R. Route

Highly efficient second-harmonic generation in buried waveguides formed by annealed and reverse proton exchange in periodically poled lithium niobate

Efficient three-wave mixing devices have numerous applications, including wavelength conversion, dispersion compensation, and all-optical switching. Second-harmonic generation (SHG) is a useful diagnostic for near-degenerate operation of these devices. With buried waveguides formed in periodically poled lithium niobate by annealed and reverse proton exchange, we demonstrate what is believed to be the highest normalized conversion efficiency (150%/W cm(2)) for SHG in the 1550-nm communications band reported to date.

Green-induced infrared absorption in MgO doped LiNbO3

Green-induced infrared absorption (GRIIRA) was investigated by a photothermal technique for undoped and Mg-doped LiNbO3 crystals that have different Li/Nb ratios. Threshold effect on GRIIRA was found, threshold MgO concentrations being the same for GRIIRA and photorefraction. We suggest that GRIIRA is associated with the formation of the small polaron that is located on Nb antisite defect. The remarkable decrease of GRIIRA in Mg:LiNbO3 can then be attributed to the elimination of this intrinsic defect, Nb in Li, following the incorporation of Mg on Li sites. For nonlinear optical applications, LiNbO3 doped with MgO at concentrations over threshold has a combined advantage of having almost no GRIIRA and photorefraction.

AgGaS2 infrared parametric oscillator

We report the first operation of an optical parametric oscillator in a chalcopyrite crystal, AgGaS2. Tuning from 1.4 to 4.0 μm is demonstrated for 1.06‐μm Nd:yttrium aluminum garnet pumping. The potential tuning range extends to the 12‐μm transparency limit of the crystal.

Titania-doped tantala/silica coatings for gravitational-wave detection

Reducing thermal noise from optical coatings is crucial to reaching the required sensitivity in next generation interferometric gravitational-wave detectors. Here we show that adding TiO2 to Ta2O5 in Ta2O5/SiO2 coatings reduces the internal friction and in addition present data confirming it reduces thermal noise. We also show that TiO2-doped Ta2O5/SiO2 coatings are close to satisfying the optical absorption requirements of second generation gravitational-wave detectors.

Light-Emitting Diodes: A Novel Light Source for Phototherapy

High intensity light-emitting diodes (LEDs) are being studied as possible light sources for the phototherapy of hyperbilirubinemic neonates. These power-efficient, low heat-producing light sources have the potential to deliver high intensity light of narrow wavelength band in the blue-green portion of the visible light spectrum, which overlaps the absorption spectrum of bilirubin (BR). We compared the efficacy between single LEDs of different color and then constructed a prototype phototherapy device using 300 blue LEDs. The efficacy of this device was compared with that of conventional phototherapy devices by measuring the in vitro photodegradation of BR in human serum albumin. When blue, blue-green, green, and white LEDs were compared, the blue light was the most effective in degrading BR by 28% of dark control, followed by blue-green (18% of control), and then white light (14% of control). Green light was the least effective (11% of control). The prototype device with three focused arrays, each with 100 blue LEDs, generated greater irradiance (>200 µW·cm-2·nm-1) than any of the conventional devices tested. It also supported the greatest rate of BR photodegradation. We conclude that light from LEDs should be considered a more effective treatment for hyperbilirubinemia than light from presently used phototherapy devices. Furthermore, the unique characteristics of this light source may make it especially suitable for use in safe and lightweight home phototherapy devices.

Solution growth of barium metaborate crystals by top seeding

Abstract Single crystals of β-BaB 2 O 4 have been grown from Na 2 O solutions using the top seeded solution growth method. Crystals were grown under high and low thermal gradient conditions. High thermal gradients allowed faster growth rates and reduced the density of inclusions which are a persistent problem with solution-grown β-BaB 2 O 4 . Crystals grown along the y-axis were found to fracture along the basal cleavage planes during cooling, but otherwise yielded useful crystal dimensions and good optical quality material. Inclusions usually contained the solvent phase but voids have also been observed. Interface breakdown resulting from constitutional supercooling is thought to be related to the restricted mass transport in these high viscosity solutions.

Vapor-transport equilibrated near-stoichiometric lithium tantalate for frequency-conversion applications.

Near-stoichiometric lithium tantalate (SLT) crystals were produced from congruent lithium tantalate by a vapor-transport equilibration process. Because of the resultant increase in photoconductivity and reduction in photogalvanism, the crystals showed no observable photorefractive damage at 514.5 nm up to the highest intensity used, 2 MW/cm2. The crystals also exhibited low green-induced infrared absorption, a Curie temperature of 693 degrees C, and a coercive field of 80 V/mm. The SLT samples were periodically poled with an 8-microm-period grating, permitting first-order quasi-phase-matched second-harmonic generation of 532-nm radiation at 43 degrees C. A 17-mm-long sample generated 1.6 W of continuous-wave output power at 532 nm for 50 h. With 150-ns pulses at a 100-kHz repetition rate in the same sample, 5-W average-power, 532-nm radiation was generated for 1000 h. No damage to the crystal and no aging effects were observed during these experiments.

Mechanical loss in tantala/silica dielectric mirror coatings

Current interferometric gravitational wave detectors use test masses with mirror coatings formed from multiple layers of dielectric materials, most commonly alternating layers of SiO2 (silica) and Ta2O5 (tantala). However, mechanical loss in the Ta2O5/SiO2 coatings may limit the design sensitivity for advanced detectors. We have investigated sources of mechanical loss in the Ta2O5/SiO2 coatings, including loss associated with the coating–substrate interface, with the coating–layer interfaces and with the coating materials. Our results indicate that the loss is associated with the coating materials and that the loss of Ta2O5 is substantially larger than that of SiO2.

Recent Developments In The Growth Of Chalcopyrite Crystals For Nonlinear Infrared Applications

Improvements in crystal growth technology have made it possible to grow crack- and twin-free boules of AgGaS2 and AgGaSe2 in comparatively large dimensions, AgGaS2 to 28 mm diameter by 100 mm length and AgGaSe2 to 37 mm diameter by 100 mm length. Although the crystals grow with optical defects (micrometer-size scattering centers), postgrowth heat treatment procedures have been used to successfully eliminate the defects and produce material of near-theoretical transparency. High optical quality, oriented single crystals of AgGaS2 1 cm in cross section and more than 2 cm in length and of AgGaSe, 1 cm in cross section and more than 3.5 cm in length have been produced and are leading to new advances in IR frequency generation. The optical and phase equilibrium studies as well as details of the crystal growth technology that led to this advance in materials technology are described.

Vertical bridgman growth of CdGeAs2 with control of interface shape and orientation

Abstract Several factors influencing the quality of Bridgman-grown CdGeAs 2 have been studied. Decanting experiments have revealed that an unfavorable concave melt-solid interface shape occurs under normal growth conditions. Modification of the interface shape to a flat profile by the use of appropriate temperature gradients has been shown to greatly reduce the occurrence of polycrystallinity and consequent cracking due to thermal expansion anisotropy. Other mechanisms causing cracking in single crystals have been identified: sticking due to oxides and additional phases such as Cd 3 As 2 on the boule surfaces, and the possibility of bending due to off-axis growth. Experiments to reduce surface contaminants and to control the growth direction by seeding are described. Conflicts in temperature profile requirements make it difficult to achieve optimum conditions in both areas simultaneously, however, and for this reason the yield of uncracked boules has remained low.