3D-printed miniature spectrometer for the visible range with a ......the visible range from 490 nm...

11
Article Open Access 3D-printed miniature spectrometer for the visible range with a 100 × 100 μm 2 footprint Andrea Toulouse 1,2 , Johannes Drozella 1,2 , Simon Thiele 1,2 , Harald Giessen 2,3 and Alois Herkommer 1,2 Abstract The miniaturisation of spectroscopic measurement devices opens novel information channels for size critical ap- plications such as endoscopy or consumer electronics. Computational spectrometers in the micrometre size range have been demonstrated, however, these are calibration sensitive and based on complex reconstruction al- gorithms. Herein we present an angle-insensitive 3D-printed miniature spectrometer with a direct separated spa- tial-spectral response. The spectrometer was fabricated via two-photon direct laser writing combined with a super- fine inkjet process. It has a volume of less than 100 × 100 × 300 μm 3 . Its tailored and chirped high-frequency grat- ing enables strongly dispersive behaviour. The miniature spectrometer features a wavelength range of 200 nm in the visible range from 490 nm to 690 nm. It has a spectral resolution of 9.2 ± 1.1 nm at 532 nm and 17.8 ± 1.7 nm at a wavelength of 633 nm. Printing this spectrometer directly onto camera sensors is feasible and can be replicated for use as a macro-pixel of a snapshot hyperspectral camera. Introduction The field of micro-optics has been transformed by the use of femtosecond direct laser writing as a 3D printing technology since the early 2000s. Both the complexity and surface quality have advanced from simple microlenses 1,2 and achromats 3 to multi-lens and multi-aperture objectives 4,5 . A similar development has occurred with diffractive optics, which have evolved from simple gratings 6 to stacked dif- fractive microlens systems 7 . Along with imaging optics, photonic crystals 8 , waveguides 9,10 , and collimators 11 , com- plex beam shapers 1214 have been demonstrated. This rapid development reflects the significant potential of 3D print- ing technology in micro-optics. Advancements have en- abled access to the millimetre scale and larger with 3D printing technology 15,16 . Meanwhile, the almost unlimited optical design freedom that it offers makes further mini- aturisation possible, thereby increasing the given function- ality in decreasing volumes on the micrometre scale. The aim of this study was to enhance both the complexity and miniaturisation of 3D-printed micro-optics and to create an entire integrated measurement system-a spectrometer-in a 100 × 100 × 300 μm 3 volume. The miniaturisation of spectroscopic measurement devices opens novel information channels for size-critical applications. For instance, medical engineering, consumer electronics, and downscaled chemical engineering could benefit from a cost-effective, efficient, and readily integ- rated spectroscopic micro-device. Spectra could be re- trieved from the tip of a distal-chip endoscope with a bend- ing radius smaller than that of an optical fibre to explore regions that are otherwise inaccessible. With the size of one or two orders of magnitude smaller than a smartphone camera objective, integration into consumer electronics is apparently realisable for applications such as skin disease diagnosis 17 and counterfeit bank note detection 18 . Further- © The Author(s) 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, dis- tribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the articles Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the articles Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Correspondence: Andrea Toulouse ([email protected]) 1 Institute of Applied Optics (ITO), University of Stuttgart, Pfaffenwaldring 9, 70569 Stuttgart, Germany 2 Research Center SCoPE, University of Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany Full list of author information is available at the end of the article. Toulouse et al. Light: Advanced Manufacturing (2020)1:5 Official journal of the JHL 2689-9620 https://doi.org/10.37188/lam.2020.005 www.light-am.com

Transcript of 3D-printed miniature spectrometer for the visible range with a ......the visible range from 490 nm...

  •  

    Article Open Access

    3D-printed miniature spectrometer for thevisible range with a 100 × 100 μm2 footprintAndrea Toulouse1,2, Johannes Drozella1,2, Simon Thiele1,2, Harald Giessen2,3 and Alois Herkommer1,2

    Abstract

    The miniaturisation of spectroscopic measurement devices opens novel information channels for size critical ap-plications such as endoscopy or consumer electronics. Computational spectrometers in the micrometre size rangehave been demonstrated, however, these are calibration sensitive and based on complex reconstruction al-gorithms. Herein we present an angle-insensitive 3D-printed miniature spectrometer with a direct separated spa-tial-spectral response. The spectrometer was fabricated via two-photon direct laser writing combined with a super-fine inkjet process. It has a volume of less than 100 × 100 × 300 μm3. Its tailored and chirped high-frequency grat-ing enables strongly dispersive behaviour. The miniature spectrometer features a wavelength range of 200 nm inthe visible range from 490 nm to 690 nm. It has a spectral resolution of 9.2 ± 1.1 nm at 532 nm and 17.8 ± 1.7 nm ata wavelength of 633 nm. Printing this spectrometer directly onto camera sensors is feasible and can be replicatedfor use as a macro-pixel of a snapshot hyperspectral camera.

     

    IntroductionThe  field  of  micro-optics  has  been  transformed  by  the

    use  of  femtosecond  direct  laser  writing  as  a  3D  printingtechnology since the early 2000s. Both the complexity andsurface  quality  have  advanced  from  simple  microlenses1,2

    and achromats3 to multi-lens and multi-aperture objectives4,5.A similar development has occurred with diffractive optics,which  have  evolved  from  simple  gratings6  to stacked   dif-fractive  microlens  systems7.  Along  with  imaging  optics,photonic  crystals8,  waveguides9,10,  and  collimators11,  com-plex beam shapers12–14 have been demonstrated.  This  rapiddevelopment reflects  the  significant  potential  of  3D print-ing technology  in  micro-optics.  Advancements  have   en-abled  access  to  the  millimetre  scale  and  larger  with  3Dprinting  technology15,16.  Meanwhile,  the  almost  unlimited

    optical design  freedom  that  it  offers  makes  further   mini-aturisation possible, thereby increasing the given function-ality  in  decreasing  volumes  on  the  micrometre  scale.  Theaim of this study was to enhance both the complexity andminiaturisation of 3D-printed micro-optics and to create anentire  integrated  measurement  system-a  spectrometer-in  a100 × 100 × 300 μm3 volume.The  miniaturisation  of  spectroscopic  measurement

    devices  opens  novel  information  channels  for  size-criticalapplications.  For  instance,  medical  engineering,  consumerelectronics,  and  downscaled  chemical  engineering  couldbenefit from  a  cost-effective,  efficient,  and  readily   integ-rated spectroscopic  micro-device.  Spectra  could  be   re-trieved from the tip of a distal-chip endoscope with a bend-ing  radius  smaller  than  that  of  an  optical  fibre  to  exploreregions  that  are  otherwise  inaccessible.  With  the  size  ofone or two orders of magnitude smaller than a smartphonecamera  objective,  integration  into  consumer  electronics  isapparently  realisable  for  applications  such  as  skin  diseasediagnosis17  and  counterfeit  bank  note  detection18.  Further-

     

    © The Author(s) 2020Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, dis-tribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a

    link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article′sCreative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article′s Creative Commons license andyour intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. 

