1
© 2009 HORIBA, Ltd. All rights reserved.
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Dr. Bernd Bleisteiner
Application Scientist
Raman Spectroscopy
HORIBA Scientific
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Outline
Dispersive Raman Instruments
Principal setup of dispersive Raman instruments
Laser Source – UV-VIS-NIR optics
Polarisation control
Collection geometries & reduction of Rayleigh scattering
– single and triple spectrometers
Collection of Raman photons – Macro- & Micro-approach
Confocal Raman Microscope and spatial resolution
Spectral resolution and spectral coverage
Sensitivity in subject to the detector
Light flux in subject to dispersion
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© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Principal setup of dispersive Raman instruments
A Raman spectrometer requires five elements:
1.
2.
3.
4.
5.
Light source
Collecting optics
Straylight rejection filter
Wavelength selector and
Detector
A dispersive Raman spectrometer uses a monochromator or a
spectrograph as wavelength selector
4
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Principal setup of dispersive Raman instruments
Filter
Collecting lens
Detector
Wavelength
Selector
Sample
Light source
5
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Principal setup of dispersive Raman instruments
Depending on the filtering technique and power of spectral resolution
Raman instruments can be divided into two principal groups
1. Single stage instruments which suppress the Rayleigh light by notch or
edge filters and
2. Double or triple stage instruments which suppress the Rayleigh light by
an intermediate slit
Double and triple stage instruments
Single stage instruments
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Dispersive Raman Instruments
The Light Source
Scattering spectroscopy (Raman) experiences much smaller cross
section compared to absorption spectroscopy (IR). As a result,
Raman scattering intensity is much weaker than that IR absorption
intensity. Therefore, a powerful light source is essential.
While the first Raman was observed using sun light, all modern
Raman spectrometers use lasers exclusively as the light source.
The laser light is called the excitation light or incident beam. The
optical path from laser to sample is called the Incident Beam Path.
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© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Laser Sources
Dispersive Raman instruments can be equipped with lasers sources
starting from the UV up to the NIR
Most common are laser sources in the VIS region
Possible wavelengths are 227, 244, 257, 325, 364, 413, 442, 457,
473, 488, 514, 532, 633, 647, 660, 785, 830, 1064 nm
-1
Horizontal lines indicate a relative Raman Shift of 3800 cm
244
325
457
488
514
532
633
785
830
1064
200
400
600
800
1000 1200 1400 1600 1800
Wavelength [nm]
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Dispersive Raman Instruments
Laser Sources
The main reason for switching the laser source is to avoid
fluorescence, which is interfering with the Raman spectrum
Laser wavelength, λ3
Laser wavelength, λ3
Laser
wavelength, λ1
Fluorescence
Raman shift, λ1-1+∆
Laser wavelength: λ3 < λ2 < λ1
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Raman shift, λ3-1+∆
9
60 000
Green spectrum: 532 nm laser
Red spectrum: 633 nm laser
Brown spectrum: 785 nm laser
50 000
40 000
Intensity (cnt)
30 000
20 000
10 000
60 000
0
50 000
600
800
Wavelength (nm)
1 000
40 000
Intensity (cnt)
Dispersive Raman Instruments
Laser Sources
30 000
Fluorescence is wavelength dependent
Ordinary Raman is wavelength independent
20 000
10 000
0
1 000
2 000
Raman Shift (cm-1)
3 000
10
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Laser Sources
A second important reason for switching the laser source is to do
resonance Raman spectroscopy
Electronical States
Virtual States
Polarizability
is particularly high if excitation
occurs in electronic resonance
(Resonance Raman)
Excitation light, λ0
Resonance Raman
∆>0
ν
Stokes Scattering: λ > λ0
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Resonance Raman of carotinoids in cyano bacteria
17_Cyanobakterium Strang_D3_473nm_H1000_S100_SW_600gr_1x30s
8 000
-20
-10
Y (µm)
