585:" (the colloquial term for the lack of techniques in the THz frequency range) was that electronics routinely have limited operation at frequencies at and above 10 Hz. Two experimental parameters make such measurement possible in THz-TDS with LT-GaAs antennas: the femtosecond “gating” pulses and the < 1 ps lifetimes of the charge carriers in the antenna (effectively determining the antenna's “on” time). When all optical path lengths have fixed length, an effective dc current results at the detection electronics due to their low time resolution. Picosecond time resolution does not come from fast electronic or optical techniques, but from the ability to adjust optical path lengths on the micrometer (μm) scale. To measure a particular segment of a THz pulse, the optical path lengths are fixed and the (effective dc) current at the detector due to the particular segment of electric field of the THz pulse.
149:
1298:
60:
645:. There is a difficulty in applying the Kramers-Kronig relations as written, because information about the sample (reflected power, for example) must be obtained at all frequencies. In practice, far separated frequency regions do not have significant influence on each other, and reasonable limiting conditions can be applied at high and low frequency, outside of the measured range.
458:) are primarily due to the rapid rise of the photo-induced current in the semiconductor and short carrier lifetime semiconductor materials (e.g., LT-GaAs). This current may persist for only a few hundred femtoseconds to several nanoseconds depending on the substrate material. This is not the only means of generation but is currently (as of 2008) the most common.
17:
230:
597:
is usually amplified with a low-bandwidth amplifier. This amplified current is the measured parameter that corresponds to the THz field strength. Again, the carriers in the semiconductor substrate have an extremely short lifetime. Thus, the THz electric field strength is only sampled for an extremely narrow slice (
648:
THz-TDS, in contrast, does not require use of
Kramers-Kronig relations. By measuring the electric field of a THz pulse in the time-domain, the amplitude and phase of each frequency component of the THz pulse are known (in contrast to the single piece of information known by a power measurement). Thus
636:
THz-TDS measures the electric field of a pulse and not just the power. Thus, THz-TDS measures both the amplitude and phase information of the frequency components it contains. In contrast, measuring only the power at each frequency is essentially a photon counting technique; information regarding the
596:
Photoconductive detection is similar to photoconductive generation. Here, the voltage bias across the antenna leads is generated by the electric field of the THz pulse focused onto the antenna, rather than some external generation. The THz electric field drives current across the antenna leads, which
325:
When an ultra-short (100 femtoseconds or shorter) optical pulse illuminates a semiconductor and its wavelength (energy) is above the energy band-gap of the material, it photogenerates mobile carriers. Most carriers are generated near the surface of the material (typically within 1 micrometre) because
356:
material. This incident laser pulse abruptly changes the antenna from an insulating state into a conducting state. Due to an electric bias applied across the antenna, a sudden electric current transmits across the antenna. This changing current lasts for about a picosecond, and thus emits terahertz
190:
is used to divide a single ultrashort optical pulse into two separate beams. A 50/50 beamsplitter is often used, supplying equal optical power to the terahertz generator and detector, though it is common to provide the terahertz generation path with more power given the inefficiency of the terahertz
161:
Constructing a THz-TDS experiment using low temperature grown GaAs (LT-GaAs) based antennas requires a laser whose photon energy exceeds the band gap of the material. Ti:sapphire lasers tuned to around 800 nm, matching the energy gap in LT-GaAs, are ideal as they can generate optical pulses as
297:
Though not strictly a spectroscopic technique, the ultrashort width of THz radiation pulses allows for measurements (e.g., thickness, density, defect location) on difficult-to-probe materials like foam. These measurement capabilities share many similarities to those of pulsed ultrasonic systems as
545:
of the laser pulse and the terahertz pulse inside the crystal. Typically, a thicker crystal will generate higher intensities, but lower THz frequencies. With this technique, it is possible to boost the generated frequencies to 40 THz (7.5 ÎĽm) or higher, although 2 THz (150 ÎĽm) is more
237:
Off-axis parabolic mirrors are commonly used to collimate and focus THz radiation. Radiation from an effective point source, such as from a low-temperature gallium arsenide (LT-GaAs) antenna (active region ~5 ÎĽm) incident on an off-axis parabolic mirror becomes collimated, while collimated
220:
of water molecules. Alternatively, nitrogen, as a diatomic molecule, has no electric dipole moment, and does not (for the purposes of typical THz-TDS) absorb THz radiation. Thus, a purge box may be filled with nitrogen gas so no unintended discrete absorptions in the THz frequency range occur.
