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.

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.

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 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.

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 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.

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.

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.

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 μm 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.

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.

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 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.

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.

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.

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 based on a 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 vibrational states. The near infrared light will probe the coherence and result in FWM based on third order nonlinearity through different pathways. One pathway 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 mixed 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 tools in studying of physical chemistry, dynamics of complicated molecular system, bioimaging and so on.