1818 OPTICS LETTERS / Vol. 30, No. 14 / July 15, 2005

Upconversion multichannel infrared spectrometer

Matthew F. DeCamp and Andrei TokmakoffDepartment of Chemistry and George R. Harrison Spectroscopy Laboratory, Massachusetts Institute of Technology,

77 Massachusetts Avenue, Cambridge, Massachusetts 02139

Received February 8, 2005

A multichannel IR spectrometer using a standard silicon CCD array is demonstrated. Sum frequency gen-eration between an ultrafast optical pulse and a frequency-dispersed IR beam generates a spatially ex-tended optical signal that is collected on a generic CCD video camera. This method provides an inexpensiveand efficient alternative to conventional multichannel IR arrays. © 2005 Optical Society of America

OCIS codes: 120.6200, 300.6340, 300.6500.

Time-resolved IR spectroscopy is an important tool inphysical chemistry and condensed matter physics. Inparticular, mid-IR vibrational resonances from 3 to20 mm provide a probe of molecular structure thatcan be combined with time-resolved techniques to ex-tract information on molecular dynamics. Due to in-terest in characterizing spectroscopic transients atmultiple vibrational frequencies in femtosecond IRexperiments, there has been a need for multichannelIR array detectors. In the mid-IR spectral region, ar-ray elements are typically based upon liquid nitrogencooled photocurrent semiconductors, e.g., HgCdTe,InSb, or PbSe.1,2 However, these arrays are atpresent limited by their finite size (typically 32 to 128elements in linear arrays3–5) and by their high cost.The use of inexpensive silicon CCD arrays could beadvantageous, even though the ,1.1 mm bandgap ofsilicon makes CCD technology unable to directly de-tect mid-IR radiation. In this Letter, we present anapproach for detection of spectrally dispersed mid-IRtransients with a silicon CCD that makes use of up-conversion methods.

Nonlinear optical mixing techniques, such as sumfrequency generation (SFG), provide an efficientmethod of upconverting mid-IR radiation to the vis-ible, allowing silicon-based detectors to be a viable al-ternative to standard IR detectors. SFG techniquesfor use in IR spectroscopy were to our knowledge firstdemonstrated by Heilweil.6,7 This technique usesSFG between a narrowband optical pulse and abroadband IR pulse to generate an optical signalwhose frequency bandwidth is equal to that of the IRpulse. Provided that the narrowband laser intensityis constant, the SFG intensity is proportional to theIR intensity. The original IR spectrum can then be re-trieved by measuring the SFG spectrum with an op-tical spectrometer. While easy to implement, thismethod has several technical challenges. To achieve areasonable conversion efficiency, the narrowband op-tical pulse must be intense. The optical pulse is rela-tively long, requiring that the optical fluence be inthe tens of mJ/cm2. Also, the frequency resolution,limited to the crystal thickness and the bandwidth ofthe long optical pulse, is typically a few cm−1.

In this work, we use a different approach, femto-second upconversion. A femtosecond optical pulse ismixed with a spectrally dispersed IR pulse to gener-ate a spatially dispersed SFG signal. In this case, the

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optical pulse is temporally short such that optical flu-ences of ,mJ/cm2 will yield good SFG efficiency. TheSFG signal is collected by a generic CCD video cam-era. We find that the frequency resolution and band-width of this technique are limited only by the opticalcomponents, not the input laser characteristics. Withsimple optical components, this method can be a rela-tively inexpensive alternative to traditional IR arraydetectors.

