High resolution on-chip spectroscopy based on miniaturized microdonut resonators.

Abstract

We experimentally demonstrate a high resolution integrated spectrometer on silicon on insulator (SOI) substrate using a large-scale array of microdonut resonators. Through top-view imaging and processing, the measured spectral response of the spectrometer shows a linewidth of ~0.6 nm with an operating bandwidth of ~50 nm. This high resolution and bandwidth is achieved in a compact size using miniaturized microdonut resonators (radius ~2 μm) with a high quality factor, single-mode operation, and a large free spectral range. The microspectrometer is realized using silicon process compatible fabrication and has a great potential as a high-resolution, large dynamic range, light-weight, compact, high-speed, and versatile microspectrometer.

Full text

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High resolution on-chip spectroscopy based on
miniaturized microdonut resonators
Zhixuan Xia, Ali Asghar Eftekhar, Mohammad Soltani, Babak Momeni, Qing Li,
Maysamreza Chamanzar, Siva Yegnanarayanan, and Ali Adibi *
School of Electrical and Computer Engineering, Georgia Institute of Technology, 777 Atlantic Drive NW, Atlanta,
Georgia 30332, USA
*adibi@ece.gatech.edu
Abstract: We experimentally demonstrate a high resolution integrated
spectrometer on silicon on insulator (SOI) substrate using a large-scale array
of microdonut resonators. Through top-view imaging and processing, the
measured spectral response of the spectrometer shows a linewidth of ~0.6
nm with an operating bandwidth of ~50 nm. This high resolution and
bandwidth is achieved in a compact size using miniaturized microdonut
resonators (radius ~2μm) with a high quality factor, single-mode operation,
and a large free spectral range. The microspectrometer is realized using
silicon process compatible fabrication and has a great potential as a high-
resolution, large dynamic range, light-weight, compact, high-speed, and
versatile microspectrometer.
©2011 Optical Society of America
OCIS codes: (130.3120) Integrated optics devices; (230.5750) Resonators; (300.6190)
Spectrometers.
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1. Introduction
Integrated photonic sensors have received considerable attention in recent years due to their
inherent advantages, including high sensitivity, low cost of fabrication, versatility,
compactness, and low power consumption [1–3]. The progress of integrated photonic sensors
has reached the point that requires integration of different system components together for an
overall efficient sensing system. One of the major building blocks for such lab-on-chip
sensing systems is an integrated spectrometer that enables on-chip spectral analysis.
Motivated by this demand, there has been significant progress in the realization of
integrated microspectrometers in different configurations such as arrayed-waveguide gratings
[4,5], grating spectrometers [6–8], superprism-based spectrometers [9,10], and the recently
demonstrated diffractive grating spectrometer combined with the thermally tunable microring
resonators [11]. However, the main challenge of the integrated spectrometers that rely on
dispersive components is the trade-off between the resolution and the size of the structure
[12]. As an alternative, a filter array with Fabry-Perot cavities has been proposed to provide
high spectral resolution (1.7 – 3.8 nm) in the near infrared region [13]. In this approach,
different spectral components of the incoming signal are individually captured by their
corresponding filters with a varying vertical cavity length obtained by multiple film
deposition steps. An array of detectors is used to measure the output of all filtered spectral
component simultaneously and sent the data to an electronic processing device in real time.
This method, without any moving parts and conventional dispersive structures, enables
parallel spectral processing in a very short time, which is critical for a variety of biological
and biomedical applications. However, the aforementioned Fabry-Perot filter array suffers
from the complexity of fabricating highly reflective and parallel mirrors with a series of
different cavity lengths, and its resolution is limited by the available reflectance of the cavity
mirrors. More importantly, the trade-off between the resolution and free spectral range (FSR)
of the Fabry-Perot cavity is a limiting factor for applications requiring large operating
bandwidth [14].
In this paper, we propose a new microspectrometer architecture based on a large array of
compact microdonut resonators to address the challenges of existing on-chip spectrometers.
