Second-order Nonlinear Frequency Conversion Research Articles

Chirality-controlled second-order nonlinear frequency conversion in lithium niobate film metasurfaces.

The high-quality factor resonant metasurfaces have extensive applications in enhancing nonlinear frequency conversion efficiency at the subwavelength scale. However, methods for actively modulating the frequency conversion process are limited. We design a chiral lithium niobate film metasurface and investigate the photonic spin as a new degree of freedom to dynamically control the second-order nonlinear frequency conversion, without reconfiguring the structure by using external stimuli. The chiral resonance with circular dichroism (CD) of 0.62 gives rise to a high nonlinear CD of 0.84 in second-harmonic generation efficiency. Interestingly, combining the chiral resonance and an achiral quasi-bound state in the continuum enables us to investigate the photonic-spin-controlled sum-frequency generation and the photon pair generation from the spontaneous parametric downconversion process. Owing to the ultrahigh quality factor exceeding 103 both for two resonances, the second-order nonlinear frequency conversion occurs at a wavelength region of 0.2 nm, suggesting good monochromaticity. Our work opens new, to our knowledge, avenues for practical implementation of dynamically controlled nonlinear optical devices and will find utility in holography, switchable light sources, and information processing.

Broadband long-wavelength upconversion in ultra-short nonlinear crystals.

Since the inception of the second-order nonlinear frequency conversion in 1961, enhancing the inherent low conversion efficiency has been a primary objective. This goal has been successfully accomplished through the utilization of cm-long nonlinear crystals characterized by high quality and nonlinearity, coupled with versatile phase-matching strategies and high-power mixing lasers. However, the reliance on lengthy nonlinear crystals and the necessity for precise phase-matching introduce stringent tolerances on acceptance angles and spectral bandwidths for the interacting fields, thereby constraining its widespread applicability in scientific and industrial domains. This challenge is addressed by combining a broadly tunable ∼5 mW quantum cascade laser operating in the 9.5-12.5 µm range with upconversion detection in ∼100 µm long AGS crystals. Using a tightly focused continuous wave Nd:YVO4 laser with 20 mW output power and spatial filtering of the upconverted beam lead to a SNR of 55 for 50 µs averaging time sufficient for many applications.

Difference-frequency generation in optically poled silicon nitride waveguides.

Difference-frequency generation (DFG) is elemental for nonlinear parametric processes such as optical parametric oscillation and is instrumental for generating coherent light at long wavelengths, especially in the middle infrared. Second-order nonlinear frequency conversion processes like DFG require a second-order susceptibility χ (2), which is absent in centrosymmetric materials, e.g. silicon-based platforms. All-optical poling is a versatile method for inducing an effective χ (2) in centrosymmetric materials through periodic self-organization of charges. Such all-optically inscribed grating can compensate for the absence of the inherent second-order nonlinearity in integrated photonics platforms. Relying on this induced effective χ (2) in stoichiometric silicon nitride (Si3N4) waveguides, second-order nonlinear frequency conversion processes, such as second-harmonic generation, were previously demonstrated. However up to now, DFG remained out of reach. Here, we report both near- and non-degenerate DFG in all-optically poled Si3N4 waveguides. Exploiting dispersion engineering, particularly rethinking how dispersion can be leveraged to satisfy multiple processes simultaneously, we unlock nonlinear frequency conversion near 2 μm relying on all-optical poling at telecommunication wavelengths. The experimental results are in excellent agreement with theoretically predicted behaviours, validating our approach and opening the way for the design of new types of integrated sources in silicon photonics.

