This PDF file contains the front matter associated with SPIE Proceedings Volume 12405, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

We present our recent results on the producing of ultra-broadband frequency combs in the mid-IR – THz range and their applications in dual-comb spectroscopy (DCS) that feature: sub-Doppler resolution, sensitivity down to part-per-billion level, and up to video-rate acquisition speed. We describe two techniques for generating frequency combs: (i) subharmonic generation in a sync-pumped optical parametric oscillator (OPO), which converts the carrier of ultrafast pulses to half that frequency and augments the spectrum to more than one octave, and (ii) optical rectification from few-optical-cycle 2.4- μm mode-locked lasers that produces a frequency downconverted output in the whole range from 1.5 to 50 THz and with the frequency span up to two octaves (e.g. 7.5–30 μm). We describe two approaches to the spectroscopic technique: (i) DCS with two mutually coherent mid-IR beams, and (ii) DCS with electro-optic sampling (EOS).

Using multiplexed broadband ptychography (MBP), we characterize the EUV light in high-order harmonic generation (HHG). MBP can measure spectrally resolved complex beam profiles for different harmonic outputs without spectral dispersion. Through a simple change to the experimental setup, we also characterize the driving laser for the process. The experimentally measured driving laser profile is used in an SFA+ with dipole approximation simulation of high harmonic generation. The simulated results are compared to the experimentally measured high harmonics produced with the characterized driving laser. By characterizing the input and output in HHG we can further understand the generation mechanics to better control the process. We also can investigate how control of the fundamental laser can lead to control of the output harmonics.

Multi-octave spanning conical emission has been numerically predicted to be generated from ultrafast LWIR pulse propagation in various bulk gaseous media. The gUPPEcore propagator was used to simulate the filamentation collapse in xenon. A flat dispersive landscape near the fundamental at 10 μm allows for efficient high-harmonic generation and slow walkoff of generated spectral components due to a high cutoff frequency and slowly varying GVD. Enough energy is converted to higher harmonics that many of the generated harmonics carry enough power to propagate nonlinearly themselves. As the pulse collapses into a filament, the evolution of the far-field, (angle-resolved) spectrum reveals a conical emission feature that is localized around many high harmonics and generates a tail that spans more than four octaves after the collapse. The x-wave dispersion relation was used to fit three distinct conical emission features generated from three different high harmonics (5th, 7th, and 9th) during collapse. The integrated spectrum exhibits a supercontinuum during collapse, but not the on-axis spectrum, indicating that most of the spectral contribution between harmonics comes from the off-axis conical emission. Pulses with various durations (34 − 500 fs) exhibit the broadband far-field spectral feature, but the signal is stronger with shorter pulses due to spectral broadening. We conclude that there exists a conical emission feature with a tail that spans multiple octaves that is formed from the interference of conical emission generated from individual harmonics using an ultrafast 10 μm pulse as a seed.

A tabletop optical parametric chirped pulse amplification (OPCPA) system is developed based on ZnGeP2 crystals to generate 3.2-mJ, 92-fs pulses centered at 3.1 μm, operating at 1 kHz. Pumped by a 2-μm chirped pulse amplifier (CPA) with a top-hat beam profile, the amplifier achieves a 16.5% overall efficiency, which is the highest efficiency achieved by OPCPA at this wavelength. The spectral bandwidth of the pulse can be further broadened by converting the OPCPA to a non-collinear configuration.

High brilliance coherent light sources are sought for a diverse range of investigations and applications, e.g., ultrasensitive spectroscopy, standoff detection, chemical diagnostics, material science, ultrafast imaging and attosecond science. Here, we present such a carrier-envelope-phase stable light source, seeded by a mid-IR frequency comb, with simultaneous spectral coverage across 7 optical octaves, from the UV (340 nm) into the THz (40,000 nm). The brightness of this source exceeds the brightness of synchrotrons across the entire 7 octave spectrum and by up to 5 orders of magnitude.

We revisit self-difference frequency generation –– nonlinear mixing of the lasing field induced by stimulated emission with its pump source in a laser gain medium –– by employing transition-metal doped chalcogenides along with the standard birefringent phase-matching techniques for mid-infrared applications. The use of an a-cut Cr:CdSe crystal under the noncritical PM condition has significantly improved the conversion efficiency compared to the previous results obtained in a Cr:ZnSe slab with Fresnel phase-matching. To generalize the self-mixing design, the phase-matching conditions of solid solutions CdS_xSe_1-x doped with Cr²⁺ and Fe²⁺ ions are characterized in the mid-infrared spectral range.