7 000
6 000
0
10
Intensity (cnt)
Dispersive Raman Instruments
Laser Sources – Resonance Raman
5 000
20
4 000
4 µm
-20
0
X (µm)
20
3 000
2 000
1 000
0
500
1 000
1 500
2 000
Raman Shift [cm-1]
2 500
3 000
3 500
Laser was attenuated with a density filter OD3. Laser
energy was only 16 µW@sample
12
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Laser Sources – VIS optics
For Raman instruments equipped with VIS laser sources one optical
path is sufficient

Confocal hole
CCD
Microscope
Lens
 Slit
Mirror
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Dispersive Raman Instruments
Laser Sources – UV-VIS optics
For Raman instruments equipped with UV and VIS laser sources the
optical path has to be adapted to the spectral range
For UV instruments a switchable path with (Aluminum) mirrors
avoids optical mismatch by chromatic aberrations
Special collecting optics (objectives) with a high transmission in the
UV are necessary
CCD
CCD
Optional
UV optimized CCD
Microscope
With lenses:
Chromatic aberrations in UV
Microscope
Reflective optics:
Achromatic
λ1, λ2, λ3 focus at
same position
© 2009 HORIBA, Ltd. All rights reserved.
14
Dispersive Raman Instruments
Laser Sources – VIS-NIR optics
For Raman instruments equipped with VIS and NIR laser sources
(> 785 nm) the optical path has to be adapted to the spectral range
For NIR instruments a switchable path with (Silver) mirrors are
necessary
Special collecting optics (objectives) with a high transmission in the
NIR are necessary
An InGaAs detector is a prerequisite for detecting Raman spectra in
the NIR
CCD
CCD
InGaAs
Microscope
Microscope
15
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Dispersive Raman Instruments
Polarization control
The polarization of the Raman scattered light can contain useful
information
This property can be measured by using polarized excitation and a
polarization analyzer
Because the laser light is normally polarized a polarizer in the
excitation path is not necessary
For polarization experiments the polarization direction of the incident
laser path can be controlled by a lambda half wave plate
In the Raman path an analyzer set at both perpendicular and
parallel to the excitation is necessary
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Dispersive Raman Instruments
Polarization control
Filter
Collecting lens
Detector
Wavelength
Selector
Sample
Analyzer
perpendicular or
parallel to the
excitation plane
Polarized
Laser source
λ/2-plate
λ/2-plate
to turn the
polarization of
the polarized
laser
17
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Dispersive Raman Instruments
Polarization resolved measurements
Depending on isotropic or anisotropic samples polarization
experiments showing a different result in subject to the relative
direction of the excitation polarization and the analyzer
Isotropic Samples: Gases and liquids, in which the molecules are
randomly oriented
Anisotropic Samples: Oriented single crystals, in which the atoms
or molecules oriented in a fixed position
Attention: Solid amorphous phases or micro crystals are randomly
oriented. For the same chemical species, the observed Raman
spectra depends on the spatial orientation of the micro sized
particles.
18
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Dispersive Raman Instruments
Polarization resolved measurements
Polarizer
Analyzer
Isotropic sample
Anisotropic Sample
same spectra
different spectra
same spectra
different spectra
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Isotropic Sample: Cyclohexane
8000
Normalized spectra
6000
Intensity (a.u.)
Dispersive Raman Instruments
Polarization resolved measurements
4000
2000
0
800
1000
1200
1400
1600
Wavenumber (cm-1)
20
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Anisotropic Sample: Acetylic salicylic acid
8000
Normalized spectra
6000
Intensity (a.u.)
Dispersive Raman Instruments
Polarization resolved measurements
4000
2000
1000
1050
1100
1150
Wavenumber (cm-1)
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Dispersive Raman Instruments
Raman Scattering Collection Geometry
Theoretically, Raman scattering
can be observed from any angle,
because Raman scattering is
generated in all directions
Practically and historically, three
angles were favored; 0°, 90° and
180°.