Triple wavelength-switchable lasing in yellow-green based on frequency mixing of self-Raman operation

An efficient Nd:YVO<sub>4</sub> crystal self-Raman laser combined with second-order nonlinear frequency conversion is demonstrated to achieve an switchable output of three wavelengths in the yellow-green band. In order to improve the thermal effect and increase the length of Raman medium, a three-stage diffusion-bonded YVO<sub>4</sub>/Nd:YVO<sub>4</sub>/YVO<sub>4</sub> crystal is designed for high power and efficient self-Raman laser operation. Selective frequency mixing mechanisms between the fundamental wave and the first Stokes wave using the LiB<sub>3</sub>O<sub>5</sub> (LBO) and BaB<sub>2</sub>O<sub>4</sub> (BBO) crystals are comparatively studied by temperature tuning and angle tuning, respectively. Considering the frequency mixing conversion efficiency and a relatively fast wavelength switching, the BBO crystal with critical phase matching is selected as the second order nonlinear optical crystal for frequency conversion. It only needs to fine-tune the phase match angle of BBO crystal within 1.4°, and thus successfully realizing all second harmonic and sum frequency generation between the fundamental wave and the first Stokes wave. Therefore the efficient-switchable output of the three wavelengths of 532 nm green light, 559 nm lime light and 588 nm yellow light is obtained. Under the incident pump power of 19.5 W and the pulse repetition rate of 60 kHz, maximum average output power of 4.37 W at 532 nm, 2.03 W at 559 nm, 3.43 W at 588 nm are achieved. The conversion efficiency values of the corresponding pump light to visible light are 22.4%, 10.4% and 17.6%, respectively. The corresponding pulse widths are 36 ns, 12.2 ns and 12.7 ns, respectively. The results show that the selective frequency mixing of self-Raman operation is an efficient approach to achieving the wavelength-switchable emission in visible waveband. This wavelength-switchable laser source has important applications in the areas of laser therapy, visual display, spectral imaging and biological medicine.

Efficient Terahertz Generation Via GaAs Hybrid Ridge Waveguides

As a common semiconductor material with high performance in the terahertz band, GaAs is hardly employed in second-order nonlinear frequency conversion (NFC) directly due to its isotropy. Although thin waveguides can break the symmetry and induce birefringence for phase matching (PM), more research seems to be needed with GaAs-based structures. In this paper, a category of ridge waveguide is demonstrated with a hybrid structure composed of GaAs and SiN in different sizes. By using the difference frequency generation (DFG) technique with 2 μm and 10 μm pump lasers, simulations suggest that the hybrid waveguides are able to generate continuous-wave (CW) monochromatic terahertz waves from 1.59 THz to 2.66 THz. The output power reach 56.16 μW, corresponding to a conversion efficiency of 5.62 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , which is much higher than that using bulk GaAs crystals. Such integrated hybrid waveguides have great potential in efficient on-chip terahertz systems.

Third-order nonlinear optical properties of calcite crystal in UV region

Different from cascaded second-order nonlinear frequency conversion which is a common way to obtain ultraviolet (UV) laser only through noncentrosymmetric crystal, the optical properties of the phase-matched direct third-harmonic generation (THG) in the centrosymmetric calcite crystal was studied. A femtosecond laser was taken as the pumping source in the type-I (ooo→e) and type-II (ooe→e) phase-matched experiment, and a nanosecond laser was also used for comparison. Polarization angle ρ, phase-matching angle θ and azimuth angle φ in direct THG were determined and verified. Strong third-harmonic at 266 nm was observed in calcite crystal pumped by femtosecond laser. Compared with type-I phase-matched, Type-II was more efficient for energy conversion. The shortest harmonic wavelength obtained was about 213 nm, which was close to deep-UV region. The maximal output energy was 19.3 μJ (average power, ～19.3 mW) and the maximal conversion efficiency was 1.25% at 266 nm, which was the highest conversion efficiency at present. These results may indicate a wider application field for centrosymmetric crystals and another method to produce laser in UV and even deep-UV region.

Universal modeling of second-order nonlinear frequency conversion in three-dimensional nonlinear photonic crystals.

The second-order nonlinear frequency conversion in three-dimensional nonlinear photonic crystals is theoretically studied using coupled wave equations. A universal theoretical model is obtained, with a unified expression combining birefringence phase match, quasi-phase match, nonlinear Raman-Nath diffraction, nonlinear Čerenkov radiation and nonlinear Bragg diffraction. They are demonstrated in the numerical simulation. With the phase-matching conditions in lower dimensions extended to three dimensions, more various phenomena can be seen and corresponding mechanisms can be explained. This research enables the control of second-harmonic generation more efficiently and has potential applications in more complicated nonlinear photonic crystals.

