**3. Femtosecond laser microprocessing of composite right/left handed (CRLH) metamaterials for millimeter wave devices**

#### **3.1 Metamaterial CRLH millimeter wave devices**

Metamaterials (MTM) are propagation media presenting simultaneously both negative permittivities ( < 0) and negative permeabilities ( < 0). These media, with unusual properties not readily available in nature, are called **L**eft **H**anded **M**aterials (LHM). For a LHM, the triade formed by the electric field, magnetic field and Poynting vector of an electromagnetic field propagating through this media has a "left hand" orientation, different from common materials where this orientation is a "right hand" one. As a result, LHM support propagation of an electromagnetic wave where the group velocity is antiparallel to the phase velocity. This phenomenon associates with a negative refraction index.

Although presented as a theoretical curiosity since 1968 (Veselago, 1968), practical applications of "left handed" media appeared 30 years later when the first experimental investigations were made (Pendry, 1999; Shelby et al., 2001). Since that time, a great variety of media with metamaterial characteristics and subsequent applications from microwave to the visible domain were developed.

In the microwave and millimeter wave frequency domain, the main conventional propagation media are the transmission lines (microstrips, coplanar waveguides). The right handed form of these transmission line structures may be assimilated to a large enough number of cascaded cells, each cell being made of *series inductor – parallel capacitor*. For this

Ultrashort Pulsed Lasers – Efficient Tools for Materials Micro-Processing 273

CCs are the series connected interdigital capacitors and LLp is the ground connected inductor. At high frequencies, LCs (parasitic inductance of the interdigital capacitor), CCp (parasitic capacitance to the ground of the interdigital capacitor), and CLp (parasitic capacitance of the inductive stub) become significant circuit elements. The CRLH-TL elementary cell configuration is determined by the working frequency and the materials used. The values of capacitances and inductances forming the CRLH cell (see figure 5b) were obtained with appropriate computer software (Microwave Office, HFSS). These programs also give the geometrical dimensions of the CRLH cell, as they are presented in figure 5a, where: wC, lC, and sC are the width, length and distance between two digits of the interdigital capacitor, respectively; wL and sL are the width and distance to the ground of the inductive CPW stub,

At high frequencies (ten-hundred GHz), the cell geometrical details reach the limits of classical photolithography and the laser ablation becomes a valuable technique to process

This antenna consists of an array of CRLH cells. Each cell is made of two series connected CPW interdigital capacitors and two parallel conected short-ended CPW inductive transmission lines. The layout of an elementary CRLH cell and its equivalent circuit are presented in figure 5. The designed antenna is a zeroth-order resonance device configuration consisting of an open-ended array of CRLH cells. Using CPW transmission lines, the circuit area of the CRLH antenna could

For an open-ended CRLH antenna, the zeroth-order resonance is at the frequency fsh = 1/[2(LLpCLp)1/2] which is the parallel resonance due to the two CPW short-ended inductive transmission lines. Also, there are resonance frequencies corresponding to the right-hand (RH) and the left-hand (LH) CRLH behaviour (Caloz & Itoh, 2006). For the operating frequency of the zeroth-order antenna, fsh , = 0 where is the equivalent phase constant of the CRLH cell, this frequency being the highest one for the LH frequency range. A zeroth-order resonance CPW CRLH antenna working at 28 GHz frequency was designed. It consists of three resonant T – shaped CRLH cells processed on a high resistivity silicon wafer substrate. The conditions and mathematical relations for the CRLH cells design are presented in literature (Caloz & Itoh, 2006; Sajin et al., 2007). For future integration in a more complex circuit, the CRLH cell was designed on silicon substrate, using CPW transmission lines. The substrate was a 500 μm thick silicon wafer (εr,Si = 11.9) of 5 kcm resistivity, covered with 1 μm SiO2 layer (εr,SiO2 = 4.7) grown by thermal oxidation. The Si wafer was plated by a sputtering process with a metallic layer of 3000 Å Au / 500 Å Cr. The backside of the silicon wafer was not metalized. The calculated dimensions of the interdigital capacitors and inductive stubs for these CRLH cells are the following: sC = 5 μm, lC = 250 μm, wC = 10 μm, wL = 42 μm, sL = 10 μm, lL = 212 μm, gC = 65 μm. Each capacitor has 10 digits. The antenna access line has 3400 μm length with a geometry computed to match the

