Since Purcell's seminal report 75 years ago, electromagnetic resonators have been used to control light-matter interactions to make brighter radiation sources and unleash unprecedented control over quantum states of light and matter. Indeed, optical resonators such as microcavities and plasmonic antennas offer excellent control but only over a limited spectral range. Strategies to mutually tune and match emission and resonator frequency are often required, which is intricate and precludes the possibility of enhancing multiple transitions simultaneously. In this letter, we report a strong radiative emission rate enhancement of Er3+-ions across the telecommunications C-band in a single plasmonic waveguide based on the Purcell effect. Our gap waveguide uses a reverse nanofocusing approach to efficiently enhance, extract and guide emission from the nanoscale to a photonic waveguide while keeping plasmonic losses at a minimum. Remarkably, the large and broadband Purcell enhancement allows us to resolve Stark-split electric dipole transitions, which are typically only observed under cryogenic conditions. Simultaneous radiative emission enhancement of multiple quantum states is of great interest for photonic quantum networks and on-chip data communications.

Since Purcell's seminal report 75 years ago, electromagnetic resonators have been used to control light-matter interactions to make brighter radiation sources and unleash unprecedented control over quantum states of light and matter. Indeed, optical resonators such as microcavities and plasmonic nanostructures offer excellent control but only over a limited spectral range. Strategies to tune both emission and the resonator are often required, which preclude the possibility of enhancing multiple transitions simultaneously. In this letter, we report a more than 590-fold radiative emission enhancement across the telecommunications emission band of Erbium-ions in silica using a single non-resonant plasmonic waveguide. Our plasmonic waveguide uses a novel reverse nanofocusing approach to efficiently collect emission, making these devices brighter than all non-plasmonic control samples considered. Remarkably, the high broadband Purcell factor allows us to resolve the Stark-split electric dipole transitions, which are typically only observed under cryogenic conditions. Simultaneous Purcell enhancement of multiple quantum states is of interest for photonic quantum networks as well as on-chip data communications.

We report the broadband fluorescence enhancement of erbium ions embedded in a single non-resonant reverse nano-focusing waveguide. We measure a large radiative Purcell enhancement of a total emission rate enhancement of > 250 across the entire measured spectrum including the prominent telecoms C-band. Further, we observe the enhancement of single electric dipole transitions from Stark-split levels at room temperature.

In this work we investigated the role of free carriers in the interaction of a wide-band gap semiconductor with strong light fields at long wavelengths. Motivated by the beneficial scaling law of the pondermotive potential (U p ~ Iλ 2 ), the interaction of intense long wavelength laser pulses with condensed matter has attracted huge attention over the last decade [Kruchinin] . After excitation of quasi free electrons in the conduction band (CB) via multiphoton absorption or tunnelling the strong pondermotive force leads to highly energetic free electrons. Bound electrons can be collisionally excited if the energy of the free electrons exceed the band gap energy. Here, we use the onset of near ultraviolet (NUV) stimulated emission in ZnO thin films ( Fig. 1a ) to study off-resonance light absorption and the role of free carriers thereby.

The interaction of laser pulses with condensed matter forms the basis of light-wave-driven electronics potentially enabling tera- and petahertz switching rate applications. Carrier control using near- and midinfrared pulses is appealing for integration into existing platforms. Toward this end, a fundamental understanding of the complexity of phenomena concerning sub-band-gap driven semiconductors such as high harmonic generation, carrier excitation due to multiphoton absorption, and interband tunneling as well as carrier-carrier interactions due to strong acceleration in infrared transients is important. Here, stimulated emission from polycrystalline ZnO thin films for pump wavelengths between 1.2 \ensuremath{\mu}m (1 eV) and 10 \ensuremath{\mu}m (0.12 eV) is observed. Contrary to the expected higher intensity threshold for longer wavelengths, the lowest threshold pump intensity for stimulated emission is obtained for the longest pump wavelength corroborating the importance of collisional excitation upon intraband electron acceleration.