640:
Even when measuring only the power reflected from a sample, the complex optical response constant of the material can be obtained. This is so because the complex nature of an optical constant is not arbitrary. The real and imaginary parts of an optical constant are related by the
330:. Secondly, the presence of a surface creates a break of symmetry that causes carriers to move (on average) only into the bulk of the semiconductor. This phenomenon, combined with the difference of mobilities of electrons and holes, also produces a dipole. This is known as the
203:
to redirect the beam along a well-defined output path but following a delay. Movement of the stage holding the retroreflector corresponds to an adjustment of path length and consequently the time at which the terahertz detector is gated relative to the source terahertz pulse.
326:
pulses are absorbed exponentially with respect to depth. This has two main effects. Firstly, it generates a band bending that has the effect of accelerating carriers of different signs in opposite directions (normal to the surface), creating a dipole. This effect is known as
554:
The electrical field of terahertz pulses is measured in a detector simultaneously illuminated with an ultrashort laser pulse. Two common detection schemes are used in THz-TDS: photoconductive sampling and electro-optical sampling. The power of THz pulses can be detected by
117:, the measurement must cover each point in time (delay-line offset) of the resulting test pulse. The response of a test sample can be calibrated by dividing its spectrum so obtained by the spectrum of the terahertz pulse obtained with the sample removed, for instance.
89:
components covering much of the terahertz range, often from 0.05 to 4 THz, though the use of an air plasma can yield frequency components up to 40 THz. After THz pulse generation, the pulse is directed by optical techniques, focused through a sample, then measured.
573:(S/N) of the resulting time-domain waveform depends on experimental conditions (e.g., averaging time). However due to the coherent sampling techniques described, high S/N values (>70 dB) are routinely observed with 1 minute averaging times.
649:
the real and imaginary parts of an optical constant can be known at every frequency within the usable bandwidth of a THz pulse, without need of frequencies outside the usable bandwidth or
Kramers-Kronig relations.
242:
such as metal-coated mirrors as well as lenses made from materials that are transparent at THz wavelengths. Samples for spectroscopy are commonly placed at a focus where the terahertz beam is most concentrated.
84:
that are accelerated to create the terahertz pulse. In the use of non-linear crystals as a source, a high-intensity ultrashort pulse produces THz radiation from the crystal. A single terahertz pulse can contain
152:
A typical THz time domain spectroscopy (THz-TDS) system. Half waveplate (HWP), polarizing beamsplitter (PBS), steering mirrors (M#), photoconductive antenna, parabolic mirrors (PM#), quarter waveplate (QWP).
559:(heat detectors cooled to liquid-helium temperatures), but since bolometers can only measure the total energy of a terahertz pulse rather than its electric field over time, they are unsuitable for THz-TDS.
212:
A purge box is typically used so that absorption of THz radiation by gaseous water molecules is minimized. A dry air source is often used for this purpose, however, a nitrogen gas source may also be used.
540:
The bandwidth of pulses generated by optical rectification is limited by the laser pulse duration, terahertz absorption in the crystal material, the thickness of the crystal, and a mismatch between the
178:
Silver-coated mirrors are optimum for use as steering mirrors for infrared pulses around 800 nm. Their reflectivity is higher than gold and much higher than aluminum at that wavelength.
113:(technically, the measurement obtains the convolution of the test signal and the time-domain response of the strobed detector). To obtain the resulting frequency domain response using the
428:
sources emitting at a center wavelength of 1550 nm. Therefore, the photoconductive emitters must be based on semiconductor materials with smaller band gaps of approximately 0.74
894:
L. Duvillaret; F. Garet; J.-F. Roux; J.-L. Coutaz (2001). "Analytical modeling and optimization of terahertz time-domain spectroscopy experiments, using photoswitches as antennas".
267:). Many interesting materials have unique spectral fingerprints in the terahertz range that may be used for identification. Demonstrated examples include several different types of
109:
the optical probe pulse sent to it. By varying the path length traversed by the probe pulse, the test signal is thereby measured as a function of time—the same principle as a
352:
When generating THz radiation via a photoconductive emitter, an ultrafast pulse (typically 100 femtoseconds or shorter) creates charge carriers (electron-hole pairs) in a
566:
radiation. Additionally, because the time slice of the measurement is extremely narrow, the noise contribution to the measurement is extremely low.
934:
1258:
680:"Tunable broadband THz emission from air plasma pumped by femtosecond pulses composed of a fundamental frequency with its detuned second harmonic"
280:
717:
Niessen, K. A.; Xu, M.; George, D. K.; Chen, M. C.; Ferre-D-Amare, A. R.; Snell, E. H.; Cody, V.; Pace, J.; Schmidt, M.; Markelz, A. G. (2019).
