Difference Frequency Generation Process Research Articles

Dual-Wavelength Mid-Infrared Generation Using Intracavity Stimulated Raman Scattering of PPLN

We report, to the best of our knowledge, the first demonstration of a continuous-wave, dual-wavelength (DW) mid-infrared optical parametric oscillator (OPO) based on the stimulated Raman scattering (SRS) effect of periodically poled lithium niobate (PPLN). The OPO was pumped by two independent fiber lasers, one being fixed at 1018 nm and the other being tunable. The high-power 1018-nm source built parametric oscillation, induced SRS effect in PPLN and generated 1495-nm signal beam as well as 1506-nm Raman beam. The tunable fiber laser was set at 1080 nm and the phase-matched difference frequency generation (DFG) occurred between the 1506-nm Raman beam and 1080-nm pump beam. Finally, DW idler beam located at 3189 and 3819 nm was obtained whose power ranged from 8.044 to 8.115 W, in which the 3189-nm idler power made up the majority of about 8 W and the maximum 3819-nm idler power reached 0.154 W. The slope and conversion efficiency of DFG and OPO processes was also discussed separately.

THz generation from DFG in a noncollinear phase-matched GaP crystal pumped with a compact diode-pumped two-frequency LiYF4:Nd laser

Pumping with a nanosecond two-frequency compact diode-pumped LiYF4:Nd laser emitting at 1047 and 1053 nm, a THz wave has been produced in a GaP crystal, attributed to the difference frequency generation process $$\frac{1}{{{\lambda _3}}} - \frac{1}{{{\lambda _2}}}=\frac{1}{{{\lambda _{{\text{TH}}z}}}}$$ . In a first set-up, the two pumping beams had a tunable angular separation provided by a sapphire prism. The phase matching angle of the noncollinear interaction has been determined to be 0.202° external angle and the refractive index 3.377 at 1.6 THz. The angular acceptance was rather broad, due to the fact that the thickness of the nonlinear crystal was only 3 mm. With a second set-up using a beam displacer constituted by YVO4 plates of different thicknesses, smaller angular separations were explored under other directions of polarizations of the pump beams. Because at the three wavelengths involved in this study the velocity of the nonlinear polarization wave inside GaP is higher than the THz wave phase velocity, a Cerenkov phase matching is possible. The plane-wave calculations show that the THz radiation under this process exits from the output face of the crystal, as it is experimentally observed.

Difference frequency generation of surface plasmon-polaritons in Landau quantized graphene

We develop a rigorous quantum-mechanical theory of the nonlinear optical process of difference frequency generation of surface plasmon-polaritons in Landau-quantized graphene. Although forbidden in the electric-dipole approximation, the second-order susceptibility is surprisingly high, equivalent to the bulk magnitude above $10^{-3}$ m/V. We consider the graphene monolayer as a nonlinear optical component of a monolithic photonic chip with integrated pump fields. The nonlinear power conversion efficiency of the order of tens $\mu$W/W$^2$ is predicted from structures of $10-100$ $\mu$m size. We investigate a variety of waveguide configurations to identify the optimal geometry for maximum efficiency.

Theoretical analysis of difference frequency generation for terahertz generation in a sheet microcavity from the CO2 laser

A scheme for efficient terahertz generation in a GaAs sheet microcavity from the CO2 laser is presented, in which the process of difference frequency generation that considered cavity phase matching is derived theoretically. This model achieves its optimum working condition when the cavity length corresponds to a coherent length and the resonant condition of pump wave is satisfied. The results show that the efficiency can realize an effective improvement when the CO2 lasers pump wavelengths are 9.55 μm and 9.79 μm, respectively, with a peak power conversion efficiency of 2.04%, which corresponds to a photon conversion efficiency of 83%. This work is expected to provide a theoretical basis for relevant experiments involving doubly and even triply resonant structures.

Quantum frequency conversion of memory-compatible single photons from 606 nm to the telecom C-band

We report on quantum frequency conversion of memory-compatible narrow-bandwidth photons at 606 nm to the telecom C-band at 1552$\,$nm. The 200$\,$ns long photons, compatible with Praseodymium-based solid-state quantum memories are frequency converted using a single-step difference frequency generation process in a periodically poled Lithium Niobate waveguide. We characterize the noise processes involved in the conversion and by applying strong spectral filtering of the noise, we demonstrate high signal-to-noise ratio conversion at the single photon level (SNR$\,>\,$100 for a mean input photon number per pulse of 1). We finally observe that a memory compatible heralded single photon with a bandwidth of 1.8$\,$MHz, obtained from a spontaneous parametric down conversion pair source, still shows a strong non-classical behavior after conversion. We first demonstrate that correlations between heralding and converted heralded photons stay in the non-classical regime. Moreover, we measured the heralded autocorrelation function of the heralded photon using the converter device as a frequency-domain beam splitter, yielding a value of $0.19\pm0.07$. The presented work represents a step towards the connection of several quantum memory systems emitting narrow-band visible photons to the telecommunication wavelengths.

