We have shown both experimentally and theoretically that the effect of intermediate-state resonance enhancement causes highly nondegenerate 2-photon absorption, 2PA, to be strongly enhanced in direct-gap semiconductors. Calculations indicate an additional 10x increase in this enhancement is possible for quantum-well semiconductors. This enhancement leads to interesting applications of 2PA, such as mid-infrared detection, where uncooled, large-gap photodiodes can rival the sensitivity of cooled MCT detectors (for short pulses). Additionally, mid-IR imaging and tomography based on this effect have been shown. Even larger enhancement of 3PA is calculated and observed. In the case of optically-pumped semiconductors, we have now demonstrated that the complementary process of nondegenerate 2-photon stimulated emission can be observed. Theoretically, this results in 2-photon gain (2PG) that is enhanced as much as 2PA, leading to the possibility of large gap devices with tunable mid-infrared gain. However, the effect of nondegenerate enhancement of 3PA can be detrimental to the observation of this gain. Additionally, by causality, Kramers-Kronig relations predict that the enhancement of 2PA is accompanied by an enhancement of the nonlinear refractive index, n2, which is very highly dispersive in the region of 2PA. Our latest experimental results confirm this enhancement and strong dispersion.
We utilize a single-photon sensitive electron multiplying CCD camera as a massively parallel coincidence counting apparatus to study spatial entanglement of photon pairs. This allows rapid measurement of transverse spatial entanglement in a fraction of the time required with traditional point-scanning techniques. We apply this technique to quantum experiments on entangled photon pairs: characterization of the evolution of entanglement upon propagation, and measurement of one- and two-photon portions of the state transmitted through non-unitary (lossy) objects, and quantum phase imaging.
Two-photon absorption, 2PA, in semiconductors is enhanced by two orders of magnitude due to intermediate-state resonance enhancement, ISRE, for very nondegenerate (ND) photon energies. Associated with this enhancement in loss is enhancement of the nonlinear refractive index, n2. Even larger enhancement of three-photon absorption is calculated and observed. These large nonlinearities have implications for applications including ND two-photon gain and twophoton semiconductor lasers. Calculations for enhancement of ND-2PA in quantum wells is also presented showing another order of magnitude increase in 2PA. Potential devices include room temperature gated infrared detectors for LIDAR and all-optical switches.
We present measurements of the temporal and polarization dependence of the nonlinear optical (NLO) response of selected organic solvents using our beam deflection (BD) method. These measurements allow us to separately determine the bound-electronic and nuclear responses which then determines the NLO response function. With this NLO response function the outcome of other experiments such as Z-scan as a function of pulse-width can be predicted. By performing similar measurements on the gas phase of these solvents we can compare the hyper-polarizabilities in the two phases.
All optical switching (AOS) applications require materials with a large nonlinear refractive index (n2) but relatively small linear and nonlinear absorption loss. The figure-of-merit (FOM), defined as the ratio between the real and imaginary parts of the second hyperpolarizability (γ), is widely used to evaluate the operating efficiency of AOS materials. By using an essential-state model, we describe the general dispersion behavior of γ of symmetric organic molecules and predict that the optimized wavelength range for a large FOM is near its linear absorption edge for cyanine-like dyes. Experimental studies are normally performed on organic solutes in solution which becomes problematic when the solvent nonlinearity dominates the total signal. This has been overcome using a Dual-arm Z-scan methodology to measure the solution and solvent simultaneously on two identical Z-scan arms and discriminating their small nonlinear signal difference. This technique significantly reduces the measurement uncertainty by correlating the excitation noise in both arms, leading to nearly an order-of-magnitude increase in sensitivity. Here we investigate the n2 and two-photon absorption (2PA) spectra of several classes of cyanine-like organic molecules and find that the results for most molecules agree qualitatively and quantitatively with the essential-state model. Many cyanine-like molecules show a relatively small FOM due to the presence of large 2PA bands near the linear absorption edge; however, an exception is found for a thiopyrylium polymethine molecule of which the maximum FOM can be < 400, making it an excellent candidate for AOS.
Eye-safe, high power tunable narrow linewidth lasers are important for various applications such as atmospheric
propagation measurements. We have investigated two techniques of generating narrow linewidth thulium 2-μm fiber
lasers, utilizing a reflective volume Bragg grating (VBG), and a guided mode resonance filter (GMRF) as a cavity end
mirror. A stable narrow linewidth (50 pm), tunable (from 2004 nm to 2054 nm) thulium doped fiber laser using a
reflective VBGg was demonstrated. A CW power of 17 W was achieved. Using a GMRF as an end mirror we showed a
narrow linewidth (~30 pm) laser with an output power of 5.8W, and at a slope efficiency of 44%.