Thomas M. Pollak

Mid-infrared ZnGeP 2 parametric oscillator directly pumped by a pulsed 2 μm Tm-doped fiber laser

We have demonstrated what we believe to be the first mid-infrared optical parametric oscillator (OPO) pumped directly by a pulsed Tm-doped fiber laser. The Tm-fiber pump laser produces 30 ns pulses with a repetition rate of 30 kHz at a wavelength of 2 μm. The ZnGeP2 (ZGP) OPO produces 20 ns mid-IR pulses in the 3.4-3.9 μm and 4.1-4.7 μm spectral regions simultaneously. More than 658 mW of mid-IR output power has been generated with a total OPO slope efficiency greater than 35%.

Ultralow Gradient HGF-Grown ZnGeP 2 and CdGeAs 2 and Their Optical Properties

ZnGeP 2 and CdGeAs 2 have long been recognized as promising crystals for infrared frequency generation. They exhibit the highest nonlinear optical coefficients ( d 36 equals 75 pm/V and 236 pm/V for ZnGeP 2 and CdGeAs 2 , respectively) among all known compounds that possess adequate birefringence for phase matching. ZnGeP 2 s transparency range (0.62−13 μ m) makes it the optimum material for shifting the wavelength of 2- μ m pump lasers into the 3–5- μ m range via optical parametric oscillation (OPO), whereas that of CdGeAs 2 (2.3–18 μ m) is better suited for doubling the frequency of CO 2 lasers (9–11 μ m) into the same range via second-harmonic generation. In both cases however, the application of these materials has been hindered by great difficulty in achieving crack-free single crystals, and by large defect-related absorption losses. The horizontal-gradient-freeze (HGF) growth technique has been instrumental in overcoming these difficulties. “Ultralow” axial gradients (1–3°C/cm) have been used to control stoichiometry by minimizing vapor transport as well as to eliminate cracking due to anisotropic thermal expansion. (The a -axis and c -axis thermal-expansion coefficients of ZnGeP 2 differ by a factor of two, whereas those of CdGeAs 2 differ by a factor of 15.) In addition, oriented seeds were used to ensure monocrystalline nucleation (because even a small degree of polycrystallinity can lead to cracking even in low gradients) and growth along preferred directions to facilitate fabrication of device crystals. Finally growth was performed in a two-zone, transparent furnace in order to monitor and control the seeding-and-growth process.

Efficient 1645-nm Er:YAG laser

We report a resonantly fiber-laser-pumped Er:YAG laser operating at the eye-safe wavelength of 1645 nm, exhibiting 43% optical efficiency and 54% incident slope efficiency and emitting 7-W average power when repetitively Q switched at 10 kHz. To our knowledge, this is the best performance (conversion efficiency and average power) obtained from a bulk solid-state Q-switched erbium laser. At a 1.1-kHz pulse repetition frequency the laser produces 3.4-mJ pulses with a corresponding peak power of 162 kW. Frequency doubling to produce 822.5-nm, 4.7-kW pulses at 10 kHz was performed to demonstrate the lasers utility.

Noncritical singly resonant optical parametric oscillator operation near 6.2 μm based on a CdSiP 2 crystal pumped at 1064 nm

CdSiP(2) is employed in a nanosecond, 90 degrees -phase-matched, singly resonant optical parametric oscillator pumped at 1064 nm to produce idler pulses near 6.2 microm with an energy as high as 470 microJ at 10 Hz.

Direct measurement of ZnGeP2 birefringence from 0.66 to 12.2 μm using polarized light interference

The birefringence (Δn) of ZnGeP2 has been measured directly from polarized light interference spectra obtained in transmittance over the 0.66–12 μm wavelength range from samples of six different thicknesses. The Δn values were determined from the positions of fringe maxima (Δn=kλ/t) and then compared to previously published data which were obtained by a different technique. It was found that the interference fringe method results in values of Δn accurate to ±0.00005. The data are shown to exhibit much less scatter as a function of wavelength than previous results and can lead to more accurate calculations of phase‐matching angles for second‐harmonic generation applications.