Highly efficient cascaded difference‐frequency generation in periodically poled LiNbO3 devices with resonators

A method for highly efficient cascaded difference frequency generation (DFG) using a periodically poled LiNbO3 (PPLN) device with resonators is proposed, where the function of a PPLN device with resonators for second‐harmonic generation (SHG) put forward by Fujimura et al. is modified so that it can fit the cascaded DFG process including intermediate SHG. This modification successfully provides about 50 times higher efficiency than that without resonators in cascaded DFG by power enhancement in intermediate SHG with perfect resonators, although the SHG PPLN device of Fujimura et al. provided only eight times higher efficiency than that without resonators because of imperfect resonators. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Tunable terahertz wave difference frequency generation in a graphene/AlGaAs surface plasmon waveguide

Graphene-based surface plasmon waveguides (SPWs) show high confinement well beyond the diffraction limit at terahertz frequencies. By combining a graphene SPW and nonlinear material, we propose a novel graphene/AlGaAs SPW structure for terahertz wave difference frequency generation (DFG) under near-infrared pumps. The composite waveguide, which supports single-mode operation at terahertz frequencies and guides two pumps by a high-index-contrast AlGaAs/AlOx structure, can confine terahertz waves tightly and realize good mode field overlap of three waves. The phase-matching condition is satisfied via artificial birefringence in an AlGaAs/AlOx waveguide together with the tunability of graphene, and the phase-matching terahertz wave frequency varies from 4 to 7 THz when the Fermi energy level of graphene changes from 0.848 to 2.456 eV. Based on the coupled-mode theory, we investigate the power-normalized conversion efficiency for the tunable terahertz wave DFG process by using the finite difference method under continuous wave pumps, where the tunable bandwidth can reach 2 THz with considerable conversion efficiency. To exploit the high peak powers of pulses, we also discuss optical pulse evolutions for pulse-pumped terahertz wave DFG processes.

Tuning parametric processes in semiconductor diode lasers

We present GaAs/AlGaAs semiconductor lasers in which second-order nonlinearities are phase matched for efficient second-order nonlinear conversion. A comprehensive study of difference frequency generation (DFG) is presented, and the process is characterized for tuning, efficiency, and tolerances. External nonlinear conversion efficiency of 1.84×10−2%/W/cm2 is measured for the DFG process. The effects of carrier injection and temperature variation on DFG wavelength are studied, and the two effects are deconvolved for better understanding of carrier effects on nonlinear conversion. A wide DFG tuning range for the device operation is experimentally demonstrated where the idler wavelength can be tuned more than 30 nm for every 1-nm span of the pump wavelength.

Theoretical analysis of terahertz generation from a compact optical parametric oscillator based on adhesive-free-bonded periodically inverted KTiOPO4 plates

Terahertz (THz) generation by difference frequency generation (DFG) processes with dual signal waves is theoretically analyzed. The dual signal waves are generated by an optical parametric oscillator (OPO) with periodically inverted KTiOPO4 (KTP) plates based on adhesive-free-bonded (AFB) technology. The phase-matching conditions in a same AFB KTP composite for the OPO generating signals and idlers and for the DFG generating THz wave can be simultaneously satisfied by selecting the thickness of each KTP plate. Moreover, 4-order cascaded DFG processes can be realized in the same AFB KTP composite. The cascaded Stokes interaction processes generating THz photons and the cascaded anti-Stokes interaction processes consuming THz photons are investigated from coupled wave equations. Take an example of 3.106 THz which locates in the vicinity of polariton resonances, THz intensities and quantum conversion efficiencies are calculated. Compared with non-cascaded DFG processes, THz intensities of 3.106 THz in 4-order cascaded DFG processes increase to 5.56 times. When the pump intensity equals 20 MW mm−2, the quantum conversion efficiency of 259% in 4-order cascaded DFG processes can be realized, which exceeds the Manley–Rowe limit.

