D. Heiss

Optically programmable electron spin memory using semiconductor quantum dots

The spin of a single electron subject to a static magnetic field provides a natural two-level system that is suitable for use as a quantum bit, the fundamental logical unit in a quantum computer. Semiconductor quantum dots fabricated by strain driven self-assembly are particularly attractive for the realization of spin quantum bits, as they can be controllably positioned, electronically coupled and embedded into active devices. It has been predicted that the atomic-like electronic structure of such quantum dots suppresses coupling of the spin to the solid-state quantum dot environment, thus protecting the ‘spin’ quantum information against decoherence. Here we demonstrate a single electron spin memory device in which the electron spin can be programmed by frequency selective optical excitation. We use the device to prepare single electron spins in semiconductor quantum dots with a well defined orientation, and directly measure the intrinsic spin flip time and its dependence on magnetic field. A very long spin lifetime is obtained, with a lower limit of about 20 milliseconds at a magnetic field of 4 tesla and at 1 kelvin.

An introduction to InP-based generic integration technology

Photonic integrated circuits (PICs) are considered as the way to make photonic systems or subsystems cheap and ubiquitous. PICs still are several orders of magnitude more expensive than their microelectronic counterparts, which has restricted their application to a few niche markets. Recently, a novel approach in photonic integration is emerging which will reduce the R&D and prototyping costs and the throughput time of PICs by more than an order of magnitude. It will bring the application of PICs that integrate complex and advanced photonic functionality on a single chip within reach for a large number of small and larger companies and initiate a breakthrough in the application of Photonic ICs. The paper explains the concept of generic photonic integration technology using the technology developed by the COBRA research institute of TU Eindhoven as an example, and it describes the current status and prospects of generic InP-based integration technology.

Observation of extremely slow hole spin relaxation in self-assembled quantum dots

We report the measurement of extremely slow hole spin relaxation dynamics in small ensembles of self-assembled InGaAs quantum dots. Individual spin oriented holes are optically created in the lowest orbital state of each dot and read out after a defined storage time using spin memory devices. The resulting luminescence signal exhibits a pronounced polarization memory effect that vanishes for long storage times. The hole spin relaxation dynamics are measured as a function of external magnetic field and lattice temperature. We show that hole spin relaxation can occur over remarkably long time scales in strongly confined quantum dots (up to similar to 270 mu s), as predicted by recent theory. Our findings are supported by calculations that reproduce both the observed magnetic field and temperature dependencies. The results suggest that hole spin relaxation in strongly confined quantum dots is due to spin-orbit-mediated phonon scattering between Zeeman levels, in marked contrast to higher-dimensional nanostructures where it is limited by valence band mixing.

InP photonic circuits using generic integration [Invited]

InP integrated photonics has become a critical enabler for modern telecommunications, and is poised to revolutionize data communications, precision metrology, spectrometry, and imaging. The possibility to integrate high-performance amplifiers, lasers, modulators, and detectors in combination with interferometers within one chip is enabling game-changing performance advances, energy savings, and cost reductions. Generic integration accelerates progress through the separation of applications from a common technology development. In this paper, we review the current status in InP integrated photonics and the efforts to integrate the next generation of high-performance functionality on a common substrate using the generic methodology.

A Charge and Spin Readout Scheme For Single Self-Assembled Quantum Dots

We propose an all optical spin initialization and readout concept for single self assembled quantum dots and demonstrate its feasibility. Our approach is based on a gateable single dot photodiode structure that can be switched between charge and readout mode. After optical electron generation and storage, we propose to employ a spin-conditional absorption of a circularly polarized light pulse tuned to the single negatively charged exciton transition to convert the spin information of the resident electron to charge occupancy. Switching the device to the charge readout mode then allows us to probe the charge state of the quantum dot (1e, 2e) using non-resonant luminescence. The spin orientation of the resident electron is then reflected by the photoluminescence yield of doubly and singly charged transitions in the quantum dot. To verify the feasibility of this spin readout concept, we have applied time gated photoluminescence to confirm that selective optical charging and efficient non perturbative measurement of the charge state can be performed on the same dot. The results show that, by switching the electric field in the vicinity of the quantum dot, the charging rate can be switched between a regime of efficient electron generation and a readout regime, where the charge occupancy and, therefore, the spin state of the dot can be tested via PL over millisecond timescales, without altering it.

