Picosecond time-resolved absorption in the mid-infraredby seeded optical parametric generation
Thomas M. Jedju and Lewis Rothberg
An apparatus to probe picosecond photoinduced absorption in semiconductors between 2 and 5 um withpicosecond resolution is described. The IR source is based on difference frequency mixing an amplifiedpicosecond dye laser pulse with a synchronized white light continuum pulse. The choice of liquid forcontinuum generation is discussed, and applications to transient absorption in germanium and trans-polyacetylene are presented.
Time-resolved absorption and reflection experi-ments in semiconductors have improved our under-standing of electron-hole plasma relaxation,",2 melt-ing,3 carrier trapping,4 electron-photon coupling,5 andexcitonic effects.6 Sources of picosecond pulses fur-ther into the IR for such experiments are important toextend our knowledge about these phenomena andothers. Probing further in the IR allows laser andplasma frequencies' to be matched at lower carrierdensities,' enables investigation of midgap absorp-tions like solitons,5 polarons,7 and surface states,8 andmakes the band edge dynamics of small gap semicon-ductors amenable to study.
In the present paper, we describe an apparatus usedfor time-resolved absorption in semiconductors withprobe wavelengths from 2 to 5 um. We discuss theexperimental layout, characterize the generated pico-second IR pulses, and demonstrate their utility ontransient free carrier absorption in GE and on themidgap absorption of photogenerated soliton pairs intrans-polyacetylene. The IR pulses are generated bydifference frequency mixing a strong monochromaticpicosecond pulse with a white light continuum in anonlinear crystal. This is best described as seededoptical parametric amplification and results in easilytunable picosecond IR pulses with energies suitable foruse with solid-state detectors.
The authors are with AT&T Bell Laboratories, Murray Hill, NewJersey 07974-2070.
Empirically, we find that this scheme is more effi-cient than ordinary optical parametric amplification9
where output is built up solely from noise in the nonlin-ear crystal. One is not as severely restricted by thedamage threshold of the nonlinear crystal becausecrystal noise is replaced by white light continuum gen-erated in a liquid which is not damaged by tight focus-ing. This has the added advantage that bandwidthnarrowing of the IR can be accomplished by use ofinterference filters or Fabry-Perot resonators in thevisible continuum so that crystal phase matching is notthe dominant linewidth contribution.' 0 Higher ener-gies can also be obtained by amplification of the con-tinuum.
The method we are using also has some advantagesover difference frequency mixing the outputs of twoseparate amplified dye lasers." There is no timingjitter between lasers to limit the IR pulse length and noneed to pump dyes other than rhodamines. Our appa-ratus is more convenient than stimulated electronicRaman shifting in Cs vapor12 in that it requires no Csoven and no change of dyes to cover a large spectralregion. In addition, amplified spontaneous emissionfrom Cs ovens must be filtered out, making some IRwavelengths inaccessible. Also, using electronic Ra-man scattering, the visible pump and IR probe must betuned together, not an ideal arrangement for time-resolved absorption. At many wavelengths, however,higher IR pulse energies and narrower bandwidthshave so far been obtained using Cs Raman shifting.
Figure 1 depicts the time-resolved absorption appa-ratus. A cw mode-locked Nd+3:YAG laser is used topump synchronously a rhodamine 6G based dye laserto obtain 5 ps nearly transform-limited pulses of-lnJ each at 82-MHz repetition rate. A Q-switchNd+3:YAG is synchronized with the radio frequencydriver for the mode-locking crystal, and ten pulses persecond are selected for amplification in a three-stage
Fig. 1. Schematic of the apparatus for picosecond time-resolvedabsorption in the mid-IR. DA and DB are InSb detectors, and F is a
long pass filter.
dye amplifier containing rhodamine 610. TheNd+3:YAG is operated as an unstable resonator toobtain a nearly uniform filled in beam profile, and thesecond harmonic (532 nm) is used to pump the ampli-fied stages. The first two stages use transverselypumped prism dye cells13 for uniform pumping and areseparated by a spatial filter to reject amplified sponta-neous emission. The final stage is isolated from theprevious two by a long pass glass filter which acts as asaturable absorber. This stage is pumped longitudi-nally in a nearly copropagating geometry. Like othergroups,'2"14"15 we routinely obtain -1 mJ/pulse at 580nm containing <3% amplified spontaneous emission.Half of this energy is focused by a 15-cm lens into a 3-cm liquid cell which can contain H20, D20, CH30H, orC3D60 (deuterated acetone). The cell output consistsof both white light continuum which typically extendsfrom 300 to 900 nm and residual monochromaticinput at 580 nm. This combination is loosely focusedinto a LiIO3 crystal cut at 22° from its optic axis fortype I phase matching. The continuum and mid-IRbeam polarizations are along the crystal's ordinaryaxis, while the monochromatic beam is divided be-tween ordinary and extraordinary axes to achieve theproper effective index for phase matching. The bire-fringence of the crystal causes walk-off between thetwo polarization components of the monochromatic,and the calculated walk-off angle p is9
[ n)2 1 tanO]
p =stand1 n + , 3.5° (1)
1+ (no)2 tan0
where no and rne are ordinary and extraordinary indicesof LiIO3 at 580 nm,' 6 and 0 is the crystal angel (22°).With a 100-200-,gm beam diameter, this limits theinteraction length for mixing to 1-2 mm. Our mea-surements of the walk-off between polarizations areconsistent with the above angle.
