Lasing in disordered media presents both theoretical challenges and practical opportunities.

/pdf/the-physics-and-applications-of-random-lasers-12d4cwd4vp.pdf

The physics and applications of random lasers

High aspect ratio (HAR) silicon etch is reviewed, including commonly used terms, history, main applications, different technological methods, critical challenges, and main theories of the technologies. Chronologically, HAR silicon etch has been conducted using wet etch in solution, reactive ion etch (RIE) in low density plasma, single-step etch at cryogenic conditions in inductively coupled plasma (ICP) combined with RIE, time-multiplexed deep silicon etch in ICP-RIE configuration reactor, and single-step etch in high density plasma at room or near room temperature. Key specifications are HAR, high etch rate, good trench sidewall profile with smooth surface, low aspect ratio dependent etch, and low etch loading effects. Till now, time-multiplexed etch process is a popular industrial practice but the intrinsic scalloped profile of a time-multiplexed etch process, resulting from alternating between passivation and etch, poses a challenge. Previously, HAR silicon etch was an application associated primarily with microelectromechanical systems. In recent years, through-silicon-via (TSV) etch applications for three-dimensional integrated circuit stacking technology has spurred research and development of this enabling technology. This potential large scale application requires HAR etch with high and stable throughput, controllable profile and surface properties, and low costs.

High aspect ratio silicon etch: A review

Unlike conventional lasers, diffusive random lasers (DRLs) have no resonator to trap light and no high-Q resonances to support lasing. Because of this lack of sharp resonances, the DRL has presented a challenge to conventional laser theory. We present a theory able to treat the DRL rigorously and provide results on the lasing spectra, internal fields, and output intensities of DRLs. Typically DRLs are highly multimode lasers, emitting light at a number of wavelengths. We show that the modal interactions through the gain medium in such lasers are extremely strong and lead to a uniformly spaced frequency spectrum, in agreement with recent experimental observations.

Strong Interactions in Multimode Random Lasers

An intensive study has been performed to understand and tune deep reactive ion etch (DRIE) processes for optimum results with respect to the silicon etch rate, etch profile and mask etch selectivity (in order of priority) using state-of-the-art dual power source DRIE equipment. The
research compares pulsed-mode DRIE processes (e.g. Bosch technique) and mixed-mode DRIE processes (e.g. cryostat technique). In both techniques, an inhibitor is added to
fluorine-based plasma to achieve directional etching, which is formed out of an oxide-forming (O2) or a fluorocarbon (FC) gas (C4F8 or CHF3). The inhibitor can be introduced together with the etch gas, which is named a mixed-mode DRIE process, or the inhibitor can be added in a
time-multiplexed manner, which will be termed a pulsed-mode DRIE process. Next, the most convenient mode of operation found in this study is highlighted including some remarks to
ensure proper etching (i.e. step synchronization in pulsed-mode operation and heat control of the wafer). First of all, for the fabrication ......
 Enjoy reading . Henri Jansen 18 June 2008

https://iopscience.iop.org/article/10.1088/0960-1317/19/3/033001/pdf

Black silicon method X: a review on high speed and selective plasma etching of silicon with profile control: an in-depth comparison between Bosch and cryostat DRIE processes as a roadmap to next generation equipment

Recent theoretical work on random lasing predicts that the occurrence of narrow lasing spikes can be caused by both localized and extended modes of light1,2,3. Typical random lasing spikes have been observed in powders of zinc oxide nanoparticles4, but identifying the possible degrees of localization of such modes has until now been an open issue5. In this Letter we present an experimental procedure that directly extracts the area of localization of the modes. This method relies on the investigation of micro-structured fields of zinc oxide powder, which also allows direct correlation to the local structural properties of the ensemble of subwavelength particles by scanning electron microscopy. We find that lasing from both localized and extended modes can be observed simultaneously. Our observation also corroborates the prediction that localized modes have a lower loss rate than that of extended modes5. Whether the electromagnetic fields in random lasers are localized or extended is a topic of ongoing debate. Now, the localization of modes in micro-structured ZnO powder is experimentally determined and lasing from both kinds of modes (localized and extended) shown to exist simultaneously.