Generation of octave-spanning mid-infrared pulses from cascaded second-order nonlinear processes in a single crystal

We report on experimental generation of a 6.8 μJ laser pulse spanning from 1.8 to 4.2 μm from cascaded second-order nonlinear processes in a 0.4-mm BiB3O6 (BIBO) crystal. The nonlinear processes are initiated by intra-pulse difference frequency generation (DFG) using spectrally broadened Ti:Sapphire spectrum, followed by optical parametric amplification (OPA) of the DFG pulse. The highest energy, 12.6 μJ, is achieved in a 0.8-mm BIBO crystal with a spectrum spanning from 1.8 to 3.5 μm. Such cascaded nonlinear processes are enabled by the broadband pump and the coincident phase matching angle of DFG and OPA. The spectrum is initiated from the DFG process and is thus expected to have passive stable carrier-envelope phase, which can be used to seed either a chirped pulse amplifier (CPA) or an optical parametric chirped pulse amplifier (OPCPA) for achieving high-energy few-cycle mid-infrared pulses. Such cascaded second-order nonlinear processes can be found in many other crystals such as KTA, which can extend wavelengths further into mid-infrared. We achieved a 0.8 μJ laser pulse spanning from 2.2 to 5.0 μm in KTA.

Terahertz generation by difference frequency generation from a compact optical parametric oscillator

Terahertz (THz) generation by difference frequency generation (DFG) processes with dual idler waves is theoretically analyzed. The dual idler waves are generated by a compact optical parametric oscillator (OPO) with periodically poled lithium niobate (PPLN). The phase-matching conditions in a same PPLN for the optical parametric oscillation generating signal and idler waves and for the DFG generating THz waves can be simultaneously satisfied by selecting the poling period of PPLN. Moreover, 3-order cascaded DFG processes generating THz waves can be realized in the same PPLN. To take an example of 8.341THz which locates in the vicinity of polariton resonances, THz intensities and quantum conversion efficiencies are calculated. Compared with non-cascaded DFG processes, THz intensities of 8.341THz in 3-order cascaded DFG processes increase to 2.57 times. When the pump intensity equals to 20MW/mm2, the quantum conversion efficiency of 106% in 3-order cascaded DFG processes can be realized, which exceeds the Manley-Rowe limit.

Terahertz Generation Based on Cascaded Difference Frequency Generation with Periodically-poled KTiOPO4

Terahertz (THz) generation by periodically-poled KTiOPO4 (PPKTP) with a quasi-phase-matching scheme based on cascaded difference frequency generation (DFG) processes is theoretically analyzed. The cascaded Stokes interaction processes and the cascaded anti-Stokes interaction processes are investigated from coupled wave equations. THz intensities and quantum conversion efficiency are calculated. Compared with non-cascaded DFG processes, THz intensities from 10-order cascaded DFG processes are increased to 5.53. The quantum conversion efficiency of 479.4% in cascaded processes, which exceeds the Manley-Rowe limit, can be realized.

Highly efficient terahertz generation from periodically-poled RbTiOPO4

Terahertz (THz) generation by periodically-poled RbTiOPO4 (PPRTP) with a quasi-phase-matching scheme based on cascaded difference frequency generation (DFG) processes is theoretically analyzed. The cascaded Stokes and anti-Stokes interaction processes are investigated from coupled wave equations. The THz intensities and quantum conversion efficiency are calculated. Compared with that of non-cascaded DFG processes, the THz intensity in 7-order cascaded DFG processes is increased to 2.95 times. The quantum conversion efficiency of 149.9% in cascaded processes can be realized, which exceeds the Manley-Rowe limit.

Theoretical Evaluation of Terahertz Sources Generated From SnGa4 Q7 (Q=S, Se) as Infrared Nonlinear Optical Materials.

We theoretically evaluated the integrated knowledge that contributes to conversion efficiency, including the phonon, photon, and electron properties of infrared nonlinear optical materials such as SnGa4 Q7 (Q=S, Se), which are terahertz (THz) sources. Specifically, we developed a new formula to calculate the susceptibility of the difference frequency generation (DFG) optical process. By evaluating the characteristics of the materials themselves in the THz region, we found that a larger nonlinear susceptibility or a large figure of merit resulted in a large efficiency of the THz source by comparing the findings of SnGa4 Se7 and SnGa4 S7 under the same experimental conditions; furthermore, THz absorption was found to reduce the efficiency of the THz source for the two SnGa4 Q7 (Q=S, Se) materials. The efficiency of the THz source also depended on the experimental conditions. A large crystal size, strong pump intensity, and small THz wavelength resulted in better efficiency of the THz source based on the DFG process. The efficiency was found to be a comprehensive index to evaluate the THz source based on the DFG process.

