Photonic wire microcavity light emitting devices

A photonic light emitting device comprising a relatively high refractive index photonic-wire semiconductor waveguide core in the form of an arcuate shape, linear shape and combinations thereof. The waveguide core is formed into a closed loop cavity for a light-emitting device or laser. The waveguide core is surrounded on all transverse sides by relatively low refractive index medium and comprises an active medium having major and minor sides and semiconductor guiding layers proximate the major sides. The active medium and the guiding layers are so dimensioned in a transverse direction relative to the path of light propagation through the core to provide dramatically increased spontaneous-emission coupling efficiency, leaading to low lasing threshold and high intrinsic modulation rates.

FIELD OF THE INVENTION 
The present invention relates to a light emitting device or laser and, more 
particularly, to a photonic-wire microcavity light emitting device or 
laser having high stimulated emission and hence gain and capable of high 
conversion efficiency from pumping power to optical output. When operated 
as a laser, it can have a low lasing threshold. Other advantages include 
extremely high near-ideal photon emission efficiency into the 
electromagnetic field mode of interest, small physical size, and high 
intrinsic modulation rate when operated above the lasing threshold. In 
addition, it can be coupled directly to strongly-guided waveguides, making 
it suitable for application to very-high-density photonic integrated 
circuits. 
BACKGROUND OF THE INVENTION 
Microcavity semiconductor lasers recently have been described for 
operation. Such microcavity semiconductor lasers employ an active lasing 
medium that support optical modes in a short cavity. For example, a 
microdisk laser is described by McCall et al. in "Whispering-Gallery Mode 
Microdisk Lasers" in Appl. Phys. Lett. 60, (3), 20 Jan., 1992. Such 
microcavity laser supports the electromagnetic field mode in a thin disk 
on top of a small supporting pillar. 
Microcavity semiconductor lasers are advantageous as compared to 
conventional semiconductor lasers in being much smaller in size and 
requiring substantially less minimum operating current (power) in the 
range of microwatts. Recently, in Directional Light Coupling From 
Microdisk Lasers, Appl. Phys. Lett. 62, 561 (1993) an asymmetric point was 
introduced into a microdisk to provide a location of increased quantity of 
lasing light to leak out from the point of asymmetry. Light emitted from 
such a thin disk with thickness d from the asymmetric point can undergo a 
large angle of diffraction in the directions perpendicular to the disk 
plane. According to the physical law of diffraction, the diffraction angle 
is given by Theta=2*ArcTan(lambda/d) in radians and will occur at a 
distance of 2.pi.d.sup.2 /lambda away from the edge of the disk. The thin 
disk of thickness around 0.2 micron emitting at a wavelength of 1.5 
microns will give rise to a diffraction angle of Theta=2.88 radians or 
almost 165 degrees and will occur at 0.15 micron from the disk's edge. 
This means that the light emitted from the disk will disperse away rapidly 
within a short distance of less than two tenths of a micron, which makes 
it very difficult, if not impossible, to collect a useful fraction of the 
output laser light into a semiconductor waveguide in practice. 
Microdisk lasers can have high efficiency (with a spontaneous-coupling 
factor of larger than 0.1) only in the limit of small disk radius of 1-1.5 
microns for the emission wavelength of 1.5 microns. The required disk 
radius scales linearly with the optical wavelength. This makes it very 
difficult to realize an efficient microdisk laser at short wavelength 
range, such as the visible wavelength of 0.5 microns. Visible microcavity 
lasers are important as they have applications to color display or 
high-density optical storage or sensing. According to the above scaling 
rule, at the visible wavelength of 0.5 microns, an efficient microdisk 
will need a disk radius of 0.3-0.5 microns and will be difficult to 
fabricate and suspend on a pillar. 
The difficulty of obtaining useful light from microdisk lasers and their 
small disk size needed for high cavity efficiency make it difficult to use 
microdisk lasers in many practical applications. 
An object of the present invention is to provide a photonic-wire light 
emitting device or laser which has an extremely high spontaneous-emission 
coupling efficiency or factor of for example 0.3 and larger by virtue of 
use of a photonic-wire waveguide core combined with microcavity structure 
and which can be scaled to operate at the visible wavelength range without 
the aforementioned problem. 
Another object of the present invention is to provide a photonic-wire 
microcavity light emitting device or laser amenable for coupling light out 
from the device to, for example, a semiconductor waveguide efficiently 
because of its extremely high spontaneous-emission coupling efficiency or 
factor. In addition, the photonic-wire laser can be modulated at a very 
high modulation rate, which will enable high data transmission in a 
photonic circuit. 
Yet still another object of the present invention is to provide a 
photonic-wire microcavity light emitting device or laser amenable for 
coupling light out from the device to a strongly-guided semiconductor 
waveguide, making it amenable for coupling to very-high density integrated 
optical circuits connected by such waveguides. Such strongly-guided 
waveguides can make bends of smaller than 1 micron radius with negligible 
photon loss, making it possible to integrate 1000 or more optical 
components within a 1 millimeter square area and resulting in very-high 
density photonic integrated circuits. Furthermore, the photonic-wire 
device can be integrated via direct fabrication on a wafer instead of via 
hybrid integration. Such very-high density photonic integrated circuits 
will have applications to optical communications, optical interconnects, 
optical sensing, optical signal processing, and optical computing. 
In comparison, conventional integrated optical circuits are typically made 
up of hybrid optical components consisting of semiconductor lasers with 
long cavity lengths of 300 microns (0.3 millimeters). The laser components 
are coupled to weakly-guided ridge waveguides. A weakly-guided waveguide 
cannot make a bend with a radius smaller than a few millimeters without 
incurring very high photon loss because of radiation from the bend. This 
seriously restricts the integration density of the current typical 
integrated optical circuits to at most a few components in a millimeter 
square area. 
Yet still an additional object of the present invention is to provide a 
highly efficient cavity for applications to visible semiconductor laser 
light sources or light-emitting diodes (LEDs) with edge emission, which 
are difficult to realize at present due partially to the low efficiency of 
conventional cavity designs. 
SUMMARY OF THE INVENTION 
The present provides a photonic-wire light emitting device or laser 
comprising a relatively high refractive index photonic-wire waveguide core 
which is surrounded in all directions transverse to photon propagation 
direction, such as, for example, both in a width direction and thickness 
direction, of the waveguide core by relatively low refractive index 
medium, such as air, silica or other relatively low refractive index 
material, to spatially confine photons strongly in all directions 
perpendicular to their propagation direction in the waveguide core. The 
waveguide core comprises an optically active excitable medium (hereafter 
referred to as active medium), which can give rise to radiation or 
absorption of electromagnetic field energy, and in particular gain or 
luminescence. The active medium is bordered on the major sides by guiding 
semiconductor layers. The active medium typically comprises layers of 
semiconductor quantum wells or excitable rare earth ions. The active 
medium and guiding layers are so dimensioned in transverse directions 
relative to the path of light propagation through the waveguide core as to 
provide greatly increased spontaneous-emission coupling efficiency for 
photons emitted by the quantum wells into a particular electromagetic 
field mode of interest. 