The experimental diagram for the prototype upcon-version spectrometer is shown in Fig. 1. The basegrating spectrometer is constructed of a150 groove/mm grating and a 25 cm focal-lengthmetal mirror. The input IR light is generated from ahome-built 1 kHz optical parametric amplifier (OPA)pumped by a 150 mJ, 80 fs, 800 nm laser pulse. TheOPA is centered at 2620 cm−1 with a working band-width of ,120 cm−1. The focusing resolution of thedispersed IR pulse is ,230 mm. At the focal plane ofthe spectrometer we place the SFG crystal, a type I8 mm38 mm31 mm KNbO3 crystal cut at 41°. Inthe current geometry, ,200 nJ and 40 cm−1 of the IRlight spans the crystal face, easily within the phase-matching condition of the KNbO3 crystal. Before thespectrometer focal plane, a CaF2 window coated to re-flect 800 nm was used as a dichroic beam splitter. Anunfocused 80 fs, 800 nm, 40 mJ pulse illuminates theentire crystal surface.

The SFG signal, peaked at ,655 nm, is generatedonly in locations where both IR and visible light in-teract. The resulting SFG signal has one spatial di-mension containing the IR spectral content and an-

Fig. 1. Experimental setup. G, grating; L, focusing mirror;HR1, 800 HR/IR transmit; HR2, 800 HR/optical transmit;BP, bandpass filter; thick solid line, IR beam; shaded area,800 nm beam; dashed line, SFG.

2005 Optical Society of America

July 15, 2005 / Vol. 30, No. 14 / OPTICS LETTERS 1819

other representing the focusing conditions of theincident IR beam. To ensure good SFG efficiency, theIR and optical pulses must overlap temporally. For-tunately, because the IR pulse is spectrally dispersed,the relative timing between the IR and the 800 nmpulse is relatively insensitive. As with the Heilweiltechnique, the upconverted intensity is proportionalto the product of the IR and visible pulse intensities.However, the optical beam must have a flat spatialprofile across the crystal to guarantee a flat spectralresponse for the IR. It should also be noted that thethickness of the nonlinear crystal will also limit thebandwidth (not the resolution) that can be upcon-verted on a single shot. For example, IR bandwidthslarger than 100 cm−1 may require crystal thicknessesless than 300 mm.

Due to the extended Rayleigh range of the IRbeam, the emitted SFG signal is effectively colli-mated, and thus no other optical focusing elementsare necessary prior to detection. When one is usingtighter focusing optics, a simple optical telescope canbe used to image the SFG signal onto the CCD array.A dielectric high-reflecting mirror for 800 nm and a650 nm bandpass filter were used to isolate the SFGsignal.

The signal was imaged directly onto the CCD ar-ray, a Watec LCL-902C monochrome CCD camera.The quoted sensitivity of the camera is 0.01 lx andeach pixel is 8.4 mm sHd39.8 mm sVd. Given theframe rate of the camera s30 frames/sd, a singlecount on the camera corresponds to ,100 visiblephotons.8 Each video frame is captured by a NationalInstruments NI-1407 video capture board, retrieving640 sHd3480 sVd pixels with 8-bit resolution. Withthe input parameters described above, the signal iseasily visible to the naked eye and saturates the CCDcamera. The beam is attenuated with a neutral den-sity filter, reducing the SFG signal by a factor of ,20prior to the camera. The 230 mm focusing spot sizecorresponds to about 28 (24) horizontal (vertical) pix-els, resulting in ,25 distinct frequency channels. To

Fig. 2. Raw upconverted spectral interferometry of an IRpulse train with a time delay of 16.5 ps.

collect a spectrum, the video frame was integrated

along the vertical dimension resulting in a typicalsignal-to-noise ratio of greater than 40:1s,10 m ODd per video frame. Simple extrapolation ofthe present signal levels indicates that a commercialCCD line camera, synchronized to the pulsed laser,will allow for single shot measurements.

Calibration of the spectrometer is performed withspectral interferometry.9 A Michelson interferometerwas used to generate two identical IR pulses. Thetime delay between the two pulses was controlled bya calibrated motorized delay line. The pulse train issent into the SFG spectrometer, and the resultingspectra are recorded as a function of pulse separation(Fig. 2). The temporal interferogram of each CCDpixel is Fourier transformed, resulting in the fre-quency calibration for each pixel (Fig. 3).

