Phase-matched Difference Frequency Generation Research Articles

Highly-Efficient Pulsed Mid-Infrared Generation Based on Intracavity Difference Frequency Mixing

We demonstrated highly-efficient pulsed mid-infrared generation based on intracavity difference frequency mixing inside a continuous-wave (CW) PPLN optical parametric oscillator (OPO). The near-infrared pulsed fiber source was located at 1120 nm with variable pulse width and repetition rate. The pump source of OPO was a CW linearly-polarized high-power 1060 nm fiber laser. The 1120 nm and 1060 nm pump beams were simultaneously incident into the OPO cavity. The 1060 nm pump beam built parametric oscillation firstly and generated high-power resonant signal beam inside the cavity. By controlling the PPLN temperature properly, the phase-matched difference frequency generation (DFG) occurred between the signal beam and 1120 nm pulsed pump beam in the PPLN crystal. Finally, the pulsed 1120 nm pump beam was successfully transformed to 3593 nm mid-infrared radiation at every power level and temporal characteristic. Both the slope efficiency and pump-to-idler conversion efficiency of the intracavity DFG process reached over 20%. In addition, the pulse shape of newly generated 3593 nm idler beam was basically the same as 1120 nm pump beam. The experimental results exhibited huge potential in frequency down conversion of low-power laser sources with special temporal characteristics.

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.

Optical properties of β-BBO and potential for THz applications

The anisotropy of optical properties of high quality beta barium borate crystal (β-BaB2O4, β-BBO) was studied in the main transparency window by using classic spectroscopic methods and in the range of 0.2 – 2 THz by using THz time-domain spectroscopy. β-BBO crystals were grown by the top-seeded solution technique in a highly resistive furnace with a heat field of 3-fold axis symmetry. At room temperature (RT), absorption coefficient in the maximal transparency window in grown crystals did not exceed 0.05 cm-1. Strong absorption anisotropy was observed in 3 – 5 μm and the THz range. At 1 THz absorption coefficients for e and o wave were, respectively, 7 cm-1 and 21 cm-1 at RT; 2 cm-1 and 10 cm-1 at 81 K. At the most attractive for out-of-door applications range < 0.4 THz the absorption coefficient is found to be very low: below 0.2 cm-1 at RT and 1 cm-1 at 81 K. Refractive indices dispersions measured by THz-TDS were approximated in the form of Sellmeier equations. Birefringence is found quite large for phase matched difference frequency generation (DFG) or down-conversion into the THz range (THz-DFG) under near IR pump at RT and 81 K. Type II (oe-o and eo-o), and type I (ee-e) three wave interactions can be realized at RT. THz-DFG of Nd:YAG laser and KTP OPO can be realized by type II (oe-o) three-wave interaction. For selected spectral ranges of femtosecond Ti:Sapphire laser efficient phase matched and group velocity matched optical rectification can be realized by another two types of three wave interactions. Accounting other well-known attractive physical properties of β-BBO crystal, wide application in THz technique can be forecasted.

Study of middle infrared difference frequency generation using a femtosecond laser source in LGT.

We demonstrate phase-matched difference frequency generation in the emerging nonlinear crystal La3Ga5.5Ta0.5O14. Tunable wavelengths between 1.4 and 4.7μm are generated by using femtosecond sources. We also report on the measurements of the optical damage threshold in the femtosecond regime and on the nonlinear refractive index n2.

Coherent field transients below 15 THz from phase-matched difference frequency generation in 4H-SiC

We experimentally demonstrate tunable, phase-matched difference frequency generation covering the spectral region below 15THz using 4H-SiC as a nonlinear crystal. This material combines a non-centrosymmetric lattice and strong birefringence with broadband transparency at low optical frequencies. Thorough refractive index measurements in the terahertz spectral range allow us to calculate phase-matching conditions for any near-infrared pump laser source. 4H-SiC is also exploited as a detector crystal for electro-optic sampling. The results allow us to estimate the effective second-order nonlinear coefficient.

