Nonlinear Frequency Generation and Conversion: Materials and Devices XXIII

In a recent study over 170 human breath samples, we combined optical frequency combs with machine learning to achieve excellent detection accuracy for COVID-19. We have since upgraded the detection capability by greatly expanding the spectral coverage which now covers molecules with C-H, N-H, O-H, C≡C and C≡N bonds. Along with excellent sensitivity, we can now detect a vastly expanded volume of chemical information from each breath sample. This next-generation comb breathalyzer is under test with new medical studies to understand how the diagnostic power can be further expanded and improved.

Ultrabroadband electro-optic sampling with few-cycle optical pulses is known to be an extremely sensitive technique to detect electric field amplitudes. Until now, there has been a trade-off between spectral resolution, spectral coverage, and acquisition speed using this technique. Here we demonstrate dual-comb spectroscopy (DCS) that feature: sub-Doppler resolution, ultra-broadband coverage simultaneously in the mid-IR – THz range, and up to video-rate acquisition speed. As a driving source, we use a pair of Cr:ZnS frequency combs. One comb is frequency downconverted via optical rectification, while the second comb is frequency doubled to produce a near-IR probe comb for electro-optic sampling (EOS).

Here we study the noise correlations of a birefringent crystal based single-cavity dual-comb laser operating at 160 MHz repetition rate. We characterize the temporal fluctuations of one RF comb line, the repetition-rate difference Δf_rep, the f_CEO of the individual combs, and their difference Δf_CEO. This is achieved by coupling both combs into a single f-2f interferometer and simultaneously measuring heterodyne beat-notes with cw lasers. We find that the dual-comb Δf_CEO fluctuations are 20-dB lower than those of f_CEO, indicating highly correlated combs. Furthermore, we show that Δf_CEO and Δf_rep fluctuations are almost fully anti-correlated, enabling narrow linewidth in free-running operation.

We present a novel detection scheme for high-sensitivity spectroscopy in the mid-infrared. We use a low-noise and low-complexity dual-comb source based on a PPLN OPO and an Yb:YAG pump. Both the laser and OPO are spatially-multiplexed single-cavity dual-comb sources. At a repetition rate of 250 MHz and ps-long pump pulses, high power per comb line of >120 W is achieved at 3000 nm (idler). The idler is tunable from 2700 nm to 5170 nm. The system enables comb-line-resolved dual-comb spectroscopy measurements in free-running operation. With our detection scheme, we achieve a spectral coefficient SNR/\sqrt\tau > 10000 \sqrt{Hz} (40 dB) at 3 µm.

We demonstrate that resonant phase-modulation of circular Quantum Cascade Laser cavities gives rise to a novel kind of frequency comb, that is remarkably stable, fully tunable and broadband. When the backscattering in such ring cavities is sufficiently low, unidirectional lasing in the free-running device yields single-mode emission. As soon as resonant RF injection is enabled, the spectrum continuously and predictably broadens to span up to 100 cm-1 with nearly-flatted topped spectra. The bandwidth of the resulting comb is fully governed by the depth of the modulation and reaches the fundamental limit dictated by dispersion.

Integrated chip-scale frequency combs have diverse applications in communication, frequency synthesis, optical ranging, and gas spectroscopy. In this study, we explore the formation of possible frequency comb in the terahertz domain, in a device consisting of a distributed feedback laser (DFB), a bus waveguide, and a ring quantum cascade laser (QCL) , all fabricated from the same GaAs/AlGaAs heterostructure. The DFB's optical modes were designed to match with those of the ring QCL, facilitating the injection of light from the DFB into the ring QCL through the bus. The experimental results revealed a comb-like broadening of the ring spectra upon the injection of light from the DFB laser. In the absence of light injection, the ring output was observed to be in a single-mode state.

We present our last numerical and experimental results on a mid-infrared source based on a tunable Yb-based hybrid MOPA pump and a backward wave optical parametric oscillators (BWOPO). The BWOPO has a record-low oscillation threshold of 19.2 MW/cm² and generates mJ-level output with an overall conversion efficiency exceeding 70%. The BWOPO acts a frequency shifter of the pump radiation toward the forward wave, maintaining the pump spectral properties. The demonstrated tuning range of 10 GHz is already compliant for DIAL applications. We have also developed advanced numerical modelling of the BWOPO taking into account spectral and, for the first time, spatial beam profiles.

