Nonlinear Frequency Conversion Research Articles

High-efficiency 100-W Kerr-lens mode-locked Yb:YAG thin-disk oscillator

We demonstrate a Kerr-lens mode-locked femtosecond Yb:YAG thin-disk oscillator and investigate the approach to increase the optical-to-optical efficiency based on the scheme of direct multiple passes of the laser beam through the thin-disk medium. With twelve passes through the thin disk, 266-fs pulses were delivered from the oscillator with an average power of 105.6 W at a repetition rate of 20 MHz. The corresponding optical-to-optical efficiency is 31.1%, which is, to the best of our knowledge, the highest efficiency of any mode-locked thin-disk oscillator with pulse duration below 300 fs. This demonstration paves the way to even more efficient mode-locked femtosecond thin-disk oscillators, and provides an excellent laser source for the applications such as non-linear frequency conversion and high-precision industrial processing.

High-Dimensional Photonic Orbital-Angular-Momentum Frequency Interface

Frequency interface, namely, converting the colors of photons while retaining their other features, is indispensable for numerous applications ranging from up-conversion imaging to the multimode quantum network. However, limited by the nonuniform nonlinear frequency-conversion efficiency, constructing the frequency interface for high-dimensional spatial modes is still challenging. Here, to break this fundamental barrier, we construct a high-dimensional (HD) orbital-angular-momentum (OAM) frequency interface via conducting sum-frequency generation between HD OAM states to be converted and a radial structured pump. Specifically, by replacing the conventional Gaussian pump with an optimized coherent superposition of HD Laguerre-Gaussian radial modes (i.e., its azimuthal number is zero), we experimentally achieve the OAM frequency interface in nine dimensions with a record fidelity up to 90.38 %. Additionally, by increasing the number of radial modes, our scheme can be flexibly extended to higher dimension. Such a high-dimensional frequency interface could find potential applications in the high-capacity free-space optical communication and would also pave the way for constructing high-dimensional quantum networks.

Nonlinear optics in 2D materials: focus on the contributions from Latin America

This paper reviews the work carried out in the field of nonlinear optics in two-dimensional and ultrathin lamellar materials, with a special focus on contributions authored and co-authored by researchers affiliated with Latin American institutions. It is shown that important and impactful contributions have emerged from Latin America, which has a consolidated and active optics community. Nonlinear optical refraction, nonlinear optical absorption, and nonlinear frequency conversion in materials such as graphene, transition metal dichalcogenides, and black phosphorus are among the reviewed themes.

Aluminum nitride photonic integrated circuits: from piezo-optomechanics to nonlinear optics

The commercial success of radio-frequency acoustic filters in wireless communication systems has launched aluminum nitride (AlN) as one of the most widely used semiconductors across the globe. Over recent years, AlN has also been investigated as an attractive photonic integrated platform due to its excellent characteristics, such as enormous bandgaps (∼6.2 eV), quadratic and cubic optical nonlinearities, Pockels electro-optic effects, and compatibility with the complementary metal-oxide semiconductor technology. In parallel, AlN possesses outstanding piezoelectric and mechanical performances, which can provide new aspects for controlling phonons and photons at the wavelength scale using nanophotonic architectures. These characteristics pose AlN as a promising candidate to address the drawbacks in conventional silicon and silicon nitride platforms. In this review, we aim to present recent advances achieved in AlN photonic integrated circuits ranging from material processing and passive optical routing to active functionality implementation such as electro-optics, piezo-optomechanics, and all-optical nonlinear frequency conversion. Finally, we highlight the challenges and future prospects existing in AlN nanophotonic chips.

Soliton-sinc optical pulse propagation in the presence of high-order effects.

