An unstable color of certain purple-, pink-, yellow-, and orange-colored natural sapphires has been noticed in the jewelry industry, and inconsistency in color severely affects their market value [1]. For example, the orange–pink hue of padparadscha sapphires, among the most valuable gemstones, could result in an unstable color owing to photochromism. Under visual evaluation, the orange coloration increases under illumination with long-wave ultraviolet (UV) lamps (365 nm) and is restored with a strong daylight equivalent source. Unlike other photochromic gemstones, the mechanism of this reversible orange coloration and the condition that initiates the photochromism have remained unresolved [1–7]. The Gemology Society is looking forward to deciphering the photochromic effect in natural sapphires to improve the consistency in color evaluation.

Two different photochromic mechanisms were previously reported in synthetic sapphire ($\alpha$-$\text {Al}_{\text {2}}\text {O}_{\text {3}}$). The first is due to the absorption of a magnesium-induced trapped-hole center, which can be created by oxidizing the sample at above 1900 K to display photochromism only below 210 K [8]. The other is due to charge redistribution between oxygen interstitials created by heat treatment after irradiation, which introduces photochromism at 773 K or above [9]. Neither of these situations exhibits the orange photochromic effect at ambient temperatures.

In this Letter, we report an investigation of the photochromic effect in natural gemstone quality sapphires at room temperature. Experimental observations of the light-induced absorption and the spectral and temporal dependence of the photochromic effect are reported. A photochromic sapphire’s hue level is confirmed to be wavelength-dependent and an equilibrium state is achievable under continuous illumination. Comparing our investigations in emission studies with several photochromic sapphires, this photochromic effect is believed to be associated with magnesium-induced trapped-hole centers and chromium.

The absorption spectra of sapphire used in this report were collected by integrating sphere-based UV-to-visible (UV–VIS) absorption spectroscopy for faceted gemstones [10]. Samples were placed above a Teflon aperture with an opening of 1-mm diameter and enclosed by a spectralon-coated integrating sphere (AvaSphere-50, Avantes) to avoid ambient light interference. Light from a shutter-controlled halogen lamp (HL-2000-FHSA, Ocean Insight) or a deuterium and tungsten halogen lamp (DH-2000-BAL, Ocean Insight) was guided by a fiber through the aperture to illuminate the sample for absorption measurement. The lamp’s transmitted light was collected from the integrating sphere’s side output port and sent to a spectrometer (QE-Pro, Ocean Insight) for analysis. A tunable light source (Hyperchromator, Mountain Photonic) was used as the excitation light to study the photochromic effect; the excitation light was guided by a fiber at an 8$^{\circ }$ incident angle to the top to illuminate the entire sample. All fibers had a core diameter of 1000 $\mathrm{\mu}$m (Avantes). The system is presented in Fig. 1.

 

Fig. 1. Experimental setup of in situ absorption spectroscopy for photochromic gemstone analysis.

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The primary advantage of this in situ spectroscopy is that the sample stays stationary during the experiment, to avoid any variation in absorption caused by misalignment or different sample orientations. The adaption of the integrating sphere in transmitted light collection also averages the absorption of the faceted gemstone to achieve $2\times 10^{-4}$ standard deviation in relative absorption. As a result, the system sensitivity to the minor photochromic-induced absorption change can be significantly improved. The corresponding photochromic absorption bands can also be identified using this system. The recorded absorption spectra can be converted to color to estimate the degree of color variation due to illumination [11]. In addition, analysis of materials with complex reflection and refraction processes would also benefit from this in situ spectroscopy. Finally, a significant amount of time can be saved, compared with the traditional wavelength-scanning absorption measurements, to enable the time-dependence analysis of the photochromic effect.

Figure 2 shows the UV–VIS absorption spectra of the selected natural orange–pink sapphire and the variation in absorption due to photochromism at ambient temperatures. The spectra were truncated at 660 nm, since chromium fluorescence began to dominate the spectrum and the longer wavelengths have a minimal influence on the visual color. Before excitation, the original body color of this sample was pink and chromium absorption bands at 410 nm and 570 nm were observed in the spectra [3]. After 20 min of illumination by 325-nm light with a 25-nm bandwidth and 0.82-mW/cm$^{2}$ power, an orange color was generated, with an increase in overall absorption. The differential spectrum indicates that the broad absorption with bands located in the VIS range at 470 nm was the origin of the enhanced orange hue. The 370-nm absorption band was previously reported as an iron-related center in low-iron yellow sapphires that created an unstable yellow color, which is different from this orange photochromic center and is outside of the scope of this study [3,12]. Finally, illuminating the sample for 120 min with 450-nm light of 25-nm bandwidth and 2.52-mW/cm$^{2}$ power faded the orange color.

