Light Amplification and Nonlinear Microscopy by Stimulated Raman

Scattering

M. A. Ferrara

, A. D’Arco

1,2

, M. Indolfi

, N. Brancati

, L. Zeni

and L. Sirleto

National Research Council (CNR) - Institute for Microelectronics and Microsystems, I-80131 Napoli, Italy

Second University of Naples (SUN), Department of Information Engineering, I-81031 Aversa, Italy

National Research Council (CNR) – Istituto di Calcolo e Reti ad Alte Prestazioni, I-80131 Napoli, Italy

Keywords: Nanophotonics, Biophotonics, Nonlinear Optics, Stimulated Raman Scattering.

Abstract: The Stimulated Raman Scattering has important connections with nanophotonics and biophotonics.

Concerning nanophotonics, one of the most recent challenges is the investigation of ‘nonlinear optical

phenomena at nanoscale’. Among them, stimulated Raman scattering is one of the most interesting, due to

its significant implications from both fundamental and applicative point of view. In this paper, comparison

among experimental investigations of stimulated Raman scattering in amorphous silicon nanoparticles and

in silicon micro- and nano-crystals, at the wavelengths of interest for telecommunications, are reported. In

addition, concerning biophotonics, first, the implementation of femtosecond Stimulated Raman

Spectroscopy (f-SRS), as a single point of scanning microscopy, is described. Then, the integration of f-SRS

in a laser scanning microscope and label free imaging of polystyrene-beads are demonstrated.

1 INTRODUCTION

In silicon photonics and silicon nanophotonics,

amplification and light emission are still the most

challenging goals to get. In Stimulated Raman

Scattering (SRS), a pump laser beam enters a

nonlinear medium and spontaneous generation and

amplification lead to a beam at a frequency different

from the pump. Raman amplification, demonstrated

in the early 1970s, is an interesting approach for

optical amplification, because it is only restricted by

the pump wavelength and Raman active modes of

the gain medium (Shen and Bloembergen, 1965).

Raman lasing can be achieved by using SRS

phenomenon, which permits, in principle, the

amplification in a wide interval of wavelengths,

from the ultraviolet to the infrared (Stolen, 2004).

SRS is used in tunable laser development, high

energy pulse compression, etc. Considering bulk

semiconductors, lasing by SRS was first discovered

in GaP (Nishizawa and Suto, 1980), whereas SRS

from spherical droplets and microspheres, with

diameters 5-20 µm, has been observed using both

pulsed and continuous wave probe beams (Spillane

et al., 2002). Raman lasers have been also

demonstrated in silicon micro-waveguides (Rong et

al., 2005).

The "ideal" material for Raman amplification

should have wide, flat and high Raman gain into all

the range of interest in telecommunications (from

1270 to 1650 nm). Unfortunately, as a general rule,

due to the physics behind the Raman effect, there is

a tradeoff for Raman amplification. In nature, we

have material, for example silicon, with high Raman

efficiency and small bandwidth, and material, for

example silica, with a large bandwidth but with

small amplification (see figure 1). The above-

mentioned tradeoff is a fundamental limitation

towards the realization of micro-/nano-sources with

large emission spectra. Therefore, the investigation

of new materials possessing both large Raman gain

coefficients and spectral bandwidth is becoming

mandatory in order to satisfy the increasing

telecommunications demands (Refi, 1999). In order

to overcome these limitations, a possible option is to

consider nanocomposite and nanostructured

materials.

Raman scattering in electrons-confined and

photons confined materials is a fascinating research

field of great importance from both fundamental and

applicative point of view. Concerning the

fundamental one, there have been a number of

investigations both experimental and theoretical, but

the question is still "open" (Gaponenko et al., 2002),

while from an applicative point of view, there are

some important prospective, for example to realize

micro/nano source, with improved performances,

based on SRS.

The phenomenon of strong resonant and local

enhancement of visible electro-magnetic (EM)

radiation when incident on the surface of metallic

particles and films resulting from surface plasmon

resonances, continues to attract significant attention

for fundamental and applied interests (Kawata,

2009). However, the possibility of EM radiation

enhancement from semiconducting and insulating

materials, particularly in silicon, is noteworthy for

silicon-based optoelectronic applications owing to

the potential for monolithically integrating photonic

technology and semiconductor electronics (Cao et

al., 2006). Except for a report of SRS from

individual single walled carbon nanotubes (Zhang et

al., 2006), and the observation of SRS from

semiconductor nanowires (Wu et al., 2009), we find

no other evidence for this important nonlinear optic

effect in nanostructured materials.

