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Far-ultraviolet (UV) light sources have attracted much attention for human safe viral inactivation and bacterial disinfection. Due to large optical nonlinearity and transparency to this wavelength region, second harmonic generation (SHG) device made of AlN can be a promising candidate. In this study, a transverse quasi-phase matched AlN channel waveguide with vertical polarity inversion was designed and fabricated. From wavelength spectra and a pump power dependence of an SH intensity, far-UV SHG via a largest nonlinear optical tensor component d 3333 was successfully confirmed under ultrashort pulse laser excitation.

A unique design of our ultracompact microcavity wavelength conversion device exploits the simple principle that the wavelength conversion efficiency is proportional to the square of the electric field amplitude of enhanced pump light in the microcavity, and expands the range of suitable device materials to include crystals that do not exhibit birefringence or ferroelectricity. Here, as a first step toward practical applications of all-solid-state ultracompact deep-ultraviolet coherent light sources, we adopted a low-birefringence paraelectric SrB4O7 crystal with great potential for wavelength conversion and high transparency down to 130 nm as our device material, and demonstrated 234 nm deep-ultraviolet coherent light generation, whose wavelength band is expected to be used for on-demand disinfection tools that can irradiate the human body.

Advances in lighting and quantum computing will require new degrees of control over the emission of photons, where localized defects and the quantum confinement of carriers can be utilized. In this contribution, recent developments in the controlled redistribution of energy in rare earth (RE)–doped nanosystems, such as quantum dots or within bulk insulating and semiconducting hosts, will be reviewed. In their trivalent form, RE ions are particularly useful dopants because they retain much of their atomic nature regardless of their environment; however, in systems such as GaN and Si, the electronic states of the RE ions couple strongly to those of the host material by forming nanocomplexes. This coupling facilities fast energy transfer (ET) (

Multiple exciton generation (MEG) takes place in competition to other hot carrier cooling processes. While the determination of carrier cooling rates is well established, direct information on MEG dynamics has been lacking. Here, we present a methodology to obtain the MEG rate directly in the initial ultrafast transient absorption dynamics. This method is most effective to systems with slow carrier cooling rates. Perovskite quantum dots exhibit this property and are used to illustrate this approach. They show a delayed carrier concentration buildup following an excitation pulse above the MEG threshold energy, which is accompanied by a faster carrier relaxation, providing a direct evidence of the MEG process. Numerical modeling within a simple framework of two competing cooling mechanisms allows us to extract the MEG rate and carrier energy cooling rates for this material. The presented methodology could provide new insights in carrier generation physics and valuable information for MEG investigations.

GaN-based micro-LEDs typically suffer from a size-dependent efficiency due to the relatively long carrier lifetime and sidewall-related recombination effects. We demonstrate that for red-emitting Eu-doped GaN, sidewall-related recombination is hardly an issue for emission efficiency. We determine the photoluminescence quantum efficiency (PL QE) of Eu-related emission as a function of the size of square structures ranging from 3 to 192 µm. With the support of finite-difference time-domain simulations, we show that the light extraction efficiency and material losses are responsible for the decrease in PL QE for large sizes. For sizes smaller than 24 µm, there is an influence of the sidewall-related non-radiative recombination of carriers on the PL QE; however, it is only minor as a result of the limited carrier diffusion lengths in the Eu-doped material. These properties combined with the high efficiency of luminescence indicate the potential of this material for micro-LED applications.

We demonstrate a GaN:Eu,O-based resonant-cavity light emitting diode (RCLED) fabricated with a n-type conductive Al0.82In0.18N/GaN bottom-distributed Bragg reflector (DBR) and a ZrO2/SiO2 top-DBR a...

In this study, we shed light on the properties of a photonic ring resonator made up of a closed array of circular dielectric nanorods arranged periodically in a background material. This type of resonator can reach high-quality factors (Q-factor) for specific transverse-magnetic (TM)-like modes, while maintaining a small footprint. We validate this by full 3D finite difference time domain simulations. The properties of the mode most interesting for applications are determined for various parameters of the resonator for the material parameters of GaN. This study provides design guidelines for the realization of this type of photonic nano-resonator and proposes and analyses two practical implementations.

