Frequency Conversion Processes Research Articles

Temperature-insensitive frequency conversion achieved by phase-matching outside principal planes and thermally induced phase-mismatch compensation

To enhance the thermal stability and scale up the output power of frequency conversion, a temperature-insensitive frequency conversion method is proposed, which is achieved by phase-matching outside principal planes and thermally induced phase-mismatch compensation. In the method, there are three cascaded crystals. Two crystals at the ends are phase matched with propagation directions outside principal planes. The other crystal, with an opposite sign of first temperature derivative of phase mismatch, is sandwiched in the middle and used for compensating the thermally induced phase mismatch generated in the first crystal. In a proof-of-principle study, two KTiOPO4 (KTP) crystals cut at (θ = 78.28 deg, φ = 40.99 deg) and a compensation crystal LiB3O5 (LBO) are employed for temperature-insensitive second harmonic generation of 1064 nm. As a result, the temperature bandwidth and maximum conversion efficiency, respectively, reach at 49.3°C and 56.8% with an interaction length of 14 mm. The proposed method is capable of increasing thermal stability for various frequency conversion processes across a broad spectral band.

Widely tunable, continuous-wave, intra-cavity optical parametric oscillator based on an Yb-doped fiber laser.

We report an optical parametric oscillator (OPO) intra-cavity pumped by an Yb-doped fiber laser. In comparison to an intra-cavity OPO based on a solid-state laser gain medium, the benefits of using a fiber-based scheme include a superior beam quality of the generated mid-infrared idler light at high power levels, a more efficient process of nonlinear frequency conversion, and the prospect of scaling to higher power. In a preliminary experiment, we obtained a slope efficiency of the down-converted power of as high as 66% with respect to the absorbed laser diode power. To the best of our knowledge, this is the first demonstration of an intra-cavity OPO based on a fiber laser.

Quantum frequency conversion for multiplexed entangled states generated from micro-ring silicon chip.

Silicon-on-chip photonic circuits are among some very promising platforms for generating nonclassical photonic quantum state, because of its low loss, small footprint, and compatibility with complementary metal-oxide-semiconductor (CMOS) and telecommunications techniques. Dense wavelength division multiplexing (DWDM) is a leading technique for enhancing the transmission capacity of both classical and quantum communications. To bridge the frequency gap between silicon-chip and other quantum systems, such as quantum memories, a quantum interface is indispensable. Here, we demonstrate a quantum interface for multiplexed energy-time entanglement states, which are generated on a silicon micro-ring cavity that is based on frequency up-conversion. By switching the pump wavelength, energy-time entanglement from any channel can be selected at will after being up-converted. The high visibilities of two-photon interference over three channels after frequency up-conversion clearly prove that the entanglement is fully preserved during the quantum frequency conversion (QFC) process. Our work provides new perspectives regarding channel capacity enhancement in quantum communications and for quantum resources being transferred between two different quantum systems.

Particle production in ultrastrong-coupling waveguide QED

Understanding large-scale interacting quantum matter requires dealing with the huge number of quanta that are produced by scattering even a few particles against a complex quantum object. Prominent examples are found from high energy cosmic ray showers to the optical or electrical driving of degenerate Fermi gases. We tackle this challenge in the context of many-body quantum optics, as motivated by the recent developments of circuit quantum electrodynamics at ultrastrong coupling. The issue of particle production is addressed quantitatively with a simple yet powerful concept rooted in the quantum superposition principle. This key idea is illustrated by the study of multi-photon emission from a single two-level artificial atom coupled to a high impedance waveguide. We find surprisingly that the off-resonant inelastic emission lineshape is dominated by broadband particle production, due to the large phase space associated with contributions that do not conserve the number of excitations. Such frequency conversion processes produce striking signatures in time correlation measurements, which can be tested experimentally in quantum waveguides. These ideas open new directions for the simulation of a variety of physical systems, from polaron dynamics in solids to complex superconducting quantum architectures.

Frequency Down-Conversion of Dual-Wavelength Raman Fiber Laser in PPLN-Based Optical Parametric Oscillator

We report on frequency down-conversion of dual-wavelength (DW) Raman fiber laser in a periodically poled lithium niobate-based optical parametric oscillator. The DW pump source was fixed at 1060 and 1111 nm that was obtained based on stimulated Raman scattering effect by combining a home-made linearly polarized 1060-nm fiber laser and 137-m-long polarization-maintaining passive fiber. The total pump power went through three stages, in the latter two of which the 1111-nm wave appeared. In the entire experiment, the 1060-nm pump beam achieved parametric oscillation and generated 1626-nm signal beam and 3056-nm idler beam. The 1111-nm pump beam generated 3503-nm idler beam based on difference frequency generation (DFG) between itself and 1626-nm signal beam in the second stage. With power enhancement, it built independent parametric oscillation in the third stage and generated 1621-nm signal beam as well as 3530-nm idler beam. The DW idler power ranged from 3.94 to 7.78 W in the latter two stages. The power and efficiency characteristics in frequency conversion processes of 1060- and 1111-nm pump beams were also analyzed separately.