    Correspondence: Andrea Toulouse ([email protected])1Institute of Applied Optics (ITO), University of Stuttgart, Pfaffenwaldring9, 70569 Stuttgart, Germany2Research Center SCoPE, University of Stuttgart, Pfaffenwaldring 57,70569 Stuttgart, GermanyFull list of author information is available at the end of the article.

    Toulouse et al. Light: Advanced Manufacturing (2020)1:5 Official journal of the JHL 2689-9620https://doi.org/10.37188/lam.2020.005 www.light-am.com

     

    http://creativecommons.org/licenses/by/4.0/mailto:[email protected]

  • more,  chemical  micro-reactors  (such  as  that  proposed  byPotyrailo  et  al.19)  could  be  further  miniaturised  by  preciseabsorption  and  emission  line  observation  via  3D-printedminiature spectrometers integrated into a microreactor plat-form.Various  concepts  for  miniaturised  spectrometers  have

    been  demonstrated  and  commercialised.  There  are  threemain categories  for  these  spectrometers:  direct,   computa-tional  and  filter-based.  Spectrometers  in  the  first  categorycreate a  spatial-spectral  response  by  redistributing   incom-ing light,  which can be measured directly by a line or im-age  sensor.  Spectrometers  in  the  second  category  create  amixed  but  spectrum-dependent  unique  signal  from  whichthe original  spectrum  can  be  computationally   reconstruc-ted (wavelength  multiplexing).  The  third  category   com-bines  all  concepts  with  narrowband  wavelength  filteringper  filter  patch.  The  3D-printed  spectrometer  presented  inthis paper is assigned to the first class of direct spectromet-ers.To  the  best  of  our  knowledge,  direct  spectrometers

    presented in the literature possess a footprint area that is atleast  two  orders  of  magnitude  larger  than  our  3D-printedspectrometer  (see  Fig.  S1).  These  range  from commercialgrating  spectrometers20,21  and  scanning  micro-opto-electro-mechanical  system  spectrometers22  to integrated   ap-proaches  with  digital  planar  holograms23  or  ring-resonatorenhancement24.  An  exceptionally  high  ratio  of  bandwidthper resolution  has  been  reached  with  a  grating  multiplex-ing  approach  in  a  centimetre-size  range25.  Meanwhile,  thefootprint  is  reduced  by  two  to  six  orders  of  magnitude  inour 3D-printed  spectrometer,  whereas  the  spectral   band-width per resolution ratio of these direct spectrometers canbe one to three orders of magnitude higher.In the computational spectrometer category, several new

    technologies  have  been  proposed,  such  as  colloidalquantum  dot26  or  photonic  crystal  slab27  patches  forwavelength  multiplexing.  The  single  patches  are  in  the100-micrometre size range; however, an entire spectromet-er consists of multiple patches. Thus, the filter size must bemultiplied  accordingly.  It  is  our  understanding  that  onlytwo  spectrometers  have  been  demonstrated  in  which  thesystem  sizes  are  of  the  same  order  of  magnitude  as  ourfootprint of 100 × 100 μm2. They are based on nanowires28,29

    or  disordered  photonic  structures30, and  they  have  a  band-width  per  resolution  ratio  that  is  similar  to  the  one  in  ourapproach. However, owing to wavelength multiplexing, thesystem must  be  calibrated  with  an  iterative  reconstructionalgorithm to deduce the original  spectrum. In contrast,  wepresent an  angularly  insensitive  spectrometer  with  a   dir-ectly separated spatial-spectral response.The third category-filter-based spectrometers-is based on

    traditional  concepts,  such  as  dyes  or  Fabry-Perot  etalons,along with  recent  concepts,  such  as  nanowire  grids,   plas-monic  nanohole  arrays,  quantum dots31,  colloidal  quantumdot sensors combined with Bragg mirror filter arrays32, andultrafast  pyroelectric  metasurfaces33.  However,  owing  totheir narrow bandwidth filtering, a considerable portion ofthe signal  is  lost.  In  contrast,  our  3D-printed spectrometercollects light with a numerical aperture (NA) that is largerthan  0.4-a  light  collection  area  of  50 × 50  μm2  at  the  firstlens-and redistributes the wavelengths with minimal losses.Overall, the  3D-printed  miniature  spectrometer   presen-

    ted  herein  marks  an  innovation  for  3D-printed  micro-op-tics  in  terms  of  complexity,  while  exploiting  for  the  firsttime the micrometre size range for direct spectrometers.

    Results

    y

    We  assessed  the  performance  of  the  spectrometer  in  aminiature volume of 100 × 100 × 300 μm3 for a wavelengthrange of  490 nm to  690 nm.  In  the  design concept Fig. 1,the strengths  of  3D printing  are  considered:  freeform sur-faces and inherent  near-perfect  alignment  owing to  simul-taneous  mount  and  lens  fabrication,  which  enable  heavilytilted and asymmetrical  surfaces.  Incoming light  is  collec-ted  by  a  lens  with  a  quadratic  aperture  of  50 × 50  μm2,which  corresponds  to  an  NA of  0.42  in  the  -direction  atthe slit. The slit is realised not in air but in a photopolymerto  serve  simultaneously  as  an  ink  barrier.  The  divergingrays exiting  the  slit  are  collimated  by  the  subsequent   sur-face. This collimation lens is terminated by a tilted surfaceto  compensate  in  advance  the  grating  deflection  angle,thereby  maintaining  the  spectrometer  within  its  narrowfootprint.  The  grating  surface  is  then  illuminated  by  thecollimated rays. The last surface serves as an imaging lensto separate the wavelengths at an output plane nearby, suchas for an image sensor in future applications. The full spec-trometer  thus  has  a  length  below 300 μm,  which is  insidethe  high-precision  piezo  z-stage  range  of  the  3D  printer.This  last  surface  is  also  tilted  with  respect  to  the  gratingsurface and thus restricts the chief ray angles (CRAs) at theoutput image plane to a maximum angle of 16° at a 690 nmwavelength. Therefore,  each  field  could  be  coupled   effi-ciently  without  crosstalk  into  an  image  sensor  pixel  withCRA limitations.The implementation of the described design principle is

    shown as a sequential  ray-tracing model  in Fig. 1b. In ex-tending  the  general  design  concept,  slight  curvatures  andaspherical extensions  were  permitted  for  the  tilt  and  grat-ing surfaces to minimise aberrations. Detailed surface typedescriptions are  given  in  the  Materials  and  Methods   sec-tion. The most important functional surfaces of the spectro-meter are  highlighted  on  the  right  side  of  the  figure.   Be-

