Nonlinear Frequency Generation and Conversion: Materials and Devices XXIV

Since the groundbreaking achievement of ignition and self-sustaining fuel burn at the U.S. National Ignition Facility (NIF), the field of fusion, specifically laser inertial fusion energy (IFE), has rapidly accelerated and transformed. Numerous countries are investing more heavily or initiating new fusion programs, with significant collaborative efforts from international research institutions and the private sector accelerating the path to practical fusion energy. The implications for the photonics market include an increased demand for lasers, optics, optical materials, diagnostics, and other key technologies, creating new opportunities for photonics companies and shifting market dynamics. Future challenges and strategies for achieving higher energy yields and commercial viability are outlined, emphasizing the critical role of photonics in enabling the next generation of fusion energy solutions.

The interaction of light and matter can create bonding structural and morphological changes in nano/micro-scale from the surfaces of diverse materials, sometimes even deep within them. This feature has been utilized in laser processing to produce new value for both science and industry. Recent advances in high-power, ultrashort pulsed laser and fast beam delivery technologies are rapidly expanding the possibilities of laser processing. At the same time, the number of parameters to be controlled has become enormous, which is why we have introduced Data Science. In this talk, we will discuss new data-driven laser processing utilizing high-speed data acquisition and AI data optimization for higher throughput and quality. We also aim for this technology to contribute to sustainable manufacturing and society in the future.

Optical frequency combs have revolutionized time and frequency metrology by providing rulers in frequency space that measure large optical frequency differences and/or straightforwardly link microwave and optical frequencies. One of the most successful uses of frequency combs beyond their original purpose has been dual-comb interferometry. An interferometer can be formed using two frequency combs of slightly different line spacing. Dual-comb interferometers without moving parts have no geometric limitations to resolution, therefore miniaturized devices using integrated optics can be envisioned. Dual-comb interferometers outperform state-of-the-art devices in an increasing number of fields including spectroscopy and holography, offering unique features such as direct frequency measurements, accuracy, precision, and speed.

Today, approximately 12,000 satellites orbit Earth. By 2030, estimates show numbers above 60,000. Today, we service spacecraft when absolutely necessary. By 2030’s, in-space services will be routine; refueling, repair, relocation, assembly, and manufacturing. Advances are underway to realizing this future, enabling a sustainable version will require photonics technologies.

Depressed cladding waveguides, capable of guiding both transverse magnetic and transverse electric polarizations, offer significant benefits for efficient frequency conversion and high power usage due to their large mode field. However, strong birefringence in materials like LiNbO3 poses challenges in the z-cut orientation. We report on the successful fabrication of these waveguides in birefringent nonlinear optical crystals using tightly focused ultrashort laser pulses. Experimental investigations reveal their potential in nonlinear optical processes such as second harmonic generation, with the flexibility to adjust the core size for specific applications.

β-Barium Borate (β-BBO) is crucial for generating UV light and advancing nonlinear optical processes. Transitioning from bulk crystals to waveguides enhances efficiency and capabilities. In this study, we present the first successful fabrication of laser-written depressed-cladding waveguides in β-BBO which demonstrates good confinement for both transverse magnetic (TM) and transverse electric (TE) polarizations. These waveguides were created using ultrashort laser pulses, forming a low-index cladding around the core. This significant breakthrough is particularly important for nonlinear optical processes like second harmonic and sum frequency generation. The ability to adjust the core size enhances their suitability for specific applications, indicating substantial potential for efficient frequency conversion and high-power photonic applications.

We fabricate a 20mm periodical poled lithium niobate ridge waveguide for third harmonic generation in C band using the cascade single + single poling structure. Combine with a dual temperature control technique, both SHG and SFG can be optimized individually to have better THG output. The previous reported issues such as the SHG/SFG temperature mismatch due to the material uncertainty and the green absorption under high green output power can therefore be overcome. Temperature-optimized THG therefore can be achieved in one single waveguide unit from low to high power, we achieve >130%/W/W intrinsic THG conversion efficiency in low input power, and >350mW THG output in 520nm under <1W coupled 1560nm input, both the efficiency and the output power are the highest value in the single pass single waveguide configuration to the best of our knowledge.

This talk addresses the challenges and breakthroughs in creating sub-micron ferroelectric domain gratings, crucial for nonlinear optical devices generating counter-propagating photons. Traditional materials and poling techniques are inadequate for these novel optical interactions, limiting their use in classical and quantum applications. The presentation highlights a significant advancement in periodic poling techniques for KTP isomorphs, enabling the creation of bulk structures with domain sizes as small as 200 nm. This progress stems from the use of coercive-field gratings formed through ion exchange, which significantly influences domain formation. Our findings emphasize that sub-micron domain creation is controlled by the properties of the ion-exchanged region, independent of the poling period, paving the way for smaller and more complex domain-engineered devices.

Efficiently detecting wireless terahertz signals relies on developing chip-scale devices that provide high signal to noise ratios at these extended frequencies spanning from 82-400 GHz. We present a terahertz-speed electro-optic phase modulator based on thin film lithium niobate, which is compatible with a wide terahertz bandwidth and efficiently collects terahertz directly from free space. The key element of our approach is an on-chip antenna, which couples the free-space terahertz radiation to a millimeter-long transmission line, which supports the propagation of the terahertz mode. An optical waveguide placed in between the gold electrodes is used to deliver the optical probe to detect terahertz radiation via phase-matched terahertz-optical frequency conversion. In addition, the high reflectivity at the open-circuit end of the transmission line and antenna enables the realization of an on-chip cavity for the terahertz fields, enhancing the electro-optic modulation efficiency.

Nonlinear metasurfaces have recently emerged as promising candidates to enable nanoscale/flat nonlinear optics. Here, we discuss our latest work to develop more efficient nonlinear metamaterials by using high-Q-factor collective lattice excitations known as surface lattice resonances (SLRs). We also demonstrate how conversion efficiencies of metasurfaces can be further improved by embedding nonlinear metasurfaces into multipass cells. Superlinear dependence of second-harmonic generation (SHG) on the number of passes through the metasurface has been measured confirming succesful phasematching during the proof-of-principle experiment.