The processing technology applied to obtain the antenna structure consists of two steps by combining two technologies: photolithography and direct laser writing. In the first step, the Au/Cr metallization was removed from the large areas of the structure by standard wet photolithography. Since the laser structures writing by ablation is a time consuming scanning method, the lithography was preferred for large size area processing in order to

be much smaller compared to the microstrip antenna where microstrip lines are used.

respectively; gC is the distance to the ground planes of the CPW line.

**3.2 Zeroth-order resonant CRLH antenna on silicon substrate** 

50 Ω characteristic impedance of the measuring system.

such MMW devices.

type of line, called **R**ight **H**anded **T**ransmission **L**ine (RH-TL), the group velocity and the phase velocity have positive values.

The left handed transmission lines are made by cascading a large enough number of cells, each cell composed of *series capacitor – parallel inductor*. For this type of line, called **L**eft **H**anded **T**ransmission **L**ine (LH-TL), the group velocity has positive values while the phase velocity has, in some conditions, negative values. In practice LH-TLs are made as artificial lines using lumped or distributed circuit elements. Due to the parasitic inductances and capacitances associated with series capacitors and parallel inductors respectively, the line configuration will present, actually, cells of type *series LC – parallel LC*, the result being a composite kind of line named **C**omposite **R**ight / **L**eft **H**anded Transmission Line (CRLH-TL).

CRLH structures act as LH-TLs at frequencies where the guided wavelength is larger than the cell size and as RH-TLs at high frequencies, where the guided wavelength is smaller than the cell size. Between these frequency domains there is a forbidden frequency band where the propagation is cut. These are so called unbalanced CRLH structures. If this forbidden band is reduced to a single point, a balanced CRLH structure is obtained and this is one of the conditions for designing some CRLH devices for microwaves and millimeter waves.

Based on the CRLH structures, a large class of microwave and millimeter wave devices (MMW) has been developed (Caloz & Itoh, 2006). Among such devices, made on the basis of metamaterial approach, various types of directional couplers (Caloz et al., 2004; Wang et al., 2006), filter configurations (Li et al., 2007; Liu et al., 2009), antennas (Sajin et al., 2007; Seongmin et al., 2008; Ziolkowski et al., 2009; Basharin & Balabukha, 2009; Eggermont et al., 2009) were reported. The metamaterial CRLH-TL approach allows substantial space reduction compared to the standard devices. Moreover, combined with other metamaterial devices and circuits, it offers the possibility to develop a new and different kind of microwave and mm-wave circuitry.

Complex monolithically integrated circuits can be fabricated using coplanar waveguide (CPW) configurations on semiconductor substrates. The design of the MTM microwave and millimeter wave devices is based on CRLH-TL elementary cells. A CPW CRLH elementary cell with distributed parameters is usually composed of series interdigital capacitors and parallel inductive stubs to the ground. The schematic layout of an elementary CPW CRLH cell is shown in figure 5a, while the equivalent circuit is given in figure 5b.

Fig. 5. Layout of an elementary CRLH cell (a) and the equivalent circuit (b).

type of line, called **R**ight **H**anded **T**ransmission **L**ine (RH-TL), the group velocity and the

The left handed transmission lines are made by cascading a large enough number of cells, each cell composed of *series capacitor – parallel inductor*. For this type of line, called **L**eft **H**anded **T**ransmission **L**ine (LH-TL), the group velocity has positive values while the phase velocity has, in some conditions, negative values. In practice LH-TLs are made as artificial lines using lumped or distributed circuit elements. Due to the parasitic inductances and capacitances associated with series capacitors and parallel inductors respectively, the line configuration will present, actually, cells of type *series LC – parallel LC*, the result being a composite kind of line

CRLH structures act as LH-TLs at frequencies where the guided wavelength is larger than the cell size and as RH-TLs at high frequencies, where the guided wavelength is smaller than the cell size. Between these frequency domains there is a forbidden frequency band where the propagation is cut. These are so called unbalanced CRLH structures. If this forbidden band is reduced to a single point, a balanced CRLH structure is obtained and this is one of the conditions for designing some CRLH devices for microwaves and millimeter

