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Circularly polarized luminescence (CPL) spectroscopy measures the difference in luminescence intensity between left- and right-circularly polarized light, and is often used to analyze the structure of chiral molecules in their excited state. Recently, it has found an increasing range of applications in the analysis of molecules that emit circularly polarized light and can be employed in 3D displays. Thus, the number of articles focusing on CPL spectroscopy has increased dramatically. However, since the luminescence dissymmetry factor (glum) for organic compounds is generally

Selective area growth technique is very promising for the realization of optoelectronic nano-devices based on InGaN/GaN quantum disks, as it allows precise positioning of the nano-objects on the substrate. However, this fabrication method induces a pronounced pyramidal shape of the nano-columnar heterostructures. To understand how the optical properties of these heterostructures are affected by this shape, we investigated the linear polarization of the luminescence from 0-dimensional localization centers included in their active layer. Our experimental results and our simulation show that a complex strain distribution exist in the active layer and also that quantum dot-like objects can be used to probe the local strain distribution through nano-scale heterostructures.

Reduced emission efficiency (``efficiency droop'') at high current is a serious problem for solid-state lighting based on In${}_{x}$Ga${}_{1-x}$N LEDs. Auger recombination may be the culprit, yet fluctuations in alloy composition also seem to matter. The authors use a scanning near-field optical microscope to study the photoluminescence of blue- and green-emitting In-Ga-N quantum wells, and find that increasing excitation density causes carrier overflow from potential minima. The mechanism behind droop depends on emission color (indium content), and carrier-activated nonradiative recombination is key in green emitters.

The propagation of surface plasmon polaritons (SPPs) on Ag waveguides with two different widths is directly observed using dual-probe scanning near-field optical microscopy (DSNOM). We find that the waveguide structure strongly affects the propagation of locally excited SPPs. SPPs in a flat plane structure spread radially, whereas SPPs in a 3.4-μm wide waveguide structure form two-dimensional interference fringes due to the multiple reflections at the side edges. The experimental results agree well with finite-difference time-domain calculations. The results suggest that the DSNOM technique can visualize the nanoscale characteristics of the SPP waves in various plasmonic waveguides.

We investigated lateral charge carrier transport in indium gallium nitride InGaN/GaN multi-quantum wells for two different samples, one sample emitting green light at about 510 nm and the other emitting cyan light at about 470 nm. For the cyan light emitting sample we found a diffusion constant of 1.2 cm2/s and for the green light emitting sample 0.25 cm2/s. The large difference in diffusion constant is due to a higher point defect density in the green light emitting quantum wells (QWs) as high indium incorporation tends to reduce material quality.

Nanoscopic optical characterization using scanning near-field optical microscopy was performed on both InxGa1-xN/GaN single quantum wells (SQWs) grown on polar (0001) orientation and a semipolar (1122) microfacet SQW fabricated by a re-growth technique. The photoluminescence intensity of a conventional (0001) SQW emitting in the blue was completely independent of the threading dislocations (TDs) due to the small diffusion length less than 100 nm. In contrast, the photoluminescence intensity was well correlated with the TDs in the sample emitting in the green due to the association of In-rich clusters with dislocations, and the effect was enhanced by the larger diffusion length contribution from the longer radiative recombination lifetime. It was found that in a (1122) SQW, the suppression of the piezoelectric field leads to orders-of-magnitude faster radiative lifetime and consequently, a shorter diffusion length. In addition, the highest internal quantum efficiency was approximately 50% at 520 nm, which is about 50 nm longer than in (0001) QWs, suggesting that (1122) QWs are suitable for green emitters.

