Abstract: 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 mu m (1 eV) and 10 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.
Abstract: 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 mu m. 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.
Abstract: 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.
Abstract: In this contribution we experimental examine the ellipticity dependence of strong off resonance absorption and high harmonic generation (HHG) in the wide band gap semiconductor ZnO upon irradiation with intense mid-IR laser pulses. HHG in semiconductors originates from nonlinear intraband currents and interband transitions of the electrons driven by the strong mid-IR fields [Ghimire]. Here, we show that the intra- and interband contribution to HHG in a ZnO thin film are affected differently by the driving laser ellipticity.
Abstract: High harmonic generation (HHG) in a single-atomic-layer non-centrosymmetric semiconductor is investigated experimentally for different driving laser field polarizations. The ellipticity enhanced even-order HHG for certain crystal orientations reveals linked laser and valley polarizations.
Abstract: 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.
Abstract: In this work, we demonstrate a discrete dispersion scan scheme using a low number of flat windows to vary the dispersion of laser pulses in discrete steps. Monte Carlo simulations indicate that the pulse duration can be retrieved accurately with less than 10 dispersion steps, which we verify experimentally by measuring few-cycle pulses and material dispersion curves at 3 and 10 µm wavelength. This minimal measuring scheme using only five optical components without the need for linear positioners and interferometric alignment can be readily implemented in many wavelength ranges and situations.
Abstract: High-harmonic generation (HHG) in crystals offers a simple, affordable and easily accessible route to carrier-envelope phase (CEP) measurements, which scales favorably towards longer wavelengths. We present measurements of HHG in ZnO using few-cycle pulses at 3.1 µm. Thanks to the broad bandwidth of the driving laser pulses, spectral overlap between adjacent harmonic orders is achieved. The resulting spectral interference pattern provides access to the relative harmonic phase, and hence, the CEP.
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2020)
Abstract: This thesis studies extreme nonlinear optical phenomena in highly excited ZnO semiconductor samples. ZnO with a band gap of 3.2 eV, in the near-ultraviolet spectral range, is irradiated with far-off resonance strong light fields in the near (0.8 µm, 1.5 eV) to the far-infrared (10 µm, 0.13 eV). Specifically, the coherent conversion of laser light into high orders of the fundamental frequency, also known as high harmonic generation (HHG) and optically pumped lasing were investigated.
Abstract: 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 100 nm diameter wires with an average distance of 200 nm.
Abstract: 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.
Abstract: We present a detailed investigation of X-ray emission from both flat and nanowire zinc oxide targets irradiated by 60 fs 5E16 W/cm^2 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 the development of compact ultrafast X-ray sources with an enhanced flux of hard X-ray emission that could find wide applications in high energy density (HED) physics.
Abstract: Semiconductor nanowire (NW) lasers attract a lot of attention as potential elements of nanophotonic circuits and lab-on-a chip devices. Here, we report on the experimental investigation of stimulated near ultraviolet (NUV) emission, pumped by three-photon absorption from near infrared femtosecond laser pulses, from ZnO NW arrays of different morphologies and compare it to the bulk. The spectrally and temporally resolved measurements of the NUV emission show both strong enhancements in the absorption and emission properties of the nanowire arrays compared to bulk samples. Thus, we determine a many times higher three-photon absorption in the nanostructure morphology compared to the bulk material. Furthermore, the threshold pumping intensity for stimulated emission in a vertically oriented nanowire array is twice lower and the emission onset time is shorter than in randomly oriented arrays, revealing strong influence of the macroscopic nanowire arrangement.