    Toulouse et al. Light: Advanced Manufacturing (2020)1:5 Page 2 of 11

     

  • cause the spectrometer size is in the micrometre range, dif-fraction  must  be  considered  for  the  slit  design.  The  slitwidth was designed to be 1.3 μm to create a single slit dif-fraction  pattern  with  an  opening  angle  that  is  suitable  forthe  collection  NA of  0.42  (with  regard  to  its  first  minimapositions at 550 nm).The  grating  is  designed  to  consist  of  as  many  lines  as

    can be manufactured to maximise the spectrometer resolu-tion. The fabrication process generally accommodates fea-ture sizes down to 100 nm34. However, if the photoresist isprinted as a bulk material, as in the case of the grating, theresolution decreases owing to the accumulated laser power,and thus polymerisation increases in the laser focus vicin-ity.  Consequently,  the  grating  period  was  restricted  to  aminimum period of 650 nm to be sufficiently resolved (seeFig. S3).The  first  diffraction  order  of  the  grating  is  used  as  the

    spectrometer  measurement  signal.  The  grating  topographywas thus established for maximum diffraction into the firstdiffraction order. Both the zeroth and second order are spa-tially separated such that the measurement signal is not af-fected.  The  period  is  chirped  and  spans  from  650  nm  to860 nm across the grating width of 38 μm. This is a consid-erably smaller portion of the 100 μm footprint for two reas-ons.  First,  the total  length of the spectrometer is  restrictedto 

  • from the first order of interest without exceeding the foot-print. Accordingly,  multiple  spectrometers  could  be   prin-ted in  close  vicinity  without  crosstalk.  The  first-order  dif-fraction  is  visible  as  being  coloured  with  separated  foci,while the non-dispersed zeroth order is initially reflected atthe right lens edge and redirected to the right part of the slitimage  plane,  which  is  well  separated  from  the  first-orderfoci.

    z

    Furthermore, the focal length of the last lens was optim-ised  such  that  the  first  diffraction  order  is  focused  withinthe working distance of the printing objective in the  -dir-ection (

  • Dip39 combined with the spectral distribution of the mono-chromator that was used for the measurement (see Fig. S2).The  average  slit  width  at  full  width  half  maximum(FWHM) is  1.3  μm,  which  is  in  good agreement  with  thedesign.  To  quantify  the  signal-to-noise  ratio,  a  sinc2  func-tion with a FWHM of 1.3 μm is subtracted from the meas-urement data, and the remaining positive signal is regardedas noise. The maximum noise level is 15%, while the aver-age noise level is 5% ± 3%.

    f = 2

    The miniature  spectrometer  was  experimentally   charac-terised  using  the  setup  depicted  in Fig. 3.  A fibre-coupledmonochromator or  laser  (only  for  the  linewidth  measure-ment)  is  used  as  illumination  source.  The  output  facet  ofthe  fiber  is  imaged with  a  100x microscope objective  (fo-cal  length   mm) to  the  front  of  the  3D-printed  spec-trometer.  This  configuration  is  used  for  multiple  reasons.First, the  illumination  is  concentrated  on  the  region of   in-terest,  namely  the  spectrometer.  Second,  stray  light  fromoutside  the  100 × 100  μm2  footprint  is  suppressed.  In  areal-world  application,  stray  light  could  be  shielded  by  achromium  aperture  without  tight  alignment  tolerances  atthe  spectrometer  front.  Third,  the  angular  extent  of  thefibre facet  emission  is  increased  owing  to  the  magnifica-tion of −0.13 and the high NA of 0.9 of the described ima-ging  setup.  Accordingly,  the  angular  insensitivity  of  thespectrometer, that is, the successful spatial filtering, can bedemonstrated.On  the  microscope  side  of  the  measurement  setup

    Fig. 3b,  the  dispersed  slit  image  plane  is  recorded  with  a50x  microscope  objective  with  an  NA  of  0.55  combined

    y

    with a tube lens and a monochromatic image sensor. In therecorded  images,  the  first  diffraction  order  is  visible  as  atight  line  that  moves  in  the  -direction  when  shifting  thewavelength. In contrast, the zeroth order is stationary at theleft  end  of  the  recorded  images.  To  measure  the  spatial-spectral  response,  an  intensity  profile  across  the  centre  ofthe first-order movement range is recorded, as indicated bythe white dashed line.In  Fig. 4a,  the  normalised  spatial-spectral  response

    measured  at  the  slit  image  plane  of  the  spectrometer  isshown for a wavelength range from 490 nm to 690 nm in10 nm steps. The peaks of the single profiles are well sep-arated, and the medium noise level is on the order of 5% to20%.  Each  profile  can  be  fitted  with  a  sinc2  function  toeliminate the noise, as depicted in Fig. 4b. The width of thesingle  wavelength  profiles  underestimates  the  resolutioncapacity of  the  miniature  spectrometer  because  the  mono-chromator-line  spectral  width  has  a  similar  magnitude  asthe resolution capacity to be measured. Fig. 4e depicts theresolution capacity.

    yThe  wavelength-dependent  centre  positions  of  the  sinc2

    fits  are plotted as  -positions or wavelength shifts  per mi-crometre, respectively, in Fig. 4c and d. These fitted meas-urement data are consistent with the respective ray-tracingsimulation data;  however,  the  wavelength  shift  per  micro-metre is  larger  in the measurement than in the simulation.This is presumably due to the shrinkage of the photoresist,which leads to a focus shift towards the last lens block andthus to closer spacing of the wavelengths.The  spectral  resolution  of  the  miniature  spectrometer

     

    a bMonochromator

    or laser

    Multimode fiber

    λi

    Image of fiber facet Dispersed slit image

    Monochromatic

    image sensor

    Illumination

    100× objective 50× objective

    Miniature

    spectrometer

    Counts

    Pix

    els

    Tube lens

    Microscope

    λn

    λ1 λnλ1

    λi

    Fig. 3      Measurement  setup. a  Illumination  side:  A  fibre-coupled  monochromator  or  laser  is  imaged  to  the  front  of  the  miniature  spectrometer.b Microscope side: The image plane of the spectrometer is magnified with a microscope. The dispersed image of the slit (first diffraction order, right) shifts correspond-ing to the illumination wavelength while the zeroth diffraction order (left) is stationary and spatially separated from the region of interest. The profile position for fur-ther evaluations is indicated as a white dashed line. The monochromatic measurements of the fibre facet and slit images are coloured for visualisation. Scale bars 20 μm

    Toulouse et al. Light: Advanced Manufacturing (2020)1:5 Page 5 of 11

     

