Nonlinear Frequency Generation and Conversion: Materials and Devices XXI

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

We report a mid-infrared (MIR) source emitting around 3 μm, employing a novel χ3/χ2 cascaded nonlinear conversion architecture. A χ3-based stage utilising four-wave mixing (4WM) in photonic crystal fibre is used to generate synchronised signal pulses, which are used to seed a χ2-based periodically poled lithium niobate (PPLN) optical parametric amplifier (OPA). The signal pulses exiting the 4WM stage are temporally and spatially overlapped, negating the need for any beam delay and combination optics commonly employed in OPAs. The PCF output is then directly focused into the χ2 PPLN crystal for efficient conversion of light to the MIR through OPA.

We report on an ultra-broadband tunable mid-IR source delivering 110 nJ, 50 fs-long pulses at 250 kHz repetition rate. The experiment starts with a Yb-doped fiber amplifier system delivering 200 μJ, 300 fs-long pulses, followed by a 70% high-efficiency dual-stage nonlinear compression based on a multipass cell and a 1 m-long capillary filled with argon. The 9.5 fs-long resulting pulses drive intrapulse difference-frequency generation (iDFG) in a 1 mm-thick LiGaS2 crystal and a specially designed waveplate is used prior to this crystal to further enhance the iDFG yield. This source paves the way for new experiments in 2D ultrafast spectroscopy.

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

In this study, we present the designed and construction of an electric field induced second harmonic generation system that was used to investigate the importance of laser polarization concerning this static field, and the effect temperatures below room temperature have on electric field induced second harmonic generation measurements in atmospheric air. Since the second harmonic light is not typically created by media with inversion symmetry, a DC power supply was used to facilitate a break in symmetry. This breakdown was then probed using a Spectra-Physics amplified femtosecond system and the temperature of these electrodes were cooled to 273 K and 194.7 K, respectively.

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

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

We present updated Sellmeier equations for CdGa2S4 that reproduce well the phase-matching angles for Yb:KGd(WO4)2 and Cr:foresterite femtosecond-amplifier-pumped Hg0.35Cd0.65Ga2S4 optical parametric amplifiers (OPAs) and a Ti:Al2O3 femtosecond-amplifier-pumped Hg0.51Cd0.49Ga2S4 OPA in the 5.6–11.5 μm range, when combined with our previously derived Sellmeier equations for HgGa2S4.

We perform transmission measurements in the 0.5-25-µm spectral range on thin (0.2-0.4-mm) unpatterned GaAsP layers grown from the vapor phase by HVPE on GaAs. The multiple-reflection effect which is considerable for high refractive index materials has been taken into account both in the regions of clear transparency and in the presence of absorption. The actual sample thickness is derived from the observed interference fringes and the refractive index which is intrerpolated using available data on GaAs and GaP. For some compositions, e.g. 33% P, we observe nice clear transmission plateau extending up to 13 µm with almost zero absorption losses.

This past August, a record-breaking shot with 1.3 megajoules of fusion yield was achieved on the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. This experimental result, decades in the making, is a significant breakthrough for laser-driven inertial fusion. This talk will review the experimental results, the photonics advancements and many more technologies that made this breakthrough possible, and the implications for future research. Furthermore, these recent game-changing results on the NIF now lay the groundwork to explore laser inertial fusion as a path for clean energy and energy security.

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

Today’s quantum technology relies on the realization of large-scale non-classical systems in practical formats to enable quantum-accelerated computing, secure communications and enhanced sensing. Optical on-chip quantum frequency combs, characterized by many equidistantly spaced frequency modes, allow the storage of large amounts of quantum information. The combination with control techniques, using accessible state-of-the-art telecommunications infrastructure, can constitute a powerful frequency-domain quantum circuit with new functionalities and represents an approach towards realizing practical large-scale controllable quantum systems. In this presentation, we will review approaches for the efficient realization of quantum frequency combs in on-chip waveguide structures and micro-resonators. We will show their applicability for the realization of quantum systems with considerably enhanced complexity, particularly generating and manipulating on-chip multi-photon and high-dimensional quantum states as well as discrete high-dimensional cluster states, laying at the basis of measurement based-quantum computing. Building on this, the realization of frequency-domain Hong-Ou-Mandel interference of independent photons, fundamental to quantum information processing, as well as an outlook on frequency-domain circuits will be discussed.