In biophotonics, confocal and multiphotons

fluorescence microscopy are important and powerful

techniques for imaging of biological samples.

However, these microscopic techniques show some

limitations, indeed, they require chemical labels that

could interfere with biological functionalities;

additionally the photo-bleaching introduces artefacts

and limits the measurement repeatability. Therefore,

it is necessary to introduce and implement a new

multiphotons microscopy technique suited for real

time imaging with high three dimensional spatial

resolution and chemical specificity of unlabeled

living cells. Raman microscopy can be used as a

contrast mechanism based on vibrational properties.

A typical Raman spectrum makes available

information on the molecular and chemical structure

of the sample, offering an intrinsic chemical

selectivity. Nevertheless, linear Raman microscopy

is limited to weak signals, so, to obtain an image

acquisition times are very long.

It is worth noting that, due to the recent

femtoseconds laser technological development,

nonlinear techniques have found application in soft

matter and in particular in biological materials.

Femtoseconds laser allows to obtain an average

power, incident onto the sample, lower than the

photodamage limit and a high enough pump peak

power to ensure the triggering of the nonlinear

effects. In addition, the range of pulses wavelengths,

generated into the range between 680 nm and 1300

nm, permits to work in the window of water

transparency, significantly reducing the absorption.

Coherent Raman Scattering (CRS) techniques

are sensitive to the same molecular vibrations

probed in spontaneous Raman spectroscopy, but

unlike linear Raman spectroscopy, CRS techniques

exhibit a nonlinear dependence on the incoming

light fields and produce coherent radiation. In CRS,

two collinear laser beams (pump and probe) at

different frequencies excite the sample. When the

difference in frequencies is equal to a molecular

vibration, a stimulated and coherent excitation of

molecular bond vibration modes (third order non-

linear process) occur and a significant increase of

Raman signal is observed. This latter property has

popularized CRS as a microscopy modality, as it is

intimately related to the technique’s strong optical

signals that enable fast imaging applications. CRS

microscopy makes it possible to achieve images

based on vibrational Raman contrast at imaging

speeds much faster than attained with conventional

Raman microscopes. Clearly, this attribute is very

attractive for biological imaging, where imaging

speed is an important experimental parameter

(Ploetz et al., 2007; Freudiger et al., 2008; Ozeki et

al., 2009; Nandakumar et al., 2009; Fu et al., 2013).

CRS includes two techniques: coherent anti-

Stokes Raman scattering (CARS) and SRS. We note

that a CARS spectrum is different from its

corresponding spontaneous Raman spectrum due to

a non-resonant background, which complicates

spectral assignment, causes difficulties in image

interpretation, and limits detection sensitivity (Ploetz

et al., 2007; Freudiger et al., 2008; Ozeki et al.,

2009; Nandakumar et al., 2009).

The recent development of SRS microscopy

overcame these limitations and provided better

imaging contrast mechanism (vibrational) contrast.

SRS eliminates the non-resonant background

problem because the generated third order SRS

nonlinear polarization is directly heterodyne mixed

and amplified by the input beam with the exact same

phase, therefore always resulting in a zero non-

resonant contribution. Definitely, SRS is free from

the non-resonant background, exhibiting an identical

spectrum as the spontaneous Raman it is linearly

proportional to the concentration of the analyte, and

therefore it allows straightforward quantification. In

such situations, it is natural to consider the

application of SRS to biological microscopy. When

SRS microscopy was proposed (Ploetz et al., 2007;

Freudiger et al., 2008; Ozeki et al., 2009), two

transform-limited picosecond (ps) lasers with narrow

spectral bandwidth were used to excite a single

Raman-active vibrational mode for fast imaging

with high spectral resolution. With this ps–ps

excitation sources it is not possible to distinguish

mixed chemical species with overlapped Raman

bands in the sample because other vibrational modes

of the sample are not excited. However, in many live

biological and biomedical applications, simultaneous

mapping of different chemical species in the same

sample is extremely important for the investigation

of the co-distribution or dynamic correlation

between pairs of biomolecules. Therefore,

multicolor imaging with multiple chemical contrasts

is considered necessary. We note that multicolor

imaging can be realized only taking advantage of

femtosecond laser source

(Nandakumar et al., 2009).

