RESULTS  AND  DISCUSSION  (CONT’D)

LASER  ABSORBING  MATERIALSLeong  Feng  Ping  Angela1,  Ke  Meicong1,  Yang  Qian  Asarelah1  and  Koh  Yaw  Koon2

1NUS  High  School  of  MathemaKcs  and  Science,  20  ClemenK  Avenue  1  Singapore  1299572DSO  NaKonal  Laboratories,  20  Science  Park  Drive  Singapore  118230

PURPOSEThe  sol-­‐gel  process  has  been  used  to  produce  silica  glass  doped  with  samarium  (Sm3+)  ions  as  the  base  material.  The  Sm3+  ion  shows  great  absorpKon  around  1064  nm  as  the  difference  in  energy  level  between  its  orbitals  corresponds  to  the  energy  of  light  with  a  wavelength  of  1064  nm.  We  aimed  to  determine  the  op8mum  thickness  and  hea8ng  protocol  for  samarium-­‐doped  glass  to  achieve  maximum  absorp8on  efficiency  at  a  wavelength  of  1070nm.

CONCLUSION• HeaKng  increases  absorpKon  efficiency  at  1070  nm  for  all  three  thicknesses.• HeaKng   appears   to   reverse   the   relaKonship   between   thickness   of   the   film   and   absorpKon  efficiency,  such  that  thinner  films  display  higher  absorpKon  efficiencies.

A  thinner  layer  of  silica  gel  could  be  used  to  achieve  higher  laser  absorp8on,  simply    by  hea8ng  during  the  solidifying  process.  This  allows  huge  cost  savings  when  

scaled  up  for  use  in  the  field.  

RESULTS  AND  DISCUSSION

FUTURE  WORK• Verify  the  relaKonship  between  film  thickness  and  absorpKon  efficiency  with  a   larger  range  of  thicknesses,  and  analyse  the  transmibance/absorbance  spectrum  in  greater  detail• Test  a  wider  range  of  hea8ng  protocols,  to  opKmise  heaKng  protocols  for  specific  wavelengths  and  for  other  laser-­‐absorbing  materials• OpKmise   absorpKon   efficiency   by   adjusKng   both   gel   thickness   and   samarium   concentraKon:  electrostaKc   repulsion   exists   between   the   Sm3+   ions   within   the   silica,   thus   the   op8mal  concentra8on  of  samarium   is  the  trade-­‐off  between  the  maximum  absorpKon  density  of  Sm3+  and  minimum  repulsion  between  the  Sm3+  ions.  • Study   the   use   of   samarium-­‐doped   silica   gel   for   the   absorp8on   of   lasers   emiNed   at   other  wavelengths,  as  the  greatest  drop  in  transmibance  occurs  at  1400  .

ACKNOWLEDGEMENTSThis  work  was  supported  by  the  Young  Defence  ScienKsts  Programme  (YDSP),  Ministry  of  Defence,  Singapore.  We  would  like  to  Dr  Koh  Yaw  Koon  from  Defence  Science  OrganisaKon  (DSO),  for  guiding  us  through  the  experiments,  instrucKng  me  in  the  background  knowledge,  and  clarifying  my  doubts,  as  well  as  Mr.  Chiam  Sher-­‐Yi  for  his  guidance  during  the  ediKng  of  the  report,  and  aiding  with  administraKve  details.

*All  images  and  graphs  are  self-­‐taken  and  self-­‐drawn  unless  otherwise  stated.

REFERENCESREFERENCESREFERENCES

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INTRODUCTIONThe   ubiquity   of   laser-­‐guided   devices   in   the   military   has   compelled   the   evoluKon   of   laser  concealment  and  laser-­‐jamming  techniques.  Passive  laser-­‐absorbing  materials  can  be  divided  into  organic  materials  and  inorganic  materials,  of  which  organic  materials  are  more  versa8le,  but  are  unstable   under   UV   light   and   involve   complicated   synthesis   methods.   The   inorganic  metamaterial  is  of  a  parKcular  interest  because  it  passively  interferes  with  electromagneKc  (EM)  waves   and   is   able   to   absorb   at   various   wavelengths,   but   much   research   has   focused   on  broadband   absorp8on   and  on   the   paNerning   of   base  materials.   However,   the  most   common  laser  in  military  use  –  the  Nd:YAG  laser  –  is  emibed  at  a  specific  wavelength  of  1064  nm.

