Quantum well photodetector in a free-standing photonic crystal slab

The fascinating optical properties of photonic crystals (PCs) have lead to numerous new devices or ways to improve existing device characteristics [1]. Photonic crystal slabs (PCS) are of particular interest due to their strong resonant behavior and three-dimensional light confinement, while being compatible with standard semiconductor technology. In this work, we present a photodetector within a photonic crystal slab (PCS), fabricated from a quantum well infrared photodetector (QWIP). Our results show that the photon lifetime in the detection area is increased and the photoresponse is significantly enhanced. By tilting our device with respect to the incident radiation, it was also possible to directly measure the photonic band structure of the PCS. The QWIP is an infrared photodetector, which is desirable for its extremely fast photoresponse and the high flexibility in design. However, for specific applications it is limited by the thermally generated background noise. To improve its characteristics, it is necessary to increase the photosensitivity without the generation of additional thermal noise. By fabricating a very thin suspended membrane of detector material, the light is confined to the photosensitive area (inset fig. 1). The photonic crystal is used to resonantly couple light into this slab and increase the photon lifetime in the detection area, hereby improving the photoresponse without increasing thermal noise. To collect the photocurrent, contacts are applied to the top and bottom of the slab. A schematic device structure is show in figure 1. The device was fabricated by processing a hexagonal lattice into the epitaxially grown QWIP and a selective underetching of the underlying AlGaAs layer (Fig. 2). The slab thickness is 2 μm and the width ranged up to 250 micrometers. The PC was designed so that the first dipole mode coincides with the peak absorption of the QWIP material (a=3.1μm and r=0.2a). The dashed line in figure 3 is the spectral photoresponse of the standard QWIP and the solid line represents the spectrum of the PCS-QWIP. Each resonance peak corresponds to one photonic band. The magnitude of the peaks depends on the mode symmetry. Modes with dipole-like mode profiles couple more efficiently to external fields and the resonances are therefore more pronounced. For the strongest resonances, we measured Q-factors of around 300, which is proportional to the photon lifetime increase in the detection area and the signal improvement. Theoretical predictions suggest even larger possible Q-factors [2]. Although the physical structure of a PCS is similar to an infinitely extended two-dimensional PC, the photonic band structure is very different. The modes are not purely TM or TE anymore and leak out into the surrounding space. Here we present a way to directly measure the real band structure of a PCS. By tilting the angle of the incident light, the photonic bands of the PCS can be mapped out directly (Fig. 4) [3]. The resonance peaks correspond to the photonic bands and the in-plane k-vector of the photons is provided by the tilting. Due to the strong resonant signal enhancement, higher photonic bands are also visible, even though the QWIP absorption is already very low. This method can be used to study any arbitrary PC structure as long as it supports a stable membrane.