BragGrate™ Raman Filters Volume Bragg Gratings for Ultra-low Frequency Raman Spectroscopy

OptiGrate Proprietary and Confidential Ultra Low Frequency (ULF) Raman Spectroscopy

Low-frequency Raman is an indispensable analytical tool in multiple areas of scientific research . Low-frequency Raman bands (lower than 50 cm-1) exist in certain proteins. They are dependent upon the conformation of the protein molecule, but are relatively independent of the form of the sample, i.e., whether it is a film or a crystal. . In amorphous glasses, most of the Raman spectra present a low frequency response called "boson peak". . Much minerals present low frequency vibration modes, i.e. sulfur between 0 and 250 cm-1, or organic materials like L-Cystine between 0 and 800 cm-1. . Single-wall and multi-wall carbon nanotubes exhibit radial breathing mode (RBM) vibrations in the range 150–200 cm-1 which are used to characterize diameter distribution and overall quality of nanotubes as well as influence of external factors. . Quality of semiconductor multi-layered structures (superlattices) is assessed by observing folded acoustic (FA) modes in the range 0–100 cm-1. . The Relaxation modes in liquids, binary mixtures and solutions, in the range 0–400 cm-1, help to determine their dynamic structure.

Selected applications of ULF Raman spectroscopy . Pharmaceutical polymorphs . LA modes of polymer . Semiconductor lattices and nanostructures . Material : phase/structure . Metal Halides . Gases . Carbon nanotubes . Micro, nano-crystallites www.optigrate.com OptiGrate Confidential 2 Notch Filter Linewidth and Low Frequency Raman Spectroscopy

Notch Filter rejects Rayleigh light

Notch filters reject scattered Rayleigh light that “overshadows” good signal Bandwidth of notch filter limits the lower range of frequencies to be measured BragGrate™ Notch Filters enable Rayleigh light rejection as close as 5 cm-1 from the laser line

Source: world wide web

www.optigrate.com OptiGrate Confidential 3 Volume Bragg Gratings as narrow line optical Raman filters

Reflecting Volume Bragg Grating (RBG) Spectral width of RBG

10 [nm]

Wavelength, nm 500 1000 1500 1 (FWHM)

Width 0.1 Spectral 0.01 0246810 Reflecting Volume Bragg Gratings as Raman Notch Filters Thickness [mm] Diffraction Efficiency (DE) up to 99.99% (standard optical density: OD>3 and OD>4) Spectral Bandwidth (FWHM) < 5 cm-1 ADVANTAGES of BragGrate™ Raman Filters Angular Selectivity (FWHM) < 5 mrad Ultra-low frequency measurement down to 5 cm-1 with Wavelength range 400 nm to 2 µm single stage spectrometers . Standard: 488, 514, 532, 633, 785, 1064 nm Simultaneous measurements of both Stokes and anti- Stokes Raman bands . In production: 405, 442, 458, 473, 491, 552, 561, 568, 588, 594, 660, 880, 1550 nm No polarization dependence . Any custom wavelength can be fabricated Environmentally stable, no humidity degradation Grating Thickness: 2-3 mm No degradation up to 400ºC Standard BNF dimensions: 11×11 and 12.5×12.5 mm2 (up Stable to any type of optical and ionizing radiation to 25×25 mm2) No time degradation: stable up to 400C and any type of optical and ionizing radiation www.optigrate.com OptiGrate Confidential 4 Linewidth of BragGrate™ notch filter vs thin film notch filter

Wavelength [nm] Wavelength [nm] 760 770 780 790 800 810 784.5784.75 785 785.25785.5 1E+00 1E+00 1E-01 1E-01 1E-02 1E-02 1E-03 1E-03 1E-04 1E-05 1E-04

Optical Density 1E-06 1E-05 1E-07 1E-08

Optical Density 1E-06 TFF Notch 1E-07 2x BNF 1E-08

Spectral profiles of the narrowest thin film filter available on the market and BNFs can be (optional) mounted BragGrate Notch Filter (BNF). The bandwidth of a typical BNF is about 100- in Ø1” round aluminum holders 200 pm, whereas bandwidth of TF filters can’t be narrower than 2-3 nm. for easy use with standard opto- Optical density of single BNF is limited to about OD4 and, thus, to provide mechanical assemblies sufficient Rayleigh light suppression depending on the measurement wavelength 2 to 3 filters have to be used in sequence

www.optigrate.com OptiGrate Confidential 5 BragGrate™ Bandpass Filter (laser line cleaning)

