Henning Höpfner

Ultrafast spin-induced polarization oscillations with tunable lifetime in vertical-cavity surface-emitting lasers

We report spin-induced polarization oscillations in vertical-cavity surface-emitting lasers above threshold and at room temperature. The oscillation frequency is 11.6 GHz, which is significantly higher than the modulation bandwidth of less than 4 GHz in the device. The oscillation frequency is determined by an additional resonance frequency in birefringence containing microcavities, which is potentially much higher than the conventional relaxation oscillation frequency. The damping of the oscillations can be controlled by the current, allowing for oscillation lifetimes much longer than the spin lifetime in the device as well as for short bursts potentially interesting for information transmission.

Photorefractive two-wave mixing for image amplification in digital holography

We use photorefractive two-wave mixing for coherent amplification of the object beam in digital holographic recording. Both amplitude and phase reconstruction benefit from the prior amplification as they have an increased SNR. We experimentally verify that the amplification process does not affect the phase of the wavefield. This allows for digital holographic phase analysis after amplification. As the grating formation in photorefractive crystals is just driven by coherent light, the crystal works as a coherence gate. Thus the proposed combination allows for applying digital holography for imaging through scattering media, after the image bearing light is coherence gated and filtered out of scattered background. We show experimental proof-of principle results.

Controlled switching of ultrafast circular polarization oscillations in spin-polarized vertical-cavity surface-emitting lasers

We demonstrate a scheme for controlled switching of polarization oscillations in spin-polarized vertical-cavity surface-emitting lasers (spin-VCSEL). Under hybrid electrical and optical pumping conditions, our VCSEL devices show polarization oscillations with frequencies far above the VCSELs electrical modulation bandwidth. Using multiple optical pulses, we are able to excite and amplify these polarization oscillations. When specific phase and amplitude conditions for the optical excitation pulses are met, destructive interference leads to switch-off of the polarization oscillation, enabling the generation of controlled short polarization bursts.

Depth-filtered digital holography

We introduce depth-filtered digital holography (DFDH) as a method for quantitative tomographic phase imaging of buried layers in multilayer samples. The procedure is based on the acquisition of multiple holograms for different wavelengths. Analyzing the intensity over wavelength pixel wise and using an inverse Fourier transform leads to a depth-profile of the multilayered sample. Applying a windowed Fourier transform with a narrow window, we choose a depth-of interest (DOI) which is used to synthesize filtered interference patterns that just contain information of this limited depth. We use the angular spectrum method to introduce an additional spatial filtering and to reconstruct the corresponding holograms. After a short theoretical framework we show experimental proof-of-principle results for the method.

Magnetic field dependence of the spin relaxation length in spin light-emitting diodes

We investigate the spin relaxation length during vertical electron transport in spin-light emitting diode devices as a function of magnetic field strength at room temperature. In most publications on spin relaxation in optoelectronic devices, strong magnetic fields are used to achieve perpendicular-to-plane magnetization of the spin injection contacts. We show experimentally that high magnetic field strengths significantly reduce spin relaxation during transport to the active region of the device. We obtain a spin relaxation length of 27(3) nm in magnetic remanence and at room temperature, which nearly doubles at 2 T magnetic field strength.

Polarization dynamics in spin-polarized vertical-cavity surface-emitting lasers

Spin-polarized lasers and especially spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) are at- tractive novel spintronic devices providing functionalities and characteristics superior to their conventional purely charge-based counterparts. This applies in particular to ultrafast dynamics, modulation capability and chirp control of directly modulated lasers. Here we demonstrate that ultrafast oscillations of the circular polarization degree can be generated in VCSELs by pulsed spin injection which have the potential to reach frequencies beyond 100 GHz. These oscillations are due to the coupling of the carrier-spin-photon system via the optical birefringence for the linearly polarized laser modes in the micro-cavity and are principally decoupled from conventional relaxation oscillations of the carrier-photon system. Utilizing these polarization oscillations is a very promising path to ultrafast directly modulated spin-VCSELs in the near future as long as an effective concept can be developed to modulate or switch these polarization oscillations. After briefly reviewing the state of research in the emerging field of spin-VCSELs, we present a novel concept for controlled switching of polarization oscillations by use of multiple optical spin injection pulses. Depending on the amplitude and phase conditions of the excitation pulses, constructive or destructive interference of polarization oscillations leads to an excitation, stabilization or switch-off of these oscillations. Furthermore even short single polarization bursts can be generated with pulse widths only limited by the resonance frequency of the polarization oscillation. Consequently, this concept is an important building block for using spin controlled polarization oscillations for future communication applications.