  • λ = 532λ = 633

    532633

    was  assessed  with  narrow bandwidth  illumination  sources(green  laser  at    nm  and  helium  neon  laser  at

     nm) (Fig. 4e). These two laser wavelengths samplethe  resolution  in  the  shorter  wavelength  range  (   nm)and  the  longer  wavelength  range  (   nm)  of  the  totalspectral range of our spectrometer. For speckle reduction, avibrating  diffuser  was  integrated  before  coupling  into  themultimode fibre,  and  multiple  measurements  were   ob-tained  for  temporal  averaging.  At  a  532  nm  illuminationwavelength, the measured profile has a centre peak with acomparable  and  even  slightly  narrower  FWHM linewidththan the profile simulated with the WPM. In exchange, theside  lobes  are  more  pronounced.  This  behaviour  may  bedue  the  non-uniform  illumination  of  the  slit  or  grating,comparable to  the  intensity  distribution  of  a  Bessel   func-tion as the diffraction pattern of a ring pupil. In such a case,the FWHM width is smaller than that of the correspondingairy disc, while the side lobes carry more energy40. The res-olution  at  532  nm  is  determined  from  the  FWHM  of1.01 μm indicated in the graph.From Fig. 4d,  the  measurement  data  of  the  wavelength

    range from 500 nm to 560 nm are averaged to calculate the

    medium wavelength shift  per  micrometre  around the  laserwavelength of 532 nm. Multiplication of this value by theFWHM yields a spectral resolution of 9.2 ± 1.1 nm, whichis close to the diffraction limit of 10.4 nm and the wave-op-tical linewidth simulation of 12.5 ± 1.5 nm. The linewidthmeasurement at  the  633  nm  wavelength  is  evaluated   ac-cordingly. Here,  the  medium  wavelength  shift  per  micro-metre in the range from 600 nm to 660 nm is considered. Ityields  a  spectral  resolution  of  17.8  ±  1.7  nm  at  awavelength of 633 nm. The wave-optical linewidth simula-tion suggests a slightly better resolution of 14.5 ± 1.4 nm.This deviation could be due to imperfections in the refract-ive  optical  surfaces  printed  with  the  high-resolutionphotoresist IP-Dip, which has some deficiencies in terms ofsmoothness.Finally, the spectrum of an unknown light source (broad-

    band  multiple  LED  light)  was  recorded  and  compared  tothat of a commercial spectrometer (see Fig. 5). Because ofthe relatively high noise level, an averaged, weighted noisesignal  was  subtracted  from  all  the  measured  spectra  (seethe Materials and Methods section and Supplementary Ma-terial  for  further  information).  A  wavelength-dependent

     

    a

    b

    c e

    d

    1.01 μm

    2.48 μm

    690 nm490 nm1.0

    0.8

    0.6

    0.4

    0.2

    0

    −10 0 10 20y (μm)

    y (μ

    m)

    ΔλΔy

    −1/(n

    m μ

    m−1

    )

    −10 0 10y (μm)

    I (a.

    u.)

    1

    0

    I (a.

    u.)

    Raytracing simulationMeasurement

    Wave-optical simulationMeasurement

    λ = 633 nm

    λ = 532 nm

    500 550 600 650 700

    λ (nm)

    1.0

    0.8

    0.6

    0.4

    0.2

    0

    −10 0 10 20y (μm)

    −10 0 10y (μm)

    I (a.

    u.)

    1

    0

    I (a.

    u.)

    500 550 600 650 700

    λ (nm)

    40

    20

    20

    25

    10

    15

    0

    5

    0

    −20

    Fig. 4    Spatial-spectral response of the miniature spectrometer. a Measured normalised intensity profiles at the image plane of the spectrometer for illu-mination wavelengths ranging from 490 nm to 690 nm in 10 nm steps (monochromator, profile position is indicated in Fig. 3b). b Sinc2 fits of the intensity profiles froma. c Centre positions of the sinc2 fits per wavelength. d Wavelength shift per micrometre deduced from c. e Linewidth simulation and measurement with a red or greenlaser, respectively. The measured full width at half maximum is indicated with a pair of arrows. The combination of measurements d and e yield a spectral resolution of9.2 ± 1.1 nm at 532 nm and 17.8 ± 1.7 nm at 633 nm wavelength

    Toulouse et al. Light: Advanced Manufacturing (2020)1:5 Page 6 of 11

     

  • calibration  factor  was  determined  by  the  quotient  of  aknown  spectrum  (halogen  lamp)  that  was  measured  withboth our spectrometer and a commercial spectrometer. Themeasurement  of  the  unknown  light  source  was  multipliedby this calibration factor. The unknown spectrum measure-ment  shows  a  slightly  lower  signal  in  the  range  of  500-570  nm  and  a  slightly  higher  signal  in  the  range  of  620-670  nm,  with  overall  satisfactory  accordance,  although  ithas a  footprint  eight  orders  of  magnitude  smaller.   Espe-cially in the lower wavelength range, small features at dis-tinct wavelengths that are visible on the spectrum recordedusing the  commercial  spectrometer,  are  distinctly  percept-ible in the measurement obtained using our spectrometer.

    DiscussionThe  study  presented  herein  proves  the  feasibility  of  a

    fully  functional  spectrometer  fitted  in  a  volume  of  only100 × 100 × 300 μm3. The size comparison with a standardsmartphone  camera  lens  Fig. 6  highlights  the  potential  ofthis technology. At a size of more than one order of mag-nitude  smaller  than  the  objective,  the  spectrometer  can  beintegrated almost  non-invasively  into  a  smartphone   cam-era, i.e. printed directly onto the camera sensor, such as forsingle  spectral  measurements  or  as  individual  pixels  of  asnapshot hyperspectral camera.The  feasibility  of  two-photon  3D  printing  directly  onto

    image sensors has been demonstrated in previous studies4,5.It should  be  noted  that  the  wavelength  shift  per   micro-metre has a value of 9.1 ± 1.1 nm/μm at 532 nm and 7.2 ±0.7 nm/μm at 633 nm. Therefore, a monochromatic imagesensor with  a  pixel  pitch  below 1 μm must  be  used to   re-solve the spectral lines. Current trends in this direction areevident.  For example, Samsung has announced a 47.3 MPsensor  with  a  0.7  μm  pixel  pitch  (ISOCELL  Slim  GH1).Moreover,  high-resolution  monochromatic  image  sensors

    have  entered  the  mass  market  through  Huawei  Mate  9smartphone.

