Continuous-wave long-wavelength infrared difference-frequency generation in ZGP driven by near-infrared fiber lasers.

We report the continuous-wave (cw) difference-frequency generation (DFG) in a ZnGeP2 (ZGP) crystal that produces tunable long-wavelength infrared (LWIR) lasing. Particularly, we experimentally demonstrate the feasibility to drive DFG in ZGP by all-fiber near-infrared fiber lasers consisting of a 1.3 µm tunable cw random Raman fiber laser (RRFL) and a 1.5 µm erbium-doped fiber amplifier seeded by a tunable distributed feedback (DFB) laser, making the whole system compact and robust. As a result, the demonstrated LWIR DFG presents a broadband spectral tuning range spanning from 9.5 to 11.5 µm, and the output powers in the spectral range of 9.5-11 µm are larger than 40 µW pumped by watt-level fiber lasers. Meanwhile, as a typical application, a proof-of-concept demonstration of gas sensing of SF6 is executed based on the generated cw LWIR source. Our work demonstrates that the combination of ZGP crystal and fiber lasers can provide an effective and robust approach for the generation of cw LWIR radiation with useful power and broadband wavelength tunability.

Highly efficient octave-spanning long-wavelength infrared generation with a 74% quantum efficiency in a χ(2) waveguide.

The realization of compact and efficient broadband mid-infrared (MIR) lasers has enormous impacts in promoting MIR spectroscopy for various important applications. A number of well-designed waveguide platforms have been demonstrated for MIR supercontinuum and frequency comb generations based on cubic nonlinearities, but unfortunately third-order nonlinear response is inherently weak. Here, we propose and demonstrate for the first time a χ(2) micrometer waveguide platform based on birefringence phase matching for long-wavelength infrared (LWIR) laser generation with a high quantum efficiency. In a ZnGeP2-based waveguide platform, an octave-spanning spectrum covering 5-11 µm is generated through optical parametric generation (OPG). A quantum conversion efficiency of 74% as a new record in LWIR single-pass parametric processes is achieved. The threshold energy is measured as ~616 pJ, reduced by more than 1-order of magnitude as compared to those of MIR OPGs in bulk media. Our prototype micro-waveguide platform could be extended to other χ(2) birefringence crystals and trigger new frontiers of MIR integrated nonlinear photonics.

Wavelength scaling of high harmonic yields and cutoff energies in solids driven by mid-infrared pulses.

The effect of driving wavelengths on high harmonic generation (HHG) have long been a fundamental research topic. However, despite of abundant efforts, the investigation of wavelength scaling of HHG in solids is still confined within the scope of theoretical predictions. In this work, we for the first time to the best of our knowledge, experimentally reveal wavelength scaling of HHG yields and cutoff energy in three typical solid media (namely pristine crystals GaSe, CdTe and polycrystalline ZnSe), driven in a broad mid-infrared (MIR) range from 4.0 to 8.7 µm. It is revealed that when the driving wavelength is shorter than 6.5-7.0 µm, HHG yields decrease monotonously with the MIR driving wavelengths, while they rise abruptly by 1-3 orders of magnitude driven at longer wavelength and exhibit a crest at 7.5 µm. In addition, the cutoff energies are found independent on driving wavelengths across the broad MIR pump spectral range. We propose that the interband mechanism dominates the HHG process when the driving wavelength is shorter than 6.5-7.0 µm, and as the driving wavelength increases, intraband contribution leads to an abrupt rise of the HHG yields, which is verified by the HHG polarization measurement driven at 3.0 and 7.0 µm. This work not only experimentally demonstrate the wavelength scaling of HHG in solids, but more importantly blazes the trail for optimizing the HHG performance by choosing a driving wavelength and provides experimental method to distinguish the interband and intraband dynamics.

Tunable phase-mismatched mid-infrared difference-frequency generation between 6 and 17†µm in CdTe.

