Silver Gallium Sulfide Research Articles

Mid-infrared photon pair generation in AgGaS2

We demonstrate nondegenerate photon pair generation by spontaneous parametric downconversion in a silver gallium sulfide AgGaS2 crystal. By tuning the pump wavelength, we achieve phase matching over a large spectral range. This allows to generate idler photons in the mid-infrared spectral range above a 6 μm wavelength with corresponding signal photons in the visible. Also, we show photon pair generation with broad spectral bandwidth. These results are a valuable step toward the development of quantum imaging and sensing techniques in the mid-infrared.

Ternary chalcogenides XGaS2 (X = Ag or Cu) for photocatalytic hydrogen generation from water splitting under irradiation of visible light

AbstractTernary chalcogenide silver gallium sulfide (AgGaS2), which has an orthorhombic structure, was already synthesized. However, the feasibility of using the crystal for hydrogen production through photocatalytic water splitting has not been explored. Here, we systematically investigated the structural, electronic, optical, and transport properties of XGaS2 (X = Ag or Cu) with orthorhombic structure by using the first principles calculations. The band alignments indicate that all calculated absolute potentials of the valence and conduction band edges met the requirement of photocatalytic water splitting reaction. The presence of 2.64 and 2.56 eV direct band energy gaps and obvious optical absorption within the visible light range imply that XGaS2 can correspond to solar light. Moreover, the large electron mobility and the obvious differences between electron mobility and hole mobility were identified in XGaS2 structures, which is beneficial to the photocatalytic performance of the water splitting reaction. The present findings can provide a helpful reference for developing novel photocatalytic materials with XGaS2 for hydrogen generation from water splitting under irradiation of visible light.

Upconversion detection of long-wave infrared radiation from a quantum cascade laser

Broadly tunable upconversion is demonstrated for long-wave infrared (LWIR) detection. The upconversion system is evaluated by the detection of 50 ns pulses from a narrow linewidth tunable quantum cascade laser (QCL) in the 9.4 to 12 µm range. The LWIR signal is mixed with a 1064 nm laser beam in a silver gallium sulfide (AgGaS2) crystal, resulting in an upconverted signal in the 956 to 977 nm range, using angle tuning for optimal phase-matching. This allows for efficient, high speed detection using a standard silicon detector. A theoretical model including absorption and diffraction shows qualitative agreement with experimental data.

Synthesis and characterization of AgGaS2 nanoparticles: a study of growth and fluorescence.

Silver gallium sulfide nanocrystals were synthesized and characterized by fluorescence, TEM, EDS, and XRD to better understand the material system with an ideal band gap of 2.7 eV. The crystals were found to nucleate in the monoclinic structure, and develop two fluorescence peaks (650, 460 nm) influenced by stoichiometry.

Synthesis, growth and physical properties of silver gallium sulfide single crystals

The temperature oscillation method was used to synthesize dense single-phase polycrystalline AgGaS2 from high purity elements. AgGaS2 single crystal of 8mm diameter and 45mm length, free of voids and crack was obtained by the descending ampoule with steady ampoule rotation method using the synthesized polycrystalline charge. The grown crystal was subjected to powder X-ray diffraction and single crystal X-ray diffraction. The AgGaS2 has been studied using differential scanning calorimetry (DSC) technique. The single crystal has high transmission of 75% in the Mid IR region. The band gap energy was calculated using absorption spectrum. The stoichiometric composition of AgGaS2 was measured using energy dispersive spectrometry (EDAX). The structural and compositional uniformities of AgGaS2 were studied using micro-Raman scattering spectroscopy at room temperature. The photoluminescence behavior of AgGaS2 has been analyzed. It shows the maximum emission at 538nm. The resistivity of the grown single crystal has been measured.