Fabrication of nanoscale lithium niobate waveguides for second-harmonic generation.

Nanoscale waveguides are basic building blocks of integrated optical devices. Especially, waveguides made from nonlinear optical materials, such as lithium niobate, allow access to a broad range of applications using second-order nonlinear frequency conversion processes. Based on a lithium niobate on insulator substrate, millimeter-long nanoscale waveguides were fabricated with widths as small as 200 nm. The fabrication was done by means of potassium hydroxide-assisted ion-beam-enhanced etching. The waveguides were optically characterized in the near infrared wavelength range showing phase-matched second-harmonic generation.

Yb-Fiber-Laser-Pumped Ultrafast Frequency Conversion Sources From the Mid-Infrared to the Ultraviolet

We describe the development of versatile, practical, and high-power ultrafast optical sources based on second-order nonlinear frequency conversion in bulk materials pumped by the mode-locked Yb-fiber laser at 1064 nm, and covering spectral regions from the near- to mid-infrared (mid-IR) to the visible and ultraviolet (UV). By exploiting synchronously-pumped optical parametric oscillators (SPOPOs) based on quasi-phase-matched nonlinear materials of MgO:PPLN and MgO:sPPLT, in combination with internal and external single-pass harmonic generation in birefringent crystals of BiB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> and β-BaB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , we have generated tunable picosecond pulses in spectral regions from above ~4 μm in the mid-IR, into the visible, and down to 317 nm in the UV. The demonstrated sources can deliver average output powers of up to 11.7 W in the near- to mid-IR, 3.5 W in the visible, and 30 mW in the UV, in picosecond pulses at ~80 MHz repetition rate, with high passive stability, excellent output characteristics, and the potential for further power scaling with increased fiber laser pump power. The described sources represent an effective approach for the extension of fiber laser technology across the entire wavelength range from the UV to mid-IR spectrum, and ultimately the THz spectral range.

Yb-Fiber-Laser-Pumped Continuous-Wave Frequency Conversion Sources from the Mid-Infrared to the Ultraviolet

We describe a wide range of versatile, practical and high-power sources of coherent continuous-wave (cw) radiation from the near- to mid-infrared (mid-IR) to the visible and ultraviolet (UV) developed in our laboratory by exploiting second-order nonlinear frequency conversion in novel quasi-phase-matched (QPM) and birefringent materials pumped by the Yb-fiber laser at 1064 nm. By exploiting optical parametric oscillators in combination with internal and external single-pass harmonic generation in QPM nonlinear materials of MgO:PPLN and MgO:sPPLT, and birefriengent crystal of BiB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> , we have generated stable, high-power cw radiation covering broad spectral regions from ~4 μm in the mid-IR, to 532 nm in the visible, down to 355 nm in the UV. The developed sources can deliver total output powers of up to 17.5 W in the near- to mid-IR, 13 W in the visible, and 68 mW in the UV, at exceptional efficiencies, with excellent passive stability, high spectral and spatial beam quality, in compact, portable and practical design. We also demonstrate successful realization of cw fiber-laser-pumped Ti:sapphire lasers enabled by the development of simplified high-power cw green sources at 532 nm. The described techniques represent a highly versatile, practical, and effective approach for the extension of fiber laser technology across the entire UV to mid-IR spectrum, and ultimately the THz spectral range, also offering the potential for further power scaling with the increase in the fiber laser pump power.

Second-order nonlinear frequency conversion processes in plasmonic slot waveguides

We present a theoretical study on the possibilities to achieve nonlinear frequency conversion in plasmonic slot waveguides having a &#x3C7;(2) nonlinear medium as its core. Second-harmonic generation is used as a referential process to discuss the possibilities in achieving strong nonlinear interactions. We show that geometrical dispersion allows for the possibility of modal phase matching without resorting to other mechanisms like birefringence phase matching or periodic poling of the nonlinear medium. We disclose that in strongly dissipative systems two effects, the damping of individual modes and the phase-matching condition, have to be carefully balanced to assure an efficient energy conversion. Besides second-harmonic generation, emphasis is put on exploring the application of potentially more importance: the parametric amplification in the waveguide with the purpose to enhance its propagation length.