We present a nanosecond, non-resonant optical parametric oscillator (NRO) based on a 20 mm long periodically-poled LiNbO₃ (PPLN) crystal operating at 30-70 kHz. Pumping with a nanosecond Nd:YVO4 laser at 1064 nm in a double-pass configuration, the signal plus idler average output power reached 1.2 W for a pump level of 4 W (at 35 kHz repetition rate). Narrowband seeding with a Tm-fiber laser is employed to narrow the linewidths of the signal to 0.8 nm and the idler to 2 nm. Continuous-wave seed levels as low as 2 mW were sufficient to produce the effect which means that this technique could be useful for single-frequency operation using DFB seed laser diodes. At higher pump power levels > 4 W, the linewidth narrowing effect produced by the seeding was insufficient to prevent broader linewidth operation of the NRO signal and idler outputs. Pumping the NRO at higher repetition rates to scale the average output power of the NRO whilst remaining in the narrow linewidth operation mode is discussed.

Broadband Sum Frequency Generation (BB-SFG) spectrometer is a complete solution for femtosecond vibrational spectroscopy designed and manufactured by EKSPLA. System produces optically coupled femtosecond broadband midinfrared pulse covering the spectral range of the molecular "fingerprint" region and narrowband visible pulse which are directed to the sample to produce sum frequency signal. Ultrashort pulses of high intensity allow to get better signal to noise ratio using lower pulse energy thus reducing the possibility of sample modification. It is especially important for aqueous and biological samples. Single measurement can cover up to 800 cm-1 bandwidth which shortens the measurement time and lets obtaining spectrum of the same sample state at the beginning and at the end of measurement, which can be different in case of scanning. The design of BB-SFG helps to overcome the main shortcoming of common broadband SFG spectrometers: a complex and energy inefficient narrowband visible pulse channel formation. We use a single all-fiber mode-locked oscillator to generate ultrashort pulses for both broadband femtosecond mid-IR and narrowband picosecond visible channels. Up to 10 μJ energy tunable broadband (>300 cm^-1) pulses in 2.5-10 μm spectral range are mixed at the sample with less than 2 cm^-1 bandwidth visible (532 nm) picosecond pulses in order to produce SFG signal at kHz repetition rates. Real-time spectral scanning technique allows even broader simultaneous spectral acquisition.

Terahertz time-domain spectroscopy (THz-TDS) is a method used in research and industry for non-invasive characterization of products and materials. Many THz-TDS systems rely on parametric conversion in semiconductor crystals to generate and detect phase-locked THz pulses, providing reliable access to frequencies below 3 THz. Accessing higher frequencies, however, often requires a sophisticated near-infrared (NIR) source delivering sub- 100 fs pulses to access the required spectral bandwidth and thin nonlinear crystals (few hundred micrometers thick) to minimize phase mismatch during both the THz generation and detection processes. As a result, broadband THz- TDS configurations rely on laser systems which are often bulky and costly, resulting in inefficient THz generation and detection processes due to a limited nonlinear interaction length in the crystals. To overcome these limitations, we introduce three modules to a THz-TDS system employing a compact and cost-effective pulsed laser. First, a fiberbased component is used to broaden the output laser spectrum and compress the pulse duration. This module provides the NIR frequency content needed for broadband THz generation through optical rectification and a pulse duration short enough to efficiently resolve high THz frequencies during electro-optic sampling. The other two modules utilize a thick nonlinear crystal with a periodically patterned surface to optimize the efficiencies of the broadband THz generation and detection processes. In this configuration, a long nonlinear interaction length is guaranteed while noncollinear phase matching provides access to a broad spectral range. The combination of these modules extends the THz spectrum from 3 THz to beyond 6 THz with a peak dynamic range >50 dB at 3.5 THz.

We demonstrate that the all-optical inscription of second-order nonlinearity through the coherent photogalvanic effect allows not only degenerate but also non-degenerate sum-frequency generation in silicon nitride waveguides. Several multiphoton absorption processes can occur simultaneously, such that their quantum interference allows for the inscription of distinct charge gratings for quasi-phase matching of several second-order nonlinear processes within the same waveguide. In this work, we investigate the effect theoretically and experimentally validate the generalized sum-frequency generation.