The present invention provides in an embodiment a light emitting device 
having Beta(tot) much larger than that available from conventional 
semiconductor laser cavity design and from which a working device can be 
realized. For example, a typical prior semiconductor laser cavity with a 
300 micron long linear cavity defined by a ridge waveguide exhibits a 
Beta(tot) value of about 0.00001 for a Beta(space)=0.004 and 
Beta(Freq)=0.003. The present invention provides a light emitting device 
having a Beta(tot) which can be as large as 0.3 and higher achieved 
through combined use of photonic-wire waveguides, microcavity structure, 
and an appropriate active medium, such as quantum wells. 
A particular photonic-wire light emitting device in accordance with an 
embodiment of the present invention comprises a relatively high refractive 
index photonic-wire semiconductor waveguide core. The photonic-wire 
waveguide core is formed into an arcuate shape, linear shape and 
combinations thereof. As described in more detail below, when the 
photonic-wire waveguide is closed onto itself, forming a closed loop 
cavity, a high Q and high gain cavity is provided. The high Q cavity can 
be an efficient cavity for the light-emitting device or laser and, in 
particular, can be a lasing microcavity. 
The high refractive index semiconductor waveguide core is surrounded by a 
relatively low refractive index medium on all sides. This confines photons 
tightly in directions perpendicular to their direction of propagation. The 
strong confinement forms strong confining potential walls for photons, and 
the waveguide is called a photonic-wire waveguide. The strong potential is 
necessary to affect the emission properties of the active medium of the 
waveguide, and can be used to dramatically increase the percentage of 
emission into one particular waveguiding mode of interest. For example, 
typical semiconductor waveguide core materials for use in practicing the 
invention have a refractive index n.sub.core greater than about 2.5, such 
as from about 3 to about 3.5 and above for InGaAsP, AlGaAs, etc. 
materials, whereas typical low-refractive index mediums for use in 
practicing the invention have refractive index n.sub.low below about 2.0, 
such as from about 1.6 to about 1.0 for silica, silicon nitride, acrylic, 
polyimide, aluminum oxide, epoxy, photoresist, PMMA, spin-on glass, 
polymers with low absorption at the emission wavelength, or air. The ratio 
of the refractive indices n.sub.core /n.sub.low has to be larger than 
about 1.3 to obtain substantial advantages of the invention. 
The waveguide core comprises an active medium having major and minor sides 
and semiconductor guiding layers proximate the major sides to define a 
generally rectangular core cross-section comprising a width dimension and 
thickness dimension. The width dimension or direction is parallel to the 
substrate on which the waveguide is disposed, while the thickness 
dimension or direction is perpendicular to the substrate. 
The strong photon confinement is provided in the width direction and the 
thickness direction in one embodiment. Low refractive index materials 
described above are the materials residing on all sides of the waveguide 
core along the width direction and thickness direction. 
Optical coupling of the device described hereabove to an output can be 
effected by an output-coupled waveguide proximate a portion of the 
periphery of the waveguide core in a manner to achieve resonant photon 
tunneling. A typical design can consist of a cavity coupled to a U-shaped 
output-coupled waveguide encircling part of the cavity with a small 
low-refractive-index gap between the outputcoupled waveguide and the 
photonic-wire waveguide of the cavity. In order to achieve resonant photon 
tunneling of photons, the mode width for the electromagnetic field mode 
propagating in the output-coupled waveguide must be close to the mode 
width for the electromagnetic field mode propagating in the cavity. This 
will ensure that the field modes propagating in the output-coupled 
waveguide and the cavity have similar propagating velocities so that they 
can couple to each other with maximal efficiency. 
The vertical layer structure of the output-coupled waveguide can be the 
same as that of the photonic-wire waveguide that forms the cavity. In that 
case, the output waveguide is excited by optical or electrical pumping to 
avoid absorption of light by the unexcited active layer(s) in the 
waveguide. To avoid the need for pumping the output-coupled waveguide, its 
vertical structure may be the same as that of the cavity's photonic-wire 
waveguide but without the active medium. The output-coupled waveguide can 
be formed at a level on a substrate compatible with an integrated optical 
circuit present on the substrate so as to provide light output signals to 
the optical circuit. 
A typical single light emitting device (as opposed to integrated circuit 
device) useful for practical applications can be provided via cleaving the 
ends of the U-shaped waveguide. The cleaved ends will provide radiation of 
the light in the output-coupled waveguide into free space (i.e. air). The 
spatial mode profile of this free-space radiation can be manipulated as 
described below. 
The above objects and advantages of the present invention will become more 
readily apparent from the following detailed description taken with the 
following drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1 and 2, a photonic-wire light emitting device 10 in 
accordance with one embodiment of the invention is illustrated 
schematically as including a relatively high refractive index 
photonic-wire waveguide core 20 which is surrounded by a relatively low 
refractive index medium 23 in a width direction of the core 20 and a 
relatively low refractive medium 33, 35 in a thickness direction of the 
core 20. In particular, the high refractive index semiconductor waveguide 
core 20 is surrounded on lateral sides 20a, 20b by relatively low 
refractive index medium 23 on two sides 20a, 20b defining the width 
dimension ds of the waveguide core by the relatively low refractive index 
medium. The waveguide core 20 also is surrounded on the top and bottom 
sides 20c, 20d by relatively low refractive index medium 33, 35 on top and 
bottom sides defining the thickness dimension dw of the waveguide core 20 
by the relatively low refractive index medium 33, 35. The surrounding of 
the waveguide core 20 in both of these dimensions or directions is 
effective such that photons are tightly confined in all directions 
perpendicular to their direction of propagation. The strong confinement 
forms strong confining potential walls for photons and restricts photons 
to move relatively freely only in one direction, and the waveguide is 
called a photonic-wire waveguide. The strong potential is necessary to 
affect the emission properties of the optically active, excitable medium 
of the waveguide, and can be used to dramatically increase the percentage 
of emission into one particular waveguiding mode of interest. 
For example, typical semiconductor waveguide core materials for use in 
practicing the invention have a refractive index n.sub.core greater than 
about 2.5, such as from about 3 to about 3.5 and above for InGaAsP, AlGaAs 
or InGaN materials, whereas typical low refractive index mediums described 
below for use in practicing the invention have refractive index n.sub.low 
below about 2.0, preferably below 1.6, such as from about 1.5 to about 
1.0. The ratio of the refractive indices n.sub.core /n.sub.low is larger 
than about 1.3 to obtain substantial advantages of the invention, and 
preferably larger than 2.0. 
For purposes of illustration and not limitation, in FIGS. 1 and 2, the 
waveguide core 20 is disposed on top of the substrate 26, which may 
comprise InP (indium phosphide) or other suitable substrate material, such 
as GaAs (gallium arsenide). The relatively low refractive index medium 23 
includes air (refractive index of 1) that borders the left and right sides 
20a, 20b of the waveguide core 20 in FIG. 1. The relatively low refractive 
index medium 23, 33, 35 includes air (refractive index of 1.0) in FIG. 1. 
The low refractive index mediums 23, 33, 35 serve to spatially confine 
photons tightly in the width and thickness directions perpendicular to 
photon circumferential propagation direction in the waveguide core. Other 
low refractive index mediums that may be used include acrylic, epoxy, 
silicon dioxide (SiO.sub.2), aluminum oxide, silicon nitride, spin-on 
glass, polymers with low absorption at the emission wavelength, 
photoresist, poly-methyl metacrorate, and polyimide. The same or different 
low refractive index mediums 23, 33, 35 can be used on the sides 20a, 20b, 
20c, and 20d. 