Spectral discrete diffraction with non-Hermitian coupling

We investigate the frequency conversion in a waveguide by dynamically modulating the real and imaginary parts of permittivity, namely, the complex modulation. A single frequency of incident light could convert into many discrete frequencies with equivalent intervals, which is analogous to discrete diffraction in spatial waveguide arrays. We develop a coupled mode theory to analyze the frequency conversion process and show the complex modulation results in a complex-valued coupling coefficient between adjacent frequency channels. The coupling strength can be flexibly tuned by controlling the modulation amplitudes. Moreover, the complex coupling becomes asymmetric with increasing the modulation phase difference between the real and imaginary parts of permittivity. The capability to control the complex coupling leads to new features in discrete diffraction in the frequency dimension, including the unidirectional transport and equalization of light intensities at different channels. The study may find interesting applications in frequency comb generators and frequency controllers.

About the Implementation of Frequency Conversion Processes in Solar Cell Device Simulations.

Solar cells are electrical devices that can directly convert sunlight into electricity. While solar cells are a mature technology, their efficiencies are still far below the theoretical limit. The major losses in a typical semiconductor solar cell are due to the thermalization of electrons in the UV and visible range of the solar spectrum, the inability of a solar cell to absorb photons with energies below the electronic band gap, and losses due to the recombination of electrons and holes, which mainly occur at the contacts. These prevent the realization of the theoretical efficiency limit of 85% for a generic photovoltaic device. A promising strategy to harness light with minimum thermal losses outside the typical frequency range of a single junction solar cell could be frequency conversion using rare earth ions, as suggested by Trupke. In this work, we discuss the modelling of generic frequency conversion processes in the context of solar cell device simulations, which can be used to supplement experimental studies. In the spirit of a proof-of-concept study, we limit the discussion to up-conversion and restrict ourselves to a simple rare earth model system, together with a basic diode model for a crystalline silicon solar cell. The results of this show that these simulations are very useful for the development of new types of highly efficient solar cells.

Field enhancement and spectral features of hexagonal necklaces of silver nanoparticles for enhanced nonlinear optical processes.

The nonlinear properties of hybrid metallic-dielectric systems are attracting great interest due to their potential for the enhancement of frequency conversion processes at nanoscale dimensions. In this work, we theoretically and experimentally address the correlation between the near field distribution of hexagonal plasmonic necklaces of silver nanoparticles formed on the surface of a LiNbO3 crystal and the second harmonic generation (SHG) produced by this nonlinear crystal in the vicinities of the necklaces. The spectral response of the hexagonal necklaces does not depend on the polarization direction and is characterized by two main modes, the absorptive high-energy mode located in the UV spectral region and the lower energy mode, which is strongly radiant and extends from the visible to the near infrared region. We show that the spatial distribution of the enhanced SHG is consistent with the local field related to the low energy plasmon mode, which spectrally overlaps the fundamental beam. The results are in agreement with the low absorption losses of this mode and the two-photon character of the nonlinear process and provide deeper insight in the connection between the linear and nonlinear optical properties of the hybrid plasmonic-ferroelectric system. The study also highlights the potential of hexagonal necklaces as useful plasmonic platforms for enhanced optical processes at the nanoscale.

Second Harmonic Generation in CdSiP2 Nanowires in the Optical Frequency Regime

We report on the second harmonic generation of light in a cadmium silicon phosphide (CdSiP2, CSP) nanowire waveguide excited at the telecommunication wavelength of 1550 nm. This frequency-conversion process is studied by means of time-domain simulations that solve for the full vector fields of Maxwell’s equations and which incorporate the complete second-order nonlinear coefficient tensor of the CSP crystal. Over the short length of 30 $\mu \text{m}$ , the CSP nanowire waveguide produces electric field pulses having a peak-to-peak value of 116 kV/cm at a conversion efficiency of $1.6\times 10^{-3}$ . This electric field value is five times larger than that obtained in a lithium niobate nanowire waveguide having the same footprint, whereas the conversion efficiency value is 11 times higher than what is achieved in the lithium niobate nanowire. These results suggest that CSP nanowire waveguides have the potential to be used for on-chip sources of coherent optical radiation that are highly efficient given their compact structure.

Nonlinear generation of Airy vortex beam.

