Summary of physics doctoral thesis: Study of organic dye lasers nanogold doped active medium for generation of short pulses by distributed feedback lasers

Research purpose: Preparation and characterization of GNPs-doped active medium based on dye molecules in PMMA applied to generate short pulses in the range of pico-seconds by using distributed feedback dye lases (DFDL) configuration are aimed.

MINISTRY OF EDUCATION

AND TRAINING

VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY

GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY

…… ….***…………

NGUYEN THI MY AN

STUDY OF ORGANIC DYE LASERS NANOGOLD-DOPED ACTIVE MEDIUM FOR GENERATION OF SHORT PULSES BY DISTRIBUTED FEEDBACK LASERS

Code: 9440110

SUMMARY OF PHYSICS DOCTORAL THESIS

The work was completed at the Center for Quantum Electronics, Institute of Physics, Vietnam Academy of Science and Technology

Supervisor:

1 Assoc Prof Dr Do Quang Hoa

2 Dr Nghiem Thi Ha Lien

The thesis can be found at the library:

- National Library of Hanoi

- Library of Institute of Physics, VAST

INTRODUCTION

1 Recent necessary of the topics

Short pulse dye lasers recently become necessary instruments in many laboratories in Vietnam and in the world, also However, the investigation for developing laser active medium is still attracted in many laboratories on optics and photonics Moreover, the achievements

in nanostructured materials have been bringing numerous applications

in both the science and human life Especially, gold nanoparticles (GNPs) with different sizes have become attractive subjects due to their distinguished properties Thus, in this research, the study and preparation of new laser active medium to be used for the laser resonance cavity included the mixture of the dye and nanostructured metallic particles is focused

Research purpose:

Preparation and characterization of GNPs-doped active medium based on dye molecules in PMMA applied to generate short pulses in the range of pico-seconds by using distributed feedback dye lases (DFDL) configuration are aimed

CHAPTER 1 OVERVIEW OF THE DYE LASERS, LUMINESCENT ORGANIC DYES AND GOLD

NANOPARTICLES 1.1 Dye lasers

A dye laser is a typical laser which uses an organic dye as the lasing medium Due to these laser dyes contained double bonds conjuncted to functional group, its could be strongly able to absorb in the wide spectral band from ultratviolet to visible

In this thesis, we used the dye DCM methyl-6-(4-dimethylaminostyryl)-4H-pyran) for the study It can be explained by special properties of DCM such as: the DCM molecule possesses both donor and acceptor behavior, leading to a large range of emission wavelengths (~ 100 nm) in visible light; DCM molecules strongly absorbs in shorter wavelengths than the peak of absorption resonance plasmon band of GNPs, therefore it is suitable for our research

(4-(Dicyanomethylene)-2-on the mixture medium of dyes and GNPs Besides, the lasers having ability of the wavelength selectivity could be easy choose desired continuous wavelengths in the emission range of DCM

1.2 Optical properties of nanostructured metallic materials, Gold nanoparticles

As known, nanostructured materials possess most special properties Due to a small size (much smaller than the wavelengths of ultra-violet and visible range), all the laws of classic optics used to explain the phenomena occured when light interacts with the materials are more unsatisfied The resonance oscilation of the electron cloud

on the surface of metallic nanoparticles (surface plasmon resonance - SPR) has been applied to the explaination of quantum confinement and quantum effect occured on the nanomaterials

At the interface between nanostructure metals and vincinity medium, surface plasmonic effect exists in a smaller space than the typical optical materials In other side, metallic nanoparticles strongly influence on the optical properties of the medium, like receiver and

emitter “anten” For example, a nanoparticles of the precious metal

with 10 nm diameter possesses a extinction coefficient of ca 107 M

-1cm-1 or larger in two orders of magnitude in comparison with a typical value of the organic laser dye

1.3 Short pulse dye laser

1.3.1 Working principle of dye laser

Dye laser performances on the gain medium having two large energy levels up-down, that can emit a large band

1.3.2 Several types of configurations of dye lasers emiting second pulses:

In this section several configurations of dye lasers emiting second pulses were presented

pico-1.3.3 Distributed feedback (DFB) dye laser

Distributed feedback (DFB) dye laser is based on the Bragg reflect effect without mirrors in resonance cavity

The optic resonance occured when light beam propagates in a medium existed the modulation of gain and refractive index to be suitable to light wavelength, which leads to burn out the laser emission

Fig 1.1: Schema of working principle of a DFDL laser

CHAPTER 2: PREPARATION OF ACTIVE MEDIUMS FOR

2.1 Initial materials and equipments used

2.1.3 Preparation of GNPs and attachment of HS-PEG-COOH

Sphere-shape GNPs were prepared by Turkevich method

2.1.4 Changing active medium for Gold nanoparticles

GNPs dispersed in water have been re-dispersed in MMA solvent for avoiding water, because water was not soluble MMA, moreover DCM molecules were easy decomposed in water

Interf erence pattern

Biến điệu gain

) ) T t T n t

2.2 Active medium in solution for dye lasers

2.2.2 Doped medium of DCM/GNPs dye

Table 2.3: Concentration of DCM and GNPs in ethanol

Sample DCM concentration

(mol/L)

