By using a compact duo of few-cycle Cr:ZnS lasers as a pump source and using optical rectification in ZGP crystal, we demonstrate high-resolution dual-comb spectroscopy in the long-wave infrared (LWIR) region. By recording a sequence of 1500 interferograms, we resolved the comb modes with the finesse exceeding 1000; LWIR spectra of several molecules including methanol, nitrous oxide, and ammonia were recorded in real time (1-10 sec) with 80-MHz (comb spacing) resolution with ~300,000 spectral points (comb modes), and the signal-to-noise ratio of ~100.

We present a low-noise and high power per comb line dual-comb source based on a PPLN OPO and an Yb:YAG pump. Both the laser and OPO are spatially-multiplexed single-cavity dual-comb sources. The system operates at a repetition rate of 250 MHz, and the relatively long pump pulse duration of around 900 fs leads to a high power per comb line of >60 μW in the idler at 3500 nm. The idler is tunable from 2900 nm to 4170 nm. The system runs at over 15 kHz repetition rate difference, enabling comb-line-resolved dual-comb spectroscopy measurements in free-running operation.

We used a mid-infrared frequency comb to detect SARS-CoV-2 infection by measuring exhaled breath molecular contents. A trial study was conducted for the first time and excellent diagnostic power was found. Mid-infrared laser sensing technologies can have unprecedented potential for non-invasive optical diagnoses of some of the most pressing medical cases.

A robust, high-power amplifier design allows for generation of >1 MW, few-cycle pulses from a 100 MHz, 1550 nm Er:fiber frequency comb. With this source, we explore non-perturbative harmonic generation in the UV-visible in solids. In zinc oxide (ZnO) crystal, nearly continuous spectra spanning 200-700 nm are achieved and the dependence on the carrier-envelope-phase is observed and analyzed using carrier-envelope amplitude modulation spectroscopy (CAMS).

This conference presentation was prepared for SPIE LASE 2023.

Temporal shaping of picosecond duration pulses is challenging due to limitations of direct shaping with electro-optical technologies or spectral shaping because of limited spectral content. We present an experimental implementation of a non-collinear sum frequency generation scheme wherein picosecond duration pulses with tailored temporal profiles are derived from femtosecond pulses with modified spectral phase. We demonstrate temporally shaped pulses with >20 ps duration, flat-top profile, and near transform-limited spectral content while maintaining upwards of 40% conversion efficiency. Additionally, we provide a framework for extending this technique to arbitrary temporal profiles and wavelengths.

UV supercontinuum based on gas filled anti resonant hollow core fibers is demonstrated to have passed a major milestone by providing spectral properties comparable to those of plasma arc lamps, namely a broad, flat, low noise, and stable spectrum. The primary advancement is the use of pump modulation which flattens the spectrum by more than 20dB. As proof of concept, results from scatterometry measurements, with both UV supercontinuum and plasma arc lamps are shown to produce comparable results. However, UV supercontinuum can meet additional requirements making it suitable for many cutting-edge UV metrology applications such as imaging and spectroscopy.

We report on mid-infrared supercontinuum generation from 4 to 9 µm in orientation-patterned gallium-arsenide waveguides pumped by nanojoule-class ultrafast fiber lasers. The QPM waveguide and the laser source are optimized in tandem to pump the waveguides close to the degeneracy by means of sub-picosecond pulses at 2760 nm. The use of a waveguide geometry drastically reduces the required energy to the nanojoule level, thereby opening supercontinuum generation in GaAs platforms to fiber lasers.

We present a few-cycle, actively carrier-enveloped phase stabilized OPCPA system operating at a central wavelength of 900 nm on a compact footprint of only 120 cm x 80 cm including the Yb:YAG pump system. The system delivers sub 9 fs pulses with pulse energies exceeding 30 µJ at a 200 kHz repetition rate. The layout allows the compactification of modern attosecond spectroscopy setups and increase stability.

We present an active imaging system using an upconversion detection scheme to detect the infrared light with a low noise visible camera. The system comprises one single fiber amplification chain to generate both the illumination pulse signal at 2 µm and the pump pulse at 1.55 µm for sum-frequency generation in a PPLN crystal. The 10 nm wide pump spectrum expands the phase matching condition of the system and increases the number of spatial modes signal up to 128x128 pixels, compatible with active imaging requirements. The whole setup is movable and we will present images from outdoor scenes.

Using a combination of three techniques, ellipsometry, interferometry and transmission measurements, the temperature dependence of refractive index (n) and absorption coefficient (α) of CdSiP2 for light polarization along and perpendicular to the c-axis of the crystal was determined. GaAs was also studied using the same techniques to determine n and α expanded over the spectral and temperature ranges available in the literature (0.9 to 25 μm for n, over temperature range of 77 to 500 K). values of α in the 0.85 to 0.9 μm region of GaAs and in the 0.5 to 0.6 μm band-edge region of CdSiP2.