Based on the CRLH structures, a large class of microwave and millimeter wave devices (MMW) has been developed (Caloz & Itoh, 2006). Among such devices, made on the basis of metamaterial approach, various types of directional couplers (Caloz et al., 2004; Wang et al., 2006), filter configurations (Li et al., 2007; Liu et al., 2009), antennas (Sajin et al., 2007; Seongmin et al., 2008; Ziolkowski et al., 2009; Basharin & Balabukha, 2009; Eggermont et al., 2009) were reported. The metamaterial CRLH-TL approach allows substantial space reduction compared to the standard devices. Moreover, combined with other metamaterial devices and circuits, it offers the possibility to develop a new and different kind of

Complex monolithically integrated circuits can be fabricated using coplanar waveguide (CPW) configurations on semiconductor substrates. The design of the MTM microwave and millimeter wave devices is based on CRLH-TL elementary cells. A CPW CRLH elementary cell with distributed parameters is usually composed of series interdigital capacitors and parallel inductive stubs to the ground. The schematic layout of an elementary CPW CRLH

(a) (b)

cell is shown in figure 5a, while the equivalent circuit is given in figure 5b.

Fig. 5. Layout of an elementary CRLH cell (a) and the equivalent circuit (b).

named **C**omposite **R**ight / **L**eft **H**anded Transmission Line (CRLH-TL).

phase velocity have positive values.

microwave and mm-wave circuitry.

waves.

CCs are the series connected interdigital capacitors and LLp is the ground connected inductor. At high frequencies, LCs (parasitic inductance of the interdigital capacitor), CCp (parasitic capacitance to the ground of the interdigital capacitor), and CLp (parasitic capacitance of the inductive stub) become significant circuit elements. The CRLH-TL elementary cell configuration is determined by the working frequency and the materials used. The values of capacitances and inductances forming the CRLH cell (see figure 5b) were obtained with appropriate computer software (Microwave Office, HFSS). These programs also give the geometrical dimensions of the CRLH cell, as they are presented in figure 5a, where: wC, lC, and sC are the width, length and distance between two digits of the interdigital capacitor, respectively; wL and sL are the width and distance to the ground of the inductive CPW stub, respectively; gC is the distance to the ground planes of the CPW line.

At high frequencies (ten-hundred GHz), the cell geometrical details reach the limits of classical photolithography and the laser ablation becomes a valuable technique to process such MMW devices.

#### **3.2 Zeroth-order resonant CRLH antenna on silicon substrate**

This antenna consists of an array of CRLH cells. Each cell is made of two series connected CPW interdigital capacitors and two parallel conected short-ended CPW inductive transmission lines. The layout of an elementary CRLH cell and its equivalent circuit are presented in figure 5. The designed antenna is a zeroth-order resonance device configuration consisting of an open-ended array of CRLH cells. Using CPW transmission lines, the circuit area of the CRLH antenna could be much smaller compared to the microstrip antenna where microstrip lines are used.

For an open-ended CRLH antenna, the zeroth-order resonance is at the frequency fsh = 1/[2(LLpCLp)1/2] which is the parallel resonance due to the two CPW short-ended inductive transmission lines. Also, there are resonance frequencies corresponding to the right-hand (RH) and the left-hand (LH) CRLH behaviour (Caloz & Itoh, 2006). For the operating frequency of the zeroth-order antenna, fsh , = 0 where is the equivalent phase constant of the CRLH cell, this frequency being the highest one for the LH frequency range.

