超快光学 第14章 超快光谱

1、Ultrafast Laser Spectroscopy,How and why ultrafast laser spectroscopy? Generic ultrafast spectroscopy experiment The excite-probe experiment Lock-in detection Transient-grating spectroscopy Ultrafast polarization spectroscopy Optical heterodyne detection (OHD) Spectrally resolved excite-probe spectr

2、oscopy,Ultrafast laser spectroscopy: Why,Most events that occur in atoms and molecules occur on fs and ps time scales because the length scales are very small. Fluorescence occurs on a ns time scale, but competing non-radiative processes only speed things up because relaxation rates add,Biologically

3、 important processes utilize excitation energy for purposes other than fluorescence and hence must be very fast. Collisions in room-temperature liquids occur on a few-fs time scale, so nearly all processes in liquids are ultrafast. Semiconductor processes of technological interest are necessarily ul

4、trafast or we wouldnt be interested,1/tex = 1/tfl + 1/tnr,Ultrafast laser spectroscopy: How,Ultrafast laser spectroscopy involves studying ultrafast events that take place in a medium using ultrashort pulses and delays for time resolution. It usually involves exciting the medium with one (or more) u

5、ltrashort laser pulse(s) and probing it a variable delay later with another,The signal pulse energy (or change in energy) is plotted vs. delay. The experimental temporal resolution is the pulse length,Whats going on in spectroscopy measurements,The excite pulse(s) excite(s) molecules into excited st

6、ates, which changes the mediums absorption coefficient and refractive index,The excited states only live for a finite time (this is the quantity wed like to find!), so the absorption and refractive index recover,The simplest ultrafast spectroscopy method is the Excite-Probe Technique,Excite the samp

7、le with one pulse; probe it with another a variable delay later; and measure the change in the transmitted probe pulse energy or average power vs. delay,The excite and probe pulses can be different colors. This technique is also called the Pump-Probe Technique,The excite pulse changes the sample abs

8、orption seen by the probe pulse,Modeling excite-probe measurements,Let the unexcited medium have an absorption coefficient, a0. Immediately after excitation, the absorption decreases by Da0. Excited states usually decay exponentially: Da(t) = Da0 exp(t /tex) for t 0 where t is the delay after excita

9、tion, and tex is the excited-state lifetime. So the transmitted probe-beam intensityand hence pulse energy and average powerwill depend on the delay, t, and the lifetime, tex: Itransmitted(t) = Iincident expa0 Da0exp(t /tex)L where L = sample length = Iincident expa0L expDa0exp(t /tex)L Iincident ex

10、pa0L 1+Da0exp(t /tex)L assuming Da0 L 1 Itransmitted(-) 1+Da0exp(t /tex)L,DT(t) / T0 = Itransmitted(t) - Itransmitted() / Itransmitted(,Modeling excite-probe measurements (contd,The relative change in transmitted intensity vs. delay, t, is,Itransmitted(t) Itransmitted(-) 1+Da0exp(t /tex)L,DT(t) / T0

11、 Da0 exp(t /tex) L,Modeling excite-probe measurements (contd,More complex decays occur if intermediate states are populated or if the motion is complex. Imagine probing an intermediate transition, whose states temporarily fill with molecules on their way back down to the ground state,Lock-in Detecti

12、on greatly increases the sensitivity in excite-probe experiments,This involves chopping the excite pulse at a given frequency and detecting at that frequency with a lock-in detector,Chopped excite pulse train,Probe pulse train,Lock-in detection automatically subtracts off the transmitted power in th

13、e absence of the excite pulse. With high-rep-rate lasers, it increases sensitivity by several orders of magnitude,The excite pulse periodically changes the sample absorption seen by the probe pulse,Chopper,Lock-in detector,The lock-in detects only one frequency component of the detector voltagechose

14、n to be that of the chopper,Excite-probe studies of bacterio-rhodopsin,Rhodopsin is the main molecule involved in vision. After absorbing a photon, rhodopsin undergoes a many-step process, whose first three steps occur on fs or ps time scales and are poorly understood,Excitation populates a new stat

15、e, which absorbs at 460nm and emits at 860nm. It is thought that this state involves motion of the carbon atoms (12, 13, 14). An artificial version of rhodopsin, with those atoms held in place, reveals this change on a much slower time scale, confirming this theory,Zhong, et al., Ultrafast Phenomena

