Difference Frequency Generation Efficiency Research Articles

A cascaded difference frequency generation method combined with cavity phase matching and quasi phase matching for high-efficiency terahertz generation

A cascaded difference frequency generation (DFG) method for high-efficiency terahertz generation is presented, which is combined with cavity phase matching and quasi phase matching. The results show that a ten-orders cascade effect can be approximately satisfied, with a peak photon conversion efficiency of 447.6%, which exceeds the Manley-Rowe limit. This method is expected to contribute to improving the conversion efficiency of DFG for terahertz generation.

Quasi-phase matched difference frequency generation in corrugation-reduced GaAs/AlGaAs periodically-inverted waveguides

We have evaluated the conversion efficiency of difference frequency generation (DFG) around 3.4 μm in a periodically-inverted GaAs/AlGaAs waveguide whose corrugation height was reduced by using chemical mechanical polishing (CMP) process. The total propagation loss for interacting waves was decreased by suppressing the corrugations at interfaces between guiding and cladding layers. The internal conversion efficiency of DFG without Fabry–Perot enhancement was estimated to be 3.0% W−1 in a 2.1 mm long CMP waveguide. Guidelines for further increasing conversion efficiencies are also given.

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.

Broadband pulsed difference frequency generation laser source centered 3326 nm based on ring fiber lasers

A broadband pulsed mid-infrared difference frequency generation (DFG) laser source based on MgO-doped congruent LiNbO3 bulk is experimentally demonstrated, which employs a homemade pulsed ytterbium-doped ring fiber laser and a continuous wave erbium-doped ring fiber laser to act as seed sources. The experimental results indicate that the perfect phase match crystal temperature is about 74.5[Formula: see text]C. The maximum spectrum bandwidth of idler is about 60 nm with suitable polarization states of fundamental lights. The central wavelength of idlers varies from 3293 nm to 3333 nm over the crystal temperature ranges of 70.4–76[Formula: see text]C. A jump of central wavelength exists around crystal temperature of 72[Formula: see text]C with variation of about 30 nm. The conversion efficiency of DFG can be tuned with the crystal temperature and polarization states of fundamental lights.

Widely-tunable high-repetition-rate terahertz generation in GaSe with a compact dual-wavelength KTP OPO around 2 μm.

A compact efficient high-repetition-rate doubly-resonant dual-wavelength KTP optical parametric oscillator (OPO), with output power up to 3.65 W and tuning ranges of 2.088-2.133 μm/2.171-2.122 μm for signal/idler waves, was deployed for terahertz (THz) generation in a GaSe crystal. Based on difference frequency generation (DFG), the THz wave was continuously tunable from 730.9 μm (0.41 THz) to 80.8 μm (3.71 THz), believed to be the first report of a compact high-repetition-rate widely-tunable THz source. The maximum THz average power reached 1.2 μW at 1.54 THz and the corresponding DFG efficiency was 7.8 × 10-7, entirely suitable for portable applications. The utility of the THz source was also demonstrated through spectroscopy and imaging experiments.

Difference-frequency generation of optical radiation from two-color x-ray pulses.

We describe the process of difference-frequency generation of short optical pulses from two-color X-ray pulses. By assuming 10¹¹ photons per X-ray pulse, we predict that the optical count rate can exceed 10⁷ photons per pulse. Similar to other effects involving nonlinear interactions of X-rays and optical radiation, the effect we describe can be used for microscopic studies of chemical bonds and as a probe for light-matter interactions on the atomic scale. Since the X-ray damage threshold is much higher than the optical damage threshold, the efficiency of difference-frequency generation from two X-ray pulses is expected to be orders of magnitude higher than the efficiency of effects such as sum/difference-frequency mixing between X-rays and optical intense short-pulse sources.