We performed near-field photoluminescence (PL) mapping and atomic force microscopy at the same scanning area to clarify relationship between luminescence dynamics and threading dislocations in violet and blue light emitting InGaN single-quantum-well structures on epitaxially laterally overgrown GaN templates at room temperature. Clear correlation has been found between the distribution of dark area in PL mapping data and threading dislocations with screw and mixed types in the violet sample. On the other hand, such correlation has not been found in the blue sample. These results indicated that the carriers/excitons are captured easily in nonradiative recombination centers (NRCs) originating from the threading dislocations in the violet sample, because the lateral diffusion length of carriers/excitons is large due to a small potential fluctuation induced by compositional fluctuation of In comparing to those in the blue sample. However, the capture to such NRCs is hindered effectively in the blue one because of the localization to potential minima. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spatial distribution of photoluminescence (PL) spectra has been assessed in violet light emitting InGaN single-quantum-well (SQW) structures by employing a scanning near-field optical microscope (SNOM) at room temperature. Clear correlation has been found between PL intensity and PL peak wavelength under illumination collection (I-C) mode, where strong PL was observed in the area emitting at lower energy bands. However, dark spots having diamter of about a few hundreds nm appeared also in lower energy region. Moreover, multi-mode SNOM-PL with a spatial resolution of 200 nm revealed that the difference of PL-intensity image between I-C mode and illumination (I) mode is not so large comparing with a blue light emitting InGaN-SQW. These findings suggest that threading dislocations act as nonradiative recombination centers in the violet sample unlike blue ones, and that the diffusion length to such centers is larger than that to localized centers leading to radiative recombination. Furthermore, nonradiative recombination centers originating from point defects have been spatially identified in the violet sample grown on epitaxially laterally overgrown (ELO) GaN template. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spatial distribution of InGaN photoluminescence (PL) at room temperature has been examined by observation through an illumination-transmission mode scanning near-field optical microscope (SNOM). The near-field PL mapping revealed a disuniform pattern of sharp or blunt emission peaks from few hundreds of nanometer extension to the micrometer scale range. The peaks’ average amplitude size and distances from neighboring peaks have been measured in a large number of experiments for three different samples at dissimilar indium doses, and their results compared. A general trend has been found and the influence of indium concentration is shown in a final plot. We describe the experimental approach adopted and present full details on the results obtained. Data were gathered from more than 100 different measurements; low-cost optical SNOM nanoprobes were originally fabricated and we briefly explain the fabrication procedure.

Photoluminescence (PL) spectra and time-resolved PL are measured from around 10 to 300 K for the InGaN/GaN single quantum wells (SQWs) with well widths of 1.5, 2.5, 4 and 5 nm. For the SQWs with the well widths of 1.5 and 2.5 nm, the peak position of PL exhibits an S-shaped shift with increasing temperature. The radiative recombination time τ RAD begins to increase at the temperature for the position to change from the red-shift to the blue-shift. The steep increase of τ RAD is observed beyond the temperature from the blue-shift to the red-shift. For the SQWs with the well widths of 4 and 5 nm, the peak position of PL exhibits a monotonic red-shift. τ RAD decreases at first and then increases with temperature. It is about 100-times longer in the low temperature region and about 10-times longer at room temperature as compared with those of the SQWs with narrower widths.

Optical properties of GaCrN, GaDyN and GaGdN have been studied by means of photoluminescence spectroscopy having spectral and time resolution. Photoluminescence (PL) emission at around 3.29 eV was determined to be the band-to-band transition of GaCrN. The PL decay time of GaCrN depends on emission energy. For GaDyN, Dy3+ ions in host GaN exhibited several sharp peaks originating from the intra-4f orbital transitions of the series of 4F9/2−6HJ (J = 15/2,13/2,...), and the lifetime of the 4F9/2−6H13/2 transition was as long as 87 µs at 13 K. The broad defect level for GaDyN provides an efficient pathway for the carrier-mediated energy transfer between Dy3+ ions and the GaN host. However, no such evidence is found between Gd ions and GaN in the GaGdN layer.

Optical properties induced by two major effects, potential fluctuation and piezoelectric fields, have been assessed to interpret the emission mechanism in low-dimensional nitride semiconductors because the former leads to the exciton/carrier localization, and the latter to the quantum confined Stark effect (QCSE). Degenerated white-light pump-and-probe spectroscopy has been employed to assess which factor plays an important role in the series of In x Ga 1-x N multiple quantum well (MQW) structures whose well widths are 3 nm, 5 nm and 10 nm. Moreover, photoluminescence (PL) mapping with scanning near-field optical microscopy (SNOM) has revealed the dense distribution of island-like structures, the size of which ranges from 20 nm to 70 nm in a 3 nm-thick In x Ga 1-x N single quantum well (SQW) structure emitting at blue spectral region.

