CuCl Microcavities Research Articles

Q-factor dependence of angle-resolved transmission spectra in CuCl microcavities

We have investigated the angle-resolved transmission spectra of the CuCl λ-and λ/2-microcavities composed of cavity layers sandwiched by distributed Bragg reflectors (DBRs) with various numbers of periods of the DBR layer which affect the degree of confinement of the photons in the microcavity, so-called quality factors (Q factors). The angle-resolved transmission spectra of the CuCl microcavities drastically change with the Q factor and the thickness of the cavity layer. The incident angle dependence of the peak energies obtained from the angle-resolved transmission spectra indicates that the cavity polariton modes are observed in the CuCl microcavities, except for the λ-microcavity with 2 periods of the DBR layer. The change of the angle-resolved transmission spectra in the microcavities will result from the difference in the coupling state between the excitons and photons in the microcavities.

Cavity effect on a biexciton in a CuCl microcavity

We studied the population and coherence dynamics of a biexciton in a planar CuCl microcavity. We developed a time-resolved two-photon polarization spectroscopy technique, by which we extracted the population dynamics of the biexciton even when the biexciton was spectrally overlapped with the lower cavity polariton. The observed lifetime of the cavity biexciton in the weak-coupling regime was much shorter than that of a bare CuCl thin film without a cavity, which directly demonstrates cavity enhancement of the radiative decay rate of the biexciton. The cavity enhancement originates from the hybridization of the unbound two-cavity-polariton state to the biexciton wave function by the biexciton-cavity coupling. By comparing the population and coherence decay curves of the biexciton, we found that the coherence of the cavity biexciton was limited by the radiative population decay.

Observation of bound and antibound states of cavity polariton pairs in a CuCl microcavity

We observed the antibound state, as well as the bound state, for cavity polariton pairs in a planar CuCl microcavity by spectrally resolved four-wave mixing. We obtained dispersion curves of the bound and antibound states by changing the incident angle of the pump pulses corresponding to the cavity detuning. The dispersion curve for the bound state suggests that the bound state is mainly composed of a bare biexciton and is weakly coupled to the cavity photons. The dephasing time of the bound state was faster than that of a bare biexciton in a thin sample, supporting the hypothesis that the bound state is coupled to the cavity photons. On the other hand, the antibound state consists of two lower polaritons having the same spin. The clear observation of the antibound state can be qualitatively explained by the phase-space filling, which reduces the Rabi splitting.

Optical properties of CuCl microcavities with fluctuations in their refractive index profiles along the cavity structures

We report on the optical properties of the CuCl microcavities that exhibit fluctuations in their refractive index profiles along the cavity structures. The angle-resolved reflectance spectra of the CuCl microcavities with a cavity length of half-integer multiples for the ${Z}_{3}$ exciton wavelength clearly show three cavity-polariton modes and minor modes. The Rabi splitting energies observed in the cavity-polariton modes are widely controlled by changing the cavity length of the microcavity. To analyze the spectral profile and the Rabi splitting energies qualitatively, the refractive index profiles of the constituent layers in the CuCl microcavities are evaluated using spectroscopic ellipsometry, and the reflectance spectra of the CuCl microcavities with the fluctuations in the refractive index profiles of the microcavity structures are calculated using the nonlocal response theory. The calculated reflectance spectra reproduce very well the cavity-polariton modes and the minor modes, including their energies and spectral profiles, in the observed reflectance spectra, which indicates an essential role of the fluctuation effect of the refractive indices of constituent layers in controlling the Rabi splitting energies and the quality factor of the microcavity.

Optical properties of asymmetric coupled CuCl microcavities

We report the optical properties of the asymmetric coupled CuCl microcavities, consisting of one optically active CuCl-cavity and one empty-cavity, fabricated by vacuum deposition method. It is found that the energies of the peaks observed in the angle-resolved transmittance spectra of the coupled microcavity change with the incident angle of light, and show the anticrossing behaviors around the crossing points between the exciton modes of the CuCl-cavity and the photon mode of the empty-cavity. We discuss the incident angle dependence by taking account of the coupling between two photon modes of the CuCl-cavity and the empty-cavity.

Photon-field-shape effects on Rabi splitting energies in CuCl microcavities

We have investigated the photon-field-shape effects on Rabi splitting energies in CuCl microcavities with HfO2/SiO2 distributed Bragg reflectors (DBRs). The CuCl active layer was prepared by vacuum deposition, while HfO2 and SiO2 layers were prepared by rf magnetron sputtering. The photon-field shape was tuned to a node-type or an antinode-type by changing the order of the refractive indices in the DBR. In order to control of the Rabi splitting energies, the active-layer thickness was changed from λ/12 to 9λ/20. In angle-resolved reflectance spectra at 10 K, three cavity polaritons resulting from the strong coupling between the Z3 and Z1,2 excitons and cavity photon were clearly detected. We estimated the energies of the exciton-photon interaction, the so-called vacuum Rabi splitting energies, from the analysis of the cavity polariton dispersions using a phenomenological Hamiltonian for the strong exciton-photon coupling. The active-layer-thickness dependence of the Rabi splitting energies are explained by a semi-quantitative analysis taking account of the overlap between the exciton and photon-field wave functions. We have demonstrated that the photon-field shape drastically affects the active-layer-thickness dependence of the Rabi splitting energies.