The relatively high refractive index photonic-wire waveguide core 20 
comprises an optically active excitable medium (hereafter active medium) 
30 which can give rise to radiation or absorption of electromagnetic field 
energy and, in particular, gain or luminescence, when pumped or excited 
optically or electrically. The active medium 30 typically comprises layers 
31 of semiconductor quantum well, quantum wire, or quantum dot (layers 
shown in FIG. 2-1) having major sides 31A parallel to one another and 
minor sides that are defined by the thickness of each layer. In order to 
more effectively radiate into the guided mode, the active medium is 
located generally around the midpoint of the core thickness dimension dw 
and each may comprise In.sub.0.53 Ga.sub.0.47 As or other suitable active 
quantum well, quantum wire, or quantum dot material (see FIG. 2A). 
Disposed proximate the major sides of the active medium are major sides of 
guiding semiconductor layers such as layers 32a, 32b which also have minor 
sides defined by the thickness of the layers involved. As shown in FIG. 2, 
barrier layers 31B (two shown) are disposed between the quantum well 
layers 31. The guiding or barrier layers each may comprise In.sub.0.84 
Ga.sub.0.16 As.sub.0.67 P.sub.0.33 or other suitable passive guiding or 
barrier materials (see FIG. 2A). The refractive index of the guiding 
layers 32a, 32b and active medium 30 is at least 1.3 times as great as 
that of relatively low refractive index mediums 23, 33, 35 bordering the 
sides 20a, 20b, 20c, 20d of the waveguide core. 
The active medium 30 alternately may comprise one or more optically active 
rare earth ion containing layers in lieu of the quantum well layers 31. 
The rare earth ion containing layers are located generally at the midpoint 
of the core thickness dimension dw and each can comprise rare earth ion 
doped semiconductor material (e.g. Ert.sup.+3 doped InP) or other suitable 
optically active rare earth ion containing material. When the rare earth 
ion containing layers are employed, the guiding layers comprise the same 
material described above. 
In practicing embodiments of the invention, the photonic-wire waveguide can 
comprise semiconductor materials In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y or 
In.sub.x Al.sub.1-x-y Ga.sub.y As as the n.sub.core material and an 
aforementioned material with a refractive index of about 1.6 or lower as 
the n.sub.low material. Alternately, the photonic-wire waveguide can 
comprise semiconductor materials In.sub.x Ga.sub.1-x N or Al.sub.x 
Ga.sub.1-x N as the n.sub.core and n.sub.high materials and a material 
with a refractive index of about 1.6 or lower as the n.sub.low material. 
Still further, the photonic-well waveguide can comprise semiconductor 
materials Al.sub.x G.sub.1-x As or In.sub.x Ga.sub.1-x P as the n.sub.core 
and n.sub.high materials and a material with a refractive index of about 
1.6 or lower as the n.sub.low material. 
The active medium 30 can be excited by suitable means such as optically 
(e.g. by a pumping laser providing pulsed light of appropriate duty cycle) 
or electrically (e.g. by electrical current pulses of appropriate duty 
cycle comprising injection current pumping through a p-n diode junction) 
as is known. Injection current is supplied to core 20 via conventional 
transparent conductor (not shown) such as indium tin oxide at suitable 
location in the waveguide structure. 
The layers 31, 32 define in FIG. 1 a circular shape waveguide core 20 
having an inner diametral rim or surface and outer diametral rim or 
surface. When the circular shape is closed onto itself as such, forming a 
closed loop, a high Q and high gain cavity is provided. Let the length of 
this cavity be Lc, defined approximately by the averaged physical length 
of the outer and inner perimeters of the closed loop. The normalized 
cavity length is then defined by Lc=Lc/(lambda/n.sub.c), where lambda is 
the emission wavelength of the active layers and n.sub.c is the effective 
refractive index for the electromagnetic field mode propagating in the 
waveguide core 20 (n.sub.c typically is well approximated by the 
refractive index of the waveguide core material n.sub.core but may fall to 
below n.sub.core, such as about 0.7 n.sub.core when the normalized 
thickness is thin, such as about 0.4). The normalized length of the cavity 
is typically smaller than 1000. The high Q (Q being quality value of the 
cavity at transparency or in the absence of material absorption) cavity 
can be an efficient laser cavity and in particular, can be a lasing 
microcavity. The invention is not limited to circular shaped waveguide 
cores and can be practiced with linear shaped waveguide cores and 
waveguide cores having arcuate segments and linear segments. 
For efficient operation of the light emitting device or laser, it is 
desireable to capture as much emission power into the electromagnetic 
field mode of interest (usually a guided TE mode or TM mode of the 
waveguide) both spatially and spectrally. The spatial mode is usually 
defined by the spatial profile of the guided mode. The spectral modes are 
clearly defined in a cavity situation for which there are discrete cavity 
resonance frequency modes. The spectral modes are then the cavity 
resonance frequency modes. 
The percentage of emission captured spatially into the electromagnetic 
field mode of interest will be denoted by Beta(space). The emission from 
an optically active medium usually spans a certain frequency range given 
by an effective spectral width, dfe. For those emission that is captured 
spatially into the field mode of interest, there are many cavity resonance 
frequency modes it can emit into, the percentage of which captured into 
the resonance frequency mode of interest being denoted by Beta(Freq). Let 
the frequency spacing between two adjacent resonance frequency modes be 
given by dfc. For the case where the spectral width of the emission is 
larger than dfc (i.e. dfe greater than dfc), the value of Beta(Freq) is 
approximately given by Beta(Freq)=(dfc/dfe)*(2/.pi.). For high Beta(Freq) 
value, it is generally desirable to have narrow emission spectral width, 
such as that typically provided by semiconductor quantum wells, quantum 
wires, quantum dots, or rare earth ions. 
The total percentage of emission captured into the desired field mode both 
spatially and spectrally is the product of Beta(space) and Beta(Freq) and 
is denoted by Beta(tot). That is, Beta(tot)=Beta(space)*Beta (Freq). 
Beta(tot) is called the spontaneous emission coupling efficiency. 
The value of dfc is related to the cavity length via a simple relation of 
dfc=c/(n.sub.c Lc), where c is the speed of light in vacuum (10.sub.8 
meters per second), Lc is the round trip physical length of the cavity, 
and n.sub.c is the effective propagation refractive index for the modes 
propagating in the cavity (or more accurately n.sub.c Lc is the effective 
round trip optical path length of the cavity). An optical cavity can be 
said to be an ideal microcavity when the cavity size Lc is so small as to 
give a large dfc value so that Beta(Freq) approaches unity (i.e. when dfc 
is almost as large as dfe so that (BetaFreq)=1.0). In practice, an optical 
cavity can be said to be a microcavity if it's (BetaFreq) is larger than 
approximately 0.03 (10 times larger than that of the conventional 
semiconductor laser structure described). It can be said to be a good 
microcavity if Beta(Freq) is larger than 0.1. 
