Formation of microcavity polaritons in ZnO nanoparticles

We report the formation of microcavity polaritons in a dielectric microcavity embedded with solution processed ZnO nanoparticles. Evidence of strong coupling between the excitons and cavity photons is demonstrated via anticrossing in the dispersion of the polariton states. At low temperatures (<150K), multiple polariton states arising due to coupling between different excitonic states and the cavity mode is observed. Rabi splitting of ~90 meV is shown to persist even at room temperature in the ZnO – dielectric microcavity. ©2013 Optical Society of America OCIS codes: (240.5420) Polaritons; (140.3945) Microcavities. References and links 1. C. Weisbuch, M. Nishioka, A. Ishikawa, and Y. Arakawa, “Observation of the coupled exciton-photon mode splitting in a semiconductor quantum microcavity,” Phys. Rev. Lett. 69(23), 3314–3317 (1992). 2. H. Deng and Y. Yamamoto, “Exciton-polariton Bose-Einstein condensation,” Rev. Mod. Phys. 82(2), 1489– 1537 (2010). 3. K. J. 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Introduction
Since the pioneering work of Weisbuch et al. [1], the strong-coupling regime in semiconductor microcavities (MC) has been extensively investigated due to the rich underlying physics and potential applications [2].This regime is characterized by the formation of half-matter half-light quasiparticles called MC polaritons and form an ideal test bed to study solid-state cavity quantum electrodynamics (QED) [3].Most of the observation of MC polaritons and associated novel effects such as Bose-Einstein like condensation, polariton lasing, parametric oscillations, and superfluidity have been reported at cryogenic temperatures due to the small exciton binding energies of the material systems (< 10 meV).Hence, there has been interest in exploring MC polaritons in systems with large exciton binding energy such as organic materials [4] and wide bandgap inorganic semiconductors such as GaN and ZnO for room temperature (RT) operation [5][6][7][8].ZnO, a wide band gap (3.378 eV) inorganic semiconductor, whose exciton binding energy (~60 meV in the bulk) is even larger than that of nitride-based materials [9] has been touted to be an excellent candidate for studying room temperature polaritonic phenomena [5,6].Strong coupling at room temperature has been observed from bulk ZnO based microcavities and more recently, polariton lasing has also been reported using bulk and nanowire systems [10][11][12][13].
Currently, the growth of crystalline ZnO materials is usually carried out using molecular beam epitaxy, metal organic chemical vapor deposition or pulsed laser deposition.An alternate low cost approach is solution processing using ZnO nanoparticles which has the following advantages: allows large area device fabrication, mechanical flexibility and most importantly ease of fabrication.Here we demonstrate the formation of MC polaritons in a dielectric microcavity embedded with colloidal ZnO nanoparticles.

Sample preparation and experimental details
ZnO nanoparticles dispersed in ethanol with an average size of (35 ± 10) nm and 40% weight concentration (~10% volume concentration) was obtained from Sigma Aldrich.The dispersion was diluted to 0.7% volume concentration and spun-cast at 10,000 rpm for 3 minutes, and then baked at 350°C for one hour to remove the residual organic solvent.The spun-cast nanoparticles form a close packed optically smooth thin film as shown in the scanning electron microscope image (inset of Fig. 1(a)).The thickness of the film was estimated to be (105 ± 5) nm and the refractive index of the spun-cast ZnO film determined using ellipsometry was found to vary between 1.6 and 1.8 (real part) and 0.22 and 0.05 (imaginary part) in the wavelength range of interest (360-390 nm).The refractive index obtained for the quasi-thin film is less than the reported values for bulk ZnO which is attributed to the voids within the thin film [14][15][16].The temperature-dependent absorption spectra of the ZnO nanoparticles are shown in Fig. 1(a).The absorption spectrum at 10K consists of three peaks corresponding to the different exciton types in ZnO, namely the free exciton A (FX A ), B (FX B ) and C (FX C ).In addition, the absorption spectrum shows continuous absorption at higher energies due to the manifold of higher energy states of FX A , FX B and FX C [9,[17][18][19].The spectrum also shows a low energy tail that can be attributed to defect states and shallow neural acceptor bound excitons [9].As the temperature increases, homogeneous broadening starts to convolute and damp the exciton peaks.The exciton energies determined from absorption are shown in Table 1.These exciton energies are consistent with those reported for crystalline bulk ZnO [9,17] indicating that the nanoparticles used in the present work have large domain sizes that are crystalline in nature.Furthermore, their sizes are much larger than the Bohr radius of ZnO exciton (~2nm) [9,20], implying that there are no quantum confinement effects.Angle-resolved reflectivity and photoluminescence (PL) measurements are carried out using a home built goniometer set up attached to a cryostat with an angular resolution of 1°.A deuterium lamp is used as the light source and a charge-coupled device-based fiber coupled spectrometer is used to collect the optical signals in reflectivity measurements.A He-Cd laser (325 nm) is used as the excitation source and a monochromator-photomultiplier tube combination with spectral resolution of 0.5 nm is for the detection in the PL measurements.