Effect of Waveguide Inhomogeneity in a χ(2)-Based Pulsed Optical Parametric Amplifier

The effect of waveguide inhomogeneity in transmitted pulses of a χ(2)-based optical parametric amplifier is investigated. Periodically poled lithium niobate (PPLN) channel waveguides are used to achieve optical parametric amplification (OPA) via cascaded second harmonic generation and difference frequency generation (cSHG/DFG) process at telecommunication bandwidth. To access high input power, a Q-switched laser of low duty cycle was used for the fundamental source. The ns-pulsed fundamental generates the pump for the DFG (OPA) process via SHG. The signal amplification and the wavelength converted idler generation are achieved via DFG process between the SH and the signal. It is shown that inhomogeneity of the waveguide leads to change in the relative phase between interacting waves along the waveguide. Consequently, transmitted pulses are disturbed for an inhomogeneous waveguide. Experimental and theoretical results of transmitted pulses from a homogeneous and an inhomogeneous PPLN channel waveguides are presented. In the homogeneous channel guide with Gaussian-shaped transmitted pulses, more than 20 dB of parametric gain was achieved in an 88-mm-long waveguide when ∼1.7 W of coupled fundamental peak power is launched to the waveguide.

Maxwell–Hydrodynamic Model for Simulating Nonlinear Terahertz Generation From Plasmonic Metasurfaces

The interaction between the electromagnetic field and plasmonic nanostructures leads to both the strong linear response and inherent nonlinear behavior. In this paper, a time-domain hydrodynamic model for describing the motion of electrons in plasmonic nanostructures is presented, in which both surface and bulk contributions of nonlinearity are considered. A coupled Maxwell-hydrodynamic system capturing full-wave physics and free electron dynamics is numerically solved with the parallel finite-difference time-domain (FDTD) method. The validation of the proposed method is presented to simulate linear and nonlinear responses from a plasmonic metasurface. The linear response is compared with the Drude dispersion model and the nonlinear terahertz emission from a difference-frequency generation process is validated with theoretical analyses. The proposed scheme is fundamentally important to design nonlinear plasmonic nanodevices, especially for efficient and broadband THz emitters.

Non-degenerate optical parametric amplification analysis of surface plasmon polariton wave in a silver coated PPLN planar waveguide

Propagation length enhancement of surface plasmon polariton (SPP) wave in telecom c-band through nonlinear χ(2)-based nondegenerate difference frequency generation (ndDFG) process is investigated theoretically. The SPP wave is excited in a silver coated planar periodically poled lithium niobate (PPLN) waveguide. Quasi phase matching is used to compensate the phase mismatch between the interacting excited SPP and guided modes. Domain inversion with proper domain wavelength of Λ = 101.9 μm leads to nonlinear efficient mode interaction of guided↔SPP. With 31 MW/cm coupled pump intensity a SPP amplification of about 16 dB is achievable which results in enhancement of the propagation length about 11 mm. The acceptance SPP amplification bandwidth is pump power dependent. In a low pump depletion regime, a 3-dB bandwidth is about 75 nm whereas in more depletion the bandwidth becomes narrower but not less than 25 nm.

Frequency down-conversion of a dual-wavelength fiber laser

The experimental study of frequency down-conversion processof a dual-wavelength fiber laser by using an MgO:PPLN nonlinear crystal was presented. The dual-wavelength fiber laser was a home-made linearly polarized fiber laser with a maximal output power of 74.1W at 1060nm and 1080nm. A total idler output power of 6.36W with dual idler-waves at 3125nm and 3323nm was obtained. The corresponding pump-to-idler slope efficiency was 11.4%. In the experiment, only one signal wavelength of 1603nm was observed, which indicated that only one optical parametric oscillation occurred while the other pump laser was contributed to the intra-cavity difference frequency generation (DFG) process. This work provides an effective approach to generate dual-wavelength mid-infrared (MIR) lasers for its features of convenience and controllability.

Flexible All-Optical Wavelength Shifters Using Strong Focusing in a Wideband Engineered PPLN

Wavelength shift of modified second-harmonic bandwidth in a 20-mm-long engineered chirped periodically poled lithium niobate is experimentally demonstrated by focal-point displacement of a strongly focused pump beam of a 30- $\mu \text{m}$ beam waist. Utilizing this non-confocal arrangement in a cascaded second harmonic generation and difference frequency generation process, a CW probe as a signal wavelength is variably shifted within an 8-nm span by pump tuning using a $\sim 7$ -nm second-harmonic bandwidth.

High-Power High-Brightness Terahertz Source Based on Nonlinear Optical Crystal Fiber

A nonlinear crystal fiber converter for terahertz (THz) difference frequency generation (DFG) is proposed. This axisymmetric structure consists of a periodically inverted GaAs core clad with index-matched chalcogenide glass, which functions as a single-mode fiber for THz radiation to prevent its diffraction and achieve efficient optical-to-THz conversion. The process of DFG between ps-pulses in the crystal fiber is investigated by developing the guided-wave coupled-mode equations for quasi-monochromatic pulses. The output power, spectral power density, and brightness are analyzed, based on the calculations of the dynamic of energy conversion and the characteristics of the THz beam focusing. High output power as well as excellent beam quality contributes to the high brightness of the presented THz crystal fiber source.