The key building element used to construct the microspectrometer is a miniaturized
microdonut resonator [15,16] offering single mode operation with a high quality factor (Q)
and a large FSR.
The proposed spectrometer employs a filter array of microdonut resonators, which is
coupled to an input bus waveguide, as is schematically shown in Fig. 1(a). The microdonut
resonators are carefully designed such that each of the resonators only taps a small portion of
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the incoming spectrum that overlaps with its resonance lineshape, as is shown in Fig. 1(b). In
this architecture, each resonator corresponds to a unique spectral channel. The input signal
power in each spectral channel can be coupled to a separate output waveguide or directly
coupled out of plane through the resonator. By imaging the output power of different spectral
channels using a detector array or a charge-coupled detector (CCD) chip on top of the
resonator array device, the entire input signal spectrum can be measured in real time. This on-
chip spectrometer can be fabricated using CMOS-compatible manufacturing techniques and
thus can be readily integrated with integrated sensors, optoelectronics, microelectronics and
microfluidic channels. This enables the realization of a system-on-a-chip suitable for several
applications in biological, chemical, medical, and pharmaceutical industries. The rest of the
paper is organized as follows: major properties of microdonut resonators are presented in
Section 2. Section 3 is dedicated to the design and fabrication of the on-chip spectrometers.
The experimental results are presented and discussed in Sections 4 and 5, respectively. Final
conclusions are made in Section 6.

Fig. 1. (a) Configuration of the resonator-array spectrometer: a 1-D array of small microdonut
resonators samples different spectral channels of the input signal propagating in the bus
waveguide. Each spectral channel is coupled by one resonator to a corresponding drop
waveguide and then scattered out-of-plane in an arrangement prepared for a detector array on
top of the structure. (b) Working principle of the resonator-array spectrometer: the unknown
input spectrum is sampled by the series of resonances provided by the resonators in the array
followed by data processing to obtain the reconstructed spectrum of the input signal.
2. Microdonut resonators
To simultaneously achieve a high Q and a large FSR, a miniaturized microdonut resonator is
used [16]. As shown in Fig. 2(a), a microdonut resonator is essentially a microdisk with an
inner hole perforated at the center. In this configuration, the fundamental radial mode of the
resonator is mostly confined around the outer perimeter of the microdonut (see Fig. 2(c))
while the higher-order radial modes are pushed into the leaky zone, leading to a single-mode
operation. The major difference between the microdonut resonator and the miniaturized
microring resonator [16] is that by adjusting the radius of the inner hole, the fundamental
radial mode of a microdonut resonator interacts only with the outer sidewall of the resonator,
while such a mode always interacts with two sidewalls in microring resonators. Consequently,
a higher quality factor for the fundamental mode is expected when using microdonut
resonators. Figure 2(a) shows the scanning electron microscope (SEM) image of an add/drop
filter based on the compact microdonut resonator with a radius of ~2 µm and an oxide
cladding. A linewidth of ~50 pm is measured from the drop port transmission for TE
polarization (i.e., electric field confined in the plane of the device) (see Fig. 2(b)), leading to a
loaded Q (QL) of ~30,000 (corresponding to an intrinsic Q of Qi ~80,000). This Qi is twice
that achieved with microring resonator with a similar diameter [17]. Figure 2(c) confirms the
single-mode operation of the miniaturized microdonut resonator with an FSR of ~57 nm.
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Fig. 2. (a) The SEM image of a microdonut resonator in an add/drop configuration: the width
and thickness for both waveguides are 400 nm and 230 nm, respectively. The outer radius of
the microdonut resonator is rout = 1.97 µm with a center hole with a radius of rin = 0.6 µm. The
gap between the waveguide and the resonator is 240 nm. The microdonut resonator has a 2 µm
thick oxide cladding layer. (b) The experimental transmission spectrum of the drop port of the
resonator in Part (a) for TE polarization showing two resonances belonging to the fundamental
radial modes with different azimuth mode numbers (m) specified in the figure. The measured
linewidth is ~50 pm and the FSR is ~57 nm. (c) Simulated fundamental TE mode profile of the
microdonut resonator with m = 18, indicating a majority of light is confined at the outer
perimeter of the resonator.
