ENHANCED JOSEPHSON RADIATION AND LINEWIDTH-NARROWING OF SUPERCONDUCTING MICROBRIDGES IN A RESONANT CAVITY

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ENHANCED JOSEPHSON RADIATION AND

LINEWIDTH-NARROWING OF

SUPERCONDUCTING MICROBRIDGES IN A

RESONANT CAVITY

M. Stern, W. Howard, Y. Kao

To cite this version:

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JOURNAL DE PHYSIQUE _{Colloque C6, suppliment au no 8, Tome 39, aoiit 1978, page ~ 6 - 5 7 7}

ENHANCED JOSEPHSON R A D I A T I O N AND LINEWIDTH-NARROWING

OF

SUPERCONDUCTING M I C R O B R I D G E S I N A RESONANT C A V I T Y

M. S t e r n , W. Howard and Y.H. Kao

Physics Department, S t a t e U n i v e r s i t y o f New Y o r k , Stony Brook, N . Y . 11 794,

U.

S. A .

R6sumQ.- Les r a d i a t i o n s Josephson b i s e s p a r d e s microponts s u p r a - c ~ n d u c t e u r s e t d e s j o n c t i o n s P deux p o n t s couplds o n t Q t Q b t u d i d e s dans l e domaine d e s hyperfrequences. La l a r g e u r Af de l a r a i e e t l a puissance dmise P v a r i e n t avec l e couplage e x t d r i e u r contrSlC p a r un plongeur accordant dans l a c a v i t d ; c e l a indique d e s e f f e t s de c o n t r e - r d a c t i o n dus aux champs de r a d i a t i o n . Nous trouvons (Af)min % 900 kHz, Pmax % 0 , 9 x 10-l2 W pour un simple micropont e t (Af) % 60 kHz,

,min

n, 5 x 10-l2 W pour l e s y s t k e coupl6. Les maxima dans l a p u i s s a n c e emme e t l e s r e t r e c i s s e m e n t s k ? a i e s dans l e s y s t h e d e s ponts couples s o n t a t t r i b u e s 1 d e s e f f e t s c o h e r e n t s dans 1 e s j o n c t i o n s . A b s t r a c t . - Josephson r a d i a t i o n e m i t t e d from superconducting microbridges and coupled two-bridge j u n c t i o n s has been i n v e s t i g a t e d a t microwave f r e q u e n c i e s . The r a d i a t i o n l i n e w i d t h Af and t o t a l r a - d i a t i o n power P a r e found t o v a r y with t h e o u t p u t coupling c o n t r o l l e d by a tuning s t u b i n t h e cavity, i n d i c a t i n g feedback-induced e f f e c t s due t o t h e r a d i a t i o n f i e l d s , We have found (Af)min % 900 kHz,

P % 0.9 x 10-l2 W f o r t h e s i n g l e microbridge and (Af) % 60 kHz,

pmaX % 5 x 10-l2 W f o r t h e coupled b r i d g e s . The enhancemzk? of r a d i a t e d power and line-narrowing i n tEsxcoupled b r i d g e s a r e a t t r i b u t e d t o c o h e r e n t behaviour of t h e j u n c t i o n s .

1 . INTRODUCTION.- I n t h i s experiment we have inves- t i g a t e d t h e e f f e c t s on r a d i a t e d power and l i n e w i d t h of microbridge Josephson j u n c t i o n s a r i s i n g from non- l i n e a r coupling of t h e j u n c t i o n c u r r e n t t o a r e s o - n a n t c a v i t y a s w e l l a s i n t e r a c t i o n s between two j u n c t i o n s .