Recently, hybrid beams have sparked considerable interest because of their properties coming from different kinds of beams at the same time. Here, we experimentally demonstrate Airy vortex beam generation in the nonlinear frequency conversion process when the fundamental wave with its phase modulated by a spatial light modulator is incident into a homogeneous nonlinear medium. In our experiments, second harmonic Airy circle vortex beams and Airy ellipse vortex beams were generated and the topological charge was also measured. The parabolic trajectory of those Airy vortex beams can be easily adjusted by altering the fundamental wave phase. This study provides a simple way to generate second harmonic Airy vortex beams, which may broaden its future use in optical manipulation and light-sheet microscopy.

Optical frequency comb generation based on the dual-mode square microlaser and a nonlinear fiber loop

A novel approach using a dual-mode square microlaser as the pump source is demonstrated to produce wideband optical frequency comb (OFC). The enhanced nonlinear frequency conversion processes are accomplished in a nonlinear fiber loop, which can reduce the stimulated Brillouin scattering threshold and then generate a dual-mode Brillouin laser with improved optical signal-to-noise ratio. An OFC with 130 nm bandwidth and 76 GHz repetition rate is successfully generated under the four-wave mixing, and the number of the comb lines is enhanced by ~ 26 times compared with the system without fiber loop. In addition, the repetition rate of the comb can be adjusted by changing the injection current of the microlaser. The pulse width of the comb spectrum is also compressed from 3 to 1 ps with an extra amplification-nonlinear process.

Two-step frequency conversion for connecting distant quantum memories by transmission through an optical fiber

Long-distance quantum communication requires entanglement between distant quantum memories. For this purpose, photon transmission is necessary to connect the distant memories. Here, for the first time, we develop a two-step frequency conversion process (from a visible wavelength to a telecommunication wavelength and back) involving the use of independent two-frequency conversion media where the target quantum memories are nitrogen-vacancy centers in diamonds (with an emission/absorption wavelength of 637.2 nm), and experimentally characterize the performance of this process acting on light from an attenuated CW laser. A total conversion efficiency of approximately 7% is achieved. The noise generated in the frequency conversion processes is measured, and the signal-to-noise ratio is estimated for a single photon signal emitted by a nitrogen-vacancy (NV) center. The developed frequency conversion system has future applications via transmission through a long optical fiber channel at a telecommunication wavelength for a quantum repeater network.

Nonlinear frequency doubling characteristics of asymmetric vortices of tunable, broad orbital angular momentum spectrum

We report on a simple experimental scheme to generate and control the orbital angular momentum (OAM) spectrum of the asymmetric vortex beams in a nonlinear frequency conversion process. Using a spiral phase plate (SPP) and adjusting the transverse shift of the SPP with respect to the incident Gaussian beam axis, we have transformed the symmetric (intensity distribution) optical vortex of order l into an asymmetric vortex beam of measured broad spectrum of OAM modes of orders l, l − 1, l − 2, …, 0 (Gaussian mode). While the position of the SPP determines the distribution of the OAM modes, we have also observed that the modal distribution of the vortex beam changes with the shift of the SPP of all orders and finally results in a Gaussian beam (l = 0). Using single-pass frequency doubling of the asymmetric vortices, we have transferred the pump OAM spectra, l, l − 1, l − 2, …, 0, into the broad spectra of higher order OAM modes, 2l, 2l − 1, 2l − 2, …, 0 at green wavelength, owing to OAM conservation in nonlinear processes. We also observed an increase in single-pass conversion efficiency with the increase in asymmetry of the pump vortices producing a higher power vortex beam of mixed OAM modes at a new wavelength than that of the pure OAM mode.

Efficient frequency conversion for cubic harmonic generation at 266 nm in centrosymmetric α-BBO crystal.

Direct third-harmonic generation (THG) is a third-order nonlinear process without the restriction to the symmetric characteristic of crystals. It is of great interest for setting up a χ(3) optical parametric oscillator which can be used in multiphoton quantum correlation. To obtain pure and strong THG, we proposed to elect the centrosymmetric crystal with delocalized π bond as the nonlinear media during the frequency conversion process. An unprecedented cubic harmonic energy 37.6μJ (average power ∼37.6 mW, conversion efficiency ∼2.5%) at 266nm was generated in α-BBO crystal (not the β-BBO), indicating direct THG in practical application. These results also demonstrated a succinct and efficient way to generate deep-UV laser and another development direction of nonlinear crystals in UV region.

ALOHA—Astronomical Light Optical Hybrid Analysis

This paper gives an overview of the Astronomical Light Optical Hybrid Analysis (ALOHA) project dedicated to investigate a new method for high resolution imaging in mid infrared astronomy. This proposal aims to use a non-linear frequency conversion process to shift the thermal infrared radiation to a shorter wavelength domain compatible with proven technology such as guided optics and detectors. After a description of the principle, we summarise the evolution of our study from the high flux seminal experiments to the latest results in the photon counting regime.