Nonlinear optical materials convert incident lights to photonic signals of higher frequencies. For ultra-sensitive photon counting devices, such as SNSPD (Superconductor Nanowire Single Photon Detector), or Superconductor tunnel junction photon counters, it is essential to understand the nonlinear optical properties of related superconductor metal oxides and nitrides. In this report, we present nonlinear spectral analysis for various superconductor oxides (LiNbO3, Nb2O5, Ta2O5), other metal oxides (Al2O3, WO3), and Niobium Nitride (NbN), in the UV, visible, and near infrared regions.

We use first principle DFT-based microscopic many-body models to investigate essential electro-optical properties of bulk tellurium. Absorption/gain and spontaneous emission spectra are calculated using the semiconductor Bloch and luminescence equations. High harmonic generation due to off-resonant excitation and its propagation dependence are studied by coupling the microscopic models to a pulse propagator. Limitations due to intrinsic carrier losses via radiative- and Auger-recombination processes are determined solving quantum-Boltzmann type scattering equations. The strong directional- as well as density- and temperature-dependence of the properties is demonstrated.

We report the experimental results on the temperature-dependent phase-matching properties of BaGa₂GeS₆ for second-harmonic generation (SHG) and sum-frequency generation (SFG) of Nd:YAG laser-pumped KTiOPO₄ (KTP) and HgGa₂S₄ (HGS) optical parametric oscillators (OPOs) and a CO₂ laser in the 0.7674-10.5910 μm range. The experimental values for type-2 90° phase-matching SHG wavelengths in the short and long wavelength branches were measured to be λ1 = 2.0376 and 7.6740 μm, respectively. By using a BaGa₂GeS₆ crystal cut at θ = 48.1˚ and Φ = 0˚, we measured the type-1 and type-2 phase-matching angles for SHG and SFG of a Nd:YAG laser-pumped KTP OPO at λ = 3.1092 μm and HGS OPO at λ = 7.6740 μm as well as those for SHG and SFG of a waveguide CO₂ laser. In addition, we derived the Sellmeier equations that provide a good reproduction of the experimental results for the phase-matching data. The temperature phase-matching bandwidths (FWHM) were also obtained in eight different phase-matching conditions by using the measured temperature variation of the pump wavelengths, the measured temperature dependence of the phase-matching angles (Δθ_ext/ΔT) from 20℃ to 120℃ and the angular acceptance (Δθ_ext‧l) calculated with our Sellmeier equations. Moreover, the temperature phase-matching bandwidth (FWHM) for type-2 90˚ phase-matching SHG was determined to be ΔT‧l = 12.2 ℃‧cm from the measured value of dλ₁/dT = +0.3 nm/℃ and the calculated spectral bandwidth (Δλ₁‧l). From these experimental results and the dn/dT measured at five wavelengths, we derived a thermo-optic dispersion formula valid in the 0.7674 − 10.5910 μm spectral range.

Photonic upconversion from the infrared regime to the visible spectrum can occur through sum-frequency generation (SFG). A second-order nonlinear optical response, such as SFG, can be produced from a nonlinear material, in this case an ABC nanolaminate. Optimization of a metamaterial consisting of a plasmonic nanolaminate device can maximize the SFG from incident wavelengths. Utilization of a deep learning framework removes the need for traditional guess and check methods and creates new possibilities for plasmonic geometries. Applications of this research include low-cost night vision or low light imaging systems for defense, autonomous vehicles, and other commercial uses.

Third harmonic generation (THG) achieved by the simultaneous phase matching of multiple quadratic nonlinear processes in a single crystal has been studied previously both theoretically and experimentally, achieving modest conversion efficiencies. As opposed to the common practice of using second harmonic generation (SHG) and sum frequency generation (SFG) stages in series, performing these processes in parallel results in a narrow material parameter range in which the transfer of energy to the third harmonic can be fully efficient, even under conditions of perfect phase matching. This is largely due to the presence of back-conversion in the conversion dynamics, which limits the efficiency for spatio-temporally non-uniform beams. Here we demonstrate an unconventional method of THG where the fundamental wave is first frequency-quadrupled through cascaded SHG before being down-converted to the third harmonic. When the three associated phase matching conditions are met simultaneously, we find that energy is transferred robustly and efficiently from the fundamental wave to the third harmonic with inhibited back-conversion. This is an example of convergent dynamical behavior emerging from hybridized nonlinear optical processes as recently observed in hybridized parametric amplification. These dynamics are achievable over a broad parameter range that correspond to common nonlinear crystals. Using a spatio-temporal numerical investigation, we find that 25-ps CO₂ laser pulses with Gaussian spatio-temporal profiles and 10-μm wavelength can be converted to their third harmonic with over 80% conversion efficiency in a realizable monolithic orientation-patterned GaAs device. High performance realizable domain-poled KTP and LNB devices are also investigated.