Electroluminescence (EL) mapping has successfully been performed for In x Ga 1-x N single quantum well (SQW)-based light emitting diodes (LEDs) by employing scanning near-field optical microscopy (SNOM) at room temperature. The relative EL intensity fluctuates in the range from 1 to 4 with the spatial scale less than a few hundred nanometers. Clear correlation has been found between EL peak wavelengths and EL intensities where the regions of long wavelength correspond to those of strong EL intensity, suggesting that the injected carriers redistribute toward local potential minima within In x Ga 1-x N SQWs.

We performed time-resolved photoluminescence (TRPL) measurements for high-quality Al-rich AlGaN/AlN multiple quantum wells (MQWs) under selective excitation condition in order to investigate carrier recombination dynamics around the Mott transition. Fast and slow lifetime components (12 ps and 210 ps) were definitely observed under high excitation conditions at 5.5 K and were attributed to electron-hole plasma and the exciton many-body effect, respectively. It was found that the remarkable difference in the radiative lifetimes caused strongly excitation-power dependent internal quantum efficiencies. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Carrier dynamics in GaN and InGaN/GaN SQW structures were observed by using the transient grating (TG) method with sub-picosecond pulsed laser at room temperature. The diffusion coefficients (D) of photo-created carriers were estimated by the decay rate time of TG signals and the photoluminescence (PL) lifetime. It was found that D depends on the emission wavelength (In composition). The relationship between the emission efficiencies and carrier diffusion was considered in terms of the spatial inhomogeneity of In composition.

Spatial distribution of photoluminescence (PL) spectra has been assessed in an InGaN single quantum well (SQW) structure by means of fluorescence microscopy and scanning near-field optical microscopy (SNOM) under illumination-collection mode. The PL intensity of fluorescence image is uniform at 77 K, but the dark spot areas were extended with increasing temperature. The near-field PL images revealed the variation of both peak energy and intensity in PL spectra according to the probing location with the scale less than a few hundreds nm.

Recombination dynamics of spontaneous and stimulated emissions have been assessed in InGaN-based light emitting diodes (LEDs) and laser diodes (LDs), by employing time-resolved photoluminescence and pump and probe spectroscopy. As for an In0.02Ga0.98N ultraviolet LED, excitons are weakly localized by 15 meV at low temperature, but they become almost free at room temperature (RT). It was found that addition of a small amount of In results in the reduction of nonradiative recombination centres originating from point defects. The internal electric field does exist in InGaN active layers, and induces a large modification of excitonic transitions. However, it alone does not explain the feature of spontaneous emission observed in an In0.3Ga0.7N blue LED such as an anomalous temperature dependence of peak energy, almost temperature independence of radiative lifetimes and mobility-edge type behaviour, indicating an important role of exciton localization. The lasing mechanism was investigated for In0.1Ga0.9N near ultraviolet (390 nm), In0.2Ga0.8N violet-blue (420 nm) and In0.3Ga0.7N blue (440 nm) LDs. The optical gain was contributed from the nearly delocalized states (the lowest quantized levels (LQLs) within quantum wells) in the violet LD, while it was from highly localized levels with respect to the LQL by 250 meV for the violet-blue LD, and by 500 meV for the blue LD. It was found that the photo-generated carriers rapidly (less than 1 ps) transferred to the LQL, and then relaxed to the localized tail within the timescale of a few ps, giving rise to the optical gain. Such gain spectra were saturated and other bands appeared in the vicinity of the LQL under higher photo-excitation.

Time-resolved optical characterization is an indispensable tool to study the recombination mechanisms of excitons and/or carriers based on radiative, non-radiative, localization and many-body processes. In this paper, we review the instrumentation of various spectroscopic techniques for the assessment of In x Ga 1 -x N-based semiconductors such as time-resolved photoluminescence (TRPL), time-resolved electroluminescence (TREL), transient grating (TG) method to probe photothermal processes, microscopic TRPL using optical microscope, submicroscopic TRPL using scanning near field optical microscopy (SNOM) and pump-and-probe spectroscopy for the measurement of transient absorption/gain spectra. The obtained results are cited in the references.

