Abstract: For holographic memory applications, the photorefraction of well-known ferroelectric such as lithium niobate doped with
different transition metals is very important. First principles study assumes
special significance in this context, as to why certain transition metal
atoms are better than the other. In this work, Nb atom in LiNbO3 was substituted
with transition elements having valency greater than/equal to +5 for UV
photorefraction applications, and atomistic first-principles calculations were
done using HSE06 functionals. The d-states of the transitional elements were found
to decrease the band-gap of the host material having implications for a
suitable material design. Minimum band-gap was obtained for W, while Ta showed a
maximum value. Absorption coefficients were estimated for each material
and based on their low values at 351 nm (i.e. for holographic applications) that
is the usual UV photo-refraction wavelength, the elements found suitable were V, W, Ta, Mo. Then birefringence properties for these crystals were also studied
to predict that V and W were good candidates.
Some Descriptions: Lithium Niobate (LN) has gone through intensive research for its volume holographic data storage capability (Ashkin et al., App. Phys. Lett. 1966. 9, 72). The theory of the volume phase holograms in the birefringent electro-optic crystals (LN) has been developed a long time ago via electrically controlled light diffraction and pattern reconstruction at a "READ" wavelength different from that of the "WRITING" beams (Petrov et al, Ferroelectrics 1978, 21, 631).
Some Descriptions: Lithium Niobate (LN) has gone through intensive research for its volume holographic data storage capability (Ashkin et al., App. Phys. Lett. 1966. 9, 72). The theory of the volume phase holograms in the birefringent electro-optic crystals (LN) has been developed a long time ago via electrically controlled light diffraction and pattern reconstruction at a "READ" wavelength different from that of the "WRITING" beams (Petrov et al, Ferroelectrics 1978, 21, 631).
Holographic
data storage is a technique that can store information at high density inside
the crystals with the volume of the recording media used for data storage
instead of just the surface. Opening up the inside of a material for storage,
rather than just using its surface, yields huge improvements in capacity. In
addition, unlike conventional technologies, which record and recover digital
data bit by bit, holography has the ability to "read and write"
millions of bits of data with a single pulse of light, enabling data-transfer
rates of billions of bits per second with storing capacity of several Tbs.
This subject remained dormant for
quite some time. After a long break, the subject has been revived by an
intensive activity on LN crystals doped with different transition elements (McMillen
et al, Opt. Exp. 1998, 2, 491). The large crystal size of LN allows: (a) high capacity,
(b) angular selectivity, (c) long storage lifetime, and (d) high diffraction
efficiency. LN can also be used in optical "correlator type holographic
data storage" applications, where analog images are accessed at high
speed. In particular, angle-multiplexing type storage in LN has been observed
where ‘optical correlation’ is used for data storage (Mok et al., Opt. Lett.
1993, 18, 915).
Transition
metal doped LN ferroelectric material is used for photo-refraction and volume
data-storage. Although a multitude of such materials have been characterized in
details, the explanation as to why only a few of them are preferred compared to
others is still to be known. In the field of condensed matter physics, one of the well-studied
materials is ferroelectric, which has a variety of important applications in
nonlinear optics, such as electro-optics and second harmonic generation, and
non-volatile memory devices (see Bandyopadhyay et al, Phys. Rev. 2010, B81,
064104, and references therein). Ferroelectrics have also emerged as important
materials as piezoelectric transducers, pyroelectric detectors, surface
acoustic wave (SAW) devices, four-phase mixing doublers. Such applications and
many more as 'nano devices' are mentioned in the latter reference given above. One
of the most important ferroelectric materials is LN that is widely used as an
optical material for its various high frequency photonic and optoelectronics
applications.
