High-power ultrafast laser technology has seen extremely fast-paced progress in the last decades, giving momentum to many fields. Nowadays, laser systems delivering hundreds of watts to kilowatts of average power with pulse energies ranging from microjoules to hundreds of millijoules become increasingly available, based on fiber, slabs and disk laser geometries. In this talk, we will discuss a recent hot topic enabled by progress in high-power ultrafast laser sources, that is the demonstration of table-top sources of few-cycle THz radiation with extremely high average power – reaching a performance level which was so far restricted to accelerator facilities. We will discuss new possibilities opened by these unique sources both in research and applied fields.

CdSiP2 (CSP) is a nonlinear optical crystal developed as a wider-band-gap analog of ZnGeP2 (ZGP) to enable mid-infrared generation. A direct comparison of the performance of ZGP and CSP crystals in mid-IR generating OPOs was performed with a 4 W Tm:YAP pump laser. CSP was shown to outperform ZGP in this configuration. A ring OPO using CSP with a 2.09 micron pump and 80W of power was used to generate 27 W of mid-IR light demonstrating CSP’s viability for high average power generation. An OPO seeded OPA was then used to directly compare CSP and ZGP with this same source as well as with an upgraded 140W source.

A fast tunable, Spontaneous Parametric Down Conversion (SPDC) light source in the saturated regime is presented. A 25 mm long, fanned pp:LN crystal combined with a translation stage provides a scanning speed of 100 nm/s. The light source is characterized in terms of scanning speed, spectral coverage, wavelength repeatability and power. Based on recently developed theory for SPDC light sources in the saturated regime, we discuss slope efficiency and pump power threshold for entering the high gain regime.

We demonstrate a compact mid-IR system using difference frequency generation based on a femtosecond Ytterbium fiber laser system operating at a repetition rate of 100 MHz. By utilizing the fundamental 1 µm emission of the Yb-laser system and the widely wavelength-tuneable soliton emission of a PCF, high output power levels of more than 100 mW are realized over a large tuning range from 3 µm to 4.9 µm. This system demonstrates a powerful and widely tuneable femtosecond mid-IR source in a compact and robust design for applications such as infrared microscopy and spectroscopy.

Wavefront sensing is essential for many applications related to imaging and remote sensing. Most of the existing methods of wavefront sensing are based on interferometry and rely on precise optical alignment, which are often hard or impossible to realize in a real-life setting. On the other hand, nonlinear optical conversion, such as second harmonic generation, is sensitive to the spatial phase information of the beam profile, and, as we have shown lately, can be optimized through external beam shaper. More recently, we have demonstrated that the inverse problem can be successfully solved by acquiring second harmonic images of the beam and retrieving through a computer algorithm, enhanced by machine learning, the incident beam's wavefront. In this report, I will outline our recent results in this field, discuss the sensitivity and acquisition speed of the newly proposed method and outline potential applications related to remote sensing and biological imaging.

When driven at infrared resonances, crystals possess giant Raman scattering susceptibilities due to nonlinear lattice coupling in the polarizability. A first-principles study of SrTiO3 reveals Stokes and anti-Stokes peaks in the cubic susceptibility with values >10^(-15) m^2/V^2, and large values of chi^(3) representing cross-phase and cross-amplitude modulation that persist from THz to visible frequencies. This implies a new route for achieving strong nonlinear frequency conversion in the infrared over short length scales, and a route for strong light-driven control of material optical properties on ultrafast timescales covering a hyperspectral range. We additionally discuss implications for non-centrosymmetric materials such as LiNbO3.