A zeroth-order resonance CPW CRLH antenna working at 28 GHz frequency was designed. It consists of three resonant T – shaped CRLH cells processed on a high resistivity silicon wafer substrate. The conditions and mathematical relations for the CRLH cells design are presented in literature (Caloz & Itoh, 2006; Sajin et al., 2007). For future integration in a more complex circuit, the CRLH cell was designed on silicon substrate, using CPW transmission lines. The substrate was a 500 μm thick silicon wafer (εr,Si = 11.9) of 5 kcm resistivity, covered with 1 μm SiO2 layer (εr,SiO2 = 4.7) grown by thermal oxidation. The Si wafer was plated by a sputtering process with a metallic layer of 3000 Å Au / 500 Å Cr. The backside of the silicon wafer was not metalized. The calculated dimensions of the interdigital capacitors and inductive stubs for these CRLH cells are the following: sC = 5 μm, lC = 250 μm, wC = 10 μm, wL = 42 μm, sL = 10 μm, lL = 212 μm, gC = 65 μm. Each capacitor has 10 digits. The antenna access line has 3400 μm length with a geometry computed to match the 50 Ω characteristic impedance of the measuring system.

The processing technology applied to obtain the antenna structure consists of two steps by combining two technologies: photolithography and direct laser writing. In the first step, the Au/Cr metallization was removed from the large areas of the structure by standard wet photolithography. Since the laser structures writing by ablation is a time consuming scanning method, the lithography was preferred for large size area processing in order to

Ultrashort Pulsed Lasers – Efficient Tools for Materials Micro-Processing 275

In order to measure the radiation characteristic, antenna chips were mounted on specialized test fixtures. The radiation characteristic was measured at 28.7 GHz frequency where the antenna return losses have a minimum. The measuring setup allows the rotation of the

**11\_1**

25 30 35 Frequency (GHz)

The radiation characteristic rated to the maximum power in the transversal plane of the CRLH antenna structure is shown in figure 8a, while the characteristic in the longitudinal plane is presented in figure 8b. The -3 dB beam-width of the radiation characteristic is about 37 degrees in the transversal plane and 25 degrees in the longitudinal plane. The

(a) (b) Fig. 8. Radiation characteristic in the transversal plane (a) and in the longitudinal plane (b)

Fig. 7. Return loss of the CRLH antenna for a frequency sweep between 25-35 GHz

longitudinal radiation characteristic has the maximum at the angle of ~ 24 degrees.

28.78 GHz -27.05 dB

29.65 GHz -15 dB

DB(|S(1,1)|)

11

CRLH emitting antenna both in transversal and longitudinal planes.

28.01 GHz -15 dB


for the measured CRLH antenna




S11 (dB)



0

obtain the main layout of the future device. Also, the removal of the metallic film by laser ablation from large area could result in a high quantity of ablated material re-deposited on the sample surface, contaminating the active area of the device. In the second step, for the fine details of the interdigital capacitor, the precise laser ablation was used.

The samples were laser ablated by tightly focusing the femtosecond laser pulses with 200 fs duration, 775 nm wavelength, tens of nJ pulse energy, and 2-kHz repetition rate. A 20x microscope objective, 0.4 NA, was used for the Au/Cr layer ablation, obtaining interdigital spaces of 5 m, as designed. The 2D structures were generated according to a computer controlled algorithm by precisely translating the sample with a resolution below 1 µm.

An optical microscopy image of the active part of the antenna structure is presented in figure 6a, where the grounded lines forming the inductive stubs and the interdigital capacitors, processed by laser ablation, can be observed. A Scanning Electron Microscopy (SEM) image showing details of the antenna interdigital capacitors is presented in figure 6b.

Fig. 6. (a) Optical microscopy image of the active part of the CRLH antenna. (b) SEM image showing a detail of the interdigital capacitor made by femtosecond laser processing.

Many electrical properties such as gain, return loss, voltage standing wave ratio, reflection coefficient, and amplifier stability of microwave and mm-wave networks may be expressed using S-parameters (Kurokawa, 1965). In the context of S-parameters, scattering refers to which way the traveling currents and voltages in a transmission line are affected when a discontinuity is inserted. These losses appear when the incident wave meets an impedance which differs from the line's characteristic impedance.

The input return loss (RL) parameter of the CRLH antenna is presented in figure 7. It represents a scalar measure of how close the actual input impedance of the device is to the nominal system impedance value and, expressed in decibels, is given by RLin (dB) = |20×log10|S11|| where, by definition, |S11|, is equivalent to the reflected voltage magnitude divided by the incident voltage magnitude.

