WO2014191344A1

WO2014191344A1 - Terahertz laser system

Info

Publication number: WO2014191344A1
Application number: PCT/EP2014/060806
Authority: WO
Inventors: Christoph Peter HAURI; Clemens RUCHERT; Carlo VICARIO
Original assignee: Paul Scherrer Institut
Priority date: 2013-05-28
Filing date: 2014-05-26
Publication date: 2014-12-04

Abstract

It is the objective of the present invention to provide a Terahertz laser system. This objective is achieved according to the invention by a Terahertz laser apparatus, for example for use in skin cancer treatments, comprising: • a) a pump laser for generating a collimated near/mid-infrared pump laser pulse; • b) an organic crystal which receives the collimated near/mid-infrared pump laser pulse thereby generating Terahertz radiation by optical rectification in the nonlinear organic crystal; and • c) a low-pass filter for separating collimated near/mid-infrared pump laser pulse from the generated Terahertz radiation.

Description

Terahertz laser system

The present invention relates to a Terahertz laser system that can be used for example in skin cancer treatment applications.

It has been a challenge in the past to cover the Terahertz range between 1-20 Terahertz (so-called THz gap) with high- field (i.e. > MV/cm) and table-top Terahertz lasers. The Terahertz frequency band lies between the optical

frequencies (>20 THz) and the electronically accessible frequency range (<0.1 THz) . In particular, the production of intense single-cycle and half-cycle pulses in this THz gap has been difficult to achieve. Such pulses require an ultra- broadband spectrum of several octaves. Conventional

techniques do not allow the formation of ultra-broadband spectra covering the THz gap and simultaneously delivering a high peak power. The most promising technique is based on a χ ^{(2 )} process, such as optical rectification (OR) , or a χ ^{( 3 )} process, such as difference frequency generation.

Very recently, high-power THz pulse generation was

demonstrated by use of a LiNbC>3 crystal as THz emitter.

However, this approach is limited to <3 THz and does not cover the full THz gap. Furthermore, complicated pulse front tilting is required to achieve best phase-matching between the driving laser and the THz emission. Another drawback is that the generated THz beam is elliptical and has a

different divergence in x and y. This makes it difficult to achieve tight focusing and yet a real challenge to achieve high fields .

It is therefore the objective of the present invention to provide a Terahertz laser system that provides a Terahertz radiation in the frequency of the so-called THz gap having high fields and a sufficient focusing of the radiation.

This objective is achieved according to the invention by a Terahertz laser apparatus, for example for use in skin cancer treatments, comprising:

a) a pump laser for generating a collimated near/mid- infrared pump laser pulse, preferably delivering up to 20 mJ/cm²;

b) an organic crystal which receives the collimated

near/mid-infrared pump laser pulse thereby generating

Terahertz radiation by optical rectification in the

nonlinear organic crystal; and

c) a low-pass filter for separating collimated near/mid- infrared pump laser pulse from the generated Terahertz radiation .

The generation scheme of the system is very simple. A femtosecond near/mid-infrared pump laser pulse is collimated and sent through an organic crystal. Terahertz radiation is produced by optical rectification in the nonlinear organic crystal. The pump laser is separated from the THz radiation by a low-pass filter. The THz output offers excellent characteristics for focusing. The THz divergence is the same in x and y, and the emitted THz offers a round, symmetric beam profile. Therefore, a diffraction-limited spot size can be achieved thanks to the excellent wave front properties. The use of an ultra-short pulse (i.e. <40 fs) allows the generation of pulses carrying a spectrum from 1 to 20 THz.