Photoreflectance study has been performed to determine the E 0 ′, E 1 and E 2 critical-point (CP) energies in crystalline silicon at temperatures T between 77 and 300 K using an Ar + -ion laser as the modulation light source. The measured photoreflectance spectra have been successfully explained by a standard CP line shape. The temperature dependence of the E 0 ′, E 1 and E 2 CP parameters (band-gap energy, amplitude and broadening parameter) has been analyzed using the Varshni equation and an empirical expression of Bose–Einstein type.

Photoreflectance (PR) measurements have been carried out to determine the E1 and E1+Δ1 critical-point (CP) parameters in CdTe at temperatures between T=77 and 300 K using a He–Ne laser as modulation light source. The measured PR spectra give very weak, but distinct, structures at ∼3.5 eV(E1) and ∼4 eV(E1+Δ1). When an Ar+-ion laser is illuminated as bias light, the spectra measured for T⩽150 K become very strong. This effect is considered to be due to a bias-laser-induced band bending. The experimental PR spectra can be successfully explained by an excitonic model of the interband transitions. The temperature dependence of the CP parameters (energy, amplitude, and broadening parameter) are analyzed using the Varshni equation and an empirical expression of Bose–Einstein type.

Photoreflectance spectra have been measured to determine the E 1 and E 1 + 1 energies in ZnTe at temperatures T between 77 and 300 K using an Ar+ -ion laser as a modulation light source. The measured photoreflectance spectra can be successfully explained by an excitonic line shape. The temperature dependence of the E 1 and E 1 + 1 critical-point parameters (energy, amplitude and broadening parameter) have been analysed using the Varshni equation and an empirical expression of the Bose-Einstein type. A comment correcting this article has been published: Molina, Espinosa-Luna and Adachi 2004 J. Phys. D: Appl. Phys. 37 2047

Photoreflectance (PR) spectra have been measured to determine the fundamental-absorption-edge (E0) structure of hexagonal CdSe single crystal for both E c and E|| c polarizations at temperatures T between 77 and 300 K. The measured PR spectra are found to be successfully explained by an excitonic line shape for temperatures T below ~200 K and by a three-dimensional one-electron line shape for T>200 K. No clear evidence of the exciton interference effect has been observed within our experimental temperature range T = 77-300 K. The temperature dependence of the E0-exciton parameters (exciton energy, amplitude and broadening parameter) has been determined and analysed using the Varshni equation and an empirical expression of Bose-Einstein type.

Reflectance spectroscopy was performed for nonpolar and semipolar bulk aluminum nitride (AlN) substrates under uniaxial stress. The exciton-polariton theory was applied to interpret the reflectivity spectra, while the Bir-Pikus deformation potential theory was used to analyze the stress dependence of exciton transition energies. This approach allowed all the deformation potentials in AlN to be determined without the quasicubic approximation.

Precise identification of recombination dynamics based on local, radiative, and nonradiative recombination has been achieved at room temperature in a blue-light-emitting InxGa1−xN/GaN single-quantum-well structure by comparing the photoluminescence (PL) spectra taken by illumination-collection mode (I-C mode) and those by illumination mode (I-mode) in scanning near-field microscopy. The PL data mapped with PL lifetimes, as well as with PL spectra, revealed that the probed area could be classified into four different regions whose dominating processes are (1) radiative recombination within a probing aperture, (2) nonradiative recombination within an aperture, (3) diffusion of photogenerated excitons/carriers out of an aperture resulting in localized luminescence, and (4) the same diffusion process as (3), but resulting in nonradiative recombination.

In this paper, micromachining inside of direct and indirect semiconductor, such as zinc oxide crystal (ZnO) and single-crystalline silicon(c-Si) using femtosecond laser pulses is successfully demonstrated. In the case of ZnO, oxygen vacancy or interstitial zinc was three-dimensionally induced by the near-infrared femtosecond laser pulse irradiation. The threshold energy for oxygen defect formation increased with increasing in a pulse width. The mechanism of the pulsewidth dependence of the damage threshold inside ZnO could be interpreted in terms of the excitonic Mott transition to the electron-hole plasma which depends on the electron plasma density induced by the laser irradiation. We have also successfully micromachined inside c-Si using infrared ultrashort laser pulses (λ = 1.24 μm). Optical microscope observation under an infrared lamp illumination indicates low density material or scattering structure was formed in the vicinity of the focal spot.