Frequency conversion in the acentric tetragonal nonlinear crystal NaI_3O_8

We performed the first study of phase-matched second harmonic generation and difference frequency generation in the acentric nonlinear crystal NaI3O8. By fitting all the recorded phase-matching tuning curves, we determined the Sellmeier equations describing the wavelength dispersion of the ordinary and extraordinary principal refractive indices. We also measured the absolute value of the quadratic nonlinear coefficients and we calculated the conditions of supercontinuum generation in NaI3O8.

Design of a hybrid As₂S₃-Ti:LiNbO₃ optical waveguide for phase-matched difference frequency generation at mid-infrared.

Based on arsenic tri-sulfide films on titanium-diffused lithium niobate, we designed a hybrid optical waveguide for efficient mid-infrared emission by phase-matched difference frequency generation (DFG). The hybrid waveguide structure possesses a low-index magnesium fluoride buffer layer sandwiched between two high-index As(2)S(3) slabs, so that pump and signal waves are tightly confined by titanium-diffused waveguide while the DFG output idler wave at mid-infrared is confined by the whole hybrid waveguide structure. On a 1 mm-long hybrid waveguide pumped at 50 mW powers, a normalized power conversion efficiency of 20.52%W(-1)cm(-2) was theoretically predicted, which is the highest record for mid-infrared DFG waveguides based on lithium niobate crystal, to the best of our knowledge. Using a tunable near-infrared pump laser at 1.38-1.47 µm or a tunable signal laser at 1.95-2.15 µm, a broad mid-infrared tuning range from 4.0 µm to 4.9 µm can be achieved. Such hybrid optical waveguides are feasible for mid-infrared emission with mW powers and sub-nanometer linewidths.

Theory of monochromatic terahertz generation via Cherenkov phase-matched difference frequency generation in LiNbO3 crystal

A theory of Cherenkov phase-matched monochromatic terahertz (THz)-wave generation via difference frequency generation in a nonlinear crystal is developed. An experimental situation (LiNbO3 pumped by dual-wavelength near-infrared nanosecond pulses) is considered. This theory accounts for the finite size of pump beam and allows us to explore the generation of transverse THz wave vector. The output characteristic of this THz source is analyzed based on the analytical expression, including radiation pattern, conversion efficiency, and tuning range. Calculated tuning curves are presented, which reasonably agree with previous experimental results. The influence of divergence of the focused pump beam on total radiated energy is studied in detail. Optimal pump beam size that maximizing generated THz energy is obtained.

THz-Wave Generation from GaP THz Photonic Crystal Waveguides under Difference-Frequency Mixing

GaP terahertz (THz) two-dimensional (2D) photonic crystal (PC) waveguides with line defects were fabricated using inductively-coupled plasma reactive-ion etching (ICP-RIE) in Ar/Cl2 gas chemistries. THz-wave generation from the fabricated PC waveguides was demonstrated under collinear phase-matched difference-frequency generation (DFG), using Cr:Forsterite (Cr:F) lasers as the incident source. We compared the THz-wave output characteristics of the PC waveguides with that of GaP planar waveguides. The collinear phase-matching conditions in the DFG process were satisfied for 300- and 500-μm-wide PC waveguide structures at 0.7 and 0.6 THz, respectively. The additional output peaks that appeared near the edge of the photonic band gap, around 0.5 THz, were attributed to the guiding modes in the PC waveguide; no such peaks appeared in the non-patterned ridge waveguides. These experimental results suggest that the phonon-polariton confinement in THz-PC waveguides based on the GaP crystal could be used to enhance the nonlinear optical effect for THz-wave generation.

Tunable and coherent nanosecond 7.2–12.2 μm mid-infrared generation based on difference frequency mixing in ZnGeP2 crystal

A coherent mid-infrared laser source, which can be tuned from 7.2 μm to 12.2 μm based on the type-II phase-matched difference frequency generation (DFG) in an uncoated ZnGeP2 (ZGP) crystal, is reported. The two pump waves are from a type-II phase-matched dual-wavelength KTP optical parametric oscillator (OPO) of which the signal and idler waves are tuned during 1.85–1.96 μm (extraordinary wave) and 2.5–2.33 μm (ordinary wave), respectively. The maximum energy of the generated mid-infrared laser is 10 μJ at 9.22 μm, corresponding to the peak power of 2.2 kW.