Difference frequency output is obtained whosewavelength is determined by which components of the
vI20
mai
15
v')
Z 10
5
2.5 3.0 3.5 4.0 4.5 5.0WAVELENGTH (im)
Fig. 2. Infrared output as a function of wavelength for four differ-ent continuum generation liquids. The dominant Raman frequen-cies for the liquids are labeled at the top of the figure. The thermaldetector noise is <0.1 unit on the scale at the left. Lines connecting
data points are merely to guide the eye.
continuum are phase matched at the given crystalangle. Figure 2 summarizes the output as a function ofwavelength (crystal angle) for the different continuumliquids. The IR outputs are peaked around the Ra-man allowed vibrational modes'7 of the various liquids,as illustrated in the figure. In H20 and D20, the highfrequency Raman modes are diffuse due to inhomo-geneous broadening from hydrogen bonding. Usingdifferent liquids provides a way of optimizing the IRintensity and stability for a given wavelength.
As the laser pulses become shorter, self-phase modu-lation is more effective and stimulated Raman scatter-ing less so as depicted in Fig. 3. Figure 3(a) shows amonochromator trace of the IR output using methanolwhen the dye laser cavity length is misadjusted so thatthe amplified laser pulses are 50 ps long. StimulatedRaman scattering dominates the output so that, afterdifference frequency mixing, a sharp line at the Ramanfrequency is observed. In these conditions, the IRoutput is so strong that it can be measured with apyroelectric detector. For mixing -40-gJ first Stokesoutput S with -100 ,gJ of residual 580-nm light, weobtain -40 nJ in the IR. The photon conversion effi-ciency is -5 X 10-3. When stimulated Raman scatter-ing is important, a significant amount of mixing be-tween the S line and the white light continuum canoccur so that more than one difference frequency canbe phase matched at a given crystal angle. For somecrystal angles we observe two IR output frequencieswith methanol, one of which persists when a long passfilter cutting out the 580-nm light is introduced beforethe crystal. This is even true for 5-ps pulses where westill see an effect of stimulated Raman scattering [Fig.3(b)] on the output. In general, we do not use metha-
Fig. 3. Spectrum of the IR pulses for (a) 50-ps pulses using metha-nol, (b) 5-ps pulses using methanol, (c) 5-ps pulses using watercontinuum. Note the transition from stimulated Raman scattering
to continuum generation dominated mixing.
nol for our time-resolved absorption studies, and, infact, the data of Fig. 2 are taken through a variablebandpass filter to avoid spurious polychromatic out-put.
Figure 3(c) is the case of 5-ps pulses in water for thesame wavelength region. In water, stimulated Ramanscattering does not occur because the OH stretchingmodes are diffuse due to hydrogen bonding. The posi-tion of these modes nonetheless determines the opti-mal output range and is accordingly shifted by use ofheavy water. Approximately 50-100 nJ/nm is avail-able in the continuum to be downshifted, and outputsof 500 pJ-1 nJ are expected for the photon conversionefficiency quoted above and the 130 + 30-cm-1 band-width observed in Fig. 3(c). These energies are consis-tent with calculations from the observed photocurrentand quoted efficiency in the InSb detector.
The phase matched bandwidth of -130 + 30 cm-l isapproximately constant from 2.5 to 4 Am in all theliquids. This is about what one expects from the shortinteraction length 1- 1-2 mm due to walk-off. Calcu-lations of the condition AMl < v/2 show that an -100-cm-' bandwidth is phase matched at these frequen-cies. Some contribution to the bandwidth comes fromthe beam divergence, which allows the beams to sam-ple several crystal angles. This contribution is calcu-lated to be within our error for beams of divergence of10 mrad or less. Experimentally, we find the band-width to be relatively insensitive to the focusing condi-tions.
g. 30
° 25a.O 20In
o 15
a 10zI-
ij
0 5
0
* * 0 I
I-
F.