The present invention provides in an embodiment a light emitting device 
having Beta(tot) much larger than that available from the usual 
semiconductor laser cavity design and from which a working device can be 
realized. For example, a typical prior semiconductor laser cavity design 
with a 300 micron long linear cavity defined by a ridge waveguide 
exhibiting a Beta(tot) value of about 0.00001 for a Beta(space)=0.004 and 
Beta(Freq)=0.003. The present invention provides a light emitting device 
having a Beta(tot) which can be as large as 0.3 and higher achieved 
through combined use of photonic-wire waveguides, microcavity structure 
and an appropriate active medium, such as quantum wells. 
A particular photonic-wire light emitting device in accordance with an 
embodiment of the present invention comprises a relatively high refractive 
index photonic-wire semiconductor waveguide core 20. The photonic-wire 
waveguide core is formed into an arcuate shape, linear shape, and 
combinations thereof. As described in more detail below, when the 
photonic-wire waveguide 12 is closed onto itself forming a closed loop 
cavity, a high Q and high gain cavity is provided. The details of the 
cavity design are essential for achieving high Q value and will be 
described below. The high Q cavity can be an efficient cavity for the 
light-emitting device or laser and in particular, can be a lasing 
microcavity. 
The high refractive index semiconductor waveguide core is surrounded by a 
relatively low refractive index medium on all sides 20a, 20b, 20c, and 
20d. This confines photons tightly in all directions perpendicular to 
their direction of propagation. The strong confinement forms strong 
confining potential walls for photons and restricts the photons to move 
relatively freely only in one direction, and the waveguide is called a 
photonic-wire waveguide. The strong potential is necessary to affect the 
emission properties of the optically active layers of the waveguide core, 
and can be used to drastically increase the percentage of emission into 
one particular waveguiding mode of interest. For example, typical 
semiconductor waveguide core materials for use in practicing the invention 
have a refractive index n.sub.core greater than about 2.5, such as from 
about 3 to about 3.5 and above for InGaAsP or AlGaAs materials, whereas 
typical low-refractive index media for use in practicing the invention 
have refractive index n.sub.low below about 2.0, such as from about 1.6 to 
about 1.0 for silica or air. The ratio of the refractive indices 
n.sub.core /n.sub.low has to be larger than about 1.3 to obtain 
substantial advantages of the invention. 
The waveguide core 20 comprises an active medium 30 having major and minor 
sides and semiconductor guiding layers proximate the major sides to define 
a generally rectangular core cross-section comprising a width dimension ds 
and thickness dimension dw. The width dimension or direction is parallel 
to the substrate on which the waveguide is disposed, while the thickness 
dimension or direction is perpendicular to the substrate. 
The strong photons confinement is provided in the width and thickness 
directions. The low refractive index materials described above are the 
materials residing on all sides 20a, 20b, 20c, and 20d of the waveguide 
core 20 along the width and thickness direction. A normalized width 
ds.sub.n greater than 0.6 and not exceeding 1.1 and a normalized thickness 
dw.sub.n greater than 0.1 and not exceeding 0.5 will provide optimal 
emission efficiency into the lowest-order guided mode, giving a 
Beta(space) of about 0.25 or larger. However, a normalized width of 
typically 1.0 and up to 4.0 and a normalized thickness of typically 0.1 up 
to 0.6 will still provide reasonable efficiency. A normalized thickness 
exceeding 0.5 and up to 1.1 and a normalized width of larger than 0.5 and 
less than 4.0 will still provide device of interest, although it will 
generally be operating with multiple waveguide modes, and will have 
relatively low emission into the lowest-order mode. This requirement may 
be relaxed to have normalized wavelength width larger than 4.0 in the case 
where the waveguide is in the form of a circular arc with a large 
curvature as described in detail below. The normalized width dimension 
ds.sub.n for the strong confinement direction is expressed as ds.sub.n 
=ds/ds.sub.lambda with ds.sub.lambda =Lambda/n.sup.s.sub.eff, where Lambda 
is the active-medium emission wavelength in free space (i.e. vacuum or in 
the absence of material medium) and n.sup.s.sub.eff is the effective 
refractive index given by n.sup.s.sub.eff =Sqrt(n.sub.core.sup.2 
-n.sub.low.sup.2), where n.sub.core is the refractive index of the 
waveguide core, n.sub.low is the refractive index of the materials 
surrounding the waveguide core, and Sqrt denotes square rooting. The 
normalized thickness dimension dw.sub.n for this weak confinement 
direction is expressed as dw.sub.n =dw/dw.sub.lambda with dw.sub.lambda 
=Lambda/n.sup.w.sub.eff, where Lambda is the active-layer emission 
wavelength in free space (i.e. vacuum or in the absence of material 
medium) and n.sup.w.sub.eff is the effective refractive index given by 
n.sup.w.sub.eff=Sqrt(n.sub.core.sup.2 -n.sub.low.sup.2). 
In a particular embodiment, the normalized width of the photonic-wire 
waveguide ds.sub.n is about 0.85 and the normalized thickness dw.sub.n is 
about 0.4 with the index ratio n.sub.core /n.sub.low larger than 2.0. 
In the limit of small enough photonic-wire waveguide core dimensions, the 
strong confinement of photons in the photonic-wire waveguide core will 
give rise to a drastic modification of the photonic density of states 
(quantum states for which the photons are allowed to be emitted into) in 
such waveguide. This drastic modification of the photonic density of 
states will cut off certain dipole emissions from the active medium and 
relatively enhances emission from one particular dipole orientation. 
Specifically, when the active medium is excited, the emission from the 
active medium can be modeled as radiation from three types of mutually 
orthogonal electric dipoles; namely, one type of dipole pointing in the 
thickness direction called the vertical dipole, one type of dipole 
pointing in the width direction called the horizontal dipole, and one type 
of dipole pointing along the axis of the waveguide (or direction of photon 
propagation of the guided mode) called the axial dipole. In addition to 
having emission from three types of dipoles (vertical, horizontal, and 
axial dipoles), the emission energy from each type of dipole can go either 
into the TE (transverse electric) polarization or the TM (transverse 
magnetic) polarization of the guided modes. 
Normally only one of the dipole types contributes most of the energy into 
the guided modes of interest and the emission power from the other two 
dipole types is almost fully wasted. If the unused emission power from 
these other two dipole types can be strongly reduced, then one can save 
the pumping power supplied to the active medium by a factor of 60% or 
larger. 
As part of the invention, we have found that with use of photonic-wire 
structure, we can substantially reduce or eliminate some of these unwanted 
dipole emissions, leading to a great increase in efficiency. 
Via the use of photonic-wire waveguide, if the normalized width dimension 
ds.sub.n is larger than about 0.6 and smaller than about 0.85 (or up to 
about 1.1) and if the normalized thickness dimension dw.sub.n is larger 
than about 0.1 and smaller than about 0.5, the emission power from the 
vertical dipole and axial dipole can be strongly reduced. This means that 
most of the pumping power supplied to active medium will end up as 
emission power from the horizontal dipole. Furthermore, most of the power 
from the horizontal dipole will go to excite mainly the TE polarization 
modes in the waveguide. Moreover, with the chosen dimensions of ds.sub.n 
and dw.sub.n, the waveguide can only support the lowest order guided mode, 
and the strong confinement in the waveguide ensures that the horizontal 
dipole will not radiate much to the radiation modes that escape the 
waveguide. Combining the aforementioned effects means that most of the 
emission power from the active medium will go into only one single 
waveguide mode, namely the lowest-order guided mode with TE polarization. 