Results and discussion
Angle-resolved reflectivity spectra of TM polarized light obtained at 10K from the MC sample are shown in Fig. 2  At smaller angles, polariton branches corresponding to strong coupling between cavityphotons and the FX A , FX B excitons are observed and at larger angles (>25°) an additional mode corresponding to strong coupling with FX C excitons is also observed as indicated by the arrows.Similar reflectivity spectra are also obtained with TE polarization (not shown here).The polariton dispersions are similar for both polarizations for angles below 45° but start diverging at larger angles due to the difference in cavity dispersion.temperature increases from 10K to 77K the four polariton branches red shift due to the shift in exciton resonances.At 150K, due to the homogenous broadening of the exciton resonances, the middle polariton branches are resolved by deconvolving the experimental reflectivity spectrum.The four polariton branches can still be resolved at 150K.However at room temperature, only the lower polariton branch (LPB) is visible for the smaller angles (< 40°) and the LPB flattens out as it approaches the exciton energy.At larger angles, the upper polariton branch (UPB) becomes visible.Only one UPB is observed and the polaritons branches arising from coupling between different excitonic states are not observed.Thanks to the large exciton binding energy in ZnO, we are able to observe strong coupling even at room temperature.The reflectivity dispersion at various temperatures is extracted from the spectra as shown in Fig. 4. Since three exciton types are observed in the absorption spectra at low temperatures, a four-coupled oscillator model is used to fit the experimentally extracted polariton dispersion.The solid curves in Fig. 4 correspond to the fits obtained using the coupled oscillator model.From these fits, Rabi splitting is extracted as shown in Fig. 4. The room temperature dispersion is fit to a two-coupled oscillator model since only one exciton feature is visible in the absorption spectrum.The cavity mode was determined as ( )  To further investigate the strong coupling effects at room temperature, the PL spectra for various collection angles at RT are studied (Fig. 5).The PL collected at 7.5° with respect to normal to the sample peaks at 3.2eV, which is consistent with the reflectivity dip at the same collection angle.As the collection angle increases, the PL peak also blue-shifts.The PL dispersion approaches the free exciton energy and flattens out, distinguishing from the weakcoupling regime where PL peaks simply follow the cavity dispersion.Only the LPB emission is observed in PL due to the large Rabi splitting which pushes the UPB to continuum states and phonon complexes [6,11,21].

Conclusion
In summary we show the formation of microcavity polaritons in a dielectric microcavity embedded with ZnO nanoparticles.Four polariton branches are observed at low temperatures due to strong coupling of cavity mode and three different excitonic states.At room temperature only two polariton branches are observed with a Rabi Splitting of ~90meV.The demonstration of strong coupling effects from cryogenic to room temperature using solution processed ZnO nanoparticles indicates the potential of these nanomaterials for developing practical and flexible polaritonic devices and circuits.

Fig. 1 .
Fig. 1.(a) Absorption spectra of the ZnO nanoparticle film at various temperatures.The inset shows the SEM image of spun-cast ZnO nanoparticle film; the scale bar corresponds to 500 nm.(b) Schematic of the microcavity structure.

Table 1 .
Free exciton energies determined from absorptionExciton energies E A (eV) E B (eV) E C (eV) of the microcavity structure is shown in Fig.1(b).The structure consists of colloidal ZnO nanoparticle cavity layer sandwiched between a 12.5 pairs SiO 2 /Si 3 N 4 bottom distributed Bragg reflector (DBR), and a 7.5 pairs top DBR resulting in a cavity with linewdith of 50 meV and Q factor of 65.The DBR is fabricated by plasma enhanced chemical vapor deposition (PECVD) on a silicon substrate.
(a).The vertical dashed lines correspond to the absorption energies #193991 -$15.00USD Received 16 Jul 2013; revised 12 Aug 2013; accepted 12 Aug 2013; published 27 Aug 2013 (C) 2013 OSA of three excitonic states and the dashed curves trace the polariton dispersion.Expanded views indicating the different polariton branches at three typical angles are also shown in Fig. 2(b).

Fig. 2 .
Fig. 2. (a) Angle-resolved reflectivity spectra for TM polarized light at 10K.The spectra are stacked with a constant offset of 0.25 between each adjacent spectrum; the vertical dash-dotted lines correspond to the free exciton energies and the dashed curves trace the reflectivity dispersion.(b) Expanded views showing the polariton reflectivity dips at 20°, 30° and 40°.

Fig. 3 .
Fig. 3. Angle-resolved reflectivity spectra for TM polarization at (a) 77K, (b) 150K and (c) RT; the vertical dash-dotted lines corresponds to the free exciton energies; the dashed curves trace the reflectivity dispersions.The temperature dependence of ZnO polaritons is shown in Fig.3, where the TM polarized reflectivity spectra are shown for 77K, 150K, and room temperature.As the

Fig. 4 .
Fig.4.The dispersions of the polariton branches extracted from the reflectivity spectra for TM polarization at (a) 10K, (b) 77K, (c) 150K and (d) room temperature.The dashed lines correspond to the free exciton energies; the dashed curve corresponds to the cavity mode; the data points represent the experimental reflectivity dips; and the solid lines correspond to the polariton dispersion obtained using coupled oscillator model.The calculated Rabi splitting is also indicated for the various polariton modes.

Fig. 5 .
Fig. 5. Angle-resolved PL contour plot at room temperature (RT).The white dashed line at 3.378eV represents the ZnO exciton (FX A ) energy; the white dashed curve represents the cavity dispersion; and the white solid line traces the PL peak dispersion.
[10,21,22]n into top and bottom DBR and index difference between cavity layer and adjacent layers.The observed anticrossings clearly indicate the presence of strongly coupled polariton states at all temperatures, with a large Rabi splitting of ~90 meV at room temperature.This is consistent with other reports on bulk ZnO based MC[10,21,22].