- Email: [email protected]
Ion implanted optical waveguides in KNbO3 for efficient blue light second harmonic generation
Nuclear Instrumenta and Methods in Physics Research B80/81 (19"3) 115!1-1153 North-Holland
1111-Interactions with Msterïals a, Atoms
Ion implanted optical waveguides in KNbO,, for efficient biue light second harmonic generation M . F[euster
Ch . Buchal ", D. Fluck ' and P. Günter "
a, htattt1a ffir Schrciu- ued lonenwcluuh, KFA Jülich, D--5170lühch, Ger»tany d Insthute of Qunutum E1ectronies, S, -- Federal Itistntute of 7eehuology, ETH-Hiinggerberg, CH-8093 Zürich, Swtt .erhoid
Single crystals of KNbO, h :, .u ua- " .",t puleaiiai for nonlinear optical applications. especially for the conversion of near infrared to hlue : : °- light . V:.- _Ye useu :t . ions of 2 to 3 MeV to fabricate low loos permanent waveguides m KNbO,. We present results on thi- % _ . ,.,iuc ,I- = za.ior, end derronsuatr efficient guided wave second hart inic generation "if blcr at 43,+ uu, . a planar waveguide as well as Cherenkov-type frequency doubling in a channel waveguide at 111 ) nm .
1 . Introduction The generation of coherent blue light by a solid state device is of considerable technological interest u, optical data storage, xerography, and spectroscopy. At present, KNb0, is one of the best materials for optical frequency conversion into the blue and green spectral range [1-31, thanks to its combination of noncritical phase-matchability, its transparency down to 390 nm, the large nonlinear optical coefficients (e .g. d,, = 20 .3 pin/V at A=860 nm),and its very high threshold to optical damage. Second harmonic generation (SHG) in optical waveguides is a promising approach, because the tie?! b,am confinement over iong interaction lengths enables high cow~ersion efficiencies even at the power 'evui, of presently avaiiabie laser diodes . Conventional methods for the fabrication of waveguides (as ion indifussion or ion exchange, successful for LiN60,) arc not applicable to KNbO, because of its structural phase transition at 217°C which prohibits high temperature treatment . So far, ion implantation is the only method to produce permanent optical waveguides in KNbO, [4,51. In this article we report the fabrication of low loss planar and channel waveguides by a low-dose He implantation process, the optical characterization of these waveguides and their performance for SHG .
2 . Waveguide fabrication He ion implantation with energies of typically 1 te, 10 MeV has been shown to produce optical waveguides for a large group of insulating crystals [6,71 . The colli0168-S<3X/93/$06 .00 0 1993 -
lion damage induced by the implanted ions causes a reduction in d- ity of these crystalline materials accomoanied by a decrease m the refractive index so tûat the profile of the refractive index is directly related to the depth profile of the nuclear deposited energy. Due to the fact that light He ions at MeV energies undergo most of the collisions at the end of the track ("end-of-range damage") the implantation generates a low-index bairicr in a depth of a few microns which separates the waveguiding region from the substrate . The exact shape of the index profile depends on the ion enerjKv, the ?on dose, and the implantation temperature . In KNb0, single crystals were cut perpendicular to the crystallographic b- and c-axis and were carefully polished . Planar waveguides (fig. la) were formed by irradiating the samples at RT with He ions (2-3 MeV, a)l'-lt .'l'' cm -- ') . 'The incidence of the He ions was slightly off normal to avoid channeling and the beam was scanned to ensure homogeneous implantation . For the formation of channel waveguides we used a mask made of tungsten wires (13 It m) or SiO, fibers (8 Win) 'o completely shield stripes of the planar waveguide from further He ion bombardment and hence from further refractive index modification . This is a fast approach which does not need time consuming lithog., ecially difficult for the thick masks de" raphy, c manded by MeV He implantation. The side barriers of the channel waveguides were produced by subsequent implantation of ions with lower energies (fig . lb). These additional implantation steps also decrease the refractive index, so that the shielded region is surrounded by a region of reduced index and the light is confined in two dimensions .