By engineering the geometry of the microdonut resonator (i.e., the outer and inner radii),
its resonance wavelength can be adjusted. Because of the small mode volume and high field
intensity of the resonant mode at the outer perimeter (see Fig. 2(c)), the resonance is very
sensitive to the variations of the outer radius. Simulations using three-dimensional finite
element method (3D FEM) show that every 1 nm variation in the outer radius of a microdonut
with a radius of rout ~2 μm and a thickness of 230 nm corresponds to a 0.6 nm change in its
resonance wavelength near 1550 nm. Meanwhile, the fine-tuning of the resonance (better than
10 pm wavelength accuracy) can be achieved by adjusting the inner radius of the microdonut
resonator. The spectral linewidth of the resonance (and thus its QL) is controlled by
engineering the coupling between the bus waveguide and the microdonut resonator.
Therefore, combined with the high Qi and large FSR, the ability to independently tune the
resonance location and spectral resolution makes the microdonut resonator an excellent
building-block device to construct a large-scale resonator array suitable for on-chip
spectroscopy.
3. Design and fabrication of on-chip spectrometer with microdonut resonators
The micrograph of an on-chip resonator-array spectrometer composed of an array of
resonators side coupled to a bus waveguide is shown in Fig. 3(a). The spectrometer is
designed to work with TE polarization. The input light signal (with a target spectrum to be
detected) is coupled through the bus waveguide from the lower left in Fig. 3(a). Each of the
resonators filters the input signal in a narrow spectral window and sends it via its
corresponding side coupled drop waveguide to a scatterer located at the end of the drop
waveguide. In this architecture, each resonator, its drop waveguide and the corresponding
scatterer describe a spectral channel that measures a unique spectral portion of the target
spectrum. The response of each spectral channel is indicated by the out-of-plane radiation
from the scatterer of that channel, and it is measured by imaging the signal from the scatterer
array onto a detector array, as shown in Figs. 3(b)–3(d).
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Fig. 3. (a) Micrograph of the proposed spectrometer: incoming light is coupled to the structure
through the input waveguide on the lower left of the figure. Each of the resonators is side
coupled to the input waveguide from one side and to a drop waveguide from the other side to
filter the input signal in a narrow spectral window. The filtered signal is scattered out of the
chip by a scatterer at the end of the drop waveguide. (b) The 2-D array of the scatterers with
channel numbers labeled. (c) The SEM image of two microdonut resonators coupled to the
input waveguide and their corresponding drop waveguides. The center-to-center distance
between the two adjacent microdonut resonators is 10.0 μm, the gap between the microdonut
and each waveguide is 130 nm, and the widths of the input and bus waveguides are 400 nm. (d)
The SEM image of a portion of the 2-D array of the scatterers.
The number of the resonators in the array and their resonance wavelengths and linewidths
are adjusted to cover one full FSR of a single resonator. Tuning of the resonance wavelength
is achieved by varying the outer radius (initially ~2 μm) with 1 nm steps, while keeping the
inner radius fixed at 0.9 μm to ensure the single-mode operation at around 1550 nm. This
leads to a ~0.6 nm increment in the resonance wavelengths of adjacent spectral channels as
calculated from 3D FEM in the COMSOL environment. Considering an FSR of ~60 nm, the
operating bandwidth is designed to be ~50 nm to avoid the interference of resonances from
different azimuthal modes. A total number of 84 microdonut resonators are needed to cover
the operating bandwidth without too much overlap among the neighboring resonances. The
outer radii of the resonators are chosen ranging from 1.950 μm to 2.033 μm. The resonance
linewidth of the resonators is set to be ~0.3 nm, corresponding to a Q of QL ~5000. This
linewidth is achieved by setting the gap size between the resonators and the bus waveguide to
130 nm. According to 3D FEM calculations, the effective coupling Q of QC is ~10,000.
