Name: rapid repetition rate fluctuation measurement of soliton crystals in a microresonator
Uploaded: 2021-12-15T13:00:00.000+00:00
Duration: 7 min 42 s
Description: Watch this Scientific Journal Video about Rapid Repetition Rate Fluctuation Measurement of Soliton Crystals in a Microresonator at JoVE.com

This work was supported by the National Natural Science Foundation of China (NSFC) (Grant 62075238, 61675231) and&#160;the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB24030600).

1. Optical coupling
<ol>
	<li>Polish the end-face of the MRR on a grinding plate using 1.5 &#956;m abrasive powders (aluminum oxide) mixed with water for 5 min.</li>
	<li>Fix the MRR with a chip fixture and place an eight-channel FA on a six-axis coupling stage, which includes three linear stages with a resolution of 50 nm and three angle stages with a resolution of 0.003&#176;. The patches of the MRR and FA are 250 &#956;m.</li>
	<li>Use a 1,550 nm laser as an optical source for real-time monitoring of the coupling efficiency. Carefully adjust the position of the FA until the inset loss reaches the minimum value, typically less than 6 dB, corresponding to a coupling loss of less than 3 dB per facet.</li>
	<li>Use an ultraviolet (UV) curved adhesive (Table of Materials) to glue the MRR and FA. Place the adhesive on the side edge of the contact surface, which ensures that there is no glue on the optical path.</li>
	<li>Expose the UV-curved adhesive to a UV lamp for 150 s and bake in a chamber at 120 &#176;C for more than 1 h.</li>
</ol>
2. Device packaging
<ol>
	<li>Conglutinate a 10.2 mm x 6.05 mm TEC chip with a maximum power of 3.9 W to the baseplate of a standard 14-pin butterfly package using silver glue. Solder the two electrodes of the TEC chip to two pins of the butterfly package.</li>
	<li>Paste a 5 mm &#215; 5 mm &#215; 1 mm tungsten plate to the surface of the TEC chip using silver glue. Use the tungsten plate as a heat sink to fill the gap between the TEC and MRR.</li>
	<li>Paste the MRR device to the top of the tungsten plate using silver glue and fix the pigtail of the FA to the output port of the butterfly package.</li>
	<li>Paste a thermistor chip to the surface of the TEC chip using silver glue. Connect one electrode of the thermistor to the top surface of TEC chip. Wire bond the other electrode of the thermistor and the top surface of TEC chip to two pins of the butterfly package using gold thread.</li>
	<li>Bake the packaged device at 100 &#176;C for 1 h to solidify the silver glue.</li>
	<li>Seal the butterfly package. Figure 1 shows the packaged device.</li>
</ol>
3. SCs generation
<ol>
	<li>Figure 2 shows the set-up of the experiments. Use an erbium-doped fiber amplifier (EDFA) to boost the pump for micro-comb generation. Control the polarization state of the pump using a fiber polarization controller (FPC). Connect all the devices using single mode fibers (SMF).</li>
	<li>Fix the wavelength of the pump laser at 1,556.3 nm. Manually tune the operation temperature through an external commercial TEC controller.</li>
	<li>Monitor the output optical spectrum with an optical spectrum analyzer. Detect the output power trace with a 3 GHz photodetector and record with an oscilloscope.</li>
	<li>Set the output of the EDFA to 34 dBm, corresponding to an on-chip power of 30.5 dBm (considering the coupling loss of the MRR and FA, insert loss of the FPC), which ensures that there is enough power coupled into the MRR for micro-comb generation.</li>
	<li>Set the thermistor to 2 k&#937;, corresponding to an operating temperature of 66 &#176;C. Then slowly decrease the operating temperature by changing the set value of the thermistor. In these experiments, when the thermistor was set to 5.8 k&#937;, corresponding to 38 &#176;C, one resonance of the MRR passed through the pump and a triangular shape power trace was recorded.</li>
	<li>Tune the polarization of the pump by the FPC until an SC step is observed at the falling edge of the triangular transmission power trace. Figure 3 shows a typical optical transmission power trace.</li>
	<li>Slowly decrease the operation temperature from ~66 &#176;C and stop when a palm-like optical spectrum is observed on the optical spectrum analyzer. The value of the thermistor was around 5.6 k&#937; in these experiments. Figure 4A and Figure 5B show the optical spectra of perfect SCs and SCs with a single vacancy, respectively.</li>
</ol>
4. Repetition rate fluctuation measurement
<ol>
	<li>Connect the generated SCs to a tunable bandpass filter (TBPF) to extract an individual comb line. Set the passband of the TBPF to 0.1 nm. Its central wavelength can be tuned over the full C and L band. The filter slope is 400 dB/nm.</li>
	<li>Couple the selected comb line to an asymmetric Mach-Zehnder interferometer (AMZI). The optical frequency in one arm of the AMZI is shifted by 200 MHz using an acousto-optic modulator (AOM). The optical field in the other arm is delayed by a segment of optical fiber. Delay fibers of 2 km and 25 km are used in these experiments.</li>
	<li>Detect the output optical signal with a photodiode and analyze the PSD spectrum using an electrical spectrum analyzer.</li>
	<li>Tune the central wavelength of the TBPF. Measure the PSDs of every comb line using the described method. Figure 4B,C shows the PSD spectra for comb lines S1 and S2 of perfect SCs with the 2 km and 25 km delay optical fibers, respectively.</li>
	<li>Using the same method, measure the PSD curves of SCs with a vacancy. Record the 3 dB bandwidth of the PSD curve and linearly fit it piecewise as shown in Figure 5B,C. Repetition rate fluctuations of ~53.24 Hz within 10 &#181;s and ~509.32 Hz within 125 &#181;s were derived.</li>
</ol>