Optical frequency mixing through nanoantenna enhanced difference frequency generation: Metatronic mixer

A design for a subwavelength all-optical frequency mixer is proposed. The method relies on enhanced difference-frequency generation, which is achieved in two steps with the help of plasmonic nanoantennas. The interaction of the two input signals with the nonlinear material is increased through the use of input nanoantennas, which focus the incident energy of two different frequencies onto the nanoparticle formed by a nonlinear material. Next, the difference-frequency emission is enhanced through the Purcell effect by the use of a separate output nanoantenna that is resonant at the difference frequency. The application of this twofold approach allows for a significant enhancement in the difference-frequency generation efficiency. Simulation results are presented highlighting the features of the method. This multi-element nanostructure is indeed an optical mixer circuit element in the metatronic paradigm.

2D simulation of generation of difference-frequency radiation by few-cycle laser pulse in thin crystal of GaAs

We present the results of theoretical study of generation of difference frequency radiation in interaction of few-cycle laser pulse with a GaAs thin crystal. 2D numerical time-integration of system of nonlinear Maxwell equations for the TEy mode has been performed by the finite-difference method. We consider the interaction of linearly-polarized pulse having the central wavelength of 1.98 μm, duration of 17.16 fs and the electric field amplitude E x0,max = 100 MV/m, propagating along the normal to the ?110? plane in the 14.74 μm-thickness GaAs crystal. The beam waist of initial linearly polarized pulse varies from 5.94 μm to 9.9 μm. The process of generation of difference frequency radiation (DFR) pulses arising at the output of nonlinear crystal via spectral filtration of formed supercontinuum is studied. The dependence of difference frequency generation efficiency on the beam waist is examined.

Generation of difference-frequency radiation in GaAs in the field of a few-cycle laser pulse in the regime of weakly expressed chromatic dispersion

We derive a system of coupled modified Korteweg-de Vries equations in the approximation of unidirectional waves which describe the evolution of electric fields of mutually-orthogonal linearly polarized laser pulses with duration of several optical oscillations in a nonlinear crystal with quadratic nonlinearity in the regime of weakly expressed chromatic dispersion. An isotropic GaAs crystal is considered as a nonlinear crystal, and the direction of propagation of interacting pulses is assumed to coincide with the normal to the 〈110〉 plane of the crystal. The derived system of equations was solved numerically by the finite difference method. We obtained the dependence of the relative shift of the central wavelength of pumping pulse spectrum on the distance travelled. It is shown that the decrease in the crystal thickness down to values commensurate with the coherence length of difference-frequency radiation leads to an increase in conversion efficiency. In particular, it is shown that the efficiency of difference-frequency generation at 4.1 μm and 10 μm for the 30 fs duration of pumping at 1.98 μm and the 100 MV/m amplitude of the electric field is −51 dB and −14 dB, respectively.

Spectral distribution of the efficiency of terahertz difference frequency generation upon collinear propagation of interacting waves in semiconductor crystals

Dispersion phase matching curves and spectral distributions of the efficiency of difference frequency generation in the terahertz range are calculated for collinear propagation of interacting waves in zinc blende semiconductor crystals (ZnTe, CdTe, GaP, GaAs). The effect of the pump wavelength, the nonlinear crystal length and absorption in the terahertz range on the spectral distribution of the efficiency of difference frequency generation is analysed.

Analysis of THz-wave surface-emitted difference-frequency generation in periodically poled lithium niobate waveguide

It was shown that the periodically poled LiNbO3-waveguide with period of poling λ≈λ/ng (λ is the wavelength of emitted THz-wave, ng is a refractive index corresponding to optical group velocity) emits THz-wave difference-frequency generation (DFG) in the direction normal to the surface of the planar waveguide. The 5% distinction between the manufactured and required periods of gratings results only in a small deflection (∼6°) of the output THz-beam from the normal direction. The dependence of DFG efficiency on mode size is analyzed. The output THz power at λ=150 μm is estimated as 2 mW, taking into account imperfections in coupling incident beams with guided modes. It was shown that the efficiency of THz-wave DFG in surface-emitting geometry is more than for collinear geometry in bulk crystal, especially in the high-absorption wavelength region.

Sum- and difference-frequency generation for broadband input fields.