628:
As with the generation, the bandwidth of the detection is dependent on the laser pulse duration, material properties, and crystal thickness.
199:
An optical delay-line is implemented using a movable stage to vary the path length of one of the two beam paths. A delay stage uses a moving
658:
855:"64 ÎĽW pulsed terahertz emission from growth optimized InGaAs/InAlAs heterostructures with separated photoconductive and trapping regions"
238:
radiation incident on a parabolic mirror is focused to a point (see diagram). Terahertz radiation can thus be manipulated spatially using
294:
Since many materials are transparent to THz radiation, underlying materials can be accessed through visually opaque intervening layers.
488:
of the resulting THz pulse is primarily limited by the duration of the laser pulse, while the frequency position of the maximum of the
609:
The materials used for generation of terahertz radiation by optical rectification can also be used for its detection by using the
418:
with pulse energies of about 1 mJ, the electrode gap can be increased to several centimeters with a bias voltage of up to 200 kV.
997:
284:
1167:
287:
forms of many compounds used as active pharmaceutical ingredients (API) in commercial medications as well as several illegal
1327:
485:
642:
1332:
1223:
526:
415:
298:
the depth of buried structures can be inferred through timing of their reflections of these short terahertz pulses.
1027:
815:
M.Suzuki and M. Tonouchi (2005). "Fe-implanted InGaAs terahertz emitters for 1.56 ÎĽm wavelength excitation".
514:
passes through a transparent crystal material that emits a terahertz pulse without any applied voltages. It is a
105:. The probe pulse strobes the detector that is sensitive to the electric field of the resulting terahertz signal
102:
637:
phase of the light is not obtained. Thus, the waveform is not uniquely determined by such a power measurement.
621:
of the detection pulse, proportional to the terahertz electric-field strength. With the help of polarizers and
445:
441:
437:
411:
327:
306:
There are three widely used techniques for generating terahertz pulses, all based on ultrashort pulses from
1197:
1017:
511:
260:
41:
1085:
191:
generation process compared to the detection efficiency of infrared (typically 800 nm wavelength) light.
138:
1322:
965:
570:
507:
501:
93:
THz-TDS requires generation of an ultrafast (thus, large bandwidth) terahertz pulse from an even faster
613:, where particular crystalline materials become birefringent in the presence of an electric field. The
679:
1192:
1075:
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903:
866:
824:
730:
618:
519:
782:
Davies, A. Giles; Burnett, Andrew D.; Fan, Wenhui; Linfield, Edmund H.; Cunningham, John E. (2008).
101:. That optical pulse is first split to provide a probe pulse whose path length is adjusted using an
1243:
1162:
1102:
1022:
563:
380:
45:
764:
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radiation since the
Fourier transform of a picosecond length signal will contain THz components.
110:
1263:
1137:
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at high optical intensities. This changing electrical polarization emits terahertz radiation.
489:
307:
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256:
167:
142:
114:
98:
94:
525:
Because of the high laser intensities that are necessary, this technique is mostly used with
76:
laser is used in the terahertz pulse generation process. In the use of low-temperature grown
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1301:
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255:. Many materials are transparent at terahertz wavelengths, and this radiation is safe for
81:
461:
Pulses produced by this method have average power levels on the order of several tens of
907:
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828:
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148:
126:
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783:
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laser with photon energies of 1.55 eV and pulse energies of about 10 nJ. For use with
48:. The generation and detection scheme is sensitive to the sample's effect on both the
1316:
1233:
1213:
1154:
1080:
614:
582:
421:
More recent advances towards cost-efficient and compact THz-TDS systems are based on
400:
353:
347:
134:
130:
125:
Components of a typical THz-TDS instrument, as illustrated in the figure, include an
53:
768:
233:
A parabolic mirror is shown with important focal lengths and several exemplary rays.
1253:
1122:
1117:
1058:
617:
caused by the electric field of a terahertz pulse leads to a change in the optical
598:
429:
422:
388:
331:
311:
252:
187:
37:
383:. In a commonly used scheme, the electrodes are formed into the shape of a simple
59:
695:
1283:
1144:
1127:
1107:
425:
410:
of the semiconductor substrate. This scheme is suitable for illumination with a
314:
163:
742:
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622:
473:
455:
396:
268:
703:
1248:
1090:
1070:
1053:
556:
469:
361:
216:
Water is known to have many discrete absorptions in the THz region that are
86:
49:
961:
760:
334:
effect and is particularly strong in high-mobility semiconductors such as
403:
288:
915:
407:
25:
953:
879:
854:
836:
16:
462:
264:
239:
935:"Exploring dynamics in the far-infrared with terahertz spectroscopy"
562:
Because the measurement technique is coherent, it naturally rejects
472:
during pulses can be many orders of magnitude higher due to the low
229:
1006:
481:
392:
228:
147:
58:
15:
588:
THz-TDS measurements are typically not single-shot measurements.
