Re-excitation Processes Research Articles

Unity yield of deterministically positioned quantum dot single photon sources

We report on a platform for the production of single photon devices with a fabrication yield of 100%. The sources are based on InAsP quantum dots embedded within position-controlled bottom-up InP nanowires. Using optimized growth conditions, we produce large arrays of structures having highly uniform geometries. Collection efficiencies are as high as 83% and multiphoton emission probabilities as low as 0.6% with the distribution away from optimal values associated with the excitation of other charge complexes and re-excitation processes, respectively, inherent to the above-band excitation employed. Importantly, emission peak lineshapes have Lorentzian profiles indicating that linewidths are not limited by inhomogeneous broadening but rather pure dephasing, likely elastic carrier-phonon scattering due to a high phonon occupation. This work establishes nanowire-based devices as a viable route for the scalable fabrication of efficient single photon sources and provides a valuable resource for hybrid on-chip platforms currently being developed.

Ultrafast wavelength-dependent carrier dynamics related to metastable defects in Cu(In,Ga)Se2 solar cells with chemically deposited Zn(O,S) buffer layer

The Cu(In,Ga)Se2 (CIGS) solar cells were fabricated with a Zn(O,S) buffer layer via chemical bath deposition method (CBD) using different thiourea (TU) mole concentration. The solar cells showed a substantial increment in the performance after light soaking treatment for 1 h. The performance increment was found to be in proportion to the TU mole concentration used in Z(O,S) layer deposition. To understand the cause of increased efficiency, we examined the metastable defect states that were reversible and reproducible in the CIGS solar cell. Firstly, to understand the film quality and the interface between CBD-Zn(O,S) and the CIGS layer, chemical states and band alignment along the depth direction was studied from the results of X-ray photoemission spectroscopy (XPS). It was observed that as the TU mole concentration increased, the distribution of S atoms in the CBD-Zn(O,S) layer changed from Gaussian to almost straight line due to the existence of numerous Zn–S bonds at the interface. With the actively diffused S atoms toward CIGS, a hollow band region was formed at the interface of the CIGS/CBD-Zn(O,S) layers, suppressing the electron-hole recombination, and thus enhancing the cell efficiency. Further, to measure the metastable defect states, optical pump-THz probe (OPTP) spectroscopy was utilized with two pumps having beam energies of 400 nm and 800 nm, which were sensitive enough to detect the defect states at an ultrafast time scale. We observed a unique and unusual decay curve of a re-excitation process after the initial ultrafast decay. The decay was contributed by “(VSe–VCu) divacancy complex,” which is considered a metastable defect state predominantly found near the surface of CIGS layer. The (VSe–VCu) metastable defect states were filled by photocarriers and then the re-excited photocarriers contributed to the expeditious flow of the photocurrent, giving rise to the increment of the cell efficiency.

Picosecond time-resolved dynamics of energy transfer between GaN and the various excited states of Eu3+ ions

To elucidate the energy transfer and reexcitation processes in Eu-doped GaN layers that are used in recently developed, highly efficient red light-emitting diodes, a systematic series of photoluminescence and time-resolved photoluminescence (TR-PL) measurements was performed. Critical insights on how ``slow'' Eu processes (\ensuremath{\sim}\textmu{}s) can compete against fast semiconductor processes (\ensuremath{\sim}ps) are revealed using TR-PL with a high temporal resolution, as it is found that the initial energy transfer from GaN to the $\mathrm{E}{\mathrm{u}}^{3+}$ ions takes place rapidly, on a timescale of 100 ps. Below band-gap resonant excitation was used to identify the states into which the energy transfer occurs. For the most efficient Eu defect complexes, this transfer dominantly occurs directly into the $^{5}D_{0}$ state of $\mathrm{E}{\mathrm{u}}^{3+}$. Less efficient complexes also exhibit transfer into the $^{5}D_{2}$ state, the emission of which can be detected using photoluminescence at low temperature, indicating the importance of the excitation pathway on device efficiency. Under high excitation intensity, reexcitation can also occur, leading to a redistribution of population into the $^{5}D_{2}$, $^{5}D_{1}$, or $^{5}D_{0}$ states.

Spin-independent and spin-dependent re-excitation in CdZnTe studied by pump-probe spectroscopy

Time-resolved pump-probe spectroscopy is used to study the re-excitation processes of spin-balanced and spin-polarized electrons trapped by the defect levels in Cd0.86Zn0.14Te. In CdTe, Cd0.96Zn0.04Te, and Cd0.86Zn0.14Te, the electron trapping time and lifetime decrease with the increasing defect density of states that depends on the Zn composition. The abnormal linearly and circularly polarized pump-probe spectroscopies, respectively, demonstrate the presence of spin-independent and spin-dependent re-excitation processes in Cd0.86Zn0.14Te which have high defect density of states.

Emission-Frequency Separated High Quality Single-Photon Sources Enabled by Phonons.

We demonstrate theoretically that the single-photon purity of photons emitted from a quantum dot exciton prepared by phonon-assisted off-resonant excitation can be significantly higher in a wide range of parameters than that obtained by resonant preparation for otherwise identical conditions. Despite the off-resonant excitation, the brightness stays on a high level. These surprising findings exploit the fact that the phonon-assisted preparation is a two-step process where phonons first lead to a relaxation between laser-dressed states while high exciton occupations are reached only with a delay to the laser pulse maximum by adiabatically undressing the dot states. Due to this delay, possible subsequent processes, in particular multiphoton excitations, appear at a time when the laser pulse is almost gone. The resulting suppression of reexcitation processes increases the single-photon purity. Due to the spectral separation of the signal photons from the laser frequencies this enables the emission of high quality single photons not disturbed by a laser background while taking advantage of the robustness of the phonon assisted scheme.

