Determination of the Best Empiric Method to Quantify the Amplified Spontaneous Emission Threshold in Polymeric Active Waveguides

Abstract

<jats:p>Amplified Spontaneous Emission (ASE) threshold represents a crucial parameter often used to establish if a material is a good candidate for applications to lasers. Even if the ASE properties of conjugated polymers have been widely investigated, the specific literature is characterized by several methods to determine the ASE threshold, making comparison among the obtained values impossible. We quantitatively compare 9 different methods employed in literature to determine the ASE threshold, in order to find out the best candidate to determine the most accurate estimate of it. The experiment has been performed on thin films of an homopolymer, a copolymer and a host:guest polymer blend, namely poly(9,9-dioctylfluorene) (PFO), poly(9,9-dioctylfluorene-cobenzothiadiazole) (F8BT) and F8BT:poly(3- hexylthiophene) (F8BT:rrP3HT), applying the Variable Pump Intensity (VPI) and the Variable Stripe Length (VSL) methods. We demonstrate that, among all the spectral features affected by the presence of ASE, the most sensitive is the spectral linewidth and that the best way to estimate the ASE threshold is to determine the excitation density at the beginning of the line narrowing. We also show that the methods most frequently used in literature always overestimate the threshold up to more than one order of magnitude.</jats:p>

Amplified Spontaneous Emission Threshold Dependence on Determination Method in Dye-Doped Polymer and Lead Halide Perovskite Waveguides

“…Among them, the visual method is the most rapid to apply, as it does not need any data analysis, but only an experimental system allowing a fine control of the excitation density, in order to be more sensitive to spectral modifications. The fact that the FWHM narr method often provides low ASE thresholds confirms the high sensitivity of the spectral linewidth to the presence of ASE, as previously observed in polymeric waveguides [35]. However, in light of the evidence that the FWHM can depend on the excitation density even when only spontaneous emission is present (see Figures 2f and S2c) and that the ASE appearance could also result in an initial FHWM increase (see Figure 3c), we suggest a FWHM deviation (FWHM dev ) method, searching for the lowest pump density that provides a variation with respect to the initial conditions; this can be a narrowing, a broadening, or a slope change in the FWHM plot.…”

“…In the I TOT method, the procedure is applied to the plot of the total intensity, integrated across the sample emission range. In the I ASE method, only the ASE integrated intensity is considered, after separating its contribution from the spectrally integrated spontaneous emission one, as described in Ref [35]. Finally, in the I peak method, the intensity at the ASE peak wavelength is considered.…”

“…However, the ASE threshold does not have a unique definition and several methods have been adopted to determine it, making comparison between different values extremely difficult. A first step in the investigation of the ASE threshold dependence on the determination method was performed on polymeric active waveguides [35], evidencing differences of up to one order of magnitude between the threshold values obtained with different methods for the same film.…”

Near‐Infrared Lasing in Four‐Zigzag Edged Nanographenes by 1D versus 2D Electronic π‐Conjugation

“…Regarding the ASE thresholds of the investigated NG-based films, they were determined from plots of I out and linewidth (defined as the full width at half maximum, FWHM) versus E pump (see illustrative example in Figure S4, Supporting Information). Among the different ways reported in literature for determining E th-ASE , [30] we have chosen the one based on the emission linewidth (FWHM) evolution with the pump energy density to enable comparisons to previously reported values with similar compounds, for which this method was used. [22,26] Thus, E th-ASE corresponds to the pump energy density at which the FWHM reaches the average value between the ones observed at low and high excitation density, which approximately coincides with that at which a drastic slope change in I out is seen.…”

Organic Semiconductor Micro/Nanocrystals for Laser Applications

“…ASE does not require optical feedback because light amplification takes place by a single pass along the optical gain medium. ASE is typical of organic waveguides (slabs, 1D planar waveguides or optical fibers) [ 107 , 108 , 109 , 110 , 111 , 112 ] but can also be supported by organic crystals [ 68 , 113 , 114 ]. The ASE output is constituted by a spectrally broader emission linewidth (~10 nm) corresponding to the amplified waveguided mode along the crystal.…”