We propose and demonstrate the experimental feasibility of a versatile geometric phase (GP) based mirror integrated in a doubly resonant optical parametric oscillator (DRO) as an output coupler. The GP mirror exploits the non-cyclic geometric phase acquired by beams in the Sagnac interferometer in presence of wave plates for easy dynamic control of the transmission across 0 - 100% in alignment free architecture. Using the GP-mirror as the output coupler of a continuous-wave, green-pumped, doubly-resonant optical parametric oscillator (DRO) based on a 30-mm-long MgO:sPPLT crystal, we have generated an output power of 2.45 W at an extraction efficiency as high as 49% when operated at optimum output coupling. The DRO shows a maximum pump depletion of 89% and delivers an optimum output power across a tuning range ≥90 nm. The novel concept can be used for any coherent sources tunable across different spectral regions and in all time-scales.

We demonstrate that thick (3-mm) periodically-poled LiNbO3 (PPLN) enables energy scaling of a non-resonant optical parametric oscillator (NRO) operated in the narrowband mode with a volume Bragg grating (VBG) at the signal wavelength. Utilizing the full available pump power at 1064 nm we obtained maximum average powers of 2.25 and 2.08 W for the signal (1822 nm) and idler (2383 nm) at 10 kHz, at a conversion efficiency of 32.8%, i.e. a two-fold increase in terms of pulse energies. The signal and idler linewidths were ⁓1 nm, the pulse lengths ⁓6 ns and the idler beam propagation factor ⁓5.

Many space applications (debris tracking for measuring velocity and distance simultaneously, landing on the Moon, differential absorption lidar for detection/quantification of greenhouse gases, sodium lidar for study Earth’s mesosphere, Doppler wind lidar for mapping Hurricane field, and so on) need single-frequency and high-energy pulsed lasers as their transmitters, but a conventional single-frequency/high-energy pulsed lasers consisting of continuous-wave master laser and Q-switched slave laser plus injection seeding device are complicated and difficult for air-/space-borne applications. Based on a Brewster-cut and prism-shaped acousto-optic Q-switch, a compact single-frequency/high-energy pulsed laser has been demonstrated in a in a unique (two-mirror) ring-cavity configuration which just consists of the gain medium and the Q-switch. The frequency of laser pulses is tunable. Two-mirror ring-cavity optical parametric oscillator (OPO) is demonstrating frequency conversion. Such a single-frequency pulsed laser/OPO is benefit for the lidar applications in SmallSats and CubeSats.

We present a compact-cavity, picosecond, mid-infrared optical parametric oscillator (OPO) employing a length of hollow-core-fiber (HCF) inside the cavity and operating at 1-MHz repetition rate for high pulse energy. Pumped by an ytterbium-doped fiber laser, the periodically-poled-lithium-niobate-based OPO generates output beam with tunable wavelengths ranging from 1.3 µm to 4.8 µm. The OPO provides 137-ps pulses with maximum energies of 10 µJ for signal output at 1.6 µm and 5 µJ for idler output at 3 µm, respectively. Output power performance with respect to the wavelength tunability and optimization of beam quality for the OPO are numerically and experimentally investigated.

Measurement of second harmonic generation efficiency of a 100 ns duration, 9.271 μm CO2 laser in orientation patterned GaAs (OPGaAs) crystal (3 mm x 7.5 mm x 39.7 mm), and in crystals of AgGaSe2, ZnGeP2 and CdGeAs2 in their largest dimensions currently available will be presented. At maximum fundamental beam fluence of 2.5 J/cm^2, 15 % conversion efficiency was achieved with the OPGaAs crystal, which was the highest among the materials studied in this work. All samples were at the ambient laboratory temperature of 23 C and the radius of the incident beam was 0.7 mm (HWe^(-1)M of intensity).

We report on our latest results in nonlinear frequency conversion based on the nonlinear material ZGP. The setup is based on a linear OPO cavity. To avoid residual absorption of the 2 µm absorption edge of ZGP, a Q-switched Tm3+:Ho3+-codoped fiber laser was developed emitting at 2.1 µm. Based on this pump source, mid-IR output powers up to 12.2 W were reached with pulse energies up to 271 µJ and a conversion efficiency of 43.4 %. This improves on the currently published fiber-laser-pumped power records by Lorenz et al. (2023) and Dalloz et al. (2019) by 50 %.

We report on a 58 W Ho3+:YAG laser used to pump a linear zinc germanium phosphide (ZGP) optical parametric oscillator (OPO). At 25 kHz repetition rate, 2.2 mJ, 20 ns Q-switched pulses are delivered resulting in a pulse peak power of 108 kW. The beam quality is near-diffraction-limited. The ZGP OPO is investigated in a conventional linear resonator and in a novel design. With this design, a total output power of 14.1 W is achieved and an improved beam quality of 2.1 and 3.3 (2.4 and 3.5) in the x- and y-axis of the signal (idler) beam compared to the conventional linear resonator is shown.