Single-photon light-emitting diodes based on preselected quantum dots using a deterministic lithography technique

In the present study, we developed a deterministic fabrication process for an electrically driven single-photon light-emitting diode based on InP quantum dots (QDs) emitting in the red spectral range, where the wavelength of interest coincides with the high efficiency window of most commonly available standard Si avalanche photodiodes (APDs). A deterministic lithography technique allowed for the preselection of a suitable QD, here exclusively operated under electrical carrier injection. The final device was characterized under micro-electroluminescence in the direct current as well as in pulsed excitation mode. In particular, under pulsed excitation of one device, single-photon emission has been observed with graw(2)(0)=0.42 ± 0.02, where the nonzero g(2)-value is mainly caused by background contribution in the spectrum and re-excitation processes due to the electrical pulse length. The obtained results constitute an important step forward in the fabrication of electrically driven single-photon sources, where deterministic lithography techniques can be used to sensibly improve the device performances. In principle, the developed process can be extended to any desired emitter wavelength above 600 nm up to the telecom bands.

On-demand generation of background-free single photons from a solid-state source

True on-demand high-repetition-rate single-photon sources are highly sought after for quantum information processing applications. However, any coherently driven two-level quantum system suffers from a finite re-excitation probability under pulsed excitation, causing undesirable multi-photon emission. Here, we present a solid-state source of on-demand single photons yielding a raw second-order coherence of g(2)(0)=(7.5±1.6)×10−5 without any background subtraction or data processing. To this date, this is the lowest value of g(2)(0) reported for any single-photon source even compared to the previously reported best background subtracted values. We achieve this result on GaAs/AlGaAs quantum dots embedded in a low-Q planar cavity by employing (i) a two-photon excitation process and (ii) a filtering and detection setup featuring two superconducting single-photon detectors with ultralow dark-count rates of (0.0056±0.0007) s−1 and (0.017±0.001) s−1, respectively. Re-excitation processes are dramatically suppressed by (i), while (ii) removes false coincidences resulting in a negligibly low noise floor.

Enhanced photoluminescence intensity by modifying the surface nanostructure of Nd3+-doped (Pb, La)(Zr, Ti)O3 ceramics.

Quasi-period cylindrical nanostructures with both diameters and intervals of about 100nm are manufactured on the surfaces of Nd3+-doped lanthanum lead zirconate titanate ceramics by femtosecond laser irradiation under SF6 atmosphere. A light-emission enhancement of more than 20 times is investigated, accompanied by an extremely long trailing-off time of light emission and lower threshold. A specific polarization state of the light emission is achieved and tuned by changing the incident regions of the pumping source. The increased absorption coefficient of the specimen is discussed based on multiple scattering and weak localization of light. In addition, both the scatterers provided by the laser-machined nanostructure and the recurrent photoinduced trapping and re-excitation process participated in the enhancement of the light emission. This Letter offers new insight to improve the luminescence property of laser materials, as well as to broaden the range of exploring the weak localization of light and random lasers.

Signatures of two-photon pulses from a quantum two-level system

The theoretical community has found interest in the ability of a two-level atom to generate a strong many-body interaction with light under pulsed excitation. Single-photon generation is the most well-known effect, where a short Gaussian laser pulse is converted into a Lorentzian single-photon wavepacket. However, recent proposals have surprisingly suggested that scattering with intense laser fields off a two-level atom may generate oscillations in two-photon emission that are out of phase with its Rabi oscillations, as the power of the pulse increases. Here, we provide an intuitive explanation for these oscillations using a quantum trajectory approach and show how they may preferentially result in emission of two-photon pulses. Experimentally, we observe signatures of these oscillations by measuring the bunching of photon pulses scattered off a two-level quantum system. Our theory and measurements provide crucial insight into the re-excitation process that plagues on-demand single-photon sources while suggesting the production of novel multi-photon states.

高E/n放電空間における振動準位の高いN<SUB>2</SUB>(C<SUP>3</SUP>Π<SUB>u</SUB>)の生成過程

The authors have studied on generation processes of N2(C3Πu) in discharge of high E/n region. In this investigation, the spatial distribution of N2 Second Positive Bands was measured and was compared with the calculated results obtained by the Monte-Carlo method. From the results, it is suggested that N2(C3Πu) was generated by any processes including direct excitation from the ground state. To confirm the generation process except the direct one, time after glow method was used. As a result, slow decay component on intensity of N2 Second Positive Bands was detected. Its relaxation time was about 0.5ms. The relaxation time is similar to the effective life time of N2(A3Σu+). Then, in discharge of high E/n region, it is shown that the rate of N2(C3Πu, υ'=3, 4) generated by re-excitation process of N2(A3Σu+) mutual collision is high.

Analytical Characteristics of a Spark-Sampled/Pulsed Radio-Frequency Re-Excited Analytical Discharge

A high-voltage analytical spark is used to “sample” a metallic electrode and to convert that sample into a desirable form (toroidally shaped post-atomic vapor). This sample is then inductively re-excited with a short burst of radio-frequency (200 MHz) energy. Emission resulting from this re-excitation process is simplified, is free from background continuum, and shows narrow linewidths, when compared with direct spark emission. Analytical working curves for lead and iron in copper-based alloys show good linearity and range. Detection limits of 10 ppm are also shown for several elements in metallic samples.