Considering the two coupling regions (corresponding to the input and the drop ports) as well
as the much higher intrinsic quality factor Qi of the microdonut resonator, the estimated
loaded QL would be ~5000 (1/QL = 2/QC + 1/Qi2/QC). In this configuration, the spectrometer
samples the incoming target spectrum with 84 sharp Lorentzian-like peaks with a constant
spacing of ~0.6 nm as schematically shown in Fig. 1(a). The width of the straight waveguide
is chosen to be 400 nm to ensure single mode operation at the wavelength region of 1500 nm-
1640 nm. The drop waveguides are designed to uniformly and efficiently transmit the
spectrum channel light energy to the scatterers. To ensure that all the scatterers fall within the
field-of-view of the CCD, the drop waveguides and their scatterers are packed in the center
(from the top and bottom) of the device as shown in Figs. 3(a) and 3(b). This leads to a small
size for the 84-element scatterers array (~200 μm by 50 μm in total) that can be readily
imaged within the field-of-view of the CCD using a 20x objective lens. The scatterers are
placed in a 2D array, where the scatterers with different parities are located in different rows,
as shown in Fig. 3(b). Therefore, one scatter of a certain channel is always placed in a
different row from both of its immediate neighbor channels. This helps to lower the crosstalk
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among the neighboring channels and to easily identify the channel number in the spectrum
estimation.
The spectrometer structure was fabricated on an SOI wafer with a Si layer thickness of
230 nm on top of a 1μm buried oxide layer. The device was patterned using a JBX-9300FS
electron beam lithography (EBL) system with 6% dilute hydrogen silsesquioxane (HSQ)
negative electron resist and etched in an inductively-coupled plasma etching system with Cl2
chemistry. The details of the fabrication process have been reported elsewhere [18]. A 1 μm
thick layer of SiO2 is subsequently deposited on top of the structure using plasma enhanced
chemical vapor deposition (PECVD) as the cladding and passivation layer.
4. Experimental results
The characterization of the spectrometer is first conducted by coupling light from a tunable
laser (81640A, Agilent Technologies) to the input bus waveguide via a tapered fiber. The
output power of the through port is collected by a photodetector (Thorlabs PDB 150C)
through another tapered fiber. The measured transmission spectrum from the through port (see
Fig. 3(a)) waveguide is shown in Fig. 4(a). The FSR of the smallest microdonut resonator
(outer radius ~1.950 µm) is measured to be ~60 nm, which corresponds to the resonance
wavelength spacing between the azimuth modes m = 17 and m = 18 of the fundamental radial
TE mode. The resonances of the resonator array densely fill up the FSR, indicating the
capability of sampling a target spectrum. However, among the total 84 microdonut resonators
in the array, three of the designed resonances are missing and a total of 81 resonances are
observed in both the through port transmission and the top view IR camera imaging that will
be discussed later. For the functioning 81 channels, the typical value of the measured loaded
Q is QL ~5000, which is in good agreement with the original design. Figure 4(b) plots the
measured resonances of the resonator array obtained directly from Fig. 4(a), showing the
channel-to-channel uniformity of the 81 resonances. The measured resonance spacing varies
between ~0.2 nm and ~1.0 nm, due to the high sensitivity of the resonance wavelength to
changes in the outer radius of the microdonut resonators, the imperfections in the device
fabrication, and possibly the thickness variation of the wafer. The solid line and the inset
formula in Fig. 4(b) show the linear regression model obtained by the measured data points.
The shift of the measured resonances from their projected values with the linear fitting model
is depicted by the standard error of ~0.400 nm. The good linearity with an average resonance
spacing of ~0.636 nm agrees well with the design value of ~0.6 nm.