    12·200 nm ·

    (1∆λ532

    +1∆λ633

    )= 16.5

    Furthermore, the  implementation of  a  spectrometer   fab-ricated strictly by 3D printing can be highlighted as a nov-elty.  The  measured  ratio  of  bandwidth  per  resolution  of

      is  in  a  margin  similar

    to those  of  other  spectrometers  that  have  been   demon-strated in this size range28,30. In the category of direct spec-trometers, the microspectrometer presented here is the firstof its kind in this size range (see Fig. S1).

    z

    Nevertheless, our spectrometer has a limitation in that arelatively  high  noise  level  can  be  identified.  A  possiblesolution could be the design of an ink basin with improvedsuppression enabled by an increase in the ink layer that  isclose  to  the  slit.  However,  a  slit  elongated  in  the    direc-tion  for  improved  absorptive  ink  layer  thickness  wouldlikely  decrease  the  spectrometer  light  efficiency.  Anotherpart  of  the  noise  could  originate  in  the  utilisation  of  thehigh-resolution  photoresist  IP-Dip,  which  enables  smallvoxel sizes and is thus well suited for the creation of high-frequency grating structures. However, a deficiency of thismaterial is the roughness of the fabricated surfaces. Kirch-ner et al.41 reported the surface roughness of IP-Dip on theorder of 100 nm, which makes this photoresist type disad-vantageous for  refractive  lenses.  Since  diffractive  and   re-fractive structures are both part of the spectrometer, the re-fractive surfaces are compromised.  The use of  a  smootherphotoresist, for example, IP-S, which can exhibit an optic-al-quality root  mean  square  surface  roughness  that  is   be-low  10  nm14,  could  be  examined  in  future  work.  Thisphotoresist may be beneficial for spectrometers with a lar-

     

    1.0

    0.8

    0.6

    0.4

    0.2

    0

    500 550 600 650 700

    λ (nm)

    3D-printed spectrometer

    Commercial spectrometer

    I (

    a.u.)

    Fig. 5      Comparison  of  a  spectrum measured  with  the  3D-prin-ted spectrometer and with a commercial spectrometer 

     

    1 mm

    100 μm

    Fig. 6      Size  comparison  of  the  3D-printed  spectrometer  (whitebox) with  an  iPhone  5s  camera  lens.  The  inset  shows  a   micro-scope  image  of  the  fabricated  spectrometer  (left)  and  its  opticaldesign principle (right) 

    Toulouse et al. Light: Advanced Manufacturing (2020)1:5 Page 7 of 11

     

  • ger footprint and lower grating frequencies. Multi-materialprinting,  as  demonstrated  by  Schmid  et  al.3  and  Mayer  etal.42,  could  resolve  these  conflicting  requirements.  Furtheroptimisation  of  3D-printing  parameters,  as  proposed  byChidambaram  et  al.43,  could  lead  to  smoother  surfaces  atthe expense of printing time. The printing parameters havethus far been optimised with a focus on the slit and grating,which both require high resolution instead of smoothness.As  presented  in  the  introduction,  our  spectrometer  is

    classified in the direct spectrometer category because of itsdirect  spatial-spectral  response.  However,  in  combinationwith a computational approach, the (static) noise can be in-terpreted  as  a  multiplexed  wavelength  signal.  An  iterativereconstruction algorithm, such as that used by Bao et al.26,could  be  employed  to  sharpen  the  spectral  lines  of  ourspectrometer. In its current configuration, the spectrometercan readily be used as a wavemeter because the centres ofthe sinc2 fits yield distinct results in the presence of noise.As  a  miniature  wavemeter,  it  could  be  employed  as  ahighly integratable laser wavelength stabiliser.Additionally,  the  wavelength  range  of  the  spectrometer

    presented  here  is  extendable.  3D  printing  comes  with  ahigh  degree  of  individuality  at  a  comparably  low  cost.Therefore,  an  array  of  slightly  different  spectrometerscould be fabricated, with each being optimised for anotherpart of the spectrum. This is a major advantage with regardto spectrometers fabricated using non-classical approaches,such  as  colloidal  quantum  dots26,  photonic  crystal  slabs27,nanowires28,  and  disordered  photonic  structures30.  Thesemethods have either a rather small bandwidth or cannot beintrinsically extended to the infrared region. In our classic-al  grating  spectrometer  approach,  the  only  limit  forwavelengths in  the  infrared  region  is  either  the   transmit-tance of the photoresist or the absorption of the sensor thatis used to record the signal. The photoresist IP-Dip has anextinction  coefficient  well  below 0.1  mm−1  in  the  infraredregion up to a 1 600 nm wavelength39.  Thus, our approachcould readily be employed in this region.Moreover, the  design  can  be  based  on  the  same   prin-

    ciple,  as  presented  in  Fig. 1. By  leveraging  minor   adap-tions  of  surface  tilts,  the  grating  phase  profile,  and  re-op-timisation of  the  aspherical  surface  coefficients,  the   spec-tral  cut-out  could  be  shifted  towards  longer  wavelengths.For instance, a grid of four individual spectrometers couldenable a spectrometer with a wavelength range of 800 nmon a footprint of 200 × 200 μm2.

    ConclusionIn this study, the feasibility of a 3D-printed spectromet-

    er in a miniature volume of 100 × 100 × 300 μm3 was the-oretically assessed and experimentally proven.  The fabric-

    ated spectrometer has a wavelength range of 200 nm in thevisible  range,  and a  spectral  resolution of  9.2  ± 1.1  nm at532 nm and 17.8 ± 1.7 nm at a 633 nm wavelength. To thebest of the authors' knowledge, the presented spectrometeris the  first  spectrometer  with  a  direct  spatial-spectral   re-sponse that has been demonstrated in this size range. Com-pared to computational and filter-based approaches, its an-gular  insensitivity  and  relatively  large  aperture  of  50  ×50 μm2 are emphasised. A hybrid direct computational ap-proach could significantly improve the average noise levelof 5 to 20%. Wavelength range extensions of 200 nm per a100 × 100 μm2 footprint were proposed in a multi-apertureapproach  by  implementing  the  same  design  principle  foradjacent parts of the spectrum. Possible applications rangefrom  endoscopy  devices  and  consumer  electronics  tochemical microreactors.

