| Surface plasmon polaritons?SPPs?is a two-dimensional bound wave formed by light waves coupled to oscillations of free electrons on the metal surface,which can propagate along the metal-dielectric interface.The field decreases exponentially in both sides of the medium perpendicular to the interface direction.It features shorter effective wavelength,localized field distribution,breaking through the limit of diffraction,and sensitive to the shape of metal structure,properties of dielectric materials,polarization of incident light.It has potential applications in the field of sub-wavelength optics,ultra-high density optical storage,super-resolution microscopic imaging,micro-nano sensing detection and new energy fields.For metals coated with the same dielectric,when the metal film is very thick?tens of nanometers?,the surface plasmon waves at the upper and lower interfaces of metal-dielectric will couple with each other,and the oscillation will be strengthened.This kind of waves can propagate for a long distance,which called long-range surface plasmon?LRSPP?waveguide.Compared with SPP waves in single metal-dielectric interface,LRSPP waveguide has advantages of smaller transmission loss,larger mode field size,easy single-mode fiber coupling.It can transmit not only optical signals but also electrical signals.It can be used in optical communication,data transmission,biochemical sensing and other fields.In this paper,polymer optical waveguide devices based on surface plasmon resonance are mainly studied,including long range surface plasmon waveguide devices,waveguide intermode interferometers,and surface plasmon polaritons based integrated waveguide refractive index sensors.The related theories of the device are analyzed,the geometric parameters of the device are optimized,and the photomask are made.The devices are fabricated by CMOS processes such as photolithography and etching.Then the devices are measured and analyzed.This paper mainly studies from the following aspects:1.Optical signal processing devices based on long range surface plasmon polariton waveguideTaking SU-8/Au/SU-8 metal core planar waveguide as an example,the mode and transmission characteristics of LRSPP waveguide are studied.LRSPP mode has a weak ability to restrict the mode field.Most of the mode fields are distributed in the dielectric,and the transmission loss is low.With the increase of thickness of the gold film,the mode confinement of metal increases,and the transmission loss increases.A LRSPP waveguide-dielectric waveguide vertical coupler is designed.When the effective refractive index of LRSPP waveguide is close to that of dielectric waveguide,the optical field will couple between them.TM light will be coupled from dielectric waveguide to LRSPP waveguide,and output from gold waveguide,while TE mode light will continue to output along dielectric waveguide,since LRSPP waveguide only supports TM mode.The numerical results show that the optical bandwidth of TM wave can reach 250 nm when the optical power difference between the cross and direct output ports is 10 dB.The device has a large tolerance to the thickness of dielectric waveguide and gold film.It can be used in optical modulator,polarization beam splitter,sensor and other fields.A long period grating filter based on LRSPP waveguide is designed and fabricated.The grating structure is on the upper surface of the cladding layer,and the photobleaching method is used to fabricate the grating.The gold thin films were fabricated by thermal evaporation method,and metal strip waveguides with finite width were fabricated by wet etching technology.Under the action of grating,the LRSPP mode is coupled into cladding mode when the phase matching condition is satisfied.A resonance peak of-23.5 dB is observed at 1553 nm wavelength,and the thermal tuning efficiency of temperature is about 2.5 nm/?.The device can be used in filtering,sensing and optoelectronic integration fields.2.Waveguide Intermode Interference Displacement Sensor Based on Plasma Enhancement EffectThe intermode interference effect of dielectric waveguide is studied.Because the mode field diameter of the optical fiber is larger than that of the waveguide,the cladding mode of the waveguide will be excited when using end-fire excitation method.Because the effective refractive index of cladding mode is close to that of waveguide core mode,when the two modes coexist,they will couple with each other and form an interferometric spectra which varies approximately sinusoidally with the wavelength.It is found that a stable mode excitation mechanism can be formed by introducing grating structure at the front of the waveguide.When the input fiber deviates from the center of the waveguide,the core mode and cladding mode of the waveguide can still be excited to form a uniform and stable interference pattern of the output spectrum.The sensitivity of the resonant wavelength to the position offset of the input optical fiber is about 2.3 nm/?m.Intermode interference effects in LRSPP waveguides are studied.The interference spectra with two resonance peaks were observed in C band by measuring8 mm long LRSPP waveguide device.Within a certain range,the resonance peak of the spectrum varies linearly with the deviation of the incident fiber from the center of the waveguide.The sensitivity of the resonant wavelength to the position offset of the input optical fiber is about 1.7 nm/?m.It can be used in sensing and optoelectronic integration fields.3.Surface Plasmon polariton based integrated waveguide refractive index sensorThe refractive index sensor based on long-range surface plasmon waveguide is studied.The silicon is adopted as substrate and polymer SU-8 as the cladding layer.When LRSPP waves propagate along the gold waveguide,the smaller the refractive index difference between the analyte and the SU-8 cladding,the lower the transmission loss of the waveguide.With the increase of the length of the gold film,the sensitivity,as well as the transmission loss of the sensor,increases.For a 2 mm long devices with 5?m width and 40 nm thick gold film,when the refractive index of the analyte increases from 1.57 to 1.575,the sensitivity is 938.4 dB/RIU.A refractive index sensor based on the coupling between dielectric waveguide mode and SPP mode is studied.The device consists of silicon dioxide on silicon as the underlying cladding layer,polymer SU-8 as the waveguide layer and PMMA as the buffer layer.For a certain wavelength,when the effective refractive index of SU-8waveguide mode is close to that of SPR mode,the waveguide mode will couple to SPR mode,forming a SPR resonance trough.The influence of metal film thickness,PMMA buffer layer thickness and waveguide layer thickness on the sensing performance of the device was studied by eigenmode expansion method.For this device,with the increase of the thickness of metal film,the resonance peak moves to the short wavelength direction,the resonance strength decreases and the sensitivity decreases;with the increase of the thickness of PMMA buffer layer,the resonance strength decreases,and the resonance peak moves slightly to the short wavelength,with little change in sensitivity;with the decrease of the thickness of waveguide layer,the resonance intensity increases and the resonance peak position almost remains unchanged,which has little effect on the sensitivity.When the refractive index of metal surface changes,the position of absorption peak will change and the stability is high.It can be used in biochemical sensor and photoelectric integrated sensor chip.An integrated waveguide liquid refractive index sensor based on SPR effect is studied.The light intensity is used as the detection parameter.Polymer waveguide structures were fabricated by photobleaching using silica as the underlying layer and polymer SU-8 as the waveguide layer.The effects of geometric parameters such as thickness of gold film,length of gold film and material of waveguide layer on the sensitivity,insertion loss and output power of the device are simulated theoretically,and the size of the device is determined.The binary mixture of benzaldehyde and cinnamaldehyde was mearsured and analyzed.The results show that the sensitivity of the device is about 875 dB/RIU,the resolution is about 1×10-6.By changing the material of the waveguide layer,the sensing range of the sensor can be adjusted.The sensitivity of SPR sensor can be improved by optimizing the waveguide structure.By using light intensity detection method,it is easy to fabricate optoelectronic integrated sensor chips. |
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