Refractive indices of paratellurite and lithium iodate in the visible and ultraviolet regions

Volume 23, number 2

OPTICS COMMUNICATIONS

November 1977

REFRACTIVE INDICES OF PARATELLURITE AND LITHIUM IODATE IN THE VISIBLE AND ULTRAVIOLET REGIONS K. TAKIZAWA, M. OKADA and S. IEIRI Broadcasting Science Research Laboratories, NHK (Japan Broadcasting Corporation), 1-10-11, Kinuta, Setagaya, Tokyo 157, Japan Received 2 August 1977

The refractive indices of TeO2 and LilO3 have been measured in the wavelength region between 0.3547 um and 0.6438 um by the method of minimum deviation. A thin fluorescent screen has been set at the focal plane of the objective of telescope to observe the ultraviolet light, which was obtained by sum-frequency mixing between a Nd:YAG laser and a tunable dye laser. The accuracies of the data are + 2.1 X 10 --4 in TeO2 and + 2.5 X 10 -4 in LiIO3.

Paratellurite (TeO2) is an attractive crystal in its acoustic [1], piezoelectric [2] and optical properties: it is applied in many acousto-optic devices and further it has recently been shown to have interesting properties in nonlinear optics [ 3 - 6 ] . a-lithium iodate (LiIO3) is one of the most available crystals in nonlinear optical devices such as the second harmonic generator [7,8], sum- and differencefrequency generator [9] and parametric oscillator. Both crystals are transparent from the near ultraviolet (UV) to infrared (IR), 0.3/am ~ 6/am, and the refractive indices in the visible and near IR regions have been reported by many authors [3,7,10--13]. However, the refractive indices of both crystals in the near UV region are not measured. This paper describes the dispersion characteristics of the refractive indices of TeO 2 and LiIO 3 in the wavelength range from 0.35 tim to 0.64/am, where the light in the near UV was obtained by sum-frequency generation using a Nd:YAG laser at 1.064/am and a dye laser tunable from 0.55 /am to 0.64/am. TeO 2 prism was prepared from a colorless single crystal of good optical quality grown by the Czochralski method. The axis of prism was parallel to the crystallographic e-axis detennined by X-ray method. The prism has an apex angle of 29°59 ', height of 10.5 mm and width of base of 6.5 mm. LiIO 3 prism was also obtained from a high quality crystal grown in an aqueous solution kept at 70°C. The axis of prism was

parallel to the c-axis, and the apex angle, height and width of base were 30°02 ', 20 mm and 10 ram, respectively. The refractive indices were measured at room temperature by the method of minimum deviation. The schematic arrangement for measurements is shown in fig. 1. The prism is irradiated by the collimated coherent UV light, which is generated in a phase-matched LilO 3 crystal by optically mixing between a beam

~

---~ Q-SW / Nd:YAG -J LASER

E

BNN

BS~~ /

G DYE . cELL

LilO~

PRISM(TeOz,Lil03) COLLIMATOR/""--~

L~N~/~.

Fig. 1. Experimental arrangement for measuring the refractive indices of TeO2 and LilO3 in the UV region. BNN: 90° phase-matched Ba2NaNbsOts crystal, BS: Beam splitter which reflects the IR beam and transmits the visible beam, G: Grating with 1800 grooves/mm, M: Mirror, F: Filter, CL: Cylindrical lens, S: Slit, L: Lens, FF: Fluorescent film. 279

Volume 23, number 2

OPTICS COMMUNICATIONS

Novcmb< !977

Table 1 Measured and calculated refractive indices of TeO2. The calculated values designated by I, 2 and 3 are obtained from the dispersion equations given by Uchida [101, Singh et al. [3] and Berny et al. [11], respectively. Wavelength X (~m)

Observed no

ne

Calculated 1 no nc

Calculated 2 no ne

Calculated 3 no tze

0.3547 0.3639 0.3712 0.3795 0.3877 0.3996 0.4800 0,5320 0.6400

2.5679 2.5333 2.5104 2.4868 2.4665 2.4407 2.3359 2.3004 2.2565

2.7775 2.7365 2.7088 2.6806 2.6567 2.6264 2.5025 2.4605 2.4088

2.5700 2.5356 2.5120 2.4882 2.4677 2.4418 2.3367 2.3007 2.2577

2.5198 2.4967 2.4802 2.4628 2.4472 2.4269 2.3355 2.2900 2.2571

2.5595 2.5284 2.5067 2.4845 2.4650 2.4402 2.3365 2.3003 2.2566

2.7799 2.7390 2,7111 2.6828 2.6585 2.6278 2.5034 2.4609 2.4100

from a repetitively Q-switched N d : Y A G laser at 1.064 /xm and a b e a m from R h o d a m i n e 6G and R h o d a m i n e B dye lasers p u m p e d by the second h a r m o n i c o f the N d : Y A G laser p r o d u c e d in Ba2NaNb 5 O15 (BNN) crystal. The tuning range o f UV light extends from 0.3547 /~m to 0 . 3 9 9 6 / x m , corresponding to the second harm o n i c o f the N d : Y A G laser and to the tuning range o f t h e dye laser from 0.55/xm to 0.64/~m. A thin fluorescent screen is placed at the focal plane o f the objective o f telescope to observe an U V slit image. The refractive indices in the visible region were also measured by this m e t h o d and c o m p a r e d with the result obtained by a conventional technique w i t h o u t the

