NONLINEAR REFRACTION AND INCREASING ABSORPTION IN HgCdTe OPTICAL BISTABILITY AT ROOM TEMPERATURE : AN EXPERIMENTAL STUDY

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NONLINEAR REFRACTION AND INCREASING ABSORPTION IN HgCdTe OPTICAL BISTABILITY AT ROOM TEMPERATURE : AN EXPERIMENTAL

STUDY

S. Cecchi, P. Coppo, P. Salieri, F. Arecchi

To cite this version:

S. Cecchi, P. Coppo, P. Salieri, F. Arecchi. NONLINEAR REFRACTION AND INCREAS- ING ABSORPTION IN HgCdTe OPTICAL BISTABILITY AT ROOM TEMPERATURE : AN EXPERIMENTAL STUDY. Journal de Physique Colloques, 1988, 49 (C2), pp.C2-135-C2-136.

�10.1051/jphyscol:1988231�. �jpa-00227648�

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JOURNAL DE PHYSIQUE

Colloque C2, Suppl6ment au n06, Tome 49, juin 1988

NONLINEAR REFRACTION AND INCREASING ABSORPTION IN HgCdTe OPTICAL BISTABILITY AT ROOM TEMPERATURE : AN EXPERIMENTAL STUDY

S. CECCHI, P.M. COPPO, P. SALIERI and F.T. ARECCHI

I s t i t u t o N a z i o n a l e

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O t t i c a . Largo

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F e r m i ,

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I-50125 P i r e n z e , I t a l y

Abstract

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In room temperature Hg Cd Te samples, nonlinear absorpLion is observed 0.815. 0.185

to induce in trasmission, upon increasing input intensity, a transition from anticlockwise to clockwise hysteresis loops.

1

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INTRODUCTION

Many authors have experimentally examined the refractive nonlinearities of HgCdTe: in particular optical bistability (OB) has been investigated under different conditions (1). In this com- munication we extend the analysis of thermally induced room temperature OB at low incident power:

we show, for the first time in HgCdTe, the appearance of clockwise hysteresis cycles in transmis-- sion as the incident intensity is increased, due to thermally induced nonlinear absorption. We can account for both absorptive and refractive nonlinear contributions by simultaneous monitoring of the sample transmission and reflection.

2 - EXPERI:?ENTAL RESULTS

16 -3 Our set of Hg 815Cd0 185Te plates with thickness D=280 um and doping concentration p=10 cm was prepared b$ ~lettbonica SPA, Rome. We used a single mode CO laser actively intensity and

frequency stabilized. The in ut Seam was focused on the sample to 2 400 um spot size and collected in trasmission onto a 0.1 cm pyroelectric detector, the portion reflected was also monitored by

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a similar detector: these signals were then syncronously detected and calibrated. The samples under investigation were placed without heatsinking in a thermally controlled environment: the incident laser intensity was scanned slowly (20-30 minutes) to allow the sample to thermalize at each increment. Bistability was observed over a broad frequency range in the 10.6 um band.

In Fig.1 we show a set of transmission and reflection data: the three curves differ in cavity mistuning. As intensity is increased, below 70 mtl the observed behavior is characteristic of dispersive bistability, with.conterclockwise (ccw) hysteresis cycles in transmission and clockwisp (CW) cycles in reflection. At higher power the non linear increase in absorption induces a different phenomenology (cw cycles observed both in transmission and reflection). In correspondance to the first ccw cycle we measured by a 2mm resolution close focus optical pyrometer a strong thermal gradient with a maximum temperature Increase AT=27OC. A set of transmission and reflection measurements performed at different bath temperatures keeplng fixed the position of the beam on the sample is summarized in Fig.2: as temperature is increased the position of the hysteresis cycles associated to a given interference order shifts to lower power: a refraction

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index increment of dn/dT=-2.8*10 K was deduced. To determine the intrinsic nonlinear absorption we AR coated the rear face of the sample and measured the absorption coefficient as function of the induced temperature difference (Fig.3a).Using this measured absorption we solved numerical- ly the nonlinear Fabry Perot equations, assuming an induced nonlinear phase shift propotional to the absorbed power (2): the calculated curves of Fig.3 must be compared with the experimental data of Fig.lb. The observed increasing absorption is not due to bandtail effects in presence of a shrinking band gap as for x=.185, dE /dT is known to be positive: wc attribute this effec?

to an increase with temperature of free cargier concentration.

REFERENCES

/1/ D. Craig. M.R. Dyball and A. Miller,:Opt. Comm.? (1985) 83.

/2/ S. Cecchi, P.M. Coppo, P. Salieri, and F.T. Arecchi to be "ublished in Opt. Comm..

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988231

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JOURNAL

DE

PHYSIQUE

Fig.1

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Transmitted and reflected (Pt and Pr) vs input power (Pi).

Fig.2

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Shift with temperature of nonlinearities associated to different cavity orders: as in Fig. lc

0

corresponds to first cycle,

h

to a switch up, to a switch down.

Fig.3 a

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Transrnissi.on function o$a sample AR coated on its rear face.

b

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Calculated reflection characteristics.

c

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Calculated transmission characteristics.