Elsevier Science Publishers B .V . Ali rights reserved
M. Fleaslcr et al. / lcur unplanted optical 1saregurdes ar KiVI Ot
1151 DEPTH (pin)
1
REGION OF REDUCED INDEX FORMED BY PLANAR IMPLANTATION
x ô
PLANAR WAVE~UIDE
2
3
4 nc
0
W Û Q _1
~
C n6
w ~ O w r,
z
U
b
. p 1I
.L 0
o 20
10
'
n 30
"
n 40
50
60
SQUARE OF MODE NUMBER tin + 1) t
Fig. 2. Elfective refractive mode indices of a 2 MeV planar waveguide measured at 632 nm ver- the mode number (0, ". lovxr quadratic scale) together with the reconstructed index eepth profile (---, --, upper linear scale).
(bl
STRIP WAVEGUIDES
Fig. 1 . (a) Formation of a planar waveguiding layer by single energy He ion implantation . (b) Creation of optical channel w-guides by additional implantations at lower dapths with appropriate masking . 3. Waveguide characterization The optical characterization of the planar ,uides was performed by dark line spectroscopy [8]. All observable modes were used to reconstruct a reasonable refractive index profile [9]. (The basic idea of this method is to model the dark mode positions of a parameterized analytical function and then to use a least square optimization routine to secure the best-fit profile .) Fig. 2 shows the measured effective mode indices together with the reconstructed index depth profiles of a planar waveguide formed by the implantation of fie ions with -in -n, rgy of 2 .0 McV and a dose of 7 .5 x 10 14 cm -2 . A depth profile of the nuclear energy loss was calculated using the TRIM'89 Monte Carlo code 1101 and waà found to coincide with the refractive index profile [11]. To get a better understanding of the influence of the ion dose on the shape of the index profile we fabricated a series of planar waveguides with 2 MeV and increasing dose and reconstructed the profiles . In fig. 3 the refractive index in the barrier is plotted as c function of dose . All three indices decrease with increasing dose and merge into a common value of 2 .119 at a dose of 10 1 " cm -2 which can be explained by the formation of an isotropic layer, indicating the full amorphization of the crystal in the barrier . This view is
supported by RBS/channeling measurements (fig . 3) which show that the relative Iatice damage is close to 100% at 10 1 " cm -2 [11] . The search for the optimum dose - high doses give good beam confinement, hot result in increasing absorption and scattering losses led to a dose of 7.5 x 10 1' cm -=. The optical waveguidcs exhibit waveguiding without subsequent annealing and show propagation losses of typically 1-3 dB/cm at 860 nm. Up to now tie chatinets confine modes with the polarization par-tllel to the IONIC ENFRGY DENSITY (OW-3 )
aî W
n z
0 W
S
s
DOSE (em,3)
Fig . 3 . Refractive indices n W, n" (0), it,. ( " ) in the harrier and the lattice danidge (0) as a function of dose for 2 MeV He implantation into KNb0 1 [11]. 1Vb . INSULATOR MODIFICATION (b)
1152
M. Ffe(rsfer er a1 / 1,,,,
Fig . 4. Configuration for nonrnlcal phase-matched frequency-doubling m a KNhO 3 planar waveguide (inset) and the generated SH peak power at 434 nm as a function of the fundamental peak power together with the theoretical prediction (--) [131 .
b-axis only [121 . We suspect that the small index chatigc of n,. (see fig. 3) is insufficient for mode confinement in a channel waveguide . 4. Second harmonie blue light generation For efficient SHG it is necessary that the fundamental (w) and the harmonic mode (2w) propagate with the same phase velocity to ensure constructive interference of the, blue light generated at differem positions in the waveguide . This leads to the phasematching condition :
n(w) =n(2w).
u-
,, KNbO,
of the wavcguidc [13]. The experimental result~ are in good agreement with the theoretical prediction . A maximum conversion efficiency of 299 was achieved for a peak power of 1 .3 kW in the wavcguidc, giving a blue fight output of 385 W. In the low power limit whem the .î:I ;tower depends on the square of the fundamental power, we measured 0 .38 mW blue output for i87 m'v"v infrared input, giving a normalized efficiency of 1 .1%/W. As already mentioned, modes with the polarization parallel to the c -axis are not guided in the channel wavcguidcs . This prohibits guided wave SHG, but permits a very interesting approach, the Cherenkov-type SHG. In this configuration (sec inset of fig . 5) the fundam( ntal guided mode is wriverîcd ini -"_, blue radiation which is emitted into the substrate at an angle which is determined by the phase-matching condition . Only at this so-called Cherenkov angle all blue radiation interferes constructively . The great advantage of this geometry lies in the fact that Cherenkov SHG is free from the stringent phase-matching condition of guided wave SHG, because small changes of the temperature or the fundamental wavelength will be compensated automatically by a very small change in the Cherenkov angle . In our waveguide the Cherenkov angle is typically 2-3° . The efficiency of the conversion is shown in fig. 5 . Over the entire range the SH power is proportional to the square of the fundamental power giving a normalized efficiency of 12%/W . With 97 mW of fundamental power in the wavcguidc we get 1 .1 mW of blue light at 430 nm [141. A conical lens which allows to focus the blue light again to a diffraction limited spot has recently been proposed [151.
