Amplified Spontaneous Emission in Paper

Abstract

Industrially produced copy paper is presented as a novel optical material for narrow-band stimulated emission. Fluorescent brightening agents (FBA) such as Calcofluor White provide enough gain to exceed the threshold for amplified spontaneous emission. By applying an additional dye such as Coumarin 307, or simply a highlighter pen, the emission line can be shifted from blue (~440 nm), towards turquoise (>480 nm), up to orange (590 nm) which can be useful for spectroscopic applications. These properties are demonstrated in two papers, a commercial copy paper and a FBA-stained calligraphy paper.

Figure 1

Emission spectra and threshold determination. Normalized emission spectra of (a) copy paper (blue) and (b) calligraphic paper stained with Calcofluor White (orange) at given input energy. It is clearly visible, that the FWHM decreases with increasing input intensity. For clarity the spectra at 2.83 µJ were smoothed with a Savitzky-Golay filter using a third order polynomial and taking a total of 20 points into account. (c) Shows the full width at half maximum at given energy density for paper A and B, respectively. The inflection point of the logistic fit function (see supplementary information) represents the ASE threshold. (d) From the outside to the inside: Fluorescence spectrum of paper B stained with Calcofluor White, emission spectra for a single fibre, a bundle of fibres taken from a stained calligraphic paper as well as the paper itself.

Figure 2

Fluorescence microscopic images of the analysed cellulose fibre (a), a bundle of fibres. (b) The tape can be seen in the background. Paper B is depicted in (c). The emission spectra are presented in Fig. 1(d).

Figure 3

ASE emission spectra of the original paper A (blue) and B (purple) with only the corresponding FBA as well as colourized with Coumarin 307 (black lines).

Figure 4

(a) Used highlighter pens as well as ready to use paper A samples. The resulting ASE spectra as well as the emission from paper A and paper A stained with Coumarin 307 are shown in (b). (c,d) The graphs show the FWHM at given (logarithmic) energy densities for threshold determination of highlighter dyes on paper A and paper B, respectively.