Polarization dependence of four‐wave mixing via biexcitons in CuCl microcavities

AbstractWe studied the CuCl microcavity designed for the enhancement of the biexciton‐photon coupling, in terms of spectrally resolved four‐wave mixing. We observed the strong dependence of the spectrum on the polarization of the pump pulses and the polarization correlation between the pump pulses and the four‐wave mixing signal, suggesting that the biexciton state is successfully formed in the CuCl microcavity. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependence of cavity-polariton energies in ZnO and CuCl microcavities

We have investigated the temperature dependence of the cavity-polariton energies in ZnO and CuCl microcavities with HfO2/SiO2 distributed Bragg reflectors. From angle-resolved reflectance spectra at 10 K, we experimentally confirmed the cavity-polariton dispersions. The experimental dispersions were analyzed with a phenomenological Hamiltonian for the exciton-photon strong coupling. Furthermore, the temperature dependence of the angle-resolved reflectance spectra was measured. We analyzed the experiment results of the temperature dependence of the energies of the cavity polaritons on the basis of the phenomenological Hamiltonian, taking account of Varshni's law to treat the temperature dependence of the band-gap energy in a bulk crystal. Note that the temperature dependence of the exciton energies in a CuCl bulk crystal is extraordinary, which is in reverse to that in a ZnO bulk crystal. We have revealed that the temperature dependence of the energies of the cavity polaritons, which deviates from that of the exciton in the bulk crystal, considerably depends on the relative fractions of the exciton and cavity-photon components in the cavity polaritons.

Control of Rabi Splitting Energies in CuCl Microcavities with HfO2/SiO2 Distributed Bragg Reflectors

Abstract The exciton-photon interaction in CuCl microcavities with HfO2/SiO2 distributed Bragg reflectors has been investigated from the viewpoint of the active-layer-thickness dependence of the interaction energy, the so-called vacuum Rabi splitting energy. The active layer thickness was changed from A/32 to A/5, where A corresponds to an effective resonant wavelength of the lowest-lying Z3 exciton. We performed angle-resolved reflectance measurements, and clearly detected three cavity-polariton modes originating from the lower, middle, and upper polariton branches in a strong coupling regime of the Z3 and Z1,2 excitons, and cavity photon. The incidence-angle dependence of the cavity-polariton energies was analyzed with a phenomenological Hamiltonian for the strong coupling. The vacuum Rabi splitting energies are systematically controlled from 22 (37) to 68 (122) meV for the Z3 (Z1,2) exciton with an increase in the active layer thickness. The active-layer-thickness dependence of the Rabi splitting energy is quantitatively explained using a simple model for quantum-well microcavities.

Control of exciton-photon interactions in CuCl microcavities

We have investigated the active-layer-thickness dependence of exciton-photon interactions in planar CuCl microcavities with HfO${}_{2}/$SiO${}_{2}$ distributed Bragg reflectors. The active layer thickness was changed from $\ensuremath{\lambda}/$32 to $\ensuremath{\lambda}/$4, while the cavity length was fixed at $\ensuremath{\lambda}/$2. We performed angle-resolved reflectance measurements and clearly detected three cavity-polariton modes, originating from the lower, middle, and upper polariton branches, in a strong-coupling regime of the ${Z}_{3}$ and ${Z}_{1,2}$ excitons and cavity photon. The incidence-angle dependence of the cavity-polariton modes was analyzed using a phenomenological Hamiltonian for the strong coupling. It was found that the interaction energies of the cavity-polariton modes, the so-called vacuum Rabi splitting energies, are systematically controlled from 22(37) to 71(124) meV for the ${Z}_{3}$(${Z}_{1,2}$) exciton by changing the active layer thickness from $\ensuremath{\lambda}/$32 to $\ensuremath{\lambda}/$4. The active-layer-thickness dependence of the Rabi splitting energy is quantitatively explained by a simple theory for quantum-well microcavities.

