Methodology for computing Fourier-transform infrared spectroscopy interferograms.

With the utilization of Fourier-transform infrared (FTIR) spectroscopy for a multitude of commercial applications, a robust methodology for designing, implementing, and servicing these systems in commercial settings is becoming increasingly paramount. Here we present a method allowing for the numerical evaluation of the interferogram signal in a FTIR spectroscopy system, in which the incident electric field can exhibit any arbitrary spectral content. The developed model assesses multiple internal reflections occurring within a beam splitter (BS) and compensating plate (CP), allows for the presence or absence of the CP, and obtains the interferogram in absolute units. The interferogram is evaluated for the representative scenarios of an incident electric field having a blackbody spectral distribution (interacting with a BS and CP composed of S i O 2) and a randomly chosen spectral distribution (interacting with a BS composed of ZnSe and no CP).

Intra-pulse difference frequency generation in ZnGeP2 for high-frequency terahertz radiation generation.

The highly-nonlinear chalcopyrite crystal family has experienced remarkable success as source crystals in the mid-infrared spectral range, such that these crystals are primary candidates for producing high terahertz frequency (i.e., [Formula: see text] 10 THz) electric fields. A phase-resolved terahertz electric field pulse is produced via intra-pulse difference frequency generation in a chalcopyrite (110) ZnGeP2 crystal, with phase-matching being satisfied by the excitation electric field pulse having polarizations along both the ordinary and extraordinary crystal axes. While maximum spectral power is observed at the frequency of 24.5 THz (in agreement with intra-pulse phase-matching calculations), generation nonetheless occurs across the wide spectral range of 23-30 THz. To our knowledge, this is the first time a chalcopyrite ZnGeP2 crystal has been used for the generation of phase-resolved high-frequency terahertz electric fields.

Generation of 17-32 THz radiation from a CdSiP2 crystal.

A phase-resolved electric field pulse is produced through the second-order nonlinear process of intra-pulse difference frequency generation (DFG) in a (110) CdSiP2 chalcopyrite crystal. The generated electric field pulse exhibits a duration of several picoseconds and contains frequency components within the high-frequency terahertz regime of ∼17-32 THz. The intra-pulse DFG signal is shown to be influenced by single-phonon and two-phonon absorption, the nonlinear phase-matching criterion, and temporal spreading of the excitation electric field pulse. To date, this is the first investigation in which a CdSiP2 chalcopyrite crystal is used to produce radiation within the aforementioned spectral range.

Emission and sensing of high-frequency terahertz electric fields using a GaSe crystal.

A GaSe crystal cut along the (001) crystallographic plane is investigated for the emission and detection of high-frequency (i.e. up to ∼20 THz) electric fields. To date, a comprehensive analysis on high-frequency difference frequency generation and electro-optic sensing in GaSe has not been performed and should consider aspects such as electric field polarization orientation, symmetries inherent to the crystal structure, and the various possible generation and detection phase-matching arrangements. Herein, terahertz radiation generation is investigated for various excitation electric field polarizations as the GaSe crystal is rotated in the (001) plane. Subsequently, the crystal is rotated out-of-plane to investigate the difference frequency generation and electro-optic sampling phase-matching conditions for various arrangements. The measured terahertz radiation spectra show peak generation at the frequencies of 10, 16, and 18 THz (dependent on the GaSe crystal orientation), in agreement with the frequencies exhibiting perfect phase-matching. GaSe has the potential to emerge as the primary crystal for the emission and detection of high-frequency electric fields, such that this comprehensive analysis is necessary for the widespread adoption and practical implementation of GaSe as a high-frequency source crystal.

Generation of narrowband terahertz radiation via phonon mode enhanced nonlinearities in a BaGa4Se7 crystal.

We report on narrowband terahertz (THz) radiation generation via optical rectification from a BaGa4Se7 crystal. The dense phonon mode distribution of the BaGa4Se7 crystal causes narrow transmission bands in the THz frequency range with enhanced nonlinear susceptibility magnitudes, thus permitting strong narrowband THz radiation generation at the frequencies of 1.97 and 2.34 THz. In comparison to THz radiation generated from a ZnTe crystal, the narrowband THz radiation produced by the BaGa4Se7 crystal is 4.5 times higher at 1.97 THz and 63% higher at 2.34 THz, thus making BaGa4Se7 a viable crystal for use in such areas as security and medicine.

Backward terahertz difference frequency generation via modal phase-matching in a planar LiNbO3 waveguide.

The backward difference frequency generation process is used to produce narrowband terahertz radiation via modal phase-matching in a SiO2-LiNbO3-air planar waveguide. The TM0 pump mode, TE0 signal mode, and TE0 or TE2 idler modes are selected to satisfy the backward difference frequency generation phase-matching condition, thus allowing narrowband (i.e., <100GHz linewidth) terahertz radiation generation in the spectral range of 2.4-3.2 THz. To date, this is the first investigation of terahertz radiation generation in a waveguide via the modal phase-matched backward difference frequency generation process.

Dependence on excitation polarization and crystal orientation for terahertz radiation generation in a BaGa4Se7 crystal.

Optical rectification is experimentally investigated in a biaxial BaGa4Se7 crystal by considering various combinations of near-infrared excitation polarizations and crystal orientations. The highest terahertz radiation is produced along the Z crystallo-physical direction of the BaGa4Se7 crystal. Despite the optical complexity of the BaGa4Se7 crystal in the terahertz spectral regime, this systematic experimental investigation determines the optimal excitation polarization and crystal orientation for the optical rectification process.

