Nonlinear frequency conversion via three-wave mixing in ${\chi }^{(2)}$ media is one of the cornerstones of laser technology. Most of the breakthroughs in this field in the last two decades were related to the development of quasi-phase-matching (QPM) techniques. QPM devices based on periodically poled ferroelectric crystals and waveguides removed limitations on nonlinear conversion efficiency, set by birefringent phase matching [1]. Furthermore, QPM in orientation-patterned gallium arsenide (OP-GaAs) has enabled frequency combs with exceptionally broad spectral spans in the mid-IR [2,3]. The limitations on the use of the QPM technique with new materials are mostly associated with challenges in the fabrication of crystals with a periodically inverted domain structure.

An alternative approach to phase-matching relies on the phenomenon of random phase matching (RPM) in a disordered ${\chi }^{(2)}$ crystalline material [4,5]. RPM eliminates the need for orientation patterning and, most important, allows three-wave interactions with extremely large bandwidths. A broadband and flat response in RPM is the result of phase randomization due to arbitrary distribution of the crystalline domains, which eliminates destructive interference. The price to pay, however, is a slow growth of the output signal, which scales linearly with sample length, as opposed to the quadratic dependence for perfect phase (or quasi-phase) matching. The main distinctive features of RPM were predicted in Ref. [4] and then confirmed in a proof-of-principle experiment [5]. Although RPM is much less efficient than the conventional QPM process in continuous-wave and nanosecond laser regimes, we claim that RPM is very well suited for three-wave processes when few-cycle laser pulses with broad spectra are used. Most recent achievements with RPM include second-harmonic generation (SHG) of femtosecond ($fs$) pulses with up to 0.5 W average power observed directly in the gain elements of $fs$ Cr:ZnS lasers and amplifiers [6]. Here we report, to the best of our knowledge, the first optical parametric oscillator (OPO) based on RPM, which we believe opens a new direction in nonlinear optics.

Polycrystalline ZnSe is a perfect candidate for the implementation of RPM nonlinear devices. ZnSe, a cubic symmetry semiconductor with a bandgap of 2.7 eV, has remarkable transparency (0.55–20 μm), high nonlinearity ($d_{14}=20\mathrm{pm}/\mathrm{V}$), high optical damage threshold, and good mechanical properties. With its low group velocity dispersion (GVD) in the mid-IR (GVD zero crossing is at 4.8 μm), this material is uniquely suited for mid-IR OPO applications under pumping by newly available $fs$ lasers at 2.4 μm. Last, but not least, high-optical-quality chemical vapor deposition (CVD)-grown polycrystalline material is readily available.

Our ZnSe samples were prepared as follows: $11\mathrm{mm}\times 6\mathrm{mm}\times 3\mathrm{mm}$ commercial CVD-grown ceramic ZnSe samples were sealed in quartz ampoules under ${10}^{-5}\mathrm{Torr}$ vacuum and annealed at 900°C. For nine sets of samples, the annealing time varied from 6 to 10 days with a half-day interval. After annealing, the samples were chemically etched and the grain size distribution was measured using a microscope. While the average grain size of untreated samples was between 50 and 60 μm, annealing the samples resulted in increasing grain size with time, and reached 100 μm after 8 days of annealing. We used the resulting annealed samples in our experiments, since the grain size was close to the coherence length (107 μm) for our desired three-wave interaction—the optimal condition for RPM [5].

First, we did $x–y$ mapping of our ZnSe samples by SHG from 4.7 to 2.35 μm (inverse process with respect to an OPO), using a nanosecond $\lambda =4.7\mathrm{\mu m}$ source. A typical result of SHG mapping is shown in Fig. 1. The histogram reveals a broad SHG signal distribution related to variations of the alignment and size of crystalline domains. There are “hot” spots where the efficiency of SHG is 2.5–3 times higher than the average (these hot spots were used to achieve oscillation of the OPO). In terms of average (over the histogram) SHG output, we observed a linear dependence on sample length, in full accord with RPM theory [5].