GNPs volume (particles/ml)

Sample 1 1.0x10-4 M 5.0x109

Sample 2 1.0x10-4 M 1.0x1010

Sample 3 1.0x10-4 M 1.5x1010

Sample 4 1.0x10-4 M 2.0x1010

Table 2.4: Concentration of DCM and GNPs in MMA solution Sample DCM concentration

(mol/L)

GNPs volume (particles/ml)

2.3 Preparation active medium for dye laser with doping GNPs

in PMMA matrices (DCM/GNPs/PMMA)

2.3.1 Active medium with PMMA matrice

Dye laser solid state active medium

was prepared by polymerization of MMA

monomers

2.3.2 Template for preparation

Solid state active medium was

prepared in a cubic shape of 1x1x2,5 cm3

size (similar to cuvet) Synthesis process

for polymers was carried-out at

temperature of ~ 500C (Fig 2.1)

2.3.3 Preparation of doped solid state active mediums

Solid state active mediums were prepared by polymerization of MMA doped DCM dye

a) Preparation of white samples

Fig 2.1 Template for

preparation solid state active mediums

The initial materials have been used: monomer MMA and catalytic AIBN MMA solution for each experimental sample is 2000

µl There are 5 samples with different weight of AIBN

Table 2.5: Materials and concentration

b) Preparation of DCM/PMMA active medium

The aim: Preparation of solid state samples for studying of the active mediums with different DCM concentration

Table 2.6: Initial materials and concentration of DCM/PMMA

Sample DCM/MMA (M) DCM/MMA (µl) AIBN (mg)

2.2.3.2 Preparation of PMMA/DCM doped with GNPs

The GNPs “Au@PEG-COOH” were dispersed in MMA such

as introduced in the first step of the samples preparation

Table 2.7: DCM/GNPs/PMMA samples with DCM of 10-3 M

Sample DCM

(mol/l)

GNPs/MMA (µl)

DCM/MMA (µl)

AIBN (mg)

DCM/MMA (µl)

AIBN (mg)

2.4 The determination of parameters of the samples and applied techniques

In this section we listed the methods and equipments to detect different parameters of the samples for researching, such as the UV-Vis absorption, fluorescence spectra, fluorescence lifetime, autocorrelation etc

CHAPTER 3: INVESTIGATION OF ACTIVE MEDIUM OF

GNPs-DOPED DYE MOLECULES

In this chapter we presented recent results of the investigation

on the quenching and enhancement effects of laser dye in the active medium of GNPs-doped DCM

3.1 Optical properties of active mediums in dye laser with doping sphere-shape GNPs

3.1.1 Samples preparation

The samples were prepared according to describing in Chapter 2 Nd:YAG laser was used for pumping DFDL, dye and GNPs-doped PMMA samples were prepared in a bulk with a size of 1×1×2.5 cm3

3.1.2 Optical characteristics of DCM in solution and solid-state medium

3.1.2.1 Absorption spectra of DCM dye in ethanol and MMA

Absorption spectra of DCM dye in ethanol and MMA without GNPs dopant are presented in Fig 3.1a and Fig 3.1b, respectively These spectra have a similar shape, however the bandwidth of the absorption spectra of DCM in ethanol is narrower than that in MMA, and the spectral intensity fast decay in the long wavelength side This can be explained due to the weak interaction between dye

molecules and the solvent, which did not expand or change the states

of the upper and lower energy states of the DCM molecules

3.1.2.2 Absorption spectra of the DCM dye in PMMA matrice

In the solid state matrice of PMMA the mobility of the DCM molecules is smaller than in solution Thus the absorption spectra are broaden in the long wavelength side (Fig 3.3)

3.1.3 Fluorescent spectra of GNPs-doped DCM in ethanol

(DCM/GNPs/ethanol)

Fig 3.3: Absorption spectra of DCM dye in PMMA.

Fig 3.1: Absorption spectra of DCM dye in ethanol (a)

quenching is occurred due to the Foster and SET energy transfer

3.1.4 Optical properties of GNPs-doped DCM in PMMA matrice

and the concentration of

GNPs was varied The

intensity of absorption

spectra of the DCM

slightly increased with the

Fig 3.6: Fluorescent spectra of

DCM/GNPs in ethanol

0 50 100 150 200 250 300 350 400

450 (1) DCM 1x10 -4 M

(2) DCM+5x10 9

hat/ml (3) DCM+1x10 10 hat/ml (4) DCM+1,5x10 10 hat/ml (5) DCM+2,0x10 10

hat/ml (1)

(2) (3)

(5) (4)