Waveguide-coupled nanopillar metal-cavity light-emitting diodes on silicon

Nanoscale light sources using metal cavities have been proposed to enable high integration density, efficient operation at low energy per bit and ultra-fast modulation, which would make them attractive for future low-power optical interconnects. For this application, such devices are required to be efficient, waveguide-coupled and integrated on a silicon substrate. We demonstrate a metal-cavity light-emitting diode coupled to a waveguide on silicon. The cavity consists of a metal-coated III–V semiconductor nanopillar which funnels a large fraction of spontaneous emission into the fundamental mode of an InP waveguide bonded to a silicon wafer showing full compatibility with membrane-on-Si photonic integration platforms. The device was characterized through a grating coupler and shows on-chip external quantum efficiency in the 10−4–10−2 range at tens of microamp current injection levels, which greatly exceeds the performance of any waveguide-coupled nanoscale light source integrated on silicon in this current range. Furthermore, direct modulation experiments reveal sub-nanosecond electro-optical response with the potential for multi gigabit per second modulation speeds.

Realization of efficient metal grating couplers for membrane-based integrated photonics

Grating couplers are widely used to couple light between photonic integrated circuits and optical fibers. Here, we fabricate and characterize a device based on a buried metal grating. In contrast to dielectric gratings, simulations predict strongly reduced parasitic leakage of light to the substrate and are performance independent of the optical buffer thickness, while using standard fabrication processes with high yield. The gratings show a 3 dB bandwidth of 61 nm and chip-to-fiber coupling efficiency of 54%, which makes them attractive building blocks for on-wafer testing and dense optical interconnects.

Photonic integration in Indium-Phosphide Membranes on Silicon (IMOS)

A new photonic integration technique is presented, based on the use of an indium phosphide membrane on top of a silicon chip. This can provide electronic chips (CMOS) with an added optical layer (IMOS) for resolving the communication bottleneck. A major advantage of InP is the possibility to integrate passive and active components (SOAs, lasers) in a single membrane. In this paper we describe progress achieved in both the passive and active components. For the passive part of the circuit we succeeded to bring the propagation loss of our circuits close to the values obtained with silicon; we achieved propagation loss as low as 3.3 dB/cm through optimization of the lithography and the introduction of C60 (fullerene) in an electro resist. Further we report the smallest polarisation converter reported for membrane waveguides ( <10 μm) with low-loss (< 1 dB from 1520- 1550 nm), > 95% polarisation conversion efficiency over the whole C-band and tolerant fabrication. We also demonstrate an InP-membrane wavelength demultiplexer with a loss of 2.8 dB, a crosstalk level of better than 18 dB and a uniformity over the 8 channels of better than 1.2 dB. For the integration of active components we are testing a twin guide integration scheme. We present our design based on optical and electrical simulations and the fabrication techniques.

Observation of an electrically tunable exciton g factor in InGaAs/GaAs quantum dots

The electric field dependence of the exciton g factor and the fine structure splitting in self-assembled InGaAs/GaAs quantum dots grown via a flush-overgrowth technique is studied by photocurrent and photoluminescence experiments. Both the fine structure and the Zeeman splitting can be tuned over a wide range via electric fields applied in growth direction of the quantum dot. For the g factor, a tunability of 250% is demonstrated from g=0.12 to 0.42.

Low-optical-loss, low-resistance Ag/Ge based ohmic contacts to n-type InP for membrane based waveguide devices

We present the development of Ag/Ge based ohmic contacts to n-type InP with both low contact resistances and relatively low optical losses. A specific contact resistance as low as 1.5×10−6 Ω cm2 is achieved by optimizing the Ge layer thickness and annealing conditions. The use of Ge instead of metal as the first deposited layer results in a low optical absorption loss in the telecommunication wavelength range. Compared to Au based contacts, the Ag based metallization also shows considerably reduced spiking effects after annealing. Contacts with different lengths are deposited on top of InP membrane waveguides to characterize the optical loss before and after annealing. A factor of 5 reduction of the propagation loss compared to the conventional Au/Ge/Ni contact is demonstrated. This allows for much more optimized designs for membrane photonic devices.