Rabi splitting in CuCl microcavity with DBR consisting of PbCl2/NaF multilayers

AbstractWe have investigated exciton‐photon coupling phenomena in bulk CuCl microcavities with distributed Bragg reflectors consisting of PbCl2 and NaF multilayers. The CuCl microcavities, which have the cavity‐length of half‐integer multiples of Z3 exciton wavelength, were fabricated by the vacuum deposition in the consistent process. From angle‐resolved reflectance spectra, the cavity polaritons originating from Z3, Z1,2 excitons and cavity photon were clearly obtained in each sample. We assumed the three polariton modes derived from 3×3 phenomenological Hamiltonian with two vacuum Rabi splitting energies. From the analysis, the obtained Rabi splitting energies for λ‐cavity are 110 and 195 meV for Z3 and Z1,2 excitons, respectively. On the other hand, we have determined the complex refractive indices of PbCl2/NaF layers by spectral ellipsometry. Using these optical constants, the reflectance spectrum of the sample was calculated by nonlocal theory with spatial structures of wavefunction of excitons. The spectrum and Rabi splitting energies estimated by the calculations agree well with the experimental results (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of Distributed Bragg Reflectors on Temporal Stability of CuCl Microcavities

We have investigated the characteristics of exciton polaritons in a CuCl microcavity with distributed Bragg reflectors (DBRs). Two sets of multilayers, PbBr2/PbF2 and HfO2/SiO2, were adopted as the DBRs in order to study the temporal stability of the CuCl microcavity. The thickness of the CuCl active layer was fixed to an effective 3λ/2 length. Angle-resolved reflectance spectra clearly demonstrate the formation of the cavity polaritons. From the phenomenological analysis with a 3×3 Hamiltonian for the cavity-polariton modes originating from the Z3 exciton, Z1,2 exciton, and cavity photon, the Rabi splitting energies are evaluated to be 97 and 162 meV for the Z3 and Z1,2 excitons, respectively, in the fresh CuCl microcavity with the PbBr2/PbF2 DBR. However, the Rabi splitting energies remarkably decrease within 6 days from the sample preparation, which is due to the degradation of the DBR resulting from alloying of PbBr2 and PbF2. On the other hand, in the CuCl microcavity with the HfO2/SiO2 DBR, the Rabi splitting energies of 105 and 168 meV for the Z3 and Z1,2 excitons, respectively, hardly change during 360 days from the sample preparation. This indicates that the stability of the oxide materials of HfO2 and SiO2 prevents the degradation of the DBR and CuCl active layer. Thus, a stable CuCl microcavity can be prepared by adopting the multilayer of HfO2/SiO2 as the DBR, which is a merit in applications.

Efficient generation of energy-tunable entangled photons in a semiconductor microcavity

A scheme is proposed for the generation of polarization- and frequency-entangled photons in a semiconductor planar microcavity. Photon entanglement can be achieved by converting two lower polariton branches with symmetric in-plane momenta into lower and upper polariton branches with zero in-plane momentum via a biexciton state. The phase-matching condition is satisfied for a microcavity with cavity detuning equal to the biexciton binding energy. The generation efficiency and energy control of the entangled photons are demonstrated for a CuCl microcavity as an example.

Exciton polaritons in bulk CuCl microcavities grown by vacuum deposition

AbstractWe demonstrate the realization of cavity polariton states in bulk CuCl microcavities. The CuCl microcavities made from a bulk CuCl layer and distributed Bragg reflectors consisting of PbF2 and PbBr2 layers prepared by vacuum deposition. Angle‐resolved reflectance spectra show the strong coupling characteristics of the excitons and photon mode; namely, the energies of reflectance structures exhibit incident‐angle dependence accompanying with an anticrossing behaviour peculiar to the cavity polaritons. From the phenomenological analysis with a 3×3 Hamiltonian of the cavity‐polariton modes originating from the Z3 exciton, Z1,2 exciton, and cavity photon, it has been revealed that the vacuum Rabi splitting energies in the λ‐thick CuCl active layer are 97 meV and 162 meV for the Z3 and Z1,2 excitons, respectively. We also indicate the potential to control the Rabi‐splitting energy by changing the active layer thickness. Furthermore, the spectral profile of reflectance is reasonably explained by calculations based on a theory of non‐local linear optical response. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Giant Rabi splitting in a bulk CuCl microcavity

We have investigated exciton-photon coupling phenomena in a bulk CuCl microcavity with distributed Bragg reflectors consisting of ${\text{PbF}}_{2}$ and ${\text{PbBr}}_{2}$ layers prepared by vacuum deposition. Reflectance spectra observed at various incident-light angles demonstrate the strong-coupling behavior of the exciton and photon modes; namely, the energies of reflectance dips exhibit incident-angle dependence accompanied by an anticrossing behavior peculiar to the cavity polaritons. In addition, the emission from the lower polariton branch was detected with angle-resolved photoluminescence spectroscopy. From the phenomenological analysis with a $3\ifmmode\times\else\texttimes\fi{}3$ Hamiltonian of the cavity-polariton modes originating from the ${Z}_{3}$ exciton, ${Z}_{1,2}$ exciton, and photon, it has been revealed that the vacuum-Rabi-splitting energies are 97 and 162 meV for the ${Z}_{3}$ and ${Z}_{1,2}$ excitons, respectively. These giant Rabi-splitting energies result from the large oscillator strengths of the relevant excitons. Furthermore, the spectral profile of reflectance is reasonably explained by calculations based on a theory of nonlocal linear optical response.