Nicholas J. Condon

Optical cooling in Er3+:KPb2Cl5.

For the first time, optical cooling has been observed in the (4)I(13/2) excited state of erbium(III), using the low phonon energy host materal, potassium lead chloride (KPb(2)Cl(2)). Cooling was observed when samples were pumped at wavelengths longer than 1557 nm, 17 nm longer than the mean fluorescence wavelength of 1540 nm, which implies a nonradiative heat load of 1.1% for the (4)I(13/2)-->(4)I(15/2) transition. When pumped at 1568 nm, the total cooling efficiency was 0.38% of the absorbed power. These results highlight the potential of Er(3+):KPb(2)Cl(5) as a material for lasers operating in an eye safe spectral region.

Minimizing Heat Generation in Solid-State Lasers

Novel high-power ytterbium YAG lasers are described. These lasers incorporate the principle of anti-Stokes fluorescence cooling to reduce or eliminate detrimental heating. Lasers with net heating and net cooling are demonstrated. By balancing the spontaneous and stimulated emission, we have reduced the net thermal loading to below 0.01% of the lasers average output power. Design, testing, and analysis are reported for lasers up to 500 W average power and pulsed operation up to 30 s. Issues and limitations of this approach are discussed.

Development of periodically oriented gallium nitride for non-linear optics [Invited]

Methods for growing periodically alternating polarities of GaN on GaN substrates have been developed. The resulting periodically oriented samples demonstrate feasibility of using this method to produce structures of utility in optical parametric generation.

Nonlinear Two-Dimensional Vibrational Spectroscopy

There is currently great interest in developing the vibrational analog to two-dimensional NMR spectroscopy. One approach to implementing two-dimensional vibrational spectroscopy is to use doubly vibrationally enhanced (DOVE) four-wave mixing (FWM). Nonlinear signals occur because of correlations and mode coupling that are induced by interactions involving the driven modes. Since cross peaks do not occur between modes if interactions are absent, spectral congestion is removed and only the coupled modes remain. We describe the development of a two-dimensional doubly vibrationally enhanced four wave mixing method that extends the doubly resonant nonlinear spectroscopies to vibrational nonlinearities. We demonstrate the selective enhancements of coupled modes that are possible with the double resonances where the intensity of the enhancements reflects the strength of the interactions that are responsible for the vibrational mode coupling. We also demonstrate the capabilities for selectively enhancing specific sample components in an isotopic mixture. Since biological applications of DOVE require aqueous environments, we have examined the ability of DOVE methods to discriminate against the strong water absorption and have found that water has a small vibrational nonlinearity that allows DOVE of the solutes. Our results demonstrate the feasibility and features required to make DOVE methods practical for a wide range of scientific applications where identification of intra- and intermolecular interactions is important.

Ho-doped fiber for high energy laser applications

Ho-doped fiber lasers are of interest for high energy laser applications because they operate in the eye safer wavelength range and in a window of high atmospheric transmission. Because they can be resonantly pumped for low quantum defect operation, thermal management issues are anticipated to be tractable. A key issue that must be addressed in order to achieve high efficiency and minimize thermal issues is parasitic absorption in the fiber itself. Hydroxyl contamination arising from the process for making the Ho-doped fiber core is the principal offender due to a combination band of Si-O and O-H vibrations that absorbs at 2.2 μm in the Ho3+ emission wavelength region. We report significant progress in lowering the OH content to 0.16 ppm, which we believe is a record level. Fiber experiments using a 1.94 μm thulium fiber laser to resonantly clad pump a triple clad Ho-doped core fiber have shown a slope efficiency of 62%, which we also believe is a record for a cladding-pumped laser. Although pump-power limited, the results of these studies demonstrate the feasibility of power scaling Ho-doped fiber lasers well above the currently-reported 400-W level.1

Interference, dephasing, and vibrational coupling effects between coherence pathways in doubly vibrationally enhanced nonlinear spectroscopies

Abstract Doubly vibrationally enhanced (DOVE) four-wave mixing is capable of producing coherent two-dimensional (2D) vibrational spectra with cross-peaks that reflect intramolecular and intermolecular mode coupling. These methods are based on double vibrational coherences that are analogous to the double spin coherences in 2D nuclear magnetic resonance methods. In particular, there are two pathways for DOVE infrared spectroscopy that interfere to change the line shapes of doubly resonant cross-peaks. The interference depends on the relative dephasing rates of the coherences involved in the different pathways. We derive relationships between the dephasing rates of the different coherences in DOVE pathways and show how these relationships affect the line shapes of DOVE resonances. Comparisons are made with experimental 2D DOVE spectra.

Optically-pumped mid-IR phosphor using Tm 3+ -sensitized Pr 3+ -doped KPb 2 Cl 5

Efficient energy transfer at room temperature from Tm3+ to Pr3+ has been demonstrated in co-doped KPb2Cl5. Because of the low phonon energies in KPb2Cl5, the energy transfer processes result in the conversion of 805 nm laser diode pump energy to a broad band of mid-IR radiation between 3500to5500 nm. Energy transfer pathways, rates, and quantum efficiencies are evaluated. Results show that the material is suitable as a phosphor for the 4to5⁢ μm spectral range that can be optically pumped with low-cost 0.8⁢ μm laser diodes.

Isotope and mode selectivity in two-dimensional vibrational four wave mixing spectroscopy

Abstract Two-dimensional doubly vibrationally enhanced four wave mixing (2D-DOVE-FWM) spectra have been recorded for a mixture of CH3CN, CD3CN, and C6D6. Cross-peaks in these spectra indicate coupling between molecular vibrations that involve common vibrational modes. Modes are coupled by through-bond interactions and therefore the strongest cross-peaks are observed for vibrations on the same molecule. This coupling produces the vibrational anharmonicities that are required for multi-resonant four wave mixing. The absence of cross-peaks for different molecules demonstrates the isotope selectivity possible with doubly vibrationally enhanced (DOVE) spectroscopy. Model spectra were generated that account for the resonant spectral features observed.

Non-radiative decay of holmium-doped laser materials

Anti-Stokes fluorescence cooling has been demonstrated in a number rare earth doped materials. Ytterbium doped oxides and fluorides, such as ZBLAN, YLF, and YAG, were the first materials to exhibit cooling.1,2,3 These materials were originally developed as laser gain media and fluorescence cooling was eventually incorporated into the 1μm lasers to reduce detrimental thermal loading.4 Anti-Stokes cooling can offset quantum defect heating allowing laser power to be scaled to very high average powers. Since the early work in ytterbium, fluorescence cooling has been demonstrated in both erbium and thulium doped materials.5,6 These materials were also initially developed as lasing media and their fluorescence cooling could be used to increase laser powers at 1.5μm and 2.0μm. In this study we examine the radiative efficiency of holmium and ask the question, “Can anti-Stokes fluorescence cooling be extended beyond 2μm?”

Diode-pumped dysprosium laser materials

We are investigating materials for direct blue solid-state lasers assuming UV excitation with GaN based laser diodes. Room temperature spectroscopy is reported relevant to a proposed quasi-three level laser from the 4F9/2 level in trivalent dysprosium. Modeling based on these measurements suggests this is a promising new laser transition.