We demonstrate ultra-sensitive chemical sensing in the mid-infrared spectral regime with a combination of quantum cascade lasers (QCLs) with GaAs/Al(0.2)Ga(0.8)As strip waveguides fabricated via metal-organic vapor-phase epitaxy (MOVPE) and reactive ion etching (RIE) using evanescent field absorption spectroscopy. These strip waveguides have been designed with a width of 200 μm, thereby facilitating 2-D confinement and mode-matched propagation of mid-infrared radiation emitted from a distributed feedback (DFB) QCL at a wavelength of 10.3 μm. Acetic anhydride was detected with a limit of detection (LOD) of 18 pL (19.4 ng) deposited at the waveguide surface by overlapping of the vibrational absorption of the methyl group with the emission frequency of the QCL. The obtained results indicate a remarkable enhancement in sensitivity by three orders of magnitude compared to previously reported multimode planar silver halide waveguides. Further reduction of the waveguide strip width to 50 μm resulted in an additional sensitivity enhancement yielding a calculated LOD of 0.05 pL for the exemplary analyte acetic anhydride, which is among the most sensitive evanescent field absorption measurements with a miniaturized mid-infrared sensor system reported to date.

We report on in-lab free space quantum key distribution (QKD) experiments over 40 cm distance using highly efficient electrically driven quantum dot single-photon sources emitting in the red as well as near-infrared spectral range. In the case of infrared emitting devices, we achieve sifted key rates of 27.2 kbit s−1 (35.4 kbit s−1) at a quantum bit error rate (QBER) of 3.9% (3.8%) and a g(2)(0) value of 0.35 (0.49) at moderate (high) excitation. The red emitting diodes generate sifted keys at a rate of 95.0 kbit s−1 at a QBER of 4.1% and a g(2)(0) value of 0.49. This first successful proof of principle QKD experiment based on electrically operated semiconductor single-photon sources can be considered as a major step toward practical and efficient quantum cryptography scenarios.

Emission from a resonantly excited quantum emitter is a fascinating research topic within the field of quantum optics and is a useful source for different types of quantum light fields. The resonance spectrum consists of a single spectral line that develops into a triplet above saturation of the quantum emitter1,2,3. The three closely spaced photon channels from the resonance fluorescence have different photon statistical signatures4. We present a detailed photon statistics analysis of the resonance fluorescence emission triplet from a solid-state-based artificial atom, that is, a semiconductor quantum dot. The photon correlation measurements demonstrate both ‘single’ and ‘cascaded’ photon emission from the Mollow triplet sidebands5. The bright and narrow sideband emission (5.9 × 106 photons per second into the first lens) can be conveniently frequency-tuned by laser detuning over 15 times its linewidth (Δv ≈ 1.0 GHz). These unique properties make the Mollow triplet sideband emission a valuable light source for quantum light spectroscopy and quantum information applications, for example. Researchers demonstrate that an individual Mollow sideband channel of the resonance fluorescence from an InGaAs quantum dot can act as an efficient single-photon source. The central frequency of the bright and narrow sideband emission can be changed by laser detuning over a range spanning 15 times the emission linewidth.

The wide range of applications in biophotonics, television or projectors, spectroscopy and lithography made the vertical external cavity surface-emitting lasers an important category of power scalable lasers. The possibility of bandgap engineering, inserting frequency selective and converting elements into the external laser cavity and laser emission in the fundamental Gaussian mode leads to ongoing growth of the area of applications for tunable laser sources. We present an intra cavity frequency-doubled VECSEL with emission wavelength around 330 nm and a maximum tuning range of more than 7 nm with output powers exeeding 100 mW. Frequency doubling is realized with an anti-reflection coated beta barium borate crystal, while a birefringent filter, placed inside the laser cavity under Brewster's angle, is used for frequency tuning. The fundamental laser, pumped by a 532nm Nd:YAG laser under an angle of 50 degrees normal to the surface, is realized by a multi quantum well structure consisting of 20 compressively strained GaInP quantum wells in an AlxGa1-xInP separate confinement heterostructure and it emits around 660 nm. The VECSEL-chip with its n-lambda cavity is completed by a 55 lambda/4 pairs Al0.50Ga0.50As/AlAs distributed Bragg reflector. Next to the optical properties of the device, we show results of different arrangements of the quantum wells, namely five times four and ten times two packages.

For a single quantum dot under excitation with short electrical pulses the dependence of the photon anti-bunching on pulse width and excitation strength is studied in a theory-experiment collaboration.