The position and width of the calibration surfacegive the frequency calibration and resolution of thespectrometer. The stepwise pattern in the pixel cali-bration is a consequence of the discrete fast Fouriertransform (FFT). The average width of the FFT pro-vides a frequency resolution of 1.81±0.03 cm−1. Theslope of the calibration surface results in a pixel cali-bration of 0.0666 cm−1/pixel. The theoretical instru-ment frequency resolution of the spectrometer, dic-tated by the focal spot size and grating, is 1.83 cm−1,implying that the frequency resolution of the spec-trometer is limited only by the optics. Reductions inthe f-number of the system (i.e., focusing tighter) orincreases in the grating’s groove density by a factor of2 (experiment not shown) improve the frequencyresolution accordingly. Reduction of the f-numberwill also effectively increase the number of usablespectrometer channels.

While the CCD array can measure single spectraeasily, multiple independent spectra can also betaken simultaneously without any additional opticalcomponents. Multiple independent IR beams, verti-cally displaced, can be inserted into the spectrometer.The resultant SFG signal has multiple distinctstripes separated vertically. Figure 4 shows a singleframe image from the CCD for two independent IRspectra taken simultaneously. The upper stripeshows the spectrum of an IR pulse transmitted

Fig. 3. (left) Frequency calibration of CCD array. (right)

FFT of pixel 293.

1820 OPTICS LETTERS / Vol. 30, No. 14 / July 15, 2005

through acetonitrile, whereas the lower stripe showsthe spectrum of the undisturbed IR pulse. Acetoni-trile has a weak absorption line at 2627 cm−1

(FWHM ,20 cm−1). The SFG spectrometer shows aclear absorption maximum at 2627 cm−1 in the upperstripe, correctly corresponding to the acetonitrileresonance.

On closer inspection, there are two sources of sys-tematic noise in the SFG signal. First, the CCD de-fects cause circular patterns that are randomly dis-persed across the array. Second, diffraction of theoptical radiation on the edge of the KNbO3 crystalcauses vertical diffraction lines in the upconvertedsignal, leading to spatial distortions of the 800 nmlight. To eliminate these two defects, the data wereconvoluted with a two-dimensional Gaussian whosewidth was 15 pixels in each direction. Because thewidth of the convolution is much smaller than the fo-cusing limit of the IR, this smoothing does not affectthe retrieved spectral information. After normalizingfor the spatial profile of the optical pulse and inte-grating each spectrum separately, a difference spec-trum was retrieved. Figure 4(a) shows the raw andsmoothed absorption spectrum of the acetonitrile

Fig. 4. Dual channel spectrometer. (a) Differential absor-bance of vibrational resonance in acetonitrile. The upper(lower) curve is from a raw (smoothed) CCD image. Thesolid (dashed) curve is from an upconverted CCD image(FTIR). (b) Raw CCD image.

resonance overlaid with a trace taken with a com-mercial Fourier transform IR spectrometer. Both thepeak position and the asymmetric line shape of theabsorption are accurately measured.

In summary, we have described a new method ofacquiring mid-IR spectra using a silicon CCD cam-era. The demonstration here was made with an inex-pensive 8-bit video-rate camera, but higher-end ar-rays with kHz data acquisition rates willdramatically improve the signal-to-noise ratio. WithTi:sapphire pulses, standard nonlinear crystals (e.g.,KNbO3, LiNbO3, and BBO) can be used to upconvert15.5 mm radiation. Upconversion of longer wave-lengths can be accomplished with other ultrafastsources (e.g., near-IR OPA radiation). With the appro-priate choice of nonlinear crystals and optics, thistechnique can be retrofitted to any IR spectrometer,providing a cheaper alternative to liquid nitrogencooled mid-IR arrays.

We thank Joe Loparo for stimulating discussions.This project received support from the Basic EnergySciences of the U.S. Department of Energy (grantDE-FG02-99ER14988). A. Tokmakoff thanks theDavid and Lucile Packard Foundation for fellowshipsupport. M. F. DeCamp’s e-mail address ismfdecamp@mit.edu.

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