 

Fig. 2. UV–VIS absorption spectra of selected natural orange–pink sapphire before experiment, after 20 min of 325-nm illumination and after 120 min of 450-nm illumination, and differential absorption spectrum before and after 325-nm excitation.

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To evaluate the wavelength dependency of the photochromic effect, the gemstone sample was illuminated under approximately 7-nm bandwidth tunable light from 600 nm to 250 nm in 10-nm steps for 30 min at each wavelength. An equilibrium state of absorption is achievable under continuous illumination. The absorption spectrum at the end of each illumination was converted to a pseudo-color under illuminant-A and D65 white reference; the hue angle (object’s basic color) in Fig. 3 represents the color variation [11]. We note that this color conversion only estimates the pseudo-color of the gemstone, owing to the complex light propagation inside the faceted gemstone; however, since the sample stays stationary during the experiment, this color conversion can still be used to estimate the color variation.

 

Fig. 3. Hue angle of selected orange–pink sapphire over a range of excitation wavelengths. A particular wavelength of light drives the sapphire to a particular color.

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Major hue angle variation was observed when illuminating the sample with 370-nm and 250-nm light, indicating the minimum energy required to start the two different transitions. The hue angle is stable under VIS light illumination, indicating that the recovery transition outperforms the enhancement transition and dominates the response. Since the photochromic effect is reversible, the color is independent of the history of the illumination, indicating that the color is optically controllable.

The time dependency of the photochromic effect was evaluated by illuminating the sample under different intensities. Figure 4(a) shows the time dependency of color enhancement under four different intensities at 325 nm. According to the results, the required time to achieve a saturated orange color under intensities of 0.81, 0.41, 0.23, and 0.08 mW/cm$^{2}$ was 48, 98, 168, and 300 min, respectively. Diminishing the color requires a significantly longer time than color enhancement. Figure 4(b) shows the color fading process from the saturated orange color over 60 min under a dark environment and four different intensities of 450-nm light. The decay was approximated by the exponential decay equation:

 

Fig. 4. Orange color enhancement and decay under various illumination intensities in selected natural orange–pink sapphire: (a) color enhancement speed under 325-nm illumination at four different intensities; (b) color decay speed under 450-nm illumination at five different intensities for 60 min. All color enhancement and decay curves are normalized to their maximum colorization.

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Table 1. Decay Parameters and Recovery Times for Color Fading of Sapphires Under Various Lighting Conditions

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The recovery time is defined as when the difference in the absorption is less than 1% compared with the original level. Based on this definition, the photochromic orange hue can be preserved in a dark environment for more than 26 days, while VIS light illumination can significantly shorten the color recovery time from weeks to several hours. We note that the 3-mW/cm$^{2}$ halogen light contributed to additional color decay in the experiment, so the actual decay rate will be lower than our calculation. Based on these results, we can conclude that both the color enhancement and diminishing rate are directly proportional to the illumination intensity.

Seven sapphires of a variety of colors, classified by their photochromic color changes as major, moderate, and minor, respectively, were used to compare their differential absorption, as shown in Fig. 5; pictures are presented in the supplementary material. Similar 470- and 370-nm absorption bands appeared in all samples. Overall, the orange–pink samples have a stronger 470-nm band than the 370-nm band, while the yellowish sapphires show the opposite. Based on this result, the 470-nm photochromic absorption band exists in unstable color sapphires with a variety of colors that introduce photochromic orange hue.

 

Fig. 5. Differential absorption spectra of pink, orange, yellow, and purple sapphires with major (Samples A and B), moderate (Samples C, D, and E), and minor (Samples F and G) levels of color enhancements under 20 min of 325-nm UV exposure.

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Although the mechanism of this reversible photochromic effect is still unknown, its differential absorption spectrum shows great similarity to sapphire’s stable orange color center, which is caused by a trapped hole related defect associated with chromium and magnesium [3,13,14]. A few parts per million atoms of this strong chromophore, which is approximately 50 times lower in concentration than $\text {Cr}^{\text {3+}}$, can create intense coloration [3,14]. The orange coloration can be introduced by annealing chromium- and magnesium-doped synthetic sapphire under a high oxygen pressure environment or by diffusing beryllium, along with a heat treatment of over 1800${^\circ }$C [3,4,14]. Once the randomly distributed electron acceptor replaces the aluminum ($\text {Al}^{\text {3+}}$) ion in the sapphire lattice, a trapped hole corresponding to one of the six neighboring oxygens is created to remain electrically neutral and to satisfy the aluminum site requirement [14,15]. In addition, in chromium-containing sapphires, the hole tends to be trapped at the $\text {Cr}^{\text {3+}}$ ion rather than $\text {Mg}^{\text {2+}}$, since $\text {Cr}^{\text {3+}}$ lies at a higher energy in the bandgap and holes are preferentially charge-compensated by ions with the highest energy states [16]. Therefore, an orange color is generated instead of a yellow-brown color [4,8]. On illuminating the sample with VIS light, the trapped-hole centers are either compensated by electrons or released [17]. It was proposed that a high localized concentration of the trapped holes also stabilizes under such conditions and prevents the trapped hole from being released by photoionization [8,18,19]. It is also possible that a lack of minority donors in sapphires gives rise to a stable orange color. Although photochromism in sapphires could also arise from charge redistribution between oxygen interstitials and direct charge transfer from impurities [9,16,20–24], photochromism induced by oxygen interstitial redistribution is accompanied with strong irradiation absorption features, which are not observed in natural sapphires [9,20,21]. Direct charge transfer in sapphires requires significantly higher energy and a high concentration of defects to initiate the absorption changes [16,23].