Theoretical formalism for off-normal angular coupling into selective and parity-dependent modes in a planar waveguide.

A theoretical formalism is presented to describe coupling of an electromagnetic field into the modes of a planar waveguide, where the electromagnetic field has a non-uniform transverse profile and is incident at an arbitrary angle. The theoretical approach is used to investigate coupling of a Gaussian electromagnetic field into a ${{\rm LiNbO}_3}$LiNbO3 planar waveguide, where the calculations are shown to be in excellent agreement with finite-different time-domain simulations. This formalism is essential to phase-matched frequency-conversion waveguides based on nonlinear optical phenomena, which can rely on coupling the excitation field into selective higher-order waveguide modes of either even or odd parity.

A modeling of dispersive tensorial second-order nonlinear effects for the finite-difference time-domain method.

A generalized formalism is developed to model second-order nonlinear processes in finite-difference time-domain (FDTD) simulations. The method is capable of modeling frequency-conversion from all 18 elements of the second-order nonlinear tensor, where dispersion of the tensor elements is included at both the pump and generated frequencies. The model is validated by considering frequency-conversion in a LiNbO3 crystal, which has highly dispersive second-order nonlinear susceptibilities near the phonon resonances. The developed nonlinear formalism is able to model any arbitrary excitation polarization state and can be applied to investigate second-order nonlinear processes in type I or type II phase-matching. This generalized second-order nonlinear formalism represents an advancement for the FDTD computational technique and can provide more realistic modeling of second-order nonlinear interactions in nanoscale devices and waveguiding structures.

Second harmonic generation in metal-LiNbO3-metal and LiNbO3 hybrid-plasmonic waveguides.

Nanoplasmonic waveguides based on lithium niobate (LN) are shown to provide the light-matter interaction required for next-generation developments in nonlinear frequency-conversion nanostructures. Here, we numerically investigate second harmonic generation of a 1550 nm, 100 fs pulse in metal-LN-metal (MLNM) nanoplasmonic and LN hybrid-plasmonic (LNHP) waveguides. In comparison to a photonic LN waveguide, a 2.1 µm-long LNHP waveguide exhibits a conversion efficiency improvement of 11 times, whereas a 20 µm-long MLNM nanoplasmonic waveguide is shown to have a conversion efficiency of 1.1 × 10-4. The MLNM nanoplasmonic and LNHP waveguides have the potential to operate as sources of optical radiation for on-chip photonic systems.

Enhanced broadband terahertz radiation generation near the reststrahlen band in sub-wavelength leaky-mode LiNbO3 waveguides.

The generation of terahertz (THz) radiation in a compact geometry is crucial for the implementation of on-chip, coherent THz radiation sources. Here, via numerical time-domain simulations, we show that LiNbO3 (LN) waveguides having sub-wavelength core widths can provide highly efficient optical-to-THz radiation frequency conversion over short lengths. By exploiting the nonlinear susceptibility, χ33(2), enhancement of LN near its phonon reststrahlen band and utilizing THz leaky-mode guidance to minimize reststrahlen band absorption and improve phase matching, we show that broadband (0.2-11.6 THz) electric field pulses of 3.4 kV/cm can be generated at an optical-to-THz conversion efficiency of 2.5×10-4. These sub-wavelength leaky-mode waveguides provide a compact platform for wideband, coherent THz radiation sources.

Investigation of ultra-broadband terahertz generation from sub-wavelength lithium niobate waveguides excited by few-cycle femtosecond laser pulses.

The generation of coherent, ultra-broadband terahertz (THz) radiation pulses spanning more than a few octaves is vital to understanding the ultrafast response of elementary excitations, molecules, nanostructures, materials, and explore device functionality across a wide spectrum. In this work, we use 2D finite-difference time-domain simulations to show that ultra-broadband (0.18-106 THz) Cherenkov radiation can be produced from SiO2:MgO-LiNbO3:SiO2 waveguides having core dimensions that are sub-wavelength with respect to the optical pump pulse being guided. These sub-wavelength core dimensions allow the ultra-broad Cherenkov radiation to be emitted at an angle between 47.2° and 47.5° (dictated by the Si cladding layer dispersion), making these waveguide structures superior to the THz generation arrangements in bulk MgO-LiNbO3 crystals. When excited by a 7 fs, 780 nm laser pulse having an energy of 2 nJ, a 300 µm-long waveguide with transverse core dimensions of 500 nm × 2 mm can generate a sub-ps, kV/cm electric field pulse. Unlike THz pulse generation in bulk MgO-LiNbO3 crystals, having sub-wavelength core dimensions reduce the absorption from the MgO-LiNbO3 reststrahlen bands. These sub-wavelength SiO2:MgO-LiNbO3:SiO2 waveguides are ideal for on-chip applications that require ultra-broadband, compact THz sources.

Generation of broadband terahertz pulses via optical rectification in a chalcopyrite CdSiP2 crystal.

We report on the generation of broadband (0.07-6 THz) terahertz (THz) radiation via optical rectification in a ⟨110⟩ CdSiP2 (CSP) crystal pumped by a 50 fs, 780 nm central wavelength optical pulse. By measuring the THz phase refractive index and the optical pump group refractive index, good phase matching can be achieved for a crystal thickness ≲200 µm. Due to this crystal's high second order nonlinearity and low absorption losses, it is envisioned that THz generation from CSP could be further enhanced by confining the optical pump pulse to sub-wavelength waveguides.