The long-wave infrared (LWIR) spectral region spanning from 8 to 12 μm is useful for many scientific and industrial applications. Many of these applications require use of either a bandpass or a bandstop filter that can be realized by the guided-mode resonance (GMR) effect with subwavelength periodic features in layered dielectric materials transparent in the LWIR. The GMR filters operating in the LWIR region are fabricated by depositing an amorphous germanium (Ge) film to form a zero-contrast (ZC) waveguide-grating (WGG) on a polished zinc selenide (ZnSe) substrate. In general, the backside of a ZnSe substrate with refractive index 2.41 is uncoated causing a 17% Fresnel-reflection loss in the light transmitted through the filter due to a large impedance mismatch at the ZnSe/air interface. Because we use such filters in the LWIR laser experiments for notch filtering, to improve the filter transmittance we used ZnSe substrates coated on one-side with broadband antireflection coating (ARC) covering the 7 to 12 μm spectral range to fabricate GMRFs with one-dimensional (1D) Ge ZC WGG. We employed high-spatial resolution e-beam lithography and reactive-ion etching nanofabrication techniques to achieve high-performance large-area (12 × 12 mm2) 1D notch filters with subwavelength periods. We characterized polarization dependent spectral performance of the prototype filters with both coherent and incoherent incident light using a tunable quantum cascade laser system that spans the 7 to 12 μm region, and a Fourier transform infrared spectrometer with collimated incident beam to achieve close to 15% improvement in the peak transmittance as well as significant reduction in coherent noise compared to our earlier results with GMRFs without ARC. Here, we present the filter design simulation and measurement results.
The longwave infrared (LWIR) spectral region from 8 to 12 µm is widely used for day/night sensing and imaging applications as it corresponds to an atmospheric window as well as the peak region of the terrestrial blackbody emission. Some of these applications require use of compact spectrally tunable notch or bandstop filters. We are developing such spectral filters based on dielectric metasurfaces that utilize the guided-mode resonance (GMR) effect to provide a resonant wavelength that can be tuned by either changing the incidence angle or the grating period. We describe development of spectrally tunable micro-engineered filters with the device structure consisting of a subwavelength dielectric grating on top of a planar homogeneous layer using high-index dielectric transparent materials, i.e., germanium (Ge) with a refractive index of 4.0 and zinc selenide (ZnSe) with refractive index of 2.4. The filters are designed to reflect the incident broadband light at one (or more) narrow spectral band while fully transmitting the rest of the light. Filters based on one-dimensional (1-d) gratings are polarization dependent and those based on two-dimensional (2-d) gratings are polarization independent for normal incidence of light while polarization sensitive at non-normal incidence. The filter designs were carried out using the rigorous coupled-wave analysis (RCWA) algorithm. We designed, fabricated and characterized a number of filters by carrying out direct transmission measurements using a tunable quantum cascade laser (QCL) system. We will present the simulation and experimental results for both the 1-d and 2-d grating GMR filters.
The long-wave infrared (LWIR) spectroscopy has emerged as a promising technique for applications ranging from medical diagnosis to satellite imaging and terrestrial imaging. However, traditional optical elements are not realized well for the LWIR region. In this work, notch filters for LWIR spectral range (8 ~ 12 μm) based on the guided mode resonance (GMR) effect were designed, fabricated, and characterized with Germanium (Ge) thin film on Zinc Selenide (ZnSe) substrates. In contrast to the typical photolithography process, which faces challenge of resolution limit, we used e-beam lithography process to pattern the grating. By using a reactive ion etching process with a mixture of etching and passivation gases, we fabricated grating with well-defined vertical sidewalls. Finite-difference time-domain (FDTD) method was used to calculate the optical responses and model the geometry of the notch filters, and the optical transmittance of fabricated filters agrees well with the calculations. Moreover, we demonstrate the improved notch filtering response with reduced Fabry-Perot noise by introducing anti-reflection layer on the bottom of the ZnSe substrate. Therefore, findings of this work will be useful for various filter fabrications that prefer high spatial resolution in the LWIR spectral region.
