Iii-nitride tunable cup-cavities supporting quasi whispering gallery modes from ultraviolet to infrared

ABSTRACT Rapidly developing nanophotonics needs microresonators for different spectral ranges, formed by chip-compatible technologies. In addition, the tunable ones are much in demand. Here,

we present site-controlled III-nitride monocrystal cup-cavities grown by molecular beam epitaxy. The cup-cavities can operate from ultraviolet to near-infrared, supporting quasi whispering

gallery modes up to room temperature. Besides, their energies are identical in large ’ripened’ crystals. In these cavities, the refractive index variation near an absorption edge causes the

remarkable effect of mode switching, which is accompanied by the spatial redistribution of electric field intensity with concentration of light into a subwavelength volume. Our results shed

light on the mode behavior in semiconductor cavities and open the way for single-growth-run manufacturing the devices comprising an active region and a cavity with tunable mode frequencies.

SIMILAR CONTENT BEING VIEWED BY OTHERS HIGH-QUALITY NANOCAVITIES THROUGH MULTIMODAL CONFINEMENT OF HYPERBOLIC POLARITONS IN HEXAGONAL BORON NITRIDE Article 06 February 2024 PREDESIGNED

PEROVSKITE CRYSTAL WAVEGUIDES FOR ROOM-TEMPERATURE EXCITON–POLARITON CONDENSATION AND EDGE LASING Article 19 August 2024 HIGH DENSITY LITHIUM NIOBATE PHOTONIC INTEGRATED CIRCUITS Article

Open access 10 August 2023 INTRODUCTION Whispering gallery mode (WGM) microresonators have attracted growing attention as essential building blocks of nano emitters1,2,3, detectors,

biological and chemical sensors4,5,6. They are especially advantageous for structures made from wide-gap III-nitrides7,8,9,10,11, because manufacturing the planar Bragg microcavities is

difficult with decreasing the thicknesses of constituent layers in the ultraviolet (UV) range. Such a problem is not essential for a WGM microresonator, since the working wavelength

determines the characteristic size of its body as a whole. On the other end of the spectral range, the narrow-gap III-nitrides could cover the near infrared (NIR) range corresponding to the

minimal losses in telecommunication networks. However, the structural quality of these materials is usually not too good. Thus, the challenge still remains of fabricating the WGM cavities

from all the III-nitrides which have good prospects for nanophotonics in general. The emergence of narrow lines in the optical spectra of the WGM microresonators is usually associated with

the Purcell effect12,13,14, when the spontaneous emission probability of a dipole resonantly coupled to a cavity mode is enhanced by a factor . Here λ is the wavelength in free space, _n_ is

the refractive index of the cavity material and is the effective volume of the mode, which gives the level of its confinement. The quality (_Q_) factor of a particular resonance is defined

as , where is the width of the WGM-related narrow line. It is commonly believed that the Purcell effect always plays a positive role of increasing the emission efficiency, that is not

unconditionally valid. As demonstrated by Kleppner15, the inhibition of spontaneous decay rate by a factor of order can take place when the density of final photon states or cavity size are

too small. Similar effect appears with detuning from the mode resonance frequency16. The ratio γ of spontaneous emission rate of an emitter having the wavelength λ_e_ located at position _r_

in the cavity to the rate in a homogeneous medium can be expressed as neglecting the decay channel into leaky modes16,17. Here the electric field amplitude distribution in the cavity has a

maximum magnitude . This equation shows that to achieve noticeable enhancement the mode resonance should be tuned to the emitter resonance and the emitter itself should be placed to a region

where the mode with high _Q_-factor is strongly confined. We assume that the distribution of electromagnetic mode intensity in a WGM cavity, which is spatially inhomogeneous in principle,

will control the local manifestation of the Purcell effect via either enhancement or inhibition of emission. Little attention was paid to this fact in the investigations of intensity

distribution in the WGM cavities18,19. Currently, there is a need for a cavity design which satisfies three basic requirements: i) high _Q_-factor, ii) tunability of working frequencies and