Interband transitions in asymmetric multi quantum well (MQW) semiconductor heterostructures allow for tailorable second order optical nonlinearities at telecom wavelengths. However, the nonlinear response requires the presence of an electric field with components polarized both parallel and perpendicular to the quantum wells due to quantum mechanical selection rules. To overcome this limitation, we utilize metasurfaces to achieve second harmonic generation at normal incidence in a III-V superlattice structure. We experimentally demonstrate polarization conversion and field enhancement of incident light into MQWs, enabling wavelength selective second harmonic generation at normal incidence.

We investigated symmetry-broken metal-insulator-metal (MIM) nanolaminated plasmonic nanoantenna arrays that enable broadband multiresonant enhancement of multiple upconversion emission pathways. By breaking the axial symmetry of these vertically stacked nanocavities, we facilitate efficient coupling between multiple plasmon modes and emergent multipolar magnetic modes, resulting in enhanced light absorption and optical field intensities across a broad wavelength range (1000-1600 nm). This innovative design allows for the simultaneous enhancement of multiple nonlinear optical processes, including second harmonic generation, third harmonic generation, and multiphoton photoluminescence, in a compact and tunable platform for potential nanophotonic and biophotonic applications.

Theoretical and experimental results demonstrating reversible and irreversible regimes of light-metasurface interactions will be described. I will describe how all-dielectric metasurfaces based on the materials with and without inversion symmetry can be used for the efficient generation of odd and even harmonics at modest laser intensities. The peculiar features of the non-perturbative regime of laser-matter interactions, where rapid electron-hole generation and large electron excursions in the laser field dominate, will be emphasized. For even higher laser intensities, I will demonstrate how laser-assisted material nanostructuring using femtosecond mid-infrared laser pulses can be used to generate high aspect-ratio features in pre-fabricated semiconductor microstructures. Finally, I will discuss how time-varying metasurfaces can be used to produce exotic optical effects such as retro-reflection.

Recent advances in nonlinear nanophotonics have enabled optical frequency conversion at energy scales remarkably close to the few-photon regime of quantum optics. A key technique in these efforts is dispersion engineering, allowing dynamical confinement of light into ultrashort pulses with high peak intensities. In this talk, we explore quantum effects expected to emerge in next-generation devices, with a focus on understanding how these phenomena interplay with the multimode physics of femtosecond pulses. Drawing inspiration from recent experiments in thin-film lithium-niobate nanophotonics, we present several numerical studies of broadband frequency conversion in chi(2)-nonlinear waveguides. Our analysis provides a detailed understanding of the dynamics and modal structure of quantum noise and entanglement, e.g., in parametric and supercontinuum generation. This talk also surveys some recent results we obtained for harnessing exotic quantum non-Gaussian states of light in ultrafast nonlinear nanophotonics. Proper engineering of these quantum effects may generate entirely new functionalities for nonlinear optics at and beyond the classical-quantum transition.

We introduce a novel method for measuring the geometric phase using the stimulated second harmonic generation (SSHG) method, which provides greater stability against dynamic phase fluctuations than traditional interferometric techniques. As we know, the SSHG efficiency depends on the relative phase between the seed and pump beams into the 2nd crystal for a double crystal single pass SSHG process. Here, we controlled this relative phase by the geometric phase components that are quarter wave-plate (Q), half waveplate (H), and quarter wave-plate(Q) placed at angles of +45˚, θ˚, and +45 ̊, respectively, with respect to the fast axis. Therefore, the geometric phase introduced by these components can be measured by observing the SSHG efficiency on rotating the H at the output directly, making this method a robust and reliable approach for applications in quantum optics and photonics.

Nonlinear optics is the workhorse for countless applications in classical and quantum optics, from optical bistability to single photon pair generation. However, the intrinsic weakness of optical nonlinearity has largely limited the efficiency of nonlinear frequency conversion. Here, motivated by recent advances in using non-Hermitian photonics to enable non-reciprocal light transport, we explore how the interplay between non-Hermiticity and optical nonlinearity leads to a fundamentally new regime of nonlinear frequency conversion. We describe how nonreciprocity in coupling between discrete frequency modes can be engineered to yield unidirectional energy flow towards the boundary mode of a frequency comb, closely resembling a non-Hermitian skin effect. Applying this mechanism to an IR comb with cascaded second-order nonlinearity, we demonstrate high (>85%) nonlinear conversion efficiency into the “skin” mode and high-power THz generation through enhancement of THz-creating nonlinear processes. We also show how these effects are robust to defects and disorder in the comb and can be harnessed to generate stable limit cycles and comb generation at IR and THz frequencies.

Robust frequency conversion and generation devices are essential for ultrafast and broadband light sources, multiphoton imaging, and the robust production of entangled photon pairs. However, the sensitivity of these processes to physical variations presents challenges in maintaining perfect phase-matching, thus resulting in a lack of robustness. In my talk, I will introduce the detuning modulated composite design scheme that enhances the efficiency and stability of frequency conversion and generation processes. The composite method, inspired by nuclear magnetic resonance and quantum optics, addresses the inherent vulnerabilities of traditional systems, offering a more reliable and robust solution for both bulk optics and integrated photonics. I will highlight our recent experimental demonstration of a new family of designs for enhancing Spontaneous Parametric Down Conversion (SPDC), showing over a 10-fold improvement in robustness to temperature and angle-of-incidence variations. The composite methods in nonlinear optics promise to advance applications in nonlinear photonics and quantum technologies.

Waveguides with second-order nonlinearities can realize an incredible variety of ultrafast behaviors by combining dispersive effects with multi-wave interactions. These systems have many degrees of freedom, including the phase-mismatch and group-velocity mismatch of the interacting waves. In this talk we discuss recently developed approaches for generating broadband coherent light in nonlinear nanowaveguides by engineering the dispersion relations of the interacting waves. We first discuss supercontinuum generation by group-velocity-matched second-harmonic generation. Here, spectral broadening occurs due to an interplay between phase-mismatch and pump depletion to form femtosecond amplitude modulations on the interacting fundamental and second harmonic. This approach has been used to realize octaves of bandwidth using less than 10 picojoules of in-coupled fundamental. In the second part of this talk, we discuss an approach to parametric down-conversion that relies on an interplay between parametric gain and temporal walk-off to realize arbitrarily long interaction lengths. This approach enables large (> 50 dB) parametric gains with picojoules of pump pulse energy.