Hence, through a combination of quantum effect which strongly reduces 
unwanted emission from the two dipole types, and strong photon confinement 
which allows the emission from the remaining horizontal dipole to couple 
only into the guided mode and not the radiation modes that escape the 
waveguide, it is possible to get a huge percentage of the emission power 
into a single TE polarized mode of interest. 
This can lead to an extremely large spatial spontaneous emission factor 
Beta(space) of larger than 0.30. The theoretical maximal value of 
Beta(space) is 0.5 due to the existence of two opposite directions of 
propagation for the waveguide modes (i.e. at most only half the emitted 
power can go into one of the propagating mode directions). Hence, the 
Beta(space) value for the photonic-wire waveguide is close to the ideal 
value of 0.5. This is much larger than the Beta(space) value of a typical 
conventional ridge waveguide used in a typical light-emitting device, for 
which Beta(space) is about 0.004. 
The extremely large Beta(space) value leads to a significantly enhanced 
optical gain for the active medium and efficiency for the conversion of 
the pumping power, which makes it possible to achieve lasing with a small 
cavity. In this invention we have realized such a photonic laser and show 
that lasing can be achieved in a microcavity with an extremely small 
cavity volume of less than 0.3 cubic micrometers or 10.sup.-18 cubic 
meters, which is about the smallest cavity ever made to lase. We also show 
that in a particular realization the pumping threshold needed for lasing 
in such a small cavity can be as low as 100 microWatts, which is about 100 
times lower than the threshold pumping power for a typical 300 micron long 
semiconductor laser. 
When the dimensions of the photonic-wire waveguide are larger than the 
above prescribed dimensions, the spontaneous emission coupling efficiency 
into the lowest-order spatial mode of opportunity is approximately: 
Beta(space)=cos.sub.-1 (1-1/rs)*cos.sub.-1 (1/rw)/(3 pi.sup.2 *fw*fs) where 
fw=dw.sub.n if dw.sub.n is greater than 1/Sqrt(3)=0.577 and fw=1 if 
dw.sub.n less than 1/Sqrt(3), and fs=ds.sub.n if ds.sub.n is greater than 
1/Sqrt(3) and fs=1 if ds.sub.n less than 1/Sqrt(3) where Sqrt is square 
root and where cos.sup.-1 is the inverse cosine or arc-cosine and 
rs=n.sub.core /n.sub.low and rw=n.sub.core /n.sub.high. The Beta(space) 
defined is a function of emission captured spatially into one particular 
polarization (TE and TM) of the lowest-order guided mode that is 
propagating in a particular direction of propagation of the waveguide 
(these are two mutually opposite directions for which a wave can propagate 
in the wave guide). The above formula gives a good estimation of 
Beta(space) generally except in structures where ds.sub.n is less than 0.5 
or dw.sub.n is less than 0.6. When ds.sub.n is less than 0.5, for example, 
emission into one of the two polarizations may be cutoff, leading to a 
factor of 2 increase in the Beta(space) factor for the remaining 
polarization. When dw.sub.n and ds.sub.n are both less than 0.1, emission 
may be reduced or cutoff, leading to a drastic decrease in the Beta(space) 
value. A more exact value for Beta(space) can be calculated based on 
quantum electrodynamic calculations which can deviate (typically larger) 
from this simple estimation by a few times especially for the case where 
ds.sub.n or dw.sub.n isless than 0.5. 
This Beta(space) value of up to about about 0.3 and higher attributed to 
the photonic-wire waveguide core in accordance with the present invention 
is substantially larger than that of the typical semiconductor laser 
waveguide structure with Beta(space)=0.004. 
Moreover, this large Beta(space) value coupled with the high Q value 
micro-cavity design for which Beta(Freq) is close to 1, provides Beta(tot) 
up to about 0.03 for the waveguide core in accordance with the present 
invention, which is much larger than the Beta(tot) factor of typical 
semiconductor laser waveguide structure with Beta(tot)=0.00001. The 
Beta(tot) will still be as large as 0.0035 (350 times larger than that of 
the typical semiconductor laser structure) even when the cavity is not at 
the ideal microcavity limit but is reasonably "micro" so that Beta(Freq) 
is greater than 0.05. 
A reasonable microcavity with Beta(Freq) greater than 0.05 can be achieved 
by closing the photonic-wire waveguide 12 on itself to form a small closed 
loop cavity. Let the cavity's physical length be denoted by Lc and the 
effective propagating refractive index for mode in the cavity be denoted 
by n.sub.c (n.sub.c is closely approximated by n.sub.core except when ds 
is small so that ds.sub.n is less than 1.0 at which n.sub.c can fall 
substantially below n.sub.core). The normalized cavity length is defined 
by Lcn=Lc/(lambda/n.sub.c ). A normalized cavity length of typically 300 
or smaller is needed to achieve Beta(Freq) of about 0.05 and larger 
(assuming the emission width of the optically active medium is about 
10.sup.13 Hertzs (or 70 nanometers), which is typical for that of a 
quantum well emitting at 1.5 microns, Lcn=300, and n.sub.c =2.4 (for 
ds.sub.n =0.3), gives dfe=10.sup.13 Hertzs, dfc=1.7*10.sup.12 Hertzs, and 
Beta(Freq)=0.05. 
Generally, the microcavity with cavity Lc specified above provides a 
Beta(Freq) of about 0.05 and larger when the emission frequency width from 
the active medium is equal to about 1/20 of the optical emission frequency 
exhibited by semiconductor quantum wells. 
One particular embodiment of the invention with near optimal properties for 
operation at 1.1 microns to 1.6 microns wavelength range employs dimension 
ds that is about 0.6 micron or smaller and dimension dw that is 0.3 micron 
and smaller with ds being larger than about 0.2 micron and dw being larger 
than about 0.03 micron. 
Another particular embodiment of the invention for operation at 0.2 micron 
to 0.7 micron wavelength range employs a dimension ds that is about 0.3 
micron or smaller and dimension dw that is about 0.3 micron and smaller 
with ds being larger than about 0.04 micron and dw being larger than about 
0.01 micron. 
Still a further particular embodiment of the invention for operation at 0.6 
micron to 1.0 micron wavelength range employs a dimension ds that is about 
0.4 micron or smaller and dimension dw that is about 0.2 micron and 
smaller with ds being larger than about 0.1 micron and dw being larger 
than about 0.02 micron. 
The general design for such high-Q cavity is illustrated in FIG. 3, which 
schematically shows a cavity formed by a closed loop of photonic-wire 
waveguide 12 with one or more arcuate shapes or one or more linear shapes, 
or combinations thereof. Each arcuate segment can be formed by one or more 
circular segments or some times by many linear segments of sufficiently 
short lengths joined to each other with a small bend. The simplest design 
involves no linear section and is in the form of a perfect circle as shown 
in FIG. 4. A more complicated variation consisting of an arcuate segment 
and a linear segment is shown in FIG. 5, while a variation consisting of 
two arcuate segments and two linear segments are shown in FIG. 6. 
In order to achieve low loss and hence high Q for the cavity, the curvature 
of the arcuate segments cannot be arbitrary but must satisfy a minimal 
radius of curvature. In addition, two adjacent segments joined to each 
other must have the same tangents for their line sections at the joint. 