A New Optical Parametric Amplifier Based on Lithium Thioindate Used for Sum Frequency Generation Vibrational Spectroscopic Studies of the Amide I Mode of an Interfacial Model Peptide

We describe a new optical parametric amplifier (OPA) that employs lithium thioindate, LiInS2 (LIS), to create tunable infrared light between 1500 cm(-1) and 2000 cm(-1). The OPA based on LIS described within provides intense infrared light with a good beam profile relative to similar OPAs built on silver gallium sulfide, AgGaS2 (AGS), or silver gallium selenide, AgGaSe2 (AGSe). We have used the new LIS OPA to perform surface-specific sum frequency generation (SFG) vibrational spectroscopy of the amide I vibrational mode of a model peptide at the hydrophobic deuterated polystyrene (d8-PS)-phosphate buffered saline interface. This model polypeptide (which is known to be an alpha-helix in the bulk solution under the high ionic strength conditions employed here) contains hydrophobic leucyl (L) residues and hydrophilic lysyl (K) residues, with sequence Ac-LKKLLKLLKKLLKL-NH2. The amide I mode at the d8-PS-buffer interface was found to be centered around 1655 cm(-1). This can be interpreted as the peptide having maintained its alpha-helical structure when adsorbed on the hydrophobic surface, although other interpretations are discussed.

Temperature dependence of the photoluminescence spectra in AgGaS 2

The Silver Gallium Sulfide (AgGaS 2) ternary compound is a wide bandgap semiconductor (about 2.7 eV) whose photoluminescence properties are characterized by excitons and donor–acceptor pairs recombinations. We have performed photoluminescence (PL) measurement exciting with the third harmonic (3.5 eV) of a Nd:YAG laser from room temperature down to 10 K at different excitation power. In this work we report the dependence of the ‘green band’ on the excitation power at various temperatures.

Pulse compression in a silver gallium sulfide, midinfrared, synchronously pumped optical parametric oscillator

We present experiments and numerical modeling of optical-pulse compression in a silver gallium sulfide, synchronously pumped optical parametric oscillator. In the experiments, 10 ps pump pulses from a pulsed Nd:YAG laser interacted in the parametric oscillator to generate infrared output pulses as short as 500 fs, yielding over 20-fold compression. Parametric conversion and pulse compression were studied by numerical simulation of the nonlinear wave equations and found to be in excellent agreement. The compression mechanism is related to group-velocity walkoff in the nonlinear interaction. Dispersion measurements and simulations of pulse compression in the wavelength range of 2.5-4.0 µm indicate that compression is inversely related to the signal-idler group-velocity difference. Although compression requires group-velocity walkoff between the pump and signal waves, the compression is most strongly controlled by the mismatch between the signal and idler group velocities.

Temperature tuning of 5–12 μm by difference frequency mixing of OPO outputs in a AgGaS2 crystal

We report a temperature tuned mid-IR generating system ranging from 5 to 12 μm by difference frequency mixing of output waves from two optical parametric oscillators (OPOs) in a silver gallium sulfide (AgGaS2) crystal. One of the OPOs generate signal wave of 1.6 μm, while the other produced tunable radiation from 1.846 to 2.353 μm. The AgGaS2 crystal is temperature tuned to allow phasematching for tunable output from 5 to 12 μm.

Toward a 3:1 frequency divider based on parametric oscillation using AgGaS/sub 2/ and PPLN crystals

Frequency divide-by-two (2:1) and divide-by-three (3:1) optical parametric oscillators (OPOs) are basic devices for the implementation of future accurate optical frequency division chains. We report our latest development toward a phase-locked 3:1 frequency division of a radiation at lambda(p) approximately 843 nm (355.9 THz), using doubly resonant oscillators (DROs) based on silver gallium sulfide (AgGaS(2 ) or AGS) and multigrating periodically poled lithium niobate (PPLN). Although stable single-mode pair operation is achievable without any active cavity length servo with the AGS-DRO, because of a strong passive thermal feedback servo, the PPLN-DRO requires an active intensity side-of-fringe locking servo to maintain long-term, single-mode pair operation. To overcome the limited idler output power (<1 mW) due to the weak mirror transmissivity at 2.53 mum, a CaF (2) Brewster-oriented plate was inserted in a longer-cavity PPLN-DRO as a variable output coupler. About 3 mW of idler wave is thus coupled outside the cavity, yielding 15 nW of doubled-idler. We obtained a 30 dB signal-to-noise ratio beatnote in a 100 kHz resolution bandwidth of a spectrum analyser. This beat signal will be used to phase-lock the divider.

