Strong Exciton-photon Coupling Research Articles

Recent Advances in Organic One‐Dimensional Composite Materials: Design, Construction, and Photonic Elements for Information Processing

Many recent activities in the use of one-dimensional nanostructures as photonic elements for optical information processing are explained by huge advantages that photonic circuits possess over traditional silicon-based electronic ones in bandwidth, heat dissipation, and resistance to electromagnetic wave interference. Organic materials are a promising candidate to support these optical-related applications, as they combine the properties of plastics with broad spectral tunability, high optical cross-section, easy fabrication, as well as low cost. Their outstanding compatibility allows organic composite structures which are made of two or more kinds of materials combined together, showing great superiority to single-component materials due to the introduced interactions among multiple constituents, such as energy transfer, electron transfer, exciton coupling, etc. The easy processability of organic 1D crystalline heterostructures enables a fine topological control of both composition and geometry, which offsets the intrinsic deficiencies of individual material. At the same time, the strong exciton-photon coupling and exciton-exciton interaction impart the excellent confinement of photons in organic microstructures, thus light can be manipulated according to our intention to realize specific functions. These collective properties indicate a potential utility of organic heterogeneous material for miniaturized photonic circuitry. Herein, focus is given on recent advances of 1D organic crystalline heterostructures, with special emphasis on the novel design, controllable construction, diverse performance, as well as wide applications in isolated photonic elements for integration. It is proposed that the highly coupled, hybrid optical networks would be an important material basis towards the creation of on-chip optical information processing.

Room-Temperature Polariton Lasing from GaN Nanowire Array Clad by Dielectric Microcavity

Room-temperature polariton lasing from a GaN-dielectric microcavity is demonstrated with optical excitation. The device is fabricated with a GaN nanowire array clad by Si3N4/SiO2-distributed Bragg reflectors. The nanowire array is initially grown on silicon substrate by molecular beam epitaxy. A distinct nonlinearity in the lower polariton emission is observed at a threshold optical energy density of 625 nJ/cm(2), accompanied by significant line width narrowing to 5 meV and a small blue shift of ~1 meV. The measured polariton dispersion is characterized by a Rabi splitting of 40 meV and a cavity exciton detuning of -17 meV. The device described here is a demonstration of exciton-photon strong coupling phenomenon in an array of light emitters and paves the way for the realization of a room temperature electrically injected polariton laser.

Microcavity controlled coupling of excitonic qubits

Controlled non-local energy and coherence transfer enables light harvesting in photosynthesis and non-local logical operations in quantum computing. This process is intuitively pictured by a pair of mechanical oscillators, coupled by a spring, allowing for a reversible exchange of excitation. On a microscopic level, the most relevant mechanism of coherent coupling of distant quantum bits—like trapped ions, superconducting qubits or excitons confined in semiconductor quantum dots—is coupling via the electromagnetic field. Here we demonstrate the controlled coherent coupling of spatially separated quantum dots via the photon mode of a solid state microresonator using the strong exciton–photon coupling regime. This is enabled by two-dimensional spectroscopy of the sample’s coherent response, a sensitive probe of the coherent coupling. The results are quantitatively understood in a rigorous description of the cavity-mediated coupling of the quantum dot excitons. This mechanism can be used, for instance in photonic crystal cavity networks, to enable a long-range, non-local coherent coupling.

Control of Rabi-splitting energies of exciton polaritons in CuI microcavities

We have investigated the active-layer-thickness dependence of exciton-photon interactions in CuI microcavities. The active layer thickness was changed from λ/2 to 2λ, where λ corresponds to an effective resonant wavelength of the lowest-lying exciton. In the CuI active layer, thermal strain removes the degeneracy of the heavy-hole (HH) and light-hole (LH) excitons at the Γ point. Angle-resolved reflectance spectra measured at 10 K demonstrate the strong coupling between the HH and LH excitons and cavity photon, resulting in the formation of three cavity-polariton branches: the lower, middle, and upper polariton branches. The energies of the three cavity-polariton modes as a function of incidence angle are reasonably explained using a phenomenological Hamiltonian to describe the exciton-photon strong coupling. It is found that the interaction energies of the cavity-polariton modes, the so-called vacuum Rabi-splitting energies, are systematically controlled from 29 (50) to 48 (84) meV for the LH (HH) exciton by changing the active layer thickness from λ/2 to 2λ. The active-layer-thickness dependence of the Rabi-splitting energies is semi-quantitatively explained by a simple model.

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.

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.