In this paper, in paragraph 2 a comparison

between our experimental results of SRS obtained

on different samples of nanostructured amorphous

silicon clusters, and of silicon micro-crystals (Si-µc)

and nano-crystals (Si-nc) are reported. The two main

figure of merit (Raman gain and bandwidth) are

compared to an ideal material and we highlight a

possible trend to get the best performance.

In paragraph 3, the details of experimental set up

and the main experimental issues of femtosecond

Stimulated Raman Spectroscopy (f-SRS)

implementation are reported. In addition, steps

towards nonlinear microscopy, i.e. the integration of

f-SRS in a laser scanning microscope, is described.

Finally, label free imaging of polystyrene-beads are

demonstrated.

2 LIGHT AMPLIFICATION BY

STIMULATED RAMAN

SCATTERING

In our previous papers (Sirleto et al., 2004 and 2006;

Ferrara et al., 2008), some advantages of silicon

nanostructure with respect to silicon were

demonstrated. Experimental results, proving

spontaneous Raman scattering in silicon

nanostructures at the wavelength of interest for

telecommunications (1.54 µm), were reported in

Refs. (Sirleto et al., 2004 and 2006; Ferrara et al.,

2008). According to phonon confinement model in

Refs. (Sirleto et al., 2004 and 2006; Ferrara et al.,

2008), two significant improvement of Raman

approach in silicon quantum dots with respect to

silicon were demonstrated: the broadening of

spontaneous Raman emission and the tuning of the

Stokes shift. Considering silicon quantum dots

having crystal size of 2 nm, a significant broadening

of about 65 cm

-l

and a peak shift of about 19 cm

-l

were obtained. Because the width of C-band

telecommunication is 146 cm

-1

, taking into account

the broadening and the shift of spontaneous Raman

emission, more than the half of C-band could be

cover using silicon quantum dots, without

implementing the multi-pump scheme.

Nanocomposities are random media containing

domains or inclusions that are on the nanometric

size scale. The optical properties of composite

materials can be adjusted by controlling the

constituents and morphology of the composite

structure. The optical nanocomposite approach

offers opportunities to produce high-performance

and relatively low-cost optoelectronic media suitable

for many applications.

In our previous papers (Sirleto et al., 2004 and

2006; Ferrara et al., 2008), SRS has been measured

using as pump a CW Raman laser operating at 1427

nm. SRS net gain (G) is given by:

(1)

where I

= P/A, with P is the power incident onto the

sample and A is the effective area of pump beam.

Since the sample is transparent to the incident light,

L is taken to be equal to the thickness of the sample

along the path of the incident light. G as a function

of signal laser wavelength was measured in three

different samples:

 Silicon nanocomposites dispersed in SiO

matrix, with a probe signal at 1542.2nm

(Sirleto et al., 2009; Ferrara et al., 2011). The

mean radius of the silicon dots and the dot

density were respectively of 49nm (Si-µc) and

1.62x10

dots/cm

 Amorphous silicon nanoclusters embedded in

Si-rich Nitride/Silicon superlattice structures

(SRN/Si-SLs), with a probe signal at 1540.6nm

(Sirleto et al., 2008). The structure of the

sample consists of 10 SRN layers and 9

amorphous Si (a-Si) layers for a total thickness

of 450 nm. Amorphous silicon nanoclusters

size was about 2nm.

 Silicon nanocrystals embedded in silica matrix,

with a probe signal at 1541.3nm (Sirleto et al.,

2012). Si-nc size was about 4nm.

In particular, we focalized our study on two

different nanocomposities materials based on

amorphous or crystalline silicon. The difference

between them is related to their different

spontaneous Raman signal, indeed in amorphous

silicon Raman spectra is broadband but shows a low

intensity, while in crystalline silicon Raman spectra

is very narrowband but shows a high intensity.

Additionally, considering that Raman effect is a

volume effect, in the sense that the greater is the

volume of interaction, the higher is the Raman

signal, we aspect that different concentration of

nanoparticles could lead to different Raman gain.

Results obtained can be summarized as follows:

 In silicon nanocomposites, an amplification of

Stokes signal up to 1.4 dB/cm is reported. This

result showed a preliminary valuation of

approximately a five-fold enhancement of the

Raman gain with respect to bulk silicon.

Moreover, a threshold power reduction of

about 60% is also reported (Sirleto et al., 2009,

Ferrara et al., 2011).