In parametric conversion, phase-matching techniques such as birefringence and quasi phase-matching (PM) with the designed crystal angle or periodically poled polarities are employed to fulfill the requirement of momentum conservation. However, directly using phase-mismatched interactions in nonlinear media with large quadratic nonlinear coefficient remains unheeded. Here, for the first time to the best of our knowledge, we study the phase-mismatched difference-frequency generation (DFG) in an isotropic cadmium telluride (CdTe) crystal, with the comparison of other DFG processes based on birefringence-PM, quasi-PM, and random-quasi-PM. Long-wavelength mid-infrared (LWMIR) phase-mismatched DFG with an ultra-broadband spectral tuning range of 6-17 µm based on CdTe is demonstrated. Thanks to the giant quadratic nonlinear coefficient (∼109 pm/V) and good figure of merit in the parametric process, the output power up to 100 µW is obtained, which is comparable to or even better than the DFG output from a polycrystalline ZnSe with the same thickness facilitated by random-quasi-PM. A proof-of-concept demonstration in gas sensing of CH4 and SF6 is conducted based on the phase-mismatched DFG as a typical application. Our results demonstrate the feasibility of phase-mismatched parametric conversion in producing useful LWMIR power and ultra-broadband tunability in a simple and convenient way without the necessity of controlling the polarization, phase-matching angle, or pole periods, which could find applications in the fields of spectroscopy and metrology.

High-harmonic generation in polycrystalline CdTe nano-films via macroscopic investigations.

Bright high harmonics generation (HHG) in CMOS-compatible nano-films can provide new opportunities for integrated coherent ultra-violet sources and attosecond photonic devices. Up to now, most HHG studies have been limited to single crystals. Polycrystalline materials, which consist of many grains separated by grain boundaries and normally have random crystallographic orientations, have rarely been explored for HHG. Understanding and predicting the HHG properties in polycrystalline nano-films are important owing to its merits of low cost and diversified properties, but challenging due to their complicated electronic structures. Here, we for the first time experimentally discover the correspondence between HHG in polycrystalline matters and macroscopic material parameters, to the best of our knowledge. Pumped by a mid-infrared femtosecond laser centered at 7.1 µm wavelength, bright and long-term stable harmonics extending to 25th orders (284 nm) are demonstrated in polycrystalline cadmium telluride (CdTe) nano-films. It is found that the HHG strengths in the transmission and the reflection behave differently as a function of the material thickness in the range from 6 nm to 4 µm, which is highly correlated to the measured macroscopic conductivity. The experimental findings agree well with the recent theoretical prediction [Phys. Rev. B103(15), 155426 (2021)10.1103/PhysRevB.103.155426]. This work provides a simple gauge to study and predict HHG in complicated polycrystalline and amorphous nano-systems, and paves the way for novel strong-field nanophotonics based on polycrystalline nano-films.

Ultrabroad (3.7-17†µm) tunable femtosecond optical parametric amplifier based on BaGa4Se7 crystal.

We demonstrate the first (to the best of our knowledge) tunable femtosecond (fs) mid-infrared (MIR) optical parametric amplifier (OPA) based on BaGa4Se7 (BGSe) crystal with an ultra-broadband spectral range. Benefiting from the broad transparency range, high nonlinearity, and relatively large bandgap of BGSe, the MIR OPA pumped at 1030 nm with a repetition of 50 kHz has an output spectrum that is tunable across an extremely wide spectral range spanning from 3.7 to 17 µm. The maximum output power of the MIR laser source is measured as 10 mW at a center wavelength of 16 µm, corresponding to a quantum conversion efficiency of 5%. Power scaling is straightforwardly achieved by using a stronger pump in BGSe with an available large aperture size. A pulse width of 290 fs centered at 16 µm is supported by the BGSe OPA. Our experimental result indicates that BGSe crystal could serve as a promising nonlinear crystal for fs MIR generation with an ultra-broadband tuning spectral range via parametric downconversion for applications such as MIR ultrafast spectroscopy.