A wavelength conversion device with an input grating coupler that enables easy alignment was fabricated using a vertically polarity inverted GaN layer. The device was excited with a femtosecond laser, and a second harmonic wave with a peak wavelength of 438.4 nm was obtained. A normalized wavelength conversion efficiency of 4.7 × 10−4% W−1 and a spectral bandwidth of the second harmonic wave of 2.9 nm were comparable to the theoretical estimations taking into account serious walk-off and waveguide losses. High efficiency of 33%–34% W−1 can be expected under continuous-wave excitation.

Inorganic perovskite nanocrystals (NCs) display attractive structural and optical properties, and have perspective in a wide range of optoelectronic applications. One interesting feature is their relatively slow hot carrier cooling rate, which allows to follow carrier dynamics in great detail with ultrafast spectroscopic techniques. In this talk, we present studies of the ultrafast carrier dynamics of perovskite NCs and assembled structures. Typically, when the excitation photon energy is increased, and thus the excess energy of the created carriers, the cooling time of the carriers towards the bandedge increases along. However, for CsPbI3 NCs, when the photon energy exceeds a certain threshold energy the cooling time starts to decrease, indicating that a competing relaxation pathway opens up which funnels carriers towards the bandedge. This threshold energy indicates the onset of a carrier multiplication process, in which absorption of a single photon leads to the creation of multiple electron-hole pairs. This process can also be inferred by following the carrier concentration, which shows a delayed build-up following the excitation pulse. The carrier dynamics in this system can be modeled within a framework of two competing cooling mechanisms. With the experimentally determined carrier cooling rate and threshold energy it is possible to extract the time-constant associated with the carrier multiplication process. Experiments on highly ordered CsPbBr3 NCs, so-called supercrystals, revealed how the photophysics are altered due to the inter-NC coupling as compared to the individual NCs. Next to a red-shift of the absorption and photoluminescence peak, the supercrystals show signatures of band-like states. The observation of a reduced Stokes-shift, decreased biexciton binding energy and increased carrier cooling rates, support the formation of delocalized states resulting from the coupling between individual NCs. These results open perspectives for assembled NCs for application in optoelectronic devices, with design opportunities exceeding the level of nanocrystal and bulk material.

We report an analysis of one-dimensional rod-based photonic crystal nanocavities. These cavities offer opportunities for dielectric materials which lack a matching low-refractive index substrate or are limited in under-etching possibilities to create slab-based PhC cavities. They offer high theoretical Q -values exceeding 10 6 for transverse magnetic polarized modes with modal volumes below 2.5 ( λ / n ) 3 . For practical implementations, we propose embedding these structures in a low-refractive index polymer. An analysis of intentionally introduced variations in a rod diameter reveals which design directions should be followed in order to create cavities that are most robust for fabrication-induced variations.

The development of efficient electrically driven color-tunable solid-state light sources will enable new capabilities in lighting and display technologies. Although alternative light sources such a...

III-nitride-based two-dimensional photonic crystal (2D-PhC) cavities with high-quality factors (Q-factors) have a large potential application, however realized Q-factors in the visible wavelength regime have been relatively moderate. In this study, we demonstrate the design and fabrication of 2D-PhC cavities to achieve high Q-factors, especially in the visible range. From the comparison of numerical calculations and the experimental results, we discuss the dominant optical losses that limit the Q-factor of H3-type cavities formed in an Eu,O-codoped GaN film. Based on these results we designed 2D-PhC cavities which can effectively suppress these dominant losses. We fabricated 2D-heterostructures and show a high Q-factor of 10500 at a resonant wavelength of ∼660 nm, which is considerably larger than any existing GaN-based nano/micro-resonators in the visible region. This study provides design guidelines for the realization of high Q-factors in photonic crystal nanocavities based on III-nitride semiconductors.