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The authors declare that they have no competing financial interests.

On-chip DKSs provide novel compact coherent optical sources and exhibit excellent application prospects in optical metrology, molecular spectroscopy, and other functions. For commercial applications, compact packaged micro-comb sources are essential. This protocol provides a practical approach to make a packaged micro-comb that benefits from the reliable, low coupling loss connection between the MRR and FA, as well as a robust thermal-controlled DKS generation method. Therefore, our experiments are no longer coupling stage dependent and exhibit excellent environmental adaptation. Meanwhile, the pump is a wavelength fixed laser that can be operated with narrower linewidths, produces significantly less noise, and is much smaller compared to tunable lasers. Therefore, the protocol is a promising approach to potential commercial applications of integrated high-performance on-chip DKSs sources.
The main limitation to obtaining a fully integrated SCs source is the high pump power, which needs an EDFA. Recently, DKSs have been realized on SiN MRR with very low pump power. Therefore, we believe that practical fully integrated DKS sources will be made in the near future.
The stability of the repetition rate is one of the most important parameters to evaluate the performance of OFCs. Generally, the repetition rate stability is measured using a frequency counter. However, the repetition rates of micro-combs are typically on the order of tens of GHz to THz, which is out of the bandwidth of frequency counters and photodetectors. Therefore, indirect methods, such as a reference laser source or a modulator, are usually used for repetition rate stability measurement, which increases the complexity of the measurement system. Our protocol provides a DSHI based repetition rate fluctuation measurement scheme, where high frequency components and ultra-stable reference sources are unnecessary. The system does not have an upper repetition rate limitation. Our system measures the accumulated frequency fluctuation during the delay time, while the frequency counter-based method tests the average value in a gate time. Therefore, our scheme is complementary to frequency counter-based repetition rate stability measurement systems.
The linewidth of the pump laser is essential for a DSHI based repetition rate fluctuation measurement scheme. When an optical field
<img alt="Equation 5" src="/files/ftp_upload/60689/60689eq5.jpg" />
is measured by a DSHI scheme, the PSD spectrum of the photocurrent can be expressed as:
<img alt="Equation 6" src="/files/ftp_upload/60689/60689eq6.jpg" />
where E0 and &#969;0 are the amplitude and angular frequency, respectively; &#966;(t) is the initial phase of the optical field; &#945; is the power ratio of the two interference arms; I0 is the input optical intensity; &#964;d and &#964;c are the relative delay time and the coherent time of the optical field, respectively; and &#937; is the frequency shift of the AOM. When &#964;c is greater than &#964;d, the PSD will be the overlap of a beating signal and a Dirac function, as shown in Figure 6 (red line). However, considering the frequency fluctuation of lasers, the optical field can be expressed as
<img alt="Equation 7" src="/files/ftp_upload/60689/60689eq7.jpg" />
where &#916;&#969; is the angular frequency fluctuation. For DSHI, an additional frequency shift is added. Figure 6 (blue line) shows the calculated PSD spectrum, where the optical frequency is linearly changed 10 kHz during the delay time. In contrast, when &#964;c is less than &#964;d, the Dirac function will be negligible, and our scheme can no longer measure the repetition rate fluctuation of DKSs. Our scheme is not suitable for the DKSs generated using a pump laser with a linewidth on the order of tens of kHz. Fortunately, a laser with a linewidth of less than 1 Hz has been commercialized, and locked fixed-frequency lasers with a linewidth of less than 40 mHz have been made20. Therefore, our scheme provides a simple rapid repetition rate instability measurement method for micro-comb performance evaluation in the future.&#160;