Spatial distribution of photoluminescence (PL) with spectral, spatial, and/or time resolution has been assessed in an InxGa1−xN single-quantum-well (SQW) structure using scanning near-field optical microscope (SNOM) under illumination-collection mode at 18 K. The near-field PL images revealed the variation of both intensity and peak energy in PL spectra according to the probing location with the scale less than a few hundredths of a nanometer. PL linewidth, the value of which was about 60 meV in macroscopic PL, was as small as 11.6 meV if the aperture size was reduced to 30 nm. Clear spatial correlation was observed between PL intensity and peak wavelength, where the regions of strong PL intensity correspond to those of long wavelength. Time-resolved SNOM–PL study showed the critical evidence that supports the model of diffusion of carriers to potential minima.

As an advantage, random lasers may be elaborated from a large variety of materials and do not require any cavity oscillators that usually necessitate complicated and expensive fabrication techniques. Since the feedback process of those non-conventional laser systems is provided by light interference in a disordered medium, spectral and temporal uncertainties are usually considered as an intrinsic part of their optical proprieties. We investigated random laser action under two photon absorption experiments through an auto-organized InGaN/GaN quantum-disks ensemble. Thanks to our experimental approach, we evidence random lasing based on a gain medium constituted by point-sized structures. In such context, a stabilised and individual emission mode is observed as for conventional semiconductor lasers. By controlling the emission energy of these nanostructures, a tuneable and stable random laser may be built. Moreover, our findings suggest that disordered medium should play an important role in the conception of low cost quantum dot and up conversion laser systems.

We propose an array of submicrometer mirrors to assess luminescent nano-objects. Micromirror arrays (MMAs) are fabricated on Si (001) wafers via selectively doping Ga using the focused ion beam technique to form p-type etch stop regions, subsequent anisotropic chemical etching, and Al deposition. MMAs provide two benefits: reflection of luminescence from nano-objects within MMAs toward the Si (001) surface normal and nano-object labeling. The former increases the probability of optics collecting luminescence and is demonstrated by simulations based on the ray-tracing and finite-difference time-domain methods as well as by experiments. The latter enables different measurements to be repeatedly performed on a single nano-object located at a certain micromirror. For example, a single InGaN∕GaN nanocolumn is assessed by scanning electron microscopy and microphotoluminescence spectroscopy.

Reflectance spectroscopy was performed for nonpolar and semipolar bulk GaN substrates under uniaxial stress. The exciton-transition energies and oscillator strengths clearly depended on uniaxial stress. The results were interpreted by group theory on excitons and analyzed in terms of the effective Hamiltonian proposed by Bir and Pikus. In the Hamiltonian, the short-range Coulomb interaction was taken into consideration. This approach allows all exciton deformation potentials to be determined without the quasicubic approximation and we found that the exciton deformation potentials in GaN considerably deviate from the quasicubic approximation.

We have developed dual-probe scanning near-field optical microscopy (SNOM) to visualize detailed carrier diffusion/recombination processes and applied it to the assessment of the local carrier dynamics in an InGaN single quantum well. It is clearly demonstrated that the carrier motion is strongly affected by the potential distribution within InGaN; potential ridges prevent carriers from diffusing outside them, whereas potential peaks cause carriers to travel a roundabout route around them. As a consequence, carriers anisotropically diffuse for several hundred nanometers along a specific direction toward a strong-photoluminescence domain. Thus, the dual-probe SNOM technique is a powerful nanoscopic tool, and may be versatile for characterizing photonic materials.

We report microphotoluminescence spectroscopy performed on individual and ensemble InGaN/GaN quantum disks (Q-disks). The typical spectrum of a single Q-disk exhibited the contribution of localization centers (LCs) formed in the InGaN active layer of the Q-disks, characterized by sharp lines appearing on the low energy side of the spectra. In addition, a broader emission peak identified as the luminescence of the quasi-two-dimensional (2D) InGaN active layer surrounding the LCs appears systematically at higher energy. Time-resolved photoluminescence experiment performed on single Q-disks exhibited the excitonic transfer, from the 2D InGaN active layer to LCs, at the submicroscopic scale. Excitation power dependence studies and linear polarization analysis allowed us to identify a biexciton complex confined in a LC in a single Q-disk with a surprising positive binding energy of 13 meV. The absence of screening effect by increasing the excitation power density and the fast excitonic radiative lifetime of a few hundred picoseconds that we measured on several individual Q-disks indicate that the absence of internal electric field in the structure can explain the observed positive biexciton binding energy.