We investigate the propagation dynamics of the soliton-sinc, a kind of novel hybrid pulse, in the presence of higher-order effects with emphasis on the third-order dispersion (TOD) and Raman effects. At variance with the fundamental sech soliton, the traits of the band-limited soliton-sinc pulse can effectively manipulate the radiation process of dispersive waves (DWs) induced by the TOD. The energy enhancement and the radiated frequency tunability strongly depend on the band-limited parameter. A modified phase-matching condition is proposed for predicting the resonant frequency of the DWs emitted by soliton-sinc pulses, which is verified by the numerically calculated results. In addition, Raman-induced frequency shift (RIFS) of the soliton sinc pulse increases exponentially with a decrease of the band-limited parameter. Finally, we further discuss the simultaneous contribution of the Raman and TOD effects to the generation of the DWs emitted from the soliton-sinc pulses. The Raman effect can then either reduce or amplify the radiated DWs depending on the sign of the TOD. These results show that soliton-sinc optical pulses should be relevant for practical applications such as broadband supercontinuum spectra generation as well as nonlinear frequency conversion.

Simulating supercontinua from mixed and cascaded nonlinearities

Nonlinear optical frequency conversion is of fundamental importance in photonics and underpins countless of its applications: Sum- and difference-frequency generation in media with quadratic nonlinearity permits reaching otherwise inaccessible wavelength regimes, and the dramatic effect of supercontinuum generation through cubic nonlinearities has resulted in the synthesis of broadband multi-octave spanning spectra, much beyond what can be directly achieved with laser gain media. Chip-integrated waveguides permit to leverage both quadratic and cubic effects at the same time, creating unprecedented opportunities for multi-octave spanning spectra across the entire transparency window of a nonlinear material. Designing such waveguides often relies on numeric modeling of the underlying nonlinear processes, which, however, becomes exceedingly challenging when multiple and cascading nonlinear processes are involved. Here, to address this challenge, we report on a novel numeric simulation tool for mixed and cascaded nonlinearities that uses anti-aliasing strategies to avoid spurious light resulting from a finite simulation bandwidth. A dedicated fifth-order interaction picture Runge–Kutta solver with adaptive step-size permits efficient numeric simulation, as required for design parameter studies. The simulation results are shown to quantitatively agree with experimental data, and the simulation tool is available as an open-source Python package (pychi).

Direct generation of 3.17 mJ green pulses in a cavity-dumped Ho3+-doped fiber laser at 543 nm

High-energy pulsed lasers in the green spectral region are of tremendous interest for applications in space laser ranging, underwater detection, precise processing, and scientific research. Semiconductor pulsed lasers currently are difficult to access to the so-called “green gap,” and high-energy green pulsed lasers still heavily rely on the nonlinear frequency conversion of near-IR lasers, precluding compact and low-cost green laser systems. Here, we address this challenge by demonstrating, for the first time to the best of our knowledge, millijoule-level green pulses generated directly from a fiber laser. The green pulsed fiber laser consists of a 450 nm pump laser diode, a Ho3+-doped ZBLAN fiber, and a cavity-dumping module based on a visible wavelength acousto-optic modulator. Stable pulse operation in the cavity-dumping regime at 543 nm is observed with a tunable repetition rate in a large range of 100 Hz–3 MHz and a pulse duration of 72–116 ns. The maximum pulse energy of 3.17 mJ at 100 Hz is successfully achieved, which is three orders of magnitude higher than those of the rare-earth-doped fiber green lasers previously reported. This work provides a model for compact, high-efficiency, and high-energy visible fiber pulsed lasers.

Watt-level deep-UV subnanosecond laser system based on Nd-doped fiber at 229 nm.

We report an efficient deep-UV master-oscillator power amplifier (MOPA) laser system at 229 nm that generates 350 ps pulses at 2 MHz repetition rate with an average power of 1.2 W. The use of a polarization-maintaining large mode area neodymium-doped fiber operating on the 4F3/2→4I9/2 transition allows high-power laser emission of up to 28 W near 915 nm in the sub-nanosecond regime with low spectral broadening. Two nonlinear frequency conversion stages (LBO + BBO crystals) in a single-pass configuration directly convert the IR laser emission to deep UV. This laser demonstrates the great potential of Nd3+-doped fiber lasers to produce high-power deep-UV emission.