The steady DKSs in microresonators, where the cavity dispersion is balanced by Kerr nonlinearity, as well as the Kerr gain and cavity dissipation1, have attracted great interest in the scientific research community for their ultra-high repetition rate, compact size, and low cost2. In the time domain, DKSs are stable pulse trains that have been used for high-speed ranging measurement3 and molecular spectroscopy4. In the frequency domain, DKSs have a series of frequency lines with equal frequency spacing that are suitable for wavelength-division-multiplex (WDM) communications systems5,6, optical frequency synthesis7,8, and ultra-low noise microwave generation9,10, etc. The phase noise or linewidth of comb lines directly affects the performance of these application systems. It has been proven that all the comb lines have a similar linewidth with the pump11. Therefore, using an ultra-narrow linewidth laser as a pump is an effective approach to improve the performance of DKSs. However, the pumps of most reported DKSs are frequency sweeping external cavity diode lasers (ECDLs), which suffer from relatively high noise and have a broad linewidth on the order of tens to hundreds of kHz. Compared with tunable lasers, fixed-frequency lasers have less noise, narrower linewidths&#160;and smaller volume. For example, Menlo systems can provide ultra-stable laser products with a linewidth of less than 1 Hz. Using such a frequency fixed laser as a pump can significantly reduce the noise of the generated DKSs. Recently, microheater or thermoelectric cooler (TEC)-based thermal tuning methods have been used for DKSs generation12,13,14.
Repetition rate stability is another important parameter of DKSs. Generally, frequency counters are used to characterize the frequency stability of DKSs within a gate time, which is generally on the order of a microsecond to a thousand seconds15,16. Limited by the bandwidth of the photodetector and frequency counter, electro-optic modulators or reference lasers are typically used to lower the detected frequency when the free-spectral-range (FSR) of the DKSs is over 100 GHz. This not only increases the complexity of test systems, but also produces additional measurement errors caused by the stability of RF sources or reference lasers.
In this paper, a micro-ring resonator (MRR) is butterfly packaged with a commercial TEC chip that is used to control the operation temperature. Using a frequency fixed laser with a linewidth of 100 Hz as a pump, soliton crystals (SCs) are stably generated by manually decreasing the operating temperature; these are special DKSs that can completely fill a resonator with collectively ordered ensembles of copropagating solitons17. To the best of our knowledge, this is the narrowest linewidth pump in DKSs generation experiments. The power spectral density (PSD) spectrum of every comb line is measured based on a delayed self-heterodyne interferometer (DSHI) method. Benefitting from the ultra-narrow linewidth of the comb lines, the repetition rate instability of soliton crystals (SCs) is derived from the central frequency drift of the PSD curves. For the SC with a single vacancy, we obtained a repetition rate instability of ~53.24 Hz within 10 &#181;s and ~509.32 Hz within 125 &#181;s.
The protocol consists of several main stages: First, the MRR is coupled with a fiber array (FA) using a six-axis coupling stage. The MRR is fabricated by a high-index doped silica glass platform18,19. Then, the MRR is packaged into a 14-pin butterfly package, which increases the stability for the experiments. SCs are generated using a thermal-controlled method. Finally, the repetition rate fluctuations of SCs are measured by a DSHI method.