We introduce sidewall poled lithium niobate (SPLN) waveguides for efficient second harmonic generation, achieving milliwatt-level on-chip UV powers. The new pole-after-etch method ensures full poling of the entire ridge by reaching the poling fingers over the waveguide sidewall. The presented geometry requires a low poling voltage and allows for a reproducibly obtained 50% duty cycle. The waveguides show low propagation loss at both the fundamental and second harmonic wavelengths and a high absolute conversion efficiency of 18.5% and maximum on-chip UV power of 1.4 mW at 388 nm, paving the way for integrated UV photonics in thin-film lithium niobate.

Thin-film lithium niobate (TFLN) is an exciting platform for nonlinear frequency conversion. To implement quasi phase matching (QPM), fabrication of gratings with minimal period variation (<20nm) and control of ferroelectric domain inversion at the micron-scale along centimeter-long waveguides are necessary. Second harmonic generation microscopy (SHM) is a powerful tool for optimizing domain engineering (E-field poling). Here, we demonstrate that SHM images of the ferroelectric domains can be used to predict a waveguide's nonlinear conversion efficiency. We also show a statistical subsampling scheme that enables characterization of 5.6mm-long waveguides in approximately 5 minutes (30 seconds per field x 10 fields per waveguide). Each field is 100 microns in length, so our results indicate that sampling ~20% of the QPM grating is sufficient to predict SHG efficiency accurately. By alternating between high voltage probing and SHM imaging, we rapidly optimized poling parameters and fabricated near-ideal (50% duty cycle, ~5% variation in period) QPM gratings in long TFLN waveguides. Overall, SHM is an invaluable tool for scalable characterization of TFLN frequency converters.

There is growing interest in photonic integrated circuits (PICs) using thin-film lithium niobate on insulator (LNOI). This approach, however, involves adhering bulk LN to a substrate, which limits process integration flexibility. To overcome this, we developed RF sputtering LN thin film deposition process. On a sapphire substrate, LN thin films (~600 nm thick, (006)) were deposited at 450℃. XRD showed omega peak FWHM of 0.6°. The refractive indices were nₒ = 2.218 and nₑ = 2.129 at 1550 nm, which are equivalent to bulk LN. Very low extinction coefficient <10^-5 and low surface roughness of RMS 2.0 nm leads to low propagation loss of 0.6 dB/cm at 635 nm. These results indicate high-quality nonlinear optical film. On a SiO2/Si substrate, (006) oriented ~400 nm thick LN film was also formed. This process expands the application of LN thin film deposition for high-density, cost-effective PICs fabrication.

Doping the plus and minus c faces of a periodically-poled waveguide of lithium niobate with some ppm of transition metals or covering the waveguide with a suitable layer may provide a conductivity sufficient to short-circuit light-induced space charge fields originating from the bulk photovoltaic effect and this without sacrificing low propagation losses. Hence unwanted light-induced refractive index changes can be prevented, helping to increase the light powers up to which frequency conversion happens.

We report nonlinear broadening and pulse compression in a single multipass cell based on dielectric mirrors. The 90 fs and 200 µJ pulses from industrial-grade Pharos UP laser were compressed to 8.5 fs with 8 W average power and 80 % efficiency.

We experimentally demonstrate chirp transfer from NIR ultrashort pulses to UV through four-wave-mixing (FWM) in gas-filled hollow-capillary-fiber (HCF) . Femtosecond pulses from Yb:KGW laser fundamental radiation (ω) and its 2nd harmonic (2 ω) are used to generate an idler UV pulse of the frequency at 3ω by the FWM process 3ω=2ω+2ω-ω. Chirp of the NIR pulses can be transferred to the UV pulses with the opposite sign. The conversion is over 13% for the FWM process under the phase-matching gas pressure when the HCF is filled with argon gas. Along with experimental results, we present principles and numerical calculations for this frequency mixing and pulse shaping regime to enable applications in indirect chirp control and spectral modification.

We report a new set of the Sellmeier equations for AgGaSe2 and AgGaTe2 that provide the phase-matching properties of the mixed AgGa(Se1-xTex)2 (x = 0-0.2) crystals for type-1 second-harmonic generation (SHG) of a CO2 laser at 9.2714 and 10.5910 μm. In addition, the SHG conversion efficiencies of these crystals under the 90° phase-matching conditions are examined by using the literature values of nonlinear coefficients of AgGaSe2 and AgGaTe2.

This paper reports on the Sellmeier and thermo-optic dispersion formulas for Hg0.51Cd0.49Ga2S4 that provide a good reproduction of the temperature-dependent phase-matching conditions for SHG and SFG of a Nd:YAG laser pumped KTP and HgGa2S4 OPOs and a CO2 laser in the 0.8966-10.5910 μm spectral range that were measured with a 3 mm thick, θ=87.6°(φ＝45°) cut sample (K.Kato et al, Proc. SPIE 860411/1-7,2013, Opt Commun. 386,49-52,2017). In addition, a set of these two formulas reproduces well the temperature-dependent phase-matching conditions for a Nd:YAG laser-pumped OPG and OPA in the 4.14-13.0 μm spectral range at 20 and 100 ℃.

In the current study, we demonstrate a pulsed cascaded Raman fiber laser (CRFL) widely tunable in both the second near infrared (NIR-II) (1060-1600nm) and visible window (530-600nm, limited currently only by crystal availability). We achieved wide wavelength tunability of the CRFL by using an in-house built passively Q-switched tunable Yb-doped fiber laser as pump. The Yb-pump is followed by a large-mode area fiber amplifier for pulse-energy scaling and a Raman module for efficient Raman conversion. Subsequently, the NIR CRFL is used as a pump for generating visible light in a second-harmonic generation module, implementing a type-I cut LBO crystal. The CRFL operates at tunable high repetition rate variable from 20kHz to 80kHz. Pulse-duration can also be changed in the range from 40ns to 200ns, by minor modifications. Source is packaged for implementation in a wide range of biomedical imaging applications and hyperspectral photoacoustic imaging of lipid is demonstrated.

We report nanoindentation measurements of Hydride Vapor Phase Epitaxy grown GaAs1-xPx epilayers, demonstrating a linear trend in Young's modulus with increasing phosphorus content. The corresponding hardness trend shows significant bowing with a peak of 10.7 GPa occurring near 80% P-content. Some epilayers exhibit non-negligible spatial variation in their mechanical properties, which is investigated with complementary measurements. Photoluminescence measurements indicate high composition uniformity, while atomic force microscopy reveals a direct correlation between surface roughness and mechanical variability. Preliminary laser-induced damage threshold results are reported, and the implications for material optimization are discussed. With a 1070-nm continuous wave pump laser, GaAs0.75P0.25 and GaAs0.52P0.48 epilayers demonstrated thresholds of 600 kW/cm^2 and 300 kW/cm^2, respectively. This work supports the development of practical laser sources in the mid-infrared spectral region for applications such as remote sensing, IR countermeasures, and spectroscopy.