iii) compatibility with the chip technology. Operation of a dielectric cavity with high _Q_-factor, when the WGMs are strongly confined inside, requires either special scatters or elements

for evanescent coupling20,21. This problem is diminished in the semiconductor cavities by using the fluorescence coupling5. For that, an emitting substance (impurities, quantum wells or

dots) is inserted within the resonator7,9. Under the illumination by a short-wavelength light, the broadband emission of this substance appears at longer wavelengths, below the principal

absorption edge of cavity material. This emission launches the WGMs and couples to them. The light out-coupling, being weakly dependent on cavity orientation18, occurs mostly via the

boundary-wave and pseudointegrable leakages22. It is worth mentioning that the _Q_-factor can hardly be very high in the “open” cavities emitting light. However, it is not a fatal

shortcoming, since the resonators of the moderate quality might be quite useful for many nanophotonics applications. To tune WGMs in the cavities, there are a few methods: temperature23,

strain24 and shape variation, like in the so-called “bottle microresonators”25. In particular, the temperature-induced shift at the optical pumping reached 3.7 meV in a hybrid structure

comprising a nanoparticle and a glass microsphere26. In many cases, the WGM cavities are formed by post-growth etching7,8,9; alternatively, they can be created by epitaxy17,27,28. Both

approaches possess inherent advantages and disadvantages. The etching allows fabricating structures of large diameters, which can maintain the long-lived high order modes with enhanced

_Q_-factor. However, the number of modes existing in the vicinity of the principal one increases29, that hampers the spectral selection of discrete transitions. The rotational symmetry of

the etched discs complicates the light into- and out-coupling. Epitaxial techniques offer a nice opportunity to fabricate a cavity and an active region during a single growth run. The formed

cavities are nano- and microcrystals, which can be precisely located on the growth surface30. Their sizes are more suitable for supporting the low-order modes31 that facilitates the

spectral selection. On the other hand, these cavities have usually a shape which reproduces the crystal structure, i.e. hexagonal in III-nitrides and II-oxides. Distortion of this shape may

unpredictably change the WGM frequencies27. Here, we present a novel type of microresonators —a cup-cavity, which can be used in a wide spectral range from UV to NIR. The monocrystal

cup-cavities were fabricated by molecular beam epitaxy (MBE) from GaN and InN to prove applicability of our approach for the whole III-nitride family. The cup-cavities support the so-called

quasi-WGMs of low orders, whose frequencies and electric field intensity distribution deviate from those in discs. The impact of the WGMs on the cavity properties is justified by

comprehensive investigations done in a wide temperature range by using high-spatial-resolution spectroscopy and imaging techniques and supported by theoretical modeling. The tunability of

the quasi-WGMs is conditioned by strong optical dispersion in semiconductors materials near an absorption edge. We report on the discovered effect of mode alteration (switching), which is

accompanied by the modification of spatial mode intensity distribution and emission of terahertz (THz) quanta. In some degree, the cup-cavity operates as a parabolic mirror concentrating the

light. At room temperature, the mode energy is confined into a subwavelength volume, that can be used for selective enhancement of a limited number of quantum emitters. RESULTS SAMPLES Our

cup-cavities were grown on cone-shaped patterned _c_-sapphire substrates by MBE which has been developed to create the site-controlled monocrystals of different shapes, such as cups and

nanocolumns (Fig. 1). Note that the latter are promising for single-photon emitters30. Details on the MBE technology for both GaN and InN monocristals are given in the Methods and

illustrated by scanning electron microscopy (SEM) images in Supplementary, Fig. 1. The metal-rich conditions turn out to be preferable for the cup-cavity formation. The creation of the InN

cup-cavity was a challenge, since InN is the most problematic material for application among III-nitrides. Importantly, the wide band of near-band-edge emission in InN is very suitable to

depict the almost full set of anticipated modes by means of the fluorescence coupling. This would be impossible using the GaN crystals, where the PL line is typically narrow and can coincide

only with 1–3 modes (Fig. 1(c)). As a result, most of the results presented below concern the InN cup-cavities. The structural properties of the cup-cavities were investigated by SEM,

