A computer memory cell ordinarily stores either a zero or a one, but a newly demonstrated example could also store a two or a three. The new memory technique, described in the 13 April Physical Review Letters, uses laser pulses to switch the atomic structure of a tiny gallium particle among four states. The demonstration used only one particle at a time, but if the particles were used for digital storage, they would increase the capacity and require a tenth of the power of current systems like DVD’s.
To increase the capacity of digital storage devices, beyond what today’s flash or magnetic memories allow, researchers have been developing so-called phase-change memories. Phase-change memory relies on a material changing its optical properties when heated. For example, a DVD bit changes from a “one” to a “zero” when heated with a laser that melts the crystalline structure into an amorphous jumble of atoms. The two phases reflect light differently and so can easily be interpreted by a reading laser. But a DVD bit can’t be smaller than a wavelength of visible light–otherwise the beam couldn’t focus precisely enough.
The smallest phase-change memories, still in the research stage, use nanoscale circuits to heat bits of a nanostructure with electric currents. But these currents can use a lot of power and are slower than light pulses. Now Nikolay Zheludev and his colleagues at the University of Southampton in England have come up with an all-optical phase-change memory element made of a gallium nanoparticle–a glob of gallium just 80 nanometers across. What’s more, the particle can be placed in any of four states, doubling the storage density of ordinary materials because the particle is equivalent to two bits .
The team deposited the gallium on the fine tip of a silica fiber through which they could route pulses from several different lasers. This intimate contact with the particle allowed them to beat the usual “diffraction limit” that ordinarily requires sizes as large as a wavelength of light.
The four states correspond to three different crystal structures of solid gallium, plus the liquid, each of which reflects light differently. Heating the nanoparticle slowly in a cryostat from 100 K up to 160 K brought it through all four states sequentially, states they called 0, 1, 2, and 3. To jump quickly from 0 to 1 at 120 K, the team used a laser pulse to briefly heat the particle above the 0-to-1 transition temperature (129 K). It cooled down immediately to the ambient temperature in the cryostat (120 K) but remained in state 1, unable to switch its structure back to state 0 without cooling much lower, below 100 K. The particle acted something like water that remains liquid below the freezing point because of quick cooling. The researchers used a similar trick to jump straight from 0 to 3 at 120 K, with a higher-energy laser pulse. But going quickly down from a higher state to a lower state involves a more involved laser trick that the team will soon publish, says team member Bruno Soares.
Soares says the ultimate commercial device would be a thin film containing many such particles made of a material that changes states at more convenient temperatures, with lasers controlling everything, including the ambient temperature. “This is innovative and visionary work in the area of optical memory and data storage,” says Nader Engheta of the University of Pennsylvania in Philadelphia. He’s impressed by the combination of an all-optical element with four states that can be switched with very low power.
- Although the number of states of the system increases exponentially with the number of states per bit, storage capacity is not a measure of the number of states. 1024 bits of memory have 21024 possible states 2048 bits have 22048. But we still say that 2048 bits is twice the amount of memory as 1024 bits. Each 4-state bit is exactly equivalent to two 2-state bits, so the 4-state bits double the storage density measured in bits/area.