For a frequency domain between 28.01 GHz and 29.66 GHz, an input return loss having values RL < -15 dB can be observed. The minimum value of RL = -27.05 dB was obtained at 28.78 GHz. It represents a very good matching of the antenna structure to the 50 characteristic impedance of the millimeter wave measurement system and to the most usual microwave and millimeter devices.

obtain the main layout of the future device. Also, the removal of the metallic film by laser ablation from large area could result in a high quantity of ablated material re-deposited on the sample surface, contaminating the active area of the device. In the second step, for the

The samples were laser ablated by tightly focusing the femtosecond laser pulses with 200 fs duration, 775 nm wavelength, tens of nJ pulse energy, and 2-kHz repetition rate. A 20x microscope objective, 0.4 NA, was used for the Au/Cr layer ablation, obtaining interdigital spaces of 5 m, as designed. The 2D structures were generated according to a computer controlled algorithm by precisely translating the sample with a resolution

An optical microscopy image of the active part of the antenna structure is presented in figure 6a, where the grounded lines forming the inductive stubs and the interdigital capacitors, processed by laser ablation, can be observed. A Scanning Electron Microscopy (SEM) image showing details of the antenna interdigital capacitors is presented in figure

Fig. 6. (a) Optical microscopy image of the active part of the CRLH antenna. (b) SEM image showing a detail of the interdigital capacitor made by femtosecond laser processing.

Many electrical properties such as gain, return loss, voltage standing wave ratio, reflection coefficient, and amplifier stability of microwave and mm-wave networks may be expressed using S-parameters (Kurokawa, 1965). In the context of S-parameters, scattering refers to which way the traveling currents and voltages in a transmission line are affected when a discontinuity is inserted. These losses appear when the incident wave meets an impedance

The input return loss (RL) parameter of the CRLH antenna is presented in figure 7. It represents a scalar measure of how close the actual input impedance of the device is to the nominal system impedance value and, expressed in decibels, is given by RLin (dB) = |20×log10|S11|| where, by definition, |S11|, is equivalent to the reflected

For a frequency domain between 28.01 GHz and 29.66 GHz, an input return loss having values RL < -15 dB can be observed. The minimum value of RL = -27.05 dB was obtained at 28.78 GHz. It represents a very good matching of the antenna structure to the 50 characteristic impedance of the millimeter wave measurement system and to the most usual

which differs from the line's characteristic impedance.

microwave and millimeter devices.

voltage magnitude divided by the incident voltage magnitude.

(a) (b)

fine details of the interdigital capacitor, the precise laser ablation was used.

below 1 µm.

6b.

In order to measure the radiation characteristic, antenna chips were mounted on specialized test fixtures. The radiation characteristic was measured at 28.7 GHz frequency where the antenna return losses have a minimum. The measuring setup allows the rotation of the CRLH emitting antenna both in transversal and longitudinal planes.

Fig. 7. Return loss of the CRLH antenna for a frequency sweep between 25-35 GHz

The radiation characteristic rated to the maximum power in the transversal plane of the CRLH antenna structure is shown in figure 8a, while the characteristic in the longitudinal plane is presented in figure 8b. The -3 dB beam-width of the radiation characteristic is about 37 degrees in the transversal plane and 25 degrees in the longitudinal plane. The longitudinal radiation characteristic has the maximum at the angle of ~ 24 degrees.

Fig. 8. Radiation characteristic in the transversal plane (a) and in the longitudinal plane (b) for the measured CRLH antenna

Ultrashort Pulsed Lasers – Efficient Tools for Materials Micro-Processing 277

is limited to a minimal resolvable feature size given by the diffraction limit. Optical nearfields were explored for their ability to localize optical energy to length scales smaller than half-wavelength. This localization was achieved for ultrasensitive detection (Fischer, 1986) and for high-resolution optical microscopy and spectroscopy (Novotny & Stranick, 2006). Near-field optics research essentially determined the advance of nano-optics (Novotny & Hecht, 2006), single molecule spectroscopy (Xie & Trautman, 1998), and nanoplasmonics (Barnes et al., 2003). Micron and sub-micron patterning was performed by "direct writing" where the laser light is just projected onto a sample via a direct-contact mask or by the interference of laser beams. Another technique is based on scanning near field optical microscopy (SNOM). Here, the light is coupled into the tip of a solid or hollow fiber. By positioning the substrate within the near field of the fiber tip, one can produce patterns with widths that are beyond the diffraction limit. Structures with lateral dimension less than 30 nm, well below the radiation wavelength, could be produced underneath the tip (Gorbunov & Pompe, 1994). This technique has been employed for nanolithography, ablation, material