energy dispersive x-ray (EDX) microanalysis and micro-Raman (μ-R), whose details are given in Methods, as well as the details of spectroscopy and imaging techniques. According to SEM

studies, the crystal surface is smoother than that of a surrounding area (Fig. 2(a)). A significant part (~20%) of the monocrystals possesses the ideal shape and large characteristic

diameter (_d_). Such a yield is sufficient for practical applications; moreover, it can be further improved. Currently, the maximal _d_ ~ 2.2 μm achieved for the InN cup-cavities exceeds

that for the GaN ones (~1.2 μm). However, in both cases the low-order optical modes can be supported, since the light wavelength inside GaN (~0.12 μm) is much smaller than that in InN (~0.55

 μm). The crystals fabricated on the patterned substrate exhibit better structural quality than a reference layer grown using the identical MBE regimes on a planar substrate. In accordance

with previous investigations32, the Raman data shown in Fig. 2(d) correspond to the lower densities of structural defects and decreased concentration of free carriers. In general, the InN

tends to non-stoichiometry and spontaneous formation of metallic In nanoparticles33,34. Plasmonic resonances in such nanoparticles could enhance emission in a semiconductor matrix nearby or

induce additional losses, depending on their sizes35. When considering the amplification of the emission intensity in our cup-cavities, we should exclude the plasmonic enhancement as a

possible mechanism, because no signs of In excess were found in the large InN crystals (Fig. 2(b,c)). Micro-cathodoluminescence (μ-CL) studies showed that the brightest emission spots are

always at the crystals (Fig. 3(a)) that is consistent with their good material quality. However, this emission is not homogeneous, but reflects the spatial distribution of electromagnetic

energy inside the cavities. At low temperatures, the smaller the crystal size, the higher the intensity of emitted light. Such a behavior is consistent with the pseudointegrable light

out-coupling22, which is proportional to . In the mono-CL images (Fig. 3(b)), the brightest emission patterns were recorded at the energies of the strongest narrow lines observed in the

micro-photoluminescence (μ-PL) spectra (Fig. 4). At the high-energy side of these spectra, where the narrow lines are absent, the emission from the crystals merges into the background

emission from the surroundings. When the temperature increases, the emission from the small crystals quenches fast due to their low cavity quality, while in the large ones it can be detected

up to room temperature. The average decay time of emission is about 150 ps at low temperature (10 K). This time was considerably shortened (up to 90–100 ps) to 100 K when the radiation of

small crystals was fully quenched. Further increase in temperature did not affect markedly the decay time. One can consider this shortened decay time as characteristic of the large crystals

which still emit light. In this case, it can be ascribed in part to the acceleration of the recombination rate due to the Purcell effect. QUASI WHISPERING GALLERY MODES The μ-PL spectra

measured in the InN cup-cavities comprise narrow WGM-related lines superimposed on a wide emission band (Fig. 4). Such lines are absent with measurements from the planar area (Fig. 4(a)).

The distinctiveness and strength of these narrow emission lines are comparable with those in the disc resonators fabricated by etching7,9. Around PL band maximum, the free spectral range

(FSR) varies from 13 meV to 30 meV, that allows the selective enhancement of optical transitions. Note that the conventional expression for FSR evaluation in discs7 is not applicable to the

cup-cavities due to the more complicated pathways of light in them. The narrow-line widths are weakly varied across the InN emission band in different crystals. Taking the widths from the

μ-PL spectra, the _Q_-factor is estimated to be in the ranges 170–200 and 700–900 for the modes in the InN and GaN cup-cavities, respectively. The reduced values for the InN cavities are in

part due to the lower spectral resolution of the NIR measurements. In the cup-cavities, the narrow emission lines are observed at the maximum and lower-energy part of the emission spectra.

At the higher-energy side they are suppressed, mainly because of optical losses induced by the close absorption edge. With a temperature rise, the broadened absorption edge quenches those of

the modes, which were at the maximum. Instead, the new modes come into play at the lower-energy side of the shifted PL band, because of changing the fluorescence coupling conditions.

