 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to a saturatable absorbant with very short
switching times. It is used in optics, particularly in the production of
lasers emitting very short pulse trains and in the production of all
optical logic gates.
A saturatable absorbant is a material, whose absorption coefficient
significantly decreases when a large amount of light is applied to it.
This type of material has numerous applications, reference being made in a
non-limitative manner to the production of lasers operating under mode
locking conditions and emitting pulse trains of an extremely short width
below one picosecond and the production of all optical logic gates, in
which a first high power light beam controls the passage or stopping of a
second light beam.
Saturatable absorbants have long been constituted by amorphous materials,
such as dissolved colouring agents. However, for some years now, interest
has been attached to a new class of materials called multiple quantum
wells or MQW and to variants of these structures called superlattices.
A quantum well is obtained by inserting between two thin films of a first
conductive material a thin film of a second material having a smaller
forbidden band or gap than that of the first. In this way a potential well
is produced for the charge carriers in the central semicoductor having the
smallest gap, said well being surrounded by two potential barriers
corresponding to the two extreme films. A multiple quantum well is
obtained by superimposing such structures, without there being any
coupling between the wells (which is obtained by giving the barriers an
adequate thickness).
A superlattice is obtained when the potential barriers are sufficiently
thick for there to be a coupling between the different wells. Thus, a
superlattice is formed from a stack of films of two different
semiconductor materials having gaps of different heights. Potential wells
are produced in the films corresponding to the semiconductor with the
smallest gap and a potential barrier appears in each film corresponding to
the semiconductor with the largest gap.
FIG. 1 diagrammatically illustrates said structure and its properties. It
is possible to see in part (a), a stack of films of two semiconductor
materials SC.sub.1 and SC.sub.2. The energy level diagram is shown in part
(b), where G.sub.1 and G.sub.2 represent the gaps separating the valence
band at the bottom from the conduction band at the top. It is assumed in
FIG. 1 that semiconductor SC.sub.1 has the smallest gap and consequently
the potential wells are formed in this material. These wells have a width
Lp corresponding to the thickness of the corresponding films. In the films
of semiconductor SC.sub.2 having a thickness Lb are formed the potential
barriers. These wells are occupied by electrons in the conduction band and
by holes in the valence band.
Numerous publications deal with such superlattices. Reference is e.g. made
to two general articles entitled "solid state superlattices" published G.
H. DOHLER in Scientific American, November 1983, Vol 249, No 5, pp 144 to
151 and "Les super-reseaux artificiels" published by J. F. PALMER in
L'Echo des Recherches, No 105, July 1981, pp 41 to 48. In connection with
quantum wells, reference is also made to the article by R. M. KOLBAS et al
entitled "Man-made quantum wells: a new perspective on the finite
square-well problem" published in the American Journal of physics, 52 (5),
May 1984, pp 431-437.
Such materials have a double structural perodicity, one due to the
crystalline structure of the semiconductors used and the other due to the
regular stacking of the films. Thus, both in the valence band and in the
conduction band there are discrete energy levels (or microbands) which are
offered to the holes and electrons. FIG. 1 diagrammatically shows a level
Ee in the conduction band, which can be occupied by an electron (e) and a
level Eh located in the valence band, which can be occupied by a hole (h).
The position of these energy levels is obviously dependent on the materials
used and on the thickness of the films.
There is great interest in such structures. This has increased since it was
found that they had saturatable absorption lines. Absorption taken place
in the films corresponding to the semiconductor with the smallest gap,
where the wells are located. Since this discovery these devices have been
used in mode locking lasers. Thus, it has been possible to produce a
semiconductor laser emitting a pulse train of width equal to 1.3 ps with a
recurrence frequency equal to 1 GHz. The passive locking process is
obtained as a result of a external resonant cavity and a superlattice of
the multiple quantum well type bonded to one of the mirrors of the cavity.
This is described in the article by Y. SILBERBERG et al entitled "Passive
mode locking of a semiconductor diode laser" published in Optics Letters,
November 1984, Vol 9, No 11, pp 507-509. These devices have also been used
in optical logics. It has been possible to produce a logic NOR gate having
a switching time below 1 picosecond. Such an application is described in
the article by A. MIGUS et al entitled "One-picosecond optical NOR gate at
room temperature with a GaAs-AlGaAs multiple-quantum-well non linear
Fabry-Perot etalon" published in Applied Physics Letters, 46 (1), January
1985, pp 70-72.