:CZ ,, wrlh CW AIG .A. diode laser with CW titanium sapphire laser
An overview on the phase-matching (PM) configuratic,is for KNbO, has recently been published [3]. In our case the PM wav.-Icrsût!i is determined by Nb(Ar,,)-''C(Apm/!), where Nf, and Nr arc the effective indices uï the fundamental and second harmonic (SH) modes that are polarized along the crystallographic b- and c-axis, respectively [13]. In fig. 4 the SH power generated in a planar waveguide is shown as a function of the fundamental power coupled into the waveguide using .l pulsed Ti : AI ZO, laser at A = 868 nm. Also shown in the theoretical conversion efficiency, calculated by taking into account the waveguide losses and the linewidth of the laser source which is wider than the acceptance bandwidth
104
Fig. 5 . Configuration for Cherenkov-type SHG in a MO, channel waveguide (inset) and the generated 511 power at 43(l nm against the fundamental power coupled into the t".,aveguide 114).
M. Fleuver rt v"/. / lon
entp/mtt,l optical
5. Conclusion We have fabricated permanent low loss planar and channel waveguides in KNbO, by a low dose He ion implat ation process . We have used these waveguides for frequency doubling of near infrared light into the blue spectra range. In a planar waveguide noncritical phase-matched second harmonic generation of blue light ::.,s performed with a normali --ed c, mv-sion effi-
ciency of 1 .1%-/W. By using the Cherenkov-type SHG in a channel waveguide more than 1 mW of blue light at 430 nm was produced for a conversion efficiency of 12",,, / W. Acknowledgement We gratefully acknowledge the support from the Tandetron accelerator facility and staff at IFF of KFA Mich . References
guides in KNbO,
1153
[3] 1 . B,aggto, P. Kerkoc, L.-S . Wu, B. Zysset and P. Ciimter, J. Opt. Soc. Am . 139 (1992) 507. [41 T. Bremer, W. Heiland, B. Hellermann, P. Hertel, E. Kriitzig and D. Kollewe, Ferroelectron. Len. 9 (1988) 11. [5] L. 'hang. P.J. Chandler and P.D . Townsend, Ferroelectrott. Lett 1 t (1990) 89 . [6] P.G . Townsend, Nucl . Instr . and Meth . B46 (1990) 18. [7] P.D . Townsend, Nucl . Instr . and Meth . B65 (1992) 243 . [8] F.P. Strohkendl, P. Giinter, Ch. Buchal and R. Innseher, J. Appl. Phys. 69 (1991) 84 . [91 P.J. Chandler and F.L. Lama, Opt. Acta 33 (1986) 127. [10] 1. Biersack as-J L. `l-ggmark, Nucl . Instr. and Meth . :74 (1980) 257. [1l] D. Fluck, R. Irmscher,Ch. Buchal and P. Günter, Ferroelec!6ce 128 (1992) 79 . [l2] D. FiuA P. Günter. M. Fleuster and Ch . Buchal, J. Appl . Phys. 72 (1992) 1571 . [13] D. Fluck, B Binder, M. Küpfer, H. Looser, Ch. Buchal and P . Günter, Opt . Commun. 90 (1992) 304. [ 14] D. Fluck, J. Moll, P. Günter, M. Fleuster and Ch . Buchal, Electron. Leu. 28 (1992) 109:. [151 K. Tatsuno, If. Yamagisawa,T. Amlou andM. McLoughlin, Appl. Opt. 31 (1992) 305.
[I] P. Giinter, Appl . Phys . Lett . 34 (1979) 650. [21 J.C. Baumert, P. Günter and H. Melchior, Opt. Commun. 48 (1983) 215.
