World Journal of Engineering
LUMINESCENCE LIGHT WAVEGUIDE AND LASING PROPERTIES IN SOME
DOPED II- VISEMICONDUCTOR NANOWIRE
Bingsuo ZOU, Guozhang Dai, Ruibin Liu
Micro-nano Technology Center, Beijing Institute of Technology, Beijing 100081，China
The confinement of elementary excitation in solids
behaves as quantum state, which play an important role in
their properties. In macroscopic solid these excitations
show less quantum behavior but statistical behavior due to
scatterings by the complicated structure and boundary
conditions. In nanosystems, the quantum behavior may be
dominant for the limited space close to the critical physical
length for elementary excitation. In bulk solid, minor
doping usually affect little changes in their properties,
however, it is supposed to be large in nanosystems with
few quanta. However, up to now, we did not find many
examples for this argument [1] due to less endeavor.
The advance in nanotechnology help to study different
physical response for few-body systems within micrometer
size.[3] In this case we can follow the microscopic response
of the nanosystem, therein understand the quantum state
variation with minor composition modulation. Here we
fabricated several nanowires with minor doping, studied
their optical properties for single nanowire, found that the
confined elementary excitation in 1-d nanowire could be
easily modified by doping, novel quantum state and novel
optical properties occur. These novel optical properties of
one-dimensional (1D) nanostructures may find promising
applications in photonic devices, solar cell, quantum
Experimental:
The Mn(II) doped ZnO nanowires were synthesized
via chemical reduction techniques. Their optical
characterization are described in next section.
Related Results and Discussion:
The Photoluminescence spectra of these doped ZnO
nanowires under rising excitation powers could be obtained.
Fig.1a,b show varied profiles with increasing powers. In
the low pump power region, two PL peaks are observed,
one from free exciton and another companion-band from
2LO phonon bound exciton (arrowhead B), at 3.305eV and
3.168eV, respectively. For their energy span is 137meV.
This span is surprisingly equal to the energy of two-phonon
(the overtone of A1 LO mode), so the B band is originated
from polaronic exciton (bound excitons). With rising
excitation power, B band increases with a superlinear rate,
computation and display. Here we present an example on
exciton condensation in Mn(II) doped ZNO nanowire.
Exciton Bose Einstein condensation is highly expected
to happen in II-VI semiconductor. To realize BEC of
excitons at first needs highly stable excitons to fight against
the thermal dissociation and collision, that is why BEC
occurs at low temerature. II-VI semiconductor with large
exciton binding energy might be suitable, together with the
easy production of high-density coherent excitons by fs
laser.[12] The only question in such system is the short
lifetime of excitons due to their dipole allowed transition
rule. So this kind of exciton BEC in such 1d system should
be a quasi-equilibrium state, which can give a single mode
lasing, but the superfluidity observation seems difficult to
be seen.
Based on above analysis, excitons in semiconductor 1D micro- or nano-structures may work as a chamber to
realize exciton BEC. In this section, ZnO nanowire doped
with minimal Mn(II)(less than 10-5) was prepared, its
photoluminescence spectra by fs laser pulse excitation were
investigated at room temperature, a single mode lasing
behavior like an excitonic BEC is observed at
intermediately high excitation.
but free exciton (A) band showed a slight increase at the
start stage of rising power, then decreased swiftly in the
range of 0.3-3μJ/pulse. Then the A band diminish until
pump fluence reaches 3.5μJ/pulse whenever B band
increases with rising power. In the high power range, only
B band stay, with fast reduction of band FWHM. Moreover
the whole PL becomes single mode stimulated emission
line without any background (spontaneous radiation) after
the excitation power reach 3.5μJ/pulse. The lasing line of B
band is blue-shifted at about 50cm-1 from its wavelength at
low excitation. The shift was slightly analogy to the phase
space filling effects of excitons at high excitation [15] which
represent an exciton repulsion but not a carrier effect.
When the excitation pulse reach about 4.0μJ, the lasing
band of nanowire become much narrower than that at low
excitation. Only a super-narrow line of 3 meV full-width of
half maximum (FWHM) exists in the emission spectra,
which reflect a complete coherent emission. Further
increasing of the excitation power led to no change of
emission profile except intensity, but a second line occurs
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World Journal of Engineering
at its red-side with an energy span of 10.5meV at 7.5μJ. In
this entire excitation and emission process, the FWHM of
A and B become narrower with the increasing excitation
power, and finally the two emission bands converge to a
line near B band. The single mode emission differs from
previous lasing phenomena from bound excitons in ZnO
nanostructures, but much like an emission of BEC excitons.
The A band turning into B band has related to the
enhanced exciton-2LO phonon coupling after Mn doping,
this coupling is long range to couple more excitons when
photoexcitation power increase. No Mn doping can not
produce this behavior in ZnO nanowire, overdoped ZnO
nanowire also cannot produce this. This indicate this phase
formation need a narrow composition region for ZnO
nanowire, in which a balance among different interaction
forces is needed.
The B band appears in this doped ZnO nanowire and
enhanced exciton-phonon coupling, i.e., therein even
higher stability and smaller mobility. The fast enhancement
and narrowing of B band and its gradual and slight
blueshift with rising density reflected the coherent twophonon-two-exciton formation, condensation at final,(see
fig.2) for the excitons in paired or condensed state show
repulsion forces.
It is supposed that a critical size of nanowire may be
suitable for exciton BEC. The first, there is critical density
of excitons at quasi-equilibrium condition, so limited
volume in nanowire can avoid the diffusion and expansion
of excitons, produce a standing wave of exciton
propagation, hence work as a 1-D exciton trap. The second,
exciton binding energy should overcome the thermal effect.
The third, propagating phonons couple with excitons in
quasi-one dimensional structure, reduce the exciton motion
rate, and fasten the 1-D orientation of coupled excitons.
Such 1-D semiconductor structure is what can supply a
confined space for excitons to move only along the 1-D
axis coherently. Hence this nanowire could limit the
expansion of excitons in two directions, and facilitate the
combination of the 2LO phonon mode with excitons and
the further condensation.
Conclusion and Outlook
Quantum wire in the micro/nanometer scale is very
important systems for future photonic devices for its
electronic states and optical properties can be significantly
tuned by minor doping(《1%) within the wire. This is in
strong contrast to that in bulk. For few Mn(II) doped ZnO
naowires, the exciton-phonon coupling is enhanced by
Mn(II) doping to bind the exciton and form the bound
exciton by the longitudinal axis 2LO phonon. This
magnetic bound excitons can be coherently queued in 1-d
orientation to undergo the BEC in the wire at high fs pulse
excitation, which expand the space for light-matter
interaction and nonlinear optical effect study for future
application.
Figure Captions
(a)
(b)
Fig..1 The energy profile of the luminescence B band (a) at
3.5μJ and (b) at 4μJ
Fig. 2 the diagram of exciton and bipolaronic exciton
formation and condensation process with rising laser
fluence(each ellipse represent a dipole, big one for phonon,
small one for exciton).
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