Roman Gansch

Photonic crystal slab quantum well infrared photodetector

In this letter we present a quantum well infrared photodetector (QWIP), which is fabricated as a photonic crystal slab (PCS). With the PCS it is possible to enhance the absorption efficiency by increasing photon lifetime in the detector active region. To understand the optical properties of the device we simulate the PCS photonic band structure, which differs significantly from a real two-dimensional photonic crystal. By fabricating a PCS-QWIP with 100x less quantum well doping, compared to a standard QWIP, we are able to see strong absorption enhancement and sharp resonance peaks up to temperatures of 170 K.

Detectivity enhancement in quantum well infrared photodetectors utilizing a photonic crystal slab resonator.

We characterize the performance of a quantum well infrared photodetector (QWIP), which is fabricated as a photonic crystal slab (PCS) resonator. The strongest resonance of the PCS is designed to coincide with the absorption peak frequency at 7.6 µm of the QWIP. To accurately characterize the detector performance, it is illuminated by using single mode mid-infrared lasers. The strong resonant absorption enhancement yields a detectivity increase of up to 20 times. This enhancement is a combined effect of increased responsivity and noise current reduction. With increasing temperature, we observe a red shift of the PCS-QWIP resonance peak of -0.055 cm(-1)/K. We attribute this effect to a refractive index change and present a model based on the revised plane wave method.

Plasmonic lens enhanced mid-infrared quantum cascade detector

We demonstrate monolithic integrated quantum cascade detectors enhanced by plasmonic lenses. Surface normal incident mid-infrared radiation is coupled to surface plasmon polaritons guided to and detected by the active region of the detector. The lens extends the optical effective active area of the device up to a 5 times larger area than for standard mesa detectors or pixel devices while the electrical active region stays the same. The extended optical area increases the absorption efficiency of the presented device as well as the room temperature performance while it offers a flexible platform for various detector geometries. A photocurrent response increase at room temperature up to a factor of 6 was observed.

Photonic crystal slab quantum cascade detector

In this Letter, we demonstrate the design, fabrication, and characterization of a photonic crystal slab quantum cascade detector (PCS-QCD). By employing a specifically designed resonant cavity, the performance of the photodetector is improved in three distinct ways. The PCS makes the QCD sensitive to surface normal incident light. It resonantly enhances the photon lifetime inside the active zone, thus increasing the photocurrent significantly. And, the construction form of the device inherently decreases the noise. Finally, we compare the characteristics of the PCS-QCD to a PCS - quantum well infrared photodetector and outline the advantages for certain fields of applications.

Measurement of bound states in the continuum by a detector embedded in a photonic crystal

We directly measure optical bound states in the continuum (BICs) by embedding a photodetector into a photonic crystal slab. The BICs observed in our experiment are the result of accidental phase matching between incident, reflected and in-plane waves at seemingly random wave vectors in the photonic band structure. Our measurements were confirmed through a rigorously coupled-wave analysis simulation in conjunction with temporal coupled mode theory. Polarization mixing between photonic crystal slab modes was observed and described using a plane wave expansion simulation. The ability to probe the field intensity inside the photonic crystal and thereby to directly measure BICs represents a milestone in the development of integrated opto-electronic devices based on BICs.

Quantum cascade detector utilizing the diagonal-transition scheme for high quality cavities

A diagonal optically active transition in a quantum cascade detector is introduced as optimization parameter to obtain quality factor matching between a photodetector and a cavity. A more diagonal transition yields both higher extraction efficiency and lower noise, while the reduction of the absorption strength is compensated by the resonant cavity. The theoretical limits of such a scheme are obtained, and the impact of losses and cavity processing variations are evaluated. By optimizing the quantum design for a high quality cavity, a specific detectivity of 10(9) Jones can be calculated for λ = 8μm and T = 300K.

Optimized photonic crystal design for quantum well infrared photodetectors

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 measured. 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.

Higher order modes in photonic crystal slabs

We present a detailed investigation of higher order modes in photonic crystal slabs. In such structures the resonances exhibit a blue-shift compared to an ideal two-dimensional photonic crystal, which depends on the order of the slab mode and the polarization. By fabricating a series of photonic crystal slab photo detecting devices, with varying ratios of slab thickness to photonic crystal lattice constant, we are able to distinguish between 0th and 1st order slab modes as well as the polarization from the shift of resonances in the photocurrent spectra. This method complements the photonic band structure mapping technique for characterization of photonic crystal slabs.

Quantum cascade lasers with a tilted facet utilizing the inherent polarization purity

We report on quantum cascade lasers (QCLs) with a tilted facet utilizing their polarization property. Contrary to diode lasers, QCLs generate purely TM polarized light due to the intersubband selection rules. This property enables the utilization of reflectivity in terms of only TM polarized light (TM reflectivity). The TM reflectivity is reduced by tilting the front facet, resulting in enhanced light output power from the tilted facet. The peak output power of a QCL with a facet angle of 12° are increased by 31 %. The slope efficiency of a QCL with a facet angle of 17° are increased by 43 %. Additionally, a peculiar property of TM reflectivity, the Brewster angle, is investigated by using COMSOL simulations to find its availability in QCLs.

In-situ measurement of bound states in the continuum in photonic crystal slabs(Conference Presentation)

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 [1]. 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 [2]. 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). [1] C.W. Hsu et al. “Observation of trapped light within the radiation continuum”, Nature 499, 188 (2013) [2] Y. Yang Y et al., “Analytical Perspective for Bound States in the Continuum in Photonic Crystal Slabs”, Phys Rev Lett 113, 037401 (2014)