Between these boundaries, a set of the narrow lines can be found, whose positions are stable with increasing the temperature from 77 K to 300 K (Fig. 4(b)). It is essential that the narrow

lines have insignificant broadening in such a wide temperature range. The amazing finding is the observed identity of the mode frequencies recorded in all ‘ripened’ crystals of the largest

size, which were chosen from the different parts of a wafer (Fig. 4(c)). This identity, existing within the limits of experimental accuracy, means that variation in shape and characteristic

sizes of the large microcrystals is negligible. Such a similarity implies that the self-limitation (saturation) of the crystal sizes takes place at the prolonged MBE growth. Note that the

standardized mode frequencies are a prerequisite for any cavity application. MODE SWITCHING The remarkable effect of mode switching accompanied by changing the spatial mode intensity

distribution was discovered by the μ-CL measurements performed in the 5-300 K range. At low temperatures, the spatial distribution of the μ-CL signal corresponds to the azimuthal type, when

the highest density of electromagnetic field is at the periphery of a crystal. Such an emission pattern can be observed even in the rather small crystals (Fig. 3(a)). When the temperature

increases up to 100-150 K, the emission pattern starts to change. At room temperature, it corresponds to the radial type when the most bright spot occurs in the center of the crystal (Fig.

5(a)). To shed light on this phenomenon, let us remind that in the simplest cylindrical approximation the frequency of a resonator mode of the _m_-order obeys the law , where _c_ is the

speed of light and _m_ is integer. Therefore, with the constant geometry, i.e. when the same crystal is measured and the small thermal expansion of its size can be neglected, only variation

of the refractive index can change the type of the modes with increasing the temperature. The variation of the absorption coefficient will rather suppress the modes, as it was observed at

the higher-energy side of the μ-PL spectra. We have performed the numerical simulation of the quasi-WGMs in the cup-cavities to prove the anticipated effect of the refractive index

variation. The modeling has been done by solving the Maxwell’s equations with appropriate boundary conditions, which is the most general way to search the modes in optical microcavities36.

The modeling procedure included the fitting of experimental mode energies taken from the μ-PL spectra. The comparison of the theoretical and experimental energies is presented in Fig. 4(a)

for crystals of different sizes and in Fig. 4(b) for different temperatures. The examples of the spatial distributions of the electromagnetic field intensity inside the cavities at different

temperatures are depicted in Fig. 5(b). The extended sets of such distributions for both InN and GaN cavities are shown in the Supplementary, Fig. 2. To determine the refractive index

dispersion dependencies, needed for these simulations, we measured first the integral absorption and reflection at different temperatures; then, the complex dielectric function was extracted

by applying the Kramers-Kronig relations. The obtained dependencies, being reasonably consistent with published data37,38, were used in our preliminary calculations. More accurate analysis

was done using the refractive index as a fitting parameter. Namely, its value was varied to search a feasible mode in the vicinity of an experimentally observed narrow line. Figure 5(c)

shows the refractive index values derived from such simulations. They deviate noticeably from the dependencies (solid lines) obtained by the analysis of the integral optical spectra taken

from areas comprising both the crystals and planar layer. This difference arose because an effective absorption edge in an InN layer can shift towards lower energies due to the optical

losses induced by the metallic In nanoparticles, while the shift in the opposite direction may occur when the material is In-depleted34. Bearing that in mind, we can conclude that the

modeling of the modes represents a unique way to determine the optical parameters characteristic for the crystal material itself. For the GaN cavities, the modeling has confirmed that the

number of the modes within the emission band is limited: only three distinct modes were found. The anticipated mode at 3.450 eV coincides reasonably with the narrow line at 3.447 eV in the

experimental spectrum shown in Fig. 1(c). We assume that the two other modes, found near 3.47 eV, can be suppressed by neutral-donor bound exciton scattering which provides the strong

attenuation of propagating light in GaN39. However, the basic result of the modeling is the evidence that the azimuthal type of quasi-WGMs prevails over the radial type at low temperatures