The generation of high order harmonics from femtosecond mid-IR laser pulses in ZnO has shown great potential to reveal new insight into the ultrafast electron dynamics on a few femtosecond timescale. In this work we report on the experimental investigation of photoluminescence and high-order harmonic generation (HHG) in a ZnO single crystal and polycrystalline thin film irradiated with intense femtosecond mid-IR laser pulses. The ellipticity dependence of the HHG process is experimentally studied up to the 17th harmonic order for various driving laser wavelengths in the spectral range 3–4 µm. Interband Zener tunneling is found to exhibit a significant excitation efficiency drop for circularly polarized strong-field pump pulses. For higher harmonics with energies larger than the bandgap, the measured ellipticity dependence can be quantitatively described by numerical simulations based on the density matrix equations. The ellipticity dependence of the below and above ZnO band gap harmonics as a function of the laser wavelength provides an efficient method for distinguishing the dominant HHG mechanism for different harmonic orders.

Laser diodes are efficient light sources. However, state-of-the-art laser diode-based lighting systems rely on light-converting inorganic phosphor materials, which strongly limit the efficiency and lifetime, as well as achievable light output due to energy losses, saturation, thermal degradation, and low irradiance levels. Here, we demonstrate a macroscopically expanded, three-dimensional diffuser composed of interconnected hollow hexagonal boron nitride microtubes with nanoscopic wall-thickness, acting as an artificial solid fog, capable of withstanding ~10 times the irradiance level of remote phosphors. In contrast to phosphors, no light conversion is required as the diffuser relies solely on strong broadband (full visible range) lossless multiple light scattering events, enabled by a highly porous (>99.99%) non-absorbing nanoarchitecture, resulting in efficiencies of ~98%. This can unleash the potential of lasers for high-brightness lighting applications, such as automotive headlights, projection technology or lighting for large spaces.

We present a simple non-destructive approach for studying the polarization dependence of nonlinear absorption processes in semiconductors. The method is based on measuring the yield of the near UV photoluminescence as a function of polarization and intensity of femtosecond laser pulses. In particular, we investigated the polarization dependence of three photon laser absorption in intrinsic and Al-doped ZnO thin films. Both specimen show stronger emission for linearly polarized excitation compared to circular polarization. The ratios for the three-photon absorption coefficients are about 1.8 and independent of the doping. It is shown that Al-doped films have lower threshold for stimulated emission in comparison to the intrinsic films.

Even though intrinsic semiconductor nanowires have already extraordinary optical properties, doping with optically active impurities significantly expands the potpourri of optoelectronic applications, such as for nanowire lasers or single photon emitters. This feature article therefore supplies a snapshot of the most recent progress on the structural and optical properties of transition metal and rare earth element doped zinc oxide (ZnO) nanowires using ion beam doping. Here, ion implantation is advantageous, as concurrent defect generation and diffusion upon subsequent annealing allows the formation of defect complexes. This scenario is in many cases even inevitable for the optical activation of the intra‐shell luminescence of the implanted impurities, as density functional theory calculations demonstrate. Finally, this article also provides the optimum preparation conditions for intense optical activity and a review on the specific luminescence properties of various optical centers in ZnO nanowires.

Semiconductor optoelectronics have contributed tremendously to various aspects of the technological progress in the past. Recently, they also stimulate research in nanophotonics seeking to overcome the inherent limitations of electronic integrated circuits and satisfy the growing demand for faster on-chip communications. In particular, nanowire (NW) lasers generate coherent light at the nanoscale and meanwhile work consistently at room temperature covering a huge spectral range from the ultraviolet down to the mid-infrared depending on the NW material. The underlying physics of their electronic and photonic systems are also studied very recently, thus NW lasers become relevant as an emergent tool for a variety of practical applications. In this review, the origins determining the emission wavelength of the device are explained, and approaches toward efficient improvement of the NW laser devices in terms of emission energy tunability are summarized. The optical mode field distribution strongly influences the emission dynamics of the nanolaser device; it will be highlighted how increasing the light-matter interaction in plasmonic type nanolasers causes a significant acceleration of their temporal emission dynamics. Both spectral and temporal tuning will help to solve scientific and engineering challenges forging semiconductor nanowire lasers into powerful tools for nanosensing, nanospectroscopy, and nanotechnology.