We report on the scaling of a polarization-maintaining MOPA at a signal wavelength of 2048 nm, designed for pumping an optical parametric oscillator (OPO). By utilizing the MOPA structure to design suitable OPO pump pulses the overall mid-IR conversion efficiency is enhanced enabling the scaling of the mid-IR average power. 60 W of average power is achieved and applied to pump different ZGP OPOs. The resonator designs are investigated and compared regarding scalability and beam quality.

The Advanced Research Project Agency-Energy (ARPA-E) has the mission to advance high-impact technologies that have the potential to transform the energy industry. Fusion energy sits in the highest risk part of the ARPA-E portfolio. Accelerating the development of enabling technologies for Inertial fusion energy (IFE) is a key focus of ARPA-E. ARPA-E intends to surgically target technologies, which will significantly reduce the time to market and engineering risk of any first of a kind commercial fusion power plant. I will provide a summary of existing ARPA-E IFE related programs and the vision for future programs.

Since the elucidation of total internal reflection by Colladon, Babinet and Tyndall over 150 years ago, bound states of light have been the primary means to transport light, whether on a chip or in optical fibers. We show that light carrying sufficiently high orbital angular momentum can create a centrifugal barrier for itself, thereby enabling guidance even in a regime where a mode is normally considered “cutoff.” We will discuss how this discovery, which has parallels with why binary stars don’t collapse into each other due to gravity, has applications in diverse areas, including classical and quantum communications and computing with high dimensionality, power scaling of fiber lasers by mitigating nonlinearities and nonlinear optics with greater degrees of freedom.

We are now working on building a real machine of optical quantum computers based on quantum teleportation technology. The main ingredients are 10THz-bandwidth waveguide optical parametric amplifiers, 100GHz-bandwidth 5G/6G technologies and Wavelength Division Multiplexing(WDM), and nonlinear feedforward. By using these ingredients, we will build 100GHz-clock 100-multicore super quantum computers.

Light projection displays play an increasingly important role in our modern life. Core projection systems including liquid crystal displays and digital micromirror devices can impose spatial light modulation and actively shape light waves. Recently, the advent of metasurfaces has revolutionized the design concepts in display technologies, enabling a new family of optical elements with exceptional degrees of freedom. In this talk, we will present examples of electrically-controlled metasurfaces for dynamic holographic displays. We will also outline the possibility to achieve programmability and addressability of optical metasurface devices at the single pixel level.

We have demonstrated entangled photon generation through spontaneous parametric down-conversion for the first time at visible pump wavelengths (406 nm) as well as second harmonic generation at ultraviolet wavelengths (350 nm) with periodically poled lithium niobate nanophotonic waveguides. Entangled photon pairs have the potential to replicate pulsed laser experiments, particularly in spectroscopy but also generally for wavelengths shorter than the common IR bands. Our sources are the first step towards an on-chip entangled photon-based spectrometer.

We have previously reported robust zinc-indiffused MgO:PPLN ridge waveguides for field applications in quantum-enhanced gravimetry and navigation, generating 2.5W of 780nm light at 74% second-harmonic generation (SHG) conversion efficiency. To tailor this process for different wavelengths and interactions, the effect of fabrication parameters on the waveguide mode shape and size from UV to MIR has been studied, with the aim to optimise mode matching between pump, SHG, and optical fibres to improve conversion efficiency, and reduce insertion loss in packaged devices.

In this work, a high-power laser source with single-mode fiber output in the green spectral range is presented. A ridge waveguide laser with a distributed Bragg-reflector (DBR-RWL) and a tapered amplifier (TPA) are used as a pump source for a ridge waveguide periodically poled lithium niobate (PPLN) crystal. The whole setup fits into a butterfly housing with a footprint of only 76 mm x 44 mm. At an emission wavelength of 532 nm, hundreds milliwatt optical output in a single spectral mode with a spectral width of the order of 1 MHz is achieved.

A 27mm special wide wavelength tuning range in a single MgO-doped periodically poled lithium niobate (PPLN) ridge waveguide is fabricated. Through the “ID algorithm” design technique, an engineered quasi phase matching (QPM) structure with 7 wavelength conversion peaks is designed. Each designed peak has equally nonlinear strength and 5nm wavelength separation, locates between 1528nm to 1558nm at 20 deg. C. Combine with temperature tuning up to 70 deg. C, which shifts the phase matching wavelength more than the designed 5nm spacing, full C band tuning range is achieved with high conversion efficiency. Under CW 5.4w single frequency fundamental input, 3.6w is coupled to the waveguide, and 2.4w SHG is generated.

