Storing and retrieving arbitrary data sets in three dimensions

The physical mechanisms underlying NV charge dynamics are presented in Fig. 1A: green illumination (for example, at 532 nm) ionizes an NV− via the consecutive absorption of two photons, thus transforming the NV− into an NV0 (that is, a neutrally charged NV). Conversely, green light can drive a neutral NV into its excited state where absorption of an electron from the valence band reconverts NV0 back into NV− (Fig. 1A). Therefore, an NV center exposed to green light dynamically alters its charge state at a rate that depends on the illumination intensity (12, 13). This behavior changes with the use of red light (for example, 632 nm), because photons of this wavelength can only excite NV− but not NV0. Consequently, strong red illumination ionizes NV− to produce NV0, but the back-conversion process is largely inhibited.

Fig. 1 Charge manipulation and readout in diamond. (A) Energy diagram for NV− and NV0. In (1) and (2), the successive absorption of two photons (wavy arrows) of energy equal or greater than 1.95 eV (637 nm) propels the excess electron of an NV− into the conduction band, leaving the defect in the neutral ground state (solid arrows). In (3) and (4), an NV0 consecutively absorbs two photons of energy equal or greater than 2.16 eV (575 nm) transforming into NV−. CB, conduction band; VB, valence bands. (B) Top: A binary pattern on an NV−-rich background is imprinted via spatially selective red illumination (632 nm, 350 μW, 100 ms per pixel). Bottom: Starting from an NV−-depleted background, the pattern results from selective illumination with green laser light (532 nm, 30 μW, 5 ms per pixel). From left to right, images are the result of three successive readouts of the same original imprint via a red scan (200 and 150 μW for the upper and lower rows, respectively). In all cases, the image size is 100 × 100 pixels, and the integration time is 1 ms per pixel. kcps, kilocounts/s.

In our experiments, we used a type 1b diamond crystal with an approximate NV concentration of 0.4 parts per million (ppm). Two intuitive forms of charge patterning are presented in Fig. 1B: upon initializing the focal plane into NV− (upper row), we convert select portions into NV0 by successively parking a strong red beam at the desired pixels for a predefined time interval. Given the near quadratic dependence of the ionization rate on the illumination intensity (13), the resulting NV charge map can be revealed via a weak red laser scan. In this regime, charge ionization during readout is minimal, and the fluorescence—brighter in NV−-rich areas—correlates with NV− concentration. The lower row in Fig. 1B illustrates the converse approach where patterning is attained by parking a green beam on a “bleached” (that is, NV−-deprived) plane. Exposure to green light locally reconverts NV0 into NV−, and subsequent fluorescence imaging—via a weak red scan—unveils the expected bright pattern on an otherwise dark background.

Both encoding protocols yield comparable pixel definition (about 0.8 μm; defined here by a numerical aperture of 0.42 of the objective; see fig. S1). However, green or red imprints respond differently to multiple red laser readouts (middle and left columns in Fig. 1B). Both exhibit a gradual loss of contrast, but the impact is substantially stronger on the green imprint. Remarkably, observations on test patterns over a period of a week show no noticeable change, provided that the diamond crystal is kept in the dark. Thus, data storage in diamond must be viewed as semipermanent in the sense that a “refresh” protocol is required, conditional on the number of readouts but independent on the total elapsed time.

To derive a more quantitative metric, we compare the fluorescence response from an arbitrary (but fixed) site of the diamond crystal to multiple readouts (fig. S2). We find not only that red imprinting features a slower fluorescence decay but also that the relative contrast between “bright” and “dark” remains high over tens of readouts. This is not the case for a green imprint where the contrast first vanishes and then inverts. The physics at play is complex, and more investigation will be needed to gain a fuller understanding. However, initial work (14) indicates that the local N+ content—higher when green light is present during the encoding process—plays an important role.

