Understanding the interplay between optical pulse parameters and ultrafast material response is critical in achieving efficient and controlled laser nanomachining. In general, the key to initiate material processing is the deposition of a sufficient energy density within the electronic system. In dielectrics this critical energy density corresponds typically to a plasma frequency in the near-IR spectral region. Creating this density instantaneously with ultrashort laser pulses of a few tens of femtoseconds pulse duration in the same spectral region, the penetration depth into the material will strongly decrease with increasing electron density. Consequently, staying below this critical density will allow deep penetration depths. This calls for delayed ionization processes to deposit the energy for processing, thus introducing the temporal structure of the laser pulses as a control parameter. In this contribution we demonstrate this concept experimentally and substantiate the physical picture with numerical calculations. Bandwidth-limited pulses of 30 fs pulse duration are stretched up to 1.5 ps either temporally symmetrically or temporally asymmetrically. The interplay between pulse structure and material response is optimally exploited by the asymmetrically structured temporal Airy pulses leading to the inherently efficient creation of high aspect ratio nanochannels. Depths in the range of several micrometers and diameters around 250 nm are created within a single laser shot and without making use of selffocusing and filamentation processes. In addition to the machining of nanophotonic devices in dielectrics, the technique has the potential to enhance laser-based nanocell surgery and cell poration techniques.
Optical spectrometers and sensors have gained enormous importance in metrology and information technology, frequently involving the question of size, resolution, sensitivity, spectral range, efficiency, reliability, and cost. Nanomaterials and nanotechnological fabrication technologies have huge potential to enable an optimization between these demands, which in some cases are counteracting each other. This paper focuses on the visible and near infrared spectral range and on five types of optical sensors (optical spectrometers): classical grating-based miniaturized spectrometers, arrayed waveguide grating devices, static Fabry–Pérot (FP) filter arrays on sensor arrays, tunable microelectromechanical systems (MEMS) FP filter arrays, and MEMS tunable photonic crystal filters. The comparison between this selection of concepts concentrates on (i) linewidth and resolution, (ii) required space for a selected spectral range, (iii) efficiency in using available light, and (iv) potential of nanoimprint for cost reduction and yield increase. The main part of this review deals with our own results in the field of static FP filter arrays and MEMS tunable FP filter arrays. In addition, technology for efficiency boosting to get more of the available light is demonstrated.
Transition metal doped colloidal nanomaterials (TMDCNMs) have recently attracted attention as promising nano‐emitters due to dopant‐induced properties. However, despite ample investigations on the steady‐state and dynamic spectroscopy of TMDCNMs, experimental understandings of their performance in stimulated emission regimes are still elusive. Here, the optical gain properties of copper‐doped CdSe colloidal quantum wells (CQWs) are systemically studied with a wide range of dopant concentration for the first time. This work demonstrates that the amplified spontaneous emission (ASE) threshold in copper‐doped CQWs is a competing result between the biexciton formation, which is preferred to achieve population inversion, and the hole trapping which stymies the population inversion. An optimum amount of copper dopants enables the lowest ASE threshold of ≈7 µJ cm−2, about 8‐fold reduction from that in undoped CQWs (≈58 µJ cm−2) under sub‐nanosecond pulse excitation. Finally, a copper‐doped CQW film embedded in a vertical cavity surface‐emitting laser (VCSEL) structure yields an ultralow lasing threshold of 4.1 µJ cm−2. Exploiting optical gain from TMDCNMs may help to further boost the performance of colloidal‐based lasers.
For the first time, Gallium Nitride(GaN)-based Gunn diodes with side-contact and fieldplate technologies were fabricated and measured with reliable characteristics. A high negative differential resistance (NDR) region was characterised for the GaN Gunn effect using side-contact technology. The I-V measurement of the THz diode showed the ohmic and the Gunn effect region with high forward current of 0.65 A and high current drop of approximately 100 mA for a small ring diode width w d of 1.5 µm with 600 nm effective diode height h d at a small threshold voltage of 8.5 V. This THz diode worked stable due to good passivation as protection from electro-migration and ionisation between the electrodes as well as a better heat sink to the GaN substrate and large side-contacts. The diodes can provide for this thickness a fundamental frequency in the range of 0.3 -0.4 THz with reliable characteristics.
The lateral resolution in microscopic imaging generally depends on both, the wavelength of light and the numerical aperture of the microscope objective lens. To quantify the lateral resolution Ernst Abbe considered an optical grating illuminated by plane waves. In contrast, the Rayleigh criterion holds for two point sources or point scatterers separated by a lateral distance, which are supposed to emit spherical waves. A portion of each spherical wave is collected by the objective lens and results in an Airy disc corresponding to a diffraction limited intensity point spread function (PSF). If incoherent illumination is employed the intensity PSFs related to different scatterers on an object are added resulting in the well-known Rayleigh resolution criterion. In interference microscopy instead of the intensity the electric field scattered or diffracted by an object will be affected by the transfer function of the optical imaging system. For a reflective object the lateral resolution of an interference microscope can be again characterized by the Abbe limit if the object under investigation is a grating. However, if two irregularities on a flat surface are being imaged the resolution no longer obeys the Rayleigh criterion. Instead, it corresponds to an optical system with an annular aperture and thus surpasses the prediction given by the Rayleigh criterion. This holds true for both, amplitude as well as phase objects, as it will be elucidated in this study by theoretical considerations, simulation results and an experimental proof of principle.
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