We present a new platform for sensitive molecular detection and control spanning 1) multiplexed genetic and proteomic screening, 2) single-cellular bacterial identification and drug susceptibility testing, and 3) chiral molecular synthesis and separation, based on high-quality-factor phase gradient metasurfaces. The high-quality factor of our metasurfaces produces a large amplification of the electromagnetic field, increasing the response to targeted binding of biomarkers. Simultaneously, the optical signal is beam-steered for multiplexed detection. We develop these metasurfaces for a new respiratory panel of SARS-CoV-2, RSV, and influenza; Raman-based identification and antibiotic susceptibility testing of pathogens; and sensitive identification and purification of chiral molecules including amino acids and small-molecule pharmaceuticals and agrochemicals.

Laser-microtextured surfaces have gained an increasing interest due to their enormous spectrum of applications and industrial scalability. In this frame, several research studies have demonstrated how laser-based fabrication methods can be used to produce functional surfaces. Furthermore, it has been demonstrated is many cases, that the combination of structures with feature sizes in different ranges (e.g., microelements decorated with nanostructures) can not only further enhanced specific functions but also to provide surfaces with several functionalities. In this context, this talk shows how Direct Laser Interference Patterning (DLIP), Direct Laser Writing (DLW) and Laser Induced Periodic Surface Structures (LIPSS) can be combined, reaching advanced functionalities on technological relevant materials.

Metasurfaces allow for agile manipulation of incoming light using a single layer of resonators. However, because of the limited interaction length of a pulse traveling through a single surface, it remains challenging to generate new spectral components using nonlinear metasurfaces. Time-dependent surfaces offer an exciting alternative to overcome this limitation. Unfortunately, a self-consistent framework to describe this mechanism is lacking. Based on an analytical model and finite-difference time-domain numerical simulations, we present physical insight into the frequency-shifting process that occurs when broadband electromagnetic pulses interact with time-varying surfaces. In particular, we find an intriguing relationship between the bandwidth of the incident pulse, the targeted frequency shift, and the number of Lorentzian resonators that need to be implemented.

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

Diffractive and dispersive devices introduce a wavelength dependent propagation angle or angular dispersion to an optical pulse. These pulses are capable of producing luminal group velocities and anomalous group velocity dispersion on-axis which has led to applications in non-linear optics and dispersion compensation. Here we introduce non-differentiable angular dispersion which can realize anomalous and normal group velocity dispersion on-axis while suppressing or enhancing all other orders of dispersion as well as non-luminal group velocities ranging from -4c to 30c. These capabilities will lead to unique control over the dispersion profile and lead to novel applications in non-linear optics.

Traditional processes for the design of metamaterial structures are often computational heavy, time-consuming, and occasionally does not lead to the desired optical response. Deep learning can quickly optimize structures through inverse design, and create new geometries for devices. This research uses a deep learning framework for the inverse design of an optimal plasmonic structure to maximize the second-order nonlinear response from a nonlinear metamaterial. The thin-film nonlinear metamaterial employed is a nanolaminate, and the optimal plasmonic structure is fabricated to establish the validity of the deep learning algorithm.

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

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

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

We optimize octave-spanning supercontinuum generation from an Er fiber comb for strong output at six wavelengths by phase shaping using temperature control of a chirped fiber Bragg grating. The shaped spectrum had 160 to 340 nW per comb mode at the half harmonics of Yb, Sr and Ca transitions for optical clock frequency transfer; 70 and 230 nW lines for an f-2f octave spanning pair; and 700 nW at 1560 nm for C-band referencing. Coherence was verified by interference with a separate optically shifted supercontinuum branch showing strong contrast and 40 dB beat notes at 100 kHz resolution bandwidth.

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

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

We present the generation of broadband terahertz radiation with the two color gas-plasma scheme. An unprecedented average power of 640 mW is generated with an efficiency of 0.1 % driven by post-compressed pulses of a high-power fiber laser system operating at a repetition rate of 500 kHz. The full bandwidth of the terahertz pulses is characterized using the air-biased coherent detection scheme. The terahertz pulses have a single cycle feature and cover the spectral region from 0.1 to 30 THz.