We present a few-cycle, carrier-envelope-phase stabilized, mid-infrared source that delivers sub-25 fs pulses at the central wavelength of 2 µm with energy exceeding 300 µJ at a 40 kHz repetition rate. This source is suitable as a driver for attosecond pulse generation and for applications in ultrafast time-resolved spectroscopy.

Femtosecond vortex beams with orbital angular momentum were theoretically and experimentally investigated during non-linear inscription in the low-density polymer, poly (methyl pentene). Paraxial mode of vortex beam in the far field is analytically solved by a complex function while the spiral phase front leading to longitudinal field component at high NA was also studied. This work provides the scenario for further investigation of the angular momentum transfer of the spiral light field to the material.

We propose a novel all-optical approach to generating a space-time wavepacket in a multimode slab waveguide through the multilevel inter-band stimulated Brillouin scattering. Two pumps along with a signal source are incident into the slab waveguide. The pumps excite an acoustic mode via electrostriction. The phonons then interact with the signal mode causing an indirect inter-band photonic transition. The process results in a space-time wavepacket that is propagate-invariant in the waveguide.

We numerically model and experimentally demonstrate the Raman based amplification of optical signals in a mode-division multiplexed multimode optical fiber. We used mode multiplexers and demultiplexers based on the multi-plane light conversion technology, and an 8 km span of low-differential mode group delay specialty graded-index fiber supporting 15 modes. Inter-modal stimulated Raman scattering between signal and pump beams, coupled in distinct modes of the multimode optical fiber permits to fully compensate for transmission fiber losses.

In this presentation, we introduce FusionNet, a multi-modality deep learning network tailored to integrate and analyze temporal, frequency, and experimental parameters for predicting and understanding ultrafast nonlinear phenomena. Specifically, the model is benchmarked in gas-filled hollow-core fibers (HCFs), which serves as an ideal test that exploits unique nonlinear interactions – four-wave mixing, self-phase modulation, and cross-phase modulation – and advanced light-guiding principles to enable groundbreaking experiments in ultrafast science. Our method achieves an overall reduction in error by 73% and improves computing time by 83% compared to standard deep learning methods. Our work accelerates parametric simulations and experimental setup designs, including high-precision spectroscopy, quantum transduction, and distributed entangled interconnects.

This study focuses on studying the nonlinear optical properties of tin-doped indium oxide (ITO) nanocrystals embedded in an aluminoborosilicate glass matrix using the Z-scan technique with a 532 nm, 550 ps laser. After a 10-hour heat treatment at 650°C, which facilitated the formation of 5-10 nm ITO nanocrystals, the resulting glass-ceramic exhibited notable nonlinear optical behavior, including third- and fifth-order nonlinearities, saturable absorption, and two-photon absorption (TPA). The effective nonlinear refraction n_2^eff was 72×10^(-16) 〖cm〗^2⁄(W (≈26×10^(-13) esu)), with a nonlinear absorption coefficient β=1.09 cm/GW. High-intensity exposures induced photo-induced changes, further enhancing the response.

The generation of coherent and narrow-band radiation sources in the short spectral regions of vacuum ultraviolet (VUV, λ = 100–200 nm) and extreme ultraviolet (XUV, λ < 100 nm) is a challenging endeavor that involves the third-order nonlinear susceptibility of the Mixing medium. In this paper, the experimental details and theory of the nonlinear process to generate tunable radiation in the VUV and XUV regions based on resonantly enhanced four-wave mixing in gaseous media are presented. The optimal experimental components and modes of operation of the setup will be shown and discussed. Moreover, the theory of four-wave mixing processes latitudes and limitations in gaseous media, i.e. phase matching conditions, gaseous medium refractive index rapid oscillatory behavior on resonance, quantum efficiency of the mixing process, mixing zone, and beam focusing will also be presented.

We demonstrate that the idler linewidth of a nanosecond non-resonant optical parametric oscillator based on 3-mm thick and 50-mm long periodically-poled LiNbO3 (PPLN) can be effectively narrowed down to about 1 nm using low power (down to 5 mW) injection-seeding at the signal wavelength. Utilizing the full available pump power at 1064 nm we obtained a maximum average idler power of 3.18 W at 2377 nm (single pulse energy of 159 µJ at 20 kHz) for a seed level of 20 mW, corresponding to a quantum conversion efficiency of 39%. The idler pulse length was approximately 7.4 ns, and the M² propagation factor was 5 and 7 in the vertical and horizontal directions, respectively.

We report on the Sellmeier and thermo-optic dispersion formulas for ZnSiAs2 that provide a good reproduction of the temperature-dependent phase-matching properties for type-1 second-harmonic and sum-frequency generation of a Nd:YAG laser-pumped HgGa2S4 optical parametric oscillator (OPO) and a CO2 laser in the 2.5050 -10.5910 μm range. In addition, we report on the determination of the phase-matching conditions of a Ho:YLF laser (2.052 μm)-pumped OPO in the 9.53 -13.0 μm range by using these two dispersion formulas. We next simulated the phase-matching parameters of type-2 90-deg phase-matched OPO pumed by a Ho:YLF laser at 2.052 μm. This OPO scheme at λi = 9.53 μm gives deff = 73 pm/V, Δθext ‧L ^1/2 = 23.8 deg‧cm^1/2, and ΔT‧L = 178 ℃‧cm. It was found that these values of ZnSiAs2 are superior to type-1 and type-2 ZnGeP2 OPOs at wavelengths longer than λi = 9.53 μm. Thus, type-2 ZnSiAs2 OPO or OPG is preferable to type-1 ZnGeP2 OPO or OPG when required the narrow spectral bandwidth.

Mid-infrared photothermal microscopy is an emerging vibrational chemical imaging modality that merges the high sensitivity of mid-infrared absorption with the high spatial resolution of visible imaging. Recently, we have developed wide-field mid-infrared photothermal microscopes utilizing quantitative phase imaging techniques. Our latest system offers imaging speed beyond video rate with optimal nanosecond lasers and a high-full-well capacity image sensor. We have also demonstrated a high spatial resolution system and paved the way for mid-infrared wide-field nanoscopy.