A bond-free PPLN thin film ridge waveguide

Lithium niobate thin film waveguide is attractive in nonlinear frequency conversion, especially in the second harmonic generation (SHG). However, the conventional fabrication method of lithium niobate thin film waveguide requires expensive bonding equipment and the prepared film is easy to peel off. In this paper, a novel fabrication method of PPLN thin film ridge waveguide with a trapezoidal substrate is presented, based on glue bonding. The PPLN waveguide together with its trapezoidal substrate is surrounded with low refractive glue to modulate the vertical refractive index gradient along the waveguide length, which was proved to be useful for broadband SHG with a theoretical bandwidth above 25 nm by changing the substrate thickness from 3 μm to 17 μm. The proposed structure was verified by using a C-band ASE laser as the pump source, where the SHG laser bandwidth is about 11 nm. The power of SHG is 2.55 μw, and the normalized conversion efficiency is 10.5 %/W∙cm2. This work is beneficial to simplify the fabrication process of lithium niobate thin film ridge waveguide, and has certain promotion significance for photonic chip integration.

Extremely high-Q quasi-BICs induced by simultaneously broken out-of- and in-plane symmetries in dielectric metasurfaces of low-index materials

Generating high-Q resonances is a difficult task to realize with low-index materials as the building blocks of metasurfaces, despite of its significance. Up to date, various schemes for obtaining strong resonances with low-index materials have been reported, while the highest record of Q-factor still remains in the order of several thousands. This work shows that quasi-bound states in the continuum (quasi-BICs) with extremely high Q-factors in the orders of 105∼107 can be obtained in the low-index metasurfaces, taking lithium niobate (LN) metasurfaces supported by silica substrate as examples. In such metasurfaces, BICs with inversion symmetric distributions evolve to quasi-BICs with extremely high Q-factors by simultaneously breaking out-of- and in-plane symmetries due to multiple symmetry protectedness. Dramatically high enhancement of mode intensity in the order of 104∼106 due to the high Q-factor enables highly efficient nonlinear frequency conversion: the results based on the self-consistent approach show that the total efficiency of second-harmonic generation reaches around 9.4% for pump intensity of 0.5 W/cm2. As the first report on extremely high-Q quasi-BICs in low-index metasurfaces, the presented results pave the way towards broad applications for ultraviolet nanophotonics.

Broadband second-harmonic generation in an angle-cut lithium niobate-on-insulator waveguide by a temperature gradient.

Frequency conversion via nonlinear wave mixing is an important technology to broaden the spectral range of lasers, propelling their applications in optical communication, spectroscopy, signal processing, and quantum information. Many applications require not only a high conversion efficiency but also a broad phase matching bandwidth. Here, we demonstrate broadband birefringence phase matching (BPM) second-harmonic generation (SHG) in angle-cut lithium niobate-on-insulator (LNOI) ridge waveguides based on a temperature gradient scheme. The bandwidth and shift of the phase matching spectrum can be effectively tuned by controlling the temperature gradient of the waveguide. Broadband SHG of a telecom C-band femtosecond laser is also demonstrated. The approach may open a new avenue for tunable broadband nonlinear frequency conversion in various integrated photonics platforms.

Theoretical exploration of sensitive terahertz detection by cascaded difference-frequency generation

Nonlinear optical frequency conversion technologies, including the optical parametric process and difference-frequency and sum-frequency processes, have demonstrated good performance for terahertz (THz) wave detection. The quantum efficiencies of the above technologies cannot break through the Manley–Rowe limit. In this work, we present a novel scheme for THz wave detection based on cascaded difference-frequency generation in combination with optical parametric oscillations. By superimposing a weak THz wave and an intense pump wave in an aperiodic periodically poled lithium niobate crystal, a series of Stokes waves and a series of anti-Stokes waves are generated. The Stokes waves oscillating in a resonant cavity are transferred to high-order Stokes waves continuously and repeatedly. The high-sensitivity THz wave detector is realized by measuring the intensities of the Stokes waves or the enlarged THz wave. This research enables a comprehensive understanding of the physics of the cascading second-order nonlinear detection and provides a worthy reference for subsequent experimental design.

