A miniaturized mid-infrared spectral analyzer will find a wide range of applications as a portable device in non-invasive disease diagnosis, environmental monitoring, food safety and others. In this work, we report an integrated spectral analyzer that can be constructed by using Au subwavelength hole arrays as multispectral filters. The hole arrays were fabricated with CMOS compatible processes. The transmission peak of the subwavelength hole arrays is continuously tuned from 3 μm to 14 μm by linearly increasing the periodicity of the holes in each array. Fourier transform infrared (FTIR) microscopy was applied to spatially map out the transmission of the hole arrays. The results show that each hole array can selectively allow for transmission at a specific wavelength. We further constructed an IR spectral analyzer model based on the microhole multispectral filters to retrieve IR spectral information of two test samples. Our experimental results show that the spectra from the integrated spectral analyzer follow nearly the same pattern of the FTIR spectra of the test samples, proving the potential of the miniaturized spectral analyzer for chemical analysis.
Most chemicals have distinct absorption signatures in mid-infrared spectral range. Spectral analysis provides a highly sensitive and selective method for chemical detection, which may find a wide range of applications in gas sensing[
Surface plasmon resonances (SPR) of noble and transition metals are well known for their capability to enhance light intensity in visible and near infrared range[
In 1998, Ebbeson et al. first investigated a periodic subwavelength hole array in a silver film and observed an extraordinary optical transmission (EOT) phenomenon such as enhanced transmission of light through the holes and wavelength filtering due to the excitation of SPR [
Following the above equation, multicolor filters in visible spectrum based on nanohole arrays has been extensively investigated in the past decade[
According to Eq. (
As a multispectral filter for spectral analysis, it is desirable to have a larger transmittance and a narrower FWHM to achieve a higher signal-to-noise ratio (SNR) and spectral resolution. However, as shown in Fig. 1(a,b), a higher transmittance always comes with a wider FWHM. To balance the transmittance and FWHM, we chose a ratio of 0.5 for the aperture diameter to period (see more discussions in SI Section 2). Figure 1(c) shows the simulation results based on this ratio, at which the transmittance maximum is approximately 60% and the FWHM is about 1 µm. As the period increases (diameter also increases accordingly), the main position linearly shifts from 3.5 µm to 13.5 µm and the main peak amplitude slightly increases from 50% to ~65%. For the following experimental work, we fixed the ratio of aperture diameter to period at this value. Figure 1(d) shows that the electric field intensity of the light is mainly localized near the metal edge of the aperture as the light transmits through the aperture due to metal surface plasmon polariton. This is consistent with the previously observed EOT phenomenon at visible spectral range.
To fabricate the multispectral filters according to the simulations above, the electron beam resist ZEP520 was first spin-cast onto the sample and patterned by electron-beam lithography (EBL, Vistec EBPG-5200) at a dosage of 155 μC/cm
The transmission spectra of the fabricated Au MHAs on a Ge substrate (“sample spectra”) were measured with a microscopic FTIR spectrometer (Thermo Scientific Nicolet iN10). To remove the effects caused by atmospheric conditions and substrate, the transmission spectra of a Ge substrate (“substrate spectra”) was also recorded. The transmission spectra of the Au MHAs were obtained by dividing the sample spectra with the substrate spectra. We plot six of the total 30 spectra in Fig. 3(a). The peak amplitudes show a trend of decline as the period increases. It is probably because the small aperture size is made slightly larger than designed due to the proximity effect of electron beam exposure. The transmittance peak position as a function of the aperture period is shown in Fig. 3(b). As the period increases from 0.7 μm to 3.2 μm (in 500 nm incremental), the transmittance peak shifts from 3.5 to 13 μm, following almost exactly the same dependence with the simulations. To visualize the wavelength selectivity of the MHA filters, we image the spatial transmittance distribution of the Au film at specific wavelengths (Fig. 3(c,d,e)). The transmittance is coded in color and the image has a spatial resolution of 2 μm × 2 μm. At a wavelength of 5.3 μm, the microhole array with a period of 1.2 μm has the maximum transmittance of 60%, which is consistent with the spectrum (red dash line) in Fig. 3(b). All the other arrays show a transmittance lower than ~40% except for the adjacent two arrays. Similar patterns can be also found in the other two images in Fig. 3(d,e).(a) Experimentally measured optical transmission spectra of six (out of 30 in total) microhole arrays. (b) Transmittance peak position as a function of the aperture period in simulation and experimental results. Mid-infrared transmission 2D maps of the MHAs are plotted at three different wavelengths: 5.3 μm (c), 8.9 μm (d) and 12.9 μm (e).