In this work, we fabricate a nitride-based photonic crystal (PhC) nanocavity with an embedded active layer for unexplored red regions using simplified selective wet etching of an ${\mathrm{Al}}_{0.82}{\mathrm{In}}_{0.18}\mathrm{N}$ sacrificial layer. Eu,O-codoped $\mathrm{Ga}\mathrm{N}$ ($\mathrm{Ga}\mathrm{N}$:$\mathrm{Eu}$,$\mathrm{O}$) is embedded in L7-type two-dimensional-PhC (2D-PhC) nanocavities with hexagonal holes. Room-temperature photoluminescence (PL) shows $\mathrm{Eu}$ emission coupled to the cavity modes at wavelengths around 620 nm. The quality factor of the fundamental mode in an L7 nanocavity is 5400, which is comparable to those of state-of-the-art 2D-PhC nanocavities with embedded nitride-based active layers for the blue and ultraviolet regions. We show that the Purcell effect due to the small mode volumes of the 2D-PhC nanocavities and enhanced light extraction lead to a 54-fold increase in the integrated PL intensity per unit area with respect to the typical $\mathrm{Ga}\mathrm{N}$:$\mathrm{Eu}$,$\mathrm{O}$ emission.

Development of an efficient red LED based on GaN is pivotal to ultra-small-size, full-color, and high-resolution micro-LED displays. In a red LED using Eu-doped GaN (GaN:Eu), the peak position of the emission is extremely stable against ambient temperature and injected current. Photoluminescence quantum efficiency of the Eu emission was investigated as a function of chip size of square structures. Even for sizes smaller than 24 µm, an influence of sidewall-related non-radiative recombination of carriers on the quantum efficiency was only minor as a result of limited carrier diffusion lengths in GaN:Eu. We also demonstrated monolithic vertically stacked full-color LEDs consisting of GaN:Eu and InGaN quantum wells. These results indicate a high potential of the GaN:Eu LED for the micro-LED applications.

We proposed a nitride semiconductor transverse quasi-phase-matched wavelength conversion device with a polarity inverted structure along the vertical direction formed by surface-activated bonding and etching processes. Inductively coupled plasma etching of a GaN layer with maintaining a root mean square roughness of less than 2 nm in a 100 μm square area was achieved even after deep etching of 1 μm using Cl2/Ar mixture gas and optimizing the antenna and bias powers. This smooth etching enabled surface-activated bonding of the ultrathin GaN layer with designed thickness. The fabrication process of the GaN polarity inverted channel waveguide was established.

The optical properties of $\mathrm{Eu}$-doped $\mathrm{Ga}\mathrm{N}$ microdisks (MDs) are demonstrated. The MDs are based on OMVPE-grown $\mathrm{Eu}$-doped $\mathrm{Ga}\mathrm{N}$ by our applying a 600-nm-thick ${\mathrm{Al}}_{0.82}{\mathrm{In}}_{0.18}\mathrm{N}$ sacrificial layer. The resonant modes of 1.57-\textmu{}m-diameter MDs are coupled with indirectly excited ${\mathrm{Eu}}^{3+}$ ions and enhance their radiative rate. We show a clear correlation between the radiative rate and the Q factor of the MD, accompanied by enhanced $\mathrm{Eu}$-related emission. For a cavity mode with a Q factor of 3680, the radiative rate is increased approximately 4.5 times by the Purcell effect. Determination of the polarization of the modes is achieved by detection of emitted light in the plane of the MD, and the designation of modes is supported by numerical calculations. Temperature-dependent measurements of a MD with a diameter of 5.8 \textmu{}m show a shift of the resonant modes of 10 pm/K, while $\mathrm{Eu}$-related emission is stable (less than 1 pm/K). The resonant mode of a MD with a diameter of 1.57 \textmu{}m shows a smaller temperature dependence of 7 pm/K.

Eu-doped GaN (GaN:Eu) has recently emerged as a promising material for red luminescence on the GaN platform, which is characterized by narrow peaks at ~620 nm from Eu ions owing to the intra-4f transitions. We have reported GaN:Eu-based red light-emitting diodes (LEDs) with an output power exceeding 1 mW at 20 mA, and are currently pursuing to further enhance the output power of GaN: Eu-based LEDs. In this presentation, we will present our recent progress on the device properties of GaN:Eu-based LEDs with several nanostructure/nanocavities including resonant cavity LEDs and localized surface-plasmon enhanced LEDs utilizing metal nanoparticles.