IVb. INSULATOR MODIFICATION (b)
1111-Interactions with Msterïals a, Atoms
Ion implanted optical waveguides in KNbO,, for efficient biue light second harmonic generation M . F[euster
Ch . Buchal ", D. Fluck ' and P. Günter "
a, htattt1a ffir Schrciu- ued lonenwcluuh, KFA Jülich, D--5170lühch, Ger»tany d Insthute of Qunutum E1ectronies, S, -- Federal Itistntute of 7eehuology, ETH-Hiinggerberg, CH-8093 Zürich, Swtt .erhoid
Single crystals of KNbO, h :, .u ua- " .",t puleaiiai for nonlinear optical applications. especially for the conversion of near infrared to hlue : : °- light . V:.- _Ye useu :t . ions of 2 to 3 MeV to fabricate low loos permanent waveguides m KNbO,. We present results on thi- % _ . ,.,iuc ,I- = za.ior, end derronsuatr efficient guided wave second hart inic generation "if blcr at 43,+ uu, . a planar waveguide as well as Cherenkov-type frequency doubling in a channel waveguide at 111 ) nm .
1 . Introduction The generation of coherent blue light by a solid state device is of considerable technological interest u, optical data storage, xerography, and spectroscopy. At present, KNb0, is one of the best materials for optical frequency conversion into the blue and green spectral range [1-31, thanks to its combination of noncritical phase-matchability, its transparency down to 390 nm, the large nonlinear optical coefficients (e .g. d,, = 20 .3 pin/V at A=860 nm),and its very high threshold to optical damage. Second harmonic generation (SHG) in optical waveguides is a promising approach, because the tie?! b,am confinement over iong interaction lengths enables high cow~ersion efficiencies even at the power 'evui, of presently avaiiabie laser diodes . Conventional methods for the fabrication of waveguides (as ion indifussion or ion exchange, successful for LiN60,) arc not applicable to KNbO, because of its structural phase transition at 217°C which prohibits high temperature treatment . So far, ion implantation is the only method to produce permanent optical waveguides in KNbO, [4,51. In this article we report the fabrication of low loss planar and channel waveguides by a low-dose He implantation process, the optical characterization of these waveguides and their performance for SHG .
2 . Waveguide fabrication He ion implantation with energies of typically 1 te, 10 MeV has been shown to produce optical waveguides for a large group of insulating crystals [6,71 . The colli0168-S<3X/93/$06 .00 0 1993 -
lion damage induced by the implanted ions causes a reduction in d- ity of these crystalline materials accomoanied by a decrease m the refractive index so tûat the profile of the refractive index is directly related to the depth profile of the nuclear deposited energy. Due to the fact that light He ions at MeV energies undergo most of the collisions at the end of the track ("end-of-range damage") the implantation generates a low-index bairicr in a depth of a few microns which separates the waveguiding region from the substrate . The exact shape of the index profile depends on the ion enerjKv, the ?on dose, and the implantation temperature . In KNb0, single crystals were cut perpendicular to the crystallographic b- and c-axis and were carefully polished . Planar waveguides (fig. la) were formed by irradiating the samples at RT with He ions (2-3 MeV, a)l'-lt .'l'' cm -- ') . 'The incidence of the He ions was slightly off normal to avoid channeling and the beam was scanned to ensure homogeneous implantation . For the formation of channel waveguides we used a mask made of tungsten wires (13 It m) or SiO, fibers (8 Win) 'o completely shield stripes of the planar waveguide from further He ion bombardment and hence from further refractive index modification . This is a fast approach which does not need time consuming lithog., ecially difficult for the thick masks de" raphy, c manded by MeV He implantation. The side barriers of the channel waveguides were produced by subsequent implantation of ions with lower energies (fig . lb). These additional implantation steps also decrease the refractive index, so that the shielded region is surrounded by a region of reduced index and the light is confined in two dimensions .