Fig. 4. (a) Transmission spectrum measured from the through port waveguide, indicating an
FSR of ~60 nm. (b) Plot of the measured resonance wavelengths of different resonators
(vertical axis, y) versus their resonator number in the resonator array (horizontal axis, x, x = 1-
81, which only include the working 81 resonators with observed resonances). The inset formula
shows the linear functions fitted to the measured resonance wavelengths. The correlation
between the measured data points and the fitted linear model (R2) is 0.999.
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Fig. 5. (a) Real time images captured by the IR camera showing different channel responses at
five different input wavelengths (nm): 1582.45, 1584.80, 1589.05, 1591.25, and 1595.65, when
the input spectrum falls within channel numbers 53, 57, 63, 67, 74, respectively. Note that this
figure only shows the upper portion of the scatterer array. (b) Post-processed light pattern
scattered by channel #74 at different input wavelengths around its resonance.
The response of the spectrometer is then characterized by capturing the out-of-plane
radiation of the scatterer array with an IR camera (Sensors Unlimited SU128-1.7RT, 128 by
128 pixels with pixel size of 60 μm2), while varying the input wavelength using a tunable
laser (81640A, Agilent Technologies) at 500 µw output power. During the full wavelength
scan over one FSR from 1550 nm to 1610 nm at a step of 0.05 nm, 81 out the total 84
scatterers turned on (with strong out-of-plane radiation observed in the real time imaging).
The remaining three resonators are not observed either in the transmission or in the out of
plane image. With the transmission spectrum and the location of the scatterers that failed to
turn on, the defective channels were identified with resonator numbers #20, #21, and #50.
This can be due to the large shift of the resonance of these three channels as a result of
fabrication imperfection causing them to overlap with adjacent resonances; it can also be due
to unwanted large imperfection resulting in low intrinsic Qs for these three resonators. Figure
5(a) shows the top images of the upper portion of the scatterer array at different input
wavelengths, corresponding to the designed channels #43 - #84. The top image of the scatterer
corresponding to the spectral channel #74 for different input wavelengths is shown in Fig.
5(b). We can clearly see the total scattered power peaks at 1595.65 nm, which corresponds to
the assigned center resonance wavelength for this channel. Good extinction ratio is also
observed within ~0.5 nm from the center wavelength as shown in Fig. 5(b). By integrating the
amplitudes of the CCD pixels for each channel at different input wavelengths, the spectral
response of each spectral channel of the spectrometer can be obtained and used for the
calibration purpose. Figure 6(a) is the calibrated spectral response of the 13 channels (#72 -
#84). In this figure, the output of each channel has been normalized to its peak value. The
variation of the peaks of the 13 channels is measured to be within ~1.5 dB, showing good
power uniformity among different channels of the spectrometer. The nonlinear response of the
IR camera to different values of the light intensity is not corrected in Fig. 6(a). Therefore, the
spectral line shape of each spectral channel is a little broadened in this figure.
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The comparison between the extracted resonances from the calibration spectrum and those
obtained directly from the through port transmission measurement is shown in Fig. 6(b). A
good agreement is observed over the 13 channels under discussion, verifying the reliability of
the post-processing method. By fitting the data with a linear function, the average deviation of
the cavity resonances from the designed values for this short wavelength range of the 13
adjacent channels is ~0.176 nm, a value much smaller than the ~0.400 nm obtained for the
whole FSR covering 81 channels. We believe the smaller local variation in the resonances of
these 13 adjacent spectrum channels is mainly caused by the fabrication imperfections, while
the thickness variations of the device layer in the SOI substrate contribute to the larger
deviation measured in the long range shown in Fig. 4(b) [19,20].

