Random Lasing Control with Optical Spatial Solitons in Nematic

Liquid Crystals

Armando Piccardi

, Sreekanth Perumbilavil

, Martti Kauranen

, Giuseppe Strangi

and

Gaetano Assanto

NooEL - Nonlinear Optics and OptoElectronics Lab, University “Roma Tre”, Rome, Italy

Laboratory of Photonics, Tampere University of Technology, Tampere, Finland

Physics Dept., Case Western Reserve University, Cleveland, Ohio, U.S.A.

Keywords: Random Laser, Nematic Liquid Crystals, Optical Spatial Solitons.

Abstract: We discuss the synergy of reorientational self-focusing and random lasing in a dye- doped nematic liquid

crystalline material. The laser emission resulting from amplification and multiple scattering inside the

medium can be either modulated or triggered depending on the energy of the visible pump beam and the

power of the near-infrared spatial soliton, respectively exciting the two nonlinear responses. Moreover, the

presence of the self-induced waveguide improves the properties of the emitted beam, i. e., directionality and

profile. Finally, the laser light can be re-directed by steering the spatial soliton with the aid of an external

low-frequency electric field.

1 INTRODUCTION

Random lasing occurs when multiple recurrent

scattering inside an optically active medium

provides the necessary feedback for the stimulated

emission to reach gain higher than losses (Wiersma,

2008). Since its prediction and the first experimental

evidence (Letokhov, 1967; Lawandy, 1994), random

lasers have attracted great attention, letting

researchers envision the realization of low-cost

tunable coherent light sources. As a drawback, their

emission is randomic in direction, and the resulting

profile has poor quality with respect to standard

sources (Cao, 2003). During the last decades, a

number of materials have been employed to generate

and control random laser emission, including

powders (Leonetti, 2011), biological tissues (Polson,

2004), conjugated polymers (Tulek, 2010),

semiconductor polycrystalline films (Cao, 1998),

perovskites (Safdar, 2018), and nematic liquid

crystals (Strangi, 2006). The latter provide light

scattering due to the thermal oscillations of their

weakly linked molecules. At the same time, the low

binding forces allow reorientation of the anisotropic

molecules by optical beams, allowing nonlinear self-

focusing and the generation of optical spatial

solitons (Peccianti, 2003).

In this work, we combined the reorientational

nonlinearity with the strongly scattering behaviour

of a nematic liquid crystal mixture doped with a dye

- acting as the active medium -, generating a

nematicon and random laser emission at the same

time (Perumbilavil, 2016), and studied their mutual

interaction. The paper is organized as follows: first,

we illustrate the principle of both random laser

emission and propagation of spatial solitons in

nematic liquid crystals. Then we expose the

experimental results, while the last part is dedicated

to the discussion of the results and the conclusion.

2 RANDOM LASING AND

NEMATICONS IN DYE-DOPED

NEMATIC LIQUID CRYSTALS

Nematic Liquid Crystals (NLC) are anisotropic

materials featuring high reorientational nonlinearity

owing to the torque induced rotation of the

elongated molecules due to electric fields at either

low or optical frequencies (De Gennes, 1993). Non

uniform molecular reorientation generated by finite

size beams compensates for diffraction, allowing for

self-focusing and the formation of spatial solitons

(Stegeman, 1999), i. e., optical wavepackets with

Piccardi, A., Perumbilavil, S., Kauranen, M., Strangi, G. and Assanto, G.

Random Lasing Control with Optical Spatial Solitons in Nematic Liquid Crystals.

DOI: 10.5220/0007575102890293

In Proceedings of the 7th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2019), pages 289-293

ISBN: 978-989-758-364-3

289

invariant profile in propagation (Segev, 1992;

Peccianti, 2000; Leo, 2004; Rotschild, 2006)

The nonlocal character of the response prevents

catastrophic collapse - typical of local media -,

supporting the stable propagation of 2D spatial

solitons, operating as self-induced waveguides for

optical signals at different wavelengths (Conti,

2003). The generation and control of spatial solitons

in nematic liquid crystals, namely nematicons, have

been demonstrated in a number of configurations,

together with their possible applications in devices

for signal addressing and processing (Piccardi,

2010; Piccardi, 2016; Izdebskaya, 2017; Laudyn,

2018) .

