Universal nuclear focusing of confined electron spins

For spin-based quantum computation in semiconductors, dephasing of electron spins by a fluctuating background of nuclear spins is a main obstacle. Here we show that this nuclear background can be precisely controlled in generic quantum dots by periodically exciting electron spins. We demonstrate this universal phenomenon in many-electron GaAs/AlGaAs quantum dot ensembles using optical pump-probe spectroscopy. A feedback mechanism between the saturable electron spin polarization and the nuclear system focuses the electron spin precession frequency into discrete spin modes. Employing such control of nuclear spin polarization, the electron spin lifetime within individual dots can surpass the limit of nuclear background fluctuations, thus substantially enhancing the spin coherence time. This opens the door to achieve long electron spin coherence times also in lithographically-defined many-electron systems that can be controlled in shape, size and position

THz Intersubband Emitter based on Silicon

We present THz quantum cascade emitters realized on a Si substrate. The emission centered at 3.4 and 4.9 THz originates from L-valley transitions in strain-compensated n-type Ge/SiGe heterostructures. This is an important step towards the realization of Si-based THz quantum cascade lasers

Terahertz intersubband electroluminescence from n-type germanium quantum wells

The Quantum Cascade Laser (QCL) has been demonstrated in polar III-V semiconductor materials employing transitions between conduction band states [1] . Harnessing intersubband transitions allows lasing at mid-infrared and far-infrared wavelengths. Buried InGaAs/InAlAs QCLs unlocked the mid-infrared application space, because they are operational at room-temperature and in continuous wave [2] . However, THz QCLs remain limited up to 250 K in pulsed operation with a large dissipation [3] . The quenching of the laser emission is related to ther-mally activated LO phonon emission in polar materials. Exploiting intersubband transitions in non-polar group IV materials with weaker electron-phonon interaction is an exciting approach to realize a Si-based THz QCL and to eventually elevate the operation temperature [4]

Ultra-fast spectroscopy of terahertz quantum cascade lasers

In dieser Arbeit werden zwei Methoden zu spektralen Emissionskontrolle vorgestellt, bei denen weder der Laserwellenleiter noch das lichtverstärkende Medium abgeändert werden soll. Beide Methoden machen sich ein kleines THz-Signal zu Nutze, das in den QKL injiziert und verstärkt wird. Bei der ersten Methode werden THz-Pulse mit einer geringen spektralen Bandbreite in den QKL injiziert. Die zweite Methode der spektralen Emissionskontrolle basiert auf der Injektion von zwei bandbreiten THz Pulsen. Mittels dieses neuen spektroskopischen Verfahrens wurde die Verstärkungsdynamik des QKLs auf dem Femto- und Pikosekunden-Bereich studiert also auch nicht-lineare Prozesse im Leser selbst beobachtet

Electro-Optic Sampling of a Free-Running Terahertz Quantum-Cascade-Laser Frequency Comb

Quantum-cascade-laser (QCL) frequency combs are compact semiconductor light sources operating in the mid-IR and terahertz frequencies. Achieving subpicosecond laser pulses with high peak power is of vital importance for performing nonlinear time-resolved spectroscopy as well for exploring nonlinear phenomena. Therefore, investigation and characterization of time-resolved free-running laser emission is a key for further improvement and optimization of these intersubband devices. In this work, we demonstrate a direct electric field measurement of a free-running terahertz QCL frequency comb using electro-optic sampling in combination with computational phase correction, where we retrieve the electric field profile and access the comb parameters. The demonstrated method is of high interest for time-resolved lowpower laser emission characterization, typical for ring terahertz QCL frequency combs and investigation of phase-compensated emission via waveguide dispersion engineering for broadband comb operation.ISSN:2331-701

Two-dimensional spectroscopy on a THz quantum cascade structure

Understanding and controlling the nonlinear optical properties and coherent quantum evolution of complex multilevel systems out of equilibrium is essential for the new semiconductor device generation. In this work, we investigate the nonlinear system properties of an unbiased quantum cascade structure by performing two-dimensional THz spectroscopy. We study the time-resolved coherent quantum evolution after it is driven far from equilibrium by strong THz pulses and demonstrate the existence of multiple nonlinear signals originating from the engineered subbands and find the lifetimes of those states to be in the order of 4–8 ps. Moreover, we observe a coherent population exchange among the first four intersubband levels during the relaxation, which have been confirmed with our simulation. We model the experimental results with a time-resolved density matrix based on the master equation in Lindblad form, including both coherent and incoherent transitions between all density matrix elements. This allows us to replicate qualitatively the experimental observations and provides access to their microscopic origin

Quantum Cascade Surface Emitting Lasers

A low-cost single frequency laser, emitting in the mid-infrared spectral region and dissipating minimal electrical power, is a key ingredient for the next generation of portable gas sensors for high-volume applications involving chemical sensing of important greenhouse and pollutant gases. Herein, a Quantum Cascade Surface Emitting Laser (QCSEL) is proposed, which is implemented as a short linear cavity with high reflectivity coated end-mirrors to suppress any edge emission and employs a buried semiconductor diffraction grating to extract the light from the surface. By wafer-level testing, the cavity length scaling is investigated, mirror reflectivities larger than 0.9 are extracted, and a pulsed threshold power dissipation of 237 mW for an emission wavelength near 7.5 µm is achieved. Finally, single-mode emission with a side-mode suppression ratio larger than 33 dB is demonstrated for a 248 µm short cavity, which is mounted with the epitaxial layer up and operated in continuous wave at 20 ◦C.ISSN:1863-8880ISSN:1863-889

Non-locality and single meta-atom spectroscopy in THz Landau polaritons

We will discuss, theoretically and experimentally, the existence of a limit to the possibility of arbitrarily increasing electromagnetic confinement in polaritonic systems, where strongly sub-wavelength fields can excite a continuum of high-momenta propagative magnetoplasmons. This leads to peculiar nonlocal polaritonic effects, as certain polaritonic features disappear and the system enters in the regime of discrete-to-continuum strong coupling. We will as well present experiments reporting spectroscopy of a single, ultrastrongly coupled, highly subwavelength resonator operating at 300 GHz