Download Reflectivity measurements of a quantum well

Reflectivity measurements of a quantum well - microcavity structure
1.
Experimental setup and measurements’ principles.
The basis is of the reflectivity measurement is to observe the spectra of the broadband
light source (like white light) reflected from the sample surface. Whenever the energy of the
source coincides with an optical transition of investigated structure the part of the light is
absorbed which can be seen in a spectra as a clear minimum. The standard experimental setup
for reflectivity measurements can be seen in Fig. 1.
Fig. 1 Experimental Setup
The role of respective elements is as follows:
- halogen lamp – a source of broadband light
- aperture – changes the convergent beam coming out of the halogen lamp into divergent
imitating the point source and extracting the center of the light spot which has the biggest
intensity being most homogeneous at the same time; its position coincides with the focusing
point of the halogen lamp and focal point of the focusing lens 1
- focusing lens 1 – forms the quasi-parallel light beam
- beam splitter – reflects source light and transmits reflected light
- focusing lens 2 – focuses the illuminating light on the sample surface
- cryostat – samples are mounted in it (on the cold finger) for cooling process, the temperature
inside the cryostat is measured by a controller
- liquid helium container – used for low temperature measurements (5K), provides
opportunity to measure emission spectra as a function of temperature in a wide temperature
range – from helium to room temperature (using the heater built in the temperature
controller), liquid helium is transferred to the cryostat and cool the sample
- focusing lens 3 - focuses the signal on the entrance slit of the monochromator
- monochromator - spectrometer analyzing the radiation and projecting dispersed light on the
detector; it is equipped with three gratings enumerated by number of grooves per mm – more
scratches gives better spectral resolution, but limits the spectral range observed at a time and
reduces the amount of light (signal intensity) coming to the detector; another important
parameter of the spectrometer is the size of the entrance slit – smaller slit coincides with
better spectral resolution but smaller amount of light in the monochromator;
- linear CCD camera (InGaAs) – detector cooled with liquid nitrogen.
The spectra are collected using the computer, which also controls the detection system.
Note that the focusing lens 2 is a part of both excitation and detection system – first the laser
beam is focused by it and than the emitted radiation is collected through the same path.
Adjustment of the experimental setup before measurements:
• mount sample into the cryostat, pump the cryostat (vacuum is required inside the
cryostat for helium not to condensate), transfer liquid helium into the cryostat;
• cool the detector – pour liquid nitrogen into the CCD camera container
caution: the camera controller need to be on, because the work temperature of the
camera (160K) is higher than the liquid nitrogen temperature (77K);
• check the optical path of excitation beam and emitted radiation – they should be on the
same height coinciding with the height of entrance slit of the monochromator, parallel
to the optical table and centrally illuminating all optical elements in• open the measurement software (SynerJy icon on the desktop), choose CCD spectra
and fill in proper (adequate to investigated structures) central emission wavelength,
aquisition time, grating and entrance slit size;
• by moving focusing lens 2 in horizontal direction find the position when the spot of
white light is focused on the sample surface;
• by moving focusing lens 3 in all three directions find the maximum of the signal from
the sample (first using the brighter grating and shorter integration time and afterwards
for fine tuning - darker grating and increased acquisition time).
2.
Investigated structures.
•
microcavity with 20 lower and 16 upper GaAs/AlGaAs mirror pairs (DBR)
Microcavity – optical resonator for light that can confine it in a micro- or even submicro
scale. Microcavities can be realized e.g. with tiny semiconductor or glass structures, such as
microtoroids or micropillars, or as defect structures in photonic crystals.
Cavity quality factor (Q) - a measure of the damping of resonator modes. Can be defined via
resonance bandwidth: the Q factor is the ratio of the resonance frequency ν0 and the full width
at half-maximum (FWHM) bandwidth δν of the resonance:
.
Distributed Bragg Reflector (DBR) or Bragg mirror – is a mirror structure based on Bragg
reflection at a period structure which consists of an alternating sequence of layers of two
different optical materials. The most frequently used design is that of a quarter-wave mirror,
where each optical layer thickness corresponding to one quarter of the wavelength for which
the mirror is designed. The latter condition holds for normal incidence; if the mirror is
designed for larger angles of incidence, accordingly thicker layers are needed. The principle
of operation can be understood as follows. Each interface between the two materials
contributes a reflection. For the design wavelength, the optical path length difference between
reflections from subsequent interfaces is half the wavelength; in addition, the reflection
coefficients for the interfaces have alternating signs. Therefore, all reflected components from
the interfaces interfere constructively, which results in a strong reflection. The reflectivity
achieved is determined by the number of layer pairs and by the refractive index contrast
between the layer materials.
•
3 stacks of 4 GaInNAs quantum wells (QW)
Fig. 2 Schematic band alignment of a single QW.
Fig. 3 TEM cross section of a quantum
well – microcavity structure.
The sample has been cut from the wafer radially. During the growth process the material
is deposited on the center of the wafer and then move towards the edges resulting in different
amount of material at different distances from the center. This difference can be used as a
tuning parameter between the cavity mode and the QW exciton as the optical path influence
the mode energy (main effect) and the thickness of the QW (very subtle) changes the
excitonic emission energy. In investigated structures we will examine the emission spectra as
a function of the position on the sample surface and try to find the optimum conditions for the
resonance.