Figure 1: Geometry of the NLC sample where both the

visible pump and the self-confined near-infrared beams

are indicated: (a) top and (b) side view of the sample. The

blue ellipses represent the NLC molecules. (c)

Experimental set-up. P: polarizer, S: sample, OBJ:

microscope objective, BS: beam splitter, SM:

spectrometer. Acquired images of the emitted radiation

without (d) and with (e) a 6mW nematicon. The near-

infrared radiation has been filtered out.

Due to their unique optical and mechanical

properties, NLC can provide various kinds of

nonlinear behaviors, including random laser

emission when samples are doped with a fluorescent

dye (Ferjani, 2006). In fact, the strong scattering

provided by thermal molecular oscillations

combined with dye fluorescence can provide

spontaneous and eventually stimulated emission

when properly pumped, allowing amplification and

random lasing (Bolis, 2016; Perumbilavil, 2016;

Perumbilavil, 2018-1; Perumbilavil, 2018-2).

Moreover, this class of materials demonstrated

to be highly versatile, since the emission can be

easily controlled by exploiting temperature

variations or applied voltages (Wiersma, 2001; Lee,

2011), as the properties depend on molecular

distribution.

3 EXPERIMENTAL RESULTS

The set-up employed for the experiments is sketched

in Fig. 1. The sample, sketched in fig.1(a)-(b) - top

and side view, respectively - is a 100m thick cell

whose top and bottom surfaces have been rubbed in

order to obtain a uniform molecular reorientation at

45° with respect to the z axis, thus maximizing the

reorientational response. The sample is filled with a

mixture of E7 (n

=1.71 n



=1.52 the refractive

indices at =1064nm for extraordinary and ordinary

polarization, respectively) doped with 3%wt of

Pyrromethene 597 dye. As visible in fig. 1(c), a near

infrared cw beam at =1064nm (out of the

absorption band of the dye) is injected into the

sample with wave vector along z, a waist of about

3m and polarization along y - extraordinary

polarization -, and it generates a nematicon. A

second beam from a pulsed source at =532nm

(close to the absorption peak of the dye), with pulse

duration of 6ns and repetition rate 20Hz, is focused

with wave vector collinear with the near infrared

beam, and yields random lasing.

Figure 2: Acquired single shot spectra of output intensity -

arbitrary units, a.u. - (a) below (E=0.40J) and (b) above

(E=48J) threshold for various nematicon powers.

Preliminary measurements showed that the ordinary

pump polarization (along x) maximizes the emitted

PHOTOPTICS 2019 - 7th International Conference on Photonics, Optics and Laser Technology

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radiation.

The evolution of the beams is observed by

collecting the light scattered out of the propagation

plane (yz) with a CCD camera, while the output

profile is recorded by a second camera imaging the

output with a microscope objective. Finally, a

spectrometer detects the output spectrum with a

resolution <1nm.

We excited the random laser with the green

beam and varied the near infrared power to

investigate the role of the self-induced waveguide on

the laser light. Fig. 1(d) and (e) show the

propagation of the green beam alone and when co-

propagating with a near infrared beam of power P

close to 6mW, respectively. As it can be seen, the

emission is confined within the nematicon

waveguide. Fig. 2 shows single shot spectra from the

output of the sample at various pump energies and

nematicon powers.

Figure 3: Acquired spectra averaged over 100 ms (a)

below (E=40J) and (b) above (E=48J) threshold. (c) In-

out characteristic versus pump energy at several

nematicon powers.

