Low-cost, compact, automated optical microsystems for chemical analysis, such as microflow cytometers for identification of individual biological cells, require monolithically integrated microlenses for focusing in microfluidic channels, to enable high-resolution scattering and fluorescence measurements. The multimode interference device (MMI), which makes use of self-imaging in multimode waveguides, is shown to be a simple and effective alternative to the microlens for microflow cytometry. The MMIs have been designed, realized, and integrated with microfluidic channels in a silica-based glass waveguide material system. Focal spot sizes of 2.4 µm for MMIs have been measured at foci as far as 43.7 µm into the microfluidic channel

Ateya

Camou

Dongre

Godin

Hamish C. Hunt

Hunt

James S. Wilkinson

Moreno

Soldano

English

Crossref

Optics Letters

Multimode interference devices for focusing in microfluidic channels

Hunt, Hamish C.

Wilkinson, James S.

Southampton (e-Prints Soton)

sputtered  on  top  of  the  etched  waveguide  cores. Conventional  photolithography  and  Ar  ion  milling  was then used to etch a microfluidic channel of depth 9  m and width 100  m through the centre of the multimode region of the MMIs, perpendicular to the waveguides.  A microfluidic channel width of 100  m was chosen to allow easier experimental determination of the focal position. BPM  simulations  show  that  widening  the  microfluidic channel  to  this  extent  does  not  significantly  affect  the focal position with respect to the center of the microfluidic channel. Finally, to seal the microfluidic channel a glass cover  slip  was  attached  on  top.  For  the  purposes  of comparison, single mode waveguides crossing the same microfluidic channel were also included in the design.  The  microfluidic  channel  was  filled  with  an  aqueous solution of Cy 5.5 fluorophore with a concentration of 50  M, to allow visualization of light propagating across it. Light  at  a  wavelength  of  633  nm  was  coupled  into  a central  input  waveguide  and  fluorescence  images  were taken from above the chip using a microscope equipped with a CCD camera and a filter to remove 633 nm light but pass fluorescent light at wavelengths beyond 675 nm. Figure 4 shows fluorescent images taken for the beam from (a) a simple monomode channel waveguide crossing the microfluidic channel and (b) a general imaging MMI of length L=4960  m. Figures 4 (c) and (d) show these intensity data normalized to power at each cross section (to eliminate the effects of absorption and scattering) with the  dashed  black  line  fit  indicating  the  1/e2  intensity contours acquired by fitting the equation for a Gaussian beam  to  the  spotsizes  measured  along  the  beam.  The spotsizes (1/e2 intensity) themselves were found by fitting Gaussian curves to the intensity distribution in the x axis.  Fig. 4. (Color online) Fluorescent images of microfluidic channels for a channel waveguide (a) and an MMI (b) and Gaussian beam best fit (c) and (d), respectively. In  these  images  of  the  100   m  wide  microfluidic channel, the left hand side (z=0) represents one wall of the microfluidic channel while the right hand side (z=100  m) represents the further wall of the microfluidic channel, so that only the light in the microfluidic channel is shown. The spotsize of the beam waist formed in the microfluidic channel for this MMI with L=4960  m, was 2.4  m and the position of the focus was 43.7  m into the microfluidic channel.  Thus  the  focal  point was  ~5004   m  from  the beginning of the MMI, compared with the initial BPM design distance of 4998  m (4988  m to the channel wall), an error of 6  m. This error is small considering the non ideal cross sections of the fabricated waveguides and the strong  dependence  of  focal  position  on  MMI  width described by Equations 1 and 2. The waveguide modal spotsize along the y axis is 1.6 m which, in an aqueous medium, will diffract in the y direction to 2.5  m over a 20  m microfluidic channel width and to 9.7  m over a 100  m microfluidic channel. For comparison, the spotsize at the  microfluidic  channel  wall  of  a  channel  waveguide mode emerging directly into the microfluidic channel was 2.5 ± 0.5  m, and this mode diverged into the microfluidic channel so that it exhibited a spotsize of 9.4 ± 0.5  m at a distance of 43.7  m into the microfluidic channel, where the MMI formed its focus. The MMI clearly shows the ability  to  reimage  the  spot  from  the  end  of  a  channel waveguide  at  a  distance  into  the  microfluidic  channel selected by MMI length. These MMIs are estimated to exhibit an excess loss of less than 0.2 dB, and inclusion of an MMI in a cytometer chip, replacing a short length of waveguide  between  the  chip  edge  and  the  microfluidic channel,  is  not  expected  to  increase  insertion  loss measurably. In conclusion, the multimode interference device (MMI) has been presented as an alternative to the micro lens for use  in  the  lab on a chip,  showing  good  performance  in imaging  a  spot  in  a  microfluidic  channel  for  microflow cytometry.  The  design  was  initiated  using  simple theoretical  expressions  and  then  optimized  by  BPM simulation. MMIs were realized in a silica glass material system and characterized to show imaging of a spot into the microfluidic channel. These devices pave the way to the  full  integration  of  more  robust  and  complex microfluidic microflow cytometers. The  authors  would  like  to  thank  Dr.  P.  Hua  for preparation of the fluorescent media and Dr. D. 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Microlenses for flow cytometry,”

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The good, the bad, and the tiny: a review of microflow cytometry,”

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Multimode interference devices for focusing in microfluidic channels

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