| Quantum well infrared photodetector(QWIP) has been one of the hot candidates in infrared detecting field, as a result of the characteristics of high uniformity and operability, high impedance and fast response time. However, the inter-subband transition mechanism leads to the non-absorption of the normal incident photons and the low quantum efficiency, which restrict the development of the QWIP. The quantum efficiency of QW has been improved by integrating optical coupling scheme to enhance the optical field in lots of works. To date such works almost have been based on the single element with large photosensitive area(about 200′200 mm2). At the same time, infrared imaging technology has been developed into the 3rd generation with multi-color and large format focal plane array(FPA), and the staring imaging of object can be obtained. Thus, study on improving the performance of infrared photodetector on the pixel level with optical coupling structures are of very importance for obtaining better staring imaging of object.Plasmonic microcavity owns both the ability of the surface plasmons to manipulate the photons beyond diffractive limit and the nature of the microcavity to capture the photons. Consequently, it can squeeze the photons into a subwavelength-sized volume. The focus of our work is by combing the plasmonic microcavity and the infrared detecting pixel to study the photo-responsivity enhancement, the mesa effect, the pixel size effect and the angular dependence, etc. The enhanced optical field in the microcavity can be formed by the plasmonic microcavity to support an improved performance. The main work consists:1. We demonstrate the first pixel-level plasmonic microcavity infrared photodetector with a single quantum well integrated between the metal patches and a reflection layer. Greater than one order of magnitude enhancement of the peak responsivity has been observed. The significant improvement originates from the highly confined optical mode in the cavity, leading to a strong coupling between photons and the quantum well, resulting in the enhanced photo-electric conversion process. Such strong coupling from the localized surface plasmon mode inside the cavity is independent of incident angles, offering a unique solution to high-performance focal plane array devices.2. We have integrated the plasmonic microdisk into the infrared photodetector pixel(PMD-QWIP), with a single quantum well sandwiched between the metal film and the gold microdisk. Three responsivity peaks of the PMD-QWIP with the microdisk diameter of d=20.5 mm are observed. The simulated electric field distribution demonstrates that these peaks stem from the eigenmodes resonance with the azimuthal mode number of m=1, creating an enhanced optical field confined in plasmonic microdisk to support an improved photo-electric conversion process. What’s more, for a target wavelength, by establishing the relationship between the pixel size and the resonance mode, we demonstrate that the gold diameter, which is adjusted by the radial mode number, can be scaled down to 0.13l0 where a subwavelength-sized pixel can be achieved at the lowest-order mode(1, 1). Our demonstration takes an importance stage further toward a cheaper IR imager with higher resolution and high-performance by switching to wavelength-size-, even subwavelength-size-pixel.3. We propose a slab waveguide with a dielectric layer sandwiched between the metal and air(metal/dielectric/air, MDA), which is easy to be integrated into the quantum well infrared photodetectors(MDA-QWIPs). The wavelength-scale thickness of the dielectric enables waveguide modes to overlap the mode field profile with the active layer of photodetector, extracting plasmonic energies from the inevitable metal/dielectric interface into the absorptive area. The challenge of exciting the guided modes in MDA is overcome by introducing a gold disk array at the metal/dielectric interface to realize the wavevector matching. Two photo-responsivity peaks are observed in the photo-current spectrum of the MDA-QWIPs. The simulated distribution of electric field demonstrates that they can be attributed to the 1st transverse magnetic(TM) and surface plasmon polariton(SPP) guided mode resonances. An enhancement factor of 3.7 at the 1st TM mode resonance is achieved, owing to its squeezing photons into the QWs active layer. The resonance wavelength dependence on the array period and the incident angle are explored experimentally to verify the guided modes resonance. We believe that our waveguide resonator would be an ideal candidate to enable high-performance optoelectronic devices.4. We report the dependence of the near-field optical modes in metal-insulator-metal quantum well infrared photodetector(MIM-QWIP) on the incident angles. Three optical modes are observed and attributed to the 2nd- and the 3rd-order surface plasmon polariton(SPP) modes and the localized surface polariton(LSP) mode. In addition to the observation of a responsivity enhancement of 14 times by the LSP mode, the varying pattern of the three modes against the incident angle are revealed, in which the LSP mode is fixed while the 2nd SPP mode splits into two branches and the 3rd SPP mode red-shifts. The detailed mechanisms are analyzed and numerically simulated. The results fit the experiments very well, demonstrating the wavevector coupling effect between the incident light and the metal gratings on the SPP modes. Our work will pave the way to fully understanding the influence of incident angles on a detector’s response for applying the MIM-QWIP to focal plane arrays.5. We have proposed a metallic resonant cavity with the infrared pixel mesa coated by metal to the bottom contact layer. The numerical simulations show that the proposed cavity can provide a resonance mode with high Q-factor(about 82), approaching to the requirement of the hyper spectral imaging. We attribute such a resonance mode to both the wavevector matching and the F-P cavity resonance. Additionally, we have studied the effect of geometric parameters on the light coupling.
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