28X,

c-

2.7 2.6[~

Te02 \

2.5

g2 . 4

n 2 = 4.7510 + 0.11942 o X2 - 0.060803

ne = 0.35 0.40 0.45

0.50 0.55 0.60 0.65 X (/zm)

WAVELENGTH

Fig. 2. Dispersion of the refractive indices no and n e in TeO2 at room temperature. Open circles are index values observed. The full curves are calculated with eqs. (1) and (2),

280

2.7678 2.73(17 2.7049 2.6783 2.6553 2,6258 2.503 l 2.4604 2.4090

fluorescent screen. As the difference between values in two m e t h o d s is less than the fourth decimal, and the results are in good agreement with those measured by m a n y authors [3,7,10 13], it is reasonable to use the fluorescent screen for measuring the refractive indices in the UV region. The accuracies o f data measured by this technique are estimated to be +- 2.1 X 10 - 4 in TeO 2 and +- 2.5 X 10 -4 in LilO3~ Table 1 lists the ordinary (no) and extraordinary 0%) refractive indices o f TeO 2 at several wavelengths, where the observed values are from our measurements and the calculated values are those obtained from dispersion equations given by Uchida [10], Singh et al. [3] and Berny et al. I11] based on the data in the visible and near IR regions. It is seen from table 1 that the difference b e t w e e n the observed and calculated values increases with the decrease o f wavelength. We determined the dispersion equations from our index values in TeO 2 using the least square method. The results are given by

2

c~ 2.5

2.7228 2.6949 2.6749 2.6540 2.6353 2.6110 2.5021 2.4483 2.4095

5.3789

+

0.14817

(1)

(2)

X-~0.062171

where ~ is expressed in ~m. Fig. 2 shows the dispersion characteristics o f the refractive indices o f TeO 2, where open circles are the measured values and the solid curves show the indices calculated with eqs. (1) and (2). The calculated values are in good agreement

Volume 23, number 2

November 1977

OPTICS COMMUNICATIONS

Table 2 The refractive indices of LilO3 obtained in our measurements Wavelength (t~m)

Observed no

Wavelength X (~m)

Observed

ne

no

ne

0.3547 0.3669 0.3712 0.3795 0.3877 0.3996 0.4358 0.4678

1.9822 1.9706 1.9671 1.9600 1.9544 1.9464 1.9275 1.9151

1.8113 1.8026 1.8000 1.7947 1.7905 1.7842 1.7702 1.7608

0.4800 0.5086 0.5320 0.5600 0.5800 0.6000 0.6200 0.6438

1.9109 1.9031 1.8975 1.8921 1.8889 1.8859 1.8828 1.8799

1.7579 1.7514 1.7475 1.7433 1.7403 1.7383 1.7361 1.7336

2.0

Fig. 3 shows the dispersion characteristics o f the refractive indices o f L i l O 3 . The measured index values indicated b y open circles are in good agreement w i t h the dispersion curves calculated w i t h eqs. (3) and (4) over the visible to U V regions.

Li[O3 C2

no

x 1.9 w 123 z

The authors are grateful to Dr. T. Suzuki for helpful discussions.

w >

btel

" ~

ne References

1.7 , , i L L , O.3~ 0.40 0.45 0.50 055 0.60 0.C5 WAVELENGTH k ( ffm ) Fig. 3. Dispersion of the refractive indices no and n e in LilO3 at room temperature. Open circles are index values observed. The full curves are calculated with eqs. (3) and (4). with the measured ones over the entire wavelength region investigated. The refractive indices o f LilO 3 were similarly examined. The indices o f L i l O 3 o b t a i n e d in our measu r e m e n t s are shown in table 2, where the index data in the visible region agree with those given b y N a t h et al. [7], Umegaki et al. [12] and Crettez et al. [13]. The dispersion equations fitted to our data b y the least square m e t h o d are given by n 2 = 3.4095 + 0.047664 o X2 - 0.033991

(3)

n 2 = 2.9163 + 0.034514 . e X2 - 0 . 0 3 1 0 3 4

(4)

[1] N. Uchida and Y. Ohmachi, J. Appl. Phys. 40 (1969) 4692. [2] G. Arlt and H. Schweppe, Solid State Commun. 6 (1968) 783. [3] S. Singh, W.A. Bonner and L.G. van Uitert, Phys. Lett. 38A (1972) 407. [4] D.S. Chemla and J. Jerphagnon, Appl. Phys. Letters 20 (1972) 222. [5] B.F. Levine, IEEE J. Quantum Electron. QE-9 (1973) 946, [6] M. Okada, K. Takizawa and S. Ieiri, J. Appl. Phys., to be published. [7] G. Nath and S. Haussfihl, Appl. Phys. Letters 14 (1969) 154. [8] F.R. Nash, J.G. Bergman, G.D. Boyd and E.H. Turner, J. Appl. Phys. 40 (1969) 5201. [9] M. Okada and S. Ieiri, Japan. J. Appl. Phys. 10 (1971) 808. [10] N. Uchida, Phys. Rev. B4 (1971) 3736. [11] J.G. Berny, J.P. Bourgoin and B. Ayrault, Opt. Commun. 6 (1972) 383. [12] S. Umegaki, S. Tanaka, T. Uchiyama and S. Yabumoto, Opt. Commun. 3 (1971) 244. [13] J.M. Crettez, J. Comte and E. Coquet, Opt. Commun. 6 (1972) 26. 281