Fabrication of blue light-blocking optical interference coatings by the solgel method.

We report on the solgel method of fabrication of thin films of high optical quality and with tunable index of refraction. The resulting coatings are hard, durable and robust against humidity and common organic solvents. Bragg mirrors and edge filters have been made by stacking these films. A blue light-blocking edge filter made by 2×21 stacked layers shows a transmittance of less than 1% in the stop band region while maintaining a high transmittance of over 80% in the rest of the visible spectral region with an integrated photopic transmittance of 89.7%.

Linearly polarized Q-switched ceramic laser made with anisotropic nanostructured thin films.

A polarizing laser mirror was made of an alternating sequence of low and high refractive index layers of titanium oxide using glancing angle deposition (GLAD). Large refractive index contrast and large birefringence, reaching 0.5 and 0.1, respectively, could be obtained from one single raw material by changing the deposition conditions. The laser mirror could withstand a train of 2.7 ns, single-mode pulses at 680 Hz, λ=1030 nm, and peak power density of 670 MW/cm2 when used as an output coupler of a passively Q-switched (Yb0.1Y0.9)3Al5O12 ceramic laser. The polarization extinction ratio was found to be better than 30 dB both in continuous-wave and pulsed regimes. These results indicate that polarizing laser mirrors made from nanostructured thin films with GLAD, in addition to being simple to fabricate, can withstand high pulse energy density.

Compact linearly polarized ceramic laser made with anisotropic nanostructured thin films.

A novel method is proposed for the fabrication of polarizing laser mirrors for compact solid-state lasers using glancing angle deposition. Changing the inclination angle and the azimuthal orientation of the substrate during deposition allows one to create and control in-plane birefringence of a deposited thin film by changing its nanostructure. Principal refractive indices of tungsten trioxide films were determined for various deposition angles using transmission and reflection ellipsometry. High-reflectance contrast between orthogonal linear polarization directions was obtained using a single material without any additional processing steps. These birefringent films were the building blocks of a Bragg mirror that was tested as an output coupler of a (Yb0.1Y0.9)3Al5O12 ceramic laser in a laser-diode end-pumped configuration. Continuous-wave, linearly polarized, transverse single-mode laser emission was obtained at a wavelength of 1030 nm with a polarization extinction ratio higher than 973 (30 dB).

Machine learning enabled detection of COVID-19 pneumonia using exhaled breath analysis: a proof-of-concept study.

Detection of the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) relies on real-time-reverse-transcriptase polymerase chain reaction (RT-PCR) on nasopharyngeal swabs. The false-negative rate of RT-PCR can be high when viral burden and infection is localized distally in the lower airways and lung parenchyma. An alternate safe, simple and accessible method for sampling the lower airways is needed to aid in the early and rapid diagnosis of COVID-19 pneumonia. In a prospective unblinded observational study, patients admitted with a positive RT-PCR and symptoms of SARS-CoV-2 infection were enrolled from three hospitals in Ontario, Canada. Healthy individuals or hospitalized patients with negative RT-PCR and without respiratory symptoms were enrolled into the control group. Breath samples were collected and analyzed by laser absorption spectroscopy (LAS) for volatile organic compounds (VOCs) and classified by machine learning (ML) approaches to identify unique LAS-spectra patterns (breathprints) for SARS-CoV-2. Of the 135 patients enrolled, 115 patients provided analyzable breath samples. Using LAS-breathprints to train ML classifier models resulted in an accuracy of 72.2%-81.7% in differentiating between SARS-CoV2 positive and negative groups. The performance was consistent across subgroups of different age, sex, body mass index, SARS-CoV-2 variants, time of disease onset and oxygen requirement. The overall performance was higher than compared to VOC-trained classifier model, which had an accuracy of 63%-74.7%. This study demonstrates that a ML-based breathprint model using LAS analysis of exhaled breath may be a valuable non-invasive method for studying the lower airways and detecting SARS-CoV-2 and other respiratory pathogens. The technology and the ML approach can be easily deployed in any setting with minimal training. This will greatly improve access and scalability to meet surge capacity; allow early and rapid detection to inform therapy; and offers great versatility in developing new classifier models quickly for future outbreaks.