I ? I I II I
-30 -20 -10 0 10 20 30 40 50
PUMP-PROBE DELAY (PS)
Fig. 4. Onset of transient-free carrier absorption in germanium at2.8,um after the above bandgap pumping.
For time-resolved absorption, the IR is separatedwith a long pass filter and steered by gold mirrors andcalcium fluoride lenses (Fig. 1). The beam is tracedwith InSb detector and pinholes are placed so that aHe-Ne laser beam can be aligned collinear with the IR.This is used to achieve overlap with the visible pumpbeam on the sample surface. The transmitted IRbeam is collected by a cooled InSb detector. A siliconwafer is placed into the IR beam before the sample andacts as a beam splitter to collect a fraction of theincident beam for pulse-to-pulse normalization by asecond InSb detector. The ratio of the transmittedand normalization beams is accumulated by a boxcarintegrator and read by a laboratory computer. Thisdetection scheme helps to reduce our shot-to-shot fluc-tuations of -+30%, making transient absorptions ofseveral tenths of a percent feasible to measure.
The visible beam can be retarded by variable pathlength delay to study absorption dynamics. Completeprobe transmission is taken to be that when the probeprecedes the pump. Figure 4 illustrates such an ex-periment on a germanium wafer. Following the ab-sorption of above gap radiation (580 nm), the IR ab-sorption increases due to free carrier absorption. Therise time of this absorption is a measurement of theinstrumental resolution and demonstrates that the IRpulses are clean 4-5-ps pulses.
These pulses have been used to study transient ab-sorption of photogeneration charged solitons in trans-polyacetylene5 and identify rapid lattice deformationsdue to the strong electron-phonon couplings in thisquasi-1-D conjugated polymer. One of the most at-tractive features of the seeded parametric amplifier isthat tuning is simply accomplished by varying thecrystal angle. This procedure does not significantlyaffect the pump-probe timing for 5-ps pulses. ThusIR spectra of picosecond transients are easily recorded.The IR absorption of photogenerated charged solitonpairs in trans-polyacetylene is shown in Fig. 5. Thisis, to our knowledge, the first measurement of a pico-second transient spectrum in this wavelength range.The dashed line is the spectrum of long-lived extrinsiccharged solitons in trans-polyacetylene, which arise by
Fig. 5. Points with error bars are the spectrum of transient absorp-tion in trans-polyacetylene with above bandgap pump and zeropump-probe delay. The dashed line is a microsecond spectrum of
charged solitons formed by a defect mechanism (from Ref. 18).
a different mechanism and were identified by Vardenyet al.'8 The spectroscopic capability of our apparatusenables us to identify our transients unambiguouslyand show that they are similar to the long-lived soli-tons.
In summary, we have reported on difference fre-quency mixing between picosecond pulses and a whitelight continuum which allows us to do time-resolvedabsorption with 2-5gm probe wavelengths. We havefound our apparatus to be simple and reliable forstudying semiconductor dynamics. Currently, we areshortening our dye laser pulses to obtain subpicose-cond resolution. The pulses have already been suc-cessfully bandwidth narrowed by filtering the continu-um so that they can be used for vibrationalspectroscopy.1 0
It is expected that we can amplify the continuum aswell so that it is sufficiently energetic to be used as apump beam.
We thank Alex Harris for many useful suggestionsand insights.References
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* The Zentrum fur Antisemitismusforschung at the Tech-nische Universitdt Berlin is conducting a new study on theeffects of physicists' emigration from Germany after the Na-zis came to power. This four-year project, sponsored by theStiftung Volkswagenwerk, is designed to integrate social andcognitive dimensions and to write the history of emigrationas a group-biographical or "aggregate" phenomenon. Themethod of investigation is based primarily on quantitativestudies of the physics literature published between 1925 and1955 and on analyses of related paradigms and paradigm-changes in the context of their social environments. The pro-ject is divided into four parts dealing with the social andparadigmatic history of physics in Germany 1925-1933 and
its international relations, the transformation of the physicscommunities between 1933 and (roughly) 1938, the socialand cognitive impact of emigrant physicists on the physicscommunities in the countries receiving them, and the returnof some physicists to Germany after World War II. Readersof this Newsletter, and especially emigrants from German-speaking countries Who are in possession of unpublished me-moirs or other materials relevant for the study, are invited tocontact the Zentrum at TU Berlin-TEL 36, Ernst-Reuter-Platz 7, D-1000 Berlin 10, Federal Republic of Germany.The Project Manager is Prof. Dr. Herbert A. Strauss, NewYork and Berlin; principal investigator for the physics partis Dr. Klaus Fischer, Berlin.
