Christopher S. Friend

Effect of crystal nature on upconversion luminescence in Er3+:ZrO2 nanocrystals

The effects of crystal size and crystal phase on the upconverted emission of Er3+ in ZrO2 oxide nanocrystals are reported. Green (550 nm) and red (670 nm) upconversion emission were observed at room temperature from the 4S3/2 and 4F9/2 levels of Er3+:ZrO2 nanocrystals. It is found that at 850 mW of cw excitation power, the total luminescence was 11960 Cd/m2 for 1000 °C heated sample. We observed that the overall upconversion luminescence intensity depends on crystal structure and particle size. We have also confirmed that upconversion process in all these samples results from a two-photon excited-state absorption process.

Upconversion properties of a transparent Er3+-Yb3+co-doped LaF3-SiO2 glass-ceramics prepared by sol-gel method

Transparent 0.1ErF3–0.1YbF3–5LaF3–94.8SiO2 (mol%) glass-ceramics was prepared by sol–gel method. Efficient upconversion emissions at 379, 407, 450, 490, 520, 540 and 660 nm were observed under 973 nm excitation. The signal at 379 nm is only ∼4.5 times weaker than the signal at 520 nm, that is readily visible by the naked eyes even with a pump power of ∼27 mW, indicating that this sample is a very good infrared-to-ultraviolet upconverter. The data obtained from transmission electron microscopy and X-ray diffraction studies indicate that the rare-earth ions were partitioned in the low phonon energy LaF3 nanocrystals embedded in silica glass. The analysis of the dynamics of the upconversion emissions from level 4S3/2 (∼540 nm) and level 4F9/2 (∼660 nm) of Er3+ suggest that distinct energy transfer excitation pathways are responsible for populating these luminescent levels.

Highly efficient infrared-to-visible energy upconversion in Er(3+):Y(2)O(3).

Very intense green and red emission was observed at room temperature from the (4)S(3/2) and (4)F(9/2) levels of Er(3+):Y(2)O(3). A cw diode laser at 975 nm was used as a pump for resonant sequential excitation of the (4)I(11/2) and (4)F(7/2) levels. The fluorescence was easily visible to the naked eye, even with 27 mW of excitation power. It was found that at 850 mW of cw excitation power the total luminance was 39,000 cd/m(2). This corresponds to ~100 muW of green emission and 270 muW of red emission.

Photogeneration, charge transport, and photoconductivity of a novel PVK/CdS-nanocrystal polymer composite

Abstract We report on the photoconductive characteristics of an inorganic:organic hybrid composite in which PVK serves as a polymeric charge-transporting matrix and quantum dots composed of surface passivated cadmium sulfide serve as a charge-generating sensitizer. The PVK:CdS nanocomposites prepared are directly compared with a similar composite composed of PVK and C60, an extensively studied system due to its promising characteristics. We demonstrate the possibility of tuning the band-gap of these sensitizing nanocrystals through careful discretion in the method of synthesis such that their spectral response can be adjusted to suit a particular wave-length of operation. In this way materials can be fabricated which posses photoconductive characteristics exceeding those observed in the PVK:C60 system. In this paper results concerning photosensitivity will be presented and the Onsager formalism developed for organic systems will be employed in order to extract the parameters r0 and Φ0 from the photocharge generation efficiency data.

Er3+-doped multicomponent sol-gel-processed silica glass for optical signal amplification at 1.5 μm

We have developed a multicomponent K–Er codoped silica-based sol-gel glass that presents high luminescence quantum yield (∼100 times larger than that of an Er-doped silica sol-gel glass) and long emission lifetime at 1.54 μm (17 ms). The glass was heat treated to melting temperature with the optical properties of the system remaining practically unchanged. The IR absorption data shows that the hydroxyl content of this multicomponent glass is drastically reduced to a few tens of parts per million by using optimized compositions. The maximum gain coefficient expected for this glass is ∼4.5 dB/cm at 1.5 μm.

Organic–inorganic heterojunction light emitting diodes based on poly(p-phenylene vinylene)/cadmium sulfide thin films

We report on a solution processed novel electroluminescent heterostructure device consisting of an inorganic semiconductor, CdS, and a polymer emitter, poly(p-phenylene vinylene) (PPV). This configuration provides the advantage of excellent charge transporting properties of an inorganic semiconductor and the high luminescence quantum efficiency of an organic emitter. The electroluminescence spectrum obtained from this hetrostructure device is similar to the emission spectrum of pure PPV. Even with aluminum electrode, we have achieved stable electroluminescence at very low threshold voltage (3 V) and overall quantum efficiency of about 1% in these devices. The device emitted bright electroluminescence radiation (150 cd/m2) at a driving voltage of 10 V under the forward bias condition.

Nanotechnology in BioMedical Applications

Nanotechnology is rapidly expanding into the biomedical field. At the Institute for Lasers, Photonics and Biophotonics, research in nanotechnology has focused on the development of surface functionalized nanoparticles for diagnostics and targetted therapy. This technology provides a platform for the development of new imaging, diagnostic and therapeutic modalities. For bioimaging and diagnostics, nanoparticles are fabricated containing rare-earth ions which exhibit two-photon, anti-stokes luminescence by frequency up-converting infrared to visible light. We have successfully prepared the phosphor containing nanoparticles having a size ∼25 nm with a silica shell around it which helps in aqueous dispersabilty, inhibits water quenching with nanophosphors and allows functionalization for covalent binding of bioprobes for targeting. The coupling of specific peptides, proteins or nucleic acid sequences to the silica shell will allow for the selective detection of biological entities and will have applications i...

Spontaneous reduction of Eu3+ ion in Al co-doped sol-gel silica matrix during densification

Abstract Eu and Eu-Al co-doped silica glasses have been prepared by impregnating the pores of a base catalysed tetraethylorthosilicate (TEOS) gel with the nitrate salt of Eu and Al and subsequent densification around 1125 to 1150°C. Absorption, emission and excitation spectra of these glasses indicate that Eu 3+ ions are spontaneously reduced to Eu 2+ in the presence of Al 3+ during sintering of the glasses above 1000°C.

Enhancement of two-photon emission in photonic crystals

We report the influence of photonic stopgaps on two-photon excited emission from highly efficient nonlinear chromophores infiltrated into high-quality photonic crystals. We have observed a sharp decrease (filter effect) in emission within the frequency range and direction of the stopgap as well as sharp enhancement of the two-photon excited emission associated with the stopgaps edge. This effect may be important for the development of low-threshold upconversion lasers.

Optical properties of Europium doped gels during densification

Abstract Base catalyzed porous silica sol–gel monoliths fired at 1000°C were used to prepare Eu and Eu–Al co-doped glasses by post doping and subsequent densification around 1125–1150°C in air. Both Eu3+ and Eu2+ ions were found to coexist in densified Al co-doped glasses. The absorption, emission and excitation spectra of the Eu-doped glasses are significantly dependent on the alumina content. The densification temperature of the doped gel also played a major role in the reduction process. The UV excitation efficiency for the 5 D 0 → 7 F 2 transition of Eu3+ ions (614 nm) was increased in the presence of Eu2+ ions due to energy transfer from Eu2+ to Eu3+ ions.