2
fere with one another. In the event that there is conser-
vation of momentum and energy during the interaction,
this pressure then stimulates an acoustic wave. As a re-
sult of the acoustic field’s stresses altering the medium’s
dielectric characteristics and perturbing its boundaries,
light is essentially scattered across distinct optical modes.
The new proposed scheme using rare-earth aluminosil-
icate glass microwires also offers a conversion rate of
≈45%, far higher than any other scheme using silicon
based membrane as transduction medium.
II. PHYSICAL INTERACTIONS AT COUPLING
INTERFACES
Our scheme for undertaking optical-to-microwave
transduction involves two distinct physical interactions
at the coupling interfaces: opto-acoustic interactions us-
ing Brillouin scattering and dielectric-modulated mag-
netic response at the acousto-electric interface [31–33].
A. Opto-acoustic Interaction with Brillouin
Scattering
Certain underlying processes in optically isotropic
material substrates result in opto-acoustic interactions
[33]. The electromagnetic field can produce mechanical
strains in the waveguide through electrostriction [34].
The converse, in the phenomenon of photoelasticity,
is observed when stresses cause localized variations in
the dielectric permittivity [21]. Waveguide boundaries
may physically shift as a result of radiation pressure
caused by the electromagnetic waves reflecting off of
the boundaries of the structure, which in turn drives
acoustic waves. A localised variation in the electro-
magnetic characteristics is caused by the mobility of
structural boundaries brought about by mechanical
vibrations. Stimulated Brillouin Scattering (SBS) is
the nonlinear phenomenon whereby an injected pump
wave is scattered by an acoustic vibration into the
Stoke and anti-Stoke wave components [31,35–37]. In
1972, Ippen et al. achieved low-threshold stimulated
Brillouin scattering with input powers as low as 1 W [38].
When we couple and guide a coherent laser beam
into an optical microwire, the light gives rise to as well
as experiences various kinds of elastic waves that have
similar frequencies. While light is sensitive only to
longitudinal and shear bulk acoustic waves in standard
optical fibers, the light and the evanescent field access
the outer-surface as well, in the case of a sub-wavelength
optical fiber, thereby causing the shaking of the wire
due to electrostriction which leads to surface acoustic
waves (SAWs) being generated. The effective refractive
index along the microwire varies periodically due to
these ripples, leading to Bragg scattering of the light in
the backward direction as per phasematching condition.
The velocity of the bulk and surface acoustic waves
differ significantly, with the surface wave travelling at a
velocity of around 0.9 that of a shear wave and giving
rise to other optical sidebands. Beugnot et al. found
the surface acoustic wave mode to travel at a velocity
of 3400 m/s and a frequency of 6 GHz, having studied
the generation of SAWs in an 8 cm. long silica optical
microwire that was drawn from a single-mode fiber using
the heat-brush method [39].
Photon silica microwires, fabricated by tapering
optical fibers, have been seen to support stimulated
Brillouin scattering [39]. It is found that rare-earth
materials like Lanthana and Ytterbia have a finite effect
on the Brillouin characteristics of silica-based oxide glass
optical fibers, with emergent attributes such as a wide
Brillouin spectral width, low acoustic velocity and a
negative photoelastic constant [40]. Brillouin processes
can alternatively be visualised as the formation and
destruction of quasi-particles, in this case photons and
phonons [41]. It is typically advantageous to differentiate
interactions based on whether there is an absorption
or emission of the phonon. A phonon is produced
when a high-frequency photon undergoes the Stokes
Brillouin transition and changes into a lower-frequency
Stokes photon [42,43]. This transition may happen
naturally or may be stimulated. Anti-Stokes transitions,
wherein a phonon is absorbed while a lower-frequency
photon is changed to a higher-frequency photon, are also
conceivable [44]. Brillouin processes can be triggered
by either an optical seed from an auxiliary optical field
at the Stokes frequency or by thermal phonons in the
waveguide, which scatter the pump photons [45].
In almost every system of practical importance, it
is infeasible to compute Brillouin scattering from first
principles, for example by considering Maxwell’s equa-
tions nonlinearly linked to the Christoffel equation. This
is caused by a division of scales. To start, the optical
and acoustic problems’ temporal scales often differ by
five orders of magnitude. Second, interaction lengths
on the millimetre scale are necessary for integrated
waveguides that have nonpareil and superlative gain
coefficients to give a noticeable overall response - the
system scale is four orders of magnitude larger than the
acoustic wave length. A coupled mode description is
especially appropriate to Brillouin problems due to this
scaling divergence [46]. We can express the acoustic and
optical fields as eigenmodes of the waveguide, which are
weighted by envelop functions - an(z, t) for each optical
mode and b(z, t) for the acoustic field, thereby giving us
the states [47],
|ψ(x, y, z, t)i=X
n
an(z, t)|Ψn(x, y)ieiknz−iωnt+c.c,
|φ(x, y, z, t)i=b(z, t)|Φ(x, y)ieiqz−iΩnt+c.c, (1)