We present our research on utilizing weak Bragg grating reflectors to assess the uniformity of zinc-doped lithium niobate ridge waveguides, aiming to optimize frequency conversion. These gratings are fabricated through ablation using a pulsed 213nm laser within a phase-controlled interferometric system, providing sub-nanometer period accuracy. By employing gratings we spectrally and spatially characterize the modal properties of our waveguides, enabling direct analysis of process variability. Through this analysis, we aim to gain a deeper understanding of the effective index variation in periodically poled lithium niobate (PPLN) waveguides, with the ultimate goal of reducing it and improving frequency conversion.

Semiconductor optical fibers offer a unique approach to marrying the needs of silicon photonics with nonlinear and infrared fibers optics. This talk will review fiber fabrication and optimization as well as the nonlinear performance of a myriad of fibers and applications. Challenges and future prospects are also discussed

We observe optical nonlinearity in a 1.2 m-long few-layer graphene-coated antiresonant hollow-core fiber (AR-HCF) at low optical power of 100 μW from a continuous wave laser operating at 976 nm. The few-layer graphene is uniformly coated on the inner surface of the AR-HCF core using a simple fill-and-bake method. The extended light-graphene interaction staged in the graphene-coated AR-HCF presents a new efficient platform for inducing χ(2)-based nonlinear optical phenomena.

Truly portable supercontinuum sources with high spectral bandwidths are poised to advance applications such as medical imaging, chemical sensing, or light detection and ranging. Yet, limited efficiencies of traditional supercontinuum generation schemes typically require relatively high pulse energies from bulky ultrafast lasers, while the so-called 1/e bandwidths are usually rather narrow. Based on sign-alternating dispersion in a silicon nitride waveguide, we present a scheme that drastically lowers input pulse energy requirements, while generating (1/e) bandwidths of up to 1000 nm. The required pulse energies are readily served by commercially available ultra-compact fiber-based femtosecond lasers – bringing handheld devices literally within reach.

Single-mode fibers with an absorptive lead-glass core and Pyrex glass cladding were fabricated using the rod-in-tube method. The initial purpose of fabricating these fibers was for their application in Faraday devices. However, when the fibers were incorporated into a polarimetric experiment, it was observed that measurement became impossible due to amplitude fluctuations occurring at frequencies of 200 kHz and 1.5 MHz. Several measurements indicated that this noise was associated with the fiber rather than the detector. It was found that the fiber has the ability to convert laser phase noise into amplitude noise. The behavior of the fiber in converting laser phase noise to amplitude noise can be classified as an "adaptive guided-wave phenomenon."

Interest in nonlinear optical (NLO) materials for mid-infrared frequency conversion has exploded in recent years largely due to the emergence of new ultrafast laser applications ranging from frequency-comb-based spectroscopy to high harmonic and THz generation. Here we discuss how to advance the state of the art of the best commercially available materials in the mid-IR, including the bulk birefringent crystals CdSiP2, ZnGeP2, and GaSe, as well as quasi-phase-matched orientation-patterned GaAs and GaP, and we point to emerging materials that need further development to extend the performance of widely-used near-infrared pump sources deeper into the infrared (to 12 microns and beyond).

Widely tunable narrowband mid-infrared coherent sources, realized using optical parametric oscillators, play an essential role in spectroscopic investigations. A part of mid-infrared spectral region is a “fingerprint range” of solid-state materials, therefore, narrow linewidth is a particularly important feature. The most suitable linewidth of radiation to satisfy the required resolution for spectroscopy of solids is 2‒6 cm-1. The biggest challenge for the developer of the laser source is meeting customers’ needs and providing numerous parameters simultaneously from a single device: broad spectral range, high spectral resolution, fast wavelength tuning, high repetition rate, stable beam direction, nearly diffraction-limited divergence, etc. All this should be provided throughout the entire operational spectral range. These features are relevant for many applications, especially for scanning near-field optical microscopy (SNOM). This presentation will describe the architecture and applications of EKSPLA's broadly tunable commercial ns and ps laser sources, from 2 to 18 m based on OP-GaAs fan-type gratings and other mid-infrared OPO nonlinear crystals. The advantages and limitations of the crystals in different narrowband OPO setups will be presented.

There is an enduring interest from the defense and security community to “see” through atmospheric obscurants and growing interest from the automotive industry to support autonomous navigation under poor weather conditions. Recent research has shown that Lidar techniques can be used to provide an enhanced ability to image through scattering media but research has been limited to visible and short wave infrared wavebands. Longer wavelengths provide greater penetration through atmospheric obscurants (dust, fog etc) and with laser pulses of short duration it should be possible to isolate the ballistic and scattered photons. This paper/presentation will describe the development of a picosecond, synchronously pumped OPO based on optically patterned GaP as a development step towards the goal of a far infrared Lidar system.

