147 research outputs found
Low power saturation of an ISB transition by a mid-IR quantum cascade laser
We demonstrate that absorption saturation of a mid-infrared intersubband
transition can be engineered to occur at moderate light intensities of the
order of 10-20 kW. and at room temperature. The structure
consists of an array of metal-semiconductor-metal patches hosting a judiciously
designed 253~nm thick GaAs/AlGaAs semiconductor heterostructure. At low
incident intensity the structure operates in the strong light-matter coupling
regime and exhibits two absorption peaks at wavelengths close to 8.9 m.
Saturation appears as a transition to the weak coupling regime - and therefore
to a single-peaked absorption - when increasing the incident power. Comparison
with a coupled mode theory model explains the data and permits to infer the
relevant system parameters. When the pump laser is tuned at the cavity
frequency, the reflectivity decreases with increasing incident power. When
instead the laser is tuned at the polariton frequencies, the reflectivity
non-linearly increases with increasing incident power. At those wavelengths the
system therefore mimics the behavior of a saturable absorption mirror (SESAM)
in the mid-IR range, a technology that is currently missing
THz ultra-strong light-matter coupling up to 200K with continuously-graded parabolic quantum wells
Continuously graded parabolic quantum wells with excellent optical
performances are used to overcome the low-frequency and thermal limitations of
square quantum wells at terahertz frequencies. The formation of microcavity
intersubband polaritons at frequencies as low as 1.8 THz is demonstrated, with
a sustained ultra-strong coupling regime up to a temperature of 200K. It is
additionally shown that the ultra-strong coupling regime is preserved when the
active region is embedded in sub-wavelength resonators, with an estimated
relative strength . This represents an
important milestone for future studies of quantum vacuum radiation because such
resonators can be optically modulated at ultrafast rates, possibly leading to
the generation of non-classical light via the dynamic Casimir effect. Finally,
with an effective volume of , it is estimated that fewer
than 3000 electrons per resonator are ultra-strongly coupled to the quantized
electromagnetic mode, proving it is also a promising approach to explore
few-electron polaritonic systems operating at relatively high temperatures.Comment: 7 pages, 4 figure
Ultra-fast amplitude modulation of mid-IR free-space beams at room-temperature
Applications relying on mid-infrared radiation (Mid-IR, 3-30
m) have progressed at a very rapid pace in recent years, stimulated by
scientific and technological breakthroughs. Mid-IR cameras have propelled the
field of thermal imaging. And the invention of the quantum cascade laser (QCL)
has been a milestone, making compact, semiconductor-based mid-IR lasers
available to a vast range of applications. All the recent breakthrough advances
stemmed from the development of a transformative technology. In addition to the
generation and detection of light, a key functionality for most photonics
systems is the electrical control of the amplitude and/or phase of an optical
beam at ultra-fast rates (GHz or more). However, standalone, broadband,
integrated modulators are missing from the toolbox of present mid-IR photonics
integrated circuits and systems developers. We have developed a free-space
amplitude modulator for mid-IR radiation ( 10 m) that can
operate up to at least 1.5 GHz (-3dB cut-off at 750 MHz) and at
room-temperature. The device relies on a semiconductor hetero-structure
enclosed in a judiciously designed metal-metal optical resonator. At zero bias,
it operates in the strong light-matter coupling regime up to 300K. By applying
an appropriate bias, the device transitions to the weak coupling regime. The
large change in reflectivity due to the disappearance of the polaritonic states
is exploited to modulate the intensity of a mid-IR continuous-wave laser up to
speeds of more than 1.5 GHz
Excitons bound by photon exchange
In contrast to interband excitons in undoped quantum wells, doped quantum
wells do not display sharp resonances due to excitonic bound states. In these
systems the effective Coulomb interaction between electrons and holes typically
only leads to a depolarization shift of the single-electron intersubband
transitions. Non-perturbative light-matter interaction in solid-state devices
has been investigated as a pathway to tune optoelectronic properties of
materials. A recent theoretical work [Cortese et al., Optica 6, 354 (2019)]
predicted that, when the doped quantum wells are embedded in a photonic cavity,
emission-reabsorption processes of cavity photons can generate an effective
attractive interaction which binds electrons and holes together, leading to the
creation of an intraband bound exciton. Spectroscopically, this bound state
manifests itself as a novel discrete resonance which appears below the
ionisation threshold only when the coupling between light and matter is
increased above a critical value. Here we report the first experimental
observation of such a bound state using doped GaAs/AlGaAs quantum wells
embedded in metal-metal resonators whose confinement is high enough to permit
operation in strong coupling. Our result provides the first evidence of bound
states of charged particles kept together not by Coulomb interaction, but by
the exchange of transverse photons. Light-matter coupling can thus be used as a
novel tool in quantum material engineering, tuning electronic properties of
semiconductor heterostructures beyond those permitted by mere crystal
structures, with direct applications to mid-infrared optoelectronics
Detection of strong light-matter interaction in a single nano-cavity with a thermal transducer
Recently, the concept of strong light-matter coupling has been demonstrated
in semiconductor structures, and it is poised to revolutionize the design and
implementation of components, including solid state lasers and detectors. We
demonstrate an original nanospectroscopy technique that permits to study the
light-matter interaction in single subwavelength-sized nano-cavities where
far-field spectroscopy is not possible using conventional techniques. We
inserted a thin ( 150 nm) polymer layer with negligible absorption in
the mid-IR (5 m < < 12 m) inside a metal-insulator-metal
resonant cavity, where a photonic mode and the intersubband transition of a
semiconductor quantum well are strongly coupled. The intersubband transition
peaks at = 8.3 m, and the nano-cavity is overall 270 nm thick.
Acting as a non-perturbative transducer, the polymer layer introduces only a
limited alteration of the optical response while allowing to reveal the optical
power absorbed inside the concealed cavity. Spectroscopy of the cavity losses
is enabled by the polymer thermal expansion due to heat dissipation in the
active part of the cavity, and performed using an atomic force microscope
(AFM). This innovative approach allows the typical anticrossing characteristic
of the polaritonic dispersion to be identified in the cavity loss spectra at
the single nano-resonator level. Results also suggest that near-field coupling
of the external drive field to the top metal patch mediated by a metal-coated
AFM probe tip is possible, and it enables the near-field mapping of the cavity
mode symmetry including in the presence of strong light-matter interaction
- …