Femtosecond laser fabrication of metamaterials for high frequency devices

Marian Zamfirescu*a), Razvan Dabu*, Marius Dumitru*, George Sajin **, Florea Craciunoiu **

*NILPRP – Bucharest, Atomistilor 409, 077125 Magurele, Romania**IMT Bucharest, Str.Erou Iancu Nicolae 126A, 077190 Bucharest, Romaniaa) marian.zamfirescu@inflpr.ro 

Direct Laser Writing (DLW) is used for configuration of micro structures. Films of few hundreds nm of gold deposited on glass or silicon are precisely processed by tightly focusing a femtosecond laser with 150fs pulse duration, 775nm wavelength, and energy of tens of nJ. The laser fluence is hold just above the ablation threshold. Thus, the evaporated film leaves behind structures with thin lateral size of about 1µm. Using this technique the fabrication of micro-structures is demonstrated. The periodic structures are designed to behave as a left handed metamaterial in the GHz range. The fabricated metamaterials have applications for antenna, filters, couplers, etc. in the high frequency range.

Film of 100 nm Au on Si substrate(Threshold : about 50 nJ at 150 fsec)

1 µm/sec 1000 µm/sec

For a given material the quality and the resolution of the written structure depends strongly on few parameters such as:- writing speed;- focusing;- laser energy.

Pulse energy = 110 nJFocused

z = 0Unfocusedz – 1 µm

Unfocusedz + 1 µm

10 µm

lc = 50 µmls = 100 µmwidth of digits = 5 µmN = 5 pairs of fingers

Composite Right/Left-Handed Metamaterials (CRLH - MTM)

Threshold Fluency

The lateral size of structures are obtained by adjusting the laser intensity above the threshold of ablation.

Femtosecond ablation of metallic films bellow diffraction limit

DLW Setup Determination of the best parameters for DLW

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'R

1

CLse

'R

'L

1

CLsh

The CRLH-TL structure is constituted of series interdigital capacitors and inductors fabricated on metallic film.

Equivalent circuit model in a LC network

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2'R

1)(

CL

'L

2'R

1)(

LC

p

Y’

Z’

L’R p C’L/ p

C’RpL’L/ p

Conclusions

Using the actual setup, structures with resolutions below 1 µm were obtined.

Metamaterial structures can be easily fabricated on metal films by femtosecond lasers.

The resolution as small as few hundreds nm can be reached by methods based on radially polarised laser, second harmonics, or/and shorter pulse duration.

Depending on the its lateral size, the fabricated structures can behave as metamaterials in the microwave spectral region (tens to hundreds GHz).

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'L'

R

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CL

LCSnn

Negative refractive index (NRI)

2D CRLH structures

30µm

lc = 30 µmls = 30 µm

width of digits = 3 µm

Transmission Line (TL)

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shseS

,minif1

,minif1

References

LASER775nm200 fsec2KHz

Translations XYZ

MicroscopeObjective(NA>0.5)

Attenuation

DichroicMirror

/2Glan

Polarizer

PC

Spatial Filter

HeNe laser

100X

PMT

5X

AD converter

Self focusing system

Laser control

unit

GasEject

CCD

AFM measurements

The AFM measurements show the minimum lateral size of the created structure of about 650 nm, and the ablation depth of about 260 nm.

Equivalent circuit model of 2D - CRLH

- laser energy: 50 nJ- writing speed 0.1 mm/s.

1D CRLH structures

50µm

Network of interdigital capacitors and stub inductances are created on Gold thin films by tightly focusing the femtosecond laser. The desired design is obtained by PC controlled displacement of the sample with submicron precision.

Series and shunt resonance frequencies :

Permittivity and permeability of CRLH material:

[1] V.G. Veselago, Soviet Phys. Usp. 10, 509 (1968).[2] J. B. Pendry et al, J. Phys. Condens. Matter 10, 4785 (1998). [3] C. Caloz, T. Itoh. Electromagnetic metamaterials: transmission line

theory and microwave applications. John Wiley & Sons, 2006.

2 4 6 8 10 12 14

0

r < 0

r < 0

r ,

r

Frequency (GHz)

se

sh

2 4 6 8 10 12 14 16 18 20

0

Re(n)<0

Re(n)>0

se

Frequency (GHz)

refr

activ

inde

x

sh

Im(n)

LR = 0.25 nH

CR = 2 pF

LL = 0.5 nH

CL = 1 pF

n<0 up to 5 GHz

p = 12 mm

From scaling proprieties: for p=70 µm n<0 at tens of GHz

( p << )