We analyze the dynamics of sum-frequency generation (SFG) and difference-frequency generation (DFG) for broadband input fields. We demonstrate that amplitude modulation of the input fields significantly affects conversion efficiency of SFG and DFG, but frequency modulation does not affect conversion efficiency. Analytic results for SFG and DFG output-field intensities are thereby available for arbitrary fields. Self- and cross-phase modulation can affect the dynamics in the limit of very high field strengths, but these effects are of higher nonlinearity and therefore usually negligible. The quantum efficiency of DFG is shown to be generally higher than SFG, but the energy efficiency is much lower if the difference frequency is significantly less than the input frequencies. Optimal SFG quantum conversion efficiency for such systems approaches that of second-harmonic generation only if the quantum fluence (total number of photons) in the two input beams are equal. Optimal DFG quantum conversion efficiency occurs when the number of photons of frequency ${\mathrm{\ensuremath{\omega}}}_{2}$ is small yet sufficient for significant stimulation. A recently developed method for obtaining efficient SFG for multimode input fields is theoretically analyzed. The method involves using an arrangement with two or more nonlinear mixing crystals with a time-delay line situated between the crystals that delays one of the fundamental fields relative to the other. The efficiency in the second crystal depends on the cross-correlation function of the two fundamental fields upon leaving the first nonlinear crystal. The time-delay method is not effective for multimode DFG.

1.5 μm band efficient broadband wavelength conversion by difference frequency generation in a periodically domain-inverted LiNbO3 channel waveguide

Efficient 1.5 μm band wavelength conversion by difference frequency generation (DFG) in a periodically domain-inverted LiNbO3 channel waveguide is demonstrated, and DFG efficiency as high as 40% W−1 cm−2 is obtained. Nearly constant wavelength conversion efficiency has been confirmed over a broad wavelength range of 30 nm, which is the largest value ever reported.

Investigation of a method of enhancing the efficiency of difference frequency generation by stimulated Raman scattering utilizing an inhomogeneous electrostatic field

An investigation is made of resonance parametric difference-frequency generation by stimulated Raman scattering (STRS) in the presence of a periodically inhomogeneous electrostatic field (externally applied or internal field). The initial and maximum difference-frequency generation intensities are determined for arbitrary relationships between the parameters of the system (difference-frequency absorption coefficient and gain of the STRS Stokes component, wave mismatch, and period of the periodically inhomogeneous electrostatic field). It is found that by generating a periodically inhomogeneous electrostatic field, it is possible to compensate for the decrease in the difference-frequency generation efficiency not only due to the wave mismatch but also due to parametric bleaching. Conditions under which the improvement in the intensity of the difference frequency generation is several orders of magnitude are discussed. By using a periodically inhomogeneous electrostatic field applied to compressed H2, an improvement by a factor of 20–80 was obtained for pumping at λρ = 1.06μ and by a factor of 100 for pumping at λρ = 0.694 μ, in agreement with the estimates obtained using the equations derived in the present paper.

Dependence of the degree of conversion in difference frequency generation on the width of a dye laser emission line

A theoretical investigation is made of the influence of the width of a dye laser emission line on the efficiency of difference frequency generation (DFG) using pump radiation at different wavelengths. The line width must be reduced to obtain the maximum efficiency and the longer the DFG radiation wavelength, the narrower the line which is required. An experimental investigation is reported of the conversion of dye laser radiation of wavelength 590 nm and line width between 0.17 and 6.5 nm into difference frequency radiation; this was done using nitrogen laser pump radiation and a KDP crystal 3 cm long. The maximum degree of conversion was 3×10–4.

Temperature-Dependent Phase Matching for Far-Infrared Difference-Frequency Generation in InSb

The efficiency of difference-frequency generation in InSb at 100 ${\mathrm{cm}}^{\ensuremath{-}1}$, using a C${\mathrm{O}}_{2}$ laser as input, was found to be strongly temperature dependent. This dependence is shown to be due to phase matching resulting from an increase with temperature of the short-wavelength refractive indices. A value of 4.03 is derived for the refractive index at 100 \ensuremath{\mu}. The effect of astigmatism in the input optics is pointed out.