Frequency doubling of femtosecond laser pulses in three dimensional nonlinear photonic crystals

Quasi-phase matched nonlinear frequency conversion in three dimensional nonlinear photonic crystals (NPCs) has been considered for implementation of the second harmonic generation of transform-limited and chirped femtosecond laser pulses. Spatial distribution of the second harmonic intensity is shown to be caused by Cherenkov- and Raman–Nath-type nonlinear diffraction. These types of nonlinear diffraction may occur simultaneously resulting in an efficient multi-order nonlinear diffraction in the Bragg regime, which can be realized by a proper choice of the modulation period of NPC structure in longitudinal direction for considered transverse diffraction order. Three dimensional NPCs open up new possibilities for ultrafast nonlinear photonics and parametric interactions in optics.

Evolution on spatial patterns of structured laser beams: from spontaneous organization to multiple transformations

Spatial patterns are a significant characteristic of lasers. The knowledge of spatial patterns of structured laser beams is rapidly expanding, along with the progress of studies on laser physics and technology. Particularly in the last decades, owing to the in-depth attention on structured light with multiple degrees of freedom, the research on spatial and spatiotemporal structures of laser beams has been promptly developed. Such beams have hatched various breakthroughs in many fields, including imaging, microscopy, metrology, communication, optical trapping, and quantum information processing. Here, we would like to provide an overview of the extensive research on several areas relevant to spatial patterns of structured laser beams, from spontaneous organization to multiple transformations. These include the early theory of beam pattern formation based on the Maxwell–Bloch equations, the recent eigenmodes superposition theory based on the time-averaged Helmholtz equations, the beam patterns extension of ultrafast lasers to the spatiotemporal beam structures, and the structural transformations in the nonlinear frequency conversion process of structured beams.

FLASH free electron laser pump-probe laser concept based on spectral broadening of high-power ytterbium picosecond systems in multi-pass cells.

Within the FLASH2020+ upgrade, the pump-probe laser capabilities of the extreme ultraviolet and soft x-ray free-electron laser (XFEL) FLASH in Hamburg will be extended. In particular, providing wavelength tunability, shorter pulse durations, and reduced arrival time jitter will increase the scientific opportunities and the time resolution for the XFEL-optical laser pump-probe experiments. We present here a novel concept for the pump-probe laser at FLASH that is based on the post-compression of picosecond pulses emitted from high-power Ytterbium:YAG slab amplifiers. Flexible reduction of the pulse duration is facilitated by spectral broadening in pressure-tunable multi-pass cells. As an application, we show the pumping of a commercial optical parametric amplifier with 150fs post-compressed pulses. By means of an additional difference frequency generation stage, tunable spectral coverage from 1.3 to 16μm is reached with multi-μJ, sub-150fs pulses. Finally, a modular reconfiguration approach to the optical setups close to the free-electron laser instruments is implemented. This enables fast installation of the nonlinear frequency converters at the end stations for user operation and flexibility between different instruments in the two experimental halls.

Complete and robust energy conversion by sum frequency generation based on invariant engineering.

An analytical method is proposed in this paper to achieve complete energy conversion in sum frequency generation based on the Lewis-Riesenfeld invariants theory. In the proposed scheme, a quasi-adiabatic single control parameter model is established, and the value of single control parameter is selected to make the initial eigenstate perfectly converted to the final eigenstate as needed. Corresponding to the nonlinear frequency conversion process, a nonlinear crystal structure is designed by inverse engineering using the optimal control theory. It is robust against perturbations of the coupling coefficient and phase mismatch, including variations in the pump intensity and crystal polarization period, and achieves almost 100% conversion efficiency at any crystal length. Theoretical simulations show that frequency conversion can be achieved in the wavelength range of 2.6 µm-3.6 µm, and the spectral bandwidth of conversion efficiency exceeds 50% and approaches 400 nm when the crystal length L = 1 mm.