In a compact cavity, we use a CSP OPO pumped by a Ho:LLF laser (2.06 μm) to achieve more than 8 W of mid-IR power. We compare the performance of the CSP non-linear crystal to commonly used ZGP for mid-IR applications.

We propose and demonstrate a novel scheme called the advanced dual-chirped optical parametric amplification (DC-OPA) for amplifying a single-cycle mid-infrared (MIR) pulse. Based on our novel scheme, where two kinds of nonlinear crystals (Bismuth Triborate and MgO doped Lithium Niobate) are employed for DC-OPA, over one-octave bandwidth from 1.4 um to 3.1 um, carrier-to-envelope phase stable MIR laser pulses are achieved with an output pulse energy of 60 mJ. The compressed pulse duration is down to 8.5 fs, corresponding to 1.05 cycles with a center wavelength of 2.45 um and peak power of multi-TW. As far as we know, the obtained pulse energy and peak power represent the highest values for single-cycle MIR optical parametric amplification. Moreover, thanks to the energy scalability of the advanced DC-OPA scheme, the prospects of the multi-TW sub-cycle laser pulses will be discussed.

During LEMON project (Lidar Emitter and Multi-species greenhouse gases Observation iNstrument - European Union’s Horizon 2020 research and innovation program 821868), we developed and tested OPO and OPA setups, in order to provide advances for multi-species differential absorption Lidar (DIAL) emitters. The targeted gas species and ad hoc wavelengths were H2O and its isotope HDO at 1982 nm, CO2 at 2051 nm, and CH4 at 2290 nm, with the prospect of demonstrating the possibility to emit the required energies (~30 mJ levels) for future spaceborne integrated-path DIAL systems. This was enabled by the use of high aperture periodically poled KTP crystals (PPKTP), specifically developed for the project, which were also radiation tested.

Hybridized parametric amplification (HPA), a recent method to achieve high-efficiency parametric amplification by suppressing back-conversion with idler second harmonic generation, has been demonstrated so far only by birefringent phase matching. In this work, we demonstrate HPA by quasi-phase matching (QPM) in quasiperiodic-poled lithium niobate. QPM greatly expands the tuning range of HPA, while avoiding the limitations imposed by spatial walkoff of noncollinear beams. Since HPA has two concurrent processes, QPM requires a quasiperiodic grating with longitudinal spatial frequency peaks at both the parametric amplification and idler second harmonic generation phase mismatches. We investigate several grating designs for high-efficiency HPA in the near-infrared range. Quantum efficiencies of 70-80% are possible for devices pumped at 1030 nm, for both femtosecond and picosecond duration regimes. Pulse energies over 1 mJ can be accommodated using a chirped pump. However, devices must be designed carefully around the propagation effects of temporal walk-off and self-phase modulation.

The availability of single-cycle pulses at short-wavelength infrared (SWIR) and megahertz repetition rates is crucial for advancing attosecond physics, real-time field-resolved molecular sensing, and femtosecond fieldoscopy. Seeded Kerr instability amplification (KIA) in solid has been exploited to directly amplify ultrashort pulses in the visible and near-infrared spectral range however with an inherent angular chirp of the amplified seed. It has been suggested that seeded KIA at SWIR offers a broadband, high-gain amplification near the pump wavelength at collinear geometry, overcoming the limitations imposed by the angular chirp of the amplified beam. In this work, we demonstrate the octave-spanning KIA amplification of ultrashort pulses at megahertz repetition rates in a flowing thin liquid acetone film. We directly measure the electric field of the newly generated photons via various processes due to the high third-order nonlinearity of acetone. In addition to the KIA of the seed pulses, we observe the enhancement of the transmitted spectroscopic response of the sample

Nonlinear optical processes in germanium (Ge), zinc selenide (ZnSe), and zinc sulfide (ZnS) were investigated using ultrafast laser pulses with a duration of approximately 170 fs. Observations include blue-shifted odd harmonics in Ge, and 2- and 3-photon mixing in ZnSe and ZnS (ωvis ± ωMIR for SFG and DFG, ωvis ± 2ωMIR). These findings demonstrate significant advancements in nonlinear frequency generation and conversion, with potential applications in spectroscopy, imaging, and industrial processing. This research is crucial for developing advanced optical devices, enhancing optical parametric chirped pulse amplification (OPCPA), and contributing to new laser technologies.

We measured the spectral flux of the Class 5 Moonlander high harmonic source for two driver systems using similar pulse parameters but different central wavelength: a solid state multipass cell compressed Yb laser and an optical parametric amplifier system. Both laser systems provide 17 W average power at 100 kHz repetition rate at similar pulse durations, 29 fs vs 20 fs FWHM. Main difference is the central wavelength of 1030 nm for the MPC system versus the OPCPA system with a central wavelength of 800 nm. We performed high harmonic generation in with both laser systems in Argon and Krypton gas media producing broad band XUV radiation ranging from 20 to 60 eV photon energy. Using a spectrometer and an XUV diode the spectral photon flux after filtering was determined. For both systems we could reach state of the art performance with photon flux of 10^11 photons/s/eV to 10^13 photons/s/eV after filtering at the output of the light source. We will discuss the advantages and disadvantages of the different drivers for the high harmonic generation.

Next-generation nano and quantum devices have increasingly complex 3D structure. As their dimensions shrink, their performance is often governed by interface quality or precise chemical, interfacial or dopant composition. However, directly probing functional properties at high spatial and temporal resolution is challenging. High harmonic upconversion of femtosecond lasers provides an exquisite source of coherent short wavelength light, with unprecedented control over the spectral, temporal, polarization and orbital angular momentum (OAM) of the emitted waveforms, from the UV to the keV photon energy region. These advances are providing powerful new tools for near-perfect (diffraction limited) functional imaging, and for engineering the illumination to achieve high-fidelity imaging. This talk will review recent advances in generating bright high harmonic beams, and the unique capabilities they provide for measuring the structural, transport, and electronic properties of hard and soft materials.

