characterization of optical polarization converters made

Characterization of optical polarization converters made byfemtosecond laser writing

Christiaan J. de Jong1,2, Alireza Lajevardipour1, Mindaugas Gecevicius3, Martynas Beresna3,Gediminas Seniutinas1, Gediminas Gervinskas1, Ricardas Buividas1, Peter G. Kazansky3,

Yves Bellouard2, Andrew H. A. Clayton1, and Saulius Juodkazis1,4

1 Centre for Micro-Photonics, Faculty of Engineering and Industrial Sciences, SwinburneUniversity of Technology, Hawthorn, VIC 3122, Australia

2 Department of Mechanical Engineering, Eindhoven University of Technology, Postbus 513,5600MB Eindhoven, The Netherlands

3 Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ,United Kingdom

4 Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton, VIC 3168, Australia

ABSTRACT

Recently, new types of silica polarization converters fabricated by femtosecond lasers have been introduced.These devices use spatially arranged nanogratings found under certain femtosecond laser exposure conditions infused silica to create arbitrary polarization states by shaping spatially and locally the retardance of an incomingbeam. Using this principle, radial and azimuthal polarization converters were demonstrated. These devicesmake use of a large density of femtosecond laser spots, introducing localized defects, affecting the performanceof the converter. To optimize the writing and the post-processing annealing step of these kind of devices, herewe introduce a novel fluorescence lifetime imaging microscope (FLIM) working with deep UV (240-280 nm)wavelength excitations. Specifically, we demonstrate the potential of this technique and more generally, how itcan be used for characterizing a variety of femtosecond laser induced modifications in fused silica. This UV-FLIMcan be used with micro-fluidic and bio-samples to characterize temporal characteristics of fluorescence.

Keywords: FLIM, deep-UV, silica, defects

1. INTRODUCTION

Femtosecond laser direct writing is a powerful technique for fabricating microfluidic and micro-optical elements bylaser-induced polymerization, laser-induced etching and index enhancement or by form birefringent structuringof glasses.1–6 This techniques find potential applications in optofluidics, optomechanics7–9 and more recently,in nanomedicine where surface-clean Au-nanoparticles can be fabricated in water10 or scaffolds formed for celland tissue growth.11 Surface nano-texturing by fs-laser pulses also find applications in optical/magnetic memorydisks and in various fields of sensing.12–16

Recently, optical devices based on spatially arranged nano-gratings patterns so-called “polarization con-verter”17 have been used for generating different output beam modalities which carry an angular momentumdue to the spin and orbital momentum as well as intensity controlled via form birefringence.18 Some of the veryfirst demonstrations of nanogratings (sometimes referred as “bulk ripples”) were in pure silica glasses which aretransparent down to UV wavelengths (< 200 nm). Such polarization converters have potential use in micro-fluidicdevices processing UV-transparent water solutions. Angular momentum of light can also be used for inducingopto-mechanical coupling effects down to molecular level, exploiting direct absorption of deep UV light. Indeed,most of the proteins absorbs light at wavelengths shorter than 280 nm.19 However UV is usually also exciting astrong fluorescence that can cause hamper imaging and fluorescence lifetime measurements popular in bio- andmedical- research.

Corresponding authors: [email protected] (CJ) and [email protected] (GG). Further author infor-mation: [email protected] (AHAC), [email protected] (YB), [email protected] (SJ).

Micro/Nano Materials, Devices, and Systems, edited by James Friend, H. Hoe Tan, Proc. of SPIE Vol. 8923, 89231E · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2033928

Proc. of SPIE Vol. 8923 89231E-1

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UV LED

270 nm

sample

Objective lens NA = 0.7

UV jig

UV LED

spectrometer

sync.

UV jig

CCD

FLIM

modulator

CCD

Figure 1. Realization of UV-FLIM with UV-LED emitting at 270 nm. Custom designed LED controller (UV-jig) wassynchronized with FLIM electronics. Samples used: cells, water dissolved dyes in micro-fluidic capillary, laser structuredregions inside silica glass. Footprint of the LED (left) was 2× 2 mm2.

Fluorescence Cross-pol. image

λex = 474 nm ls = 4

ls = 1

Figure 2. (from left to right) Fluorescence (false color) image after long-pass filter λl = 560 nm; the excitation wavelengthwas λex = 474 nm. Cross-polarized images under white light illumination revealing the polarization converters of chargels = 4 (upper) and ls = 1 (lower). Arrays of polarization converters 3×3 of different charges and with different orientationswere recorded in a silica glass piece that forms the base of a microfluidic channel; the converters diameter is 600 µm.