When a Josephson j u n c t i o n i s s t r o n g l y cou- p l e d t o a r e s o n a n t c a v i t y , induced c u r r e n t s t e p s appear on t h e I - V c h a r a c t e r i s t i c a t v o l t a g e s cor- responding t o t h e r e s o n a n t f r e q u e n c i e s o of t h e c a v i t y a c c o r d i n g t o t h e Josephson r e l a t i o n

,2 eV = #oc. Using a p o i n t c o n t a c t j u n c t i o n , R i c h a r d s and S t e r l i n g / I / have shown t h a t t h e h e i g h t of t h e s e c u r r e n t s t e p s i s s e n s i t i v e t o e x t e r n a l r a d i a t i o n a n d t h e system can be used a s a narrow band d e t e c t o r . They observed feedback-narrowed s p e c t r a l response w i t h a Q much h i g h e r than t h a t of t h e c a v i t y mode. It i s of i n t e r e s t t o d e t e c t t h e Josephson r a d i a t i o n d i r e c t l y and observe t h e feedback narrowing e f f e c t on t h e l i n e w i d t h . Such r e s u l t s would provide i n f o r - mation on t h e frequency s e l e c t i v i t y when t h e junc-

t i o n i s used a s a d e t e c t o r o r a r a d i a t i o n source. Also o f i n t e r e s t i s t h e enhancement of t h e s e e f f e c t s when two j u n c t i o n s a r e i n t e r a c t i n g w i t h each o t h e r , r e s u l t i n g i n a c o h e r e n t s t a t e .

made on s a p p h i r e r o d s u s i n g an e l e c t r o n beam l i t h o - graphy technique. The t y p i c a l weak l i n k a r e a was about 1.0 um long, 0.4 um wide and 0.1 pm t h i c k . The sample rod was i n s e r t e d i n t o an X-band r e s o n a n t s t r u c t u r e with a movable plunger and a tuning s t u b . The s t u b t u n e r was l o c a t e d a t a d i s t a n c e 3Ag/4 from t h e j u n c t i o n . T h i s r e s o n a n t s t r u c t u r e was analogous t o t h e one used by Taur e t a l . 121.

R a d i a t i o n was observed u s i n g a narrow band r e c e i v e r tuned t o 9.05 GHz. The r e c e i v e r had a bandwidth of 30 MHz, an I F c e n t r e frequency of 60 MHz, a n o i s e f i g u r e of 8.5 dB and a g a i n of 51 dB. The r e s o n a n t frequency of t h e c a v i t y was tuned t o t h e r e c e i v e r frequency f R by minimizing t h e c r i t i c a l c u r r e n t of t h e microbridges when an i n c i d e n t microwave s i g n a l s e t a t t h e same frequency was a p p l i e d . F i n e r t u n i n g t o t h e r e c e i v e r frequency was achieved by v a r y i n g t h e s t u b and plunger p o s i t i o n s w h i l e d i s p l a y i n g t h e r e c e i v e r output on a spectrum analyser. The r a d i a t i o n a t f occurred a t t h e dc v o l t a g e _R

V

= hf /2e which concided w i t h t h e p o s i t i o n of a R R

minimum i n dV/dI o r a c u r r e n t s t e p . The c a v i t y had o t h e r resonance modes but o n l y t h e one tuned t o f

R

was used i n t h e p r e s e n t experiment. The loaded Q of t h e c a v i t y was around 350. The e f f e c t of moving t h e s t u b was mainly t o change t h e impedance seen by t h e 2. EXPERIMENTAL DETAILS.- The indium t h i n f i l m r e s o n a n t combination of t h e sample p o s t and t h e microbridge samples used i n our experiment were plunger, t h u s t h e s t u b p o s i t i o n c o n t r o l l e d t h e cou-

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p l i n g between t h e r e s o n a n t s t r u c t u r e and i t s o u t s i d e l o a d .

3. RESULTS.- Josephson r a d i a t i o n w i t h i n t h e r e c e i - v e r bandwidth e m i t t e d from t h e m i c r o b r i d g e s was measured by c o n n e c t i n g t h e r e c e i v e r o u t p u t t o a spectrum a n a l y s e r . The o u t p u t spectrum was found t o depend on t h e s t u b p o s i t i o n which determined t h e o u t p u t c o u p l i n g . F i g u r e I shows a t y p i c a l p l o t o f