 

Fig. 1. (a) Normalized SHG signal and (b) a histogram for an $L=1.5\mathrm{mm}$ ZnSe ceramic sample mapped in $x–y$ with 50 μm steps. Inset: $500\mathrm{\mu m}\times 500\mathrm{\mu m}$ cross section of a chemically etched ZnSe ceramic sample.

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The OPO was synchronously pumped by a Kerr-lens mode-locked Cr:ZnS laser with a center wavelength of 2.35 μm, 62 fs pulse duration, up to 650 mW average power, and 79 MHz repetition rate, similar to that in Ref. [3]. The bow-tie ring OPO cavity (Fig. 2) was composed of an in-coupling dielectric mirror M1 with high transmission ($>85\%$) for the 2.35 μm pump and high reflection ($>95\%$) at 3–8 μm; two gold-coated parabolic mirrors (M2 and M3) with a 30° off-axis angle and 30 mm apex radius; and five gold-coated flat mirrors (for simplicity, only M4 is shown in Fig. 2; the other four mirrors were used for folding the beams to reduce the footprint). An uncoated plane-parallel polished $L=1.5\mathrm{mm}$ ZnSe ceramic sample was placed between the two parabolas at Brewster’s angle. A 0.3-mm-thick ZnSe wedge was used inside the cavity for outcoupling the OPO signal/idler waves.

 

Fig. 2. Schematic of the ring-type OPO. M1, incoupling mirror; M2–M4, gold-coated mirrors; PZT, piezo-actuator; OC, outcoupler wedge. Inset: OPO “engine” including the ZnSe ceramic sample at Brewster’s angle located between two parabolic mirrors.

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The OPO was operating in the subharmonic frequency-divide-by-2 mode near degeneracy. In addition to lowering the pump threshold, this arrangement provides other advantages [7] such as (i) phase locking to the pump laser—a prerequisite for creating precision mid-IR frequency combs, and (ii) the possibility of achieving an extremely broadband “instantaneous” spectrum due to negligible GVD of ZnSe in this spectral range.

The OPO threshold was achieved at 90 mW of average pump power. The output spectrum, measured with a monochromator and a mercury cadmium telluride (MCT) detector, spanned 3–7.5 μm (at $-40\mathrm{dB}$ level) and was centered at the 4.7 μm subharmonic of the pump (Fig. 3). The OPO, being a doubly resonant device, is interferometrically sensitive to the cavity length adjustment, accomplished via using a piezo-actuator (Fig. 2); this is revealed in the 2D plot in Fig. 3(b), which shows the spectrum versus length dependence. At maximum pump, the average OPO power [in two beams from the outcoupler wedge (OC)] was $\sim 20\mathrm{mW}$. The observed OPO pump depletion was as high as 79%, which indicates that with optimized outcoupling one can obtain high conversion efficiency, approaching 100%, from such a device.

 

Fig. 3. (a) OPO output spectrum showing a continuous span of 3–7.5 μm. (b) 2D spectrum where $y$ axis shows cavity length detuning.

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We also operated the OPO with an off-the-shelf polycrystalline ZnSe sample from II-VI Inc. ($L=2\mathrm{mm}$, average grain size 80 μm). It demonstrated similar performance, although with twice higher (180 mW) pump threshold.

In conclusion, we report on optical parametric oscillation based on RPM in a ${\chi }^{(2)}$ disordered polycrystal, ZnSe ceramic. To the best of our knowledge, this is (i) the first OPO that utilizes random ${\chi }^{(2)}$ material and (ii) the first OPO based on ZnSe. The parameters obtained by the polycrystalline ZnSe OPO are in line with those of a conventional OP-GaAs OPO under similar pumping conditions. Very likely, RPM in ZnSe and similar random materials represents a viable route to the realization of practical devices for generation of few-cycle pulses and multioctave frequency combs in the mid-IR.