<table><tbody><tr><td>6-axis coupling stage</td><td>Suruga Seiki</td><td>KXC620G&lt;br/&gt; KGW060</td><td>Contains 3 linear motorized translation states and 3 angular motorized rotational stages.&lt;br/&gt; Linear state: Minimum stepping: 0.05 &amp;mu;m; Travel: 20mm; Max.speed: 25mm/s; Repeatability: +/-0.3 &amp;mu;m; Rotational stage:Travel: &amp;plusmn;8&amp;deg;; Resolution/pulse: 0.003 degree; Repeatability:&amp;plusmn;0.005&amp;deg;</td></tr><tr><td>Abrasive powder</td><td>Shenyang Kejing Auto-Instrument Co., LTD</td><td>2980002</td><td>Silicon carbide, granularity: 1.5 &amp;mu;m</td></tr><tr><td>Glue 3410</td><td>Electronic Materials Incorporated</td><td>Optocast 3410</td><td>Optocast 3410 is an ultra violet light and heat curable epoxy suitable for opto-electronic assembly. It cures rapidly when exposed to U.V. light in the 320-380 nm.</td></tr><tr><td>High-index doped silica glass</td><td>Home-made</td><td>-</td><td>The MRR is fabricated by a high index doped silica glass platform. The waveguide section is 2&amp;times;3 &amp;mu;m and radius is 592.1 &amp;mu;m, corresponding to FSR of 49 GHz.</td></tr><tr><td>Pump laser</td><td>NKT Photonics</td><td>E15</td><td>It is a continuous wave fiber laser with linewidth of 100 Hz.</td></tr><tr><td>Ultrastable Laser</td><td>Menlosystems</td><td>ORS</td><td>State-of-the-art linewidth (&amp;lt;1Hz) and stability (&amp;lt;2 x 10&lt;sup&gt;-15&lt;/sup&gt; Hz)</td></tr></tbody></table>

rapid repetition rate fluctuation measurement of soliton crystals in a microresonator

Temporal solitons have attracted great interest in the past decades for their behavior in a steady state, where the dispersion is balanced by the nonlinearity in a propagation Kerr medium. The development of dissipative Kerr solitons (DKSs) in high-Q microcavities drives a novel, compact, chip-scale soliton source. When DKSs serve as femtosecond pulses, the repetition rate fluctuation can be applied to ultrahigh precision metrology, high-speed optical sampling, and optical clocks, etc. In this paper, the rapid repetition rate fluctuation of soliton crystals (SCs), a special state of DKSs where particle-like solitons are tightly packed and fully occupy a resonator, is measured based on the well-known delayed self-heterodyne method. The SCs are generated using a thermal-controlled method. The pump is a frequency fixed laser with a linewidth of 100 Hz. The integral time in frequency fluctuation measurements is controlled by the length of the delay fiber. For a SC with a single vacancy, the repetition rate fluctuations are ~53.24 Hz within 10 &#181;s and ~509.32 Hz within 125 &#181;s, respectively.

