Multimodal Nonlinear Hyperspectral Chemical Imaging Using Line-Scanning Vibrational Sum-Frequency Generation Microscopy

Summary

A multimodal, rapid hyperspectral imaging framework was developed to obtain broadband vibrational sum-frequency generation (VSFG) images, along with brightfield, second harmonic generation (SHG) imaging modalities. Due to the infrared frequency being resonant with molecular vibrations, microscopic structural and mesoscopic morphology knowledge is revealed of symmetry-allowed samples.

Abstract

Vibrational sum-frequency generation (VSFG), a second-order nonlinear optical signal, has traditionally been used to study molecules at interfaces as a spectroscopy technique with a spatial resolution of ~100 µm. However, the spectroscopy is not sensitive to the heterogeneity of a sample. To study mesoscopically heterogeneous samples, we, along with others, pushed the resolution limit of VSFG spectroscopy down to ~1 µm level and constructed the VSFG microscope. This imaging technique not only can resolve sample morphologies through imaging, but also record a broadband VSFG spectrum at every pixel of the images. Being a second-order nonlinear optical technique, its selection rule enables the visualization of non-centrosymmetric or chiral self-assembled structures commonly found in biology, materials science, and bioengineering, among others. In this article, the audience will be guided through an inverted transmission design that allows for imaging unfixed samples. This work also showcases that VSFG microscopy can resolve chemical-specific geometric information of individual self-assembled sheets by combining it with a neural network function solver. Lastly, the images obtained under brightfield, SHG, and VSFG configurations of various samples briefly discuss the unique information revealed by VSFG imaging.

Introduction

Vibrational sum-frequency generation (VSFG), a second-order nonlinear optical technique^1,2, has been used extensively as a spectroscopy tool to chemically profile symmetry-allowed samples^{3,4,5,6,7,8,9,10,11,12,13,1....}

Protocol

1. Hyperspectral line-scanning VSFG microscope

1. Laser system
   1. Use a pulsed laser system (see Table of Materials) centered at 1025 nm ± 5 nm. The laser is set at 40 W, 200 kHz (200 µJ/pulse) with a pulse width of ~290 fs.
      NOTE: The exact repetition rate can vary, and a high repetition rate laser generally works better for this VSFG microscope.
   2. Guide the output of the seed laser into a commercial optical parametric amplifier (OPA) to generate a mid-infrare.......

Discussion

The most critical steps are from 1.42 to 1.44. It is critical to align the objective lens well for an optical spatial resolution. It is also important to collect the emitted signal, relay, and project the scanning beam as a line at the entrance slits. Proper alignments would guarantee the best resolution and signal-to-noise ratio. For a typical sample, like SDS@2β-CD 100 µm by 100 µm sheets, a good resolution image (~1 µm resolution) with a high signal-to-noise ratio would take 20 min. This is al.......

