Photonic crystal slabs have been subject to research for more than a decade, yet the existence of bound states in the radiation continuum (BICs) in photonic crystals has been reported only recently . A BIC is formed when the radiation from all possible channels interferes destructively, causing the overall radiation to vanish. In photonic crystals, BICs are the result of accidental phase matching between incident, reflected and in-plane waves at seemingly random wave vectors .
While BICs in photonic crystals have been discussed previously using reflection measurements, we reports for the first time in-situ measurements of the bound states in the continuum in photonic crystal slabs. By embedding a photodetector into a photonic crystal slab we were able to directly observe optical BICs. The photonic crystal slabs are processed from a GaAs/AlGaAs quantum wells heterostructure, providing intersubband absorption in the mid-infrared wavelength range. The generated photocurrent is collected via doped contact layers on top and bottom of the suspended photonic crystal slab.
We were mapping out the photonic band structure by rotating the device and by acquiring photocurrent spectra every 5°. Our measured photonic bandstructure revealed several BICs, which was confirmed with a rigorously coupled-wave analysis simulation. Since coupling to external fields is suppressed, the photocurrent measured by the photodetector vanishes at the BIC wave vector. To confirm the relation between the measured photocurrent and the Q-factor we used temporal coupled mode theory, which yielded an inverse proportional relation between the photocurrent and the out-coupling loss from the photonic crystal. Implementing a plane wave expansion simulation allowed us to identify the corresponding photonic crystal modes.
The ability to directly measure the field intensity inside the photonic crystal presents an important milestone towards integrated opto-electronic BIC devices. Potential applications range include nonlinear optics, nano-optics, sensing and optical computing.
This research was supported by the Austrian Science Fund FWF (Grant No. F2503-N17), the PLATON project 35N, the “Gesellschaft für Mikro- und Nanoelektronik” GMe and the European Research Council (Grant no. 639109).
 C.W. Hsu et al. “Observation of trapped light within the radiation continuum”, Nature 499, 188 (2013)
 Y. Yang Y et al., “Analytical Perspective for Bound States in the Continuum in Photonic Crystal Slabs”, Phys Rev Lett 113, 037401 (2014)
The authors present a technique to reduce the facet reflectivity in quantum cascade lasers (QCLs) by tilted facets. In
order to minimize the Fabry-Pérot resonances, the feedback from the laser facets into the cavity must be minimized.
Due to intersubband selection rules, the light generated inside QCLs is TM polarized. This polarization purity in
QCLs enables the reduction of the facet reflectivity through the angle of light incidence at the laser facet. We
observed a maximum threshold current density when the facet is tilted 17° towards the surface normal. This is in
agreement with the calculated Brewster's angle for the QCL heterostructure.
The performance of quantum well infrared photodetectors (QWIP) can be significantly enhanced combining it
with a photonic crystal slab (PCS) resonator. In such a system the chosen PCS mode is designed to coincide
with the absorption maximum of the photodetector by adjusting the lattice parameters. However there is a
multitude of parameter sets that exhibit the same resonance frequency of the chosen PCS mode.
We have investigated how the choice of the PC design can be exploited for a further enhancement of QWIPs.
Several sets of lattice parameters that exhibit the chosen PCS mode at the same resonance frequency have
been obtained and the finite difference time domain method was used to simulate the absorption spectra of the
different PCS. A photonic crystal slab quantum well infrared photodetector with three different photonic crystal
lattice designs that exhibit the same resonance frequency of the chosen PCS mode were designed, fabricated and
This work shows that the quality factor of a PCS-QWIP and therefore the absorption enhancement can be
increased by an optimized PCS design. The improvement is a combined effect of a changed lattice constant, PC
normalized radius and normalized slab thickness. An enhancement of the measured photocurrent of more than
a factor of two was measured.