Quantum Dot Spintronics: Fundamentals and Applications

Spintronics is a generalization of electronics: Electronics means charge carrier transport, spintronics adds to this transport the supplementary degree of freedom spin which has been neglected since the roots of electronics. In this sense, spintronics is opening a new dimension of functional devices which is even more mighty than it may look at a first glance: The electron spin and its orientation is a pure quantum mechanical phenomenon which leads in its complexity to much more information coding depth and combinatorial operations than the storage and transport of charges in classical electronics. That is why the quantum bit (qubit) concept has been introduced by Schumacher [1].

Room temperature spin relaxation in quantum dot based spin-optoelectronic devices

Spin-optoelectronic devices have become a field of intensive research in the past few years. Here we present electrical spin injection into spin light-emitting diodes both at room temperature and in magnetic remanence. Our devices consist of a Fe/Tb multilayer spin injection structure with remanent out-of-plane magnetization, a MgO tunnel barrier for efficient spin injection and an InAs quantum dot light-emitting diode. The ground state emission and first excited state emission both show circularly polarized emission in remanence, i.e. without external magnetic fields which is due to spin injection from our ferromagnetic contact. Using a series of samples with varying transport path lengths between the spin injector and the active region, we investigate the spin relaxation length during vertical carrier transport through our devices. Due to our spin injector with remanent out-of-plane magnetization this spin relaxation can be investigated without the need for external magnetic fields which would possibly influence the spin relaxation process. The decrease in circular polarization with increasing injection path length is found to be exponential, indicating drift-based transport which is in accordance with theoretic calculations. From the exponential decay the spin relaxation length of 26 nm as well as a lower bound for the spin injection efficiency of 25% are calculated. Additionally, influences of magnetic field, temperature and current density in the devices on the spin relaxation process are discussed.

Ultrafast spin-polarized vertical-cavity surface-emitting lasers

Spin-polarized lasers are highly attractive spintronic devices providing characteristics superior to their conventional purely charge-based counterparts. Spin-polarized vertical-cavity surface emitting lasers (spin-VCSELs) promise to offer lower thresholds, enhanced emission intensity, spin amplification, full polarization control, chirp control and ultrafast dynamics. In particular, the ability to control and modulate the polarization state of the laser emission with extraordinarily high frequencies is very attractive for many applications like broadband optical communication and ultrafast optical switches. After briefly reviewing the state of research in this emerging field of spintronics, we present a novel concept for ultrafast spin-VCSELs which has the potential to overcome the conventional speed limitation for directly modulated lasers and to reach modulation frequencies significantly above 100 GHz. The concept is based on the coupled spin-photon dynamics in birefringent micro-cavity lasers. By injecting spin-polarized carriers in the VCSEL, oscillations of the coupled spin-photon system can by induced which lead to oscillations of the polarization state of the laser emission. These oscillations are decoupled from conventional relaxation oscillations of the carrier-photon system and can be much faster than those. Utilizing these polarization oscillations is thus a very promising approach to develop ultrafast spin-VCSELs for high speed optical data communication in the near future.

Spin injection, transport, and relaxation in spin light-emitting diodes: magnetic field effects

Efficient electrical spin injection into semiconductor based devices at room temperature is one of the most important requirements for the development of applicable spintronic devices in the near future and is thus an important and very active research field. Here we report experimental results for the electrical spin injection in spin light-emitting diodes (spin-LEDs) without external magnetic fields at room temperature. Our devices consist of a Fe/Tb multilayer spin injector with remanent out-of-plane magnetization, an MgO tunnel barrier for efficient spin injection and an InAs quantum dot light-emitting diode. Using a series of samples with different injection path lengths allows us to experimentally determine the spin relaxation during vertical transport from the spin injector to the active region at room temperature. In combination with our concept for remanent spin injection, we are additionally able to investigate the influence of an external magnetic field on the spin relaxation process during transport. While the spin relaxation length at room temperature without external magnetic field is determined to be 27 nm, this value almost doubles if an external magnetic field of 2 Tesla is applied in Faraday geometry. This demonstrates that the results for spin injection and spin relaxation obtained with or without magnetic field can hardly be compared. The efficiency of spin-induced effects is overestimated as long as magnetic fields are involved. Since strong magnetic fields are not acceptable in application settings, this may lead to wrong conclusions and potentially impairs proper device development.