Final Report for Nano Materials Class, 2007

1. An All-Optical Silicon Transistor Utilizing Opal/Inverse Opal Based
Heterostructure as a Photonic Crystal
Mohammad Faisal Halim (Faissal)
Department of Electrical Engineering, The City College of New York,
City University of New York, 140th ST. and Convent Ave., New York, NY 10031
Abstract:
For some time photonic crystals have been used as photonic nanocavities for confining
light, both for guiding it, and for using it as an optical switch. Thus far, most of this kind
of work has been carried out using materials and structures with dimensions that lie
above the regime of nanomaterials (200-420nm). More recently, however, inverse opals
have been reported to have been used as photonic crystals, and also, inverse opals have
been reported to have been fabricated in dimensions in the nano regime. This, combined
with the interest in silicon based inverse opals opens up the possibility of rapidly
implementing all optical, ultrafast (nanosecond and picosecond time intervals), switching
technology into future computer chips, since the technology for fabricating silicon
devices is so mature, and silicon based photonic structures can be readily integrated into
silicon chips without expensive bonding technologies. This paper will investigate
photonic bandgap materials and photonic nanocavities in the light of silicon and silicon
based materials in inverse opals and opals research, and the photonic band structures
desirable for an all-optical switching device (in this case, all optical transistor) and will
suggest approaches that could be taken for the realization of such a device. This device
will use low dimensional (nano structure regime, so as to take advantage of quantum
effects) opals or inverse opals for photonic bandgap materials in which switching will be
performed by varying the applied electric field thus changing the refractive index of the
material. The advantage of using nanostructures is that the resulting quantum
confinement of light and electrons is known, from other studies, to increase the power
efficiencies of devices, having low thresholds and for having very low power
requirements.
Keywords: PC, PhC, PBG, nanocavity, inverse opals, opals, silicon photonics

2. Background
Silicon has dominated the computing landscape for a long time, and silicon devices,
interconnected by copper interconnects, are the staple of most of the microprocessor
industry. In order to extend Moore’s Law (Figure 1), however, this technology (and its
paradigm) will need to be abandoned, as silicon devices are facing more and more
problems with power and reliability as the devices are being made smaller. Also, the
copper interconnects in the chips are fast approaching their information carrying limits.
Figure 1: Moore’s Law
This situation has motivated scientists and engineers to look at other carriers of
information, and other methods of performing digital logic operations. One promising
solution is the use of light (since its speed in a dielectric is much faster than the speed of
electrons in a copper wire) as the replacement for electricity (currents and voltages),
which would mandate the use of optical switching devices for logic operations.

3. Introduction
Photonic crystals (PCs, or PhCs) have been investigated for quite some time, for use in
all optical circuits. Photonic crystals can act, both, as more efficient waveguides (using
defect modes), as well as digital logic devices. Photonic crystals, also called photonic
bandgap materials (PBGs), are macroscopic (their minimum size depends on the
wavelength ranges that the device made from the crystal will be used for) structures that
either possess transmission bands (i.e., they only allow certain ranges of EM wavelengths
through), or stop bands (i.e., they block certain ranges of wavelengths – corresponding to
only certain ranges of energies of incoming photons – from getting through the device).
Most devices encountered in academia and research laboratories that are run by
corporations focus on band pass (also called band stop) materials/crystals, and so this
paper will focus on band stop materials (see Figure 2). The bandgap has resulted in PCs
being described as optical semiconductors, since semiconductors have bandgaps.
Figure 2: Source: MODELING OF PHOTONIC BAND GAP STRUCTURES AND PROPOSED SYNTHESIS SCHEMES, By
Srivatsan Balasubramanian, RPI, 2002

4. Clearly, from Figure 2, photonic crystals need to be periodic arrays of dielectric
structures. They need to be periodic, with the periodicity chosen to block the chosen
range of wavelengths, and they need to be dielectric structures because dielectrics allow
the transmission of light. While photonic crystals with microscopic (and even
macroscopic) elements (i.e., the dimensions of the periodic structures are large, rather
than in the nanoscale) can be created, their application will be for wavelength of that
scale. In other words, PCs block wavelengths comparable to the dimensions of their
periodicities. So, such devices will be large and bulky, and impractical as a replacement
for silicon components that are already in the nanoscale. If PC devices are made for using
optical wavelengths, however, then the dimensions of the entire crystal that is being used
as a device can be shrunk to very small dimensions, and the features of the crystals,
themselves, will be in the nanoscale. Although a lot of work has been done towards the
realization of PCs, there are some major challenges in the area:
1. The easiest kind of photonic crystal that can be made is a one dimensional crystal,
and so it only has a photonic bandgap in one direction. It is physically
(structurally) identical to the simple conceptual picture of PCs shown in Figure 2.
It will be easy to fabricate these structures in the manner in which the electronics
industry manufactures diode lasers, however, that would still entail layer by layer
deposition, which is not as versatile as self assembly.
2. With two dimensional PCs the growth process may not be as cumbersome as for
the 1D PC. Also, the PC may be fabricated separately from the rest of the circuit,
and then just placed into position, for use. Such PCs have been receiving a lot of
attention in theoretical and experimental research. For example, the work “Two-