Cadmium silicon phosphide, CdSiP2 (CSP), exhibits the highest d-coefficient (d36 = 85 pm/V) among all practical nonlinear optical crystals. Its large band gap of 2.45 eV allows for 1-micron pumping with widely-available Nd- and Yb-based laser sources, and its dispersion properties are such that a 1-um pump yields non-critically phase-matched temperature-tunable output between 6.2-6.5 um (an attractive range for minimally-invasive laser surgery). However, residual 1-um absorption losses in CSP are not insignificant (0.16-0.2 cm-1). In this work we focused on identifying, and ultimately minimizing, the point defects responsible for these losses by correlating EPR spectra with polarized absorption near 1-um.

CdSiP2 (CSP) is a nonlinear optical material used for mid-infrared generation. For nonlinear optical materials, absorption bands associated with point defects often limit output power. We use electron paramagnetic resonance (EPR) to monitor paramagnetic charge states of defects. In CSP crystals, EPR shows singly ionized silicon vacancies (VSi-) initially present are eliminated by exposure to 1064 nm light. Our results suggest that 1064 nm light converts VSi- acceptors to nonparamagnetic doubly ionized (VSi2-) and neutral (VSi0) charge states. A thermal activation energy of 0.23 eV describes the recovery of the VSi- signal including at room temperature.

In this work we compared the laser damage threshold of ZGP, CSP, GaAs, GaP, OP-GaAs, and OP-GaP under identical conditions. ZGP, CSP, GaAs, and GaP samples measuring 4x5x12 mm3 were fabricated from bulk, melt-grown single crystals. Orientation-patterned GaAs and GaP grown by HVPE on MBE templates were fabricated with dimensions of 2x6x12 mm3. All samples were ground and double-side polished together on the same polishing fixture.to achieve identical surface finish, then demounting, cleaned, and anti-reflection coated together (AR @ 2 and 3-5 microns) followed by laser damage testing with typical 2-micron laser parameters of 25 kHz, 75 ns, 30-micron spot.

In this work, we demonstrate a diamond Raman oscillator at high average powers, and we study in detail the influence of resonator length on the spectral features of the output beam. Additionally, we investigate the occurrence of parasitic nonlinear effects such as four-wave mixing, Brillouin and anti-Stokes Raman scattering. Furthermore, we propose a method to suppress Brillouin scattering in diamond Raman oscillators and thus allow for further power scaling. An etalon is used to decouple parasitic spectral components, achieving a high purity output spectrum which is tested at different cavity lengths and its performance compared with an ordinary mirror.

The ternary chalcopyrite semiconductors AgGaS2, AgGaSe2, ZnGeP2, CdGeAs2, and most recently CdSiP2 are among the most efficient and widely-used nonlinear optical (NLO) crystals for mid-infrared frequency conversion involving interaction wavelengths beyond 4 microns. Here we survey the entire class of chalcopyrites – including II-IV-V2, I-III-VI2, and II-III2-VI4 (defect chalcopyrites) – in search of promising new NLO crystals offering high nonlinear coefficients, broad transparency ranges, low absorption losses, adequate birefringence for phase matching, and ideally congruent melting to allow for growth from stoichiometric melts by directional solidification. We report preliminary results on synthesis, growth, and characterization of the most promising candidates.

Barium gallium sulfide, BaGa4S7 (BGS) is an attractive nonlinear optical (NLO) crystal notable for the rare combination of wide band gap (2.64 eV), long phonon cut-off wavelength (13.7 m), and relative ease of growth from stoichiometric melts. BGS is ideal for shifting widely-available Ti:sapphire and Yb-doped femtosecond laser sources deep into the mid-IR. However, BGS is plagued by severe cracking during cool-down and post-growth processing. To overcome these difficulties, we fabricated oriented single crystal BGS cubes with faces normal to the principal axes, and determined the thermal expansion coefficient along each axis via temperature-dependent x-ray diffraction as well as dilatometry.

HVPE growth of orientation patterned GaAsP (OP-GaAsP) layers with thickness exceeding 600 m and with excellent domain fidelity is accomplished for several arsenic-phosphorus compositions to obtain desired optical properties. This novel ternary material system could be an ideal candidate as compared to those of widely explored QPM materials – GaAs and GaP, for nonlinear frequency conversion in the mid- and long-wave infrared based on some specific pump sources, such as mode-locked Er-fiber lasers. Recent demonstration of second harmonic generation in an OP-GaAs0.15P0.85 structure brings us a step closer to implement such GaAsP QPM structures as frequency conversion devices.

Ternary GaAsP alloys, with up to 50% phosphorous content, were grown by hydride vapor phase heteroepitaxy on GaAs substrates, in thicknesses exceeding 0.5 mm. After polishing off the GaAs substrate, the two-photon absorption coefficient of the material was measured at wavelengths between 1064 and 1700 nm, using a tunable picosecond duration laser. Current challenges faced by orientation patterned GaAs crystals for high power MWIR generation are expected to be alleviated through the use of the ternary alloys due to the expected reduction in the two-photon absorption coefficient values.