We report on research into the properties of supercontinuum (SC) generated from low-coherence bursts of different duration in a 1-km long P2O5 fibre. It was found out that SC with a spectral width of ~135–150 nm is formed within the ~900–1200-nm range mostly due to cascaded Raman scattering of noise-like pulses with the variable envelope duration of 36–153 ps (sub-pulse duration was ~ 300 fs) and average power of 560 mW at 1080 nm. It was discovered that the spectral width of SC is predominantly affected by the duration of interaction between the pumping and Raman pulses.

Z-Scan technology has been developed and actively applied to measure non-linear refractive index and non-linear absorption coefficient. When a single, intense beam is focused on the material, a phenomenon called 'self-focusing' can be observed. This self-focusing causes a change in the angle of divergence of the laser beam after it has passed through the material. By theoretically analyzing the change in the amount of light measured by the detector, it is possible to calculate the change in the nonlinear refractive index and the nonlinear absorption. We have investigated the nonlinear optical properties of PTS by using both Z-scan and confocal techniques in the picosecond time domain. In this presentation, the nonlinear refractive index n_2 and nonlinear coefficient β obtained from open and closed aperture Z-scan experiments will be presented.

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

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

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

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

Third-harmonic generation in silicon nitride waveguides is an ideal source of coherent visible light, suited to ultrafast pulse characterization and self-referenced comb generation, due to the large nonlinear refractive index and CMOS compatibility of silicon nitride. We demonstrate phase-matched third-harmonic generation in silicon nitride waveguides, where a fundamental transverse mode at 1596 nm is phase matched to a TM02 mode at 532 nm, observed by collecting the far-field image. Simulations confirm the waveguide width-dependent phase-matched wavelength, with a conversion efficiency of 7e-8 %/W^2, over a 400um-long interaction length.

A design study is presented for difference-frequency generation (DFG) in aluminum gallium arsenide (AlGaAs)-on-insulator waveguides. AlGaAs is a mature technology platform with large optical nonlinearities, a high refractive index contrast and the presence of second-order susceptibility as compared to silicon dioxide (silica), making it interesting for chip-based frequency conversion. Furthermore, a thorough revision of the DFG theory is provided, giving insight into the special case of a low-powered pump, which is relevant for quantum applications. A comparison is also made with state-of-the-art devices in periodically poled lithium niobate (PPLN).

Sub-picosecond green-pumped OPG/OPA using non-critical phase matching in LBO was investigated at high average powers, utilizing a novel high-power ultrafast fiber laser providing 300W of 1ps pulses at 515nm and 1.4GHz pulse repetition rate. The 4-stage collinear OPA produced about 100W of combined Signal plus Idler output powers when tuning the Signal wavelength between 740nm and 940nm. An extended spectral tuning range between 300nm and 1800nm with average power between 10W and 70W was demonstrated by using SFG of Signal plus Pump and SHG of Signal and of Idler. The OPA output pulse durations were ~0.5ps long.

We present a narrowband, non-resonant optical parametric oscillator based on 5-mm thick Rb-doped periodically-poled KTiOPO4 (PPKTP) operating in the high-energy/low repetition-rate regime. An uncoated volume Bragg grating (VBG) is employed as one of the cavity mirrors reflecting only the signal whereas the other cavity mirror is reflecting only the idler. Pumping by a Nd:YAG laser at 1.0642 μm in a double-pass, the signal plus idler output energy reached almost 5 mJ at a repetition rate of 100 Hz corresponding to a conversion efficiency of ⁓26%. Both signal and idler are narrowband with full width at half maximum (FWHM) of 0.5 nm at 1942 nm and 0.76 nm at 2355 nm, respectively.

The design considerations for a CEP stable high power optical parametric chirped-pulse amplifier (OPCPA) at 3200 nm central wavelength, pumped by a Yb-based disk amplifier are presented based on the MIR-HE laser system procured by ELI-ALPS Hungary. The design aims to provide pulses exceeding 20 mJ pulse energy and less than 2.5 cycles pulse length.

We present a versatile high repetition rate, optical-parametric chirped-pulse amplifier system (OPCPA) in combination with a high-harmonic-generation (HHG) source. Tuning of the fundamental OPCPA driver wavelength allows for higher harmonic generation within the full range between 25 and 50 eV. All energies between two adjacent odd harmonics can be addressed, making the system a powerful, gaplessly tunable extreme-ultraviolet (XUV) light source for spectroscopy.