Figure 3 shows the transmission power trace while a resonance thermal was tuned across the pump. There was an obvious power step that indicated the generation of SCs. The step had similar power compared with its precursor, the modulational instability comb. Therefore, the generation of SCs was not tuning speed dependent. The SCs exhibited a great variety of states, including vacancies (Schottky defects), Frenkel defects, and superstructure12,17. As examples, Figure 4A shows a perfect SC with 27 solitons and Figure 5A is a SC with a single vacancy.
The frequency of the &#181;th comb line equals to
<img alt="Equation 1" src="/files/ftp_upload/60689/60689eq1.jpg" />
and the frequency fluctuation of the &#181;th comb line can be expressed as:
<img alt="Equation 2" src="/files/ftp_upload/60689/60689eq2.jpg" />
where &#181; is the mode number away from the pump, frep is the repetition rate of the SCs, and &#916;fpump and &#916;frep are the frequency fluctuations of the pump laser and the repetition rate of the SCs, respectively. Therefore, the repetition rate fluctuation of the SCs was almost amplified &#181; times at the &#181;th frequency line.
For perfect SCs, Figure 4B,C shows the measured PSD spectra for the pump, S1 and S2, based on a 2 km and a 25 km delay fiber, respectively. The most notable features of the PSD curves were the flat tops, which were caused by the frequency fluctuation within the delay time. When the delay time was 10 &#181;s, the frequency fluctuations of S1 and S2 were 2.08 kHz and 3.54 kHz, respectively. When the delay fiber was 25 km, the measured frequency fluctuations of S1 and S2 were 14.31 kHz and 28.02 kHz, respectively.
Figure 5A shows the typical optical spectrum of SC with a single vacancy. There were 27 solitons circulating in the MRR. The measured frequency fluctuations of each comb line were plotted and are shown in Figure 5B,C. The piecewise linear fitting lines are plotted in blue lines which can be expressed as
<img alt="Equation 3" src="/files/ftp_upload/60689/60689eq3.jpg" /> 
<img alt="Equation 4" src="/files/ftp_upload/60689/60689eq4.jpg" />
The average slopes of the fitting lines were about 53.24 Hz/FSR and 509.32 Hz/FSR, which represent the repetition rate fluctuations of the SC within the responding delay times of 10 &#181;s and 125 &#181;s, respectively. The residual frequency fluctuations were regarded as the frequency fluctuation of the pump laser together with the frequency fluctuation of the driven radio signal of the AOM.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/60689/60689fig1.jpg" /> 
Figure 1. Butterfly Packaged MRR. (A) Model and (B) picture of the butterfly packaged MRR. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60689/60689fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/60689/60689fig2.jpg" /> 
Figure 2. The experimental setup of Kerr OFCs generation and repetition rate fluctuation measurement. The inset shows the optical spectrum of one typical SC with a single vacancy. A frequency fixed CW laser with a linewidth of 100 Hz was used as the pump. An EDFA was used to boost the pump up to 34 dBm. The frequency fluctuation was measured by the delayed self-heterodyne interferometer method. CW = continuous wave; EDFA = erbium-doped fiber amplifier; FPC = fiber polarization controller; TEC = thermoelectric cooler; MRR = micro-ring resonator; BPF = bandpass filter; AOM = acousto-optic modulator; PD = photodiode; ESA = electrical spectrum analyzer. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60689/60689fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/60689/60689fig3.jpg" /> 
Figure 3. Optical transmission power trace at the drop port. A SC step that had similar power to its precursor is clearly obtained. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60689/60689fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/60689/60689fig4.jpg" /> 
Figure 4. Perfect SC. (A) Measured optical spectrum of a perfect SC. The inset shows the uniformly distributed 27 solitons in the MRR. (B) Measured PSD curves of the pump, S1 and S2, with the 2 km delay fiber. (C) Measured PSD curves of the pump, S1 and S2, with the 25 km delay fiber. The flat-top PSD curve was caused by the rapid frequency fluctuation of the comb lines. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60689/60689fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/60689/60689fig5.jpg" /> 
Figure 5. SC with a single vacancy. (A) Optical spectrum of SC with a single vacancy. The inset shows the soliton distribution in the MRR. (B) Frequency fluctuations with the 2 km delay fiber. The repetition rate fluctuation was about 53.24 Hz within 10 &#181;s. The frequency fluctuation introduced by the pump laser and measurement system was about 500 Hz. (C) The repetition rate fluctuation with a 25 km delay fiber was about 626 Hz within 125 &#181;s. The frequency fluctuation introduced by the pump laser and measurement system was about 1 kHz. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60689/60689fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/60689/60689fig6.jpg" /> 
Figure 6. Theorical PSD of the photocurrent when the optical coherent length is larger than the relative delay time. The red line shows no central frequency fluctuation. The blue line presents the PSD with linear central frequency fluctuation. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/60689/60689fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Rapid Repetition Rate Fluctuation Measurement of Soliton Crystals in a Microresonator at JoVE.com