0°configuration
(forward scattering)
was the least favorite
because of high
Rayleigh scattering and
laser interference.
90°configuration (right angle scattering)
was an early favorite because Rayleigh
scattering was minimized.
However, this is viable only for
transparent samples such as liquids or
solutions
Sample
Molecule
Laser
180° configuration (backscattering)
is viable to all forms of samples.
However, it posed implementation
difficulty - How to guide the laser
beam to the sample without
blocking the Scattering beam path?
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© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
180°° Backscattering Collection Geometry
Is the most common collection geometry nowadays because almost
all Raman system are based on microscope optics
The laser is guided to the samples either by a beam splitter or
(especially common in single stage instruments) by injection
rejection filters (edge-, notch-filters)
The filter reflects the laser light and serving to deliver the incident
beam to the sample
Rejection filter that blocks laser
light wavelength is placed in the
incident beam path.
Sample
Molecule
The filter is transparent for scattering light
because wavelengths are different from
the laser light, and passes the Raman
signal through.
Laser
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Dispersive Raman Instruments
Notch Filter vs. Edge Filter
White light spectrum with a notch filter
White light spectrum with an edge filter
150
1500
100
1000
50
500
0
-300
-200
-100
0
100
Wavenumber (cm-1)
200
300
0
-300
-200
-100
0
100
200
300
Wavenumber (cm-1)
Zero Raman shift → Excitation laser position
• A finite life time
• Stokes and Anti-Stokes Raman
• A virtually infinite life time
• Stokes Raman only
There is a cost advantages to the edge filter die
to no aging, but Anti-Stokes-Raman is not
obtainable
24
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With double or triple spectrometers the Rayleigh line is filtered by an
intermediated slit
The cut off gained with this technique is < 5cm-1 in comparison to
100 cm-1 with an edge or notch filter
subtractive double monochromator
50000
40000
Notch Filter
20000
10000
3000
Intensity (a.u.)
30000
Intensity (a.u.)
Intensity (a.u.)
Dispersive Raman Instruments
Filter vs. Subtractive Double Monochromator
2000
1000
0
3000
SiGe spectrum
2000
1000
0
-10
-10
-5
-5
0
0
5
5
10
10
15
15
Wavenumber (cm-1)
20
20
25
25
wavenumber (cm-1)
0
0
50
100
150
200
250
300
350
Wavenumber (cm-1)
25
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
90°° Collection Geometry
90° collection geometry is used in macro Raman experiments
The incident beam path of the laser is different from the scattering
beam path (= the optical path from sample to the detector)
90° collection geometry is suitable for Raman experiments which
need very high excitation power
Raman scattering
Sample
Molecule
Macro chamber with
interchangeable collection
optics covering adjustable
collection from 90° to 180°
geometries
Laser
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© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
0°° Collection geometry – Transmission Raman
0° collection geometry is used for Raman volume measurements of
bulky opaque samples
The incident beam path of the laser is in opposite to the scattering
beam path
Raman scattering
Example: Macro -Transmissions Raman
for content uniformity control on tablets
Sample
Molecule
Laser
27
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Dispersive Raman Instruments
Collection geometry – Various collection angle
For micro Raman experiments with various excitation / collection
angles customized instruments are available
E. g. off-axis excitation for improving very low frequency
measurements on solid state samples
Sample
Molecule
Raman scattering
Laser
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Dispersive Raman Instruments
Collection optics
Depending on the experiment and the sample different collecting
optics are used
Therefore micro Raman instruments are equipped with different
objectives with different numerical aperture
Objective
N.A.
Working distance [mm]
x100
0.90
0.21
x50
0.75
0.38
x10
0.25
10.6
x100 LWD
0.80
3.4
x50 LWD
0.50
10.6
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© 2009 HORIBA, Ltd. All rights reserved.