Broadband Second Harmonic Generation in a z-Cut Lithium Niobate on Insulator Waveguide Based on Type-I Modal Phase Matching

We numerically investigate a second harmonic generation (SHG) in a z-cut lithium niobate on insulator (LNOI) waveguide based on type-I mode phase matching (MPM) between two fundamental modes. A mode overlap factor that is close to unity is achieved and the normalized SHG efficiency reaches up to 72.1% W−1cm−2 at the telecommunication band, together with a large spectral tunability of 2.5 nm/K. Moreover, a bandwidth of about 100 nm for the broad SHG in a 5 mm-long LNOI ridge waveguide is demonstrated with this scheme. This stratagem will inspire new integrated nonlinear frequency conversion methods for versatile applications.

Rotational Doppler shift tripling via third-harmonic generation of spatially structured light in a quasi-periodically poled crystal

The rotational Doppler effect—the angular version of the Doppler effect—shows a frequency shift phenomenon happening when light passes through a rotating object and its angular momentum is changed. The rotational Doppler effect has been widely studied with various light waves based on different angular momentum transform mechanisms and successfully utilized to detect the rotation of objects. However, these studies are mostly limited to linear optics. As the rotational Doppler effect is closely related to the frequency degree of freedom and frequency conversion always happens in nonlinear optics, it is of great significance to explore the rotational Doppler effect in nonlinear optics. Although nonlinear rotational Doppler phenomena have been observed in second-harmonic generation with circularly polarized Gaussian fundamental light and a rotating nonlinear crystal, there is still the open challenge of how to realize Doppler frequency shift conversion (similar to conventional nonlinear frequency conversion), such as doubling or tripling. Here we report the experiment observation of rotational Doppler shift conversion in nonlinear optics. We demonstrate the tripling of a rotational Doppler frequency shift in third-harmonic generation of a spatially structured fundamental wave. Frequency shifts can be extracted from the intensity beating signals with the interference of two components of vector fields with opposite topological charges and the measured modulation frequency of the beating signal for the generated third harmonic. Our results will excite more research on nonlinear optics with spatially structured light, and our method may create opportunities for precision measurement of frequency shifts.

LD End-Pumped Nd:YAG InnoSlab Laser at 1319 nm

The 1319 nm lasers have important research value and application prospects in optical communications, biomedicine and nonlinear frequency conversion. Currently, there are few reports of high power 1319 nm continuous lasers with high beam quality. We have demonstrated a high output power, high beam quality 1319 nm continuous-wave laser by laser diode end-pumped Nd:YAG slab with a stable-unstable hybrid resonator. With a pumping power of 477 W, an output of 96.8 W was obtained with a slope efficiency of 26.5% and stability of 0.27%. A single wavelength laser operation at 1318.7 nm was demonstrated. At an output of 91.1 W, the beam quality factors M2 in stable and unstable directions were 2.95 and 1.88, respectively.

206 nm deep ultraviolet laser generated from fifth harmonic of femtosecond fiber laser

Deep ultraviolet (DUV) femtosecond laser, which combines the advantages of high single-photon energy of DUV laser with high peak power of femtosecond laser, is widely used in scientific research, biomedicine, material processing and so on. However, in the process of generating DUV femtosecond laser based on nonlinear frequency conversion is encountered a problem that the group velocity mismatch caused by dispersion makes the temporal walk-off of the nonlinear frequency conversion larger than the pulse duration of the femtosecond laser, thus making the generation of the DUV femtosecond laser very difficult. In this work, based on a Yb-doped fiber femtosecond laser, the delay line is optimized to precisely compensate for the spatial and temporal walk-off, so DUV femtosecond laser possesses the following performances: the center wavelength is 206 nm, the repetition rate is 1 MHz, the maximum output power is 102 mW, the maximum conversion efficiency is 4.25% from near infrared to DUV, the root mean square (RMS) power stability is 0.88% within 3 h, and the peak-to-peak power stability is 3.75%. The evolution of laser spectra and beam quality in the process of second harmonic generation (SHG), fourth harmonic generation (FHG) and sum-frequency generation (SFG) are also systematically studied. The experimental results provide a basis for generating DUV femtosecond laser from femtosecond fiber lasers.