Excitonic Waveguides Research Articles

Optical properties of molecular nanocrystals consisting of J-aggregates of anionic and cationic cyanine dyes.

We report results of experimental studies of the photoabsorption, photoluminescent and photoelectric properties of a new type of multilayer molecular nanocrystals, consisting of highly ordered J-aggregates of one anionic and two cationic J-aggregates of cyanine dyes. In contrast to conventional J-aggregated dyes the multichromic nanocrystals synthesized in this work, are capable of efficient light absorption in three excitonic bands of the visible and near-IR spectral ranges. The spectral peak positions in the absorption bands can be controlled by appropriately selecting a set of dyes a molecular crystal is made of. Our investigations of the photoelectric properties of multichromic crystals have shown that each of them can potentially be used as a photosensitive layer of a photocell with photoconductivity in three peaks of excitonic absorption. The synthesized nanocrystals are attractive for the creation of thin-film organic photodetectors with a large photosensitive area and varied photoabsorption spectra, excitonic waveguides and for some other applications in organic and hybrid photonics and optoelectronics.

Room-temperature lasing of strain-compensated CdSe/ZnSSe quantum island laser structures

Efficient resonant excitonic waveguiding is achieved in laser structures, grown by metallorganic chemical vapor deposition, with stacked CdSe quantum islands which were separated by ternary ZnSSe barriers. Plastic relaxation within the stack is shown to be suppressed by adjusting the sulfur content in the barriers to compensate the strain. Excitonic lasing with low threshold intensities is demonstrated well above room temperature with Ith77 K=0.8 kW/cm2 and Ith300 K=55 kW/cm2.

Optical Properties and Lasing in CdSe-Submonolayers in a (Zn,Mg)(S,Se) Matrix

We have proposed and realized a new type of optoelectronic structures we refer to as excitonic waveguides which allow an ultimate shift towards the violet spectral range (460 nm at 300 K for the ZnMgSSe/GaAs materials system). The excitonic waveguiding effect is realized at the low energy side of the strong exciton absorption peak due to localized states, caused by ultrathin narrow-gap insertions. To realize the zero-phonon lasing mechanism resonant to the spectral region of resonantly enhanced refractive index one needs to break the k-selection rule preventing direct radiate exciton recombination having large in-plane k-vector dominating at high excitation densities and observation temperatures. This can be done by localizing excitons at nanoscale CdSe-rich islands formed by ultrathin CdSe insertions.

Gain studies and lasing in excitonic waveguides of II–VI submonolayer structures

In this paper we report on gain studies and lasing in excitonic waveguides of CdSe–ZnSe submonolayer structures. In contrast to conventional waveguides and double heterostructure lasers, no significant difference in the average refractive index between the cladding and the active layers is necessary, and these regions can be fabricated from the same matrix material. In this approach: (i) the waveguiding effect has a resonant nature and appears on the low-energy side of the strong exciton absorption peak in agreement with the Kramers–Kronig equation (ii) the absorption peak is induced by nanoscale island-like insertions of narrow-gap material in a wide-bandgap matrix. Gain spectra up to 1 kW/cm 2 demonstrate that here the excitonic-waveguide mechanism dominates. At high excitation densities the gain is additionally influenced by the formation of localized biexciton complexes.

Excitonic waveguiding and lasing in wide bandgap semiconductor

New type of structures for optoelectronics, we refer to as excitonic waveguides, are proposed and realized. Oppositely to conventional waveguides and double-heterostructure lasers, no significant difference in the average refractive index between the cladding and the active layers is necessary, and these regions can be fabricated from the same matrix material (homojunction laser). In this approach: (i) the waveguiding effect has a resonant nature and appears on the low-energy side of the strong exciton absorption peak in agreement with the Kramers-Kronig transformation; (ii) the absorption peak is induced by nanoscale island-like insertions of narrow-gap material in a wide bandgap matrix (quantum dots), preventing free-carrier screening of excitons and, simultaneously, allowing lasing resonant to the spectral range of the enhanced refractive index.

Gain to absorption conversion by increasing excitation density in excitonic waveguides

Exciton waveguiding was proposed to improve optical confinement of II–VI lasers. Strong resonant exciton absorption induces a refractive index enhancement (Δ n) on the low-energy side of the absorption peak. By stacking sheets of submonolayer (SML) CdSe insertions into a ZnSe matrix, we fabricated a region in ZnSe with strong resonant exciton absorption. In this region, the k-selection rule is lifted due to exciton localization by the SML islands. Therefore, a zero-phonon lasing mechanism resonant to the exciton waveguiding region could be realized. Using the variable-stripe-length method, we study the intensity-dependent optical gain of the ZnSe/CdSe submonolayer superlattices in a ZnS 0.06Se 0.94 matrix.

Gain to absorption conversion by increasing excitation density in excitonic waveguides

Gain to absorption conversion by increasing excitation density in excitonic waveguides