Stability and frequency tuning of thermally loaded continuous-wave AgGaS_2 optical parametric oscillators

We analyze and investigate experimentally the output stability and the frequency tuning characteristics of weakly triply resonant silver gallium sulfide (AgGaS2) optical parametric oscillators. These oscillators are subject to thermally induced bistability and passive self-frequency-locking phenomena. The robust self-frequency-locking on a single-mode pair (thermal lock) originates from the material’s ability to correct external cavity-length perturbations, which would normally cause a mode hop, by increasing or decreasing the optical path length of the cavity through the thermo-optic effect triggered by the intracavity signal–idler power fluctuations. The Fourier frequency bandwidth of this passive servo is limited to ∼1 kHz by the thermal diffusion time constant, which is proportional to the ratio of the specific heat to the thermal conductivity Cp/Kc. A thermal feedback servo gain as high as 180 is obtained, owing to the large thermal figure of merit η=(dn/dT)/Kc of AgGaS2, leading to routine passive mode-hop-free operation for more than 30 min, without the need for an external cavity-length servo. Analysis of the stability range of the thermally loaded standing-wave resonator shows that thermal lensing is less critical for shorter doubly resonant optical parametric oscillator cavities employing shorter-curvature mirrors, in agreement with experimental observations. When a doubly resonant oscillator operates near the boundary of the power stability range a self-pulsing behavior is observed on a number of axial mode pairs. This self-pulsing is found to originate from the destabilization of a self-locked cw state, and the transition from self-pulsing to the stable self-frequency-locked state is found to be controlled by the pump frequency detuning. The passive stability allows the single-parameter frequency tuning to be studied. Under pure thermal lock operation the oscillators show a tendency to resist the pump frequency and temperature tuning processes. When an external cavity-length servo is implemented continuous tuning over 850 MHz, by means of the pump frequency tuning (Δνs/Δνp≈0.66), and over 100 MHz, by means of the crystal temperature (Δνs/ΔT≈250 MHz/°C), is obtained. These tuning ranges are in good agreement with calculations based on a cold doubly resonant oscillator.

AgGaS 2 optical parametric oscillator continuously tunable from 3.9 to 11.3 μm

This is the experimental realization of a silver gallium sulfide (AgGaS2) optical parametric oscillator (OPO) with a wide mid-IR tuning range. The singly resonant angle-tuned AgGaS2 type-II OPO was pumped by 1.06 μm pulses from a nanosecond Nd:YAG laser and yielded idler wave continuously tunable from 3.9 to 11.3 μm with a linewidth of 1 cm−1. The OPO threshold was 0.03 J/cm2 corresponding to sub-MW/cm2 pump intensity and sub-100 μJ pump energy. The slope and absolute quantum conversion efficiencies reached 41% and 22%, respectively.

Mid-infrared (5–12-µm) and limited (55–85-µm) single-knob tuning generated by difference-frequency mixing in single-crystal AgGaS_2

We describe tunable 5-12-microm mid-infrared generation in single-crystal silver gallium sulfide (AgGaS(2)), from nonlinear optical difference-frequency generation. Signal and idler waves obtained from a Nd:YAG laser-pumped LiNbO(3) optical parametric oscillator (OPO) were mixed in AgGaS(2) crystal to yield difference-frequency waves. For the efficient generation of difference frequency, an unstable resonator was employed as the OPO to reduce output beam divergence. A maximum difference-frequency power of 95 microJ/pulse was obtained near 7.5 microm for a 1-cm-long AgGaS(2) crystal. Spectral noncritical phase matching within a specific tuning range was also investigated that permitted limited single-knob tuning (5.5-8.5 microm) of the difference-frequency generator.