 In SRN/Si-SLs, amplification of Stokes signal

up to 0.87 dB/cm was experimentally

demonstrated, consistent with a preliminary

valuation of approximately a four-fold

enhancement of the Raman gain with respect to

bulk silicon. Moreover, a threshold power

reduction of about 40% is also reported (Sirleto

et al., 2008).

 A giant Raman gain from the silicon

nanocrystals is obtained that is up to four

orders of magnitude greater than in bulk

crystalline silicon (Sirleto et al., 2012).

In figure 1 the Raman Gain coefficients and their

bandwidth for all the samples are reported and

compared with an ideal material. We note that si-nc

has a Raman gain value comparable to the ideal

materials, but the bandwidth is still small. Probably

if the nanostructures was embedded in a different

matrix, a greater bandwidth could be obtained, too.

Figure 1: Raman Gain coefficients and their bandwidth are

reported for different materials: silicon and silica (as 'bulk

material'), and silicon micro- and nano-particles

(amorphous and crystalline). Features for 'ideal materials'

for Raman amplification are reported, too.

3 STIMULATED RAMAN

MICROSCOPY

In SRS microscopy, pump and probe pulses at

angular frequencies of ω

and ω

(ω

> ω

) are

focused into a sample, and the intensity change of

the probe pulse due to SRS is detected. Intuitively,

SRS is caused by the optical phase modulation

induced by the time-dependent refractive index

reflecting the molecular vibration, which is

coherently driven by the intensity beat between

pump and probe pulses (Shen and Bloembergen,

1965).

Concerning SRS, an important issue is its

sensitivity, because the SRS signal is detected as a

small change of the intensity of excitation beam, and

hence is deteriorated by shot-noise and laser

intensity noise. The laser intensity noise is quite

important aspect to take into account in SRS

microscopy because it can easily surpass the shot

noise, an intrinsic property of the light source (Min

et al., 2011).

In the field of laser spectroscopy, SRS was

extensively studied as a highly sensitive tool of

vibrational spectroscopy (Owyoung, 1978) and the

detection of SRS with a shot-noise limited

sensitivity was achieved (Owyoung, 1978, Levine et

al., 1979; Heritage et al., 1980). The basic idea is to

take advantage of lock-in detection at high-

frequency. In this approach, a high-frequency

modulation transfer method to detect the signal is

used. The intensity of the pump beam is modulated

with an electro-optic modulator and the modulation

transfered to the probe beam is measured with a

lock-in amplifier (LIA) after blocking the pump

beam with an optical filter. It is preferable to

increase the modulation frequencies because the

relative intensity noise of laser pulses typically

decreases with frequency. Increasing modulation

frequency of the beam, at frequencies above 1 MHz,

it allows to reach the intrinsic limit of

photodetectors. The thermal noise can be negligible

compared to the shot noise when the optical power is

of the order of several milliWatts.

Fig. 2 shows the schematic layout of the

microscope; it requires two sources. The first one,

used as a pump beam, is a femtosecond Ti-sapphire

(Chameleon Ultra II) with a pulse duration of

approximately 100 fs with a repetition rate of

80MHz and emission wavelengths into the range

680-1080nm. The second one, used as probe beam,

is a femtosecond synchronized optical parametric

oscillator (SOPO-Chameleon Compact OPO) with a

pulse duration of approximately 200 fs with a

repetition rate of 80MHz that emits into the range of

wavelengths 1000-1600nm. An electro -optic

modulator (EOM 350-160 KD*P CONOPTICS) was

placed for the intensity modulation of the pump

pulses at a modulation frequency of 9.1 MHz. These

two beams were collinearly combined with a

dichroic mirror (Semrock FF875-Di01-25x36),

temporally overlapped by a delay line (Newport

MOD M-ILS200CC) and focused inside a sample

through a scanning microscope (Ti-eclipse Nikon).

The integration with the microscope ensures greater

stability of the system and increase the spatial

resolution of the measurement, while the presence of

scanning unit allows the analysis on a large area, i.e.

the realization of an image.

Figure 2: Experimental setup for SRS microscopy.

A 20X objective focuses beams inside a sample,

and output pulses are collected by a 60X high

numerical aperture objective. After, pump pulses are

removed by a stack of optical filter, while probe

pulses are detected by a photodetector (Thorlabs

DET 10N/M) and measured by a lock-in amplifier

(SR844- 200 MHz dual phase). The electrical signal

coming out from the lock-in amplifier is digitalized

by a PCI card, which manages and synchronizes the

lock-in amplifier and the scanning unit of

microscope in order to collect information and to

obtain a 2D image.