Rapid Repetition Rate Fluctuation Measurement of Soliton Crystals in a Microresonator

Here, we present a protocol to generate soliton crystals in a butterfly-packaged micro-ring resonator using a thermal tuned method. Further, the repetition rate fluctuations of a soliton crystal with a single vacancy are measured using a delayed self-heterodyne method.

Temporal solitons have attracted great interest in the past decades for their behavior in a steady state, where the dispersion is balanced by the ...

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3.0K Views.  Chinese Academy of Sciences (CAS). De afgelopen jaren is dissipatief Kerr-soliton een nieuwe coherente lichtbron op chipschaal geworden, die veel aandacht heeft getrokken voor de enorme waarde in solitonfysica-onderzoek en praktische toepassingen. Dissipatief Kerr-soliton heeft een hoge herhalingsfrequentie. Het is dus een uitdaging om de relatieve parameters te meten, met name de herhalingssnelheidsfluctuatie.In ons werk vinden we een manier om het te krijgen. Ondertussen is een robuust pakket noodzakelijk voor praktis...

Video: Rapid Repetition Rate Fluctuation Measurement of Soliton Crystals in a Microresonator

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals1-4 and the enhancement of both linear5-7 and nonlinear devices.8-11
Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically12 and electrically13 pumped ultra-low threshold lasers and the enhancement of nonlinear effects.14-16 Furthermore, passive filters17 and modulators18-19 have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption.
To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues.
The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum20-21 and interferometric techniques.22-25 Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.26 Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans.
When characterising photonic crystal cavities, techniques involving internal sources21 or external waveguides directly coupled to the cavity27 impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.28 and further developed by Galli et al.29

Use of photonic crystal slow light waveguides and cavities has been widely adopted by the photonics community in many differing applications. Therefore fabrication and characterization of these devices are of great interest. This paper outlines our fabrication technique and two optical characterization methods, namely: interferometric (waveguides) and resonant scattering (cavities).

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view ...

Microwave Photonics Systems Based on Whispering-gallery-mode Resonators

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various areas of technology and applied science, such as aerospace and communication engineering, sensing, metrology, nonlinear photonics, and quantum optics. In this article, we present the principal techniques used in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators. First detailed in this article is the protocol for resonator polishing, which is based on a grind-and-polish technique close to the ones used to polish optical components such as lenses or telescope mirrors. Then, a white light interferometric profilometer measures surface roughness, which is a key parameter to characterize the quality of the polishing. In order to launch light in the resonator, a tapered silica fiber with diameter in the micrometer range is used. To reach such small diameters, we adopt the "flame-brushing" technique, using simultaneously computer-controlled motors to pull the fiber apart, and a blowtorch to heat the fiber area to be tapered. The resonator and the tapered fiber are later approached to one another to visualize the resonance signal of the whispering gallery modes using a wavelength-scanning laser. By increasing the optical power in the resonator, nonlinear phenomena are triggered until the formation of a Kerr optical frequency comb is observed with a spectrum made of equidistant spectral lines. These Kerr comb spectra have exceptional characteristics that are suitable for several applications in science and technology. We consider the application related to ultra-stable microwave frequency synthesis and demonstrate the generation of a Kerr comb with GHz intermodal frequency.