Circular cavity structures are fabricated on GaAs/ AlGaAs-based epi-wafer using SiO x hard mask to realize a proposed photonic crystal (PhC) structure with GaAs core layer having embedded InAs quantum dots (QDs) towards PhC laser fabrication. We investigate the evaluations of selective dry etching of the core layer for an epi-wafer using optical measurements. Photoluminescence (PL) intensity at 1285 nm, which is related to ground-state emission of QDs, monotonously decreases with increasing the etching depth of the core layer with QDs. The change of PL intensity for epi-wafer is good agreement with the behavior of the increased etching depth of the core layer which is evaluated by the measurements of scanning electron micrograph. We demonstrate a possibility of etching end-point detection and evaluation of etching rate during the dry etching of the core layer with QDs using in-situ optical emission spectroscopy.

The carrier mobility and dynamics of nonequilibrium carriers in Eu-doped GaN have been determined by above-band-gap, optically excited, time-resolved terahertz spectroscopy and compared with that of undoped GaN. The carrier mobility of GaN:Eu was an order of magnitude smaller than that of a similarly grown undoped GaN layer. Two time constants were observed in the carrier relaxation dynamics for GaN:Eu. A fast one (2.2 ps), which we related to an efficient nonradiative carrier trap, and a slower one (\ensuremath{\sim}85 ps) related to Eu-related traps. Ultrafast photoluminescence spectroscopy shows that Eu-related emission is observed within 100 ps after a pulsed excitation. These values set an upper value to the energy-transfer time from the Eu-related trap to $\mathrm{E}{\mathrm{u}}^{3+}$ ion and underlie the high efficiency of the excitation of $\mathrm{E}{\mathrm{u}}^{3+}$ ions in GaN.

Eu-doped GaN is a promising material with a wide array of potential applications in optoelectronics, optogenetics, micro displays and quantum computing. While this system has been the subject of intense investigation for the last two decades, several questions still remain about certain aspects of its optical properties, such as the polarization dependence of the optical transitions, and the coupling between the 4f-electron configuration and bulk phonons, as well and the appearance of local phonon modes. Moreover, the origin of certain emission peaks remains under debate in the literature. In this proceeding, the results of a systematic series of “site-selective” photoluminescence measurements are presented, where the properties of pulsed and continuous-wave laser excitation, such as polarization and intensity, were controlled.

The excitation efficiency and external luminescence quantum efficiency of trivalent ${\mathrm{Eu}}^{3+}$ ions doped into gallium nitride ($\mathrm{Ga}\mathrm{N}$) is studied under optical and electrical excitation. For small pump fluences, the excitation of ${\mathrm{Eu}}^{3+}$ ions is limited by an efficient carrier trap that competes in energy transfer from the host material. For large pump fluences, the limited number of high-efficiency ${\mathrm{Eu}}^{3+}$ centers and the small excitation cross section of the majority ${\mathrm{Eu}}^{3+}$ centers limit the quantum efficiency. At low temperatures, under optimal excitation conditions, the external luminescence quantum efficiency reaches a value of 46%. These results show the high potential for this material as an efficient light emitter and demonstrate the importance of excitation conditions on the light-output efficiency.