Barium gallium selenide (BaGa4Se7) is a recently developed nonlinear optical material with a transmission window extending from 470 nm to 17 μm. A primary application of these crystals is production of tunable mid-infrared laser beams via optical parametric oscillation. Unintentional point defects, such as selenium vacancies, cation vacancies (barium and/or gallium), and trace amounts of transition-metal ions, are present in BaGa4Se7 crystals and may adversely affect device performance. Electron paramagnetic resonance (EPR) and optical absorption are used to identify and characterize active defects in BaGa4Se7 crystals grown at BAE Systems. Five distinct defects, each representing an electron trapped at a selenium vacancy, are observed with EPR (there are seven crystallographically inequivalent selenium sites in this monoclinic crystal). One defect is seen at room temperature before illumination. The other four are seen at lower temperature after exposure to 532 nm laser light. Each singly ionized selenium vacancy has a large, nearly isotropic, hyperfine interaction with 69Ga and 71Ga nuclei at one neighboring Ga site, which indicates a significant portion of the unpaired spin resides in a 4s orbital on this adjacent Ga ion. Optical absorption bands peaking between 430 and 750 nm are produced by the 532 nm light. These photoinduced bands are assigned to the selenium vacancies.

CdSiP2 (CSP) is a non-linear optical material for mid-infrared optical parametric oscillators. Previous work showed that an intrinsic acceptor (Si vacancy) produced unwanted absorption in the near-IR. The VSi concentrations are much reduced in recent growths. Other compensating defects now play an important role: iron impurities, an intrinsic donor (Si-on-Cd antisite), and a second intrinsic acceptor (Cd vacancy). We present photoinduced electron paramagnetic resonance (EPR) spectra to identify these defects. Illumination using light sources (lasers, LEDs) in the 500nm to 1064nm range can “reveal” these defects by converting them to their paramagnetic charge states. We present the wavelength dependence and thermal stability of these defects. Thermal decay data allow us to determine activation energies for various defect charge state transitions which allows us to predict decay times at room temperature of defect charge states and related absorption bands that can impact laser devices.

A new theory for Spontaneous Parametric Down Conversion (SPDC) light sources in the saturated regime is presented. The theory models quantitatively the SPDC output power and spectral distribution in detail, thus providing central information for designing new and efficient SPDC light sources as well as studying the noise properties in the presence of pump depletion. The theory is supported by experimental work based on a passively Q-switched laser and a 40 mm PPLN crystal.

Supersymmetry (SUSY) enables tuning of certain eigenvalues of quantum well without changing other states. We leverage second-order SUSY to tune the intermediate state of asymmetric coupled quantum well states. A family of such quantum wells can be used for upconversion of two bands of frequencies into a single frequency. We investigate the total nonlinear conversion of light in a metallic coupled quantum well system. We obtain non-uniform refractive index distributions required to tune the frequency of the intermediate state while leaving the ground and excited state unchanged. We repeat SUSY transformations to tune the eigenvalues required for third-order susceptibility.

We present a new approach for dual-comb optical parametric oscillators. The system uses a single-cavity dual-comb laser that pumps a single OPO cavity. The pump has 1.7 W average power per comb at 1054 nm with 80-MHz repetition rate. The OPO ring cavity is pumped in opposite directions by the two pumps. The idler beams have >280 mW average power at 3500 nm with 145-nm bandwidth. We characterize the signal noise and find shot-noise-limited performance (RIN below -155 dBc/Hz at >1 MHz frequencies). Our approach represents a low-noise solution to dual-comb spectroscopy across the short-wave infrared and mid-infrared spectral regions.

Optical frequency comb (OFC) spectroscopy in the mid-infrared (MIR) promises faster, more precise, or more sensitive molecular spectroscopy. To date, demonstrations of MIR OFCs have suffered from low power, poor wavelength coverage, or low sensitivity. Systems that do excel in these areas have high cost and complexity. The new MIR OFC generation method presented here overcomes these limitations. Phase modulation of a CW laser forms an NIR OFC, which pumps a singly resonant, single frequency optical parametric oscillator (OPO). The OPO output is an MIR OFC, which is tunable between 2200 - 4000 nm with >1 W output power.

We experimentally demonstrate a novel approach to generate a multi-frequency comb light source with a high mutual coherence in an all-fiber system. Starting from EOM combs, we exploit spatial light multiplexing in a 3-core all-normal nonlinear silica fiber at 1550 nm. Each pulse propagates in its own core to experience a nonlinear broadening but within the same fiber. We obtained 3 almost similar output flat-top spectra spanning over 14 nm with 3 nJ per pulse at 250 MHz and a flat phase noise spectrum down to -125 dBc/Hz. The signal-to-noise ratio of interferograms is about 40 dB.