Fig. 6. (a) Calibration spectrum of the 13 channels covering the wavelength range from
1594.30 nm to 1602.00 nm; (b) comparison of resonances obtained by the through port
spectrum (triangles) and the calibration spectrum based on the top-view images (squares). A
linear model is fitted to the measured resonance wavelengths. The standard deviation of the
resonance wavelengths from the linear model is only 176 pm.
5. Discussion
The demonstrated high resolution (~0.6 nm) microspectrometer can be very compact thanks to
the small size of the miniaturized microdonut resonators and the scatterers. Although the
current footprint of this spectrometer is ~1 mm2, a large portion of the real estate on-chip is
occupied by the passive waveguides, which transport the filtered spectral channel signals from
the drop port to the scatterer array. Therefore, the size of the spectrometer could be further
reduced through suitable optimized device designs for the scatterer array and the drop port
waveguides.
One key advantage of the demonstrated spectrometer is the capability of independently
controlling and configuring the resolution and the operating bandwidth of each channel of the
microspectrometer for different applications. The resolution is well controlled by engineering
the coupling between the resonator and the bus waveguide. By implementing different sets of
resonator arrays in parallel, the operating bandwidth can be extended to more than one FSR.
Ultimately, the spectral range of operation of this microspectrometer is limited by the material
absorption of Si as the device layer and SiO2 as the cladding and buffer layer. Therefore, the
proposed spectrometer can cover a wide range of wavelengths from ~1.1µm to ~3 µm [21].
The main limitation on the spectral resolution is the degree of control on the radii of
microdonut resonators. Since the intrinsic Q is very high (~80,000 for a 2 µm radius) and can
be further improved by fabrication optimization, the ultimate limit on resolution (assuming
full control on the resonator sizes) is very high. This makes the proposed spectrometer a
unique device for a wide range of practical applications.
The low insertion loss in this structure not only enables efficient use of the incident optical
power, but also maintains uniform power distribution of the signal among different channels,
which has been verified by the measured power variation of ~1.5 dB. The main challenges are
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the shift of the resonances due to the variation of the thickness in the wafer scale and the
fabrication imperfections. The former can be solved by further reducing the area of the device.
The random variation of the resonator resonant wavelength, caused by fabrication
imperfection, impose a challenge, not only in the specific application of high resolution
spectrometer as proposed in this paper, but also in many other applications of integrated
optics. Different techniques have been proposed for the post fabrication trimming of the
integrated optics structure that can reduce these fabrication induced variations and resolve this
issue [19,22,23]. Moreover, unlike the case in wavelength demultiplexers, data processing
techniques [24] used in the spectrum reconstruction can relax the requirements for the spectral
response of the spectrometer. The compatibility of the fabrication processes to the CMOS
process enables the integration of these spectrometers with microelectronic and microfluidic
circuitry to develop integrated sensing systems for various applications in biological,
chemical, medical, and pharmaceutical industries.
6. Summary
In this paper we experimentally demonstrated an 81-channel on-chip spectrometer based on
an array of miniaturized microdonut resonators. The single-mode operation of the microdonut
resonators with an outer radius of ~2 μm enables the large FSR of ~60 nm. The experimental
resolution extracted from the calibrated spectral response is ~0.6 nm. The capability of
independently engineering the resolution and the operation bandwidth makes the proposed
spectrometers suitable for measuring a large range of different target spectra. This high
resolution on-chip spectrometer with a large dynamic range shows great potentials as a light
weight, compact, high speed, and versatile spectrometer for a variety of applications and for
insertion into future lab-on-chip applications.
Acknowledgments
This work was supported by the Defense Advanced Research Projects Agency (DARPA)
under Contract No. HR 0011-10-1-0075 and by Air Force Office of Scientific Research under
Contract No. FA9550-06-01-2003 (G. Pomrenke).
#143529 - $15.00 USD
(C) 2011 OSA
Received 4 Mar 2011; revised 11 Apr 2011; accepted 4 May 2011; published 10 Jun 2011
20 June 2011 / Vol. 19, No. 13 / OPTICS EXPRESS 12364