465:
433:
77:
979:
272:
546:
commonly used since it requires less complex optical setups.
492:
is determined by the carrier lifetime of the semiconductor.
853:
R.J.B. Dietz; B. Globisch; M. Gerhard; et al. (2013).
518:
process, where an appropriate crystal material is quickly
975:
454:
The short duration of THz pulses generated (typically ~2
251:
THz radiation has several distinct advantages for use in
896:
IEEE Journal of
Selected Topics in Quantum Electronics
141:, terahertz beam focusing and collimating optics like
395:
between them. The ultrafast laser pulse must have a
1206:
1153:
1041:
678:Fan, Zhengquan; Lu, Chenhui; Liu, Yi (2022-02-15).
784:"Terahertz spectroscopy of explosives and drugs"
170:are available as commercial, turnkey systems.
991:
8:
476:of mostly >1%, which is dependent on the
80:as an antenna, the ultrashort pulse creates
387:with a gap of a few micrometers and have a
375:(SI-GaAs), or other semiconductor (such as
998:
984:
976:
719:"Protein and RNA dynamical fingerprinting"
581:The original problem responsible for the "
878:
799:
750:
601:) of the entire electric field waveform.
625:, this polarization change is measured.
1259:Multiple-prism grating laser oscillator
848:
846:
670:
281:anisotropic terahertz microspectroscopy
20:Typical pulse as measured with THz-TDS.
63:Fourier transform of the above pulse.
7:
659:Time resolved microwave conductivity
30:terahertz time-domain spectroscopy
14:
1297:
1296:
529:. Typical crystal materials are
97:optical pulse, typically from a
44:are probed with short pulses of
1168:Amplified spontaneous emission
1:
801:10.1016/s1369-7021(08)70016-6
933:C. A. Schmuttenmaer (2004).
696:10.1016/j.optcom.2021.127532
527:amplified Ti:sapphire lasers
416:amplified Ti:sapphire lasers
271:, dynamic fingerprinting of
56:of the terahertz radiation.
1224:Chirped pulse amplification
371:(LT-GaAs), semi-insulating
279:using polarization varying
1349:
1028:List of laser applications
743:10.1038/s41467-019-08926-3
499:
360:Typically the two antenna
345:
1292:
1013:
592:Photoconductive detection
643:Kramers–Kronig relations
605:Electro-optical sampling
537:, and gallium selenide.
446:indium aluminum arsenide
399:that is short enough to
342:Photoconductive emitters
308:titanium-sapphire lasers
859:Applied Physics Letters
817:Applied Physics Letters
442:indium gallium arsenide
438:indium gallium arsenide
40:technique in which the
1018:List of laser articles
520:electrically polarized
512:ultrashort laser pulse
412:Ti:sapphire oscillator
328:surface field emission
234:
153:
64:
21:
684:Optics Communications
571:signal-to-noise ratio
508:optical rectification
502:Optical rectification
496:Optical rectification
247:Uses of THz radiation
232:
151:
135:beam steering mirrors
131:optical beamsplitters
111:sampling oscilloscope
62:
19:
1328:Terahertz technology
1193:Population inversion
484:source. The maximum
42:properties of matter
1333:Explosive detection
1244:Laser beam profiler
1163:Active laser medium
1103:Free-electron laser
1023:List of laser types
916:10.1109/2944.974233
908:2001IJSTQ...7..615D
871:2013ApPhL.103f1103D
829:2005ApPhL..86e1104S
735:2019NatCo..10.1026N
510:, a high-intensity
364:are patterned on a
139:terahertz generator
46:terahertz radiation
240:optical components
235:
154:
137:, delay stages, a
103:optical delay line
65:
22:
1310:
1309:
1264:Optical amplifier
1113:Solid-state laser
954:10.1021/cr020685g
880:10.1063/1.4817797
837:10.1063/1.1861495
543:propagation speed
535:gallium phosphide
516:nonlinear-optical
277:protein molecules
257:biological tissue
225:Parabolic mirrors
157:Ti:sapphire laser
145:, and detector.