Fig. 1 : The v a r i a t i o n i n t h e l i n e w i d t h ( a ) , power s p e c t r a l d e n s i t y (b) and i n t e g r a t e d power ( c ) f o r a one-microbridge sample a s t h e s t u b t u n e r i s moved i n t o t h e waveguide a t T = 3.373 K ; X-axis : z e r o i n d i c a t e s t h e p o s i t i o n f o r b e s t c o u p l i n g t o e x t e r n a l r a d i a t i o n a t 9.05 GHz. A change of + I r e p r e s e n t s an advancement o f t h e s t u b i n t o t h e waveguide by 2 % of t h e s h o r t g u i d e dimension ; Y-axis : ( a ) l i n e w i d t h a t half-maximum (MHz, r i g h t s c a l e ) , (b) power spec- t r a l d e n s i t y x W/Hz, l e f t s c a l e ) , ( c ) i n t e g r a - t e d power x 10-l2 W, l e f t s c a l e )

l i n e w i d t h , peak power and peak power s p e c t r a l den- s i t y f o r a s i n g l e indium m i c r o b r i d g e . As t h e s t u b was moved i n t o t h e waveguide, t h e d i f f e r e n t i a l r e - s i s t a n c e and t h e l i n e w i d t h a t V _Rd e c r e a s e d monoti- c a l l y . The power i n c r e a s e d f i r s t and t h e n d e c r e a s e d . The minimum l i n e w i d t h measured w i t h t h i s sample was a b o u t 900 kHz c o r r e s p o n d i n g t o a n e f f e c t i v e Q o f a l o 4 . The h i g h e s t measured v a l u e f o r peak power

s p e c t r a l d e n s i t y was 5.35 x W/Hz and a maximum t o t a l power of 0.9 x 10-l2 W.

Measurements were a l s o c a r r i e d o u t f o r r a - d i a t i o n e m i t t e d from m u l t i - e l e m e n t a r r a y s c o n s i s t i n g o f 2 and 27 b r i d g e s . When t h e b r i d g e s i n t h e a r r a y were u n c o r r e l a t e d , e a c h one e x h i b i t e d a w e l l - r e s o l - ved r a d i a t i o n peak a s t h e b i a s c u r r e n t was swept. However, t h e peaks showed lower t o t a l power t h a n had been o b s e r v e d w i t h a s i n g l e j u n c t i o n . On t h e o t h e r hand, when t h e b r i d g e s i n t h e a r r a y were a c t i n g

c o h e r e n t l y , t h e l i n e w i d t h - n a r r o w i n g and r a d i a t e d power were s i g n i f i c a n t l y enhanced. F i g u r e 2 shows t h e r e s u l t f o r a c o h e r e n t two-bridge a r r a y . F i g . 2 : a r r a y a t Y-axis : s p e c t r a l t e g r a t e d Same a s f i g u r e I b u t f o r a t w o - m i c r o b r i d g e T = 3.368 K ; X-axis : same a s f i g u r e 1 . ( a ) l i n e w i d t h (MHz, r i g h t s c a l e ) , (b) power d e n s i t y x 10-l8 W/Hz, l e f t s c a l e ) , ( c ) i n - power x 10-l2 W , l e f t s c a l e ) . Comparing t h e p l o t w i t h f i g u r e 1 , we s e e t h a t a d e c r e a s e by one o r d e r of magnitude i n l i n e w i d t h and a n i n c r e a s e by two o r d e r s o f magnitude i n peak power s p e c t r a l d e n s i t y have been o b s e r v e d . The maximum r a d i a t e d power i n t h e a r r a y showed a n i n c r e a s e by a f a c t o r of 5.5.

4. CONCLUSION.- We have p r e s e n t e d t h e f i r s t expe- r i m e n t a l d e m o n s t r a t i o n o f feedback-narrowed r a d i a - t i o n l i n e w i d t h f o r m i c r o b r i d g e a r r a y s . The i n t e r - a c t i o n between j u n c t i o n s i n a c o h e r e n t a r r a y g i v e s r i s e t o a n enhanced r a d i a t i o n power and f u r t h e r narrowed l i n e w i d t h .

R e f e r e n c e s

/ I / R i c h a r d s , P.L. and S t e r l i n g , S.A., Appl. Phys. L e t t .

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