Spherical particles can act as spherical lenses and therefore increase the laser intensity if their diameter is bigger than the laser wavelength. Laser-induced submicron patterning of surfaces has been demonstrated by means of two-dimensional colloidal lattices of microspheres that are formed by self-assembly (Piglmayer et al., 2002). If the diameter of spherical particles is of the order of magnitude of the radiation wavelength, according with the Mie solution to Maxwell's equations (Mie, 1908), the optical field enhancement occurs quite near laser irradiated particles. Electric field intensity distributions were calculated with the finite-difference time-domain (FDTD) method using simulation software (RSoft Design Group). For colloidal sub-wavelength particles (700 nm diameter of the particle, 532 nm wavelength radiation) placed on a glass substrate, the electric field can be greatly enhanced (by a factor of ~ 8) in the near-surface field region under particles as shown in the simulation from figure 10a (Ulmeanu et al., 2009a). The enhancement value quoted above is a theoretical estimation and the actual field strength enhancement may differ due to various

influencing factors, such as surface roughness and oxidation of the thin film layer.

Fig. 10. (a) Field intensity enhancement of a 700 nm colloidal particle sitting on a glass substrate in a free space for = 532 nm (b) Experimental setup (BS – beam splitter, AT – attenuator, L – convergent lens, DET – pyroelectric detector, M – high reflectivity mirror.

We demonstrated surface patterning in the enhanced near-field by scanning a quasi-Gaussian laser beam through a self-assembled monolayer of colloidal particles onto different substrates:

etching and local reduction of oxides.

The antenna gain, which relates the intensity of an antenna radiation in a given direction to the intensity that could be produced by a hypothetical ideal antenna that radiates isotropically (equally in all directions), was computed using the De Friis formula (Balanis, 1997). The gain was nearly constant in the 28 ÷ 29 GHz frequency band, with a maximum value of 2.99 dB at 28.6 GHz. The area occupied by the antenna is 2.15 0.6 mm2. There is a considerable size reduction compared to a standard λ/2 patch antenna.

### **3.3 Millimeter wave CRLH band-pass filter and directional coupler**

The band-pass filter was made by cascading a number of identical CRLH cells. The directional coupler consists of two coupled CRLH artificial lines, each composed of two identical cascaded CRLH cells. Both band-pass filter and directional coupler (figure 9a,b) were microprocessed by the two-steps technology described in section 3.2.

The geometrical dimensions of the interdigital capacitor and inductive CPW stub of the CRLH band-pass filter at 50 GHz working frequency were calculated: sC = 5 μm, lC = 250 μm, wC = 10 μm, gC = 65 μm, the number of capacitor digits = 10, wL = 42 μm, sL = 10 μm, and lL = 212 μm (see figure 5a). Unlike the antenna CRLH cell, the filter CRLH cell has only one inductive CPW stub. A silicon substrate plated with 2000 Å Au/500 Å Cr was used.

The working parameters of the band-pass filter structure were measured in the 40-60 GHz frequency range. The S11 parameter evaluation show return loss values < -15 dB in a frequency range of 46.32 GHz ÷ 55.4 GHz, whereas losses in the same frequency range, given by S21 parameter, are around 6 ÷ 7 dB (Sajin et al., 2010a).

Fig. 9. SEM images of CRLH band pass filter (a) and directional coupler (b) structures microprocessed by laser ablation.

For the CRLH directional coupler the port 1 is the input port, while the ports 2, 3 and 4 are the transmission, coupled, and isolated ports, respectively (figure 9b). Measured return losses (S11) were better than -20 dB for a frequency domain between 24.01 GHz ÷ 38.11 GHz, whereas the isolation (S41) is greater than 30 dB in a large frequency range, exceeding the domain 20-40 GHz (Sajin et al., 2010b).
