Photoelectric Detection Based On Surface Plasmons

Plasmonic is one of the most active branches in nano-optics.It mainly studies surface plasmons,the collective oscillation of electrons generated in metal nanostructures after excitation by light or high-energy electrons,which is coupled with the evanescent electromagnetic field.Surface plasmons have many unique properties,such as field enhancement,resonant wavelength tunability,and the ability to break through the optical diffraction limit.Therefore,it has attractive applications in photonic chips and integrated optoelectronic chips.Recently,researchers have implemented a series of functional devices,such as plasmon lasers,metal waveguide networks,on-chip phase modulators,and plasmon enhanced photodetectors.Photodetectors based on surface plasmons are a key part of these studies,and mainly focus on two aspects.On the one hand,plasmon structures are used to enhance the response of traditional photodetectors,provide wavelength tunability,polarization selectivity and chiral sensitivity.On the other hand,surface plasmon circuits are adopted to change the photodetector from a vertical far-field detector to a planar on-chip detector.The on-chip plasmon detector can be used as the hardware interface between plasmon chip and semiconductor chip.Photoelectric detection based on surface plasmon is a very challenging research topic and involves many physical problems,such as photothermal conversion,hot electron process,plasmon propagation in waveguide,coupling of different optical modes.This area also contains many engineering and technical issues,such as device micro-nano fabrication process and CMOS compatible device design.This thesis focuses on photodetection based on surface plasmons in two aspects.First,we combine plasmon nanostructures with photothermoelectric effects in silicon nanobelts to achieve plasmon modulated photothermoelectric detectors.Second,we combine semiconductor nanowires with plasmon waveguides to achieve on-chip detection of surface plasmons at sub-wavelength scales.The main innovations and research results of the thesis include: 1.We experimentally and theoretically study the photoelectric response in silicon nanobelts.Photovoltaic(PV)and photothermoelectric(PTE)responses in silicon caused by different contact types(Ohmic or Schottky)are clearly distinguished from the I-V characterization and spatial resolved photocurrent scanning.In samples with hybrid contact types,PTE and PV effects coexist under laser illumination and the relative contribution from PTE and PV effects significantly changes with incident power.A multiphysics model is built that involves hot carrier generation,semiconductor transport and a two-temperature heat transfer model.Our comprehensive theoretical model successfully reproduces the photovoltage saturation observed in the experiment,and serves as a versatile tool for optimizing the system.2.We realize plasmon-modulated photothermoelectric detectors by combining plasmon gratings with silicon nanobelts.An open-circuit photovoltage response of up to ~82 mV/?W is achieved by illuminating the 633 nm laser on the nanograting.The large photovoltage responsivity stems from the large Seebeck coefficient of lightly doped Si and the resonant absorption(~10 times)in Si layer induced by plasmon excitation.The device also achieves a selective response to the wavelength and polarization of incident laser.The angular resolved reflection dispersion spectra obtained by experiment and simulation prove that the quasi-waveguide modes caused by plasmon excitation is the main mechanism for the enhanced resonant light absorption.3.We demonstrate an on-chip plasmon detector with sufficient near-field coupling efficiency.By controlling the gap between Au waveguide and Si nanobelt,the near-field coupling of Au waveguide and Si stripe is optimized,which is important for further reducing the device size.Periodic oscillations are observed in the wavelength dependent photocurrent,which is attributed to the plasmon Fabry-Perot cavity on the waveguide.We then study the effect of Si stripe thickness on coupling,and optimize the surface plasmon propagation by inserting a thin silicon underneath Au waveguide.Finally,we succeed to shrink the device size to nanowire structure with sufficient electrical on-chip detection efficiency.Our proposed device designs are promising to be used as a photoelectric conversion interface unit between the plasmonic circuit and microelectronic devices.