in the InN cup-cavities and that this situation changes to the radial type at room temperature. Notably, the variation of the refractive index in semiconductor structures may be achieved

under pumping by the laser beam or electric pulses. Moreover, these methods can change the concentration of free carriers, which in turn affects the refractive index to a considerable

extent. At the optical pumping, likely, the combination of these factors induced the mode intensity variation at the PL maximum in the spectra shown in Fig. 4(c), which were measured at

somewhat different laser beam focusing conditions. We assume that the switching between two neighboring modes has to gain the small energy excess, which can radiate as the quanta of

terahertz (THz) emission. Such an assumption is well consistent with the results of THz measurement performed using electric-pulse excitation technique40,41. As demonstrated, the electric

pumping of InN structures induces the similar broad NIR emission band42, which can launch the quasi-WGMs via the fluorescence coupling. In the spectrum of THz emission shown in Fig. 5(d),

the peak energy at 14 meV (~3.5 THz) corresponds well to the energy separation (13–15 meV) between the adjacent modes at the maximum of the IR emission band. Note that the modes are absent

at the higher-energy side of the band and that their separation increases up to 20–30 meV at the low-energy wing. Such a situation may be a reason for a strongly asymmetrical shape of the

THz spectrum. The used excitation regime by the well-separated packets of electric pulses (see Methods) was optimized to prevent the overheating of the samples. Therefore, we assume that the

increase in the carrier concentration governs the mode switching process. If the carrier concentration is increased at the electric-pulse propagation in the realistic range of  cm−3, the

modification of the real part of the complex refractive index will comprise several percents from an initial value42, which can dramatically modify the conditions of the maintenance of

modes. Other mechanisms, including nonlinear ones, should provide the THz emission in InN at different energies41. In particular, such emission could arise due to the surface plasmons

coupled to electromagnetic field at the Bragg grating formed by the cones on the patterned substrate. However, for the used cone period the emission frequency should be below 2 THz, i.e.

significantly less than the measured frequency of the THz signal in the InN structure with the cup-cavities. DISCUSSION We have demonstrated the novel cup-cavities operating in principle

from UV to NIR, which were fabricated by MBE from III-nitride semiconductors — GaN and InN, without any post-growth treatment. Realization of these cup-cavities evidences the potential of

MBE for the cavity nanotechnology. Note that up to now there was a lack of nanophotonic devices based on InN, although this semiconductor has been intensively studied since the beginning of

this century. Therefore, successful fabrication of the InN cup-cavities with stable mode frequencies and pronounced emission enhancement is a certain technological breakthrough in this

field. The cavities exploit the mechanism of fluorescence coupling by their own near-band-edge emission, which allowed us to discover new effects emerging near an absorption edge in

semiconductor cavities such as temperature-induced mode alteration and energy confinement on a subwavelength level. We highlight that the change of the refractive index, controlling the

quasi-WGMs, can be realized not only by the temperature increase, but also by laser beam and electric-pulse pumping. In contrast with the slow temperature exposure, their heating effect is

fast. Furthermore, these techniques change the free carrier concentration. In a degenerate semiconductor, like InN, it provides the Burstein-Moss effect which will push the apparent

absorption edge towards higher energies due to the filling of conduction band states. Therefore, its impact on the complex refractive index will be opposite to the temperature-induced red

shift of the absorption edge. In this case, two scenarios of the mode alteration can be realized. First, the energy of a mode will be increased by few meV with the conservation of its

spatial intensity distribution, that might be useful for fine adjustment to a resonance in a nano-emitter. Alternatively, the abrupt switching to the neighboring mode, promising for optical

switchers, can take place with changing the intensity distribution and emitting the THz quanta. In our studies, the weighty argument in favor of the later is the registration of the THz

emission. Considering the emission intensity pattern in the low-temperature μ-CL images of the crystals (Figs 3 and 5(a)) we should draw attention to the strong contrast between the emitting

periphery and dark central area, which reflects the difference in emission dynamics within these regions. In the local areas where the density of photon states determined by the electric

