Takamasa Kuroda

Enhancement of spontaneous recombination rate in a quantum well by resonant surface plasmon coupling

Using time-resolved photoluminescence measurements, the recombination rate in an

Influence of free carrier screening on the luminescence energy shift and carrier lifetime of InGaN quantum wells

{\mathrm{In}}_{0.18}{\mathrm{Ga}}_{0.82}\mathrm{N}/\mathrm{GaN}

Localized exciton dynamics in strained cubic In0.1Ga0.9N/GaN multiple quantum wells

quantum well (QW) is shown to be greatly enhanced when spontaneous emission is resonantly coupled to a silver surface plasmon. The rate of enhanced spontaneous emission into the surface plasmon was as much as 92 times faster than QW spontaneous emission into free space. A calculation, based on Fermis golden rule, reveals that the enhancement is very sensitive to silver thickness and indicates even greater enhancements are possible for QWs placed closer to the surface metal coating.

Spin relaxation dynamics in highly uniform InAs quantum dots

We studied the influence of free carrier screening on the luminescence energy shift and carrier lifetime of InGaN multiple quantum wells (MQWs) mainly in relation to a quantum-confined Stark effect. We performed a systematic time-resolved photoluminescence measurement of MQWs for various carrier densities and three different well widths (2.5, 4.0, and 5.5 nm). We show that the energy shift and the change in carrier lifetime are explained well by the free carrier screening effect which compensates for the internal electric field.

Luminescence energy shift and carrier lifetime change dependence on carrier density in In0.12Ga0.88N/In0.03Ga0.97N quantum wells

Radiative and nonradiative recombination dynamics in strained cubic (c-) In0.1Ga0.9N/c-GaN multiple quantum wells were studied using temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy. In contrast to hexagonal InGaN quantum wells, low-excitation photoluminescence peak energy increased moderately with decreasing well thickness L and the PL lifetime did not strongly depend on L. The results clearly indicated that the piezoelectric field was not acting on the transition process. The TRPL signal was well fitted as a stretched exponential decay from 10 to 300 K, showing that the spontaneous emission is due to the radiative recombination of excitons localized in disordered quantum nanostructures such as In clusters. The localized states were considered to have two-dimensional density of states at 300 K (quantum disk size), since the radiative lifetime increased with increasing temperature above 150 K.

Subpicosecond exciton spin relaxation in GaN

We have investigated carrier spin dynamics in highly uniform self-assembled InAs quantum dots. The highly uniform quantum dots allowed us to observe the spin dynamics in the ground state and that in the second state separately, without the disturbance of inhomogeneous broadening. The spin relaxation times in the ground state and the second state were measured to be 1.0 and 0.6 ns, respectively. Our measurements reveal the absence of the carrier density dependence of the spin relaxation time. The measured spin relaxation time decreases rapidly from 1.1 ns at 10 K to 200 ps at 130 K. This large change in the spin relaxation time is well explained in terms of the mechanism of acoustic phonon emission.

Coupling of spontaneous emission from GaN–AlN quantum dots into silver surface plasmons

To clarify the carrier density dependence of carrier lifetime and luminescence energy, we have performed a systematic study of time-resolved photoluminescence (PL) measurements of In0.12Ga0.88N/In0.03Ga0.97N multiple quantum wells for various carrier density. The carrier recombination rate and the PL energy, for the carrier density below 1.5×1018cm−3, are found to decrease nonlinearly with the decrease of the carrier density, although the carrier lifetime is constant and the PL energy shifts linearly for carrier density above this value. We show that the energy shift for the small carrier density and the change in the carrier lifetime are well explained by the free carrier screening effect which compensates the internal electric field. The linear energy shift for the high carrier density is attributed to the band-filling effect.

Exciton spin relaxation dynamics in InGaAs∕InP quantum wells

The spin-relaxation process of A-band exciton in GaN is observed by spin-dependent pump and probe reflectance measurement with subpicosecond time resolution. The spin-relaxation times at 150−225K are 0.47−0.25ps. These are at least one order of magnitude shorter than those of the other III-V compound semiconductors. The spin-relaxation time τs is found to be proportional to T−1.4, where T is the temperature.

A pump and probe study of photoinduced internal field screening dynamics in an AlGaN/GaN single-quantum-well structure

We have demonstrated the decay of spontaneous emission (SE) from AlN-GaN quantum dots (QDs) into silver surface plasmon (SP) modes in the ultraviolet at approximately 375-380 nm. Using time-resolved photoluminescence (PL), we show that the electron-hole recombination rate in AlN-GaN QDs is enhanced when SE is resonantly coupled to a metal SP mode, corresponding to the dip in the continuous-wave PL spectrum. Exciton recombination by means of silver SP modes is as much as 3-7 times faster than in normal QD SE and depends strongly on emission wavelength and thickness of the silver.

Band gap bowing and exciton localization in strained cubic InxGa1-xN films grown on 3C-SiC (001) by rf molecular-beam epitaxy

We have investigated the exciton spin relaxation mechanism between 13 and 300K in InGaAs∕InP quantum wells using time-resolved spin-dependent pump and probe absorption measurements. The exciton spin relaxation time, τs above 40K was found to depend on temperature, T, according to τs∝T−1.1, although the spin relaxation time is constant below 40K. The clear carrier density dependence of the exciton spin relaxation time was observed below 40K, although the carrier density dependence is weak above 40K. These results imply that the main spin relaxation mechanism above and below 40K are the D’yakonov–Perel’ process and the Bir–Aronov–Pikus process, respectively.