θ
Collection solid angle
Large for high N.A. lens
Small for low N.A. lens
High N.A. lens
Sampling volume
Small for high N.A. lens
Large for low N.A. lens
θ
Working distance
Low N.A. lens
Working distance
Dispersive Raman Instruments
Collection optics
Laser spot size
Small for high N.A. lens
Large for low N.A. lens
NA = n · sin (Θ
Θ)
n: refraction index
Θ: aperture angle
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© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Collecting Optics – what objective should be used?
A distinction between opaque and transparent samples has to be made
For opaque samples, high N.A. lens works better because there is almost no
penetration of the laser into the sample. High N.A. lens enables
- High laser power density (mW/µm3) → increases sensitivity
- Wide collection solid angle → increases sensitivity
100 %
30 000
Example for an opaque sample:
Silicon wafer
25 000
70 %
20 000
Intensity (cnt/sec)
Y (µm)
-20
Silicon
x100 – NA = 0.9 – 31.350 C/s
x50 – NA = 0.75 - 21.995 C/s
x10 – NA = 0.25 - 1.462 C/s
15 000
0
10 000
10 µm
20
5 000
5%
0
X (µm)
0
460
480
500
520
540
Raman Shif t (cm-1)
560
580
600
620
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© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Collecting Optics – what objective should be used?
A distinction between opaque and transparent samples has to be made
For transparent samples, low N.A. lens works better because of penetration of
the laser into the sample. Low N.A. lens enables
- Large sampling volume → increases sensitivity
x10 3
14
12
Sample: Cyclohexane
Instrument: ARAMIS
Red: x100LWD, 7,000 cts/s
Blue: Macro lens, 14,500 cts/s
cyclo_100xLWD
cyclo_macro
100 %
Intensity (cnt)
10
8
48 %
6
4
2
0
740
760
780
800
820
Raman Shift (cm-1 )
840
860
880
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© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Spatial resolution and confocality
The ability to axially discriminate signals using the confocal hole
is called confocality.
Decreasing the confocal hole diameter increases axial and
lateral spatial resolution.
Slit
Confocal hole
Rejection fitter
33
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Spatial resolution and confocality
Confocal hole (image plane)
Wide Hole
Objective
Sample (object plane)
Sampling
Volume
Narrow Hole
34
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Spatial resolution and confocality
Confocal z-scan against silicon as a measure for the axial resolution λexc = 633 nm
Narrow Hole:
Collecting Raman radiation that
originates only from within a
diffraction limited laser focal volume
with a dimension of:
Focus waist diameter ~ 1.22 λ / NA
Depth of laser focus ~ 4 λ / (NA)2
Laser focus
waist diameter
Depth of
laser focus
(d.o.f)
35
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Spatial resolution and confocality
Integrated Intensity
Z-scan of multi layered polymer
Film
Glue
x10 3
Confocal hole diameter
1000 µm
6
4
2
0
0
-20
-40
Glass
x10 3
Integrated Intensity
398.0
x10 3
-60
z-Axis (µm)
20
Intensity
15
Film
1730.3
10
5
Glue
-80
-100
-120
Confocal hole diameter
100 µm
6
4
2
0
0
-20
-40
-60
z-Axis (µm)
-80
-100
-120
0
500
1 000
Raman Shift (cm-1)
1 500
The narrow hole improves axial resolution.
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© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Spatial resolution and confocality
Spatial resolution especially axial resolution can be improved by
(water or oil) immersions objectives (n1 ≈ n2 or ideally n1 = n2)
With normal (air) objectives the difference in refraction index
(n1 ≠ n2) of air and material results in an optical distortion which
reduce the spatial resolution
n1 ≠ n2: d.o.f. spreads linearly with z0
n1 ≠ n2: focus is located at deep P,
which differs from “mechanical deep”
Length or depth of
Laser Focus (= d.o.f.)
Air
Sample
Data and figures from: Modeling and Measuring the Effect of Refraction on the Depth Resolution of Confocal Raman microscopy Neil. J. Everall,
Applied Spectroscopy, Vol. 54, 6, 2000
© 2009 HORIBA, Ltd. All rights reserved.