Yb:KYW laser pulses at 1030 nm were frequency-broadened and compressed by single-pass propagation in a 12 mm single-domain KTP crystal. The compression mechanism relies on refractive index modulation by the polariton shock-wave generated by impulse excitation of the lattice vibration modes, with a large dipole moment parallel to the crystal polar axis. Coherent Stokes sidebands generated by the index modulation lead to pulse compression under normal dispersion conditions. A compression ratio of about eight times was obtained for 170fs-long Yb:KYW laser pulses.

The state of the art in multicycle THz generation relies on nonlinear optical conversion in commercial periodically-poled lithium niobate crystals. THz pulse energies, however, are limited by the small apertures of available crystals as well as the low damage thresholds connected to photorefractive effects, especially at low temperatures associated with optimum efficiency. We explore three options for increasing the THz pulse energy achievable: increasing the crystal aperture to allow use of higher energy driver lasers; tuning the crystal temperature to look for an optimum; and testing an alternate medium (KTP) to mitigate photorefractive effects and push to higher intensities.

We demonstrate record conversion efficiencies of near 1% for nonlinear optical generation of narrowband (<1% bandwidth) THz pulses. These results are achieved using a novel laser source, customized for high efficiencies, with two narrow spectral lines of variable separation and pulse duration (≥250 ps). THz generation in 5% MgO doped PPLN crystals of varying poling period was explored for cryo- and room temperatures as well as different lengths. This work addresses an increasing demand for high-field THz pulses which has, up to now, been largely limited by low optical-to-THz conversion efficiencies.

Narrowband terahertz radiation at 1.97 and 2.34 THz is generated via optical rectification in a BaGa4Se7 crystal. Although broadband terahertz radiation is produced in the crystal, absorption occurs due to the dense phonon mode distribution in the low terahertz frequency regime (i.e. <4 THz), thus resulting in narrowband terahertz radiation exiting the crystal. Since this crystal exhibits numerous non-zero second-order nonlinear tensor elements, generation is investigated for various crystal orientations and excitation polarizations. The BaGa4Se7 crystal exhibits higher spectral densities at the generated frequencies of 1.97 and 2.34 THz in comparison to ZnTe. As such, BaGa4Se7 could provide benefits in the areas of security, communication, medicine, quality control, research, and on-chip applications.

We achieved to extract terahertz (THz)-waves generated by cascade process (i.e., higher-order THz-waves) in an injection-seeded THz parametric generator (is-TPG), and realized higher-power output. Because of the angular phase-matching condition, the higher-order THz-waves are generated deep in a crystal and extraction of them from the crystal was difficult due to the large absorption loss. In our setup, we used total reflection of the pump beam on the crystal surface to reduce absorption loss, and optimized the THz focusing lens and the position of Si-prism coupler. As a result, we succeeded in extracting higher-order THz waves and improved the output power.

Orientation-patterned gallium arsenide (OP-GaAs) and gallium phosphide (OP-GaP) are strategic nonlinear optical crystals, extending the many merits of quasi-phase-matching (QPM) deep into the mid-infrared spectral range (2-12 microns). Chief among the benefits of QPM are 1) long interaction lengths, enabled by non-critical phase-matching (NCPM, which eliminates birefringence walk-off) to reduce the threshold for low-peak-power applications, and 2) extremely broad-band tunability, by replacing angle tuning with simple translation across discrete- or continuously-varying grating periods. Since orientation-patterning is a vastly different mechanism from electric-field poling, a different set of design criteria exists for these materials, which are described in this talk.

Combining materials (through heteroepitaxy or forming ternaries), growth techniques, and template preparation approaches helps in resolving current limitations in developing frequency conversion sources in the MLWIR. In this study we focus on heteroepitaxy of GaAsP ternaries with different composition on close matching substrates and on orientation-patterned templates. The results are layers with excellent crystalline quality and quasi-phase matching structures with also excellent domain fidelity. Simplifying the existing template preparation techniques and improving the growth on them, and developing new ones for preparation of orientation-patterned templates on common substrates such as Si is another direction along with efforts to prove SHG and QPM frequency conversion in the grown materials.