Al_xGa_1-xAs nested waveguide heterostructures for continuously phase-matched terahertz difference frequency generation

We previously demonstrated a nested pair of single mode (pump/THz) waveguides which produced guided far-infrared (1.3 THz) light with a record power-normalized conversion efficiency of 1.3x10(-7) W(-1) by way of continuous phase matched difference frequency generation (DFG) [Opt. Express 16, 13296 (2008)]. Using the same numerical simulation tools we used to design and model this LiNbO(3)-based device, we show that a lattice-matched AlGaAs heterostructure, with its significantly lower absorption losses, can produce guided far-infrared light (3.5 THz) with a power-normalized conversion efficiency of 1.3 x 10(-5) W(-1) - some 100 times larger than achieved with the LiNbO(3) structure.

Power scaling of widely-tunable monochromatic terahertz radiation by stacking high-resistivity GaP plates

A high-resistivity GaP crystal was used to generate monochromatic THz pulses with peak output powers reaching 722 W at 108.1 μm by mixing two coherent beams at about 1 μm based on phase-matched difference-frequency generation. By stacking two and three GaP plates with their second-order nonlinear coefficients being switched between the adjacent ones, we have increased the peak power at 120.3 μm from 433.4 W to 1.36 and 2.36 kW, respectively. 2.36 kW corresponds to the photon conversion efficiency of 25%, which is two orders of magnitude higher than our previous result. In contrast, if they are stacked for having the same sign of the nonlinear coefficients, the wavelength corresponding to the highest peak power is red-shifted to 204.8 and 303.9 μm, respectively. Such a result indicates that there is an optimal interaction length for each specific output wavelength.

Enhancement of CW THz Wave Power Under Noncollinear Phase-Matching Conditions in Difference Frequency Generation

We investigated methods for improvement of continuous-wave (CW) terahertz (THz) output power by laser diode (LD) pumping in noncollinear phase-matched difference frequency generation (DFG). The effects of interaction length and beam spot size of input lasers (near-IR) in GaP crystals were studied. The THz wave power dependence on various sizes of GaP crystals was investigated and it was observed that an output power of 4 nW was obtained with a 20 mm long GaP crystal at 1.5 THz. Also, the THz wave absorption coefficient was dominant for longer GaP crystals at high frequencies (above 2.5 THz). The THz wave power dependence on beam spot size (1.2 mm-300 mum) of near-IR lasers at 1.62 THz was studied, and an improvement of THz wave power being seen with a 500 mum beam spot size, while the beam divergence effect was dominant for beam spot sizes below 500 mum.

Difference frequency generation in GaAs microdisks

Semiconductor-based whispering-gallery-mode microcavities are very promising for nonlinear optics applications, thanks to the high optical quality factors attainable with today's technology. We propose to exploit this advantage to generate cw light through phase-matched difference frequency generation in a triply resonant GaAs microdisk. A proper choice of the microdisk radius and thickness allows one to select the generated wavelength in the band of 2.5-2.9 microm. Besides illustrating the design features, we numerically show that temperature can be effectively used to compensate for wavelength shifts induced on the generated field by fabrication errors.

Efficient terahertz generation from two collinearly propagating CO2 laser pulses

Coherent terahertz pulses have been generated at 328.2μm (914.1GHz) by frequency-mixing two CO2 laser lines near 10μm based on collinearly phase-matched difference-frequency generation in GaSe crystals. The highest average output power was measured to be 260μW, corresponding to a peak output power of 510mW. Both the average powers and normalized conversion efficiencies have been significantly improved over the previous results. The generated terahertz pulses are expected to have the narrowest linewidth dictated by the pulse width through Fourier transform based on the characteristics of the two CO2 laser beams.