37
300
x100 LWD / 100 µm Hole
Intensity (cnt)
Film
Glue
Glass
The water immersion
objective improves
axial resolution
remarkably in
comparison with the
x100 LWD air
objective
200
100
0
0
-50
-100
-150
-200
Z (µm)
-250
-300
-350
x100 water immersion / 100 µm Hole
500
Intensity (cnt)
Dispersive Raman Instruments
Spatial resolution and confocality
400
300
200
100
0
0
© 2009 HORIBA, Ltd. All rights reserved.
-50
-100
-150
-200
Z (µm)
-250
-300
-350
38
Exploring lateral spatial resolution -Test on a well structured sample
-15
Circles etched by a Gallium ion beam on
silicon with different distances of
1400, 1200, 1000, 800, 600, 400, 200 nm.
-10
-5
Y (µm)
Dispersive Raman Instruments
Spatial resolution and confocality
Structure were mapped with a
mechanical table and x100 try objective
with NA = 0.9
0
5
10
1 µm
0
10
X (µm)
Sample in courtesy of Dr. Wilhelm, TU Graz
© 2009 HORIBA, Ltd. All rights reserved.
39
Dispersive Raman Instruments
Spatial resolution and confocality
Optical parameters for the lateral spatial resolution
Laser spot size
1.22 λ
dw =
NA
Diffraction limited resolution d given by the Rayleigh criterium
0.61 λ 0.61 λ
d=
=
n sin α
NA
e. g. with 633nm HeNe laser and
x100 objective with NA = 0,9
d = 430 nm
40
© 2009 HORIBA, Ltd. All rights reserved.
Spatial resolution and confocality
Y (µm)
-2
-4
-6
-8
-10
-8
0
0
X (µm)
-6
4
4
2
-1
Raman Shift [cm
]
-4
6
6
-2
8
8
0
10
10
2
12
14
16
12
0.5 µm
Intensity [Counts]
30
20
10
0
x [µm]
Sample in courtesy of Dr. Wilhelm, TU Graz
© 2009 HORIBA, Ltd. All rights reserved.
41
-6.11
-4.17
-1.70
0.97
3.63
40
6.95
10.21
Intensity [Counts]
Spatial resolution and confocality
20
0
12
10
8
6
4
2
Raman Shift [cm -1]
0
-2
-4
-6
-8
x [µm]
Real distances [nm]
Fitted FWHM [nm]
Difference
1400
1160
240
1200
1050
150
1000
780
220
800
640
160
600
520
80
400
430
-30
200
270
-70
-10
-8
-6
-4
-2
0
Spatial resolution of object structure
which can be resolved is about λ/2
Y (µm)
2
4
6
8
10
12
14
0.5 µm
16
0
X (µm)
Sample in courtesy of Dr. Wilhelm, TU Graz
© 2009 HORIBA, Ltd. All rights reserved.
42
Dispersive Raman Instruments
Spectral resolution and spectral coverage
Spectral resolution is a function of 1. dispersion, 2.
widths of entrance slit and 3. pixel size of the CCD
Dispersion is the relation between refraction of light
according to the wavelength of light
Dispersion is a function of the 1. focal length of
spectrograph the 2. groove density of the grating and
3. the (excitation) wavelength
In general, long focal length and high groove
density grating offer high spectral resolution.
