Organic Thin Film Laboratory

Wassily Kandinsky, Several Circles, 1926. Oil on canvas. The Solomon R. Guggenheim Museum.

Reflection colors from circular islands in a thin freely-suspended smecticA liquid crystal film.

In the Organic Thin Film Laboratory, we investigate soft condensed matter systems of reduced dimensionality, including ultra-thin freely suspended smectic films and substrates coated with organic monolayers. These systems are studied under conditions of varying temperature, humidity, ambient pressure, or electric field under polarized or unpolarized white incandescent, LED, or laser illumination. All the microscopes in the lab are equipped with video cameras having a range of capabilities, including a cooled CCD camera for low light conditions and two high speed cameras for fast dynamics. Moreover, we have optical tweezers that give us the ability to manipulate inclusions in smectic films and an ultra-sensitive polarimeter that can measure exceedingly small birefringence.

Time-Sharing Optical Tweezers

The optical tweezer system consists of a 2 W infrared laser and an optical system designed to bring the laser to focus on the sample plane of an Olympus microscope. The laser beam is steered by an acousto-optic modulator which is in turn controlled by a high-frequency wave generator computer card. Using a LabView program, we can fully program or interactively control the position of the optical trap. Multiple traps are obtained by time-sharing.

Oil droplets on water are arranged to spell 'CU' with optical tweezers.

High-Speed Cameras

The VisionResearch Phantom V12.1 monochrome camera is capable of capturing a million frames per second at 128x8 pixel resolution. At the full resolution of 1280x1080 pixels, its speed is 6242 fps (pixel binning may be used to achieve higher frame rates). It boasts an ISO rating of 7000 and a bit depth of 12. For a typical 2-layer smectic film with reflectivity of 0.2%, we can distinguish structures in the film even at 1000 fps.

Coalescence of smectic A islands captured at 10,000 fps with the Phantom high-speed camera.

The NAC Image Technology MEMRECAM GX-3 FPS is is a high-speed color camera capable of capturing1679 frames per second at 1280 X 1024 pixel (full-frame) resolution (pixel binning may be used to achieve higher frame rates). The camera sensitivity is ISO 5000. This camera has been used to perform fluorescence microscopy of microscopic particles suspended in thin smectic films.

High-Sensitivity Camera

The Photometrics Retiga R1 is a sensitive video camera for low light imaging (the quantum efficiency at 600 nm is 75%) with a maximum frame rate of 25 fps at the full frame resolution of 1360 X1024 (pixel binning may be used to achieve higher frame rates). The camera has thermoelectric cooling and a fast USB3 computer interface capable of 50 MHz digitization. This highly sensitive camera allows the observation and characterization of freely-suspended liquid crystal films as thin as a single molecular layer using reflected light microscopy.

Zeiss polarizing light microscope with Retiga R1 camera.

 

Fluorescence Microscopy

The Olympus BX51 microscope with NAC high-speed camera and incadescent or LED illumination can be used for visible light or fluorescence measurements. LED illumination is provided by a Lumencor Spectra-X Light Engine, which has six independently-operatable LED channels that span 380–680 nm with individually adjustable power of 50–500 mW per channel.

Fluorescence of microscopic inclusions in smectic films can be captured using the NAC high-speed camera.

High-Sensitivity Polarimeter

We have built a highly sensitive polarimeter with a large extinction ratio for measuring the orientation dynamics of monolayers. The probe light from a He-Ne laser is focused to the monolayer sample. The analyzed probe beam is transmitted through an optical fiber to a PMT interfaced with a computer. The crossed polarizer and analyzer in the polarimeter give a very high extinction ratio, up to 4.1x109, making measurements of extremely small birefringence changes possible. The analyzer is mounted on a stepper motor with a resolution of 0.001o, so a very good dark state can be achieved.

This polarimeter has been used to study the dynamics of photoinduced birefringence in azobenzene-based self-assembled monolayers (azo-SAMs). Under the irradiation of light with proper wavelength, azobenzenes undergo photoisomerization, a reversible transformation between the trans and the cis conformations. When the pump light is linearly polarized, the isomerization process is characterized by angle-dependent excitation which results in the photoselection of a preferred azobenzene dye orientation, perpendicular to the polarization of the light. On the other hand, the aligned azobenzene dye surface can be randomized by illuminating with circularly polarized light (CPL) or by thermal relaxation.

In our experiments two pump beams, which are linearly polarized and circularly polarized light at 514 nm respectively, are incident on the sample with an angle of 10o with respect to the probe beam, and can be switched on and off individually by ferroelectric liquid crystal optical shutters. The pump beam spots on the sample are much larger than the probe beam, so that the probed region is essentially uniformly illuminated.

Schematic of high-sensitivity polarimeter.

 

 

For more information about the Organic Thin Film Laboratory, contact Joe Maclennan.

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