For the sake of defining the radii of curvature of the arcuate segments, 
each arcuate segment will be approximated by one or more circular 
segments. High accuracy of approximation is achieved when the error of 
approximation involves spatial deviations smaller than one tenth the 
optical wavelength of emission in the photonic-wire waveguide. The outer 
rim of each such circular segment is formed by a circular arc drawn with a 
certain radius of curvature R from a certain center C. We can label the Rs 
and Cs of all such circular segments in FIG. 3 by R1, R2, etc. and C1, C2, 
etc., respectively, which will be referred to as their radii and centers 
of curvature. 
If the center of curvature Ri (i=1,2, etc.) for arc i is too small, photons 
in the photonic-wire waveguide can escape from the waveguide and radiate 
in a direction away from the center of curvature. This will result in 
cavity photon loss and hence low cavity Q value. To avoid loss, the design 
requires that: 
.vertline.Ri.vertline.&gt;-lambda/(2.pi.)!*Lnlambda/(2.pi.n.sub.core 
L.sub.arc)!/Lnn.sub.core /n.sub.low ! 
where L.sub.arc is the length of the circular arc and Ln is the Natural 
Logarithm. For the purpose of illustration, it is necessary to specify if 
the curved arc is around the center designated P.sub.c or away from the 
center. If it is around the center, Ri is assigned a positive sign and if 
it is away from the center, Ri is assigned a negative sign. 
More specifically, let point Pc be a point in the center region of the 
closed loop, we will assign Ri (i=1, 2, etc.) for arc i to be positive if 
its center of curvature Ci lies on the same side of arc i as point Pc, and 
negative (i.e. Ri=.vertline.Ri.vertline.) if its center lies on opposite 
side of the arc i as point pc. For the general case in FIG. 3, Ri can 
either be positive or negative (e.g. R2 is negative). 
Lasing threshold is achieved when the round trip optical gain of the 
electromagnetic field mode propagating in the cavity due to the excited 
active layer exceeds the round trip optical loss. The round trip loss can 
be due to absorption and scattering by impurities or defects in the 
waveguides, as well as roughness on the waveguide side walls. It can also 
be due to photons leaking out to the output-coupled waveguide described 
below. 
The pumping power needed to achieve lasing threshold is partially used to 
cause inversion in the active layer at which the active medium will become 
transparent and begin to contribute to optical gain. This is called the 
transparency pumping power. It is partially used to achieve additional 
gain above the transparency point so as to overcome the cavity loss as 
described above. This is called the additional-gain pumping power. Due to 
the high Q, low loss nature of the cavity, the additional-gain pumping 
power may be a small part of the total threshold pumping power. In that 
case, most of the threshold pumping power is used to make the medium 
transparent (i.e. used as transparency pumping power). The transparency 
pumping power needed is proportional to the total material volume of the 
active medium. It can be reduced by using thinner active medium or by 
reducing the area of the active medium in the cavity. Hence, the threshold 
pumping power may be reduced by having the active medium present only 
along a small section of the entire length of the waveguide that forms the 
cavity (this is illustrated in FIGS. 10, 10A, and 10B where a small 
section 40 of the waveguide has a photonic-wire structure 42 with an 
active medium 45, while the remaining length of the waveguide 50 that 
forms the cavity (40 and 50) has a photonic-wire waveguide structure 52 
sans an active medium. 
To reduce side-wall scattering loss, it is in general desirable that the 
side walls of the photonic-wire waveguide core, especially the two side 
walls 20a, 20b for the strong photon confinement in the width direction, 
be fabricated with smooth surfaces having roughness less than one tenth of 
lambda/n.sub.core. Power loss due to side-wall scattering loss is also 
dependent on the refractive index difference between the waveguide core 
and the surrounding materials, or specifically directly proportional to 
(n.sub.core.sup.2 -n.sub.low.sup.2).sup.2. 
Thus, reducing the refractive index difference between the waveguide core 
and the surrounding materials can reduce the scattering loss. However, as 
described above, it will trade-off (i.e. reduce) the efficiency of 
emission given by Beta(Freq), which is also dependent on the refractive 
index difference. These considerations must be taken into account in the 
design and fabrication of the cavity. 
The requirement of smooth waveguide side walls, however, can be relaxed for 
those highly curved arcuate segments where the Ri's are small. In that 
situation, as explained below, it may actually be desirable to have 
roughness fabricated on some of the side walls to enforce lasing in the 
lowest-order guided mode. 
The situation is illustrated for the simplest case of a circular, annular 
core cavity in FIG. 7. When the radius of curvature R1 in FIG. 7 is 
smaller than approximately 50 times the optical wavelength in the 
waveguide core 20 (i.e. R1 less than 50 lambda/n.sub.core), the strong 
curvature can cause the guided modes to squeeze towards the outer rim of 
the waveguide core as shown by the guided mode cross-section in FIG. 7A. 
The outer or inner rim of a curved segment is the side wall (20a or 20b) 
of the photonic-well waveguide that is respectively closer to or farther 
from the segment's center of curvature. Generally, the large curvatures 
squeeze the mode width of the lowest-order mode to less than about ten 
optical wavelengths in the photonic-wire waveguide, and force it to 
porpagate along the outer rim of the curved waveguide. In particular, the 
lowest-order mode (mode 1) is squeezed to a mode width d.sub.1 ' of less 
than 10 lambda/n.sub.core. If the waveguide width ds is larger than the 
mode width d.sub.1 ', the lowest-order mode will touch mainly the outer 
rim of the curved waveguide due to centrifugal force on the photons 
undergoing curvilinear propagation. The higher-order modes, such as the 
second-order mode 2, will have a mode width d.sub.2 ' larger than d.sub.1 
' and will touch more of the inner rim of the curved waveguide. As a 
result, roughness along the inner rim of the waveguide will cause much 
more loss to the higher-order modes than the lowest-order modes, and will 
eliminate lasing in the higher-order modes when sufficient loss is 
imposed. 
The waveguide width ds typically is larger than the lowest-order mode of 
propagation and at least a portion of an inner rim of the closed loop is 
roughened to increase scattering loss on higher-order modes. 
The general case is shown in FIG. 8 for which there are multiple arcuate 
segments forming a closed loop cavity. In order for this configuration to 
work, it is desirable that every segment with substantial segment length 
has a radius of curvature Ri smaller than 50 times the optical wavelength 
in the waveguide core (i.e. Ri less than 50 lambda/n.sub.core). Straight 
segments can be viewed as curved segments with an infinite radius of 
curvature and are not desirable to be present in this scheme. The 
waveguide width ds must be larger than the mode-width d.sub.1 '. A typical 
design has its normalized waveguide width ds.sub.n to be larger than 0.1 
but smaller than approximately 10.0. For each segment, roughness can be 
present on the rim closer to its center of curvature. To eliminate lasing 
in the higher-order modes, roughness present on some portions of segment 
length may be sufficient (i.e. not the entire length must have roughness 
present on such side walls). 