When the visible beam is at low energy (around

0.40J) the nematicon has negligible effects

irrespective of its power. Conversely, when the

energy overcomes the threshold value (0.48J) the

effect of the near infrared beam is to enhance the

emission, with the occurrence of a number of lasing

peaks of random wavelength and amplitude.

Figure 4: Single shot acquisition slightly below threshold:

without nematicon (black line) the system does not lase.

With a 6mW nematicon (red line) the system switches- to

the random lasing regime.

To rid of the stochastic character of the lasing

emission we gated the acquisition over 100ms (200

acquired spectra), obtaining the results shown in Fig.

3. The enhancement due to the nematicon is still

apparent, and the in/out characteristic (Fig. 3(c))

shows the typical lasing features, with a threshold

and a slope efficiency increasing with nematicon

power.

As shown in Fig. 4, when the pump energy is

close to (but under) the lasing threshold, the

nematicon favours the transition to the lasing

regime, demonstrating the possibility of switching

on the laser emission by optical means. Thus, the

nematicon does not only guide the emitted photons,

but enhances the lasing process, improving its

efficiency when increasing its near-infrared power

and the corresponding guided-wave confinement.

Being an effective waveguide for the emitted

photons, the nematicon also affects the transverse

profile of the emission. We collected both the

emitted radiation backscattered at the input and the

output signal. Fig. 5 compares the two typical

profiles: the speckled beam of the backscattered

radiation (Fig. 5(a)) and the smoother beam after

confined propagation within the nematicon (Fig.

5(b)).

Finally, the laser emission can also be re-

addressed by steering the nematicon waveguide. We

apply a voltage across the NLC sample, i. e. across

x. In this way the electric torque lifts the molecules

out of the propagation plane yz and the

nematicon walk-off chenges according to the voltage

Random Lasing Control with Optical Spatial Solitons in Nematic Liquid Crystals

291

Figure 5: Acquired photographs of the laser beam profiles

taken from (a) backscattered emission at the input and (b)

forward emission after propagation within the nematicon.

dependent orientation of the optic axis. Remarkably,

the laser emission follows the power flow of the near

infrared beam. We stress that the observable quantity is

the apparent walk-off, i. e., the projection of the actual

walk-off on the propagation plane. Fig. 6 shows both the

output laser profiles without and with applied voltage and

the plot of the resulting apparent walk-off versus voltage,

with a simple numerical fit.

Figure 6: Voltage-driven steering of random laser.

Photographs of output laser profiles for (a) V= 0V and V=

2V. (c) Acquired spectra averaged over 100ms as a

function of applied voltage. (d) Measured (black circles)

and calculated (red line) apparent walk-off versus applied

voltage.

4 CONCLUSIONS

We presented an innovative approach to random

lasing control in nematic liquid crystals, exploiting

the self-induced waveguide and the corresponding

molecular distribution of a spatial soliton to improve

the emission properties in terms of profile and

directionality, demonstrating also the possibility to

electrically control the emission direction.

The effects of the nematicon are not only to

guide the emitted photons, but also to enhance the

conversion efficiency. The corresponding guided-

wave random laser profile results smoother, and the

electric control of the nematicon walk-off allows

controlling the direction of the laser emitted

photons.

We believe this opens new perspectives on

application-oriented random lasing, introducing a

low cost source with electro-optic control.

Some open questions still request deeper

investigation: the actual role of the nematicon-

induced refractive index profile on the random laser

emission and on its bell-shape profile must be

addressed; the degree of coherence of random lasing

modes has to be verified, as well as their wavelength

dependence; moreover, the correlation between

spectral and spatial components of the random laser

emission must be investigated. Future studies will

thus tackle a model to account for the interaction

between the two nonlinearities, addressing the

modulation of the guest-host parameters (doping

percentage, sample geometry, etc.) for the

optimization of the laser. Other strategies for

direction control could also be implemented, in

either two- or three-dimensional geometries.

ACKNOWLEDGEMENT

Academy of Finland, FiDiPro grant no. 282858.

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