5. Dimensional Optical Interconnection Based on Two-Layered Optical Printed
Circuit Board,” IEEE Photon. Technol. Lett., vol. 19, no. 6, pp 411-413, Mar. 15,
2007, done by Hwang, Cho, Kang, Lee, Park, and Rho utilizes this kind of
structure.
3. 3D PCs allow the greatest flexibility in use. They have the further advantage that
nanoscale 3D PC structures can be fabricated by self assembly, thus easing, and
speeding, mass production. Further, calculations and experiments with opals and
inverted opals, with infiltrated electro-optic materials, have yielded promising
results that could be applied towards devices.
4. A further challenge is the migration to all optical, or at least party optical,
technologies. This challenge is two pronged:
a. Bringing industrial process technologies up to speed with PC
manufacturing. It will be more cost effective for the industry to make PC
from silicon based materials, since silicon process technologies are very
mature, and that will reduce the time that it takes PCs to make itto the
market.
b. Integration with the silicon on the chips. This problem can be solved, at
least in part, by doing more research work in creating PCs using silicon,
and related materials. If the PCs are made of the same of the same material
as the underlying substrate then there is no need to invest in expensive,
and cumbersome, wafer bonding technologies.
In conclusion, the industry needs silicon based 3D PCs, in order to realize the goal of all
optical computing quickly, and with the least number of technological challenges.

6. Opals and Inverted Opals
Figure 3 shows an SEM image of an inverted opal PC and is photonic bandgap (the
shaded region).
Figure 3: Source: Source: MODELING OF PHOTONIC BAND GAP STRUCTURES AND PROPOSED SYNTHESIS SCHEMES,
By Srivatsan Balasubramanian, RPI, 2002
So, if light is shone on the PC with a frequency within the shaded region then that light
will get blocked by the PC. In other words, light within the range of shaded wave-vectors
shown will be the reflected light, as shown in Figure 4.
Figure 4: Source: Inverse Opal Photonic Crystals – A Laboratory Guide, Schroden and Balakrishnan

7. Theory of Making Opals and Inverse Opals
Opals
Opals are made by depositing beads of the chosen opal material into a cavity, where they
form a regular structure, often an FCC (face centered cubic) [PHYSICAL REVIEW B
72, 205109 2005]. The spaces between the spheres/beads can then be infiltrated by a
polymer in solution, that can subsequently get the structure to set. Also, the infiltrated
material can have electro-optic properties, in which case the system can be used in
electro-optic applications, like digital logic operations.
Inverse Opals
Once the structure in an opal system is set/fixed, then the beads themselves can be
dissolved away in solution, leaving air spaces. The resulting system can be a PC with
entirely different properties. Also, if these new air spaces are infiltrated with electro-optic
materials, like liquid crystals, then the optical properties of the PC can actually be tuned.
Examples of this effect can be seen in published papers like: IEEE Transactions on
Dielectrics and Electrical Insulation Vol. 13, No. 3; June 2006.
Figure 5a shows (in green) the region that would be a occupied by beads in an opal. The
rest of Figure 5 shows the space left (in the interstices) when the bead is dissolved away,
leaving behind a structure that is an inverse opal.

8. Figure 5: Source: D. GAILLOT, T. YAMASHITA, AND C. J. SUMMERS PHYSICAL REVIEW B 72, 205109 2005)
Figure 6: The effects of Voltage on infiltrated PCs made of Opals and Inverse Opals
Figure 6 shows the electro-optic effects of infiltrated inverse opals, as published in: IEEE
Transactions on Dielectrics and Electrical Insulation Vol. 13, No. 3, June 2006.

9. Current Work at the Nanoscale
Most of the current work on opals and inverse opals have been done on materials other
than silicon related materials. This could present a significant integration challenge for
silicon technologies. Current work at the nanoscale includes work on Tungsten Nitride,
among other materials, as shown in Figure 7, from the paper Nano Lett., Vol. 3, No. 9,
2003. The system is at the nanoscale because the interstitial walls are less than 100nm in
thickness.
Figure 7.
Most current work on silicon based materials are still at a scale higher than nanoscale, but
progress is being made towards the nanoscale so that visible light may be utilized for
information processing, rather than infra red (IR). Such work can be seen in papers like
OPTICS EXPRESS 2678 / Vol. 13, No. 7 / 4 April 2005.

10. Conclusion
Although most silicon related work at the nanoscale is being done for IR wavelengths,
progress is being made towards going into the nanoscale, for utilizing visible light in
silicon based PCs, as seen in papers like:
 IEEE Transactions on Dielectrics and Electrical Insulation Vol. 13, No. 3; June
2006
 APPLIED PHYSICS LETTERS 87, 151112 (2005)
Figure 8: Source: IEEE Transactions on Dielectrics and Electrical Insulation Vol. 13, No. 3; June 2006

11. Appendix
PHYSICAL REVIEW B 72, 205109 2005
Inverse Opal Photonic Crystals – A
Laboratory Guide, Schroden and
Balakrishnan
MODELING OF PHOTONIC BAND GAP
STRUCTURES AND PROPOSED
SYNTHESIS SCHEMES, By Srivatsan
Balasubramanian, RPI, 2002
“Two-Dimensional Optical Interconnection
Based on Two-Layered Optical Printed
Circuit Board,” IEEE Photon. Technol.
Lett., vol. 19, no. 6, pp 411-413, Mar. 15,
2007, done by Hwang, Cho, Kang, Lee,
Park, and Rho
Intel Corporation
IEEE Transactions on Dielectrics and
Electrical Insulation Vol. 13, No. 3; June
2006
D. GAILLOT, T. YAMASHITA, AND C. J.
SUMMERS PHYSICAL REVIEW B 72,
205109 2005)
Nano Lett., Vol. 3, No. 9, 2003
OPTICS EXPRESS 2678 / Vol. 13, No. 7 / 4
April 2005