The establishment of the phenomena involved in these applications is
extremely short and is approximately or less than 1 picosecond, which is
the switching time which can be called "on" and which represents the
absorption saturation. However, the return to equilibrium, which
characterises a switching time which can be called "off" is much longer
and is approximately 1 nanosecond.
It is widely accepted that it is the latter time which essentially limits
the performances of such devices. Various solutions have been proposed for
obviating this deficiency and particularly the irradiation of the
absorbant by electrons, which reduces the radiative life in the structure
and therefore the return to equilibrium time. However, this is difficult
and complex to carry out.
SUMMARY OF THE INVENTION
The invention has the same objective of reducing the "off" switching time,
but uses a quite different and much simpler means. According to the
invention, instead of giving the different wells identical widths, they
are given different widths, some being wide and others narrow. A certain
number of wide wells can alternate with a certain number of narrow wells.
In an advantageous variant, every other well is wide and every other well
narrow. The saturatable absorption is obtained in the narrow wells, whilst
the wide wells are used for collecting the photoexcited carriers produced
in the adjacent narrow wells.
In the present invention, a wide well is a well having a width between 9
and 25 nm and a narrow well is a well having a width between 5 and 9 nm.
These wells are separated by barriers having a width between 5 and 9 nm.
The introduction of an enlarged well into a superlattice has already formed
the subject matter of publications, e.g. the article A. CHOMETTE et al
entitled "Enlarged Wells as Probes for Study Superlattices", published in
Superlattices and Microstructures, Vol 1, No 3, 1985, Academic Press.
The introduction of an enlarged well has the effect of producing therein,
one or more energy levels below the lowest energy level of the conduction
band and above the highest energy level of the valence band. Thus, the
carriers tend to accumulate in the wide well or wells over low energy
levels.
The application according to the invention of said phenomenon to the
reduction of the recovery time of the saturatable absorption can then be
explained in the following way.
What prevents the restoration of the absorption in a prior art device is
the presence of a large number of hot carriers. These excess carriers only
thermalize slowly before recombining even more slowly. In the structure
according to the invention, the carriers are trapped extremely rapidly in
the wide wells. If the precaution is taken of ensuring that the wide wells
are definitely wider (at least 1.5 and preferably 2.5 times) than the
narrow wells, a high energy difference is obtained of approximately 50 meV
between the electronic confinement levels in said two types of wells (and
also for the levels offered to the holes). Although the thermalization and
radiative recombination in the wide wells is not faster than in the narrow
wells, the hot carriers are maintained in the wide wells and no longer
prevent the formation of excitons in the narrow wells.
The major consequence is that the mean restoration time of the excitonic
absorption following the stopping of the control excitation is limited by
the sum two characteristic times, namely the trapping time in the enlarged
wells which, in a structure according to the invention is extremely short,
there being no difficulty in obtaining times well below 1 ps and the
cooling time of the carriers trapped at an electronic temperature below
that at which they could be reemitted in the narrow wells.
It should be noted that the cooling of hot carriers takes place by two
mechanisms having very different characteristic times:
(a) diffusion by optical phonons, which is extremely fast (below 1 ps), but
can only occur when the carriers have an energy above 36 meV;
(b) when the carriers have an energy below 36 meV, the relaxation is
controlled by interaction with acoustic phonons and is much slower.
In order that the cooling time is not prejudicial, it is necessary for the
electronic levels in the wells of each of the widths to be spaced by more
than roughly 40 meV, which is the case in the structure according to the
invention.
Thus, by using the device according to the invention, it is possible to
obtain extremely short on and off switching times (typically 1 ps), which
opens the door to very important applications.
More specifically, the present invention relates to an absorbant with
saturatable absorption and low switching times, constituted by a
superlattice formed by a stack of films of a first semiconductor material
having a first forbidden band (gap) and a second conductor material having
a second forbidden band (gap) which is wider than the first, a potential
well being produced in each film corresponding to the first semiconductor
and a potential barrier in each film corresponding to the second
semiconductor, the films corresponding to the first semiconductor having a
thickness assuming two values, one large and the other small, the large
thickness being between 9 and 25 nm and the small thickness between 5 and
9 nm, the saturatable absorption of a radiation occurring in the films of
the first semiconductor having a small thickness and a rapid restoration
of this absorption occurs in the films of the first semiconductor having a
large thickness.