Tailoring of the properties of orientation-patterned (OP) GaAs and GaP by growing mixed ternary compounds by heteroepitaxy will enable pumping by Er-fiber laser systems at 1.56 µm and idler wavelengths beyond the mid-IR limit of GaP. We will present transmission measurements and bandgap estimations related to potential two-photon absorption with 167-322-µm thick unpattrened layers of different composition with P-content of x = 0%, 33%, 39.8%, 48.3%, and 100%, after separating them from the substrate and chemically polishing to a roughness of 0.8 nm. Except for pure GaP, which exhibits also an indirect bandgap, the estimated bandgaps are well described by the empirical relation 1.424 + 1.172x + 0.186x2 for the direct band-gap, where 1.424 eV stand for GaAs. A strong absorption band is seen around 13.3 µm in GaP (present also in all ternary samples) and at 19.1 µm in GaAs. However, the parasitic absorption band in the 2-4 µm range known for pure GaP, is absent in the ternary compounds.

Using a low-power laser, and a thin film of indium tin oxide (ITO) in the Kretschmann configuration, we characterize the efficiency of optical nonlinearities of ITO as an epsilon near zero material across a wide range of wavelengths. Additionally, we explore the relationship between the zero-epsilon wavelength and the efficiency of nonlinear interaction. This was accomplished by testing multiple ITO slides with different zero-epsilon wavelengths and comparing the results.

The field of nonlinear optics (NLO), launched about 60 years ago, has gained considerable momentum over the past two decades, resulting in an enormous growth in NLO publications for a wide range of material categories, including bulk materials, 0D-1D-2D materials, metamaterials, fiber waveguiding materials, on-chip waveguiding materials, and hybrid waveguiding systems. However, an overview of NLO data collected since 2000 for these different material types was missing. Here, we present a new set of data tables showcasing a representative list of NLO properties taken from the literature since 2000 on the above-mentioned material categories. Furthermore, we provide best practices for performing and reporting NLO experiments. These best practices underpin the selection process that we used for including papers in the tables, and also form the foundation for a more adequate comparison, interpretation, and use of the NLO parameters published today and those that will be published in the future.

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 high purity HGF growth along desired phase-matching directions for simplified fabrication and maximum yield of oriented frequency conversion devices, allowing apertures up to 15x25 mm2 and lengths greater than 20 mm, as well as compare seeded vs. unseeded growth.

We study the hardness and Young’s modulus of CdSiP2 (CSP) and ZnGeP2 (ZGP), two widely used chalcopyrites for mid-IR nonlinear frequency conversion. Nanoindentation with a Berkovich tip is applied for precise load control. Microhardness values from the literature are scattered but still indicate higher hardness for the compound with wider bandgap and higher melting point, i.e. CSP. Our measurements with optically polished single-crystal samples of random orientation give nanohardness of 9.9 and 11.5 GPa, and Young’s modulus of 136 and 150 GPa, for CSP and ZGP, respectively. We compare the obtained results with GaP, the binary isoelectronic analog of ZGP.

We review the phase-matching properties of AgGa1–xInxSe2 for second-harmonic generation of a CO2 laser at 10.7186–9.2714 μm. The refined Sellmeier equations for AgInSe2 coupled with our previously published Sellmeier equations for AgGaSe2 are found to reproduce well the critical phase-matching conditions at 10.5910–9.2820 μm thus far published in the literature. In addition, these Sellmeier equations are used to clarify the reason for the discrepancy between the measured and calculated 90° phase-matching conditions at 10.6964–9.2714 μm.

We present a CO2-filled hollow-core fibre Raman laser emitting in the yellow spectral range. Taking advantage of a state-of-art inhibited-coupling hollow-core photonic crystal fibre showcasing loss below 40 dB/km in the 530-580 nm region, we were able to develop an extremely compact and simple yellow-Raman laser scheme, allowing to emit as much as 29.5 mW of average power at the 574.5 nm wavelength while using a compact, microchip laser as a pump source. This solution provides an innovative and scalable alternative for yellow lasers, which are in high demand for biophotonics due to their effective interaction with hemoglobin and melanin.

In recent years Raman lasers have proven to be efficient and valuable way to expand the spectral range of existing lasers in the SWIR where there is a lack of direct gain materials .Due to its optical and thermal properties potassium gadolinium tungstate (KGW) is a well-known Raman gain, mainly used for lasers operating in the visible range. Results that expanded utility of KGW up to 2343nm, will be presented. Conversion efficiencies of 45.3%, with energy per pulse of 2 mJ at 2263nm will be shown for the first time.