Continuously variable, electrically addressed beam splitter based on vanadium dioxide.

Vanadium dioxide (VO(2)) is used to implement an electrically addressable beam splitter with continuously variable splitting ratios. The electrical control of temperature in a thin VO(2) layer is used to vary its transmission/reflection behavior. The technique is characterized for various incidence angles, s- and p-polarizations, and the wavelength range of 400-2000 nm. Splitting ratios continuously tunable over four orders of magnitude are reported.

Biaxial thin-film coated-plate polarizing beam splitters.

We present a design for a biaxial thin-film coated-plate polarizing beam splitter that transmits the p-polarized component of a beam of light without change of direction and reflects the s-polarized component. The beam splitter has a periodic structure and is planned for fabrication by serial bideposition in mutually orthogonal planes. Recent experimental data for form-birefringent silicon is used to establish the feasibility of the design for a beam splitter to be used at 1310 nm and at an angle of 45 degrees in air.

Control of power law scaling in the growth of silicon nanocolumn pseudo-regular arrays deposited by glancing angle deposition.

Nanocolumn pseudo-regular arrays of silicon with controlled aspect ratio and porosity are fabricated by electron-beam evaporation using the glancing angle deposition (GLAD) method with vapour impinging at oblique incidence onto rapidly rotating substrates. The width W at positions y along the height of one individual column scales with y following a power law dependence W approximately y(p). We demonstrate that the scaling exponent value, p, can be modified from 0.6 to 0.3 by varying the vapour incidence angle from 75 degrees to a glancing 89 degrees from the substrate normal. This exponent is an important morphological factor for thin films, as it determines the morphological correlation length, nanocolumn profile, size, and spacing. The nanocolumn mean diameter can be varied between 12 and 40 nm, while the intercolumnar spacing can be adjusted between 37 and 85 nm via changing the incidence angle. The growth mechanism and film morphology are explored in detail.

Enhanced birefringence in vacuum evaporated silicon thin films.

We report an experimental study of enhanced optical birefringence in silicon thin films on glass substrates. Form anisotropy is introduced as an atomic-scale morphological structure through dynamic control of growth geometry. The resulting birefringence is large compared with naturally anisotropic crystals and is comparable to two-dimensional photonic crystals. The films are fabricated with serial bideposition onto a substrate held at a fixed tilt angle relative to the impinging vapor. Films were analyzed by spectroscopic ellipsometry and scanning electron microscopy, the latter clearly revealing form anisotropy in a morphology of bunched columns perpendicular to the deposition plane with dimensions of hundreds of nanometers and smaller. The observed linear birefringence varies with wavelength and tilt angle, with a maximum of 0.4 at a 630-nm wavelength and 0.25 at 1500 nm.

Vacuum evaporated porous silicon photonic interference filters.

Porous materials with nanometer-scale structure are important in a wide variety of applications including electronics, photonics, biomedicine, and chemistry. Recent interest focuses on understanding and controlling the properties of these materials. Here we demonstrate porous silicon interference filters, deposited in vacuum with a technique that enables continuous variation of the refractive index between that of bulk silicon and that of the ambient (n approximately 3.5 to 1). Nanometer-scale oscillations in porosity were introduced with glancing angle deposition, a technique that combines oblique deposition onto a flat substrate of glass or silicon in a high vacuum with computer control of substrate tilt and rotation. Complex refractive index profiles were achieved including apodized filters, with Gaussian amplitude modulations of a sinusoidal index variation, as well as filters with index matching antireflection regions. A novel quintic antireflection coating is demonstrated where the refractive index is smoothly decreased to that of the ambient, reducing reflection over a broad range of the infrared spectrum. Optical transmission characterstics of the filters were accurately predicted with effective medium modeling coupled with a calibration performed with spectroscopic ellipsometry.