Unlike photorefractive polymers (which are prone to degradation upon repeated light exposure) (15) or gold nanorods (which undergo a permanent photoinduced shape change) (16, 17), the charge state of the NV center can be reversibly altered with no accumulated effect, hence allowing one to erase and rewrite information a virtually limitless number of times. A proof-of-principle demonstration is presented in Fig. 2A (see also fig. S3): after resetting the NV− content via a strong green laser scan (step 1), we proceeded to write the focal plane through a red imprint, which we then exposed via a weak red scan (step 2); the same protocol was then repeated to encode and read out a new, different pattern (steps 3 and 4). Note that unlike Fig. 1—where the brightness in each pixel takes one of two possible values—the images in Fig. 2A are imprinted using a variable exposure time per pixel. In the present case, we bin the illumination times into five different durations, which correspondingly lead to discernible levels of fluorescence, that is, the equivalent of a multivalued bit (fig. S1). The result is a concomitant boost of the information density, illustrated here via the grayscale images. The number of levels is largely defined by the signal-to-noise ratio (SNR) of the optical detection, which, in turn, grows with the square root of the readout time and NV density. In practice, considerations such as background noise and sample homogeneity must also be taken into account. For the conditions herein, up to eight different levels seem realistic (see fig. S1), although more are conceivable, for example, if the sample is engineered to host a higher NV concentration.

Fig. 2 Diamond as a 3D read/write memory. (A) Starting from a blank ensemble of NV− centers (1), information can be written (2), erased (3), and rewritten (4). In (1) and (3), a green laser scan (1 mW at 1 ms per pixel) was used to reset the target plane to a bright state. In (2) and (4), images were imprinted via a red laser scan with a variable exposure time per pixel (from 0 to 50 ms). Note the gray scale in the resulting images corresponding to multivalued (as opposed to binary) encoding. The same scale bar applies to all four images. (B) Information can be stored and accessed in three dimensions, as demonstrated for the case of a three-level stack. Observations over a period of a week show no noticeable change in these patterns for a sample kept in the dark. In (A) and (B), readout is carried out via a red laser scan (200 μW at 1 ms per pixel). The image size is 150 × 150 pixels in all cases.

Because the illumination intensity decays with the inverse square of the distance to the focal plane, it is possible to selectively imprint the diamond at a given depth without altering the information stored elsewhere. A demonstration is presented in Fig. 2B, where we write NV charge maps on three stacked planes approximately 90 μm apart from each other. Given the thickness of the diamond sample we used (200 μm), these results indicate minimal optical aberrations throughout the crystal. On the other hand, the separation between planes—largely defined by the beam shape near the focal plane—could be reduced by resorting to beam-shaping techniques. In particular, a spatial light modulator could be used to adjust the optical wave front to reduce axial elongation in the beam profile.

Ultimately, the interplane separation results from a tradeoff between various parameters, including the required level of contrast, writing speed, and light intensity. For example, better in-plane localization is attained in the limit of low laser power, where the NV ionization rate responds quadratically to the illumination intensity (13), but the encoding time per pixel is comparatively longer. Faster writing speed can be reached with stronger laser power, but saturation of the first excited state gradually makes the NV ionization rate transition from quadratically to linearly dependent on the intensity, with the corresponding reduction of the in-plane localization (12). For a given laser power, a similar consideration applies to the light exposure time and fluorescence contrast, the latter improving with longer imprint times at the expense of a larger pixel volume (see also fig. S1). Note, however, that this tradeoff has a lesser impact on data density if the brighter fluorescence of larger pixels is binned into discrete levels to produce multivalued bits, as discussed above (Fig. 2A).

The absolute write and read times per pixel—either comparable to or greater than 1 ms—presently make NV storage comparatively slow for practical applications, although there seems to be considerable room for improvement. The most obvious route to faster writing makes use of stronger illumination intensities, though at the expense of higher power consumption and system complexity. For a constant average laser power, pulsed excitation may prove beneficial given the quadratic response of NV ionization upon green or red illumination. Along the same lines, different excitation colors can markedly exhibit different ionization efficiencies (for example, see conditions in Fig. 1 for red and green imprinting), thus calling for systematic characterization as a function of the excitation wavelength. In particular, we show below that NV− can be efficiently ionized by blue illumination (directly exciting the excess electron into the conduction band), although further work will be needed for a vis-à-vis comparison between one- and two-photon ionization in type 1b diamond. Although some of the same considerations also affect readout speed, the latter is mainly defined by SNR limitations, which, perhaps, could be ameliorated by increasing the NV content.