The present invention presents therefore a much easier and straight forward technique to generate intense and coherent THz radiation which offers excellent characteristics for tight focusing and high field production. Organic crystals suitable for the generation of the

Terahertz radiation are large-size phenolic

configurationally locked polyene OH1 (2- (3- (4- hydroxystyryl ) -5, 5-dimethylcyclohex-2- enylidene ) malononitrile ) organic bulk crystals and ionic salts such as DAST ( 4 ' -dimethylamino-iV-methyl-4-stilbazolium tosylate) and DSTMS ( 4-JV, JV-dimethylamino-4 ' -N' -methy1- stilbazolium 2 , 4 , 6-trimethylbenzene sulfonate) and other organic crystals like HMQ-TMS . Typically in these set-ups, the organic crystal can be chosen to have a clear aperture up to 12 mm at a nominal thickness of 0.5 mm. But larger sizes can be pumped by assembling different crystals to a mosaic-like structure. The nonlinear axis of the different crystals are then aligned parallel in order to achieve optical rectification in all crystals. This way crystal sizes as large as 30x30 mm2, or larger can be achieved.

In a further preferred embodiment, the pump laser is a Cr:Fosterite laser system or a TW-class Ti:Sa laser. The Cr:Fosterite laser system is particularly suited to product laser pulses being sufficiently collimated what is

advantageous in case of various assembled crystals.

Furthermore, the use of an amplitude and phase shaper in the above mentioned pump laser systems is proposed to achieve a shaped output in the THz range between 1 to 20 THz. This allows the production of arbitrary shaped THz pulses.

The novel features in the solution according to the present invention are:

a) The use of a high peak-power pump laser from a

Cr : Fosterite , a Ti:Sa laser system or from an optical parametric amplifier with a collimated beam, short pulse (<200 fs) in combination with a large size or mosaic-patterned organic crystal. This produces extremely high THz fields (>1 MV/cm) which cover the full THz gap.

b) The conversion technique offers collimated THz output with an excellent wave front and excellent beam profile. This allows to achieve focusing which is close to the diffraction limited spot size.

c) The large bandwidth and the high field open new

opportunities . Examples for these opportunities are the cancer treatment, in particular skin cancer treatment, and a THz-based ultrafast x-ray switch. Other

applications can be related to the exploration of fundamental physical phenomena and to biology and homeland security.

d) Terahertz pulse shaping across the THz gap via shaping of the pump beam allows the generation of almost arbitrarily shaped pulses.

The table below shows a summary of conventional techniques and their limitations and the last line of this table the properties achieved with the set-up according to the present invention .

LiNb0₃ Octave 1 MV/cm 0.1-3 THz Yes crystal

Other Varia « MV/cm Yes crystals

Organic several Several 0.1-20 Yes crystal octaves MV/cm THz

Table 1: Conventional techniques and THz generation with Organic Crystal according to the present invention.

For use in the above mentioned Terahertz cancer treatment, the inventive Terahertz source offers a novel approach for skin cancer treatment without affecting the surrounding healthy tissue. The idea behind is to irradiate the skin cancer cell with strong THz pulses, with a spectrum which matches the frequency "fingerprint" of the malign cells of the skin cancer. This allows deposition of laser pulse energy into THz-active modes of the malign cell leading to the destruction of the cell reproducibility. Terahertz pulses are a priori non-ionizing due to the low energy content of the individual photons. This makes them unique for skin cancer treatment.

So far, Terahertz has been used in medicine only for

diagnostic purpose (imaging) since a powerful THz laser was not available. The commercially available THz lasers provide only low power and limited THz frequencies (<3 THz) . So far, cancer treatment with THz was not considered and the

properties of skin cancer cells in the 1-20 THz frequency range have not been explored in the past. Another application for the high-peak power THz pulses is the realization of all-optical ultrafast magnetic switching. Terahertz allows a direct coupling of the magnetic field component to the magnetization. This allows to change the direction of the magnetization coherently on an ultrafast timescale .

Other applications will cover highly nonlinear processes in gases and solids, such as the generation of high-order harmonics or the creation of novel meta-states in

superconductors .