Ultracompact Asymmetric Mach–Zehnder Interferometers with High Visibility Constructed from Exciton Polariton Waveguides of Organic Dye Nanofibers

AbstractManipulation of light using subwavelength waveguides is a key technology in the development of miniaturized photonic circuits, which possess various advantages over their electronic counterparts. The novel approach presented for such waveguiding involves the propagation of exciton polaritons (EPs), which are quasi‐particles formed by strong exciton–photon coupling, along organic dye nanofibers. A self‐assembled nanofiber of thiacyanine (TC) with a width of ≈200 nm propagates the EPs created by an optical excitation over a submillimeter‐scale distance and passes through a bend with a micrometer‐scale radius with low bending loss. To demonstrate the remarkable potential of EP‐based miniaturized photonic circuits, asymmetric Mach–Zehnder interferometers (AMZIs) are fabricated with TC nanofibers by micromanipulation. The AMZIs with a footprint of ≈20 μm × 20 μm exhibit a visibility of nearly unity and function as channel drop filters with the considerably high extinction ratio of up to ≈15 dB. Such high‐performance and ultracompact channel drop filters operating in the visible wavelength region have rarely been developed with other waveguide technologies. The coherent properties of the EPs in the nanofibers are investigated using time‐resolved experiments. The coherent properties provide useful information for designing EP‐based photonic circuits and for understanding EP dynamics in a nanofiber.

Long-Range, Photon-Mediated Exciton Hybridization in an All-Organic, One-Dimensional Photonic Crystal

We demonstrate an all-organic, one-dimensional photonic crystal tuned to the regime of strong exciton-photon coupling. The structure consists of a high index of refraction light-absorbing active layer periodically distributed in a low index of refraction nonabsorbing background medium. The strongly coupled state in this structure is shared between multiple active layers separated by distances longer than the wavelength of light in the structure. In order to investigate the potential for long-range energy transfer, photon-mediated exciton hybridization is demonstrated between multiple absorbing species spatially separated in the photonic crystal.

Plasmonic effects on strong exciton-photon coupling in metal-insulator-metal microcavities

Metal-insulator-metal (MIM) microcavities consisting of a layer of polyvinyl alcohol doped and undoped with cyanine dye molecules were prepared and their reflection and transmission spectra were measured. In the doped-MIM microcavity, dispersion curves determined from the reflection data clearly showed the anticrossing behavior arising from the strong coupling between excitons in the dye molecules and photonic modes supported by the MIM microcavity. The experimental dispersion curves could be well reproduced by a standard expression of the coupled mode dispersions by appropriately taking into account the plasmonic nature of the transverse magnetic mode supported by the MIM microcavity. A large splitting between the transverse electric and transverse magnetic modes in the upper branch of the coupled modes can also be well explained by the plasmonic effects.

Strong exciton-photon coupling in microcavities containing new fluorophenethylamine based perovskite compounds

We synthetize some new perovskite thin layers: p-fluorophenethylamine tetraiodoplumbate pFC(6)H(4)C(2)H(4)NH(3))(2)PbI(4) perovskite molecules, included in a PMMA matrix. We report on the optical properties of the perovskite doped PMMA thin layers and we show that these layers are much more stable under laser illumination and present a smaller roughness than the spin-coated (C(6)H(5)C(2)H(4)NH(3))(2)PbI(4) layers. These new layers are used as the active material in vertical microcavities and the strong-coupling regime is evidenced by a clear anti-crossing appearing in the angular-resolved reflectivity experiments at room temperature.

Strong-Coupling in Inorganic-Organic Hybrid Embedded Single and Coupled Metallic Microcavities

Here we fabricated and studied highly-layered Inorganic-Organic Multiple Quantum Well structures and later placed them into metallic low quality factor microcavities. Room-temperature strong exciton-photon coupling is achieved in such single and coupled microcavities. Upon angle tuning, the broad photonic mode of the microcavities is clearly split into two modes at the exciton resonant absorption of hybrid perovskite. The large Rabi splitting upto -213 meV is obtained in good agreement with the theoretical models.

Strong exciton-photon coupling in a monolithic ZnO/(Zn,Mg)O multiple quantum well microcavity

We report on strong exciton-photon coupling in an epitaxially grown (Zn,Mg)O-based λ-microcavity (MC) containing four 3.5 nm wide ZnO quantum wells (QWs) as active layers. At 5 K, the observed Rabi splitting in absorption is 26 meV, while the inhomogeneous linewidth of A and B excitons in similar QWs without a MC is about 10 meV. The strong coupling regime (SCR) is lost between 150 K and 200 K due to additional homogeneous broadening. From transfer matrix calculations, we deduce that increasing the number of QWs per cavity length can extend the SCR up to room temperature.

Polarization splitting in polariton electroluminescence from an organic semiconductor microcavity with metallic reflectors

Organic semiconductors have received considerable attention as the active medium in microcavity devices that exploit the regime of strong exciton–photon coupling. The eigenstates of these systems are microcavity polaritons, whose properties are an admixture of the uncoupled exciton and photon. Organic microcavities are particularly interesting due to their large exciton binding energy which permits the electrical excitation of polaritons at room temperature. Measurements of electroluminescence are often facilitated through the use of metallic reflectors that form the optical microcavity and also serve as device electrodes. Here, we demonstrate that such structures exhibit a significant polarization splitting under both optical and electrical excitation. The size of the polarization splitting rivals those observed in strongly coupled microcavities based on distributed Bragg reflectors having a long optical penetration depth.

