J. Heffernan

All optical, high contrast absorptive modulation in an asymmetric Fabry–Perot étalon

We report a 27:1 switching contrast ratio with 2.5 mW of power in an asymmetric Fabry–Perot etalon. The modulation is achieved by optical saturation of the excitonic absorption profile of a 95 A GaAs/AlGaAs multiple quantum well structure grown on a high‐reflectivity dielectric stack mirror.

Optical measurement of the ambipolar diffusion length in a ZnCdSe-ZnSe single quantum well

We describe a straightforward technique for the measurement of carrier diffusion in semiconductors. Using an optical microscope we can spatially image luminescence with a resolution of ∼500 nm. We measured the ambipolar diffusion length in a Zn0.75Cd0.25Se–ZnSe single quantum well by fitting the spatially resolved luminescence profile with the solution of the two-dimensional diffusion equation. The ambipolar diffusion length was found to be 498 nm at a carrier density of ∼1×1018 cm−3 and we deduce an ambipolar diffusion constant of 1.7 cm2 s−1.

Optical gain in (Zn, Cd)Se-Zn(S, Se) quantum wells

We have investigated the mechanism of stimulated emission in ZnCdSe–ZnSSe quantum wells through optically pumped measurements of the gain spectrum in a variety of structures from 270 to 77 K. We also calculated the optical gain, using a model that includes many-body effects, and found excellent agreement between the calculated gain line shapes and our measurements. Under the conditions studied, which are close to those found in an operating laser diode, we conclude that the stimulated emission arises from an electron–hole plasma in our samples, even down to 77 K. Although our measurements do not rule out exciton gain mechanisms at other temperatures or operating conditions, sensitive line-shape fitting does not require them in our case. However, our line-shape analysis does show that Coulomb enhancement is significant, even at room temperature.

Calculation of gain-current characteristics in ZnCdSe-ZnSe quantum well structures including many body effects

The gain‐spontaneous recombination characteristics have been calculated for a 40 A Zn0.8Cd0.2Se‐ZnSe quantum well including many body effects. We examine the effect of the inclusion of the Coulomb enhancement on the gain spectra and the gain‐current relationship. We show that, in the absence of the Coulomb enhancement, the threshold current density of a 340 μm 40 A Zn0.8Cd0.2Se‐ZnSe quantum well laser is underestimated by approximately 40% and the lasing wavelength overestimated by 4 nm. Our calculation of the scattering lifetime for the first electron‐heavy hole transition gives a lifetime varying between 29 and 37 fs, and shows that the carrier‐phonon scattering mechanism in II‐VI quantum wells is more dominant than in III‐V materials. We also comment on the effect the neglect of Coulomb enhancement has on the calculation of leakage currents in a laser at threshold.

Strain effect on the optical nonlinearity in an InGaAs/GaAs asymmetric Fabry-Perot modulator

The effect of strain on the optical nonlinearities and operation of an all‐optical asymmetric Fabry–Perot etalon is investigated. A high reflectivity modulation of 60% is reported with a contrast ratio of 12.2:1 and insertion loss of 1.87 dB. High contrast is achieved through absorption matching requiring a thick active layer. The effect of a thick structure on the strain reduced saturation carrier density is measured. The saturation density is calculated to be a factor of 2.5 less than in a similar GaAs modulator, showing thicker strained devices still display the advantages of thinner structures.

A quantum light-emitting diode for the standard telecom window around 1,550 nm.