We demonstrated a widely tunable visible laser source with ~µJ pulse energy and ~100kHz repetition rate, 40-200ns pulse duration. This is the first demonstration of a high pulse energy nanosecond laser source continuously tunable from 530nm to 600nm. We generated the visible laser source by second harmonic generation from a tunable pulsed cascaded Raman fiber laser continuously tunable from 1060nm to 1600nm. We addressed the system complexity of previous demonstrations by employing a passive Q-switched mechanism. We utilized a large mode-area amplifier to amply pulse-energy without introducing temporal instability and optimized Raman module for efficient pump conversion. The wavelength tunable range is limited just by the choice of nonlinear crystal.

We demonstrate supercontinuum generation from 800 to 2000 nm on the highly nonlinear gallium phosphide GaP-on-insulator platform. The supercontinuum is generated in a dispersion engineered waveguide with a length of 13 mm. Femtosecond pulses at the telecom wavelength are broadened in the process. The long length and low loss allow the waveguide to be pumped at the picojoule level.

Optical coherence tomography systems would greatly benefit from stable mid-infrared supercontinuum sources, which can allow deeper sample penetration compared to near-infrared sources, making them highly desirable for nondestructive testing applications. Here we firstly present the development of a flexible and fully-fiberized pump consisting of a gain-switched 1950 nm semiconductor laser amplified to an average power of 1.3 W, at 1 MHz repetition rate with 11 ps pulse duration. We utilize this pump to initiate nonlinear soliton dynamics in ZBLAN fiber, subsequently generating a broad supercontinuum extending beyond 4 μm. Spectrally-resolved pulse-to-pulse energy variations are recorded for various pumping configurations in order to quantify and optimize spectral broadening and stability, which are essential factors to maximizing speed and signal-to-noise ratio in the intended application. Funding: VILLUM FONDEN (no. 00037822), TURBO (no. 101058054), ZDZW (no. 101057404), TRIAGE (no. 101015825).

A broad and flat spectrum is preferable in many applications of supercontinuum sources. However, supercontinuum generation based on modulational instability often exhibits a prominent narrow blue peak followed by a significant dip in the neighboring longer-wavelength region. In this numerical study, we present a mitigation strategy based on modulating the pump power. Using as little as three pulses in the modulation scheme, we demonstrate the ability to improve the flatness by a factor of three, while simultaneously lowering the peak power of each pulse, which is desirable from the perspective of fiber degradation.

We report on high average-power, high-energy picosecond fourth-harmonic generation in LBO. The first stages of a Yb:YAG laser chain operating at 1 kHz repetition rate generate few-picosecond 220 mJ chirped pulses at 1030 nm fundamental wavelength. They are frequency-converted in a cascade of three LBO crystals to generate the second-, third-, and fourth-harmonics at 515 nm, 343 nm and 257 nm respectively. Crystals thicknesses and angular phase-matching detuning were calculated as a function of pulse duration through broadband nonlinear optical numerical simulations. Last crystal is both conduction-cooled on edge and surface-cooled at center through forced-air flow to mitigate heating due to nonlinear absorption in the deep-UV and reduce temperature gradients. Chirped-pulse duration was experimentally adjusted to achieve stable 20% overall conversion efficiency. Near-field beam profiles were continuously recorded at 10 Hz, for all four wavelengths involved, together with corresponding energies, showing no significant beam degradation over 50 hours. Temperatures of the two last crystals were monitored, and will help optimize surface cooling for future power ramping-up.

High power continuous-wave (CW) DUV lasers are required as light sources for high-resolution laser-based angle-resolved photoemission spectroscopy. This technique provides direct observations of band structures and is utilized to research quantum solid states. We realized a high-power 213 nm light source based on fourth harmonic generation of an 852 nm light source for this purpose. Because we have placed importance on its reliability and long-term operation, we use home-made nonlinear element and a VCM cavity locker. We obtained laser power of more than 270 mW and keep emission at 50 mW for long-period operation.

We present a multi-watt red nanosecond laser based on frequency-doubling a Raman-shifted fiber laser. The Raman-shifted fiber laser operating at 1310 nm has a novel configuration where the first Raman shift is performed in a Yb-doped fiber amplifier and the second Raman shift is performed in a phosphosilicate fiber. The Raman shifting in both stages utilizes CW seeding, which enables the temporal properties of an amplified 1064 nm modulated diode to be transferred to narrow-band light at 1310 nm with very high conversion efficiency. The resulting micro-joule level, nanosecond pulses at 1310 nm are frequency-doubled to 655 nm with high conversion efficiency owing to their narrow bandwidth. The multi-watt, micro-joule level nanosecond red pulses are ideally suited to applications such as super-resolution and photoacoustic microscopy.