Note that since the lowest-order mode is squeezed to the outer rim and to a 
normalized mode-width of less than 10.0, the position of the inner rim 
would have no significant affect on its mode width if distance between the 
outer rim and inner rim is larger than the mode width d.sub.1 '. In fact, 
in that case, the clear presence of the inner rim is not needed and the 
waveguide region beyond a distance of about d.sub.1 ' away from the outer 
rim can take on different physical forms such as different thicknesses or 
sidewall roughness without affecting the mode confinement or performance 
of the device. Hence, the waveguide normalized width ds.sub.n can be 10.0 
or larger without affecting the waveguiding or gain of the lowest-order 
mode significantly from the case where ds.sub.n =10.0. Thus, in this 
situation we can relax ds.sub.n to have a large value limited only by the 
allowed diameter of the cavity size such as 100, at which the curved 
waveguide will have a radius of curvature of 50 lambda/n.sub.core. Thus, 
ds.sub.n can be larger than 0.1 and less than 100. In practice, as the 
lowest-order mode only occupies a small region close to the outer rim, it 
is economical to keep ds.sub.n to be around 4.0 or smaller such as to 
reduce the area of the active medium which can draw additional pumping 
power. 
Optical coupling of the device described hereabove to an output can be 
effected by a light output-coupled waveguide W proximate a portion of the 
periphery of the waveguide core 20 in a manner to achieve resonant photon 
tunneling. The waveguide W is spaced by a small gap of low refractive 
index material (e.g. air) from the photonic-wire waveguide 12. The 
coupling involves a short section of the cavity and the waveguide W in 
which the photons in the cavity are propagating in parallel to each other. 
The refractive index of the gap region is lower than the refractive 
indices of the waveguide material and cavity material. The gap size 
typically is smaller than 10 and larger than 0.02 optical wavelength in 
the photonic-wire waveguide. The amount of coupling or energy transfer 
between the output-coupled waveguide W and the waveguide core 20 increases 
with decreasing gap size or increasing coupling length, where the coupling 
length is the length for which the output-coupled waveguide W proximates 
the waveguide core 20. 
The vertical structure in the thickness dimension of the output-coupled 
waveguide W can be the same as that of the laser cavity. In that case, it 
has to be excited by optical or electrical pumping to avoid absorption of 
light by the unexcited active medium in the waveguide as illustrated in 
FIG. 11. To avoid the need for pumping the output-coupled waveguide W, its 
vertical structure may be the same as that of the laser cavity but without 
the active materials as illustrated in FIGS. 12, 12A, and 12B. The 
output-coupled waveguide can be formed at a level on a substrate 26 
compatible with an integrated optical circuit IC present on the substrate 
so as to provide light output signals to the optical circuit. 
The output-coupled waveguide W can also have active materials present only 
along certain sections of the waveguide as illustrated by sections 55 and 
56 of FIG. 13. The sections 55, 56 with active materials can be pumped to 
achieve population inversion, giving rise to optical gain for light 
propagating in the waveguide W. This can be used to provide further 
amplificaton of optical power from the cavity, FIG. 13A, leading to a 
higher net optical power in the waveguide. 
If the active medium section is not pumped, it can function as a detector 
in the following manner. It can absorb light in the waveguide which will 
cause population excitation of the active medium in proportion to the 
light power absorbed. The population excitation can be detected for 
example via a closed electric circuit, leading to a circuit current. The 
circuit current is then an indication of the optical power in the 
waveguide. The active medium section can also function as a modulator by 
modulating the pumping power, leading to a modulation of optical 
absorption or gain of the active medium section an d hence the optical 
power through the section. 
The cavity may be coupled to one or more output-coupled waveguides W at 
different segments of the cavity as illustrated in FIG. 14, providing six 
output ports 60, 61, 62, 63, 64, and 65. It must be noted that light in 
the closed loop cavity may be divided into two beams, one that propagates 
in the clockwise direction 70 and one that propagates in the 
counterclockwise direction 71. One or both of the beams may be brought 
above lasing threshold via external pumping. 
The output-coupled waveguide W may also serve to couple light from other 
light source or another such photonic-wire light-emitting device, into the 
cavity, thereby forming input ports to the cavity. This is illustrated in 
FIG. 15 where two input ports 70a, 71 and two output ports 72a, 73 to 
cavity 80 are shown. 
The input ports are used to introduce light in the form of pulses or 
continuous wave beams into the cavity, which can affect the properties of 
light output from the cavity. In particular, the amplitudes, phases, 
frequencies, pulse duration, and polarization of the light output from the 
output ports can depend on the amplitudes, phases, frequencies, pulse 
duration, and polarization of the light input into the input port. The 
dependence can be different depending on the excitation level or 
population inversion of the active medium in the cavity, or on whether the 
input beams in the cavity are brought above or below the lasing 
thresholds. 
This dependence comes about through the mechanisms of injection locking and 
light transmission through a nonlinear optical cavity as is known for 
which light injected into a cavity can change properties of light in the 
cavity in terms of amplitudes, phases, frequencies, pulse duration and 
polarization thereby leading to a change in the properties of the output 
light from the cavity. A change in the population excitation of the active 
medium in the cavity can lead to the change of cavity gain or loss, and 
the refractive index in the cavity, thereby affecting the mechanism of 
injection locking or light transmission through a resonant cavity, which 
then affects the properties of the output light. 
Such input-output dependence can be used for processing signals or 
information coded in the properties of the input light beams, and can be a 
useful device for application to optical communications, optical 
interconnects, optical sensing, optical signal processing, and optical 
computing. 
The presence of both input and output ports allows two or more of the 
photonic-wire cavities to connect to each other by connecting the output 
port(s) of one cavity to the input port(s) of the other cavity. This 
allows one to build up a circuit comprising one or more of such cavities, 
which by itself can be a high-density photonic-wire integrated circuit. 
The properties (amplitudes, phases, frequencies, pulse duration and 
polarization) of the output beams from the output ports can be changed (or 
modulated) by changing the excitation level of the active medium via 
modulating the pumping power supplied to excite the active medium, such as 
the external injection current ic to the active medium when electrically 
pumped. Such modulation can be generally imposed at high rate or frequency 
if the Beta(tot) value of the cavity is high, especially when operating at 
above lasing threshold. 
A typical single device useful for practical applications can consist of a 
laser cavity formed by waveguide 12 coupled to a U-shaped output-coupled 
waveguide W as shown in FIG. 9. In order to achieve resonant photon 
tunneling of photons, the mode width for the electromagnetic field mode 
propagating in the output-coupled waveguide W must be close or similar to 
the mode width for the electromagnetic field mode propagating in the 
cavity. This will ensure that the field modes propagating in the 
output-coupled waveguide and the laser cavity have similar spatial width 
or propagating velocities so that they can couple to each other with 
maximal efficiency. 
By cleaving the ends WE of the U-shaped waveguide W shown in FIG. 9, laser 
light radiation into free-space (i.e. air) can be obtained. The spatial 
mode profile in the width direction and thickness direction is determined 
by the width and thickness of the output-coupled waveguide, which can be 
changed at the ends via tapered waveguide sections shown FIG. 9A. Thus, 
the waveguide width dsg.sub.laser and thickness dwg.sub.laser at the laser 
is chosen to achieve maximal coupling of light, while the waveguide width 
dsg.sub.out and thickness dwg.sub.out at the end WE is chosen to achieve a 
certain spatial profile for the light output. In particular, the spatial 
mode profile of the free-space radiation mode in the width direction can 
be made to have the same size as the spatial mode profile in the thickness 
direction, thereby achieving a near circular output spot size. Circular 
output spot size is generally desirable to achieve efficient coupling into 
an optical fiber via use of the coupling lens as shown in FIG. 9. 