We investigated correlations between nanoscopic optical and structural properties in violet-emitting, blue-emitting, and green-emitting ${\text{In}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}/\text{GaN}$ quantum wells (QWs) by means of scanning near-field optical microscopy (SNOM) and atomic force microscopy. Only in the blue-emitting QW, threading dislocations were not major nonradiative recombination centers (NRCs). SNOM data indicated that NRCs in the blue-emitting QW are surrounded by energy levels higher than those for radiative recombination. Such potential distributions realize ``antilocalization'' of carriers to NRCs, which is the cause of high emission quantum efficiencies in blue emitters.

We investigate the carrier transport and optical properties of a thick InGaN single quantum well (SQW) where an AlGaN δ-layer is embedded. It is shown that the results of photoluminescence (PL) measurements are consistent with the numerical predictions.

Spatially resolved photoluminescence (PL) of InGaN/GaN/AlGaN-based quantum-well-structured light-emitting diodes (LEDs) with a yellow-green light (530 nm) and an amber light (600 nm) was measured by using confocal microscopy. Submicron-scale spatial inhomogeneities of both PL intensities and spectra were found in confocal micro-PL images. We also found clear correlations between PL intensities and peak wavelength for both LEDs. Such correlations for yellow-green and amber LEDs were different from the reported correlations for blue or green LEDs. This discrepancy should be due to different diffusion, localization, and recombination dynamics of electron-hole pairs generated in InGaN active layers, and should be a very important property for influencing the optical properties of LEDs. In order to explain the results, we proposed a possible carrier dynamics model based on the carrier localization and partial reduction of the quantum confinement Stark effect depending on an indium composition in InGaN active layers. By using this model, we also considered the origin of the reduction of the emission efficiencies with a longer emission wavelength of InGaN LEDs with high indium composition.

Scanning near field optical microscopy (SNOM) has been developed to assess the recombination mechanism in low-dimensional nitride semiconductors by employing spatial and temporal photoluminescence (PL) mapping under illumination-collection at cryogenic temperatures. The near-field PL images taken at an InxGa1-xN single-quantum-well (SQW) structure revealed the variation of both intensity and peak energy according to the probing location with the scale less than a few tens of a nanometer. The PL, the linewidth of which was about 60meV in macroscopic measurements, was separated into several peaks with the linewidth of about 12 meV if the SNOM-PL was taken with the aperture size of 30 nm. Clear spatial correlation was observed between PL intensity and PL peak-photon-energy, where the regions of strong PL intensity correspond to those of low PL peak-photon-energy. Time-resolved SNOM-PL study showed the important role of exciton/carrier localization in the recombination mechanism in InxGa1-xN-based quantum structures.

Recombination dynamics of spontaneous and stimulated emissions have been assessed in InGaN-based light emitting diodes (LEDs) and laser diodes (LDs), by employing time- resolved photoluminescence and pump and probe spectroscopy. As for an In0.02Ga0.98N-ultraviolet-LED, excitons are weakly localized by 15 meV at low temperature, but they become almost free at room temperature. It was found that addition of small amount of In results in the reduction of nonradiative recombination centers originating from point defects. The internal electric field does exist in InGaN active layers, and induces a large modification of excitonic transitions. However, it alone does not explain the feature of spontaneous emission observed in an In0.3Ga0.7N- blue-LED such as an anomalous temperature dependent of peak energy, almost temperature independence of radiative lifetimes and mobility-edge type behavior, indicating an important role of exciton localization.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The optical gain properties of InGaN-based green (512 nm) laser diodes fabricated on (0001) GaN substrates are investigated. Fitting simulations to the experimental gain spectra provides a Gaussian inhomogeneous broadening of 95 meV, an optical confinement factor of 0.006, and an internal loss as low as ~10/cm. The remarkable suppression of inhomogeneous broadening and internal loss compensate for the low optical confinement, leading to a low threshold current density of 2.75 kA/cm2. The suppressed inhomogeneity contributes to the highly linear gain increase with the injection carrier, while the low optical confinement results in a relatively low differential mode gain.