Electron‐nuclear double resonance of the zinc vacancy in ZnGeP2

Electron‐nuclear double resonance (ENDOR) has been used to identify the singly ionized zinc vacancy (VZn− center) in ZnGeP2. This S=1/2; defect is the dominant paramagnetic acceptor in the material, and it is associated with the absorption from 0.7 to 2.5 μm that limits the use of ZnGeP2 in optical parametric oscillators. The unpaired spin of the VZn− center is shared nearly equally by two phosphorus nuclei adjacent to the vacancy with little overlap of the wave function onto the other two phosphorus neighbors. Angular dependence of the ENDOR spectrum shows that the two primary 31P nuclei have nearly axial hyperfine matrices with unique axes pointing approximately toward the center of the vacancy. The internuclear axis for these two phosphorus makes an angle of 37.8° with the basal plane.

Electron paramagnetic resonance study of a native acceptor in as‐grown ZnGeP2

Electron paramagnetic resonance (EPR) has been used to investigate an acceptor in as‐grown single crystals of ZnGeP2. The spectra are characterized by equally spaced triplets with 1:2:1 intensity ratios representing hyperfine interactions (varying from 35 to 55 G in magnitude) with two equivalent phosphorous nuclei. Their angular dependence shows that there are four crystallographically equivalent orientations of the defect. The principal values of the g matrix are 2.002, 2.021, and 2.074 and the corresponding principal axes, at one of the four sites, are the [011], [100], and [011] directions, respectively. Two possible models are suggested for this acceptor: Either a zinc vacancy (VZn) or a zinc ion on a germanium site (ZnGe). It also is suggested that the acceptor responsible for the EPR signal is the same acceptor, namely AL1, that gives rise to a dominant near‐infrared absorption band.

Photoinduced electron paramagnetic resonance of the phosphorus vacancy in ZnGeP2

The electron paramagnetic resonance (EPR) spectrum of the neutral phosphorus vacancy has been observed in as‐grown ZnGeP2 during illumination at liquid‐helium temperatures. Without illumination, this donor is a nonparamagnetic singly ionized phosphorus vacancy (VP+ center). Either above‐band‐gap light (514.5 nm) or below‐band‐gap light (632.8 nm) can produce the paramagnetic neutral state (VP0 center) of the donor. Principal values of the g matrix for the neutral donor are 1.944, 2.046, and 2.223. The angular dependence of the EPR spectrum suggests that the unpaired spin is unequally shared by two of the zinc ions neighboring the phosphorus vacancy. These phosphorus vacancies are the dominant donor in this highly compensated material, while the previously reported zinc vacancies are the dominant acceptor.

Subnanosecond, 1 kHz, temperature-tuned, noncritical mid-infrared optical parametric oscillator based on CdSiP(2) crystal pumped at 1064 nm.

Operation of an optical parametric oscillator based on CdSiP(2) and pumped at 1064 nm is demonstrated at a repetition rate of 1 kHz. The maximum output idler energy of 24 microJ at 6.125 microm corresponds to an average power of 24 mW. Increasing the crystal temperature up to 150 degrees in the noncritical (90 degrees) configuration leads to idler wavelength tuning from 6.117 to 6.554 microm. Subnanosecond pulse durations are obtained for the signal and idler as a result of the 1 ns pulse duration of the pump, made possible by the rather short crystal and cavity lengths (approximately 1 cm).

Characterization of defect-related optical absorption in ZnGeP2

A broad optical absorption band with a peak near 1 μm is present in most single crystals of ZnGeP2. These same crystals have an electron paramagnetic resonance (EPR) signal which has been assigned to singly ionized zinc vacancies. A direct correlation between the intensity of the optical absorption at 1 μm and the intensity of the EPR signal has been established using a set of ZnGeP2 crystals where this absorption varied widely. These results suggest that the singly ionized zinc vacancy acceptor plays a direct role in the electronic transition(s) responsible for the 1 μm optical absorption. In separate experiments, it was found that illuminating the ZnGeP2 crystals with a He–Ne laser (632.8 nm) while at temperatures near 25 K produces an increase in the absorption at 1 μm and an increase in the zinc vacancy EPR spectrum. These latter results provide further evidence that the absorption at 1 μm is associated with the singly ionized zinc vacancy acceptor.