3.
The physics behind.
The physical phenomenon crucial for this experiment is the light-matter coupling – in our
case realized in a form of interaction between the microcavity mode photons and the excitons
generated in the quantum wells.
Two regimes of light-matter coupling can be distinguished:
•
weak coupling – in this regime spontaneous emission of the emitter (e.g. QW) inside the
cavity can be enhanced or reduced compared with its vacuum level by tuning discrete
cavity modes in and out of resonance with the emitter; in this regime the Purcell effect can
be observed at resonance – rapid increase of the excitonic emission intensity (because of
the decrease of exciton lifetime resulting in more emission acts at a given time) as a
consequence of its interaction with the cavity depending on the oscillator strength of the
transition and the intensity of the electromagnetic wave at the position of the emitter; the
Purcell effect can be described by the Purcell factor which is a ratio between the lifetime
of the exciton in a bulk semiconductor and its lifetime modified by the interaction with the
cavity photons; it is proportional to the quality factor of the cavity and inversely
proportional to the mode volume;
•
strong coupling – in this regime quasiparticles merging the properties of light and matter
called exciton-polaritons can be formed and (in sufficently low temperatures and high
densities) can undergo a phase transition into the condensed state characterized by the
macroscopic occupation of the ground state (polariton condensate); it is characterized by
the reversible, periodic exchange of energy between the emitter and the cavity mode (Rabi
oscillations); the fingerprint of the strong coupling regime is the anti-crossing between the
emitter exciton and cavity-mode dispersion relations occurring in photoluminescence
spectra while tuning discrete cavity modes in and out of resonance with the emitter; the
anticrossing behaviour is characterized by the vacuum Rabi splitting; the requirement for
reaching the strong coupling regime is strong enough emitter-photon interaction (larger
than the combined atomic dipole decay rate and the cavity field decay rate); to maximize
the emitter-photon interaction the cavity mode and the emitter exciton should be spectrally
and spatially matched, the quality factor Q of the cavity and the oscillator strength of the
excitonic transition should be maximize and the cavity mode volume should be
minimized;
The reflectivity spectra of a quantum well – microcavity structure has a central flat
maximum, which drops off in an oscillation fashion either side of the reflection- or stop-band
(Fig. 4).
Fig. 4 Reflection of a planar DBR microcavity consisting of a top and a bottom mirror with 15 and 21 repeats of
GaAs/AlAs. In a) a 240 nm-thick bulk GaAs cavity is incorporated, while in b)3 InGaAs QWs are incorporated
in the same region [A. V. Kavokim, J. J. Baumberg, G. Malpuech and F. P. Laussy Microcavities, Oxford
University Press (2007)].
The spectral bandwith of the mirror is set by the refractive index difference between the
materials (Fig. 5).
Fig. 5 Reflectivity of a different Bragg mirror structures. All stop-bands are centered at the same Bragg
wavelength of 1550 nm [A. V. Kavokim, J. J. Baumberg, G. Malpuech and F. P. Laussy Microcavities, Oxford
University Press (2007)].
Different mechanisms can be used to tune the cavity mode and the emitter into the resonance:
•
temperature – the emitter changes its emission energy because the energy bandgap of
the semiconductor is temperature dependent (due to the thermal expansion of the
crystal lattice) and can be described by the empirical Varshni formula; at the same
time the cavity mode energy undergo very subtle changes connected with the
temperature dependence of the refractive index and the thermal expansion of the
cavity (influencing the optical path) – both of them are very weak dependencies;
4.
•
electric field – the emitter emission energy is modified (decreases with increasing field
strength) by the quantum confined Stark effect (QCSE) while the cavity mode energy
remains constant;
•
magnetic field – the emitter undergoes diamagnetic shift and as a consequence its
emission energy increases quadratically with the magnetic field whereas the cavity
mode is unaffected.
Motivation.
Strong coupling regime changes completely the emission properties of the emitter in a
cavity leading to new physical phenomena – fundamental research on cavity quantum
electrodynamics (cQED). The effect is coherent what gives a potential for applications in
quantum information processing or control and for nonlinear optics. Almost all photonic
devices are based on the spontaneous emission rate control.
Addition of the nitrogen to the InGaAs alloy redshifts the emission energy of the QW (via
the change of the bandgap) into the more interesting in view of applications in
telecommunication spectral region of 1.3 µm. Unfortunately simultaneously the quality of the
structure is reduced as the system becomes more inhomogeneous and the excitons can be
localized what decreases the spatial overlap with the electromagnetic field, also the number of
the non-radiative recombination centers increases. What is more the design and growth of the
cavity for the 1.3 µm is more technologically challenging. The increased number of emitters
should increase the coupling (proportionally to the square root of the number of the QWs).
The long-term purpose of those investigations is the realization of polariton condensate
and its utilization for polariton lasing at 1.3 µm which has not been achieved so far. Polariton
lasers has an important advantage over the photon lasing – the treshold current is much lower
because they do not require the occupation inversion. The first step is to check weather the
strong coupling regime is achievable.