Preferred embodiments of the present invention are

hereinafter described in more detail with reference to the attached drawings which depict in:

Figure 1 schematically a Terahertz radiation system; Figure 2 schematically an experimental setup comprising a

Terahertz radiation system according to Figure 1;

Figure 3 THz energy generated from 8 mm aperture DAST

crystal as function of infrared pump energy and power density;

Figure 4 Focused THz generated in DSTMS together with

vertical and horizontal projections and Gaussian fit;

Figure 5 Temporal (left) and spectral shapes (right) of the

THz emitted by different organic crystals;

Figure 6 Direct manipulation of the absolute phase of the

THz pulse by dispersion management by means of transparent plastic; and

Figure 7 Spectrum generated in 180 ym DAST crystal and

reconstructed with first order autocorrelation. Fig. 1 shows schematically a Terahertz radiation system 2 for the THz generation with a collimated pump laser 4, an organic crystal 6 emitting collimated THz radiation 10, which is separated by a low pass filter 8 from the pump laser pulses.

The experimental setup comprising the Terahertz radiation system 2 of Figure 1 is shown in Figure 2 (a) . Figure 2 (b) shows a short pump pulse from an optical parametric

amplifier that drives optical rectification in the organic crystal 6. Standard electro-optical sampling with a short 800 nm probe pulse is used for temporal and spectral characterization as illustrated in Figure 2 (c) .

In brief, a TW-class Ti:Sa laser 4 at 100 Hz, producing 60 fs FWHM pulses, drives an optical parametric amplifier (OPA) and provides at the same time a sub- J probe at 800 nm for electro-optical sampling (EOS) . The OPA generates pulses with 70 fs FWHM duration and up to 2.5 mJ at the IR

wavelengths required for THz generation in the organic crystals 6. Alternatively, a high-power Cr:Fosterite laser is used as it provides directly the wavelength required to pump the organic crystal. The THz is emitted from thin organic crystals 6 collinearly to the pump laser 4. Behind the organic crystal 6, a thin sheet of Teflon or blackened Topas polymer selectively blocks as the low pass filter 8 the residual IR laser pulses. The Terahertz beam 10 is expanded and then tightly focused by an off-axis parabolic (OAP) mirror (f = 101.6mm) for electro-optical sampling

(EOS) . The EOS gives direct access to the THz electric field shape, the peak electric field, and the spectral content. The electro-optical spectral sensitivity for this electro- optical setup (GaP with 95 μπι thickness) decreases above 5 THz with frequency cut-off at 7 THz.

The cut-off in the detection crystal is determined mainly by the group velocity mismatching between THz and optical probe and by the electro-optical spectral response. THz pulses with spectral components beyond 5 THz are measured by means of Fourier transform first order interferometry . It is realized in a Michelson interferometer equipped with Golay cell detectors. The pellicle beam splitter installed in the interferometer is suitable for radiation in the frequency range from 0.7 to 20 THz. Absolute energy measurements are carried out by means of a calibrated Golay cell equipped with diamond entrance window (produced by Tydex) and of a pyroelectric deuterated triglycine sulfate detector (DTGS model D201 manufactured by Bruker) . The Golay cell

sensitivity has been calibrated in energy up to 12 THz using a blackbody source. The transverse beam profile is recorded with a bolometer un-cooled camera having a pixel size of 23.5 m.

In this disclosure, intense single-cycle THz transients generated in the organic crystals DAST, DSTMS, OH1 and HMQ- TMS are reported. In order to prevent crystal damage while using the maximum available pump flux, large crystals with up to 12mm aperture at a nominal thickness of 0.5mm, and a mosaic-like crystal structure of 20x20 mm2 have been utilized. On the other hand, thinner crystals are shown to be adequate for the generation of multi-octave spanning spectra covering the entire THz gap.