    Materials and methods

    x ztoroidal (y)

    The proposed spectrometer was designed using ZEMAXOpticStudio optical design software in the sequential modefor  a  wavelength  range  of  490  nm  to  690  nm.  The  glassmodel that was utilised is the photoresist IP-Dip presentedby Gissibl et al.44. For ray tracing, the spectrometer was di-vided into two parts: the light collector (part 1, top lens toslit) and the dispersing imager (part 2, slit to the dispersedslit  image  plane).  All  surfaces  except  the  grating  surfaceand the  photoresist/air  interface  behind  the  slit  are   de-scribed by a toroidal surface with a rotation radius of infin-ity.  The surfaces  have a  cylinder-like  shape with  a  50 μmextension in the  -direction. The surface sags   aredefined by

    ztoroidal (y)=cy2

    1+√

    1−c2y2+a1y2+a2y4+a3y6+a4y8+a5y10 (1)

    c ai

    x = 0

    where curvature   and coefficients   are optimisation vari-ables. Part 1 consists of a toroidal surface with coefficientsup to the second order followed by a propagation distanceof 90 μm inside the photoresist to the slit. In part 2, ray-tra-cing  for  all  fields  starts  at  the  centre  of  the  slit  with  thesame NA as the rays focused by the collector. Accordingly,spatial  filtering at  the slit  is  considered,  and part  2  can beoptimised  for  the  best  imaging  (minimum  spot  width  perwavelength  at  )  of  the  slit  plane.  After  a  distance  of4.5 μm, the photoresist/air interface is simulated by a planeto complete the ink basin described later in this section.

    zgrating (y)

    The following  optical  surfaces,  except  the  grating   sur-face,  are  described  by  Eq.  1  with  coefficients  up  to  thetenth order. The grating surface is described by the sequen-tial  surface-type  elliptical  grating  1,  and  the  surface  sags

     are given by

    Toulouse et al. Light: Advanced Manufacturing (2020)1:5 Page 8 of 11

     

  • zgrating (y) =cb2y2

    1+√

    1−b2y2+a1y+a2y2+a3y3+a4y4+a5y5

    (2)

    b, c ai

    de f f

    where  ,  and  coefficients    are  optimisation  variables.This polynomial  surface  is  superimposed  by  a  phase   pro-file resembling the variable grating period with the effect-ive grating period   defined as

    de f f (y) =1T0+αy+βy2+γy3+δy4 (3)

    T0, α, β, γ δwhere  the  optimisation  variables  are  ,  and  .This  phase  profile  was  translated  into  a  topography  usingMATLAB by applying the first-order blazed condition fora  wavelength  of  550  nm  at  a  fixed  incident  angle  of  thechief ray  to  the  field-dependent  (chirped)  grating   deflec-tion angle.

    dy = dz = 20 x = 0

    E = 0

    Subsequent to the optical design, lens mounts and an inkbasin  were  added  to  the  optimised  lens  surfaces  in  theSolidWorks  CAD  programme.  A  two-dimensional  scalarwave-optical simulation using the wave propagation meth-od35,36  (WPM)  was  performed  with  discretisation  of  thevolume  model  with    nm  at    for  a  planewave normal input that covers the full width of the spectro-meter.  To approximate the absorbing behaviour of  the inkbasin,  a  perfectly  absorbing  ( )  layer  was  introducedto the  model  at  the  inside  edges  of  the  basin.  For  the   res-ults  presented  in Fig. 1d,  a  spectrum simulation  of  490  to690  nm in  steps  of  40  nm was  performed.  Each  resultingintensity distribution was normalised to its own maximumvalue  in  the  observation  plane  and  converted  to  an  RGBrepresentation according to Bruton45 for display purposes.

    y

    In the next step, the CAD model was sliced at a distanceof  100  nm,  hatched  strictly  in  the  direction  of  the  gratinglines  ( -direction)  at  a  distance  of  250 nm,  and fabricatedby  means  of  3D  dip-in  two-photon  DLW  using  PhotonicProfessional GT2  (Nanoscribe  GmbH,  Karlsruhe,   Ger-many)  and  the  proprietary  negative-tone  photoresist  IP-Dip44  on  a  glass  substrate  with  an  ITO  coating.  The  lasersource of the 3D printer is a pulsed femtosecond fibre laserwith  a  centre  wavelength  of  780  nm,  a  specified  averagelaser power output of 120 mW, a pulse length of approxim-ately  100  fs  at  the  source,  and  an  approximate  repetitionrate of 80 MHz. The spectrometer was printed with 7.5 mWlaser power and a scan speed of 15 mm/s. The total print-ing time of the single spectrometer was just below 2 h. Thedispersed slit image plane coincided with the glass surface.The  polymerised  samples  were  developed  in  propyleneglycol methyl ether acetate for 8 min to wash out the unex-posed photoresist.  Subsequently,  the  sample was rinsed inan  isopropanol  bath  for  2  min  and  dried  with  a  nitrogenblower.

    The  slit  was  fabricated  with  the  SIJ-S030  super-fineinkjet  printer  (SIJ  Technology,  Inc.,  Tsukuba,  Japan).  Theink utilised  for  the  creation  of  the  non-transparent   struc-tures was NPSJ (Nanopaste Series,  Harimatec,  Inc.,  Geor-gia,  USA).  It  is  a  conductive  ink  that  comprises  a  silvernanoparticle  content  of  65  mass % with  a  particle  size  of12 nm.  When high  voltage  is  applied,  the  printer  can  dis-pense droplet  volumes of 0.1 fl  to 10 pl  from a needle tipwith  a  diameter  below  10  μm,  which  fits  into  the  gapbetween the collector lens and the ink basin walls. (Detailson this fabrication method are provided in Toulouse et al.37,38.)This  process  was  applied  to  fill  the  ink  basin  that  wasdefined in the CAD design.The spatial-spectral  response  measurements  were   per-

    formed  with  a  fibre-coupled  home-built  monochromatorusing  the  spectrum  of  a  dimmable  150  W quartz  halogenlamp  (model  I-150,  CUDA  Products  Corp.,  FiberopticLightsource, Florida, USA), which was used as the illumin-ation source. The linewidth measurements were performedwith  a  green  laser  source  (532  nm,  5  mW)  or  red  heliumneon  laser  (632.8  nm,  5  mW,  model  05-LLP-831,  MellesGriot,  Darmstadt,  Germany),  respectively,  in  combinationwith a vibrating diffuser coupled into the same fibre as themonochromator (300 μm, NA 0.39, model M69L02, Thor-labs, New  Jersey,  USA).  The  output  fibre  facet  was   im-aged with a 100x objective with NA 0.9 (M Plan Apo HR100X, Mitutoyo,  Kawasaki,  Japan) to the front  of  the 3D-printed spectrometer. The dispersed slit image plane of thespectrometer  was  recorded  with  a  monochromatic  videomicroscope.