Novel gas sensors have been realized by decorating clusters of tubular Aerographite with CdTe using magnetron sputtering techniques. Subsequently, individual microtubes were separated and electrically contacted on a SiO2/Si substrate with pre-patterned electrodes. Cathodoluminescence, electron microscopy and electrical characterization prove the successful formation of a polycrystalline CdTe thin film on Aerographite enabling an excellent gas response to ammonia. Furthermore, the dynamical response to ammonia exposure has been investigated, highlighting the quick response and recovery times of the sensor, which is highly beneficial for extremely short on/off cycles. Therefore, this gas sensor reveals a large potential for cheap, highly selective, reliable and low-power gas sensors, which are especially important for hazardous gases such as ammonia.

We report on stimulated emission from vertically aligned, vapor transport grown, ZnO nanowire arrays, and pumped by three-photon absorption in intense near-infrared femtosecond laser pulses. In respect to single nanowires, arrays have the advantage of a higher light absorption and emission rate. The intensity and bandwidth of the emitted ultraviolet radiation as a function of the pump intensity is compared for nanowire arrays with different wire lengths, diameters, and spacing. The measured lasing thresholds for all arrays can be well described by the geometry of individual nanowire lasers, showing that coupling effects between the individual emitters in the arrays are negligible, even for the smallest 100nm diameter wires with an average distance of 200nm.

Einstein established the quantum theory of radiation and paved the way for modern laser physics including single-photon absorption by charge carriers and finally pumping an active gain medium into population inversion. This can be easily understood in the particle picture of light. Using intense, ultrashort pulse lasers, multiphoton pumping of an active medium has been realized. In this nonlinear interaction regime, excitation and population inversion depend not only on the photon energy but also on the intensity of the incident pumping light, which can be still described solely by the particle picture of light. We demonstrate here that lowering significantly the pump photon energy further still enables population inversion and lasing in semiconductor nanowires. The extremely high electric field of the pump bends the bands and enables tunneling of electrons from the valence to the conduction band. In this regime, the light acts by the classical Coulomb force and population inversion is entirely due to the wave nature of electrons, thus the excitation becomes independent of the frequency but solely depends on the incident intensity of the pumping light.

Tunable nanoscale light emitters are essential to accomplish future multifunctional optoelectronic nano-devices. Here, we present an approach for achieving red electroluminescence from single ZnO nanowires (NWs) implanted with europium ions. The electroluminescence is emitted mainly from the end facets of ZnO NWs at room temperature under the application of an AC voltage. The corresponding electroluminescence spectrum is attributed to the radiative intrashell transitions of the Eu ions, while contributions from near band edge or deep level emission of the ZnO remain absent. The total intensity of the electroluminescence is linearly proportional to the length of the NWs, whereas there is no clear correlation with other morphology factors of the NW based device such as the diameter. Furthermore, the underlying excitation mechanism of the electroluminescence is proposed as direct-impact excitation of Eu ions by hot electrons in the ZnO NWs.

Damage formation and annealing of Eu implanted KTiOAsO4 crystals is investigated. Ion implantation was done with 400 keV Eu2+ ions and an ion fluence of 5 x 10(15) ions/cm(2) at room temperature. The damage of as implanted and annealed samples was analysed by Rutherford backscattering spectrometry in channelling configuration using 2.1 MeV He+ ions. After implantation, a thick RBS-amorphous layer is obtained. The damage in the Eu2+ implanted KTiOAsO4 layer decreases significantly after annealing at 700 degrees C for 30 min in Ar atmosphere. Interestingly, the Eu peak firstly migrated to the surface after annealing at 600 degrees C for 30 min, and then appeared narrower after annealing at 700 degrees C for 30 min. Furthermore, arsenic loss by evaporation and titanium enrichment at the surface are observed during the annealing process.