For purposes of illustration and not limitation, the photonic-wire light 
emitting device having the dimensions described above can be formed by a 
fabrication process involving nanofabrication techniques including 
electron-beam (e-beam) lithography and reactive ion etching (RIE). For 
example, an InP substrate can be coated with an epitaxial InGaAsP/InGaAs 
laser layer structure of 0.19 micron thickness. Within the layer 
structure, three 100 Angstrom thick quantum well layers (Ino.sub.0.53 
Ga.sub.0.47 As) can be separated by 100 Angstrom thick guiding or barrier 
layers (In.sub.0.84 Ga.sub.0.16 As.sub.0.33 P.sub.0.67). They can be 
sandwiched by two 700 Angstrom thick (In.sub.0.84 Ga.sub.0.16 As.sub.0.33 
P.sub.67) guiding or barrier layers on both sides. 
A wafer bonding and etching technique can be used to transfer the thin 
waveguide core 20 on top of a low index SiO.sub.2 cladding or medium on a 
GaAs substrate. First, 800 Angstrom thick SiO.sub.2 is deposited on a 
wafer via plasma enhanced chemical vapor deposition (PECVD). Electron-beam 
lithography is used to write the core pattern on PMMA (poly methyl 
methylmethacrylate) coated on top of the SiO.sub.2 layer. The pattern then 
is transferred down to the SiO.sub.2 layer by etching away the unmasked 
region using the RIE process with CHF.sub.3 as the etchant gas under 31 
millitorrs with 60 Watts plasma power and then the PMMA is removed. The 
pattern in SiO.sub.2 layer then forms the mask for subsequent etching of 
the InGaAsP layer. The RIE process is used to etch the structure down 
vertically through the 0.19 micron InGaAsP/InGaAs epitaxial layer 
structure into the InP substrate. In this step, a gas mixture of methane, 
hydrogen, and argon can be used in a ratio of 10:34:10 under a gas 
pressure of 45 millitorrs and a plasma beam power of 90 Watts plasma 
power. 
In order to achieve placement of the thin annular waveguide core 20 on a 
low-refractive index material, the substrate is removed as follows. The 
RIE etched sample is deposited with 0.75 micron thick SiO.sub.2 using 
PECVD. A piece of GaAs substrate covered with 0.75 micron thick SiO.sub.2 
deposited via PECVD is then prepared. The two substrates are SiO.sub.2 
face-to-face bonded together using acrylic. Finally, a highly selective 
HCl etchant (HCl plus H.sub.3 PO.sub.4 in 1:1 ratio) was used to remove 
the InP substrate, leaving the photonic-wire light emitting structure on 
1.5 micron thick SiO.sub.2 on the GaAs substrate. 
For purposes of further illustration and not limitation, photonic wire 
lasers were made as described above with an outer diameter of 4.5 micron s 
and with a waveguide thickness of dw of 0.19 micron. These lasers were 
made with waveguide widths of ds of 0.25 micron and 0.4 micron. The 
photonic wire lasers so made were optically pumped with a 514 nanometer 
argon-ion laser modulated with 1% duty cycle in a vacuum-assisted 
Joule-Thomson refrigerator at 85K. The beam was focused with a 40.times. 
microscope objective lens to a size larger than the size of the waveguide 
core. The scattered output was collected from the top of each waveguide 
core via the objective lens and dispersed by an optical grating 
spectrometer (resolution of 1 nanometer) and detected using a lock-in 
technique and a liquid-nitrogen cooled germanium detector. 
FIG. 16 shows the typical lasing spectra obtained from the photonic-wire 
laser of the invention with a waveguide with a width of 0.4 micron and 
thickness of 0.19 micron pumped by a 514 argon-ion laser modulated with a 
1% duty cycle in a vacuum-assisted Joule-Thomson refrigerator at 85K. FIG. 
16 indicates lasing at 1403 nanometers (nm). The dashed curve is the 
spectrum at around the lasing threshold where the peak pump power absorbed 
by the laser is approximately 95 microWatts. The lasing power as a 
function of the peak pump power is shown in the inset designated FIG. 16A. 
Its spectral linewidth at 1.5 threshold was measured to be about 0.5 nm 
with a spectrum analyzer resolution of 0.1 nm. The cavity Q value was 
estimated to be Q=300 from the emission linewidth of the enhanced photo 
luminescence below lasing threshold near the cavity resonance. This 
corresponds to a waveguide loss of 2% per round trip primarily due to the 
average surface corrugations of about 0.002 micron s on the etched 
waveguide sidewalls. 
The small area (cross-sectional area of 0.02 microns squared) of the 
photonic-wire laser provides a large, spontaneous emission coupling 
efficiency of about 35%. The mode volume of this laser was only 0.27 
micron s cubed, and the total material volume was only 1 micron cubed. 
While FIG. 16 indicates that the photonic-wire laser of waveguide width of 
0.4 micron lased at 1403 nm, the photonic-wire laser having a waveguide 
width of 0.25 micron did not lase under the same excitation conditions 
described above. The lack of lasing was attributed to suppression of the 
emission into the lowest order guided ode when ds.sub.n is about 0.65 
micron (actual ds of approximately 0.3 micron ). The large drop in 
emission into the lowest-order guided mode is attributed to a large plunge 
in the photon density of states as well as an increase in the mode area at 
the very small waveguide dimensions, although Applicants do not wish or 
intend to be bound by any theory in this regard. As the waveguide width 
dimension of 0.25 micron creates ds approximately equal to dw less than 
0.65 micron, the laser structure will have a low gain, and hence cannot 
lase. These results indicate that photonic-wire lasers of the invention 
have a minimum waveguide cross-sectional area below which the Beta value 
can become small and lasing will cease. Thus, the photonic-wire lasers of 
the invention must have a waveguide cross-sectional area greater than this 
minimum value. 
Although the active medium materials is shown in the form of layer with a 
width equal to the waveguide width, those skilled in the art will 
recognize that the specific shape and dimensions of the medium can vary 
and the gain provided by the medium is dependent mainly on its volume of 
occupation being primarily at the center region of the cross-section of 
the waveguide core. 
Although the low refractive index materials 23 and low refractive index 
materials 30, 32 are described as having a particular refractive indices, 
those skilled in the are will recognize that they are used to indicate the 
typical refractive indices possessed by these materials. For example, with 
in each material, it is possible to have small refractive index 
fluctuations or variations spatially without affecting the mode 
confinement in the waveguide core or the performance of the device. 
Although the waveguide core 20 is shown with a rectangular cross section, 
those skilled in the art will recognize that the core shape can deviate 
from a perfect rectangular shape with an averaged thickness and width 
dimension of the invention and as set in the appended claims with regard 
to mode confinement and the performance of the device. 
Although the invention has been described with respect to certain specific 
embodiments thereof, those skilled in the art will recognize that these 
embodiments are offered for purposes of illustration rather than 
limitation and that the invention is not limited thereto but rather only 
as set forth in the appended claims.