In Figure 3, the THz pulse energy generated in a 0.5mm thick DAST crystal with an aperture of 8mm is shown as a function of the infrared laser energy and power density. The energy stability of the THz pulse is comparable to the pump energy stability and equal to 1% rms . The temporal shape of the THz pulse is determined through EOS in a GaP crystal. The absolute THz energy is determined by the Golay cell

positioned after the low-pass filter and energy-calibrated THz attenuators. Maximum THz pulse energy E_THz = 45 μJ is reached when the crystal is pumped by E_m = 2.4 mJ (at 160 GW/cm2) . The experimental points confirm that the organic crystal 6 is operated far from the saturation. The pump-to- THz energy conversion (n_E = E_HTZ/E_IR) is about 2%. This corresponds to a photon conversion efficiency n_ph larger than 200% (n_ph = 100 N_THZ/N_IR, where N_THz and Ni_R are the number of photons for the THz and the IR pump pulses) . Furthermore, the shot-to-shot THz energy stability is remarkable. The energy variation recorded over 500

consecutive shots is better than 1% rms comparable to the OPA energy stability (see error bars of Figure 3) . The quasi-linear dependence of the generated energy indicates that the upscale of the THz output by increasing the source energy is further feasible. It is worth noting that the maximum power density of 160 GW/cm2 used is not inducing damages in the organic crystal. For other organic materials, the THz energy yield recorded in the same experimental conditions is approximately 1%.

The THz generation occurs over several millimeters of crystal area illuminated by the pump. The THz beam therefore has a divergence of only a few mrad. In the present setup, tight focusing is ensured by a 10 cm focal length parabola placed about 20cm from the THz organic crystal 6.

In Figure 4, the THz transverse intensity profile produced in an 8mm aperture DSTMS crystal is shown. The beam, recorded with a micro-bolometer camera, indicates vertical and horizontal FWHM of 350 and 366 μπι. The profile was recorded with an un-cooled bolometer array sensor. The focus shape is circular and not affected by astigmatism or other visible aberrations. As shown in the projected profiles, the intensity is well fitted by a Gaussian with full width half maximum (FWHM) of 360 μπι, which is close to the diffraction limit. The beam size measurement carried out for other organic crystals also resulted in excellent focusing characteristics. The foci are somewhat larger but still in the order of sub-mm.

The temporal shapes (left plots) and the spectral shapes (right plots) of the THz radiation for DAST, DSTMS, and OH1 are shown in Figure 5. the THz radiation emitted by

different organic crystals. The radiation is generated in approximately same thickness crystals: 0.43, 0.4, and 0.49mm for DAST, OH1, and DSTMS, respectively. In all the case the optical rectification process is driven by IR pump at power densities up tol60 GW/cm2. High energy per pulse

concentrated in one or two optical cycles and very tight focusing, reported before, allow for high THz field. The peak electric field is calibrated against the THz electro- optical effect and crosschecked with fluence and pulse duration measurement. The spectral intensity, right side plots in Figure 5, is calculated by Fourier transformation of the corresponding temporal evolution. For all the crystals, the THz transient approximates a single-cycle temporal oscillation with different grade of asymmetry. The maximum field recorded for DSTMS displays a peak of 1.5

MV/cm and magnetic field of 0.5 T for a single crystal and more than 6 MV/cm and 2 T for a mosaic-like crystal

structure. Even larger fields up to 100 MV/cm and 33 T are achieved when pumped by a Cr:Fosterite laser delivering 30 mJ pulse energy. For OH1 and DAST organic crystals, the peak field is slightly less than 1 MV/cm when pumped by the same pump fluence. With respect to OH1 and DAST, the higher frequencies for DSTMS result in a tighter focus and shorter THz pulse and thus in higher peak field. The calculated spectra reveal the absorption properties and phonon

resonances of each organic material. Due to these

absorptions, the temporal transients exhibit oscillations after the main pulse.