Strong exciton-photon coupling with colloidal quantum dots in a high-Q bilayer microcavity

We demonstrate evanescently coupled bilayer microcavities with Q-factors exceeding 250 fabricated by a simple spin-coating process. The cavity architecture consists of a slab waveguide lying upon a low refractive index spacer layer supported by a glass substrate. For a lossless guide layer, the cavity Q depends only on the thickness of the low index spacer and in principle can reach arbitrarily high values. We demonstrate the versatility of this approach by constructing cavities with a guide layer incorporating CdSe/ZnS core/shell quantum dots, where we observe strong coupling and hybridization between the 1S(e)-1S3/2(h) and 1S(e)-2S3/2(h) exciton states mediated by the cavity photon. This technique greatly simplifies the fabrication of high-Q planar microcavities for organic and inorganic quantum dot thin films and opens up new opportunities for the study of nonlinear optical phenomena in these materials.

Thermally activated population of microcavity polariton states under optical and electrical excitation

We examine the luminescence from optical microcavities containing an organic semiconductor in the regime of strong exciton-photon coupling. The ratio of luminescence from the upper polariton branch to the lower polariton branch is studied as a function of microcavity detuning and temperature under both optical and electrical excitation. The population of the upper polariton branch is modeled by means of thermal activation from an uncoupled exciton reservoir. Here, the activation energy for the population of the upper polariton branch is equal to the energetic separation between the exciton reservoir and the upper branch at the angle of detection. Agreement is obtained with this model under both optical and electrical excitation using measurements of angle-resolved microcavity reflectivity and luminescence.

Control of polariton scattering in resonant-tunneling double-quantum-well semiconductor microcavities

Electronic control of ultrafast optical amplification is achieved in specially designed semiconductor microcavities with double quantum well. The gain from parametric scattering of polaritons is selectively quenched by tuning the intracavity electric field to turn on and off interwell resonant tunneling. A $>90\mathrm{%}$ reduction in optical gain is observed with only 100 mV change in applied bias. The strong exciton-photon coupling regime leads here to competition between the rate of Rabi coupling and of electronic tunneling between adjacent quantum wells.

Electrical excitation of microcavity polaritons by radiative pumping from a weakly coupled organic semiconductor

In an organic semiconductor, the population of microcavity polariton states occurs via an uncoupled exciton reservoir. Consequently, this results in the inefficient excitation of the upper polariton branch, and a significant population of uncoupled excitons in the active material. Here, an alternate excitation approach is demonstrated that permits the direct population of microcavity polariton states under both electrical and optical excitation without first forming an exciton reservoir. This is realized by introducing a weakly coupled emitter into an optical microcavity containing an organic semiconductor suitable for strong exciton-photon coupling. In contrast to previous work on microcavity polariton luminescence in organic semiconductors, angle-resolved measurements of the photoluminescence and electroluminescence show variations in upper and lower branch emission intensity consistent with the branch photon character. These results confirm that the excitation of the microcavity polariton states is by radiative pumping from the weakly coupled emitter.

Polarization instability in a polariton system in semiconductor microcavities

The kinetics of a field on a quantum well in the active region of a planar microcavity with strong exciton-photon coupling has been investigated under the conditions of resonance pulse excitation by a small degree of circular polarization. It has been shown that the system of polaritons at the early stage of the development of instability induced by polariton-polariton interaction tends to transit to a circularly polarized state, but does not reach 100% circular polarization and returns to a polarized state whose polarization is close to the pump polarization. It has been shown that the observed effects are caused by the excitation of an unpolarized reservoir of excitons in quantum wells, which leads to fast relaxation of the difference between the effective resonance frequencies of excitons with different circular polarizations.

Strong exciton-photon coupling in inorganic-organic multiple quantum wells embedded low-Q microcavity

Optoelectronic-compatible heterostructures are fabricated from layered inorganic-organic multiple quantum wells (IO-MQW) of Cyclohexenyl ethyl ammonium lead iodide, (C(6)H(9)C(2)H(4)NH(3))(2)PbI(4) (CHPI). These hybrids possess strongly-resonant optical features, are thermally stable and compatible with hybrid photonics assembly. Room-temperature strong-coupling is observed when these hybrids are straightforwardly embedded in metal-air (M-A) and metal-metal (M-M) low-Q microcavities, due to the large oscillator strength of these IO-MQWs. The strength of the Rabi splitting is 130 meV for M-A and 160 meV for M-M cavities. These values are significantly higher than for J-aggregates in all-metal microcavities of similar length. These experimental results are in good agreement with transfer matrix simulations based on resonant excitons. Incorporating exciton-switching hybrids allows active control of the strong-coupling parameters by temperature, suggesting new device applications.

In-gap polaritons in uniformly filled microcavities

We examine the nature and the dispersion of new polariton modes, which appear in thepolariton gap (i.e. within the Rabi splitting), considering the spectrum of polaritons in aplanar microcavity uniformly filled with a resonant material and exhibiting the strongexciton–photon coupling regime. These additional modes are the consequence of thequantization of the excitonic states caused by the confinement. We consider these in-gapmodes in materials typically used in microcavities and link the obtained results toexperiments, where such in-gap states have indeed been observed. We show that thesplitting between the in-gap modes and their bandwidth depend on the bandwidth of theexciton out of the microcavity and on the oscillator strength of the excitonic transition.