Naoki Furukawa

Room-Temperature Red Emission from a p-Type/Europium-Doped/n-Type Gallium Nitride Light-Emitting Diode under Current Injection

We have succeeded in the growth of europium (Eu)-doped GaN layer grown by organometallic vapor-phase epitaxy (OMVPE) and demonstrated the first low-voltage operation of current-injected red emission from a p-type/Eu-doped/n-type GaN light-emitting diode (LED) at room temperature. The bright red emission was obtained with an applied voltage as low as 3 V under normal lighting conditions. At a dc current of 20 mA, the output power, integrated over the 5D0–7F2 transition in Eu3+ ions (around 621 nm), was 1.3 µW. This result suggests a novel way to realize GaN-based red LEDs and monolithic devices comprising red, green and blue GaN-based LEDs.

Improved luminescence properties of Eu-doped GaN light-emitting diodes grown by atmospheric-pressure organometallic vapor phase epitaxy

We investigated the luminescence properties of Eu-doped GaN (GaN:Eu) grown by atmospheric-pressure organometallic vapor phase epitaxy. The GaN:Eu exhibited radiant red emission due to the intra-4f shell transition of Eu3+ ions at room temperature. The intensity of the dominant peak was about 4 times higher than that in the sample grown at 10 kPa, even though the Eu concentration was only half that of the 10 kPa sample. This was mainly caused by the enhancement of the energy transfer from the GaN host to Eu ions. The enhanced energy transfer resulted in improved luminescence properties of a GaN:Eu light-emitting diode.

Femtosecond real-time pump-probe imaging spectroscopy

Real-time pump–probe imaging spectroscopy with femtosecond time resolution is demonstrated. This method allows real-time mapping of frequency- and time-resolved transient absorption of materials at once. We can clearly measure the image of the transient absorption spectra of β-carotene with wide temporal and spectral ranges in a short accumulation time. Since the typical accumulation time is about two orders of magnitude shorter than that by the conventional pump–probe technique, this method becomes a powerful spectroscopic tool to study ultrafast relaxation and dynamics on organic/biochemical materials, which easily undergo photofatigue after many repetitions of the pump–probe sequence.

Femtosecond Real-Time Pump-Probe Imaging Spectroscopy Implemented on a Single Shot Basis

Real-time pump-probe imaging spectroscopy implemented on a single shot basis applicable to photochemical reaction dynamics in biological and organic materials is demonstrated in femtosecond time regime. Using this method, the accumulation time to measure transient signals becomes dramatically reduced, and we can successfully map time–frequency two-dimensional image of the absorbance changes of β-carotene in real-time with an accumulation of 20 laser shots.

Improved Eu Luminescence Properties in Eu-Doped GaN Grown on GaN Substrates by Organometallic Vapor Phase Epitaxy

We have grown Eu-doped GaN on a freestanding GaN substrate by organometallic vapor phase epitaxy and investigated its red luminescence that is due to intra-4f shell transitions of Eu3+ ions. The optimum growth temperature for Eu luminescence was 50 °C higher than that of Eu-doped GaN on a sapphire substrate. The highest emission intensity was more intense than that on the sapphire substrate, while the Eu luminescence lifetime was identical for both substrates. These results indicate that energy transfer from the GaN host to the Eu3+ ions occurs more efficiently in Eu-doped GaN on the GaN substrate because of a low dislocation density.

Direct visualization of transient absorption by real-time pump-probe imaging spectroscopy

We report a new method to visualize ultrafast transient absorption of materials. This method enables us to simultaneously map frequency- and time-resolved absorbance changes with femtosecond time resolution in real-time.