The THz pulse generated by optical rectification is

characterized by a stable absolute phase. This is an important feature for field-sensitive applications. Equally important, for exploring nonlinear dynamics, is the

generation of the highest field at the interaction by proper control of the absolute phase of the single/few cycle THz pulses. In fact, for the highest asymmetry and zero absolute offset, the THz pulse could potentially act as a quasi- unipolar stimulus. An efficient method to directly control the absolute phase of a THz pulse by combining dispersion properties of different transparent plastics was recently demonstrated. Teflon and Topas polymer sheets with different thickness were used in order to vary the THz absolute phase and forming a fully asymmetric pulse, as shown in Figure 6. The direct manipulation of the absolute phase of the THz pulse by dispersion management by means of transparent plastic causes the asymmetry. The pulses are characterized by a carrier envelope phase offset of (a) n/3 rad, (b) n/6 rad, and (c) zero. The energy losses and the peak field reduction associated with such phase manipulation are negligible. To the best of the present inventors, the absolute phase control for THz pulses has not been

demonstrated so far for such multi-octave spanning spectra. Moreover, in the present setup, the THz field polarity can be easily inverted by 180° rotation of the organic crystal.

Both the phase velocity mismatch and the THz re-absorption define the optimal thickness of the organic crystal and therefore the THz spectral content. For the generation of high frequencies, thin crystals are necessary. In Figure 7, the broadband spectrum generated via optical rectification in 180 μπι thick DAST and reconstructed with first order autocorrelation is presented. The generated spectrum is relatively flat and extends over the full THz gap. Several absorption peaks due to phonon resonances in the organic crystal 6 are visible. To avoid the decrease of sensitivity caused by the electro-optic detection (at frequencies higher than 5 THz), the spectra are reconstructed by Fourier interferometry . Moreover, for these measurements, the low pass Teflon filter, which is transparent to up to 5 THz, is replaced by blackened home-developed Topas polymer, which is characterized by low absorption in the entire THz gap. The measured spectra are impressively large and extend far beyond 10 THz.

With organic crystals it is possible to produce high fields in the whole THz gap (0.1-10 THz) and, furthermore, it seems feasible to extend the spectral region up to frequencies only accessible by optical difference frequency generation (>20 THz) . OH1, DAST, and DSTMS organic crystals have been discussed here in view of their potential of high-field generation in the THz gap (0.1-10 THz) . All crystals turned out to be highly efficient (up to 2% energy conversion yield) and broadband THz emitters when pumped by a mJ femtosecond infrared pulse. The generated THz radiation offers multi-octave spanning, single-cycle, and phase-stable pulses with up to 1.5 MV/cm electric and 0.5 T magnetic field strength. The generation scheme based on a collimated pumping geometry provides excellent THz focusing

characteristics. A new method to efficiently control the absolute phase and the polarity of the THz field has been disclosed here, too. Ultra-broadband THz spectra covering the frequency range 1-10 THz have been realized by optical rectification in a thin organic crystal. The THz source presented here will be of benefit for field-sensitive investigations to drive extreme nonlinear phenomena in gases and solids with single-cycle , half-cycle or arbitrarily- shaped transients.

Claims

Patent Claims

1. Terahertz laser apparatus (2), for example for use in skin cancer treatments, comprising:

a) a pump laser (4) for generating a collimated near/mid- infrared pump laser pulse;

b) an organic crystal (6) which receives the collimated near/mid-infrared pump laser pulse thereby generating collimated Terahertz radiation by optical rectification in the nonlinear organic crystal (6); and

c) a low-pass filter (8) for separating collimated near/mid- infrared pump laser pulse from the generated Terahertz radiation (10) .

2. The apparatus according to claim 1, wherein the pump laser (4) is a TW-class Ti:Sa laser (with/without OPA) or a Cr:Fosterite laser.

3. The apparatus according to claim 1 or 2, wherein the organic crystal (6) is a DAST ( (diethylamino) sulfur

Trifluoride) or a DSTMS (4-N, N-dimethylamino-4 ' - ' -methy1- stilbazolium 2 , 4 , 6-trimethylbenzenesulfonate) or a OH1 (2- (3- (4-hydroxystyryl) -5, 5-dimethylcyclohex-2 - enylidene) malononitrile) crystal .

4. The apparatus according to claim 3, wherein the organic crystal (6) has an aperture up to 12 mm at a nominal thickness of 0.5 mm, or a mosaic-like patterned organic crystal of any size.