field intensity, , is close to zero, the emission is strongly inhibited, although the mode Purcell factor can be high. At room temperature, the mode energy is confined in a small

subwavelength volume in the center of the cavity (Fig. 5(a)). Its characteristic size ~500 nm is about 1/3 of the free-space wavelength λ, that is well consistent with the anticipated

smallest possible electromagnetic mode volume , achievable in a dielectric cavity44. Such mode confinement can be used to select the limited number of radiating dipoles in a semiconductor

cup-cavity instead of mesa formation by expensive electron-beam lithography. In summary, we propose the III-nitride cup-cavities exploiting quasi-WGMs with _Q_-factor approaching 900 in the

UV range, that is sufficient for enhancement of the nano-emitter efficiency and detection sensitivity. Small electromagnetic mode volume allows spatial selection of radiating dipoles in

addition to spectral selection by the narrow mode lines. Furthermore, we show that the WGMs in a semiconductor cavity can be controlled by the complex refractive index variation near the

absorption edge. This opens a way to realize the semiconductor WGM cavities with tunable frequencies controlled by optical or electric-pulse pumping. METHODS MOLECULAR BEAM EPITAXY GROWTH

III-nitride monocrystal cavities were grown by the low-temperature plasma—assisted (PA) MBE on cone-shaped patterned sapphire substrates using a Compact 21T (Riber) setup equipped with a

nitrogen plasma activator HD25 (Oxford AR). The substrates have the regularly positioned cones with a diameter and height of 3 and 1.5 μm, respectively. These cones act as pedestals for the

grown monocrystals. The formation of the crystals on their tops is mainly induced by the difference in growth rates along basic crystallographic directions, stimulated by chosen

technological parameters. The important factors which control the crystal shape are the substrate temperature , fluxes ratio and polarity, N or Ga(In), which is determined by a buffer layer.

Their variation can change the shape of the monocrystals from inverted hexagonal pyramids to nanocolumns. The procedure of their fabrication comprises consecutive growth of the thin buffer

layer and basic micrometer-sized layer at different parameters (Table 1). For the buffer growth, either the ordinary MBE or migration enhanced epitaxy (MEE) was exploited. The growth

procedures are illustrated by SEM images given in Supplementary Fig. 1. MEASUREMENTS The μ-PL studies were carried out at 77 and 300 K using Horiba Jobin Yvon T64000 and LabRAM HR

spectrometers equipped by a Linkam THMS600 temperature-controlled microscope stage. The measurements were done with cw excitation by 532 nm and 325 nm laser lines for InN and GaN structures,

respectively. We exploited the objective Mitutoyo 50x UV (NA = 0.40) and a liquid nitrogen-cooled charge-coupled detector (CCD) to measure the GaN cavity spectra with the spectral

resolution of about 0.5 meV. The Mitutoyo 100x NIR (NA = 0.50) objective and an InGaAs photodiode with a thermoelectric cooler was used to measure the InN cavity spectra with the spectral

resolution of about 1.0 meV. The beam impinging normally to the surface was focused by the objective into a spot with FWHM of 1 μm that is enough to measure separately the monocrystals. The

same objective collected the PL signal from sample surface. The μ-R measurements were done using the same setup at 300 K. The spectra of transmission and reflection were measured using a

two-monochromators setup with excitation by a tungsten lamp. The time-resolved PL measurements were performed in a closed-cycle He cryostat using a 710 nm line of a pulsed laser for

excitation. These integral cw and time-resolved spectra characterize the ~1-mm-sized areas which include both the crystals and surrounding layer. SEM, microanalysis and μ-CL studies were

done using a LEO 1550 Gemini analytic scanning electron microscope equipped by a unit for EDX and a low-temperature μ-CL stage with a liquid nitrogen-cooled Ge-detector. At the EDX

characterization performed with 0.5% accuracy, the nitrogen calibration was done using a perfect bulk GaN sample. The μ-CL studies were carried out at 10 kV beam voltage from 5 K up to 300 K