Wavelength-tuneable ultrafast laser pulses in the vacuum ultraviolet (VUV, 100-200 nm) are an important tool for next-generation ultrafast science and technology, as nearly all materials and chemical compounds exhibit strong electronic absorption resonances in this spectral region. Resonant dispersive wave (RDW) emission in gas-filled hollow-core fibres is a promising avenue to overcoming the limitations of both existing (excimer-based) laser sources and conventional nonlinear frequency-conversion methods. Microjoule-scale VUV pulses with few-femtosecond duration and near-perfect beam quality can be efficiently generated by driving RDW emission in simple gas-filled hollow capillaries. However, this has so far required a high-energy titanium-doped sapphire laser as the primary laser source. This limits the pulse repetition rate (and hence average power) and forms an important barrier to applications outside of specialised laboratories. Here, we demonstrate tuneable VUV generation using a high-power ytterbium-based laser, obtaining broadband pulses tuneable between 145 nm and 300 nm central wavelength.

In previous studies of spin-to-orbital angular momentum (AM) conversion in laser high harmonic generation (HHG) using a plasma target, one unit of spin AM is always converted into precisely one unit of OAM. In this paper, we show, through analytic theory and numerical simulations, that we can exchange one unit of SAM for a tuneable amount of OAM per harmonic step, via the use of a structured plasma target. In the process, we introduce a novel framework to study laser harmonic generation via recasting it as a beat wave process. This framework enables us to easily calculate and visualise harmonic progressions and to provide new explanations for existing HHG results. Our framework also includes a specific way to analyse simultaneously the frequency, spin and OAM content of the harmonic radiation which provides enhanced insight into this process. We will present the results of simulations of laser pulses interacting with targets having either a structured reflective surface or a structured aperture, analyse the harmonic spectra from the laser-target interactions and demonstrate frequency combs with controllable line spacing.

We report on the status of high average power, sub-ps pulse compression of Yb lasers in the range from 80W up to 300W. Spectral broadening and compression down to sub 8fs using two cascaded hollow-core fibers (HCFs) is achieved with >60% efficiency. As a subsequent application of the broadband continuum, we exploit its spectral content to derive new wavelengths. Employing sum frequency generation and difference frequency generation, respectively, we achieve a spectral extension ranging from the UV (280 nm) up to the for IR (11,000 nm).

We report on ultra-flat supercontinuum generation spanning from the deep ultraviolet to infrared based on anti-resonant hollow-core fibres filled with molecular gases. We utilise the comb-to-continuum supercontinuum technique which initially establishes a vibrational frequency comb and then merges the comb lines to form a continuous flat spectrum. We demonstrate high-power scalability, achieving spectral power densities reaching up to 10 mW/nm in the ultraviolet-visible spectral region at repetition rates exceeding 1 MHz. We have investigated many different molecular gases, pump pulse parameters (duration, wavelength) and fibre structures, and will report on optimised routes to ultimate supercontinuum performance.

Integration of on-chip supercontinuum (SC) sources benefits optical clocks, gas spectroscopy, and coherent tomography. Aluminum nitride (AlN), with a 6.2 eV band gap and transparency down to 200 nm, boasts both second- and third-order nonlinearities ideally enabling on-chip f-to-2f interferometry. By improving the quality of AlN epilayers grown on sapphire via metalorganic vapor-phase epitaxy and optimizing the waveguide fabrication, we reduced propagation losses below 1 dB/cm at 1550 nm. This enabled the generation of a gap-free SC spectrum ranging from the visible to MIR, including UV components from cascaded X(2) processes, by pumping with a 400 pJ telecom femtosecond laser. Dispersion engineering in waveguides allows precise tuning of dispersive waves, optimizing conditions for efficient second harmonic generation and f-to-2f interference.

Ultrashort light sources in the mid-infrared range are highly beneficial for applications such as gas molecular spectroscopy, remote sensing, atmospheric science, medical procedures, and various free-space light-matter interaction studies. Ultrafast lasers utilizing chromium-doped ZnS/Se (Cr:ZnS/Se) have already proven to be robust and stable solutions within this spectral region. Spectral broadening is fundamentally important for achieving single-cycle pulses, as it pushes the pulse duration limits imposed by the uncertainty principle. In this study, we demonstrate supercontinuum generation in several bulk materials, including InP, Si, GaN, GaAs, Diamond, PbMoO4, and TiO2, using radiation with up to 4W average power centered at 2.35 µm from a Cr:ZnS femtosecond MOPA system. This results in a simple and stable system for generating single-cycle pulses.

Broadband supercontinuum generation (SCG) with high peak-power ultra-short laser pulses has been widely applicable to fields such as spectroscopy and ultrafast dynamics studies. Laser-induced damage and ablation threshold (LIDT/LIAT) intensities of transparent optical materials are usually only slightly higher than those required for SCG, and materials are often damaged during SCG. In this work, we investigate how SCG in solids changes as intensities surpass the LIAT. We experimentally observed that for incident pulses above the LIAT, surface damage craters deflect some of the incident light, and the deflected light has a broadened spectrum indicating it undergoes SCG. We present progress towards the development of a model simulation that couples a finite-difference time-domain (FDTD) full-wave solver to our Bi-directional, Three-dimensional, Ultrafast Laser (BiTUL) nonlinear propagation code. Preliminary results suggest that deflected light undergoes some amount of self-focusing, breaking up into distinct beamlets in addition to SCG.

We will present a versatile design of a tunable high-power OPCPA system operating in visible and near-IR spectral regions. The tunability range can be further extended to 200 nm (6 eV) in the deep UV and 1100 nm in IR by adding harmonic generation or nonlinear spectral broadening modules. Combining different compression schemes, we achieve pulse duration below 50 fs within most of the tunability range. The design can be adapted to different femto- and pico-second Yb-based pump-laser architectures, including but not limited to InnoSlab, thin-disk and bulk-crystal regenerative amplifiers.

Laser induced damage threshold (LIDT) of recently grown BaGa4Se7 plates were measured at 1064 nm, 1550 nm and at 9569 nm using nanosecond duration laser pulses with different exposure durations. The dependence of the LIDT values on the laser spot size was explored. At 1064 nm, closest to the two-photon absorption edge, the LIDT value of BaGa4Se7 for a laser beam with Gaussian diameter of 216 "μm" was found to be about 26 J/cm^2 for single shot exposure. This value is about five times higher than that of AgGaSe2, which is one of the state-of-the art commercial materials with similar transparency range.

We combine data mining of known organic materials from the Cambridge Structural Database with DFT calculation of key molecular properties to identify new candidate organic materials for intense terahertz (THz) generation and electro-optic detection. We then validate our combined data mining and computational approach to materials discovery by synthesizing and characterizing the THz generation capability and the potential to be used as electro-optic detection crystals for new THz generating organic materials via electro-optic sampling.