Experimental evidence is provided here to support the hypothesis that the photochromic effect is chromium-dependent. Figure 6(a) shows the correlation between the calculated hue value and the chromium emission during the photochromic experiment in Sample C. The sample was held under 255-nm illumination to maximize the color enhancement for 60 min, and 450-nm illumination for 120 min. Immediately after each absorption measurement, 600-nm illumination was used to excite the $\text {Cr}^{\text {3+}}$ emission from the sample. The differential emission was generated by dividing the 693-nm emission peak by the initial level. In addition to some variation due to power fluctuation of the tunable light source, the distributions of the two curves mirror one another. As the hue enhanced, the intensity of the 693-nm emission decreased. The same correlation in the other six sapphires is presented in the supplementary material.

 

Fig. 6. Relationship between photochromic effect and chromium emission in Sample C: (a) variation in hue angle and differential emission of $\text {Cr}^{\text {3+}}$ under 255-nm illumination and 450-nm illumination; (b) PLE spectra of $\text {Cr}^{3+}$ emission before and after 255-nm illumination, and after 450-nm illumination.

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The minor reduction of the $\text {Cr}^{\text {3+}}$ concentration might not be detectable by optical absorption, since the $\text {Cr}^{\text {3+}}$ absorption bands overlap the 470-nm absorption. To increase the sensitivity, photoluminescence excitation (PLE) spectroscopy was applied to study the correlation between the $\text {Cr}^{\text {3+}}$ emission and photochromism in sapphire [25]. Figure 6(b) shows the PLE spectra of the $\text {Cr}^{\text {3+}}$ emission in Sample C before and after 60 min of 255-nm excitation and after 120 min of 450-nm excitation. The $\text {Cr}^{\text {3+}}$ emission decreased along the entire sensing range after UV illumination and recovered after VIS illumination. Since the PLE spectra correspond to the defect concentration, this result indicates that the $\text {Cr}^{\text {3+}}$ concentration reduction is due to the formation of the orange coloration, and supports the hypothesis that the photochromic effect is indeed chromium-related. Further trace element analysis between samples with different photochromic effect levels is compared in the supplementary material. We note that, in natural sapphire, the interaction between trace elements is quite complicated. Low-concentration trace elements that are insufficient to create an observable absorption may still play a significant role in charge compensation [4]. Therefore, further investigation is required to reveal the mechanism behind the photochromic effect.

To summarize, in this Letter, we report the spectral and temporal properties of a reversible photochromic effect at ambient temperature in natural gemstone sapphires. In situ absorption spectroscopy has been developed for faceted gemstone absorption analysis and the results show that illumination with UV light of $\leq$370 nm and VIS light of $\geq$410 nm introduces and removes the orange coloration, respectively, which was accompanied by an absorption band at 470 nm. The equilibrium state of colorization is achievable under continuous illumination, indicating that the photochromic effect is optically controllable. Both the color enhancement and diminishing rates are proportional to the excitation intensity. The enhanced orange color diminishes slowly over several weeks, but strong VIS light illumination may significantly accelerate the color recovery. By using absorption analysis, we experimentally confirmed that the same photochromic effect could appear in common sapphires of a variety of colors. Based on similarities in the absorption spectra, this photochromic effect is possibly due to the interaction between a magnesium-induced trapped hole and chromium. The opposite trends between the 470-nm absorption and the $\text {Cr}^{\text {3+}}$ emission, along with a reduction in the excitation efficiency of $\text {Cr}^{\text {3+}}$, support this hypothesis.

To maintain transparency in gemstone trading, color evaluation should consider photochromic effects. Since many unstable colors in sapphires are due to intentional or accidental UV irradiation occurring prior to color analysis, additional screening should be applied to confirm that the gemstones are evaluated in their stable color state. Enhanced coloration contributed by color evaluation lamps with UV components should be avoided. Similar in situ spectroscopy can be applied to study other photochromic or thermochromic effects [5,6,26]. An ideal lighting environment and protocol for each photochromic defect center, and the contribution of trace element concentrations to photochromic effects, will be evaluated in future research.