H. Lohmeyer

Confined optical modes in monolithic II-VI pillar microcavities

Monolithic II-VI pillar microcavities made of ZnSSe and MgS∕ZnCdSe supperlattices have been fabricated by molecular-beam epitaxy and focused-ion-beam etching. Discrete optical modes of the pillar microcavities are studied in photoluminescence measurements. The optical modes are identified by means of calculations based on an extended transfer matrix method. Achievable Purcell factors well above 10 can be estimated from the measured quality factors and calculated mode volumes.

Enhanced spontaneous emission of CdSe quantum dots in monolithic II-VI pillar microcavities

The emission properties of CdSe∕ZnSe quantum dots in ZnSe-based pillar microcavities are studied. All-epitaxial cavities made of ZnSSe and MgS∕ZnCdSe superlattices with a single quantum-dot sheet embedded have been grown by molecular beam epitaxy. Pillar structures with diameters down to 500nm have been realized by focused-ion-beam etching. A pronounced enhancement of the spontaneous emission rate of quantum dots coupling to the fundamental mode of the cavities is found as evidence for the Purcell effect. The enhancement by a factor of up to 3.8 depends systematically on the pillar diameter and thus on the Purcell factor of the individual pillars.

Green laser emission from monolithic II-VI-based pillar microcavities near room temperature

We report on single-mode stimulated emission at a wavelength of 510nm in monolithic high-Q ZnSe-based vertical-cavity surface-emitting laser (VCSEL) micropillars under optical excitation at 280K. Calculations reveal the influence of polariton coupling and the dynamic detuning of the gain maximum to the cavity modes on the spectral characteristics of the VCSEL emission. In accordance with theory, we find a systematic decrease in threshold excitation energy for decreasing pillar diameter to a minimum value of 0.09pJ for a pillar diameter of 1.2μm as a combined effect of reduced active area and reduced mode volume in the microcavity lasers.

Fine tuning of quantum-dot pillar microcavities by focused ion beam milling

The targeted fine tuning of semiconductor pillar microcavities by postfabrication focused ion beam milling is described for the example of ZnSe-based structures with CdSe quantum dots embedded. Using the sensitive dependence of the spectral position of the modes on the cavity diameter, the modes are precisely blueshifted by a reduction of the pillar diameter with an accuracy below 100nm. The microcavities can be tuned to match the emission energy of individual quantum dots at a certain temperature, which results in a strongly enhanced luminescence intensity of the dots.

Wide-Bandgap Quantum Dot Based Microcavity VCSEL Structures

In this contribution we report on the optical properties of planar and pillar structured GaN- and ZnSe-based monolithic microcavities. These structures reveal three-dimensional confined optical modes with high quality factors and potentially small mode volumes especially for the ZnSe-based samples. The measurements are completed with theoretical calculations. Furthermore, the optical emission properties of CdSe quantum dots embedded into microcavities have been studied. The Purcell effect demonstrated by means of the pronounced enhancement of the spontaneous emission rate of quantum dots coupled to the discrete optical modes of the cavities. This enhancement depends systematically on the pillar diameter and thus on the Purcell factor of the individual pillars.

Manipulating the optical properties of CdSe/ZnSSe quantum dot based monolithic pillar microcavities

A customization of the optical properties of pillar microcavities on the desired applications is essential for their future use as quantum-optical devices. Therefore, all-epitaxial cavities with CdSe quantum dot embedded in pillar structures with different geometries have been realized by focused-ion-beam etching. The quality factors of circularly shaped pillar microcavities have been measured and their dependence on the excitation power is discussed. As a possibility to achieve polarized light emission, asymmetrically shaped microcavities are presented. Examples of an elliptically shaped pillar as well as of photonic molecules are investigated with respect to their photoluminescence characteristics and polarization.

Enhanced Spontaneous Emission of CdSe/ZnSe Quantum Dots in Monolithic II‐VI Pillar Microcavities

The emission properties of CdSe/ZnSe quantum dots in ZnSe‐based pillar microcavities are studied. All‐epitaxial cavities made of ZnSSe and MgS/ZnCdSe superlattices with a single quantum‐dot sheet embedded have been grown by molecular‐beam epitaxy. Pillar structures with diameters down to 500 nm have been realized by focused‐ion‐beam etching. As evidence for the Purcell effect a pronounced enhancement of the spontaneous emission rate by a factor of up to 3.8 is found for quantum dots coupling to the fundamental mode of the cavities.

Room‐temperature operation of a green monolithic II‐VI vertical‐cavity surface‐emitting laser

We report on the realization of an optically pumped monolithic vertical‐cavity surface‐emitting laser operating at a wavelength of 511 nm. The structure consists of a ZnSSe λ‐cavity containing three ZnCdSSe quantum wells surrounded by two Bragg reflectors using ZnSSe for the high‐index material and MgS/ZnCdSe superlattices for the low‐index material. The microresonator has a quality factor of 3200 while the threshold excitation power density for the onset of lasing is 22 kW/cm2 at room temperature. Micropillars of different diameter fabricated out of this structure show discrete optical modes due to the three dimensional optical confinement of the optical wave.

Electron-phonon quantum kinetics beyond the second-order Born approximation

For a one‐band model of a quantum wire coupled to LO‐phonons we present results for the dynamics of the coupled electron‐phonon system calculated with quantum kinetic equations obtained in the density matrix approach. Using a correlation expansion for the truncation of the infinite hierarchy of equations of motion we go beyond the usual second‐order Born approximation by taking into account two‐particle correlations and twofold phonon‐assisted density matrices. The different approximation levels are compared among each other and with an exact solution for a simplified case.

Comparative investigation of quantum-dot-like localization centers in InGaN quantum well and quantum dot structures

We present comparative micro-photoluminescence measurements on InGaN/GaN quantum well and quantum dot samples. Single sharp emission lines were investigated for both kinds of samples as a function of temperature and excitation density. For the sharp emission lines of the quantum dots and the strong localization centers in the quantum well samples comparable experimental findings were obtained such as the independence of their spectral position of the excitation density and the observation of binding and antibinding multiexcitonic states giving clear evidence for the quantum dot nature of localization centers.