A first measurement of stimulated Raman

spectroscopy was carried out on a single point of a

drop of a water solution with a very high density of

polystyrene beads, which is placed between a

microscope slide and a coverslip; the polystyrene

beads had a diameter of 15µm. In order to

investigate a typical C-H bond of polystyrene

(Raman shift of 3054 cm

-1

), the pump signal was set

at 799 nm with a focused power of 20mW, while the

probe signal was set at 1057nm with a focused

power of 10mW. The temporal overlap of these two

beams was obtained by scanning the delay line with

steps of 0.001mm corresponding to 13,3fs time-shift.

The time constant of the LIA was set to 3 ms with a

slope of 18 dB/oct and 30μV sensitivity. The

measured values from lock-in amplifier, in terms of

phase and amplitude of SRS signal as a function of

the probe-pump delay in ps, are reported in Fig.3.

Figure 3: Amplitude and Phase of SRS signal measured by

lock-in amplifier.

Because SRS uses near-infrared excitation light,

the standard optics of a laser-scanning multiphoton

microscope are compatible with SRS modalities. In

particular, SRS uses the same high numerical

aperture (NA) lenses that are employed in

multiphoton microscopes. In fact, a SRS imaging

modality shares many of the imaging properties of a

multiphoton microscope, including fast image

acquisition and sub-micrometer resolution.

Commercial laser-scanning microscopes can be

upgraded with a SRS module with some important

modification. In its simplest form, a SRS microscope

can be constructed from a fluorescence laser-

scanning microscope by equipping a forward

detector and with proper bandpass filters and

interfacing the scanning unit of microscope with the

detector. Moreover, although standard condensers,

which have a NA of ∼0.55, suffice to capture a

significant portion of the forward- propagating

signal, better collection efficiencies are obtained

with higher-NA condensers. This is especially

important for the SRS techniques, where

photothermal and position-dependent interference

effects may introduce artifacts in the image (Popov

et al., 2012;

Chung et al., 2013). Such effects can be

mitigated by choosing a high-NA condenser.

Fast image acquisition rates are the prime

advantage of SRS imaging over spontaneous Raman

microscopy. Using high repetition rate femtosecond

pulse trains with average powers in the 10 mW

range on the sample produces SRS signals with

acceptable SNR in a few microsecond per pixel. For

an image with 512 × 512 pixels, this translates in

acquisition rates of about a frame per second. Such

imaging speeds are at par with those of linear and

multiphoton fluorescence microscopy techniques.

To demonstrate the feasibility of the proposed

device, the same sample, used for spectroscopic

investigation, is studied. The binary image shown in

fig. 4, is single recordings of 512x512 pixels with

acquisition time of 16 seconds. The time constant of

lock-in amplifier was set to 100μs with a slope of

18dB/oct and 3μV sensitivity. For the process of

binarization, an adequate threshold of gray level has

been selected with Otsu method (Otsu, 1979).

Figure 4: Binary SRS image of 15μm polystyrene beads at

3054cm

-1

at I

pump

at 7mW. Scale bar is 15μm.

4 CONCLUSIONS

In this paper, we describe some important

connections of SRS with nano- and bio-photonics.

As far as nanophotonics is concerned,

experimental investigations of stimulated Raman

scattering in nanostructured silicon based materials

are compared. Because a theoretical understanding

addressing the physical origin of enhanced Raman

gain in nanostructured materials remains to be

established, in this work we try to give some tiles for

the important open question about stimulated Raman

scattering at nanoscale. In addition, our results could

be an important step towards to silicon based Raman

laser.

As far as biophotonics is concerned,

femtosecond stimulated Raman spectroscopic and

microscopy have been implemented. As a

preliminary step for nonlinear microscopy, label free

imaging of polystyrene-beads is demonstrated. Next

step is to apply this nonlinear optical imaging

approach to biological research.

ACKNOWLEDGEMENTS

We really appreciate the useful discussions and

continuous support from Giacomo Cozzi, product

specialist - Nikon Instruments. We thank Vitaliano

Tufano from IMM-CNR, for his valuable technical

assistance.

This work was partially supported by Italian

National Operative Programs PONa3_00025

(BIOforIU) and by Euro-bioimaging large scale

pan

‐

European research infrastructure project.

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