Crystal growth, experimental and theoretical investigations of organic NLO material 4-nitrophthalimide

The adequate size of 4Nitrophthalimide (4-NPT) single crystals has been grown by solution growth slow evaporation technique at room temperature in methanol. The Vibrational and electronic properties of the crystal were analysed by analysing the FTIR,FT-Raman, UV –Vis spectrum. The Powder and single crystal XRD reveals that the crystalline structure of the grown material. Hyperpolarizability and electric dipole moment values have been calculated theoretically by using Gaussian 09 Package. TG/DTA study has been used to investigate the thermal stability of the grown crystal. By using the Kurtz and Perry method, second harmonic frequency conversion process of the crystal was examined. The band gap energy of the molecule can be examined from the HOMO – LUMO energy.

Characterization of ultrashort laser pulses employing self-phase modulation dispersion-scan technique

We present a new phase characterization technique for ultrashort laser pulses that employs self-phase modulation (SPM) in the dispersion scan approach. The method can be implemented by recording a set of nonlinearly modulated spectra generated with a set of known chirp values. The unknown phase of the pulse is retrieved by linking the recorded spectra to the initial spectrum of the pulse via a phase function guessed by a function minimization iterative algorithm. This technique has many advantages over the dispersion scan techniques that use frequency conversion processes. Mainly, the use of SPM cancels out the phase and group velocity mismatch errors and dramatically widens the spectral acceptance of the nonlinear medium and the range of working wavelength. The robustness of the technique is demonstrated with smooth and complex phase retrievals using numerical examples. The method is shown to be not affected by the spatial distribution of the beam or the presence of nonlinear absorption process. In addition, we present an efficient method for phase representation based on a summation of a set of Gaussian functions. The independence of the functions from each other prevents phase coupling of any kind and facilitates a flexible phase representation.

Adaptive pumping for spectral control of broadband second-harmonic generation.

Second-harmonic generation (SHG) is always a significant frequency conversion process in nonlinear optics for many great applications but can be limited when broadband spectral laser sources are involved, e.g.,femtosecond pulses. The conversion efficiency can be high, but the spectral control is hard because of the phase-matching (PM) limitation. Recently, a random quasi-phase-matching (QPM) scheme was proposed to make use of highly nonlinear materials that are difficult to be phase matched under traditional configurations. The spectral control is even harder in anisotropic random materials, and the coherence is completely lost. Here, we proposed an approach to solve this problem by coherent light control via feedback-based wavefront shaping. We utilized this method for spectral control of broadband SHG, which can be efficient even in strongly scattering media. Randomly selected wavelengths in the broadband spectra were enhanced with a good selectivity, and the direction was also controlled in a three-dimensional (3D) configuration. This technique paves the way for convenient spatial and spectral control of both linear and nonlinear emissions and a local enhancement of their conversion efficiency, indicating great progress in both random and ultrafast nonlinear optics.

High-Power, Solid-State, Deep Ultraviolet Laser Generation

At present, deep ultraviolet (DUV) lasers at the wavelength of fourth harmonics of 1 μm (266 nm/258 nm) and at the wavelength of 193 nm are widely utilized in science and industry. We review the generation of these DUV lasers by nonlinear frequency conversion processes using solid-state/fiber lasers as the fundamental frequency. A DUV laser at 258 nm by fourth harmonics generation (FHG) could achieve an average power of 10 W with a beam quality of M2 < 1.5. Moreover, 1 W of average power at 193 nm was obtained by sum-frequency generation (SFG). A new concept of 193-nm DUV laser generation by use of the diamond Raman laser is also introduced. A proof-of-principle experiment of the diamond Raman laser is reported with the conversion efficiency of 23% from the pump to the second Stokes wavelength, which implies the potential to generate a higher power 193 nm DUV laser in the future.

Microwave amplification based on quasiparticle SIS up and down frequency converters

Heterodyne instruments have recently attained quantum-limited low-noise performance, particularly in radio astronomy, but it is difficult to develop large heterodyne arrays such as a modern radio camera using cryogenic sensitive detectors based on microwave kinetic inductance detectors, transition edge sensors, etc. In the realization of the heterodyne array, the reduction of power dissipation for semiconductor-based amplifiers remains a major challenge. Alternatively, superconducting parametric amplifiers still seem to have several barriers to application, especially in terms of operating temperature. Here, we show a novel concept of microwave amplification based on up and down frequency-conversion processes using quasiparticle superconductor-insulator-superconductor (SIS) tunnel junctions. We demonstrate positive gain using a proof-of-concept test module, which operates with a power dissipation of several μW at a bath temperature of 4 K. The performance of the module suggests great potential for application in large arrays.