick film thermocouples, based on a combination of Pt/Ag-Pd/Ag, Pt/Au-Au and Ni-Cr. A voltage response of the element was measured as a function of the current through the heating resistor at different ambient temperatures. The transfer function K(T) of the RMS meter was found to be relatively independent of ambient temperature, which means that the voltage response is usably linear

Quasi-phase-matched second harmonic generation of UV light using AlN waveguides

As an alternative to electrically injected diodes, UV light emission can be obtained via second harmonic generation (SHG). In weakly birefringent materials such as aluminum nitride (AlN), the phase matching of the driving and second harmonic waves can be achieved by the quasi-phase-matching (QPM) technique, where the polarity of the material is periodically changed commensurate with the coherence wavelength. QPM also allows the use of the highest nonlinear susceptibility, and therefore, higher conversion efficiencies are possible. In this work, the QPM SHG of UV light in AlN lateral polar structure-based waveguides is demonstrated. The peak intensity of the frequency doubled laser light was measured at 344nm and 472nm wavelengths, in agreement with dispersion-based theoretical predictions. These results confirm the potential of III-nitride-based lateral polar structures for quasi-phase-matched nonlinear optics and for frequency doubling media for UV light generation

Quasi-phase-matched second harmonic generation of UV light using AlN waveguides

As an alternative to electrically injected diodes, UV light emission can be obtained via second harmonic generation (SHG). In weakly birefringent materials such as aluminum nitride (AlN), the phase matching of the driving and second harmonic waves can be achieved by the quasi-phase-matching (QPM) technique, where the polarity of the material is periodically changed commensurate with the coherence wavelength. QPM also allows the use of the highest nonlinear susceptibility, and therefore, higher conversion efficiencies are possible. In this work, the QPM SHG of UV light in AlN lateral polar structure-based waveguides is demonstrated. The peak intensity of the frequency doubled laser light was measured at 344nm and 472nm wavelengths, in agreement with dispersion-based theoretical predictions. These results confirm the potential of III-nitride-based lateral polar structures for quasi-phase-matched nonlinear optics and for frequency doubling media for UV light generation

Photorefractive grating fixing in KNbO3 by ferroelectric domains

We report observations of photorefractive grating fixing in potassium niobate at room temperature by means of ferroelectric domains. A grating is recorded while the crystal is simultaneously depoled. By repoling the crystal, the presence of a latent grating is revealed. This fixed grating diffracts light with an efficiency which increases when an electric field is subsequently applied and when it is uniformly illuminated. We explain these observations qualitatively using a previously developed model that involves charge compensation by domain walls, and a new aspect that invokes the creation of new trap sites at the ferroelectric domain walls

Second-Harmonic Generation of Blue Light in GaN Waveguides

Second-harmonic generation was studied in III-metal-polar GaN films grown on sapphire substrates by metalorganic chemical vapor deposition and formed into ridge waveguides. Broadband near-IR femtosecond pulses of an optical parametric amplifier system were injected by end-fire coupling and the nonlinear response was measured while tuning the central wavelength. A prominent peak was found at 450 nm for 1140 nm thick and 10 μm wide GaN waveguides. The measured second-harmonic peak was in agreement with the modal-dispersion phase matching condition calculated using the dispersion of the extraordinary refractive indices of GaN obtained by prism coupling