Thick growth of a ternary nonlinear optical material GaAsxP1-x by HVPE is accomplished to demonstrate nonlinear frequency conversion in the mid and longwave infrared. The nonlinear optical properties of the ternary material are ideal compared to those of widely explored QPM materials – GaAs and GaP – for frequency conversion. Here, we present the HVPE growth results of 500 µm or thicker GaAsxP1-x ternary layers with different arsenic composition. A full suite of optical characterization of GaAsP layers grown on both plain substrates and on orientation patterned templates is presented to demonstrate the suitability of this novel optical material for frequency conversion.

BaGa4S7 (BGS) and BaGa4Se7 (BGSe) are attractive new nonlinear optical (NLO) crystals notable for the rare combination of wide band gaps (3.54 eV and 2.64 eV), long phonon cut-off wavelengths (13.7 m and 18 m), and relative ease of growth from stoichiometric melts, making them ideal for shifting widely-available 1-micron laser sources deep into the mid-IR. Here we demonstrate seeded HGF growth along crystallographic directions optimized for simplified fabrication and maximum yield of oriented frequency conversion devices, allowing apertures up to 25x25 mm2 and lengths greater than 20 mm.

We report the development of a high-power, fiber-laser-pumped, sub-nanosecond pulsed 260 nm DUV laser and demonstrate its use for bacterial disinfection. The source generates up to 5.8 W of average power at 260 nm (585 ps pulses at a repetition rate of 1.6 MHz, corresponding to a pulse energy of 3.6 μJ and a peak power of 6.9 kW). The results represent the highest DUV output power from an all-fiberized fiber laser pumped frequency conversion source to date. We demonstrate the application of the laser system to bacterial inactivation. A survival rate of less than 1 in 100000 is demonstrated for E. Coli bacteria after exposure to a DUV dose of 7 mJ/cm2.

We accurately measured the second-order nonlinear-optical coefficients of LaBGeO5 (LBGO) at the fundamental wavelength of 532 nm using the wedge technique. The values of d33, d22 and d31(=d32) were determined to be 1.15, 1.41 and 0.75 pm/V, respectively. The values are much larger than those estimated from our measured results at 1064 nm using Miller’s rule. Moreover, d33 is smaller than d22, the magnitude relation of which is inverse at 1064 nm. Then we need to remeasure at 1064 nm as well as measure the other d components.

High harmonic generation (HHG) is readily achieved by focusing mid-infrared (mid-IR), femtosecond laser pulses into polycrystalline Zinc Selenide (p-ZnSe). In this high-power regime (>> 100 GW/cm2), the HHG harmonics are self-phase modulated into a continuum. In this talk, we explore mid-IR frequency conversion in p-ZnSe in a low-power regime (1 – 100 GW/cm2) using mid-IR, 30 picosecond pulses which results in spectrally isolated visible and near-IR harmonics. In this regime, harmonic intensities clearly decrease with harmonic order with conversion efficiencies of 10-4 to 10-12 for second to ninth harmonics while retaining a non-perturbative character consistent with frequency conversion between harmonics permitted via random quasi-phase matching.

Realizing compact picosecond Optical Parametric Oscillators (OPOs) capable of generating high-energy mid-IR pulses at MHz repetition rates is a challenge due to the correspondingly long cavity length requirements. Intracavity fiber delay lines can be used to increase the cavity length but the achievable peak powers are then severely constrained by fiber nonlinearity. Here we report a compact, ytterbium-fiber-laser pumped, periodically poled lithium niobate based OPO that incorporates a 298 m length of hollow-core-fiber as an ultralow nonlinearity intracavity delay line. The OPO is capable of generating 1-MHz, 100-ps mid-IR pulses with an energy of 1.64-μJ and 12.8-kW peak power.

We report the design for a high-power optical parametric chirped-pulse amplifier (OPCPA) at 3200 nm central wavelength for the MIR-HE laser at ELI-ALPS, aiming to provide 20 mJ pulse energy and sub-2.5 cycles pulse length.