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Cryptography based on the absorption/emission features of multicolor semiconductor nanocrystal quantum dots

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Abstract

Further to the optical coding based on fluorescent semiconductor quantum dots (QDs), a concept of using mixtures of multiple single-color QDs for creating highly secret cryptograms based on their absorption/emission properties was demonstrated. The key to readout of the optical codes is a group of excitation lights with the predetermined wavelengths programmed in a secret manner. The cryptograms can be printed on the surfaces of different objects such as valuable documents for security purposes.

©2004 Optical Society of America

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Figures (4)

Fig. 1.
Fig. 1. Absorption spectra of three single-color commercial CdSe QDs with emission (concentration) at 535±10 nm (0.25 mg/mL), 585±10 nm (0.17 mg/mL), and 640±10 nm (0.083 mg/mL), respectively. The inset photo was taken under 380 nm UV excitation.
Fig. 2.
Fig. 2. Emission spectra of a mixture of three single-color QDs in toluene excited at a λex ranging from 350 to 510 nm with a constant interval of 10 nm. The excitation peaks (λex=510, 500, 490, 480, 470, 460 and 450 nm) and three diffraction-induced second-order peaks (from λex=350, 360 and 370 nm) are also recorded in the measurement range. Inset: Two sets of selected spectral data normalized to a 10-level (0–9) intensity scale.
Fig. 3.
Fig. 3. Emission spectra of a QD/poly(methyl methacrylate)/toluene solution excited with a stepwise scanning of λex ranging from 360 to 480 nm with a constant interval of 10 nm. Inset: Three sets of selected spectral data normalized to a 10-level (0–9) intensity scale.
Fig. 4.
Fig. 4. QD-based fluorescent cryptogram – creating and reading the codes under a programmed sequence of excitation.
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