The optical properties of InGaN-based green laser structures fabricated on (0001) GaN substrates are investigated using photoluminescence (PL) spectroscopy. Both macroscopic and microscopic measurements demonstrate that the potential fluctuations are drastically suppressed in state-of-the-art (0001) green InGaN laser structures. Time-resolved PL suggests that InGaN quantum wells (QWs) become thinner than conventional QWs, which is compensated for by higher In compositions. Such QWs increase the radiative recombination probability due to a greater overlap between the electron and hole wavefunctions. The suppressed luminescence inhomogeneity and increased radiative recombination probability may be responsible for the recent remarkable reduction in the lasing threshold.

Nanoscopic photoluminescence (PL) properties of a green-emitting {2021} InGaN single quantum well (SQW) are investigated by scanning near-field optical microscopy (SNOM). Carrier/exciton diffusion outside the probe aperture of 150 nm is demonstrated by a multimode SNOM technique. The estimated diffusion lengths are ~70 nm along the [1014] direction and ~50 nm along the [1210] direction, and are in between those of (1122) and (0001) InGaN QWs. This finding is well accounted for by the difference in carrier/exciton lifetimes. Furthermore, atomic force microscopy (AFM) reveals ridge structures along the [1210] direction. Superimposing the SNOM-PL image with an AFM image, we find a clear correlation between the spatial distributions of PL peak wavelength and surface morphology.

Optical gain is assessed for a green laser diode (LD) grown on a semipolar {2021} GaN substrate. The estimated internal loss is 20±5/cm, and the maximum gain becomes saturated with the injection current, suggesting the existence of gain suppression mechanisms. The quantum-confined Stark effect due to the pn junction, state filling, and screening of polarization-induced electric fields can explain shifts in the gain and the emission peaks with the injection current. The tail states in {2021} LDs are much smaller than those in (0001) LDs, and are not the major cause of the gain suppression in {2021} LDs, in contrast to (0001) LDs.

Spatial distribution of photoluminescence (PL) spectra has been assessed in an InGaN single quantum well (SQW)-based light-emitting diode structure by near-field optical microscopy under the illumination-collection mode. The obtained PL mapping image revealed a variation in both peak and intensity of PL spectra according to the probing location with a scale less than about 200 nm. The variation in PL intensity is from 0.8 to 1.8 in arbitrary units indicating that the internal quantum efficiency fluctuates from 10% to 50% within the active layer.

Recently, we performed reflectance spectroscopy for nonpolar and semipolar bulk GaN substrates under uniaxial stress and experimentally determined all the exciton deformation potentials in GaN. Our findings indicated a breakdown of the quasicubic approximation. Herein its impact on the electronic band structures in GaN/AlGaN strained quantum wells is theoretically investigated. The calculated results significantly differ from those using the widely accepted exciton deformation potentials. A simplified Hamiltonian is given to describe the in-plane optical polarization properties of GaN/AlGaN quantum wells.

Carrier/exciton localization in InGaN quantum wells (QWs) for green laser diodes fabricated on semi-polar {2021} GaN substrates is assessed using time-resolved photoluminescence (TRPL) spectroscopy. The estimated characteristic energy, which represents the localization depth in a {2021} InGaN QW, is 15.1 meV. This value is much smaller than that reported for c-plane green InGaN QWs, indicating a high compositional homogeneity of {2021} InGaN QWs and consequently suggesting that the GaN semi-polar {2021} plane is suitable for fabricating green laser diodes.

The optical properties of InGaN/GaN quantum wells, which were nanopatterned into cylindrical shapes with diameters of 2 μm, 1 μm, or 500 nm by chemically assisted ion beam etching, were investigated. Photoluminescence (PL) and time-resolved PL measurements suggest inhomogeneous relaxation of the lattice-mismatch induced strain in the InGaN layers. By comparing to a strain distribution simulation, we found that partial stain relaxation occurs at the free side wall, but strain remains in the middle of the pillar structures. The strain relaxation leads to an enhanced radiative recombination rate by a factor of 4–8. On the other hand, nonradiative recombination processes are not strongly affected, even by postgrowth etching. Those characteristics are clearly reflected in the doughnut-shape emission patterns observed by optical microscopy.