43
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Spectral resolution and spectral coverage
Schematic diagram of a Czerny-Turner spectrograph
Slit
Collimating
mirror
Raman straylight
generated at a
certain excitation
wavelength
Grating
with a certain
groove density
Focusing mirror
CCD Detector
with a certain
pixel size
Focal Length
44
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Dispersion in respect of excitation wavelength
-1
Horizontal lines indicate a relative Raman Shift of 3800 cm
244 - 269 nm (25 nm)
325 - 371 nm (46 nm)
457 - 553 nm (96 nm)
488 - 599 nm (111 nm)
514 - 639 nm (125 nm)
532 - 667 nm (135 nm)
633 - 833 nm (200 nm)
785 - 1119 nm (334 nm)
830 - 1210 nm (380 nm)
1064 - 1768 nm (704 nm)
400
600
800
1000 1200 1400 1600 1800
Wavelength [nm]
Short excitation
wavelength
Same focal length
Same grating
CCD Detector
CCD Detector
200
Long excitation
wavelength
45
© 2009 HORIBA, Ltd. All rights reserved.
Length of CCD Chip
x10 3
473 nm – 633 nm – 785 nm
22
20
18
Same focal length
Same grating
Length of CCD Chip
16
Intensity (cnt/sec)
Dispersive Raman Instruments
Spectral coverage - dependence from excitation
wavelength
14
12
10
Length of CCD Chip
Relative Raman shift of 3100 cm-1
8
corresponds to 81 nm
6
4
Relative Raman shift of 3100 cm-1
corresponds to 154 nm
2
Relative Raman shift of 3100 cm-1
corresponds to 252 nm
0
500
600
700
800
Wavelength (nm)
900
1 000
46
© 2009 HORIBA, Ltd. All rights reserved.
CCD Detector
Short focal length
Same grating
Same excitation wavelength
Long focal length
CCD Detector
Dispersive Raman Instruments
Dispersion in respect of the focal length
47
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Dispersion in respect of the focal length
Dispersion in cm-1 / pixel
1800 gr/mm Grating
LabRAM (F = 300 mm)
LabRAM HR (F = 800 mm)
Same grating
Same excitation wavelength
48
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Dispersive Raman Instruments
Dispersion in respect of the grating
Grating Equation: sin i + sin i’ = nkλ
where n is the groove density, k the diffraction order and λ the wavelength
0th order
Polychromatic light
1st order
i’
i
i
2nd order
Grating
49
© 2009 HORIBA, Ltd. All rights reserved.
Same focal length
Same excitation wavelength
© 2009 HORIBA, Ltd. All rights reserved.
CCD Detector
Low density groove grating
CCD Detector
Dispersive Raman Instruments
Dispersion in respect of the grating
High density groove
grating
50
Dispersive Raman Instruments
Spectral resolution as a function of slit width
Slit
Slit
Slit
One of parameters that determines the spectral resolution is the entrance slit width. The narrower the slit,
the narrower the FWHM (full width at half maximum), and higher the spectral resolution.
When recording a line whose natural width is smaller than the monochromator’s
resolution, the measured width will reflect the spectrograph’s resolution.
51
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Dispersive Raman Instruments
Spectral resolution as a function of pixel size
Detector
Intensity
Detector
Because a CCD detector is
made of very small pixels, each
pixel serves as an exit slit (pixel
size = exit slit width)
For the same size CCDs, the
CCD with a larger number of
smaller pixels produces a larger
number of spectral points closer
to each other increasing the
limiting spectral resolution and
the sampling frequency
26 µm pixel vs. 52 µm pixel
(simulation)
600
650
Raman Shift (cm
700
-1)
52
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Sensitivity in subject to the detector
Comparison of different detectors concerning quantum efficiency
between 200 – 1100 nm
Front illuminated
Back illuminated UV coated
Front illuminated open electrode
Back illuminated deep depleted
53
© 2009 HORIBA, Ltd. All rights reserved.
Comparison of fingerprint region of polyethylene spectra
CCD quantum efficiency and scatter efficiency (∼
∼ ν4) has influence on
sensitivity in NIR
poly1_633nm
BLACK: 785 nm, 30s, 20mW @sample
poly1_785nm
RED: 633 nm, 5s, 10mW @sample
8 000
7 000
6 000
5 000
Intensity (cnt)
Dispersive Raman Instruments
Sensitivity in subject to the detector
4 000
3 000
2 000
1 000
0
1 000
1 100
1 200
Raman Shift (cm
-1
1 300
1 400
1 500
)
Factor 6 on measurement time, Factor 2 on laser power →
means 12 times more input in NIR
54
© 2009 HORIBA, Ltd. All rights reserved.