2 3

concentration from 5 l/ml to 20 l/ml (or from 0.5x1010 particles/ml

to 2x1010 particles/ml) (Fig 3.7) At low concentrations of GNPs, a slightly increase of the fluorescence intensity was also observed This can be explained due to the appearance of the near-field interaction Several molecules of DCM were adhered on the GNPs surface, resulting in higher absorption cross-section of DCM increased With higher concentration of GNPs, the absorption intensity of the samples

decreased

3.1.4.2 Fluorescence of the dye of DCM/GNPs/PMMA

Fluorescence spectra of DCM/GNPs/PMMA (DCM concentration of 3x10-4 M) vs GNPs concentration under an excitation wavelength of 472 nm is shown in Fig 3.8 From this figure one can see that the fluorescence intensity of DCM increased up to a maximum value at the GNPs concentration of 1.5x1010 particles/ml (Curve “2”), then decreased with increasing GNPs concentration (Curves “3, 4”)

Fig 3.8: Fluorescence spectra of DCM/GNPs/PMMA

The excitation wavelength l = 472 nm (DCM concentration is

of 3x10-4 M)

0 20

2

3 4

This can be explained due to less mobility of the DCM molecules

in the solid state host, thus the larger GNPs concentration, the smaller average distance between GNPs and DCM, resulting in clearer SET effect This behavior of GNPs can be applied for controlling the emission of the dye centers around the particles GNPs exhibited as an anten, emiting or detecting electromagnetic radiation

With low GNPs concentrations, when pumping source excited

to fluorescence is presented, GNPs play a role of emitting energy, leading to the energy transfer from GNPs to DCM molecules At higher GNPs concentration, the quenching of fluorescence radiation from DCM molecules occurred With excitation wavelength of 532

nm, only fluorescence quenching was observed (Fig 3.9) This can be explained as follows When the excitation wavelength is closed to maximum of plasmonic absorption of GNPs, the bleaching occurred for the DCM molecules located on the GNPs surface This result obtained is different from that observed in case when DCM solutions doped GNPs

Fig 3.9: Fluorescence spectra of DCM/GNPs/PMMA

The excitation wavelength l = 532 nm (DCM concentration is

3x10-5 M)

3.1.5 Fluorescence lifetime of molecules of DCM/GNPs/PMMA

For the solution

dependent on both the

solvent and doping

materials (Fig 3.10) Whereas, fluorescence lifetime of DCM in PMMA with different GNPs concentration (namely from 0 to 33 l of GNP solution of 1x1011 particles/ml) is presented in Fig 3.11

The fluorescence

of DCM molecules

exhibited similarly to

self-emission, the

transition from higher

energy levels almost

did not change Thus,

solid state materials

containing DCM doped

with GNPs can be used

for the active medium for lasers as they exist in solutions

3.2 Influence of the light-to-heat of GNPs on DCM molecules

3.2.1 Thermal conversion of plasmonic effect of GNPs

Fig 3.10: Fluorescence lifetime of DCM

doped with different GNPs in solution

Fig 3.11: PL lifetime of DCM/GNPs/PMMA

Light-to-heat effect between GNPs particles and around environment was simulated by Mie theory This simulation can be applied for explanation of the experimental results obtained when GNPs particles with a diameter of 16 nm doped in the active medium

of solid state DCM dye laser

3.2.2 Fluorescence decay of DCM/GNPs/PMMA

Light-to-heat effect

strongly affected to the

working time of the active

medium of DFDL Fig

3.13 shows the decay of

integrated fluorescence

intensity over time of the

active medium based on

Fig 3.13: Lowering process of

photoluminescence vs time of the acive medium DCM/GNPs/PMMA at

RT with cooling

0 2000 4000

3000 6000

9000 at 10 o

C room temp.

curve At temperature of (4 ± 1) °C, the unchanged laser intensity was maintained for a long time

CHAPTER 4 DISTRIBUTED FEEDBACK DYE LASER (DFDL) USING GNPs-DOPED SOLID STATE MEDIUM

- Modeling theoretical simulation for solid-state DFDL laser used DCM/GNPs/PMMA Calcultion of spectro-temporal evolution of the DFDL by Matlab language

- Studing the influence of the laser parameters on the laser properties for optimization of the performance of DFDL

- Experimentally researching the influence of some parameters of solid-state DFDL on laser properties

- Setup a DFDL equipment that can be applied in practice based on the results of both the theoretical and experimental research

4.1 Theoretical research on the solid-state DFDL dye laser

4.1.1 Rate equations of two dissimilar components

It is suggested that the peformance of a dye laser can be described by two broad energy levels (corresponding to a laser with four energy levels, as shown in Fig 4.1)

To describe the energy levels transition in a dye laser doped

GNPs, the rate equations has been shown in Ref [126], it consists of

four equations describing spectro-temporal evolution of the laser

emission of DFDL with intrinsic quenching parameters:

where n 0Au , n a are the densities of GNP and DCM molecula in 1cm3,

respectively; n Au (t), n a (t) – the average densities of GNPs and DCM at

above energy level in 1cm3, respectively; τ c – life time of a photon in

the active medium equivalent that is considered as follows:

(4.6) Equation (4.1) describes the changing rate of GNPs at the excited

state by energy pumping and laser emission energy of the DCM

molecules The change of resonance surface plasmon energy of GNPs

is shown in equation (4.2) The energy transfer from/to molecules of

dye DCM is described by equation (4.3) with K F and K s – coefficients

of the energy transfer This value is positive when the energy transfer