Here, we show a proof-of-the-principle of a UV fluorescence lifetime imaging microscope (FLIM) that usesdeep UV LEDs for direct excitation of defects in silica glass. These defects originate from the femtosecond-laser fabrication of the form-birefringent structures that act as polarization converters for the designed 532 nmwavelength. Annealing at 300◦C for 2 h fully removed the fluorescence at 650 nm and longer wavelengths withoutsuppressing the form birefringence required for polarization conversion. Our UV-fluorescence-lifetime-imagingmethod revealed the removal of long-time relaxation components from the fluorescence time decay.

2. SAMPLES AND CHARACTERIZATION

Polarization converters operating at 532 nm were fabricated by femtosecond-laser direct write in silica glass.They convert horizontally polarized light (x-pol.) into the radially and the vertically polarized (y-pol.) beaminto azimuthally polarized one. The polarization converter is a λ/2 plate and can convert a circularly polarizedlight into an optical vortex of different charge ls according to the fabricated pattern.

Figure 1 shows the schematics of our UV-FLIM which was realized on the basis of a commercial FLIM setup(LIFA, Lambert Instruments). The UV-LED-Controller (OptoTech Pty Ltd) tunes the duty cycle of the LEDemission from 1 to 1/10 for repetition rates up to 1 MHz. Thanks to this modality, the maximum current canbe as high as 20 mA for the light emission duration of the cycle. Deep-UV LEDs at 250-290 nm wavelength(SET, Inc.) spectral range has been used in experiments as reported.20 The UV-jig has a portable size of135 × 45 × 25 mm3 (l × w × h) and can be placed directly onto the microscope sample stage for illumination.Leads on standard TO-18 can housing of LED were shortened to reduce the RC-constant and to achieve highLED switching rates.



200 400 600 800 1000

0.00

0.25

0.50

0.75

1.00 LED

LED with FLIM

Converter with FLIM

Em

issio

n (

no

rma

lize

d)

Wavelength (nm)

Microscope block

274 nm LED

Figure 3. (from left to right) Emission spectra from the 270 nm LED (peak at 274 nm), emission of the same LED detectedwith FLIM microscope, and emission detected from the polarization converter in silica(inset shows an image of LED takenwith the FLIM setup) illuminated with the very same LED. The shaded region marks the wavelengths that are blockedby the microscope optics.

Fluorescence Life-time Imaging Microscopy is based on the temporal modulation of the excitation wavelengthand on the synchronized detection of the phase-shifted signal. To characterize the temporal properties of fluores-cence, the frequency-domain approach (FLIM) was utilized, wherein the lifetime is determined from the phase,φ, and modulation, M , of the fluorescence signal. Typical phasor presentation Avs.B is a convenient method toshow the phase and modulation values:

B = M cos(φ), A = M sin(φ). (1)

This method is well suited to measure fluorescence of dye - nanoparticle mixtures in water solutions10 and nowis extended to measure the fluorescence from the defects in solid state materials.

3. RESULTS

3.1 Fluorescence from pol.-converters illuminated by deep-UV LEDs

Figure 2 illustrates an inherent problem of the background fluorescence from the polarization convertor illumi-nated by a 474 nm laser diode light (one of the standard FLIM illumination sources). It is noteworthy, theconverter is fabricated for operation at 532 nm illumination but since the form birefringent nano-gratings struc-tures inside silica are inherently broadband spectral devices a polarization sensitive fluorescence from converterswas observed at the different wavelengths. Here, we investigate the unwanted background emission for the con-verter use in microscopy including FLIM. Since the form-birefringence created by nano-gratings is spectrallybroadband, the fluorescence at wavelengths longer than λl = 560 nm (a long-pass filter) show a pattern of in-tensity distribution in Fig. 2 which has a peculiar reminiscence of a phase pattern typical to an optical vortex.This, we believe, is caused by polarization dependence of the fluorescence excitation and emission due to thepresence of nano-gratings. The form-birefringent nano-gratings are creating the same pattern of the defects insilica. The linearly polarized LED illumination of such nano-gratings creates a fluorescence emission followingan intensity of Ifl ∝ cos θ2, where θ is the angle between polarization and the orientation (wavevector) of thenano-grating.