Co-existence of strongly and weakly localized random laser modes

We have experimentally studied the distribution of the spatial extent of modes and the crossover from essentially single-mode to distinctly multimode behavior inside a porous gallium phosphide random laser. This system serves as a paragon for random lasers due to its exemplary high index contrast. In the multimode regime, we observed mode competition. We have measured the distribution of spectral mode spacings in our emission spectra and found level repulsion that is well described by the Gaussian orthogonal ensemble of random-matrix theory.

/pdf/spatial-extent-of-random-laser-modes-2rzwgluext.pdf

Spatial Extent of Random Laser Modes

We report on the fabrication of periodic arrays of deep nanopores with high aspect ratios in crystalline silicon. The radii and pitches of the pores were defined in a chromium mask by means of deep UV scan and step technology. The pores were etched with a reactive ion etching process with SF6, optimized for the formation of deep nanopores. We have realized structures with pitches between 440 and 750 nm, pore diameters between 310 and 515 nm, and depth to diameter aspect ratios up to 16. To the best of our knowledge, this is the highest aspect ratio ever reported for arrays of nanopores in silicon made with a reactive ion etching process. Our experimental results show that the etching rate of the nanopores is aspect-ratio-dependent, and is mostly influenced by the angular distribution of the etching ions. Furthermore we show both experimentally and theoretically that, for sub-micrometer structures, reducing the sidewall erosion is the best way to maximize the aspect ratio of the pores. Our structures have potential applications in chemical sensors, in the control of liquid wetting of surfaces, and as capacitors in high-frequency electronics. We demonstrate by means of optical reflectivity that our high-quality structures are very well suited as photonic crystals. Since the process studied is compatible with existing CMOS semiconductor fabrication, it allows for the incorporation of the etched arrays in silicon chips.

http://iopscience.iop.org/article/10.1088/0957-4484/19/14/145304/pdf

Periodic arrays of deep nanopores made in silicon with reactive ion etching and deep UV lithography

The effects of unintended deviations from ideal inverse woodpile photonic crystals on the photonic band gap are discussed. Such deviations occur during the nanofabrication of the crystal. By computational analyses it is shown that the band gap of this type of crystal is robust to most types of deviations that relate to the radii, position, and angular alignment of the pores. However, the photonic band gap is very sensitive to tapering of the pores, i.e., conically shaped pores instead of cylindrical pores. To obtain three-dimensional inverse woodpile photonic crystals with a large volume, our work shows that with modern fabrication performances, reduction in tapering contributes most significantly to a high photonic strength.

The influence of fabrication deviations on the photonic band gap of three-dimensional inverse woodpile nanostructures

In this report the effects of unintended deviations from ideal inverse woodpile photonic crystals on the band gap are discussed. These deviations occur during the nanofabrication of the crystal. By computational analyses it is shown that the band gap of this type of crystal is robust to most types of deviations that relate to the radii, position and angular alignment of the pores. However, the photonic band gap is very sensitive to tapering of the pores, i.e., conically shaped pores instead of cylindrical pores. To obtain three-dimensional inverse woodpile photonic crystals with a large volume, our work shows that with modern fabrication performances, tapering contributes most significantly to a reduction in the photonic strength of inverse woodpile photonic crystals.

R. Willem Tjerkstra

Papers

Spatial Extent of Random Laser Modes

Periodic arrays of deep nanopores made in silicon with reactive ion etching and deep UV lithography

The influence of fabrication deviations on the photonic band gap of three-dimensional inverse woodpile nanostructures

The role of fabrication deviations on the photonic band gap of 3D inverse woodpile nanostructures