If each microhole array is equipped underneath with a broadband mid-infrared photodetector, a chip-size spectral analyzer can be realized with appropriate signal processing technology, as shown in Fig. 4(a). In such a system, we can assume a blackbody IR light source that has a spectral radiance density governed by eq. (
The photon number will be collected as photocurrent by photodetectors. For simplicity, we assume that the internal quantum efficiency of the photodetectors is 100%, meaning that all the photons at different wavelengths will be collected by the photodetector and converted to a photocurrent. For the case when the sample under test does not present in the optical path, the photocurrent I
Clearly, the photocurrent ratio Ri=IiIi0 for the ith photodetector will be equal to the transmittance X(λ
For this reason, we employ the following algorithm to reconstruct the spectral information of the sample. Eq. (
Similarly, when the test sample is present in the optical path, the photocurrent vector can be written asI=FXB,I∈ℝ30×1where X∈ℝ2000×2000 is a diagonal matrix and the (i, i) element on the diagonal is the sample spectral information at the i th wavelength. The filtering spectral vector FX is experimental measured when the sample is in the optical path.
However, the matrix X cannot be directly solved from Eq. (
We should admit that such an IR spectrometric analyzer based on MHAs cannot compete with the Fourier Transform Infrared Spectroscopy (FTIR) in terms of spectral resolution. The measured filtering spectra for the MHA filters has a FWHM of ~1.5 μm on average. This limits the spectral resolution to 0.75 μm (half of the FWHM), meaning that our spectral analyzer will not be able to resolve a spectral feature narrower than 0.75 μm in FWHM (similar to the light not able to resolve an object smaller than half of the wavelength), no matter how many MHA filters are used. But the number of MHA filters, although the more the better, shall not be smaller than a lower limit. Otherwise, the integrated spectral analyzer cannot reach its full capability. To find this lower limit, we can regard the MHA filters as probes to sample the target spectral information. According to the sampling theory, the sampling rate should be at least twice the maximum bandwidth (BW) of the target information, that is,Δk≤12BWwhere Δk is the sampling interval in wavenumber domain. The bandwidth (BW) of the target IR transmission spectrum can be found[
Let us take the IR spectrum of CaCO
Indeed, our integrated spectral analyzer cannot replace the FTIR spectral analyzer for the general purpose of spectral analysis. But it may be quite useful for some specific applications where the miniaturization of the device is highly desired while the spectral features are not complicated.
In this work, Au MHAs were successfully fabricated on Ge substrate using electron beam lithography and evaporation. The transmission spectra of the microhole arrays were measured with a microscopic FTIR spectrometer. Spatial mappings at fixed wavelengths were applied to visualize the wavelength selectivity of the Au MHA filters. An IR spectral analyzer model based on these filters was constructed to demonstrate the feasibility of retrieving the IR spectra of test materials. The successful development of such an integrated spectral analyzer may find a wide range of applications in non-invasive disease diagnosis, environmental monitoring, food safety and others.
The data that support the findings of this study are available from the authors on reasonable request, see author contributions for specific data sets.
This work is supported by the Key Laboratory of Infrared Imaging Materials and Detectors, Shanghai Institute of Technical Physics and Chinese Academy of Sciences. The microfabrication was conducted at the Center for Advanced Electronic Materials and Devices (AEMD), Shanghai Jiao Tong University. The mid-infrared spectroscopy and microscopic mapping were performed at the Instrumental Analysis Center, Shanghai Jiao Tong University.
Y.D. conceived the idea and directed the research. A.W. performed the experiments. A.W. and Y.D. analyzed the data and wrote the manuscript. All authors reviewed the manuscript.
The authors declare no competing interests.