Temperature phase-matching properties of mixed chalcopyrite AgGa1−xInxS2 crystal

AgGa1−xInxS2 with x=0.14±0.01 was found to be 90° phase-matchable for the second harmonic generation (SHG) of CO2 laser radiation at 10.591μm at 203°C. In addition, temperature-tuned 90° phase-matched difference frequency generation (DFG) at 4.02μm was demonstrated by mixing the idler output of a Nd:YAG third harmonic pumped β-BBO optical parametric oscillator and its fundamental source at 1.0642μm. The Sellmeier and thermo-optic dispersion formulas that reproduce well these experimental data are presented.

Fabrication and Characterization of GaP Photonic Crystals for Terahertz Wave Application

We have fabricated GaP two-dimensional photonic crystals (PCs) for terahertz (THz) wave generation by a reactive ion etching in Ar/Cl2 gas chemistries. We performed 75-mm-deep etching of GaP, in which Al2O3 layer is applied as a hard mask with its selectivity as high as 125. We demonstrated the THz-wave generation from the fabricated GaP slab waveguide with the PC structure as a cladding layer under a collinear phase-matched difference frequency generation. In the frequency dependence of THz output power for the PC slab waveguide is seen at around 1.1 THz. From the in-plane transmission spectrum of THz-wave, we confirmed that the THz output characteristics had relation with the photonic structure for THz wave. [doi:10.2320/matertrans.MAW200728]

Efficient THz generation and frequency upconversion in GaP crystals

We have developed a widely-tunable monochromatic THz source based on phase-matched difference-frequency generation (DFG) in a GaP crystal. By using a 20-mm-long GaP crystal the output wavelength was tuned in the range of 71.1–2830 μm whereas the highest peak power was 15.6 W. The advantage of using GaP over birefringent non-linear-optical crystals is obvious: crystal rotation is no longer required for wavelength tuning. Instead, one just needs to tune the wavelength of one mixing beam within a bandwidth of as narrow as 15.3 nm. Most recently, we observed THz frequency upconversion by mixing a THz wave with an infrared laser beam in another GaP crystal. The lowest energy per pulse measured by us was 780 pJ whereas the wavelength range of the detectable THz waves was 113.6–128.3 μm. This scheme allows us to measure the pulse energy density, wavelength, linewidth, and pulse width of a THz beam at non-cryogenic temperatures. We have also discussed a few mechanisms for further enhancing the conversion efficiencies of the two parametric processes.

Widely tunable monochromatic THz sources based on phase-matched difference-frequency generation in nonlinear-optical crystals: A novel approach

We review our up-to-date result on the development of widely tunable monochromatic THz sources, implemented based on difference-frequency generation (DFG) in GaSe, ZnGeP2, and GaP. Using a GaSe crystal, the output wavelength was tuned in the range from 66.5 μm to 5664 μm (from 150 cm−1 to 1.77 cm−1) with the highest peak power 389 W. This tuning range is the widest ever produced for a continuously tunable and coherent tabletop THz source. Moreover, the conversion efficiency 0.1% is also the highest ever achieved for a tabletop system. On the other hand, based on DFG in a ZnGeP2 crystal, the output wavelength was tuned in the ranges 83.1–1642 μm and 80.2–1416 μm for two phase-matching configurations. The output power has reached 134 W so far. Finally, using a GaP crystal, the output wavelength was tuned in the range 71.1–2830 μm, whereas the highest peak power was 15.6 W. The advantage of using GaP over GaSe and ZnGeP2 is that crystal rotation is no longer required for wavelength tuning. Instead, one just needs to tune the wavelength of one mixing beam within the bandwidth of as narrow as 15.3 nm.

Coherent terahertz waves based on difference-frequency generation in an annealed zinc–germanium phosphide crystal: improvements on tuning ranges and peak powers

We have efficiently generated a monochromatic terahertz wave continuously tunable in the ranges of 83.1–1642 μm and 80.2–1416 μm as a result of phase-matched difference-frequency generation in an annealed zinc–germanium phosphide crystal. The highest output peak power achieved by us is 134 W. Compared with our previous result, we have significantly improved the tuning ranges and peak powers by using a longer and annealed crystal with much lower absorption coefficients at the two pump wavelengths. Moreover, we have discussed the possibility of using intracavity difference-frequency generation to implement a compact THz source.