To achieve ULF Raman measurements, the laser spectral noise has to be removed as close as possible to the laser line. Standard Bandpass filters have the line width of 200-300 cm-1 and, thus, all spectral noise below 200 cm-1 would be visible in ULF Raman spectra interfering with measured Raman bands BragGrate™ Bandpass Filter (BPF) has the linewidth ~5 cm-1 FWHM)and, thus, removes laser noise down to 5 cm-1 with suppression up to -70 dB BPF is a reflecting VBG which diffraction efficiency and other parameters are optimized for best noise removal close to the laser line Wavelength Range 400 nm to 2 µm. Standard wavelengths: 405, 488, 514, 532, 633, 785, 1064 nm. Any custom wavelength in the range can be fabricated Standard BPF dimensions: 5×5×2 mm3 (785 nm filters are typically different in size) BPFs can be mounted in 1” or 0.5” mm round aluminum holders to be used with standard opto-mechanical mounts BPFs provide both spectral and spatial filtering as shown in figures below

Spatial filtering of laser light with BPF. Left panel: far field Spectral filtering of laser light with BPF. Red line: spectrum of a image of HeNe laser beam profile without cleaning. Right 785 nm diode laser with ASE background. Blue line: BPF removes panel: HeNe laser beam profile after spatial filtering with the ASE background in immediate vicinity of the laser line. LD light BPF. At the same time the laser is spectrally cleaned to -70 beam cleaned with BPF enabled ULF Raman measurement down dB as close as 5 cm-1 form the laser line to 5 cm-1 www.optigrate.com OptiGrate Confidential 6 Examples: ULF Raman measurements of L-Cystine

Ultra-low frequency measurements of L-Cystine at 4 different wavelengths: 488, 532, 633 nm (left) and 785 nm (right)

data courtesy of Horiba Jobin Yvon www.optigrate.com OptiGrate Confidential 7 Examples: ULF Raman spectrum of SiGe superlattice

60 000

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40 000

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0 -500 0 500 Raman Shift (cm -1 ) 7 7 3 000 27 27 -27 -27 -7 -7 17 17 -17 -17 -14 -14 23 23 14 14 37 37 -24 -24 -37 -37 47 47 34 34 44 44 -34 -34

2 000 -47 -47 -44 -44 53 53 57 57 -54 -54 -57 -57 64 64 67 67 -64 -64 -67 -67 1 000 73 73 77 77 -74 -74 83 83 -77 -77 8686 -83 -83

0 -80 -60 -40 -20 0 20 40 60 80 Raman Shift (cm -1 ) data courtesy of : P. H. Tan, State Key Laboratory for SL and Microstr., Institute of Semiconductors, Beijing, P. R. China; K. Brunner, University Wuerzburg, Germany www.optigrate.com OptiGrate Confidential 8 Examples: ULF Raman spectra at 5 cm-1 and below

ULF Raman spectrum of several layers of MoS2 flakes

Data courtesy of P. H. Tan, State Key Laboratory for SL and Microstr., Institute of Semiconductors, Beijing, P. R. China;

Data courtesy of HORIBA Jobin Yvon SAS; measured with LabRAM HR Evolution

www.optigrate.com OptiGrate Confidential 9 BragGrate™ Raman filters in work: selected publications

Glebov et al., "Volume Bragg gratings as ultra-narrow and multiband optical filters", Proc. SPIE Vol. 8428, 84280C (2012), invited Tan et al., “The shear mode of multilayer graphene,” Nature, Materials 11, 294–300 (2012). Ferrari et al., “Raman Spectroscopy as a versatile tool for studying the properties of graphene,” Nature, Nanotechnology 8, 236-246 (2013) Chen et at., “Electronic Raman Scattering On Individual Semiconducting Single Walled Carbon Nanotubes,” Nature, Sci. Reports 4, No. 5969 (2014)