1 Werle P, Near-and mid-infrared laser-optical sensors for gas analysis, Optics and lasers in engineering, 2002, 37, 101, 114, 10.1016/S0143-8166(01)00092-6
- 2 Klonoff DC, Mid-infrared spectroscopy for noninvasive blood glucose monitoring, IEEE Lasers Electro-optics Soc Newsletters, 1998, 12, 13, 14
- 3 Seddon AB, Mid‐infrared (IR)-A hot topic: The potential for using mid‐IR light for non‐invasive early detection of skin cancer in vivo, physica status solidi (b), 2013, 250, 1020, 1027, 10.1002/pssb.201248524
- 4 Willer U, Saraji M, Khorsandi A, Geiser P, Schade W, Near-and mid-infrared laser monitoring of industrial processes, environment and security applications, Optics and Lasers in Engineering, 2006, 44, 699, 710, 10.1016/j.optlaseng.2005.04.015
- 5 Guillen MD, Cabo N, Infrared spectroscopy in the study of edible oils and fats, Journal of the Science of Food and Agriculture, 1997, 75, 1, 11, 10.1002/(SICI)1097-0010(199709)75:1<1::AID-JSFA842>3.0.CO;2-R
- 6 Eerdenbrugh B, Taylor LS, Application of mid-IR spectroscopy for the characterization of pharmaceutical systems, International journal of pharmaceutics, 2011, 417, 3, 16, 10.1016/j.ijpharm.2010.12.011
- 7 Baker MJ, Using Fourier transform IR spectroscopy to analyze biological materials, Nature protocols, 2014, 9, 1771, 1791, 10.1038/nprot.2014.1104480339
- 8 Piegari A, Bulir J, Variable narrowband transmission filters with a wide rejection band for spectrometry, Applied optics, 2006, 45, 3768, 3773, 10.1364/AO.45.003768
- 9 Mulvaney P, Surface plasmon spectroscopy of nanosized metal particles, Langmuir, 1996, 12, 788, 800, 10.1021/la9502711
- 10 Ebbesen TW, Lezec HJ, Ghaemi H, Thio T, Wolff P, Extraordinary optical transmission through sub-wavelength hole arrays, Nature, 1998, 391, 667, 669, 10.1038/35570
- 11 Yokogawa S, Burgos SP, Atwater HA, Plasmonic color filters for CMOS image sensor applications, Nano Letters, 2012, 12, 4349, 4354, 10.1021/nl302110z
- 12 Burgos SP, Yokogawa S, Atwater HA, Color imaging via nearest neighbor hole coupling in plasmonic color filters integrated onto a complementary metal-oxide semiconductor image sensor, Acs Nano, 2013, 7, 10038, 10047, 10.1021/nn403991d
- 13 Chang C-Y, Wavelength selective quantum dot infrared photodetector with periodic metal hole arrays, Applied Physics Letters, 2007, 91, 163107, 10.1063/1.2800378
- 14 Jang W-Y, Experimental Demonstration of Adaptive Infrared Multispectral Imaging using Plasmonic Filter Array, Scientific reports, 2016, 6, 10.1038/srep348765056518
- 15 Cao H, Nahata A, Resonantly enhanced transmission of terahertz radiation through a periodic array of subwavelength apertures, Optics express, 2004, 12, 1004, 1010, 10.1364/OPEX.12.001004
- 16 McCrindle IJH, Grant JP, Gouveia LCP, Cumming DRS, Infrared plasmonic filters integrated with an optical and terahertz multi‐spectral material, physica status solidi (a), 2015, 212, 1625, 1633, 10.1002/pssa.201431943
- 17 Chang C-C, A surface plasmon enhanced infrared photodetector based on InAs quantum dots, Nano letters, 2010, 10, 1704, 1709, 10.1021/nl100081j
- 18 Brolo AG, Gordon R, Leathem B, Kavanagh KL, Surface plasmon sensor based on the enhanced light transmission through arrays of nanoholes in gold films, Langmuir, 2004, 20, 4813, 4815, 10.1021/la0493621
- 19 Genet C, Ebbesen T, Light in tiny holes, Nature, 2007, 445, 39, 46, 10.1038/nature05350
- 20 Chen Q, CMOS photodetectors integrated with plasmonic color filters, IEEE Photonics Technology Letters, 2012, 24, 197, 199, 10.1109/LPT.2011.2176333
- 21 Si G, Zhao Y, Chew AB, Liu YJ, Plasmonic color filters, Journal of Molecular and Engineering Materials, 2014, 2, 1440009, 10.1142/S2251237314400097
- 22 Tan SJ, Plasmonic color palettes for photorealistic printing with aluminum nanostructures, Nano letters, 2014, 14, 4023, 4029, 10.1021/nl501460x
- 23 Li, Z., Clark, A. W. & Cooper, J. M. Dual color plasmonic pixels create a polarization controlled nano color palette (2016).
- 24 Orwiler, B., Oscilloscope vertical amplifiers. (Tetronix, Inc., 1969).
Supplementary information accompanies this paper at 10.1038/s41598-018-29177-0.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
PHOTO (COLOR)
PHOTO (COLOR)
PHOTO (COLOR)
PHOTO (COLOR)
PHOTO (COLOR)
PHOTO (COLOR)
PHOTO (COLOR)
PHOTO (COLOR)
PHOTO (COLOR)
PHOTO (COLOR)
PHOTO (COLOR)
PHOTO (COLOR)
PHOTO (COLOR)
PHOTO (COLOR)
PHOTO (COLOR): supplementary information
By Ang Wang and Yaping Dan