Figure 3 shows the emission spectra of the 270 nm LED measured in air and through the microscope. Free-airemission of the LED has a peak centered at 274 nm and its second order diffraction is at exactly 548 nm. Whenthe same emission is measured through the FLIM microscope, a broader emission 530± 50 nm is observed in theeye-piece of the microscope and spectrometer. In this case due to filtering of UV component by microscope optics,this emission is related to the defects in LED; the same bright blue appearance of LED is recognizable by nakedas well. When polarization converter is illuminated by the 270 nm LED, by placing it close, a reddish emissionband around 670 nm is recognizable. This band is related to non-bridging oxygen hole center Si-O· emission(NBOHC; · is the dangling bond).21–23 The fluorescence image shown in Fig. 2 has the strongest emission fromNBOHC when illuminated by 270 nm.



500

0.0

0.1

0.2

0.3

0.4

A =

1-

R-T

Silica

Rayleigh

600 700

silica

conv.1 RT

conv.2 RT

conv. 300

1/λ4

Wavelength (nm)

1 mm

800

conv.1 RT

conv.2 RT

conv. 300oC

Figure 4. Absorbance, A, spectra of pol.-converters as fabricated (at room temperature RT) and annealed at 300◦C for2 h; transmission T = 10−OD = e−αd, where d is thickness and α (cm−1) is the absorption coefficient. The spectral rangeis defined by FLIM microscope transmission band. Thickness of glass was d = 1 mm and optical thickness of nano-gratings(pol.-convertor) dc ' 72 µm (dashed-box encloser in the inset). The Rayleigh scattering spectral profile is given by λ−4;R+ T +A = 1 with R accounting for reflection losses.

0 100 200 300

0

10

20

30

Flu

ore

sce

nce

(a

rb.

un

its)

Phase (degrees)

∆φ ∆m

Figure 5. The phase shifted and modulated FLIM signals from the rhodamine (�) and pol.-convertor (©). The phaseshift ∆φ and change of modulation depth ∆m are shown. The best fit to a sin-function was used to extract the lifetimeτ .

3.2 Post-annealing optical characterization of pol.-converters

After annealing of converters from 100 to 300◦C their scattering is slightly decreased and defect-related fluores-cence at 650 nm NBOHC band was fully erased. Figure 4 shows the effect of thermal annealing of pol.-convertorson their losses - scattering and absorption. The optical density (OD) spectrum showed only a slight reductionof the losses within the spectral window of the FLIM microscope. Annealing at 300◦C for 1 h is enough toerase the defect-related emission in silica.21,22 The residual losses are due to scattering in nano-gratings. It isessential to have the gratings unchanged after annealing since they are required for the λ/2-waveplate functionof pol.-converters.

Interference fringes (Fig. 4) are the signature of the nano-gratings which have recognizable (an optical side-view imaging was used) thickness of dc ' 70 µm. One can obtain comparable value from the two neighboringinterference maxima at wavelengths λ1,2 = 526, 576 nm from ∆nd = (1/λ1 − 1/λ2)−1, where ∆n ' 8.4× 10−2 isthe refractive index of silica and d = 72 µm is the apparent thickness of the pattern responsible for interference(thickness of polarization converter). This value is larger than usual modulation of refractive index due to nano-grating which is approximately ∆n ∼ 10−3. Possibly, the actual extension of the polarization converter is larger



0 200 400 600 800

1E-3

0.01

0.1

1

300oC

200oC

100oC

RT

Flu

ore

scence (

arb

. units)

Time, t (ns)

(a)

(b)

0 200 400 600 800

1E-3

0.01

0.1

1

conv. 650 nm

conv. 560 nm

LED 270 nm

rhodamine

Time, t (ns)

Figure 6. (a) Fluorescence transients of pol.-converter ls = 2 annealed at different temperatures. Excitation wavelengthis λex = 474 nm. (b) Fluorescence time decay under λex = 270 nm excitation. Rhodamine 6G showed a τ ' 4 ns decay.Pol.-converter fluorescence was filtered by band-pass (650 ±20 nm) and long-pass (560 nm) filters. The direct emissionfrom LED 270 nm measured by FLIM is also presented.

than can be judged from the obvious breakdown regions from side observation.12,23–25

3.3 FLIM measurements from defects in silica

From the phase φ and modulation M using a phasor plot A = f(B) the fluorescence lifetime τ is determined. Byselecting different modulation frequencies in the range of f = 0.1 − 10 MHz the lifetime as short as ∼ 1 ns canbe retrieved. Figure 5 shows raw experimental data and sin-fit which were used to calculate the lifetime data.