Commonly Fe-doped LN is used for writing
holograms in blue-green region of the visible spectrum, but in other part of
electromagnetic spectrum, the ‘sensitivity’ is needed for inexpensive
applications. In addition, LN:Fe has problems, such as low response speed, strong light-induced
scattering and volatility. Other transition elements such as Cu, Mn, Ni etc.
have also been investigated for holography. Early attempt of doping with more
than one transition elements in LN was made for various systems. Among
transition elements, which were found to be suitable for holographic memory
applications are: Sc, Zr, Hf, V, Mo, Mn, Fe, Ru and Cu. It was also found that
the substituents occupying Li-sites were those having valence less than 5+
Hence, others were found to occupy Nb sites and were primarily used for
studying UV-photo-refraction.
Despite intensive experimental work,
comparatively less theoretical investigation has been carried out on this
subject, as well as on density functional theory (DFT) approach for atomistic
descriptions. One of the pioneering investigations on the electronic structures
and energetics of LN in comparison with those of another ferroelectric (LT) was
done by Inbar and Cohen (Phys. Rev. 1996, B53, 1193) via ‘linearized augmented
plane-wave’ (LAPW) frozen phonon calculations in order to shed light on phase
transition and origin of ferroelectricity. Now, for the electronic and optical
properties of LN, various levels of DFT calculations such as general gradient
approximation (GGA) and GW calculations were carried out, and the "band
gap" of LN was a crucial topic in the last few decades
It has been evident that there is a necessity of studying
the atomistic electronic properties, particularly in relation to 'band-gap
engineering' of such photorefractive materials involving LN via density
functional theory (DFT) approach. This assumes more importance due to the
recent surge in the activity in the field of holographic memory applications. In
the present work, for "holographic
memory" applications, the photo-refraction of well-known ferroelectric
such as lithium niobate doped with different transition metals is very
important. First principles study assumes special significance in this context.
In this work, Nb atom in LiNbO3 was
substituted with transition elements having valency greater than/equal to +5
for UV photo-refraction applications, and atomistic first-principles
calculations were done using HSE06 functionals. The d-states of the
transitional elements were found to decrease the band-gap of the host material
having implications for a suitable material design. Absorption coefficients
were estimated for each material and based on its low values at 351 nm (for
holographic applications) that is the usual UV photo-refraction wavelength, the
elements found suitable were V, W, Ta, Mo. Then birefringence properties for
these crystals were also studied to predict that V and W were good candidates.
Here, we study the dopants that have been used (or possibly used) for photo-refraction
having valence more than 5+, namely V, Ta, Cr, Mo, W, Tc, Re, Ru, Os, Rh, Ir,
Pt by using HSE06 hybrid functional for these calculations.
Finally, in the
realm of 'band-gap engineering', the results show that tantalum (Ta) has the
minimum impact, while tungsten (W) has the maximum effect on the host crystal
on the reduction of 'band-gap' within the systems under the present
investigation. Then, it was interesting to study the density of states (DOS) of
such materials both in spin-up and spin-down conditions. It is quite pertinent
to mention about an important work by Tong et al. (Phys. Rev. 2014, B89,
064404), who observed significantly different optical responses from spin-up
and spin-down channels in case of multiferroic EuO via first principles DFT
calculations.
Moreover, among
few elements found suitable in this work, Ta is very well-known as nonlinear
optical material showing the lowest absorption coefficient for LN:Ta at 0.1,
although its band-gap is the highest (5.07 eV). However, for LN:W whose
absorption coefficient is also very low at 0.2, its band-gap is the lowest
(0.72 eV). This opens up a new field of work in the realm of "Band-Gap Engineering" of device
material formulations to suit different applications. For example, in a recent
work by Grinberg et al (Nature 2016, 503, 509), a 10% replacement of KNbO3
by nickel-niobium based material decreased the band-gap to 1.39 eV to give rise
to a very high level of photovoltaic efficiency in terms of high "photocurrent
density for solar-energy conversion" and other important applications for
this type of ferroelectric-semiconductor based materials.
It is pertinent
to mention that in the present investigation with our DFT approach, a
comparable band-gap was observed for LN:Re and LN:Rh and LN;Mo at 1.30 eV, 1.36
eV and 1.57 eV respectively. These materials could also be considered as competitive
candidates for photoconduction based "Solar Energy" applications.