The ultraviolet (UV) wavelengths are gaining attention in various applications including lithography, imaging and spectroscopy due to their high photon energy and high spatial resolution. However, the strong UV absorption has required specialized optics to beam shaping and wavefront manipulation of the UV. In this paper, a novel beam shaping of UV is demonstrated by manipulating the wavefront of a near-infrared (NIR, λ=800 nm) driving laser in harmonic generation. The preservation of spatiotemporal coherence in harmonic generation allows the transfer of the spatial beam distribution from the IR driver to the UV harmonics. To separate the UV beam from the strong NIR background, a non-collinear harmonic generation configuration is employed. By using quartz and MgO nonlinear medium, the pattern of 2nd harmonic UV (λ=400 nm) and 3rd harmonic deep-UV (λ=266 nm) waves were manipulated. Our technique allows the wavefront control of UV laser beams in UV optical metrology and patterning.

We study the generation of multicycle terahertz (THz) radiation by nonlinear down-conversion of tunable high-energy infrared pulse trains in custom large aperture periodically-poled crystals. Pulse train energies up to 100 mJ together with the large crystal apertures promise increased THz yields. We demonstrate high flexibility in tuning the pulse train parameters and investigate the dependence of the optical-to-THz conversion efficiency on the single pulse duration and find the optimum pulse number for different crystal lengths aimed at validating theoretical expectations and determining the efficiency limitations in a regime avoiding laser-induced damage.

High-dimensional spatial entanglement of photons is versatile yet difficult to harness. We demonstrate an efficient direct measurement of the OAM spectrum of an SPDC source using the stimulated parametric down-conversion in the low-gain regime.

Orthogonal optical coding is widely used in classical multiuser communication networks. Using the phase conjugation property of stimulated parametric down-conversion, we extend the current orthogonal optical coding scheme to the spatial domain to encode and decode image information. In this process, the idler beam inherits the complex conjugate of the field information encoded in the seed beam. An encoding phase mask introduced to the input seed beam blurs the image transferred to the idler. The original image is restored by passing the coded transferred image through a corrective phase mask placed in the momentum space of the idler beam. We expect that this scheme can also inspire new techniques in aberration cancellation and frequency conversion imaging.

Femtosecond Laser Irradiation followed by Chemical Etching is exploited to create microfluidic devices for High-order Harmonic Generation (HHG) in noble gases. A finetuning of the channels’ diameter and length permits the production of high-order harmonics in completely different regimes, going from the hollow waveguiding regime to the sub-mm interaction regime. We envisage that the high adaptability of our microfluidic approach will allow us to integrate more functionalities in the same integrated device thus paving the way to palm-top HHG solutions.

We experimentally identify a strong, elastic, frequency-upshifting optical effect in diamond, phonon-dressed third-harmonic generation (PD-THG). Unlike traditional electronic THG, PD-THG shows strong sensitivity to pump polarization and frequency, emphasizing its deep connection to Raman phonon degrees of freedom. We demonstrate that its cubic electric susceptibility is at least 58 times larger than the regular electronic route, boosting THG efficiency by up to three orders of magnitude. The effect is a degenerate sum-frequency analog of coherent anti-Stokes Raman scattering that works in the mid-IR and THz range, and has implications for infrared nonlinear optics and the burgeoning realm of light-driven structural control. In addition to its relevance to diamond photonics, we anticipate applications of PD-THG including THz spectroscopy, infrared-controlled nonlinear optical switching, and orientation diagnostics, as well as a temporal diagnostic for coherent structural excitation.

Free electrons in heavily doped semiconductors operate in the hydrodynamic regime, where oscillating velocity, current and electromagnetic field terms can mix and produce relatively strong nonlinear effects in the mid-infrared and terahertz ranges, where the material behaves as a free-electron system. We have designed and realized electron-doped InGaAs nanoantennas with the aim of measuring the efficiency of third harmonic generation (THG) and comparing it with the nonlinearity coefficients predicted by a hydrodynamic model. To observe THG from nanoantennas, we used a difference-frequency generation source of mid-infrared short pulses with center-wavelength tunable between 12 and 6 micrometers. Four different doping levels and several dipole antenna lengths were investigated. The volume-normalized THG efficiencies of free-electrons are much higher than those of the crystal host, as directly shown by analysis of an undoped sample. The THG efficiency is found to peak at a mid-infrared excitation wavelength that depends on the free electron concentration, mirroring the decrease of the plasma wavelength with increasing carrier concentration.