Ge et al., “Coherent Longitudinal Acoustic Phonon Approaching THz Frequency in Multilayer MoS2,” Nature, Sci. Reports 4, No. 5722 (2014) Cong et al., “Enhanced ultra-low-frequency interlayer shear modes in folded graphene layers,” Nature, Communications 5, No. 4709 (2014) Wu et al., “Resonant Raman spectroscopy of twisted multilayer graphene,” Nature, Communications 5, No. 5309 (2014) Wojcieszak et at., “Origin of the variability of the mechanical properties of silk fibers: Order/crystallinity along silkworm and spider fibers,” J. Raman Spectroscopy 45, Issue 10, pages 895–902, (2014) Reymond-Laruinaz et al., “Growth and size distribution of Au nanoparticles in annealed Au/TiO2 thin films,” Thin Solid Films 553, (2014) Tan et al., “Ultralow-frequency shear modes of 2-4 layer graphene observed in scroll structures at edges,” Phys. Rev. B 89, 235404 (2014) 12 Du et al., “Soft vibrational mode associated with incommensurate orbital order in multiferroic CaMn7O ,” Phys. Rev. B 90, 104414 (2014) Chang et al., “Imaging molecular crystal polymorphs and their polycrystalline microstructures in situ by ultralow-frequency Raman spectroscopy,” Chem. Commun. 50, pages 12973-12976, (2014) Alamendia et at., “Protective ability index measurement through Raman quantification imaging to diagnose the conservation state of weathering steel structures,” J. Raman Spectroscopy (2014) Hehlen et al., “Soft-mode dynamics in micrograin and nanograin ceramics of strontium titanate observed by hyper-Raman scattering”, Phys. Rev. B 87, 014303 (2013) Zeng et al., “Low-frequency Raman modes and electronic excitations in atomically thin MoS2 films,” Phys. Rev. B 86, 241301(R) (2012) Ibanez et al., “Raman scattering by folded acoustic phonons in InGaN/GaN superlattices,” J. Raman Spectrosc. 43, 237-240 (2012)

Zhang et al., “Raman spectroscopy of shear and layer breathing modes in multilayer MoS2,” Phys. Rev. B 87, 115413 (2013)

Saviot et al., “Quasi-free nanoparticle vibrations in a highly-compressed ZrO2 nanopowder”, J. Phys. Chem. C 116, 22043 (2012) Tsurumi et al., “Evaluation of the interlayer interactions of few layers of graphene”, Chem. Phys. Letters 557, 114–117 (2013)

Boukhicha et al., “Anharmonic phonons in few-layer MoS2: Raman spectroscopy of ultralow energy compression and shear modes,” Phys. Rev. B 87, 195316 (2013)

Crespo-Monteiro et al., “One-step microstructuring of TiO2 and Ag-TiO2 films by continuous wave laser processing in the UV and visible ranges”, J. Phys. Chem. C 116, 26857 (2012)

Zhao et al., “Interlayer Breathing and Shear Modes in Few-Trilayer MoS2 and WSe2,” Nano Lett. 13, 1007−1015 (2013) Schidt et al., “Moganite detection in silica rocks using Raman and infrared spectroscopy,” European J. Mineralogy 25, 797-805 (2013) Saviot et al., “THz nanocrystal acoustic vibrations from ZrO2 3D supercrystals,“J. Mater. Chem. C, 8108-8116 (2013) Rapaport et al., "Very Low Frequency Stokes and Anti-Stokes Raman Spectra Accessible with a Single Multichannel Spectrograph and Volume Bragg Grating Optical Filters," XXII ICORS, AIP Conference Proceedings, Volume 1267, pp. 808-809 (2010). Tsurumi, A., Naito, Y., Iwata, K., “Construction of Multichannel Low-Frequency Raman Microspectrometer of Spectral Coverage to 20 cm-1 with Gratings Formed in Glass Plates”, XXII ICORS, AIP Conference Proceedings, Volume 1267, pp. 792-793 (2010). www.optigrate.com OptiGrate Confidential 10