Optical computing may one day provide fast and efficient calculation, with complex nonlinear interactions occurring at the speed of light. Here we use supercontinuum generation as an optical computing block within a neural network. Supercontinuum generation is an interesting candidate for optical computing because it can be adjusted to operate over a large range of nonlinearity in a single device by adjusting laser pulse parameters. Here, the optical block accepts inputs in the form of actuator settings for a chirped fiber Bragg grating pulse shaper, and outputs a measured supercontinuum spectrum. A small neural network before the optical block translates the calculation data into actuator settings, and another small network after the optical block translates the spectrum into the desired output. In this architecture, the optical block acts like a static, fixed-weight neural network, while the two translation networks are trained to adapt the optical block to the particular computational problem. As first demonstrations, we train the system to perform hand-writing identification, and use the supercontinuum as the decoder in an autoencoder network. As temperature-controlled pulse shaping is slow, we first emulate the supercontinuum with a neural network trained on spectra measured at randomized heater settings. The emulation network is frozen and inserted in place of the optical block for training of the translation layers on computer. The training data is converted to spectra with the optical device for retraining of the output translator, compensating for emulation errors.

Spectral interferometry is employed to characterize a temporal-mode sorter, also known as a quantum pulse gate, which is essential to ensure efficient information encoding and processing. We present and test a model to reconstruct transfer functions and propose a feasible experiment.

Petroleum products such as gasoline or oils tend to age over time with iterative thermocycles, leading to a degradation in quality. To investigate this aging process, spectroscopy techniques involving nonlinear four-wave mixing have been recently used to shed light on the viscoelastic properties of these materials. Impulsive stimulated Brillouin scattering is an emerging spectroscopy technique for monitoring changes in the mechanical properties of materials by using a transient laser grating to create acoustic waves within the sample. A probe beam then diffracts off of this standing acoustic wave and yields in a frequency shift detected using optical heterodyning. Impulsive stimulated Brillouin scattering was used to examine mineral oils, motor oils, and a variety of different gasoline grades. The gasoline samples underwent thermocycling to 70°C and back to room temperature to observe viscoelastic differences and noticeable hysteresis.

The ability to confine and guide light makes photonic crystals (PhCs) a promising platform for large local field enhancement, which enables efficient nonlinear processes at the nanoscale. Here, we utilize optical bound states in the continuum (BICs) to engineer sharp resonances with high quality factors. By investigating the angleresolved reflection spectra, we demonstrate that two PhC slabs with different configuration but the same lattice constant support a pair of at-Γ and a pair of off-Γ resonances, respectively. In both cases, BIC-type resonances are observed at the fundamental frequency while BIC-like resonances are found at the second harmonic. This double-resonance phenomenon is subsequently used to significantly enhance the second-harmonic generation from PhC slabs. The maximum values of the SHG are several orders of magnitude larger than those corresponding to the reference slabs. We consider that our approach based on double-resonance BICs provides a novel way to realize enhanced harmonic generation in photonic nanodevices.

We report on the achieving and investigating random Raman lasing based on femtosecond (fs) pulse written randomly spaced scattering points (Rayleigh reflector) and random array of fiber Bragg gratings (FBGs) in graded-index (GRIN) fiber directly pumped by multimode laser, with much better output characteristics in the last case. The fabricated 1D-3D FBG arrays are used as a complex output coupling mirror together with input highly-reflective FBG fabricated by a phase mask technology. The 1-km GRIN fiber (100/140um) line was pumped by 940-nm laser diodes with a total power of up to 180 W and beam quality M²~34. Above threshold pump power of ~100W, random lasing of the 1^st-order Stokes beam was obtained with output power exceeding 25 W at maximum pumping. The beam quality parameter M² varies with FBG distribution in the fiber cross-section and its best value amounts to ~2, while the linewidth narrows to 0.1-0.2 nm.