Materials

Name	Company	Catalog Number	Comments
1x Camera Por	Thorlabs	WFA4100	connect a camera to a microscope or optical system
25.0 mm Right-Angle Prism Mirror, Protected Gold	Thorlabs	MRA25-M01	reflect light and produce retroreflection, redirecting light back along its original path
3” Universal Post Holder-5 Pack	Thorlabs	UPH3-P5	hold and support posts of various sizes and configurations
30 mm to 60 mm Cage Plate, 4 mm Thick	Thorlabs	LCP4S	convert between a 30 mm cage system and a 60 mm cage system
500 mm Tall Cerna Body with Epi Arm	Thorlabs	CEA1500	provide the function of enabling top illumination techniques in microscopy
60 mm Cage Mounted Ø50.0 mm Iris	Thorlabs	LCP50S	control the amount of light passing through an optical system
60 mm Cage Mounting Bracket	Thorlabs	LCP01B	mount and position a 60 mm cage system in optical setups
Air spaced Etalon	SLS Optics Ltd.	Customized	generate narrow-band 1030 nm light
Cage Plate Mounting Bracket	Thorlabs	KCB2	hold and adjust mirrors at a precise angle
CCD	Andor Technologies	Newton	2D CCD for frequency and spatial resolution
Collinear Optical Parametric Amplifier	Light Conversion	Orpheus-One-HP	Tunable MID light generator
Copper Chloride	Thermo Fischer Scientific	A16064.30	Self-assembly component
Customized Dichroic Mirror	Newport	Customized	selectively reflects or transmits light based on its wavelength or polarization
Ext to M32 Int Adapter	Thorlabs	SM1A34	provide compatibility and facilitating the connection between components with different thread types
Infinity Corrected Refractive Objective	Zeiss	420150-9900-000	Refractive Objective
Infinity Corrected Schwarzschild Objective	Pike Technologies Inc.	891-0007	Reflective objective
Laser	Carbide, Light-Conversion	C18212	Laser source
M32x0.75 External to Internal RMS	Thorlabs	M32RMSS	adapt or convert the threading size or type of microscope objectives
M32x0.75 External to M27x0.75 Internal Engraving	Thorlabs	M32M27S	adapt or convert the threading size or type of microscope objectives
Manual Mid-Height Condenser Focus Module	Thorlabs	ZFM1030	adjust the focus of an optical element
Monochromator	Andor Technologies	Shamrock 500i	Provides frequency resolution for each line scan
Motorized module with 1" Travel for Edge-Mounted Arms	Thorlabs	ZFM2020	control the vertical positon of the imaging objective
Nanopositioner	Mad City Labs Inc.	MMP3	3D sample stage
Resonant Scanner	EOPC	SC-25	325Hz resonant beam scanner
RGB Color CCD Camera	Thorlabs	DCU224C	Brightfield camera, discontinued but other cameras will work just as well
RGB tube lens	Thorlabs	ITL200	white light collection
Right Angle Kinematic Breadboard	Thorlabs	OPX2400	incorporate a sliding mechanism with two fixed positions
Right Angle Kinematic Mirror Mount, 30 mm	Thorlabs	KCB1	hold and adjust mirrors at a precise angle
Right Angle Kinematic Mirror Mount, 60 mm	Thorlabs	KCB2	hold and adjust mirrors at a precise angle
SM2, 60 mm Cage Arm for Cerna Focusing Stage	Thorlabs	CSA2100	securely mount and position condensers
Snap on Cage Cover for 60 mm Cage, 24 in Long,	Thorlabs	C60L24	enclose and protect the components inside the cage
Sodium dodecyl sulfate	Thermo Fischer Scientific	J63394.AK	Self-assembly component
Three-Chnnale Controller and Knob Box for 1" Cerna Travel Stages	Thorlabs	MCM3001	control ZFM2020
Tube lens	Thorlabs	LA1380-AB - N-BK7	SFG signal collection
Visible LED Set	Thorlabs	WFA1010	provide illumination in imaging setup
Whitelight Source	Thorlabs	WFA1010	Whitelight illumination source for brightfield imaging
WPH05M-1030 - Ø1/2" Zero-Order Half-Wave Plate, Ø1" Mount, 1030 nm	Thorlabs	WPH05M-1030	alter the polarization state of light passing through it
WPLQ05M-3500 - Ø1/2" Mounted Low-Order Quarter-Wave Plate, 3.5 µm	Thorlabs	WPLQ05M-3500	alter the polarization state of light passing through it
X axis Long Travel Steel Extended Contact Slide Stages	Optosigma	TSD-65122CUU	positioning stages that offer extended travel in the horizontal (X) direction
XT95 4in Rail Carrier	Thorlabs	XT95RC4	mount and position optical components
X-Y Axis Translation Stage w/ 360 deg. Rotation	Thorlabs	XYR1	precise movement and positioning of objects in two dimensions, along with the ability to rotate the platform
XY(1/2") Linear Translator with Central SM1 Thru Hole	Thorlabs	XYT1	provide precise movement and positioning in two dimensions
Yb doped Solid State Laser	Light Conversion	CB3-40W	Seed laser
β-Cyclodextrin	Thermo Fischer Scientific	J63161.22	Self-assembly component