    f = 200

    y

    x

    The video microscope consisted of a 50x objective withNA 0.55  (50X  Mitutoyo  Plan  Apo,  Edmund  Optics,  NewJersey, USA) in combination with a   mm tube lens(MT-4, Edmund  Optics,  New  Jersey,  USA)  and  a  mono-chromatic camera (UI-3180CP-M-GL R2, IDS, Obersulm,Germany). For  the  linewidth  measurement,  the  video  mi-croscope was focused on the minimum detectable linewidthper  wavelength,  and the  simulated linewidth  was  offset  inthe  -direction  for  the  best  fit.  Before  each  measurementand for  each wavelength,  the  camera  integration time wasadjusted  to  record  a  high  signal  without  saturation.  Foreach measurement, 20 images and 20 profiles in the  -dir-ection per image were recorded and averaged after subtrac-tion of a dark image. The slit width measurement was sim-ilarly performed; however, instead of the entire spectromet-er, only part 1 (collector lens, slit, and ink basin) was fab-ricated  and  the  video  microscope  was  focused  on  the  slit.Here, the integration time of the camera was adjusted to theoverall maximum signal and was the same for all wavelengths.For all spectrum measurements, the noise was evaluated

    and subtracted (see Supplementary Material for further in-

    Toulouse et al. Light: Advanced Manufacturing (2020)1:5 Page 9 of 11

     

  • Im (y)Im (λ)

    Ith,halogen (λ)

    k (λ) = Ith,halogen (λ)/Im,halogen (λ)

    formation).  The  wavelengths  outside  the  spectrum  of  ourspectrometer  were  filtered  with  a  long-pass  filter  (pass  >500  nm)  and  a  short-pass  filter  (pass  <  700  nm)  (modelsFEL0500 and FES0700, Thorlabs, New Jersey, USA). Thesetup  shown  in  Fig. 3  was  used.  The  measured  profile

      was  translated  into  a  wavelength-dependent  signal by evaluating the centres of the sinc2 fits (see Fig. 4b-

    e).  The  relative  intensity  calibration  of  the  spectrometerwas conducted with a 150 W quartz halogen lamp (modelI-150, CUDA Products Corp., Fiberoptic Lightsource, Flor-ida, USA). A reference spectrum   was recordedwith  a  commercial  spectrometer  (AvaSpec-ULS2048CL-EVO-FCPC, Avantes,  Mountain  Photonics  GmbH,   Land-berg am Lech, Germany) and was normalised to its maxim-um  intensity.  The  calibration  factor  was  calculated  as

    .

    Im,LED,calib (λ) =k (λ) · Im,LED (λ)

    As a  light  source  for  the  unknown  spectrum   measure-ment, a white light LED (model MCWHLP1 and collimat-or  SM2F32-A,  Thorlabs,  New  Jersey,  USA)  was  coupledinto the multimode fibre of the measurement setup. The finalspectrum  measurement  was  calibrated  as 

    .

    AcknowledgementsThis work was funded by Bundesministerium für Bildung undForschung (Printoptics, Printfunction), Baden-Württemberg StiftunggGmbH (Opterial), and the European Research Council (ComplexPlas,3DPrintedOptics). The authors thank Simon Amann for building, provid-ing, and supporting the ITOM-operated monochromator used for themeasurements. We also thank Mario Hentschel for performing the SEMmeasurements of the grating surface.

    Author details1Institute of Applied Optics (ITO), University of Stuttgart, Pfaffenwaldring9, 70569 Stuttgart, Germany. 2Research Center SCoPE, University of Stut-tgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany. 34th Physics Insti-tute (PI4), Pfaffenwaldring 57, 70569 Stuttgart, Germany

    Author contributionsAndrea Toulouse performed the optical and mechanical design, fabrica-tion steps, measurements, evaluations, interpretations, and article writ-ing. Johannes Drozella implemented and executed the wave-opticalsimulations. Simon Thiele, Harald Giessen, and Alois Herkommer de-vised the general concept together with Andrea Toulouse and providedadvice throughout the work, including the conception and writing ofthe article.

    Conflict of interestThe authors declare that they have no conflict of interest.

    Supplementary information is available for this paper at https://doi.org/10.37188/lam.2020.005.

    Received: 09 April 2020 Revised: 12 August 2020 Accepted: 31 August 2020

    Published online: 30 November 2019

    References Guo, R. et al. Log-pile photonic crystal fabricated by two-photon pho-topolymerization. Journal of Optics A: Pure and Applied Optics  7, 396-399 (2005).

    1.

     Malinauskas, M. et al. A femtosecond laser-induced two-photon photo-polymerization technique for structuring microlenses. Journal of Op-tics  12, 35204 (2010).

    2.

     Schmid, M. et al. Three-dimensional direct laser written achromaticaxicons and multi-component microlenses. Optics Letters  43, 5837-5840 (2018).

    3.

     Gissibl, T. et al. Two-photon direct laser writing of ultracompact multi-lens objectives. Nature Photonics  10, 554-560 (2016).

    4.

     Thiele, S. et al. 3D-printed eagle eye: compound microlens system forfoveated imaging. Science Advances  3, e1602655 (2017).

    5.

     Winfield, R. J. et al. Fabrication of grating structures by simultaneousmulti-spot fs laser writing. Applied Surface Science  253, 8086-8090(2007).

    6.

     Thiele, S. et al. 3D printed stacked diffractive microlenses. Optics Ex-press  27, 35621-35630 (2019).

    7.

     von Freymann, G. et al. Three-dimensional nanostructures for photon-ics. Advanced Functional Materials  20, 1038-1052 (2010).

    8.

     Schumann, M. et al. Hybrid 2D-3D optical devices for integrated opticsby direct laser writing. Light: Science & Applications  3, e175 (2014).

    9.

     Lindenmann, N. et al. Photonic wire bonding: a novel concept for chip-scale interconnects. Optics Express  20, 17667-17677 (2012).

    10.

     Thiele, S. et al. Ultra-compact on-chip LED collimation optics by 3Dfemtosecond direct laser writing. Optics Letters  41, 3029-3032 (2016).

    11.

     Schmidt, S. et al. Tailored micro-optical freeform holograms for integ-rated complex beam shaping. Optica  7, 1279-1286 (2020).

    12.

     Dietrich, P. I. et al. In situ 3D nanoprinting of free-form coupling ele-ments for hybrid photonic integration. Nature Photonics  12, 241-247(2018).

    13.

     Gissibl, T. et al. Sub-micrometre accurate free-form optics by three-di-mensional printing on single-mode fibres. Nature Communications  7,11763 (2016).

    14.

     Jonušauskas, L. et al. Mesoscale laser 3D printing. Optics Express  27,15205-15221 (2019).

    15.

     Heinrich, A. & Rank, M. 3D Printing of Optics. (SPIE, 2018).16. Kim, S. et al. Smartphone-based multispectral imaging: system devel-opment and potential for mobile skin diagnosis. Biomedical Optics Ex-press  7, 5294-5307 (2016).

    17.

     Rissanen, A. et al. MEMS FPI-based smartphone hyperspectral imager.Proceedings of SPIE 9855, Next-Generation Spectroscopic Technolo-gies IX. Baltimore, Maryland, United States: SPIE, 2016, 985507.

    18.