Elsevier Science Publishers B .V . Ali rights reserved
M. Fleaslcr et al. / lcur unplanted optical 1saregurdes ar KiVI Ot
1151 DEPTH (pin)
1
REGION OF REDUCED INDEX FORMED BY PLANAR IMPLANTATION
x ô
PLANAR WAVE~UIDE
2
3
4 nc
0
W Û Q _1
~
C n6
w ~ O w r,
z
U
b
. p 1I
.L 0
o 20
10
'
n 30
"
n 40
50
60
SQUARE OF MODE NUMBER tin + 1) t
Fig. 2. Elfective refractive mode indices of a 2 MeV planar waveguide measured at 632 nm ver- the mode number (0, ". lovxr quadratic scale) together with the reconstructed index eepth profile (---, --, upper linear scale).
(bl
STRIP WAVEGUIDES
Fig. 1 . (a) Formation of a planar waveguiding layer by single energy He ion implantation . (b) Creation of optical channel w-guides by additional implantations at lower dapths with appropriate masking . 3. Waveguide characterization The optical characterization of the planar ,uides was performed by dark line spectroscopy [8]. All observable modes were used to reconstruct a reasonable refractive index profile [9]. (The basic idea of this method is to model the dark mode positions of a parameterized analytical function and then to use a least square optimization routine to secure the best-fit profile .) Fig. 2 shows the measured effective mode indices together with the reconstructed index depth profiles of a planar waveguide formed by the implantation of fie ions with -in -n, rgy of 2 .0 McV and a dose of 7 .5 x 10 14 cm -2 . A depth profile of the nuclear energy loss was calculated using the TRIM'89 Monte Carlo code 1101 and waà found to coincide with the refractive index profile [11]. To get a better understanding of the influence of the ion dose on the shape of the index profile we fabricated a series of planar waveguides with 2 MeV and increasing dose and reconstructed the profiles . In fig. 3 the refractive index in the barrier is plotted as c function of dose . All three indices decrease with increasing dose and merge into a common value of 2 .119 at a dose of 10 1 " cm -2 which can be explained by the formation of an isotropic layer, indicating the full amorphization of the crystal in the barrier . This view is
supported by RBS/channeling measurements (fig . 3) which show that the relative Iatice damage is close to 100% at 10 1 " cm -2 [11] . The search for the optimum dose - high doses give good beam confinement, hot result in increasing absorption and scattering losses led to a dose of 7.5 x 10 1' cm -=. The optical waveguidcs exhibit waveguiding without subsequent annealing and show propagation losses of typically 1-3 dB/cm at 860 nm. Up to now tie chatinets confine modes with the polarization par-tllel to the IONIC ENFRGY DENSITY (OW-3 )
aî W
n z
0 W
S
s
DOSE (em,3)
Fig . 3 . Refractive indices n W, n" (0), it,. ( " ) in the harrier and the lattice danidge (0) as a function of dose for 2 MeV He implantation into KNb0 1 [11]. 1Vb . INSULATOR MODIFICATION (b)
1152
M. Ffe(rsfer er a1 / 1,,,,
Fig . 4. Configuration for nonrnlcal phase-matched frequency-doubling m a KNhO 3 planar waveguide (inset) and the generated SH peak power at 434 nm as a function of the fundamental peak power together with the theoretical prediction (--) [131 .
b-axis only [121 . We suspect that the small index chatigc of n,. (see fig. 3) is insufficient for mode confinement in a channel waveguide . 4. Second harmonie blue light generation For efficient SHG it is necessary that the fundamental (w) and the harmonic mode (2w) propagate with the same phase velocity to ensure constructive interference of the, blue light generated at differem positions in the waveguide . This leads to the phasematching condition :
n(w) =n(2w).
u-
,, KNbO,
of the wavcguidc [13]. The experimental result~ are in good agreement with the theoretical prediction . A maximum conversion efficiency of 299 was achieved for a peak power of 1 .3 kW in the wavcguidc, giving a blue fight output of 385 W. In the low power limit whem the .î:I ;tower depends on the square of the fundamental power, we measured 0 .38 mW blue output for i87 m'v"v infrared input, giving a normalized efficiency of 1 .1%/W. As already mentioned, modes with the polarization parallel to the c -axis are not guided in the channel wavcguidcs . This prohibits guided wave SHG, but permits a very interesting approach, the Cherenkov-type SHG. In this configuration (sec inset of fig . 5) the fundam( ntal guided mode is wriverîcd ini -"_, blue radiation which is emitted into the substrate at an angle which is determined by the phase-matching condition . Only at this so-called Cherenkov angle all blue radiation interferes constructively . The great advantage of this geometry lies in the fact that Cherenkov SHG is free from the stringent phase-matching condition of guided wave SHG, because small changes of the temperature or the fundamental wavelength will be compensated automatically by a very small change in the Cherenkov angle . In our waveguide the Cherenkov angle is typically 2-3° . The efficiency of the conversion is shown in fig. 5 . Over the entire range the SH power is proportional to the square of the fundamental power giving a normalized efficiency of 12%/W . With 97 mW of fundamental power in the wavcguidc we get 1 .1 mW of blue light at 430 nm [141. A conical lens which allows to focus the blue light again to a diffraction limited spot has recently been proposed [151.
