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RLE's Professor Rajeev
Ram and the Semiconductor Laser Group
Using optics to advance electronics
Cambridge, MA 12.1999
A high speed semiconductor laser
coupled to a lensed optical fiber.
The Semiconductor Laser group (SLG), headed by Professor
Rajeev Ram, is part of the Optics and Devices group of the
Research Laboratory of Electronics. The SLG group makes extensive
use of advanced semiconductor lasers to investigate a variety
of topics in optical physics and devices.
Professor Ram describes the group's research as taking place
on three levels: physics, devices, and systems. In an ideal
world, he says, research would progress from (a) advances
in understanding of the physics of certain phenomena to (b)
devices that take advantage of this understanding and then
to (c) new or improved systems based on these devices. In
fact, research often progresses in the opposite direction,
with an attempt to improve the performance of a system inspiring
the development of a new device which requires further investigation
of underlying physical phenomena.
A case in point is the Bipolar
Cascade Laser.
The group was working with a fiberoptic communication system,
trying to improve the overall signal-to-noise ratio of the
system. The system converts electrical signal to optical signal
and transmits it along fiberoptic cable with an ultrafast
laser. To improve the signal-to-noise ratio of the transmission,
one could increase the strength of the electrical signal going
into the system, but the laser cannot handle more than a certain
level of signal without creating distortion in the output
transmission.
Instead, the group turned to the electrical-to-optical conversion
process. By making this process more efficient, the optical
signal strength could be increased without increasing the
strength of the electrical signal going into the system. The
result was the most efficient interband electrical-to-optical
conversion device ever built, the bipolar cascade laser. This
innovative device is built on a surprisingly simple premise:
that by connecting the input electrical signal not to one
laser but to two laser sections cascaded in series one could
nearly double the output. The two laser sections were stacked
on top of one another, allowing their output to be combined
into an single, more powerful light stream.
Scanning electron micrograph
of a high speed ridge waveguide laser.
The concept of the cascade laser, though simple, was a relatively
new one, and there was no theory to describe how the paired
lasers would interact and combine. The group developed its
own theory which helped to optimize their device and proved
helpful to other groups developing cascade lasers for different
purposes. A collaboration resulted with Carlos Sitori at Thomson
CSF in Paris who was developing a quantum cascade laser for
pollution sensing systems. The Sitori group's device operates
at a wavelength of 10 microns, much longer than the 0.98 micron
wavelength of the RLE group's device, but the theory developed
at RLE was sufficiently general to apply to this device.
Research on Magnetic Force
Microscopy has followed a different path
Professor Ram and graduate student Mathew Abraham were investigating
a new technique for imaging the flow of current through electrical
wires. This technique, called magnetic force microscopy, had
more than enough spatial resolution to image electron flow
through electron wires as small as 2 microns in diameter,
but to find just how high its resolution was, they needed
a much smaller subject for imaging. The group contacted Professor
Caroline Ross of MIT's Magnetic Materials and Devices Group
who had been looking for a way to image the magnetic fields
in new material being developed for computer hard disks. This
material, fabricated by Timothy A. Savas of Professor Henry
I. Smith's NanoStructures Laboratory, consists of tiny posts,
only 57 nanometers wide, each of which can possess a magnetic
charge. The magnetic posts were imaged successfully.
Topographical (above) and magnetic
contrast (below) images of novel computer hard disk material.
The topographical images show an array of magnetic posts at
different magnifications, while the magnetic contrast images
show their magnetic fields, essentially depicting the data
stored on the disk.
Now that the magnetic fields of this advanced hard disk material
could be imaged successfully, the group embarked on a collaboration
with Professors Ross and Smith to learn more about the limits
of hard drive density. Data storage density of current hard
drives is roughly 4 Gb/in2. The materials being tested have
storage densities around 60 Gb/in2. To achieve such density,
the magnetically charged posts, each of which stores one bit,
must be made very small and very close together, about 57
nm wide and 43 nm apart. At this scale, the magnetic fields
of these posts can interact with each other, potentially creating
instability in the stored data. This interaction was predicted,
and its effect has been clearly observed using magnetic force
microscopy. Current study focuses on estimating and better
understanding these interactions and thereby learning more
about the limits of hard disk density.
Meanwhile, progress continues on the original aim of magnetic
force microscopy, imaging of currents flowing through electrical
wires. The aim of this study is to investigate hydrodynamic
behaviors of electric current. Electrons flowing at the edge
of a wire flow more slowly than those in the center, just
as water flowing close to the edge of a pipe flows slower
than water at the center. The smaller the wire, the more significant
the effect. The group hopes to use magnetic force microscopy
to observe this effect and to help develop a model for understanding
this fluid nature of electron flow. While the spatial resolution
of this imaging technique is excellent, greater sensitivity
is required to observe a current's field than to observe the
magnetic fields in hard disk material. The group hopes to
increase sensitivity by a factor of one hundred from its current
level, which would allow imaging on a quantum scale.
Left: a topographical scan of two
2 micron wide and 400 nanometer high metal wires that join
together and then split apart. Right: a current contrast
image of the junction. One can clearly see the sharper
contrast over the central strip of wire indicating the larger
current carried.
Laser Strobe
In another project, fast laser pulses are used as strobes
to image very small, quick events.
This study aims at a better basic understanding of the magnetic
property of electrons, known as spin. The spin of an electron,
either "up" or "down," denotes its magnetic
orientation, corresponding with more everyday notions of the
"north" and "south" poles of a magnet.
By sending a beam of light through a cluster of electrons,
one can force their spins to align. In most materials, this
moment of perfect alignment will be extremely brief as random
collisions with other particles change the electrons' spins.
In extremely pure semiconductors, however, there are few such
collisions, and the electrons' spins can remain aligned for
much longer. The group is investigating how long the electrons
can be made to remain in alignment, and what factors affect
this. Understanding spin could open doors to new areas of
electronics; for example, if spin can be made stable, transistors
could use electrons' spin to store information the way they
now use charge.
The SLG is led by Professor Rajeev J. Ram and includes Research
Scientist Charles H. Cox III; Postdoctoral Associate Holger
Schmidt; graduate students Mathew C. Abraham, Harry Lee, Steve
Patterson, Kevin Pipe, Farhan Rana, and Mehmet Fatih Yanik;
and undergraduates Erwin Lau and Margaret Wang.
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