Single vertically stacked pairs of InP quantum dots (QDs) with different interdot barrier width are investigated using microphotoluminescence, photoluminescence excitation spectroscopy and time-resolved techniques. The results indicate unidirectional carrier tunneling to be the main coupling mechanism between the two layers containing QDs with intentionally different size. The coupling is changing from electronhole tunneling to electron tunneling and finally vanishing tunneling with increasing barrier size.

We perform excited-state spectroscopy of single self-assembled lateral InGaAs quantum dot molecules embedded in a planar microcavity structure, which are grown using a combination of metal-organic vapor phase and solid-source molecular beam epitaxy. The individual quantum dot molecules (QDMs) consist of two single dots that are coupled along the [1 $(1) over bar $0] crystal direction via electron tunneling. By microphotoluminescence excitation spectroscopy, we investigate the presence and structure of the excited states confined in the system using a wide-band tunable Ti:sapphire laser source. We show that the excited states are very similar even for different molecules. We attribute this to the presence of a potential basin below the quantum dots that levels the differences among various QDMs.

We present excitation-pulse-width- and pump-power-dependent microelectroluminescence and photon statistics measurements on electrically driven single-photon devices based on InP/AlGaInP quantum dots (QDs). For an excitation regime far below QD saturation, the results show a characteristic decrease of the purity of the single-photon emission [${g}^{(2)}(0)$ value] with increasing excitation pulse width. For stronger excitation pulses close to QD saturation, strong antibunching is maintained for a much larger pulse width. In this case the ground-state exciton emission, which is used for the single-photon source, is inhibited during the pump pulse due to the presence of higher excited states. This prevents multiple-ground-state emission and reexcitation during long pump pulses and delays the single-photon emission to the end of the pulse, as predicted by theory and confirmed experimentally.

This chapter describes the progress in development of vertical-cavity surface-emitting lasers (VCSEL) emitting in the red spectral region around 650 nm for data transmission over polymer optical fibers (POF). First, growth issues of red VCSEL using two different material systems, namely AlGaAs and AlGaInP, are introduced. In particular, the optical and electrical state-of-the-art characteristics as low threshold currents $$({\leq} 1\,\hbox{mA})$$ and high output powers (several mW) are presented with a special focus on emission wavelength. Also the thermal budget and heat removal in the devices are pointed out with regard to the geometry of the VCSEL. Small-signal modulation response in terms of maximum resonance frequency in dependance on temperature behavior are discussed. Applications of these devices in optical interconnects are described and digital data transmission at data rates up to 2.1 Gbit/s over step-index POF is reported. These properties make red emitting VCSEL perfectly suited for high-speed low power consuming light sources for optical data communication via POF. By introducing InP quantum dots as gain material in red emitting VCSEL nearly temperature independent record low threshold current densities of around $$10\,\hbox{A}/\hbox{cm}^{2}$$ could be observed.

We report on the epitaxial growth of vertically stacked InP and In(Ga)As quantum dot (QD) layers to realize a triple dot quantum gate structure consisting of an asymmetric control double dot and a single target dot suitable for a CNOT gate structure. Structural analysis as well as studies on the optical properties are presented. For studies on control dot structures we analyze the growth of InP islands in a GaInP barrier on (100) GaAs substrates. By stacking InP QD layers with intentional asymmetric design in QD size of each layer and adjustment of the barrier width between the double dots, coupling and control of the coupling via barrier layer width‐design can be demonstrated. For defined gate action single dot spectra of aligned dots are indispensable. Therefore QD density reduction is studied. We study two possibilities to affect the QD density and control the dot site by manipulating the surface potential of InP island nucleation. (i) Growth of InP islands on top of a low density In(Ga)As QD seed layer (ii) Growth of InP islands on patterned (100) GaAs substrates. We present microsphere photolithography in combination with wet chemical etching as a fast and low‐cost method to produce regular hole arrays in a GaAs surface, which are suitable for controlled nucleation of self‐assembled InP islands.

The heteroepitaxy of III-V semiconductors on silicon is a promising approach for making silicon a photonic platform. Mismatches in material properties, however, present a major challenge, leading to high defect densities in the epitaxial layers and adversely affecting radiative recombination processes. However, nanostructures, such as quantum dots, have been found to grow defect-free even in a suboptimal environment. Here we present the first realization of indium phosphide quantum dots on exactly oriented Si(001), grown by metal-organic vapour-phase epitaxy. We report electrically driven single-photon emission in the red spectral region, meeting the wavelength range of silicon avalanche photodiodes' highest detection efficiency.