:CZ ,, wrlh CW AIG .A. diode laser with CW titanium sapphire laser
An overview on the phase-matching (PM) configuratic,is for KNbO, has recently been published [3]. In our case the PM wav.-Icrsût!i is determined by Nb(Ar,,)-''C(Apm/!), where Nf, and Nr arc the effective indices uï the fundamental and second harmonic (SH) modes that are polarized along the crystallographic b- and c-axis, respectively [13]. In fig. 4 the SH power generated in a planar waveguide is shown as a function of the fundamental power coupled into the waveguide using .l pulsed Ti : AI ZO, laser at A = 868 nm. Also shown in the theoretical conversion efficiency, calculated by taking into account the waveguide losses and the linewidth of the laser source which is wider than the acceptance bandwidth
104
Fig. 5 . Configuration for Cherenkov-type SHG in a MO, channel waveguide (inset) and the generated 511 power at 43(l nm against the fundamental power coupled into the t".,aveguide 114).
M. Fleuver rt v"/. / lon
entp/mtt,l optical
5. Conclusion We have fabricated permanent low loss planar and channel waveguides in KNbO, by a low dose He ion implat ation process . We have used these waveguides for frequency doubling of near infrared light into the blue spectra range. In a planar waveguide noncritical phase-matched second harmonic generation of blue light ::.,s performed with a normali --ed c, mv-sion effi-
ciency of 1 .1%-/W. By using the Cherenkov-type SHG in a channel waveguide more than 1 mW of blue light at 430 nm was produced for a conversion efficiency of 12",,, / W. Acknowledgement We gratefully acknowledge the support from the Tandetron accelerator facility and staff at IFF of KFA Mich . References
guides in KNbO,
1153
[3] 1 . B,aggto, P. Kerkoc, L.-S . Wu, B. Zysset and P. Ciimter, J. Opt. Soc. Am . 139 (1992) 507. [41 T. Bremer, W. Heiland, B. Hellermann, P. Hertel, E. Kriitzig and D. Kollewe, Ferroelectron. Len. 9 (1988) 11. [5] L. 'hang. P.J. Chandler and P.D . Townsend, Ferroelectrott. Lett 1 t (1990) 89 . [6] P.G . Townsend, Nucl . Instr . and Meth . B46 (1990) 18. [7] P.D . Townsend, Nucl . Instr . and Meth . B65 (1992) 243 . [8] F.P. Strohkendl, P. Giinter, Ch. Buchal and R. Innseher, J. Appl. Phys. 69 (1991) 84 . [91 P.J. Chandler and F.L. Lama, Opt. Acta 33 (1986) 127. [10] 1. Biersack as-J L. `l-ggmark, Nucl . Instr. and Meth . :74 (1980) 257. [1l] D. Fluck, R. Irmscher,Ch. Buchal and P. Günter, Ferroelec!6ce 128 (1992) 79 . [l2] D. FiuA P. Günter. M. Fleuster and Ch . Buchal, J. Appl . Phys. 72 (1992) 1571 . [13] D. Fluck, B Binder, M. Küpfer, H. Looser, Ch. Buchal and P . Günter, Opt . Commun. 90 (1992) 304. [ 14] D. Fluck, J. Moll, P. Günter, M. Fleuster and Ch . Buchal, Electron. Leu. 28 (1992) 109:. [151 K. Tatsuno, If. Yamagisawa,T. Amlou andM. McLoughlin, Appl. Opt. 31 (1992) 305.
[I] P. Giinter, Appl . Phys . Lett . 34 (1979) 650. [21 J.C. Baumert, P. Günter and H. Melchior, Opt. Commun. 48 (1983) 215.
IVb. INSULATOR MODIFICATION (b)