We present a systematic study of the nonlinear optical properties of bulk materials relevant to the design of next-generation ultrashort-pulse, high-peak-power, long-wave infrared lasers. Specifically, we explore post-amplification spectrum broadening via self-phase modulation to overcome the bandwidth limit imposed by the gain spectrum of high-pressure, mixed-isotope CO2 active medium. This effect underpins pulse post-compression, enabling unprecedented sub-picosecond pulse durations at multi-terawatt peak power near 10 microns with our unique gain medium. We discuss single-shot measurement techniques for characterizing nonlinear refraction, nonlinear absorption, and laser-induced damage thresholds in various materials. Additionally, we detail the inclusion of these results in the RefractiveIndex.INFO database and the design of an innovative bulk-material long-wave infrared post-compressor apparatus.

Novel barium gallium selenide (BGSe) and barium gallium germanium selenide (BGGSe) crystals show promising nonlinear properties and appear an attractive alternative to silver gallium sulfide (AGS), as AGS suffers from a well-known photodarkening effect, which degrades efficiency over time. In this work, we present efficient nonlinear conversion in the mid-IR range spanning from 2.3 to 10 µm and a similar photodarkening effect occurring in both BGSe and BGGSe nonlinear crystals.

BaGa4Se7 (BGSe) is an attractive nonlinear optical (NLO) crystal notable for a rare combination of wide band gap (2.64 eV), long phonon cut-off wavelength (18 um), and relative ease of growth from stoichiometric melts, making it ideal for shifting widely-available 1-micron laser sources deep into the mid-IR and long-IR. Here we demonstrate issues with crystal growth via Horizontal Gradient Freeze (HGF) growth of BGSe, and rectification of the issues through improved synthesis methods which are required for this compound. We also measured the laser damage threshold of BGSe samples at 1550 nm and compared to other NLO crystals.

CdSiP2 (CSP) is a nonlinear optical crystal developed as a wider-band-gap analog of ZnGeP2 (ZGP) to enable mid-infrared generation. Recent advances in crystal growth from stoichiometric melts using the horizontal gradient freeze (HGF) technique have resulted in the fabrication of phase match oriented parts with interaction lengths as long as 23 mm. CSP crystals of different lengths and cut from different bulk grown boules were compared using a Q-switched Tm:YAP pump laser operating at 1.94 microns. A comparison of thermal lensing behavior of several CSP crystals pumped at 1.94 microns is also presented.

Here we present our most recent results on the dual-comb spectroscopy using the electro-optic sampling technique. As a pump source, we use a pair of mutually-coherent Cr:ZnS frequency combs centered at 2.35 μm. One of the combs is frequency downconverted via optical rectification to produce a molecular ‘sensing’ comb. The second driving comb is frequency doubled to produce a short probe pulse. In addition to the unprecedented spectral coverage and the absolute frequency referencing, a low intensity and phase noise of our dual-comb system allowed to reveal all the beauty of this technique such as sub-Doppler resolution, high dynamic range, and a video-rate acquisition speed. Of special interest is the ability of our system to measure THz and mid-IR spectra simultaneously. As a demo experiment, we performed measurement of rovibrational (at 670–1000cm-1) and rotational (at 85–120cm-1) spectra of ammonia molecule with about 10-MHz spectral resolution.

We have demonstrated high-resolution dual-comb spectroscopy in the UV spectral ranges: 372 - 412 nm and 324 - 338 nm using high harmonic generation (6th and 7th harmonics) in multi-grating periodically-poled lithium niobate (PPLN) for up-conversion of the driving dual-comb Cr:ZnS laser system with a center wavelength 2350 nm. The achieved spectral coverage in the UV bands corresponds to almost a million of spectrally resolved comb modes for the 6th harmonics and ~ 500,000 modes for the 7th harmonics. Even without comb tuning the mode spacing of 80 MHz (or 30-40 fm) corresponds to an exceptional revolving power of 10^7, which is well beyond the resolution of any traditional UV spectrometer. Also, referencing of all frequencies in laser stabilization and data acquisition systems to Rb clock provides an absolute spectral accuracy. As a demo experiment, we have measured reflection and transmission spectra of a high-finesse volume Bragg grating (BragGrate™ Mirror from IPG/OptiGrate) with 95 pm (200 GHz) bandwidth. The technique can be used for high-resolution high-precision spectroscopic studies of neutral and ionized atoms and molecules in the UV spectral range.

We demonstrate dual-comb spectroscopy (DCS) in the deep ultraviolet (DUV). DCS at these wavelengths enables access to a large number of strong electronic transitions in neutral and ionized atoms and molecules. We make direct measurements of ionic and atomic transitions in a laser-produced plasma (LPP) and utilize Boltzmann and Saha-Boltzmann analysis to determine ionic and electron densities at low temperatures and at late times in the plasma evolution. The results provide new insights into low temperature plasma properties. We present results on accessing the DUV using a high average power dual-comb Yb laser system combined with BBO nonlinear crystals for frequency quadrupling, as well as latest results at vacuum-ultraviolet wavelengths (below 200nm) utilizing intra-cavity high harmonic generation (iHHG) with femtosecond enhancement cavities (sEC's).

We investigate the noise correlation properties in the repetition rates (f_rep) and the carrier envelope offset frequencies (f_ceo) of the comb pairs emitted from two low-noise Yb:CaF2 single-cavity dual-comb lasers. For a 160 MHz repetition rate polarization multiplex laser we find a high correlation for the noise in f_rep and f_cep of the two combs yielding to a 20 dB reduction in the differential quantities \Delta f_rep and \Delta f_ceo. Due to the mostly anticorrelated nature of the noise in \Delta f_rep and \Delta f_ceo a reduction of the RF comb line width in a heterodyne beat of the combs is observed. Here, we extend this study to a newly developed spatially multiplexed cavity with a transmissive biprism operating at 1 GHz repetition rate and ultra-low noise performance and compare the two multiplexing techniques. Our findings are important for practical dual-comb interferometry with spectral interleaving.