We have investigated the formation and decay of hydrogen-related donors (HDs) and irradiation-induced intrinsic defects. N-type m:Cz and FZ silicon wafers, which were irradiated with 2 MeV protons and subsequently annealed at 100–600 °C, were analyzed using spreading resistance profiling and photoluminescence (PL). HDs formed at 260 °C and then disappeared in two stages at 400–440 and 500–540 °C. This decay behavior indicates the existence of two types of HDs with different thermal stabilities. PL measurements showed interstitial silicon clusters ( W and X center), a carbon–oxygen complex ( C center), and exciton lines bound to unknown shallow centers. The origin of the HDs was investigated based on the correlation of the formation and decay temperatures between HDs and irradiation-induced defects. The predominant defects at the early stage of annealing, such as the C and W centers, are ruled out as candidates for the core defects of HDs because annealing above 260 °C is indispensable for the HD formation. In contrast, the X center was found to be thermally generated above 200 °C and disappeared at 580 °C. The similarity of the formation and decay temperatures between the X and HD centers suggests that HDs are associated with the formation of the interstitial silicon-related defects attached to hydrogen. Our results suggest that controlling the formation of interstitial silicon-related defects is important for realizing desirable doping profiles with high accuracy and reproducibility for power devices. Annealing above 400 °C exclusively provides thermally more stable HDs, leading to the realization of more rugged power devices.

We report the formation and the optical properties of Tm,Yb-codoped ZnO nanowires (NWs) grown by sputtering-assisted metalorganic chemical vapor deposition. The growth conditions of the ZnO core NWs are optimized by changing the growth temperature and the VI/II ratio. The optimal conditions are elucidated via structural analysis by scanning electron microscopy, X-ray diffractometry, and optical characterization. The ZnO:Tm,Yb/ZnO core-shell NWs are formed by growing ZnO:Tm,Yb shells on optimized ZnO core NWs. Light emission originating from the highest 1G4-3H6 transitions from Tm3+ ions is observed around 490 nm in ZnO:Tm,Yb/ZnO NWs due to the improved crystal quality of the ZnO host by utilizing the NW configuration.

The all-inorganic perovskite nanocrystals are currently in the research spotlight owing to their physical stability and superior optical properties—these features make them interesting for optoelectronic and photovoltaic applications. Here, we report on the observation of highly efficient carrier multiplication in colloidal CsPbI3 nanocrystals prepared by a hot-injection method. The carrier multiplication process counteracts thermalization of hot carriers and as such provides the potential to increase the conversion efficiency of solar cells. We demonstrate that carrier multiplication commences at the threshold excitation energy near the energy conservation limit of twice the band gap, and has step-like characteristics with an extremely high quantum yield of up to 98%. Using ultrahigh temporal resolution, we show that carrier multiplication induces a longer build-up of the free carrier concentration, thus providing important insights into the physical mechanism responsible for this phenomenon. The evidence is obtained using three independent experimental approaches, and is conclusive., In semiconductor nanocrystals, efficient carrier multiplication counteracts hot carrier thermalization, increasing the overall carrier generation yield. Here, de Weerd et al. observe a quantum yield of up to 98% in CsPbI3 nanocrystals as a result of efficient carrier multiplication.

The behavior of trivalent europium (Eu3+) ions doped into gallium nitride (GaN) was investigated under intense excitation conditions to explore the excitation energy transfer characteristics in the presence of large carrier densities. Under such conditions, strong emission from the higher excited 5D1 and 5D2 states of the Eu3+ ions was observed in highly efficient AlGaN/Eu-doped GaN multiple quantum wells grown by organometallic vapor phase epitaxy. This behavior was studied using a variety of excitation sources and conditions. Most notably, when a femtosecond-pulse laser was used, the excitation of the Eu3+ ions into the higher energy states became significant only with a second excitation pulse arriving within the lifetime of the 5D0 state. We propose that an already excited Eu3+ ion is promoted from its 5D0 excited state into the higher 5DJ states where it relaxes and can emit from the 5D1 and 5D2 states.

Eu-doped GaN (GaN:Eu) are novel candidates for red light-emitting diodes (LEDs). To further improve the luminescent efficiency of the GaN:Eu-based LED, the efficiency-droop under strong excitation conditions should be suppressed. In this paper, we demonstrate droop-free luminescence of GaN:Eu emitted from a sample-edge using a stripe excitation configuration. The Eu emission intensity clearly increases compared to the conventional surface-emission, and the enhancement is more pronounced for stronger excitation conditions. We clarify that the wavelength dependence of the enhancement agrees well with the optical gain spectrum of the GaN:Eu and is attributed to amplified spontaneous emission.