Preferably the large and small thicknesses of the wells are in a ratio R
exceeding 1.5. Advantageously, the ratio R is between 2 and 3 (e.g. 2.5).
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is described in greater detail hereinafter relative to
non-limitative embodiments and the attached drawings, wherein show:
FIG. 1, already described, diagrammatically a superlattice according to the
prior art.
FIG. 2, a superlattice according to the invention.
In part (a) of FIG. 2, it is possible to see a stack of films of two
semiconductor materials SC.sub.1 and SC.sub.2, the thickness of the films
of SC.sub.2 always being the same, but the thickness of the films of
SC.sub.1 is equal either to a low value Lp.sub.1, or to a high value
Lp.sub.2. In part (b), it is possible to see the energy level diagram of
the conduction band (the valence band is not shown, but it has a similar,
but symmetrical configuration). Barriers of width Lb separate the wells of
alternate widths Lp.sub.1 and Lp.sub.2. In the wide wells, the lowest
energy level E.sub.1 is below the level E'.sub.1, which is the lowest
level in the narrow wells.
The materials which can be used according to the invention are those
encountered in the production of multiple quantum wells and other
superlattices. Favoured materials are GaAs, AlAs Ga.sub.1-x Al.sub.x As.
In the latter case, the parameter x (which is between 0 and 1) makes it
possible to regulate the height of the gap knowing that the more x
increases, the more the band is widened. Generally x is between 0.2 and
0.4.
The Applicants have produced and studied absorbants according to this
principle in two cases:
EXAMPLE 1
Lb=5 nm
Lp.sub.1 =5 nm
Lp.sub.2 =12.7 nm
SC.sub.1 =GaAa
SC.sub.2 =Ga.sub.1-x Al.sub.x As with x=0.30
EXAMPLE 2
Lb=7 nm
Lp.sub.1 =6.5 nm
Lp.sub.2 =15.5 nm
SC.sub.1 =GaAs
SC.sub.2 =Ga.sub.1-x Al.sub.x As with x=0.30
Samples according to these two examples have been studied by the Applicants
with the aid of various experimental techniques and the following
characteristics have been verified. The trapping time in the wide wells is
extremely short. In both samples, the excitonic absorption corresponding
to the narrow well persists at 300 K. However, it is weaker in sample 1,
but the latter has a shorter trapping time. Thus, there must be an
equilibrium effect between the quality of the absorption at 300 K and the
off switching time. Conversely the excitonic resonance persists at 300 K
in the samples of the second example having a wider gap and the trapping
time still remains extremely short.
By using the saturatable absorbant according to the invention, it is
possible to considerably improve the devices described hereinbefore. In
the case of a mode locking laser, it is possible to operate at frequencies
above 100 Ghz (which makes it necessary to produce a cavity with a length
of less than 3 mm. In the case of an optical gate, it is possible to reach
a recurrence frequency up to 1000 Gbit/sec.
Other structures can obviously be used for giving the same effect, e.g. the
number of wide wells may not be equal to half the wells and instead
proportions of 1/3, 1/4 etc can be envisaged. It is also possible to
produce structures, where the narrow well is formed from an alloy
Ga.sub.1-x Al.sub.x As with x being low (the well could then be wider).
It is also possible to improve the efficiency of the transfer by bringing
into resonance the fundamental level E.sub.1 ' of the narow well and one
of the levels of the wide well E.sub.2 for example. This is carried out in
the samples and shown in FIG. 2.
It is finally pointed out that superlattice structures are already known in
which narrow wells alternate with wide wells. This is the case in certain
semiconductor lasers, like that described by H. Sakaki et al in
Electronics Letters, Apr. 12, 1984, Vol 20, No 8, pp 320-321. However, it
is necessary to understand that the dimensions and consequently the
phenomena differ. In an optical cavity like that described in the
aforementioned document, the narrow wells have a width less than 1
nanometer, which is ten times less than in the present invetion. It is out
of the question in such a well for a saturatable absorption phenomenon to
occur, because the carriers move too fast. Thus, these narrow wells only
lead to the participation of barriers in superlattice form and which
separate the wells. The width of these wells is roughly the same as that
of the barriers. It is typically 2.7 nm, i.e. it is very narrow compared
with the barriers used in the invention.
* * * * *
|
|
 |
|
 |
Description |
|