16、 X, p. 355 (1996,Excite-probe measurements in DNA,Pecourt, et al., Ultrafast Phenomena XII, p.566 (2000,DNA bases undergo photo-oxidative damage, which can yield mutations. Understanding the photo-physics of these important molecules may help to understand this process,Excite-probe measurements of H

17、ypericin, an anti-viral substance,When excited by light, Hypericin deactivates HIV. So it would be nice to understand how it works,Relative change in absorbance,These curves (for two different solvents) show the rise time for a proton-transfer process important in its biological activity,M.J. Fehr,

18、et al., Ultrafast Phenomena IX, pg. 462 (1994,Excite-probe measurements of Terawatt femtosecond UV pulses in water,Pommeret, et al., Ultrafast Phenomena XII, p. 536 (2000,High-intensity UV ultrashort pulses may someday be used in surgery. So understanding what these pulses do to water is important.

19、Hydrated electrons are formed in very high concentrations (0.01 molar,The induced absorption seen here is very high,Excite-probe reflection spectroscopy,Exciting a surface and probing its reflectivity later reveals surface physics. Here, a quantum wire is studied using ultrashort pulses in a near-fi

20、eld scanning optical microscope to yield 200nm spatial resolution, too,Emiliani, et al., Ultrafast Phenomena XII, p. 256 (2000,DR/R vs. x and y for a delay of 10ps,Excite-probe measurements can reveal quantum beats: Theory,Since ultrashort pulses have broad bandwidths, they can excite two or more ne

21、arby states simultaneously,Probing the 1-2 superposition of states can yield quantum beats in the excite-probe data,Excite-probe measurements can reveal quantum beats: Experiment,Here, two nearby vibrational states in molecular iodine interfere,These beats also indicate the motion of the molecular w

22、ave packet on its potential surface. A small fraction of the I2 molecules dissociate every period,Zadoyan, et al., Ultrafast Phenomena X, p. 194 (1996,Quantum beats in polymers using 5-fs pulses,Excite-probe measurements in polydiacetylene show several different frequencies, implying several (vibrat

23、ional) states were excited,Kobayashi, Ultrafast Phenomena XII, p. 575 (2000,lpr,The coherence spike in ultrafast spectroscopy,When the delay is zero, other nonlinear-optical processes occur, a involving coherent 4WM between the beams and generatingadditional signal not described by the simple Da mod

24、el. As in autocorrelation, its called the coherence spike or coherent artifact. Sometimes you see it; sometimes you dont,Alternate picture: the pulses induce a grating in the absorption and/or refractive index, which diffracts light from each beam into the other,Taking advantage of the induced grati

25、ng: the Transient-Grating Technique,Two simultaneous excitation pulses induce a weak diffraction grating, followed, a variable delay later, by a probe pulse. Measure the diffracted pulse energy vs. delay,This method is background-free, but the diffracted pulse energy goes as the square of the diffra

26、cted field and hence is weaker than that in excite-probe measurements,Delay,Excite pulse #1,Sample,Excite pulse #2,Probe pulse,Diffracted pulse,Intensity fringes in sample due to excitation pulses,A transient-grating measurement may still have a coherence spike,When all the pulses overlap in time, w

27、hos to say which are the excitation pulses and which is the probe pulse,A transient-grating experiment with a coherence spike,Delay,Excite pulse #1 (acting as the probe,Excite pulse #2,Probe pulse (acting as an excite pulse,Intensity fringes in sample due to an excitation pulse and the probe acting

28、as an excitation pulse,What the transient-grating technique measures,It measures the Pythagorean sum of the changes in the absorption and refractive index. The diffraction efficiency, , is given by,This is in contrast to the excite-probe technique, which is only sensitive to the change in absorption

29、 and depends on it linearly,H. Eichler, Laser-Induced Dynamic Gratings, Springer-Verlag, 1986,If the absorption grating dominates and the excite-probe decay is exp(- /ex), then the TG decay will be exp(-2 /ex,You might think that a grating can be induced only by a sinusoidal intensity pattern (cause