The customized techniques developed in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators are presented. The protocols to obtain and characterize these resonators are detailed, and an explanation of some of their applications in microwave photonics is given.

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various ...

Measurement of Coherence Decay in GaMnAs Using Femtosecond Four-wave Mixing

The application of femtosecond four-wave mixing to the study of fundamental properties of diluted magnetic semiconductors ((s,p)-d hybridization, spin-flip scattering) is described, using experiments on GaMnAs as a prototype III-Mn-V system.&#160; Spectrally-resolved and time-resolved experimental configurations are described, including the use of zero-background autocorrelation techniques for pulse optimization.&#160; The etching process used to prepare GaMnAs samples for four-wave mixing experiments is also highlighted.&#160; The high temporal resolution of this technique, afforded by the use of short (20 fsec) optical pulses, permits the rapid spin-flip scattering process in this system to be studied directly in the time domain, providing new insight into the strong exchange coupling responsible for carrier-mediated ferromagnetism.&#160; We also show that spectral resolution of the four-wave mixing signal allows one to extract clear signatures of (s,p)-d hybridization in this system, unlike linear spectroscopy techniques.&#160;&#160; This increased sensitivity is due to the nonlinearity of the technique, which suppresses defect-related contributions to the optical response. This method may be used to measure the time scale for coherence decay (tied to the fastest scattering processes) in a wide variety of semiconductor systems of interest for next generation electronics and optoelectronics.

The technique of femtosecond four-wave mixing is described, including spectrally-resolved and time-resolved configurations. We illustrate the utility of this technique for the investigation of crucial physical properties in the III-V diluted magnetic semiconductors, afforded by its nonlinearity and high temporal resolution.

The application of femtosecond four-wave mixing to the study of fundamental properties of diluted magnetic semiconductors ((s,p)-d hybridization, ...

Fabrication and Testing of Microfluidic Optomechanical Oscillators

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.

Parametric optomechanical excitations have recently been experimentally demonstrated in microfluidic optomechanical resonators by means of optical radiation pressure and stimulated Brillouin scattering. This paper describes the fabrication of these microfluidic resonators along with methodologies for generating and verifying optomechanical oscillations.

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including ...

High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we demonstrate an optical spectroscopy method that is capable of resolving spectral features beyond that of the spin fine structure and homogeneous linewidth of single quantum dots (QDs) using a standard, easy-to-use spectrometer setup. This method incorporates both laser and photoluminescence spectroscopy, combining the advantage of laser line-width limited resolution with multi-channel photoluminescence detection. Such a scheme allows for considerable improvement of resolution over that of a common single-stage spectrometer. The method uses phonons to assist in the measurement of the photoluminescence of a single quantum dot after resonant excitation of its ground state transition. The phonon&#39;s energy difference allows one to separate and filter out the laser light exciting the quantum dot. An advantageous feature of this method is its straight forward integration into standard spectroscopy setups, which are accessible to most researchers.

The manuscript describes a method of phonon-assisted quasi-resonant fluorescence spectroscopy that incorporates both laser-limited resolution and photoluminescence (PL) spectroscopy. This method utilizes optical phonons to provide linewidth-limited resolution spectra of atom-like semiconductor structures in the energy domain. The method is also easily realized with a single spectrometer optical spectroscopy setup.

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we ...

Fabrication and Characterization of Superconducting Resonators

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.

Superconducting microwave resonators are of interest for detection of light, quantum computing applications and materials characterization. This work presents a detailed procedure for fabrication and characterization of superconducting microwave resonator scattering parameters.

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors ...