We describe recent progress in the development of spectrally tunable micro-engineered notch filters operating in the longwave infrared (LWIR) region from 8 to 12 µm based on using the guided-mode resonance (GMR) effect. The device structure consists of a subwavelength dielectric grating on top of a homogeneous waveguide using high-index dielectric transparent materials, i.e., germanium (Ge) with a refractive index of 4.0 and zinc selenide (ZnSe) with a refractive index of 2.4. We design the filters to reflect the incident broadband light at one (or more) narrow spectral band while fully transmitting the rest of the light. Filters based on one-dimensional (1-D) gratings are polarization-dependent and those based on two-dimensional (2-D) gratings are less polarization-dependent. We designed and characterized both 1-D and 2- D filters. Anti-reflection coatings (ARCs) were applied on the backside of some of the filter substrates to improve transmission over the entire spectral region. We carried out transmission measurements of these filters using two separate experimental setups—an automated tunable room-temperature quantum cascade laser (QCL) system as well as a modified Fourier Transform Infrared (FTIR) spectrometer with normal incidence of light on the sample. We will present filter designs, theoretical simulation, characterization experiments and results.
Plasmonic nanocavities can control light flows and enhance light-mater interactions at subwavelength scale, and thus can potentially be used as nanoscale components in integrated optics systems either for passive optical coupling, or for active optical modulation and emission. In this work, we investigated a new type of multilayered metal-insulator optical nanocavities that can support multiple localized plasmon resonances with ultra-small mode volumes. The total number of resonance peaks and their resonance wavelengths can be freely and accurately controlled by simple geometric design rules. Multi-resonance plasmonic nanocavities can serve as a nanoscale wavelength-multiplexed optical components in integrated optics systems, such as optical couplers, light emitters, nanolasers, optical sensors, and optical modulators.
For high-power diode lasers, asymmetric reflectivities of facets are employed in order to improve slope efficiency. In recent years, the cavity lengths of these laser diodes have been increased to better distribute heat in order to improve output power. However, these two methods result in an inhomogeneous longitudinal profile of photon density, which leads to a non-uniform gain profile and is typically referred to as longitudinal spatial hole burning (LSHB). LSHB is believed to one of the limiting factors in scaling the output power of diode lasers. In this work, the LSHB effect is confirmed experimentally. The longitudinal photon density distribution, carrier density distribution, and gain distribution were calculated using a finite difference method to solve the spatially-varying rate equations in an 808 nm high-power diode laser. The experimental work was carried out by direct observation of spontaneous emission from a window patterned into the top contact of a 1.5 mm cavity length 808 nm diode laser. Because the spontaneous emission rate is proportional to the square of carrier density, the carrier density distribution could be measured for the device. The non-uniformity observed in this device agrees with the calculated carrier density profile, strongly supporting the existence of the LSHB effect in the device.
For high-power semiconductor lasers, asymmetric reflectivities of facets are employed in order to improve slope efficiency. Cavity lengths of these laser diodes have been increased to better distribute heat in order to improve output power. However, these two methods result in an inhomogeneous longitudinal profile of photon density, which leads to a nonuniform gain profile and is typically referred to as longitudinal spatial hole burning (LSHB). In this work, we developed a model to self-consistently calculate the longitudinal photon density distribution, carrier density distribution, and gain distribution in a high-power semiconductor laser. The calculation is based on modified rate equations, and a finite difference method is used to solve the differential equations. Newton’s method is employed to obtain final results with residual error below 10-6. The impact of LSHB was analyzed with different parameters, and we demonstrate that LSHB is expected to limit the maximum achievable output power of semiconductor lasers having cavity lengths in excess of several mm. The results are expected to be useful in the optimization of high-power semiconductor laser designs.
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