30、d by the interference of two parallel-polarized beams). But orthogonally polarized beams, which have a constant intensity vs. position, also induce a grating! An orientation grating. Variation of the electric field vs. position,Transient orientation gratings,Orientation gratings can also decay due t

31、o orientational relaxation,Induced gratings can also decay by diffusion,Diffusion can wash out an induced grating. Sometimes diffusion is faster than excited-state decay,Diffusion occurs on a time scale that depends on the grating fringe spacing. If the fringes are closely spaced, diffusion is very

32、fast; if the fringes are far apart, then its much slower. Varying the grating fringe spacing can determine the time scales for both decay mechanisms,where D = diffusion coeff,Transient-grating measurements in multiple quantum wells,Both concentration (amplitude) and orientation (spin) gratings induc

33、ed by excite beams with parallel and perpendicular polarizations. The orientation grating decays much faster,Time-resolved fluorescence is also useful,Exciting a sample with an ultrashort pulse and then observing the fluorescence vs. time also yields sample dynamics. This can be done by directly obs

34、erving the fluorescence or, if its too fast, by time-gating it with a probe pulse in a SFG crystal,Time-resolved fluorescence decay,When different tissues look alike (i.e., have similar absorption spectra), looking at the time-resolved fluorescence can help distinguish them,Here, a malignant tumor c

35、an be distinguished from normal tissue due to its longer decay time,Normal tissue,Malignant tumor,Svanberg, Ultrafast Phenomena IX, p. 34 (1994,Ultrafast Polarization Spectroscopy,Its also possible to change the absorption coefficient differently for the two polarizations. This is called induced dic

36、hroism. It also rotates the probe polarization and can also be used to study orientational relaxation,A 45-polarized excite pulse induces birefringence in an ordinarily isotropic sample via the Kerr effect. A variably delayed probe pulse between crossed polarizers watches the birefringence decay, re

37、vealing the sample orientational relaxation,Nice features of ultrafast polarization spectroscopy,Its even easier to set up than excite-probe (just cross two beams in space and time, and cross the polarizers to more easily see the signal beam). Its almost background-free (crossed polarizers transmit

38、as little as 10-6 of the incident light). Unlike excite-probe, it measures both absorption and phase effects. It can use lock-in detection. And simultaneously, it can use optical heterodyne detection, which optimizes the signal-to-noise ratio,Heterodyned Ultrafast Polarization Spectroscopy,This triv

39、ial (seemingly inappropriate!) change can actually improve the sensitivity by many orders of magnitude,Optical Heterodyne Detection (OHD) polarization spectroscopy involves slightly uncrossing the polarizers. This allows some of the probe pulse to leak into the detector and combine coherently with t

40、he signal pulse,Heterodyned ultrafast polarization spectroscopy,As long as the leaked probe intensity the signal intensity, we can neglect the latter,Heterodyned polarization spectroscopy adds a small amount of the probe pulse, dEprobe(t), to the (even smaller) signal pulse. As a result, we now dete

41、ct the squared magnitude of the sum of these two fields,As long as the probe intensity is stable, this yields a huge improvement in sensitivityespecially when combined with lock-in detection,This yields a signal term proportional to Esig(t), which is much larger than its squared magnitude. And it al

42、so yields its phase,usual PS signal,Heterodyned ultrafast polarization spectroscopy of liquids,Castner and Chang, Ultrafast Phenomena X, p. 296 (1996,Using different colors for the excite and probe pulses, this technique is called the Optical Heterodyne Detection-Raman-Induced Kerr Effect Spectrosco

43、py (OHD-RIKES,Sample media are various amides,Notice how very clean the data are,The excite pulse can induce a change in the refractive index seen by the probe pulse, which is enhanced when wex wpr = w10,Heterodyned ultrafast polarization spectroscopy of CS2,Loughnane, et al., Ultrafast Phenomena X,

44、 p. 304 (1996,OHD-RIKES study of CS2 at different temperatures,Temporally and spectrally resolving the fluorescence of an excited molecule,Exciting a molecule and watching its fluorescence reveals much about its potential surfaces. Ideally, one would measure the time-resolved spectrum, equivalent to its intensity and phase vs. time (or frequency,Here, excitation occurs to a predissociative state, but other situations are just as interesting. Analogous s