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To identify the simplest organic structure for an emitter, we focused on 2,5-dimethoxybenzene-1,4-dicarboaldehyde. This symmetric molecule has a very low molecular weight (MW=194), a single benzene unit, and consists of only three elements (H, C and O). It forms highly efficient and pure emitting crystals (λem=499 nm, ΦF =0.42, FWHM=42 nm) due to the rigid structure based on the single benzene framework and four intramolecular hydrogen bonds between electron-donating methoxy and electron-accepting aldehyde groups. This crystal acts as a good optical waveguide with pure green emission (FWHM=34 nm) and very low loss coefficient (0.00120 dB/μm).

We investigated the relation between Sr# [= 100 Sr/(Sr + Ca) in mol] of calcites and their Raman spectra for Sr-doped calcite samples with Sr# of 0–13.2. We selected 3 major Raman peaks observed at 150–155 cm−1 (peak A), 274–282 cm−1 (peak B), and 710–712.5 cm−1 (peak C). With increasing Sr#, the peaks shifted monotonously to lower wavenumbers. We obtained a linear expression for peak B as Sr# = −1.734 ΔνB + 76.93, where ΔνB is a separation between peak B and a neon emission line of 237.07 cm−1 in Raman shift. The present spectroscopic analysis can ascertain Sr# of calcite with precision of ±1.26 in the range of Sr# of 0–13.2.