We developed a high-power (3 W), few cycle (<20 fs), fully referenced (timing jitter < 0.1 fs) optical frequency comb source at the central wavelength 2.4 µm. The comb is based on the ultrafast polycrystalline Cr:ZnS laser platform. The comb features a unique combination of few-cycle pulses, multi-Watt power with ultra-low intensity noise (<0.1% RMS). The comb’s servo system allows for robust phase-locking to any standard narrowband reference laser with the timing jitter <0.1 fs. Using readily available nonlinear crystals, the comb can be up and down-converted to any part of the electromagnetic spectrum from the THz to PHz. High performance of the source is confirmed by the demonstrations of dual comb spectroscopy resolving millions of comb modes in the IR and UV regimes.

Optical Frequency Comb (FC) synthesizers are a key tool in frequency metrology, providing precise frequency references for lasers within their spectral range. Extending these capabilities to the terahertz (THz) domain (0.1-10 THz) marks a significant advancement for high-precision molecular spectroscopy, as this is a fingerprint region for many molecules. This study explores the use of optical rectification in Lithium Niobate (LiNbO3) crystals to generate free-space THz FCs, suitable for phase stabilization of single-wavelength and comb THz QCLs and high-precision spectroscopy. We introduce a novel THz FC generated via optical rectification in a Cherenkov configuration within LiNbO3 ridge waveguides, mechanically etched on the surface of the crystal plate. The ridge waveguide improves light confinement and, coupling the pump laser to waveguides with different transverse dimensions (5 to 8 μm), allows to assess the dependance of the bandwidth of the generated THz radiation from this parameter.

Pulsed laser operation by 3rd-order nonlinearities of saturable absorbers is a well-known technique called passive Q-switching or mode-locking. Recently, low-dimensional carbon nanostructures such as graphene and carbon nanotubes have been proposed as efficient passive nonlinear switching materials. They possess superior optical properties including intrinsic ultrabroadband absorption, high nonlinearity, and relatively simple device fabrication. The nonlinear absorption of nanocarbons has been widely investigated to successfully develop various pulsed solid-state and fiber lasers in a broad wavelength range from visible to mid-infrared. Especially for compact cavities with short round-trip time and low critical mode-locking pulse energy, carbon nanomaterials are highly advantageous thanks to ultrashort recovery times, finely controllable nonlinearity with low non-saturable loss, and flexible integration types. This talk discusses recent advances in miniaturized pulsed lasers demonstrating diverse pulsed operation regimes by unique integration of solid-state active waveguides and nanocarbon-based nonlinear photonic devices.

Four-wave mixing (FWM) in multimode fibers facilitates frequency conversion to wavelengths outside of conventional laser spectral bands with increased diversity in available pathways. In particular, FWM with stably guided orbital angular momentum (OAM) carrying fiber modes provides thousands of combinations for momentum/phase matched intermodal FWM processes without relying on material dispersion and allows for efficient transfer of ~kW peak power pulses across the near-infrared. Because FWM requires energy conservation, however, the transparency of silica glass limits the shortest possible output wavelength to ~700 nm, making these systems ill-suited to applications requiring high power pulses at visible colors. To overcome this limitation, we implement a cascaded approach to FWM: the first stage converts high power 1064 nm pulses to 814 nm. Using an 8f imaging system, we launch 11 kW pulses of 814 nm light into a second fiber to act as a pump for the second stage. Characterization of the secondary process shows spontaneously-generated red light at 680 nm which can be later seeded by its corresponding Stokes wave (1013 nm) to generate a high power, all-fiber visible laser source.

We enhanced four-wave mixing (FWM) in a photonic crystal fiber (PCF) by amplifying the FWM pump pulses within a newly designed PCF with a Yb-doped core. Utilizing a 1064 nm fiber laser for the FWM pump pulses alongside a 976 nm CW laser diode for exciting the Yb-doped core, we demonstrated significant enhancement of the FWM process. For a 1064 nm peak power of 13.1 kW, an anti-Stokes sideband around 790 nm was generated with a peak power of 2.5 kW; adding the 976 nm light increased the anti-Stokes peak power to 4.5 kW, representing an 80% enhancement.

We report on the capability of the Frequency-resolved optical switching (FROSt) method for characterizing ultrashort visible pulses using silicon thin films. We demonstrate that FROSt can effectively characterize sub-20 fs visible pulses and those with energy densities less than 1 nJ/nm using 500 nm thick Si thin films. The findings suggest that FROSt has the potential to be a valuable technique for characterizing weak solid-state high harmonics. Also, this makes it an ideal pulse characterization method for controlling and optimizing ultrashort visible pulses used in ultrafast spectroscopy.

Spectral Beam Combining(SBC) enables high power sources for materials processing and defense. However, it requires multiple seed-lasers, increasing system complexity. De-multiplexed optical frequency combs are suitable replacements, but at 1𝜇m, lack of components for high repetition-rate comb generation and high-selectivity de-multiplexing limits its application. Here, we overcome these problems by using Brillouin de-interleaving on an amplified, lower repetition-rate comb. Dual combs with differing rep-rates between 9-13GHz were amplified and sent in counter-propagating directions in HNLF. By varying the rep-rates of the combs, through vernier effects in Brillouin amplification, amplified and deinterleaved combs were obtained at user-defined spacings from 32-48GHz, limited only by number of lines in original combs. De-interleaved rep-rates were finely tunable with <0.1GHz steps. De-interleaved combs could then be de-multiplexed with a simple grating-based setup providing lines in distinct fiber with powers of few mW. This source promises to be a compact seed-source for spectral beam combining.

We present a novel SHG scheme where the wavefront of the signal beam can be controlled via that of the pump beam in a non-collinear setup. A variable radius of curvature from 50mm to infinity at 515nm is obtained by combining two 1030nm laser beams in periodically-poled lithium niobate (PPLN). This is utilised using a Fourier optic technique with potential applications in near-to-eye displays.

While matching phase velocities in optical fibers is often sufficient for continuous-wave or long pulse length frequency conversion, it is critical to consider higher-order dispersion terms for ultrashort pulse conversion. In our previous works, we already demonstrated that fibers can be specially designed to achieve simultaneous modal phase and group velocity matching for both second and third harmonic generation. However, such designs often come with the penalty of being highly sensitive to geometrical parameters. As a solution to that problem, we propose implementing an effective phase matching strategy. The approach considers the cumulative effect of dispersion terms up to second order and does not enforce strict phase and group velocity matching, instead requiring that they jointly lead to effective phase matching. Here, we present a series of fiber designs and pump pulse specifications that enable effective phase matching for both second and third harmonic generation.