Fluorescence of the defects usually has a complex stretched exponential behavior. From the FLIM data thetime dependence of fluorescence is expressed by multi-exponential expression with a characteristic time τi andcorresponding amplitude ai as:

Fluorescence(t) =

n∑i=1

aiτie− tτi . (2)

We used maximum n = 5 for the best fit of the measured FLIM data. Data are summarized in Fig. 6. Understandard FLIM excitation λex = 474 nm the effect of annealing is clearly seen. We observed a slight increase offluorescence intensity after annealing up to 100◦C and a slight change in the decay profile, however, changes wereminimal and usually within the uncertainty range of measurements. Such behavior can be explained by defectsrelaxation26,27 since fs-laser fabrication is capable to quench meta-stable states of defects with low activationenergy in crystals, glasses and polymers.28–34

Annealing up to 300◦C degrees was sufficient to strongly decrease fluorescence at times longer than 100 nsconsistent with silica defect relaxation time.22 It was demonstrated earlier that such annealing preserves theform birefringence of laser structures regions since the nano-gratings remain intact.35 A temperature of 300◦Cdegrees corresponds to the activation energy of Ea = 0.78 eV.

Under a 270 nm UV LED excitation, the optics of FLIM microscope was acting as a filter (Fig. 3) withvery fast decay times (Fig. 6(b)). Since UV FLIM has been never used before, we tested it on Rhodamine 6Gand obtained results very close to the single exponential decay of τ ' 3.97 ns as for the reference solutionfrom Lambert Instruments. This expected value shows that UV-FLIM can be used with standard dyes. Thefluorescence was filtered in two different spectral bands for which different transients were observed. Since under274 nm excitation wavelength, a wider spectrum of defects is expected to be excited, such filter-gating of emissionbands provides tool to characterize the defects induced by fs-laser fabrication.



4. DISCUSSION

Although the frequency of the FLIM measurements was only set to 0.1 - 10 Mhz, the use of phase modulationcreates a much broader time-resolution (Fig. 6 shows 1 to 800 ns) and reveals much more details in the fluorescencetransients as compared with standard pump-probe measurements which usually cannot cover such a vast timespan. Moreover, UV-FLIM measurements were performed under microscope observation. Focusing with anobjective lens (NA = 0.7) provides a high spatial resolution down to 2-3 µm. It is expected that UV-FLIMcombination with Raman scattering spectroscopy will provide deeper insights into defect and morphologicalmodifications occurring within and around the fs-laser structured volumes.36–41 This is a key for fabricationof micro-optical elements and waveguides where local mass density and defects are critically important for thedesigned function and longevity.42–46 Better understanding of fs-laser structuring is still strongly required as itcan deliver unique new materials.47–49

5. CONCLUSIONS

The first UV-FLIM setup based on a commercial FLIM microscope is demonstrated and extends excitationwavelengths to the shortest 240 nm-wavelength available state-of-the-art UV LEDs. The spectral bandwidth ofthe detection path was defined by the microscope; synchronization of UV-LED with FLIM setup was realizedby a custom built controller, which is small enough to place the illumination directly onto a sample mountedon the microscope stage. By controlling the duty cycle, the dedicated electronics allows pulsing operation ofthe UV-LED with pulse duration down to 20 ns. This setup can be integrated in microfluidics systems withUV-transparent silica tubing.

Potential use of UV-FLIM for defect characterization in fs-laser structured materials is demonstrated. Thistechnique is also promising in bio-medical research since it delivers the capability, for the first time, to useFLIM in frequency domain with light sources which directly excite proteins by absorption (λex < 280 nm).Since UV-LEDs are widely used in water disinfection, a FLIM capability can provide further information on theeffectiveness of the delivered UV dose in incapacitating bacterial, microbial, and viral agents.

ACKNOWLEDGEMENTS

We acknowledge Mircea Perte and Eckhard Wellner from OptoTech Pty Ltd who made electronics for UVLED controller and synchronized microscope setup. SJ is grateful for support via Australian Research CouncilDiscovery DP130101205 and DP120102980 grants. SJ is grateful to Etienne Brasselet for discussions on opticalvortices.

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characterization of optical polarization converters made

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