Space-time wave packets can travel in linear media invariantly, even in presence of chromatic dispersion. I review the current status of theoretical and experimental work on this ‘dispersion cancellation’ phenomenon, which is made possible by endowing the optical field with non-differentiable angular dispersion. Consequently, space-time wave packets can travel with no pulse broadening in dispersive media independently of the magnitude, sign, or order of the dispersion. These results may have useful consequences in phase-matching of nonlinear optical effects.

We theoretically explore the enhancement in nonlinear response that can be achieved by interfacing multiple graphene nanostructures in close proximity to trigger nonlocal effects associated with large gradients in the electromagnetic near-field. Our findings reveal the importance of both passive and active tuning in the design of atomically-thin nanostructures for nonlinear optical applications, and in particular emphasize the role played by nonlocal effects in generating an even-ordered nonlinear response that may contribute to other nonlinear optical processes through a cascaded interaction.

Recently an all-optical Quasi Phase Matching (QPM) scheme for High Harmonic Generation (HHG) was introduced in which the pump beam was constructed in the form of an intensity grating whose periodicity could be easily tuned. This was performed using a two-component fundamental beam, superposing a Bessel beam and a Gaussian beam. Here we present, theoretically and experimentally, a phase matched non-collinear Second Harmonic Generation (SHG) in a doped LiNbO3, using a similar easily tunable scheme. The phase matching condition depends on both the spatial properties of the fundamental beam and the thermal properties of the medium. We show that a thermally induced phase-mismatch can be compensated by choosing the spatial properties of the pump beam controlled by the SLM, resulting in a phase-matched non-collinear SHG.

We experimentally demonstrate a new technique for achieving efficient optical parametric amplification (OPA) which maintains the simplicity of conventional OPA implementation and works for common laser wavelengths using existing nonlinear media. This technique is achieved by simultaneously performing OPA and second harmonic generation at the idler wavelength. The dynamics of the two nonlinear processes hybridize, inhibiting back-conversion in the OPA and creating a long region of laser-like gain saturation. We show conversion of 2 μm picosecond pump pulses to 3.4 μm with 68% quantum efficiency, 44% internal pump to signal energy efficiency, and 48-dB gain in a bulk CSP crystal.

We demonstrated efficient frequency doubling of the ultrafast laser system at the wavelength range of 1620 – 1700 nm. The laser system was based on Er3+ fiber laser with Raman soliton self-frequency shift (SSFS) stage. The power of SSFS radiation was up to 110 mW/10.9 MHz with a pulse duration of 125 fs. PPLN crystal with several quasi-phase matching periods was used for efficient frequency doubling in the tuning range. The highest SHG power of 55 mW was obtained at 1675 nm with conversion efficiency of > 50%. The developed laser will be used for bladder cancer diagnosis by multi-photon bioimaging.

In this work, we propose a new concept of phase-matching based on multiple passes of interacting beams through the same nonlinear crystal placed inside a multi-pass cell. We show that materials such as crystalline quartz, can be quasi-phase-matched in such a multipass arrangement. Moreover, we have demonstrated enhanced conversion efficiency approaching 40% with very low, 10 MW/cm2 intensity in the 2-mm thick KTP crystal inside a multipass arrangement. Thus we present the first proof-of-concept experimental demonstration of χ(2) multipass nonlinear optics where free space birefringent phase-matching and quasi-phase matching can be realized with nearly all types of nonlinear materials.

The second-order nonlinear susceptibility of asymmetric type-II quantum well is predicted to be strongly enhanced by the large interband electric dipole moment in type-II structures as compared with that in asymmetric type-I quantum well designs. The practical lattice-matched InGaAlAs/InP materials system is used to calculate the nonlinear susceptibility enhancement of the type-II band alignment.

Optical second harmonic generation (SHG) is a process that doubles the photon frequency and is widely used to detect broken inversion symmetry and local polar order. Analytical SHG modeling is essential to connect experimental results to material properties, such as point group symmetry and SHG susceptibilities. However, complexity builds up when the crystal exhibits low symmetry, absorption, and consists of multiple interfaces. Thus, the SHG model in the literature involves many approximations, leading to a scattered dataset of reported SHG properties. Here, we have developed an open-source package called ♯SHAARP which derives analytical solutions and performs numerical simulations of reflected SHG from crystals with arbitrary symmetry group, orientation, complex and anisotropic linear dielectric tensors with frequency dispersion, a general SHG tensor, and any polarization state of the incident and SHG light. ♯SHAARP enables accurate SHG analysis of a broad range of materials.

This conference presentation was prepared for the Nonlinear Frequency Generation and Conversion: Materials and Devices XXII conference at SPIE LASE, 2023.