The formation of temporal dissipative solitons in optical microresonators enables compact, high repetition rate sources of ultra-short pulses as well as low noise, broadband optical frequency combs with smooth spectral envelopes. Here we study the influence of the resonator mode spectrum on temporal soliton formation. Using frequency comb assisted diode laser spectroscopy, the measured mode structure of crystalline MgF2 resonators are correlated with temporal soliton formation. While an overal general anomalous dispersion is required, it is found that higher order dispersion can be tolerated as long as it does not dominate the resonator's mode structure. Mode coupling induced avoided crossings in the resonator mode spectrum are found to prevent soliton formation, when affecting resonator modes close to the pump laser. The experimental observations are in excellent agreement with numerical simulations based on the nonlinear coupled mode equations, which reveal the rich interplay of mode crossings and soliton formation.Temporal dissipative solitons [1][2][3] can be formed in a Kerr-nonlinear optical microresonator [4] with anomalous dispersion that is driven by a monochromatic continuous wave pump laser. These temporal solitons are sech 2 -shaped ultra-short pulses of light circulating inside the microresonator, where the temporal width of the solitons is fully determined by the resonator dispersion and nonlinearity as well as the pump power and pump laser detuning [4,5]. It has been shown that the pump laser parameters can be used to control the number of solitons circulating in the microresonator. In particular the single soliton state, where one single soliton is circulating continuously inside the resonator, is of high interest for applications. In the time domain soliton formation in microresonators allows for the generation of periodic ultrashort femto-second pulses, which in the frequency domain correspond to a frequency comb spectrum with smooth sech 2 -shaped spectral envelope. The free spectral range (FSR) of the resonator, typically in the range of tens to hundreds of GHz, determines the pulse repetition rate (equivalent to the frequency comb line spacing). Soliton formation is related to four-wave mixing based frequency comb generation in microresonators [6][7][8][9][10][11][12][13][14][15], where low and high noise operating regimes [12,16,17] have been identified. Here, techniques such as δ − ∆-matching [17], self-injection locking [18,19] or parametric seeding [20] can be used to achieve low noise operation. In contrast to these low noise four-wave mixing based combs (also termed Kerr combs), the transition to the soliton regime [17] offers a unique combination of features, such as intrinsic low noise performance, direct pulse generation in the microresonator [4,21,22], and smooth spectral envelope as shown in Figure 1. These properties are critical to applications in e.g. telecommunications [23][24][25], low phase noise microwave generation [18,26]

Films of nanocrystalline silicon (nc-Si) were prepared from hydrogenated amorphous silicon (a-Si:H) by using rapid thermal annealing. The formed nc-Si films were subjected to stain etching in hydrofluoric acid solutions in order to passivate surfaces of nc-Si. The optical reflectance spectroscopy revealed the nc-Si formation as well as the high optical quality of the formed films. The Raman scattering spectroscopy was used to estimate the mean size and volume fraction of nc-Si in the annealed films, which were about 4 to 8 nm and 44 to 90%, respectively, depending on the annealing regime. In contrast to as-deposited a-Si:H films, the nc-Si films after stain etching exhibited efficient photoluminescence in the spectral range of 600 to 950 nm at room temperature. The photoluminescence intensity and lifetimes of the stain etched nc-Si films were similar to those for conventional porous Si formed by electrochemical etching. The obtained results indicate new possibilities to prepare luminescent thin films for Si-based optoelectronics.

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