We present experimental investigations of the transverse beam profile and polarization characteristics of GaInP-based oxide-confined vertical-cavity surface-emitting lasers in dependence on the oxide aperture size, mesa size, current, and temperature. We demonstrate that these lasers with aperture diameters of less than 6 mu m are required for stable fundamental-mode operation. The influence of operation current and external temperature on the mode shape is investigated. We experimentally present a highly stable linearly polarized GaInP-based microcavity laser emitting at around 655 nm, where the polarization characteristics originate from intrinsic material properties.

Systematic excitation power and temperature-dependent measurements on the emission lines of single self-assembled InP/(Al0.20Ga0.80)0.51In0.49P quantum dots embedded in micropillars have been performed. The quantum dots were excited optically via a pulsed laser and their luminescence was collected using a micro-photoluminescence setup. The exciton and biexciton intensity, linewidth, and spectral position was investigated in a temperature range from 4 K up to 130 K. Single-photon emission from the quantum dots is presented up to a temperature of 100 K, confirmed by photon-statistics measurements.

We present a detailed analysis of the transverse beam profile and the polarization characteristics of red-emitting oxide-confined vertical-cavity surface- emitting lasers (VCSELs). We demonstrate that oxide aperture sizes smaller than 6 mu m are required for emission in fundamental mode. We further show first steps towards integrating beam shaping optics directly into the top mirror of the VCSEL.

We demonstrate electrical pumping of self-assembled InP/Ga(0.51)In(0.49)P quantum dots embedded in a p-i-n resonant-cavity-diode structure with emission in the red spectral region. A high aluminum containing Al(0.98)Ga(0.02)As layer allows wet thermal oxidation and implementation of a current restricting oxide aperture above the active region. The intended use of these InP-quantum dots in such a resonant-cavity-LED structure as a pulsed electrically driven single-photon emitter was confirmed by measuring the second order intensity correlation function g((2))(tau) with a Hanbury-Brown and Twiss type setup. The correlation measurements performed on a single quantum dot (approximate to 40K) show a clear antibunching behavior (g((2))(0) <0.24) up to 200 MHz as expected for a single-photon emitter. (C) 2010 Elsevier B.V. All rights reserved.

We present a non-resonantly pumped red-emitting vertical external cavity surface-emitting laser system based on a multi-quantum-well structure with 20 compressively-strained GaInP quantum wells for an operation wavelength between 645-675 nm. Five quantum well packages with four quantum wells are placed in a separate confinement heterostructure in a resonant periodic gain design in quaternary AlGaInP barriers and cladding layers, respectively. The 3 lambda cavity is fabricated on a 55 lambda/4 pairs Al0.50Ga0.50As/AlAs distributed Bragg reflector. By bonding an intra-cavity diamond heatspreader to the chip, continuous-wave operation exceeding 700 mW output power at a wavelength of 662 nm with a low threshold power of 0.8 W was achieved. A thermal resistance value of R-1 = 5 K/W and R-2 = 7 K/W could be determined for our setup at operation heatsink temperatures of T-hs = -28 degrees C and T-hs = 16 degrees C, respectively. Measurements of the slope efficiency within a v-type cavity with different outcoupling mirror reflectivities lead to a cavity round-trip transmission factor of T-loss = 98.6% and an absorption efficiency of eta(abs) = 17.6%. Using a birefringent filter in a folded cavity, a maximum tuning range of 22 nm at a center wavelength of 667 nm could be shown. With this method wavelengths below 650 nm were observed. Utilizing a non-linear crystal for intra-cavity frequency doubling in this cavity geometry, coherent emission down to 322 nm could be detected. In the UV spectral range, a maximum tuning range of 10 nm could be measured at a center wavelength of 330 nm, so we could match the HeCd laser line at 325 nm.

We investigated the photoluminescence from InP quantum dots incorporated in (Ga${}_{0.51}$In${}_{0.49}$)P microdisk structures. With increasing pump power we observe a transition to stimulated emission indicated by the S shape of the input-output curve. This transition is accompanied with a concentration of the emission to one or a few modes exhibiting quality factors on the order of ${10}^{4}$ at transparency. Time-resolved measurements show that at the same time the photoluminescence decay time considerably decreases. Furthermore, in the transition regime the linewidth of the lasing mode is reduced and the second-order photon correlation function exhibits a reduction of fluctuations as previously reported for lasers with InAs quantum dots in photonic crystal, microdisk and micropillar cavities as the gain medium. The experimental findings are compared with the predictions of a microscopic theory.