High-harmonic generation (HHG) sources are vital in generating high-brilliance laser radiation in the extreme ultraviolet (EUV) to the soft-X-ray range. In this presentation, we give an overview of different design approaches for high-flux HHG sources using our most powerful laser systems based on advanced nonlinear technologies. These technologies include optical-parametric chirped-pulse amplifiers (OPCPA) and multi-pass cells (MPC) in combination with Yb-doped laser systems. This combination allows an immense scalability of average power, and adaptability to optimum HHG driver parameters for increased HHG photon flux. Laser systems at mid-infrared wavelengths allow the generation of radiation in the water-window spectral range. We demonstrate OPCPA systems designed for 2 and 3 µm wavelength, and average power up to 70 W. Further, we demonstrate laser systems operating in the near-infrared range around 800 nm, targeting the efficient generation of 13.5 nm wavelength. Our results draw a promising roadmap towards next-generation high-flux HHG sources for a wide range of scientific and industrial applications, such as materials science and metrology in the semiconductor industry.

Frequency converting pulses with bandwidths approaching an octave remains a daunting technical challenge. We show that by tailoring the frequency-dependent photon conversion position in a chirped quasi-phase matched crystal, it becomes possible to frequency shift ~10-fs, ~1 μJ pulses efficiently and uniformly, while intrinsically customizing their dispersion. We employed this technique to design frequency downshifters that apply zero group delay dispersion to a compressed input pulse. For example, we demonstrated conversion of an 11.1-fs, 680-820 nm pulse into an octave-spanning, 11.6-fs, 2-4 μm output pulse with 70% internal photon conversion. Within some constraints, it is also possible to apply custom dispersion of multiple order, or to pre- or post-compensate for other dispersive elements in an experiment. A general approach that also works for sum frequency generation, this technique thus provides significant flexibility in selecting frequency conversion pathways and designing a hyperspectral architecture with multi-color few- and single-cycle pulses.

Recently, hybridized parametric amplification (HPA) was experimentally demonstrated as a solution for high-efficiency optical parametric amplification, with 44% pump-to-signal energy conversion achieved in a high-gain mid-infrared sub-picosecond amplifier [Flemens, et al., arXiv:2207.04147 [physics.optics] (2022)]. In HPA, concurrent idler second harmonic generation (SHG) eliminates the idler during amplification [Flemens, et al., Opt. Express 29, 30590-30609 (2021)]. This produces a saturating amplifier gain, which enables highly uniform spatiotemporal conversion and thus high-efficiency high-gain amplification for bell-shaped pump beams. In this work, we analyze major practical considerations for designing and implementing an HPA system. Considerations investigated include phase-matching bandwidth, limitations due to self-phase modulation and cascaded chi(2) nonlinearity, the effect of gain guiding to overcome temporal and spatial walk-off, noise performance, and factors affecting beam quality and M-squared measurements. We find HPA to be a robust and feasible method for achieving high-efficiency parametric amplification.

Ultrashort and intense tunable Mid-IR source promise new insights into the investigation of electron dynamics in quantum materials. Recently, we have developed an OPA providing intense optical excitation in near- and mid-infrared region (1.7 to 8 µm), and it is now used as a pump for time- and angle-resolved photoemission (TR-ARPES). We will discuss preliminary TR-ARPES results on Bi2Te3 to demonstrate the exquisite peformances of our novel TR-ARPES endstation.

We study four wave mixing (FWM) generated by an ultrashort mid-infrared pulse and a near infrared pulse. The mid infrared light is near resonant with vibrational modes of molecule and it can create a coherence between the vibrational states. The near infrared light will probe the coherence and result in FWM based on third order nonlinearity through different pathways. One process is a third-order sum frequency generation (tSFG) and the other is a third-order difference frequency generation (tDFG). We report experimental investigation of a time resolved tDFG generated from plastic materials such as mixture beads and a thin low density polyethylene (LDPE) film. We compare results of the tDFG with that of the tSFG in terms of their intensities and phase matching conditions. Our results show that a vibrational spectroscopy combing the tDFG and the tSFG can be versatile tool in studying of physical chemistry, dynamics of complicated molecular system, bioimaging and so on.

We study the frequency up- and down-conversion based on the cascaded process which accompanies large phase mismatching between fundamental and intermediate waves. So, for example, we propose using the cascaded second-harmonic generation (SHG) to implement the frequency conversion process, which is similar to that occurring in a medium with cubic susceptibility. At phase matching between the fundamental wave and the third-harmonic wave, third harmonic generation (THG) occurs with high efficiency (94.5%). We demonstrate that the cascaded process may also influence negatively on the frequency conversion processes. SHG in a medium with combined quadratic and cubic nonlinear response accounting for weak THG at large phase mismatching demonstrates that in some cases , the second harmonic intensity evolution is different, whether we take into account of weak THG process or not. So, then the intensity at doubled frequency may be much lower in certain sections in comparison with those without accounting for THG. We study such an influence of weak THG both for pulses with long duration by developing analytical approach and for short pulses based on computer simulation.