Crystal Growth and Optical Properties of AgGaS2 and AgGaSe2

Silver gallium sulfide (AgGaS2), silver gallium selenide (AgGaSe2), and silver gallium indium selenide (AgGaxIn(1 − x)Se2) are unique nonlinear crystals suitable for a wide range of three-wave mixing applications. They combine strong nonlinear coupling with phase matching across a wide (0.5 − 12.5 μm) transmission range. These properties provide the basis for CO2 laser harmonic-generator (HG), visible (VIS), and near-infrared (NIR) pumped opticalparametric-oscillator (OPO) and sumand difference-frequency-generator (SFG/DFG) systems for producing tunable laser radiation from approximately 0.65 to 12 μm. A wide range of pump lasers is now available, including diode lasers capable of direct pumping of AgGaS2 and efficient diode-pumped lasers for pumping any of these crystals. Significant progress in material characteristics and nonlinear optical (NLO) performance has been accomplished. Bulk absorption for production-quality AgGaS2 is ~0.01 − 0.02 cm across the 0.8−9-μm region. Exceptional crystals of AgGaS2 have absorption &lt;0.0005 cm−1 at 1.064 μm, and even at 0.633 μm, absorption is ~0.015 cm−1. Figure 1 illustrates the late 1980s vintage and present transmission of AgGaS2. For AgGaSe2, absorption for production material ranges from 0.010 cm to 0.018 cm−1 from 1 μm to 11 μm. Certain crystals can have 0.007−0.008 cm−1 absorption at 10.6 μm. The NIR transmission curves for AgGaSe2 and AgGaxIn(1 − x)Se2 appear in Figure 2. This combination of optical properties, along with their good physical characteristics, make these two crystals versatile for NLO applications spanning the VIS to the infrared (ir).

Absolute value of the d_36 nonlinear coefficient of AgGaS_2: prospect for a low-threshold doubly resonant oscillator-based 3:1 frequency divider

We report the measurement of the absolute d36 nonlinear coefficient of silver gallium sulfide (AgGaS2) from three phase-matched nonlinear interactions spanning from 10.2 to 0.78 µm: the sum-frequency of a CO2 laser (10.2 µm) and a near-IR AlGaAs diode laser (0.842 µm), the second-harmonic generation of a KCl:Li color-center laser (2.53 µm), and the noncritically phase-matched 3ω-ω→2ω difference frequency between the AlGaAs diode laser and the KCl:Li laser to generate 1.265 µm. From the theoretical evaluation of the Gaussian-beam aperture functions for these type I interactions with arbitrary focusing parameters, beam-waist locations, and absorption losses, we have deduced from the three processes the same consistent value d36=13 (±2) pm/V for AgGaS2. Our value is independent of the growth origin of the material. In light of the trustworthy value of d36 measured from our experiments we analyze the feasibility of a continuous-wave doubly resonant parametric oscillator-(DRO-)based 3:1 frequency divider pumped by a near-IR diode laser. The predicted pump power threshold for the ring-resonator DRO lies in the range of 70–140 mW for a 14–18-mm-long sample with an absorption loss near the level of 1% cm-1 at the output frequencies.

Updated thermo-optic coefficients of AgGaS^2 from temperature-tuned noncritical 3ω ω → 2ω infrared parametric amplification

We report the temperature dependence of a continuous-wave noncritically phase-matched (θ=90°) optical parametric amplifier based on a 15-mm-long silver gallium sulfide crystal (AgGaS2) pumped by a 50-mW near-IR AlGaAs diode laser (λ3=0.843 µm). This amplifier is idler seeded by a 15-mW KCl:Li (FA-II) color-center laser (λ1=2.53 µm) and produces microwatt-range power of a signal radiation at λ2=1.265 µm. Updated linear and thermo-optic data of this chalcopyrite material, such as its room-temperature refractive indices and their temperature dependence, are devised from this type I (eoo) 3:1 frequency-division process and from other published data on noncritically phase-matched difference-frequency generation. The resulting dno,e/dT values reproduce fairly well all of the reported results on temperature phase-matched parametric mixing.

Silver Gallium Sulfide by XPS

High quality single crystal AgGaS2 has been analyzed by x-ray photoelectron spectroscopy. This material is one of a group of I–III–VI2 compounds that crystallize in a tetragonal chalcopyrite structure and have been shown to have very promising nonlinear optical properties. The spectra presented here include survey and high resolution scans of the photoemission peaks and valence band spectra.