We present a detailed investigation of X-ray emission from both flat and nanowire zinc oxide targets irradiated by 60 fs 5 × 1016 W/cm2 intensity laser pulses at a 0.8 µm wavelength. It is shown that the fluence of the emitted hard X-ray radiation in the spectral range 150–800 keV is enhanced by at least one order of magnitude for nanowire targets compared to the emission from a flat surface, whereas the characteristic Kα line emission (8.64 keV) is insensitive to the target morphology. Furthermore, we provide evidence for a dramatic increase of the fast electron flux from the front side of the nanostructured targets. We suggest that targets with nanowire morphology may advance development of compact ultrafast X-ray sources with an enhanced flux of hard X-ray emission that could find wide applications in highenergy density (HED) physics.

The atomic-scale structure and vibrational properties of semiconductor alloys are determined by the energy required for stretching and bending the individual bonds. Using temperature-dependent extended x-ray absorption fine-structure spectroscopy, we have determined the element-specific In-As and Ga-As effective bond-stretching force constants in (In,Ga)As as a function of the alloy composition. The results reveal a striking inversion of the bond strength where the originally stiffer bond in the parent materials becomes the softer bond in the alloy and vice versa. Our findings clearly demonstrate that changes of both the individual bond length and the surrounding matrix affect the bond-stretching force constants. We thus show that the previously used common assumptions about the element-specific force constants in semiconductor alloys do not reproduce the composition dependence determined experimentally for (In,Ga)As.

Combining near infrared (NIR) luminescence and magnetic resonance (MR) contrasts in a crystal host is highly desirable for contrast agents in biomedical imaging technology, as it will enable multimodal imaging processes. In the present work, biocompatible luminescent and paramagnetic fluorapatite (FAp) nanoparticles were prepared via doping with neodymium (Nd3+) and gadolinium (Gd3+), respectively. While Nd3+-doped FAp (Nd:FAp) exhibits dopant concentration-dependent photoluminescence (PL) in the NIR spectral region, Gd3+-doped FAp (Gd:FAp) shows paramagnetic behavior and strong transverse relaxation effects resulting in MR contrastive properties. Remarkably, multimodal co-doped FAp (Nd:Gd:FAp) nanoparticles combine both properties in 1 single crystal enabling luminescence as well as MR contrast.

Semiconductor optoelectronic devices have contributed tremendously to the technological progress in the past 50-60 years. Today, they also play a key role in nanophotonics stimulated by the inherent limitations of electronic integrated circuits and the growing demand for faster communications on chip. In particular, the field of 'nanowire photonics' has emerged including the search for coherent light sources with a nano-scaled footprint. The past decade has been dedicated to find suitable semiconductor nanowire (NW) materials for such nanolasers. Nowadays, such NW lasers consistently work at room temperature covering a huge spectral range from the ultraviolet down to the mid-infrared depending on the band gap of the NW material. Furthermore, first approaches towards the modification and optimization of such NW laser devices have been demonstrated. The underlying dynamics of the electronic and photonic NW systems have also been studied very recently, as they need to be understood in order to push the technological relevance of nano-scaled coherent light sources. Therefore, this review will first present novel measurement approaches in order to study the ultrafast temporal and optical mode dynamics of individual NW laser devices. Furthermore, these fundamental new insights are reviewed and deeply discussed towards the efficient control and adjustment of the dynamics in semiconductor NW lasers.

Nanoscale light sources for intense laser emission based on CdS nanowires provide both an extremely localized emission and an ultrafast response even on sub-ps timescales. These fundamental mechanisms were investigated in the past, and practical device applications, such as on-chip laser driven applications working in different temperature ranges up to values far beyond room temperature, are becoming the focus of research. Here, we present a detailed study of the power dependent performance of CdS nanowire lasers which exhibit four distinct working regimes divided by certain threshold values. These regimes are spontaneous emission, amplified spontaneous emission, lasing, and a regime of vanishing laser oscillations due to active material degradation. The three threshold values bridging these four regimes are evaluated as a function of operating temperature, enabling the determination of an upper temperature limit for stable CdS nanowire lasing, at which the degradation threshold drops below the lasing threshold. Furthermore, the degradation mechanism of the CdS nanolasers will be proposed.