Comparison of C-H region of polyethylene spectra
CCD quantum efficiency and scatter efficiency (∼
∼ ν4) has influence on
sensitivity in NIR
x10 3
BLACK: 785 nm, 30s, 20mWpoly1_633nm
@sample
poly1_785nm
RED: 633 nm, 5s, 10mW @sample
25
20
Intensity (cnt)
Dispersive Raman Instruments
Sensitivity in subject to the detector
15
10
5
0
2 600
2 700
2 800
Raman Shift (cm -1 )
2 900
3 000
3 100
25 times more signal with 633 nm despite 12 times more input in NIR
55
© 2009 HORIBA, Ltd. All rights reserved.
Comparison of polyethylene spectra acquired with 633 and 785 nm excitation.
Raman signal in the 785 nm spectrum are equal or weaker in comparison to
633 nm spectrum although Factor 6 on measurement time and factor 2 on laser
power. Mainly caused due to lower QE of the detector in the NIR region.
x10 3
CCD_QE_OP
BLACK: 785 nm, 30s, 20mW @sample
poly1_633nm
RED: 633 nm, 5s, 10mW @samplepoly1_785nm
BLUE: QE curve of OE-CCD
20
Intensity (cnt)
Dispersive Raman Instruments
Sensitivity in subject to the detector
15
10
5
0
650
700
750
800
850
900
wavelength (nm)
950
1 000
1 050
1 100
56
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Light flux in subject to dispersion
If dispersion is doubled integrated signal becomes
approximately half because every pixel is collecting only half
amount of photons
57
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Light flux in subject to dispersion
Silicon spectra acquired with 633 nm with 600 and 1200 grating.
Ratio is 1 : 2.
Dispersion is 2.07 and 0.94 cm-1/pixel @ 654 nm (according to 520,7 cm-1).
Ratio is 2.2 : 1
Integrated Raman signal of 520,7 cm-1 band is 265063 and 136982.
Ratio is 1.9 : 1
Si_633nm_1200gr
Si_633nm_600gr
30 000
Intensity (cnt/sec)
25 000
20 000
15 000
10 000
5 000
0
510
520
Raman Shift (cm-1)
530
Only grating was varied. All other acquisition conditions are unchanged
© 2009 HORIBA, Ltd. All rights reserved.
58
Reverse argument:
If dispersion is similar or equal by a combination of grating and focal length
(e. g. F = 300 mm & 1200 gr/mm and F = 600 mm & 600 gr/mm) the integrated
signal becomes approximately the same
High density groove grating
Long focal length
CCD Detector
Short focal length
CCD Detector
Dispersive Raman Instruments
Light flux in subject to dispersion
Low density groove grating
59
© 2009 HORIBA, Ltd. All rights reserved.
Dispersive Raman Instruments
Summary – What we have heard about
Principal setup of a Raman spectrometer
Single and triple stage instruments
Laser excitation – switching of excitation wavelength
Different optics for VIS, UV-VIS and VIS-NIR Raman instruments
Polarization control – isotropic vs. anisotropic samples
Collection geometry 0°, 90°, 180°or various angles
Different Rayleigh filtering techniques
Collection optics with high and low NA
Spatial resolution and confocality
Spectral resolution and spectral coverage
Dispersion in respect of the focal length, grating, wavelength
Quantum efficiency of different detectors
Wavelength dependence of quantum efficiency of a CCD detector
Light flux in respect of dispersion
60
© 2009 HORIBA, Ltd. All rights reserved.
Thank you
61
© 2009 HORIBA, Ltd. All rights reserved.
© 2009 HORIBA, Ltd. All rights reserved.