Single photons and entangled photon pairs are a key resource of many quantum secure communication and quantum computation protocols, and non-Poissonian sources emitting in the low-loss wavelength region around 1,550 nm are essential for the development of fibre-based quantum network infrastructure. However, reaching this wavelength window has been challenging for semiconductor-based quantum light sources. Here we show that quantum dot devices based on indium phosphide are capable of electrically injected single photon emission in this wavelength region. Using the biexciton cascade mechanism, they also produce entangled photons with a fidelity of 87 ± 4%, sufficient for the application of one-way error correction protocols. The material system further allows for entangled photon generation up to an operating temperature of 93 K. Our quantum photon source can be directly integrated with existing long distance quantum communication and cryptography systems, and provides a promising material platform for developing future quantum network hardware.Quantum light sources operating at telecom wavelength are a long-sought goal for quantum technologies. Here, the authors show electrically injected emission of single photons and entangled photon pairs from indium phosphide based quantum dots, operating up to a temperature of 93 K.

Universal Growth Scheme for Quantum Dots with Low Fine-Structure Splitting at Various Emission Wavelengths

Efficient sources of individual pairs of entangled photons are required for quantum networks to operate using fibre optic infrastructure. Entangled light can be generated by quantum dots (QDs) with naturally small fine-structure-splitting (FSS) between exciton eigenstates. Moreover, QDs can be engineered to emit at standard telecom wavelengths. To achieve sufficient signal intensity for applications, QDs have been incorporated into 1D optical microcavities. However, combining these properties in a single device has so far proved elusive. Here, we introduce a growth strategy to realise QDs with small FSS in the conventional telecom band, and within an optical cavity. Our approach employs ‘dropletepitaxy’ of InAs quantum dots on (001) substrates. We show the scheme improves the symmetry of the dots by 72%. Furthermore, our technique is universal, and produces low FSS QDs by molecular beam epitaxy on GaAs emitting at ~900nm, and metal-organic vapour phase epitaxy on InP emitting at ~1550 nm, with mean FSS 4x smaller than for StranskiKrastanow QDs.

Optical properties of a ZnSSe microcavity fabricated by epitaxial lift-off.

Abstract We have fabricated and studied the optical properties of a planar II–VI ZnSSe microcavity. The MBE grown II–VI epi-layers were subsequently transplanted onto a metal coated GaAs substrate using the epitaxial lift-off technique. Comparison of the emission from the processed (microcavity) and unprocessed (non-microcavity) samples show evidence of spectral filtering and modified directionality which are due to the action of the cavity on the quantum well emission. Reflectance spectra confirm the Fabry-Perot resonance and excition feature are in the same spectral region. These results show that II–VI microcavities can be achieved using lift-off techniques.

Structural, electrical, and optical characterization of as grown and oxidized zinc nitride thin films

Zinc Nitride (Zn3N2) films were grown by DC sputtering of a Zn target in a N2 plasma under a variety of different growth conditions, which resulted in the deposition of films with variable compositions. The as deposited films exhibited a polycrystalline Zn3N2 structure, which was converted to a ZnO-based structure after several weeks of ambient exposure. Zn3N2 films that were N-poor exhibited electrical properties indicative of a natively doped semiconductor and reached a minimum carrier concentration in the order of 1018 cm−3 at compositions, which approached the stoichiometric ratio of Zn3N2. A maximum carrier mobility of 33 cm2 V−1 s−1 was obtained in N-rich films due to an improved microstructure. The Zn3N2 films had an optical band gap of 1.31–1.48 eV and a refractive index of 2.3–2.7. Despite a wide range of Zn3N2 samples examined, little variation of its optical properties was observed, which suggests that they are closely related to the band structure of this material. In contrast to the as grown ...

Optical gain in ZnCdSe-ZnSe quantum well structures

We have measured the gain spectrum of an optically pumped 40 angstrom ZnCdSe-ZnSe multiple quantum well. Our calculation, which includes many body effects such as Coulomb enhancement and spectral broadening due to carrier scattering, gives excellent agreement with the experimental gain measurements. We then show the importance of the inclusion of the Coulomb enhancement for the calculation of optical gain when predicting laser threshold currents. This is emphasized by using our gain calculation as a basis to theoretically optimize a simple ZnCdSe-ZnSe quantum well laser structure incorporating the leakage current over the p-type cladding.