Diffraction-limited real-time terahertz imaging by optical frequency up-conversion in a DAST crystal

Up-Conversion Sensing of 2D Spatially-Modulated Infrared Information-Carrying Beams with Si-Based Cameras

Up-conversion sensing based on optical heterodyning of an IR (infrared) image with a local oscillator laser wave in a nonlinear optical sum-frequency mixing (SFM) process is a practical solution to circumvent some limitations of IR image sensors in terms of signal-to-noise ratio, speed, resolution, or cooling needs in some demanding applications. In this way, the spectral content of an IR image can become spectrally shifted to the visible/near infrared (VIS/NWIR) and then detected with silicon focal plane arrayed sensors (Si-FPA), such as CCD/CMOS (charge-coupled and complementary metal-oxide-semiconductor devices). This work is an extension of a previous study where we recently introduced this technique in the context of optical communications, in particular in FSOC (free-space optical communications). Herein, we present an image up-conversion system based on a 1064 nm Nd³⁺: YVO₄ solid-state laser with a KTP (potassium titanyl phosphate) nonlinear crystal located intra-cavity where a laser beam at 1550 nm 2D spatially-modulated with a binary Quick Response (QR) code is mixed, giving an up-converted code image at 631 nm that is detected with an Si-based camera. The underlying technology allows for the extension of other IR spectral allocations, construction of compact receivers at low cost, and provides a natural way for increased protection against eavesdropping.

Intracavity image upconversion system with fast and flexible electro-optic image gating based on polarization-frustrated phase-matching for range-gated applications

We report on a new image gating mechanism for intracavity nonlinear image upconversion systems that uses sum-frequency mixing of an external infrared image and a pump laser beam. Fast and flexible time duration gating of the upconverted image is achieved through transient electro-optic frustration of the phase-matching condition in a nonlinear crystal placed inside the cavity of the pump beam. The phase-matching condition is controlled by altering the polarization state of the laser cavity beam without interrupting laser oscillation, using a Pockels cell in one arm of an L-folded standing-wave resonator. In this way, an external image shutter mechanism is added to an image upconverter system that allows for using low shutter-speed EMCCDs (Electron Multiplying CCD) in range-gated imaging systems across the whole IR and potentially in the THz range.

IR-to-visible image upconverter under nonlinear crystal thermal gradient operation

In this work we study the enhancement of the field-of-view of an infrared image up-converter by means of a thermal gradient in a PPLN crystal. Our work focuses on compact upconverters, in which both a short PPLN crystal length and high numerical aperture lenses are employed. We found a qualitative increase in both wavelength and angular tolerances, compared to a constant temperature upconverter, which makes it necessary a correct IR wavelength allocation in order to effectively increase the up-converted area.

IR Image upconversion using band-limited ASE illumination fiber sources

We study the field-of-view (FOV) of an upconversion imaging system that employs an Amplified Spontaneous Emission (ASE) fiber source to illuminate a transmission target. As an intermediate case between narrowband laser and thermal illumination, an ASE fiber source allows for higher spectral intensity than thermal illumination and still keeps a broad wavelength spectrum to take advantage of an increased non-collinear phase-matching angle acceptance that enlarges the FOV of the upconversion system when compared to using narrowband laser illumination. A model is presented to predict the angular acceptance of the upconverter in terms of focusing and ASE spectral width and allocation. The model is experimentally checked in case of 1550-630 nm upconversion.

Pr-based metallic glass films used as resist for phase-change lithography

Metallic glass film of Pr₆₀Al₁₀Ni₁₀Cu₂₀ is proposed to be used as a resist of phase-change lithography (PCL). PCL is a mask-less lithography technology by using laser-direct-writing to create the intended nanopatterns. Thermal distribution in the PrAlNiCu film after exposure is calculated by finite element method (FEM). Thin films are exposed by continuous-wave laser and selective etched by nitric-acid solution, and the patterns are discerned by optical and atomic force microscope. The etching rate of as-deposited PrAlNiCu is thus nearly five times of the crystalline film. These results indicate that PrAlNiCu metallic glass film is a promising resist for phase-change lithography.