with signal collection normally to the top monocrystal surface. Both panchromatic and mono-CL regimes were used. For the later, a CL signal was registered at particular mode wavelengths

within the emission band. The THz spectra were measured at 77 K with excitation by the series of packets of rectangular pulses with 15 V pulse height, 10 μs duration and 71.5 Hz reputation

rate, as described in40,41. Wide-angle detection was done normal to the sample surface. The pulses passed through contacts formed on the top surface of a ~5-mm-long sample. Such a pulsed

excitation prevents the heating of a sample. The step-scan Fourier spectrometer has the volume of optical path evacuated down to Torr to exclude any influence of water vapor absorption on

the shape of the THz spectra. MODELING The spatial distribution of the electromagnetic field intensity inside the cavities and mode frequencies were searched for the quasi-WGMs existing

within the limits of emission bands of the cavity materials. It was done numerically by solving the Maxwell’s equations using the Comsol Multiphysics software. We adopted boundary conditions

so as to cause a rapid decay of light waves outside the crystal. The crystal dimensions were taken from the SEM data. In these simulations, the cup-like shape of the cavities was

approximated by a truncated cone. For GaN crystals, the prism-like shape was taken as more appropriate. In this case, we revealed the distinct slab (Fabry-Pérot) modes propagating between

two parallel facets in the crystals. ADDITIONAL INFORMATION HOW TO CITE THIS ARTICLE: Shubina, T. V. _et al._ III-nitride tunable cup-cavities supporting quasi whispering gallery modes from

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Institute’s authors are grateful to the Russian Science Foundation (Project # 14-22-00107) for the support of technological, optical and theoretical studies. GP and CH, involved in CL and

EDX studies, are appreciative of financial support from the Swedish Research Council (VR) and the Swedish Energy Agency. The authors thank D. V. Nechaev and N. V. Kuznetsova for growth

assistance, T. B. Popova for material characterization assistance, D. I. Kuritsyn for preliminary time-resolved measurement, A. N. Smirnov and A. O. Zakhar’ in for optical measurement

assistance. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Ioffe Institute, St. Petersburg, 194021, Russia T. V. Shubina, V. N. Jmerik, V. Yu. Davydov, A. V. Andrianov, D. R. Kazanov & S.

V. Ivanov * Department of Physics, Linköping University, Chemistry and Biology (IFM), Linköping, S-581 83, Sweden G. Pozina & C. Hemmingsson Authors * T. V. Shubina View author

publications You can also search for this author inPubMed Google Scholar * G. Pozina View author publications You can also search for this author inPubMed Google Scholar * V. N. Jmerik View

author publications You can also search for this author inPubMed Google Scholar * V. Yu. Davydov View author publications You can also search for this author inPubMed Google Scholar * C.

Hemmingsson View author publications You can also search for this author inPubMed Google Scholar * A. V. Andrianov View author publications You can also search for this author inPubMed 

Google Scholar * D. R. Kazanov View author publications You can also search for this author inPubMed Google Scholar * S. V. Ivanov View author publications You can also search for this

author inPubMed Google Scholar CONTRIBUTIONS T.V.S. conceived experiments and modeling, participated in those and wrote the paper; V.N.J. grown the samples; G.P., V.Y.D. and C.H. conducted

optical and structural studies; A.V.A. did THz measurements; D.R.K. performed simulations, S.V.I. supervised the project. All authors were involved in preparing this manuscript. ETHICS

DECLARATIONS COMPETING INTERESTS The authors declare no competing financial interests. ELECTRONIC SUPPLEMENTARY MATERIAL SUPPLEMENTARY INFORMATION RIGHTS AND PERMISSIONS This work is

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V. _et al._ III-nitride tunable cup-cavities supporting quasi whispering gallery modes from ultraviolet to infrared. _Sci Rep_ 5, 17970 (2016). https://doi.org/10.1038/srep17970 Download

citation * Received: 19 August 2015 * Accepted: 09 November 2015 * Published: 11 December 2015 * DOI: https://doi.org/10.1038/srep17970 SHARE THIS ARTICLE Anyone you share the following link

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