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb explosion imaging setup in which a hitherto-impractical camera angle is realized. In our imaging technique, diatomic molecules are irradiated with a circularly polarized strong laser pulse. The ejected atomic ions are accelerated perpendicularly to the laser propagation. The ions lying in the laser polarization plane are selected through the use of a mechanical slit and imaged with a high-throughput, 2-dimensional detector installed parallel to the polarization plane. Because a circularly polarized (isotropic) Coulomb exploding pulse is used, the observed angular distribution of the ejected ions directly corresponds to the squared rotational wave function at the time of the pulse irradiation. To create a real-time movie of molecular rotation, the present imaging technique is combined with a femtosecond pump-probe optical setup in which the pump pulses create unidirectionally rotating molecular ensembles. Due to the high image throughput of our detection system, the pump-probe experimental condition can be easily optimized by monitoring a real-time snapshot. As a result, the quality of the observed movie is sufficiently high for visualizing the detailed wave nature of motion. We also note that the present technique can be implemented in existing standard ion imaging setups, offering a new camera angle or viewpoint for the molecular systems without the need for extensive modification.

We present a protocol for creating a real-time movie of a molecular rotational wave packet using a high-resolution Coulomb explosion imaging setup.

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb ...

Chemistry

Recombination Dynamics in Thin-film Photovoltaic Materials via Time-resolved Microwave Conductivity

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials such as organo-lead halide perovskites is presented. The perovskite film thickness and absorption coefficient are initially characterized by profilometry and UV-VIS absorption spectroscopy. Calibration of both laser power and cavity sensitivity is described in detail. A protocol for performing Flash-photolysis Time Resolved Microwave Conductivity (TRMC) experiments, a non-contact method of determining the conductivity of a material, is presented. A process for identifying the real and imaginary components of the complex conductivity by performing TRMC as a function of microwave frequency is given. Charge carrier dynamics are determined under different excitation regimes (including both power and wavelength). Techniques for distinguishing between direct and trap-mediated decay processes are presented and discussed. Results are modelled and interpreted with reference to a general kinetic model of photoinduced charge carriers in a semiconductor. The techniques described are applicable to a wide range of optoelectronic materials, including organic and inorganic photovoltaic materials, nanoparticles, and conducting/semiconducting thin films.

A Time Resolved Microwave Conductivity technique for investigating direct and trap-mediated recombination dynamics and determining carrier mobilities of thin film semiconductors is presented here.

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials ...

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Silicon photonic chips have the potential to realize complex integrated quantum systems. Presented here is a method for preparing and testing a silicon photonic chip for quantum measurements.

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit ...

Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization

We present a pump-probe method for preparing vibrational coherences in polyatomic radical cations and probing their ultrafast dynamics. By shifting the wavelength of the strong-field ionizing pump pulse from the commonly used 800 nm into the near-infrared (1200-1600 nm), the contribution of adiabatic electron tunneling to the ionization process increases relative to multiphoton absorption. Adiabatic ionization results in predominant population of the ground electronic state of the ion upon electron removal, which effectively prepares a coherent vibrational state (&#34;wave packet&#34;) amenable to subsequent excitation. In our experiments, the coherent vibrational dynamics are probed with a weak-field 800 nm pulse and the time-dependent yields of dissociation products measured in a time-of-flight mass spectrometer. We present the measurements on the molecule dimethyl methylphosphonate (DMMP) to illustrate how using 1500 nm pulses for excitation enhances the amplitude of coherent oscillations in ion yields by a factor of 10 as compared to 800 nm pulses. This protocol may be implemented in existing pump-probe setups through the incorporation of an optical parametric amplifier (OPA) for wavelength conversion.

We present a protocol for probing ultrafast vibrational coherences in polyatomic radical cations that result in molecular dissociation.

We present a pump-probe method for preparing vibrational coherences in polyatomic radical cations and probing their ultrafast dynamics. By shifting the ...

Rapid Repetition Rate Fluctuation Measurement of Soliton Crystals in a Microresonator

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