EXPERIMENTAL  

RelaKve  transmibance  in  arbituary  unit  (A.U.)  of  samarium-­‐doped  silica  gel    of  different  thickness  under  different  heaKng  protocol

100  μm 200  μm 300  μm

Air-­‐dried 89.426  A.U. 88.818  A.U. 88.301  A.U.

Oven-­‐dried 85.017  A.U. 86.301  A.U. 87.148  A.U.

%  increase  due  to  heaKng

5.186% 2.917% 1.323%

Sm3+

Samarium  ions  integrate  into  the  spaces  within  the  silicate  lamce

Spectrophotometer  Results

(1a) (1b)

RelaKve  transmibance  in  arbituary  unit  (A.U.)  against  wavelength  (nm)  of  samples  of  different  thickness  when  (a)  air-­‐dried  and  (b)  over-­‐dried

RelaKve  transmibance  in  arbituary  unit  (A.U.)  against  wavelength  (nm)  of  air-­‐dried  and  oven-­‐dried  samples  of  different  thickness  (a)  100  μm  (b)  200  μm  (c)  300  μm

(2b)(2a)

(2c)

Absorp8on  efficiency  of  air-­‐dried  samples  increases  with  increasing  thickness

The   thicker   the   silica   gel,   the   longer   the   EM  wave   takes   to   propagate   through   the   gel,  increasing  the   likelihood  of  the  photons  being  absorbed  by  the  electrons  of  the  Sm3+  ions.  

Absorp8on  efficiency  of  over-­‐dried  samples  decreases  with  increasing  thickness

HeaKng   the   silica   sol   increases   the   kineKc  energy   of   Sm3+   ions   and  atoms   of   the   silicate  lamce,  causing  Sm3+  ions  to  break  and  migrate  to   other   locaKons   within   the   lamce   while  forming  new  bonds     HeaKng  may  alter  the  structure  of  the  Sm3+-­‐doped  silica  gel  such  that    its  opKcal  properKes  are  changed.

• At   all   wavelengths,   oven-­‐dried   silica   gels  absorb  a  higher  percentage  of  EM  wave  than  air-­‐dried  silica  gels  of  the  same  thickness.  • Air-­‐dried   samples   show   a   sharp   drop   in  transmibance   –   a   local   peak   of   the  absorbance  spectrum  –  just  aner  1070nm.• The   heaKng   protocol   shins   the   peak   of   the  absorpKon   spectrum   towards   shorter  wavelengths   and   towards   1070   nm,   but   the  peak  becomes  less  pronounced.

• Maximum   absorpKon   of  waves   with   wavelength  1070   nm   is   achieved   at   a  thickness  of  300  μm.• The   increase   in   absorpKon  efficiency   imparted   by  furnace   heaKng   decreases  with  increasing  thickness.

②  IncorporaKon  of  samarium  ions

Image  taken  from:  hbp://upload.wikimedia.org/wikipedia/commons/thumb/4/4b/Silica.svg/220px-­‐Silica.svg.png  

with  NH4OH  (

Literature)

                             sKrri

ngwithout  NH4OH(Ours)                      sKrring

Casting & Gelation:

Preparation of Sm2O3 sol:

Dry  at  room  temperature  for  24  hours.Heated  in  a  furnace  at  an  increase  of  5oC/min  and  held  at  650oC  for  15  mins  before  cooling  to  room  temperature.

Dry  at  room  temperature  for  24  hours.

CharacterizationBoth   the   heated   and   unheated  samples  were  characterized  using  a  UV-­‐VIS-­‐NIR   spectrophotometer  (UV3600,   Shimadzu)   at   the   near  infrared  region  (900nm  –  1200nm).  

Mica is a silicate mineral chemically inert and stable when exposed to electricity, light, moisture, and extreme temperatures.Optical and chemical properties of mica unlikely to be affected by contact with the silica gel or heating process.

white  precipitate

homogeneous  

Mechanism

①FormaKon  of  silicate  

Image  taken  from:  The Sol-Gel Preparation of Silica GelsA. M. Buckley and M. GreenblattJournal of Chemical Education 1994 71 (7), 599

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