     Potyrailo, R. A. et al. Fluorescence spectroscopy and multivariate spec-tral descriptor analysis for high-throughput multiparameter optimiza-tion of polymerization conditions of combinatorial 96-microreactor ar-rays. Journal of Combinatorial Chemistry  5, 8-17 (2003).

    19.

     Hamamatsu Datasheet. Mini-Spectrometer. SMD Series C14384MA-01(2019).

    20.

     Ibsen Photonics. PEBBLE VIS. 380-850 nm OEM Spectrometer (2019).21. Huang, J. et al. Miniaturized NIR spectrometer based on novel MOEMSscanning tilted grating. Micromachines  9, 478 (2018).

    22.

     Calafiore, G. et al. Holographic planar lightwave circuit for on-chipspectroscopy. Light: Science & Applications  3, e203 (2014).

    23.

     Kyotoku, B. B. C., Chen, L. & Lipson, M. Sub-nm resolution cavity en-hanced microspectrometer. Optics Express  18, 102-107 (2010).

    24.

     Pang, Y. J., Yao, M. L. & Liu, S. Grating multiplexing structure basedhigh-resolution infrared spectrometer. Infrared Physics & Technology 104, 103148 (2020).

    25.

     Bao, J. & Bawendi, M. G. A colloidal quantum dot spectrometer. Nature 523, 67-70 (2015).

    26.

    Toulouse et al. Light: Advanced Manufacturing (2020)1:5 Page 10 of 11

     

    https://doi.org/10.37188/lam.2020.005https://doi.org/10.37188/lam.2020.005https://doi.org/10.37188/lam.2020.005https://doi.org/10.37188/lam.2020.005ds高亮Editorial office will modify it before online

    ds高亮Please kindly revise the reference according to the following example:

    Complete book: Atkinson, K. et al. (eds) Clinical Bone Marrow and Blood Stem Cell Transplantation (Cambridge Univ. Press, 2004).

    Chapter in book: Harley, N. H. & Vivian, L. in Mechanisms of Disease 4th edn, Vol. 2 (eds Sodeman, W. A. & Smith, A.) Ch. 3 (Saunders, 1974).

    ds高亮Please kindly revise the reference according to the following example:

    Complete book: Atkinson, K. et al. (eds) Clinical Bone Marrow and Blood Stem Cell Transplantation (Cambridge Univ. Press, 2004).

    Chapter in book: Harley, N. H. & Vivian, L. in Mechanisms of Disease 4th edn, Vol. 2 (eds Sodeman, W. A. & Smith, A.) Ch. 3 (Saunders, 1974).

  •  Wang, Z. et al. Single-shot on-chip spectral sensors based on photoniccrystal slabs. Nature Communications  10, 1020 (2019).

    27.

     Yang, Z. Y. et al. Single-nanowire spectrometers. Science  365, 1017-1020 (2019).

    28.

     Zheng, B. J. et al. On‐chip measurement of photoluminescence withhigh sensitivity monolithic spectrometer. Advanced Optical Materials  8,2000191 (2020).

    29.

     Redding, B. et al. Compact spectrometer based on a disorderedphotonic chip. Nature Photonics  7, 746-751 (2013).

    30.

     Gildas, F. & Dan, Y. P. Review of nanostructure color filters. Journal ofNanophotonics  13, 020901 (2019).

    31.

     Tang, X., Ackerman, M. M. & Guyot‐Sionnest, P. Acquisition of Hyperspec-tral data with colloidal quantum dots. Laser & Photonics Reviews  13,1900165 (2019).

    32.

     Stewart, J. W. et al. Ultrafast pyroelectric photodetection with on-chipspectral filters. Nature Materials  19, 158-162 (2019).

    33.

     Thiel, M. & Hermatschweiler, M. Three-dimensional laser lithography.Optik & Photonik  6, 36-39 (2011).

    34.

     Schmidt, S. et al. Wave-optical modeling beyond the thin-element-ap-proximation. Optics Express  24, 30188-30200 (2016).

    35.

     Drozella, J. et al. Fast and comfortable GPU-accelerated wave-opticalsimulation for imaging properties and design of highly aspheric 3D-printed freeform microlens systems. Proceedings of SPIE 11105, NovelOptical Systems, Methods, and Applications XXⅡ. San Diego, California,United States: SPIE, 2019, 1110506.

    36.

     Toulouse, A. et al. Alignment-free integration of apertures and non-37.

    transparent hulls into 3D-printed micro-optics. Optics Letters  43, 5283-5286 (2018). Toulouse, A. et al. Super-fine inkjet process for alignment-free integra-tion of non-transparent structures into 3D-printed micro-optics. Pro-ceedings of SPIE 10930, Advanced Fabrication Technologies forMicro/Nano Optics and Photonics XⅡ. San Francisco, California, UnitedStates: SPIE, 2019, 109300W.

    38.

     Schmid, M., Ludescher, D. & Giessen, H. Optical properties of photores-ists for femtosecond 3D printing: refractive index, extinction, lumines-cence-dose dependence, aging, heat treatment and comparisonbetween 1-photon and 2-photon exposure. Optical Materials Express 9, 4564-4577 (2019).

    39.

     Gross, H. Handbook of Optical Systems. Volume 2: Physical ImageFormation. (Weinheim: Wiley-VCH, 2005).

    40.

     Kirchner, R. et al. Reducing the roughness of 3D micro-optics. SPIENewsroom. http://dx.doi.org/10.1117/2.1201611.006788 (2017).

    41.

     Mayer, F. et al. Multimaterial 3D laser microprinting using an integ-rated microfluidic system. Science Advances  5, eaau9160 (2019).

    42.

     Chidambaram, N. et al. Selective surface smoothening of polymer Mi-crolenses by depth confined softening. Advanced Materials Technolo-gies  2, 1700018 (2017).

    43.

     Gissibl, T. et al. Refractive index measurements of photo-resists forthree-dimensional direct laser writing. Optical Materials Express  7,2293-2298 (2017).

    44.

     Bruton, D. RGB VALUES FOR VISIBLE WAVELENGTHS. at http://www.physics.sfasu.edu/astro/color/spectra.html (1996).

    45.

    Toulouse et al. Light: Advanced Manufacturing (2020)1:5 Page 11 of 11

     

    http://dx.doi.org/10.1117/2.1201611.006788http://www.physics.sfasu.edu/astro/color/spectra.htmlhttp://www.physics.sfasu.edu/astro/color/spectra.htmlhttp://dx.doi.org/10.1117/2.1201611.006788http://www.physics.sfasu.edu/astro/color/spectra.htmlhttp://www.physics.sfasu.edu/astro/color/spectra.htmlhttp://dx.doi.org/10.1117/2.1201611.006788http://www.physics.sfasu.edu/astro/color/spectra.htmlhttp://www.physics.sfasu.edu/astro/color/spectra.html