143:parabolic mirrors
115:Fourier transform
99:Ti-sapphire laser
74:ultrashort pulsed
1340:
1300:
1299:
1274:Optical isolator
1239:Injection seeder
1219:Beam homogenizer
1198:Ultrashort pulse
1188:Lasing threshold
1000:
993:
986:
977:
972:
971:on July 8, 2007.
970:
964:. Archived from
948:(4): 1759–1779.
942:Chemical Reviews
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490:Fourier spectrum
449:heterostructures
373:gallium arsenide
369:gallium arsenide
321:Surface emitters
218:rotational modes
174:Steering mirrors
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1183:Laser linewidth
1173:Continuous wave
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1042:Types of lasers
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927:Further reading
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478:repetition rate
366:low temperature
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336:indium arsenide
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263:(as opposed to
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82:charge carriers
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1269:Optical cavity
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1229:Gain-switching
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1076:Carbon dioxide
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1064:Liquid-crystal
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1049:Chemical laser
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1033:Laser acronyms
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902:(4): 615–623.
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611:Pockels effect
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531:zinc telluride
500:Main article:
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385:dipole antenna
346:Main article:
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302:THz generation
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1234:Gaussian beam
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865:(6): 061103.
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631:
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624:
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616:
615:birefringence
612:
604:
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600:
591:
589:
586:
584:
583:Terahertz gap
576:
574:
572:
567:
565:
560:
558:
550:THz detection
549:
547:
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532:
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354:semiconductor
349:
348:Auston switch
341:
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207:
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194:
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189:
182:Beamsplitters
181:
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61:
57:
55:
51:
47:
43:
39:
38:spectroscopic
35:
31:
27:
18:
1323:Spectroscopy
1254:Mode locking
1207:Laser optics
966:the original
945:
941:
899:
895:
889:
862:
858:
820:
816:
810:
794:(3): 18–26.
791:
787:
777:
726:
722:
712:
687:
683:
673:
647:
639:
635:
627:
619:polarization
608:
599:femtoseconds
595:
587:
580:
568:
561:
553:
539:
524:
505:
460:
453:
420:
389:bias voltage
359:
351:
332:photo-Dember
324:
315:fiber lasers
305:
296:
293:
291:substances.
261:non-ionizing
253:spectroscopy
250:
236:
215:
211:
198:
188:beamsplitter
185:
177:
168:These lasers
162:short as 10
160:
124:
106:
92:
71:
33:
29:
23:
1284:Q-switching
1145:X-ray laser
1138:Ti-sapphire
1108:Laser diode
1086:Helium–neon
729:(1): 1026.
623:photodiodes
426:fiber laser
423:mode-locked
406:across the
312:mode-locked
285:polymorphic
195:Delay stage
95:femtosecond
68:Explanation
1317:Categories
690:: 127532.
665:References
632:Advantages
577:Downmixing
564:incoherent
557:bolometers
474:duty cycle
470:peak power
397:wavelength
362:electrodes
269:explosives
121:Components
1249:M squared
1071:Gas laser
1054:Dye laser
704:0030-4018
486:bandwidth
404:electrons
391:up to 40
381:substrate
208:Purge box
87:frequency
50:amplitude
1302:Category
1096:Nitrogen
962:15080711
769:70350342
761:30833555
653:See also
432:such as
289:narcotic
52:and the
1081:Excimer
904:Bibcode
867:Bibcode
825:Bibcode
752:6399446
731:Bibcode
480:of the
436:-doped
408:bandgap
36:) is a
34:THz-TDS
26:physics
1123:Nd:YAG
1118:Er:YAG
1059:Bubble
1007:Lasers
960:
767:
759:
749:
723:Nature
702:
468:. The
401:excite
265:X-rays
259:being
1128:Raman
969:(PDF)
938:(PDF)
765:S2CID
482:laser
466:watts
463:micro
54:phase
1133:Ruby
958:PMID
757:PMID
700:ISSN
569:The
275:and
78:GaAs
1091:Ion
950:doi
946:104
912:doi
875:doi
863:103
833:doi
796:doi
747:PMC
739:doi
692:doi
688:505
506:In
440:or
377:InP
310:or
273:DNA
24:In
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790:.
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727:10
725:.
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698:.
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682:.
533:,
456:ps
451:.
434:Fe
430:eV
379:)
338:.
317:.
283:,
186:A
166:.
164:fs
133:,
129:,
28:,
999:e
992:t
985:v
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914::
906::
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32:(
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