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Mid- and far-IR optoelectronic devices based on Bose-Einstein condensation

Periodic Reporting for period 2 - MIR-BOSE (Mid- and far-IR optoelectronic devices based on Bose-Einstein condensation)

Reporting period: 2019-01-01 to 2021-09-30

Optoelectronic devices typically operate in the weak coupling regime between light and matter, for
example in conventional lasers which rely on population inversion to achieve optical gain. Recently,
however, there has been a surge of interest in quantum systems operating instead in the strong
coupling regime, when the coupling strength of the light-matter interaction is so strong that new
states – cavity polaritons – are created, that are partially light, and partially material excitation. In
semiconductors, exciton-polaritons have been the most widely studied type of strongly coupled

However, recently a new phenomenon has been realized exploiting intersubband transitions. The
resulting excitations are called intersubband polaritons, and they have two remarkable properties:
(i) a bosonic character that is maintained up to high carrier densities since they are not restricted
by the Mott transition limit; and, (ii) large Rabi splittings. Although the scientific community has
explored the basic science of intersubband polaritons, their potential for future and innovative
optoelectronic devices has been entirely untapped.

The goal of MIR-BOSE is to demonstrate a new family of optoelectronic devices operating in the
strong coupling regime between light and matter. We aim at demonstrating bosonic lasers operating
in the mid-IR / THz frequency ranges of the electromagnetic spectrum. Laser action here
does not rely on population inversion, and so we temperature independent operation and high
output powers should be at reach. We will also develop non-classical/quantum light sources and
devices based on the ultra-fast modulation of the light-matter interaction. This – very perspective –
development could lead to the generation of “quantum light” in the mid-IR/THz spectral range.
During Year 1 we have started work on all the 3 WPs of MIR-BOSE. More focus has been devoted to
WP1 (polaritonic LEDs) and WP3 (Casimir radiation), since WP2 follows WP1. However, some work
has been done for WP2 too. In general we are in line with what was planned.
During Year 2 we followed up on those topics, and advanced on all aspects of the project.

In WP1 we have successfully demonstrated a mid-IR polaritonic LED under optical pumping (D1.1.)
and we have build solid bases for the THz version. A quantum theory for polaritonic LEDs and lasers
in the mid-IR has been developed, and work is being undertaken for further theoretical
developments. A pulsed laser system that can attain elevated peak powers has been developed and (i) accurate
saturation measurements of the polaritonic system have been performed, and (ii) the electro optics detection
for pump-probe measurements is now in place, to perform pump-probe experiments to verify that final-state stimulated
scattering is indeed present, and achieve laser threshold.
As part of WP1, but useful to WP2, we have demonstrated that the inhomogeneous broadening has little or no effect
on the polaritonic linewidth
Also, theory predicts that non-dispersive resonators should yield a spontaneous emissino enhancement. Such structures
have been designed and implemented. They will be measured in the next few weeks.
Theory also predicts that the threshold for laser action is proportional to the number of QWs in the structure. We have
therefore designed and started implementing devices with 2 QWs instead of 36, to reduce threshold intensity for polariton

In WP2, that is devoted to ISB polariton lasers, we have explored new growth strategies to improve
the linewidth of ISB transitions. In particular we have shown that proper use of growth interruptions
(specific durations, and specific interfaces) can improve the linewidth by 10/15%. In terms of
resonators, we have studied and designed graphene-based resonators for the THz range. Theory
shows promising properties, but fabrication proved challenging.
Eventually, we realized that the most promising path to obtain high Q factors is with metallic resonators featuring
very high filling factors, around 96% or 97%. They are being implemented.

WP3 focuses on mid-IR and THz radiation via Casimir effect. In the mid-IR, one of the enabling
element was the implementation of transparent contacts. Graphene proved not a viable solution. We
have therefore explored an alternative based on wafer bonding on low-index materials that are
transparent in the visible AND in the mid-IR. CaF2 and BaF2 possess these properties, and we have
developed a bonding process that is effective and robust.The decision point at M24 has therefore been set.

As per devices operating in the THz, they rely on the development of an ultra-fast switchable LC
resonator. We have theoretically developed the concept, and implemented devices that use GaAs as
switch material. The experiments revealed the need for a different switch material, and our choice
fell on the well known SOI (silicon-on-insulator) platform. We have characterized the insulator-to-conductor
transition of this material as a function of the ultra-fast laser power. Then we fabricated switch
devices that have been characterizations by partners of the consortium. The initial results show a switch effect,
but more measurements are necessary to confirm it beyond all reasonable doubt.

We have also delivered the management deliverables for WPs 4 and 5, namely:

D4.1 Website (M2)
D4.5 Dissemination and exploitation plan (M6)
D4.6 Update of dissemination and exploitation plan
D4.8 Complete market study
D4.12 Data management plan (M6)
D5.1 Gender action plan (M6)

We have also completed the science deliverables

D1.1 Spontaneous polaritonic emission
D1.3 Optimized parabolic QWs in the THz range
D2.1 Improved mid-IR or THz resonators
Typical dispersion diagram of intersubband polaritons in a MIM cavity.